News Article | February 15, 2017
A star -- as big or bigger than our sun -- in Pegasus constellation is expanding and contracting in 3 directions at once every 2.5 hours, the result of heating and cooling of hydrogen fuel burning 28 million degrees Fahrenheit at its core Astronomers are reporting a rare star as big -- or bigger -- than the Earth's sun and that is expanding and contracting in a unique pattern in three different directions. The star is one that pulsates and so is characterized by varying brightness over time. It's situated 7,000 light years away from the Earth in the constellation Pegasus, said astronomer Farley Ferrante, a member of the team that made the discovery at Southern Methodist University, Dallas. Called a variable star, this particular star is one of only seven known stars of its kind in our Milky Way galaxy. "It was challenging to identify it," Ferrante said. "This is the first time we'd encountered this rare type." The Milky Way has more than 100 billion stars. But just over 400,900 are catalogued as variable stars. Of those, a mere seven -- including the one identified at SMU -- are the rare intrinsic variable star called a Triple Mode 'high amplitude delta Scuti' (pronounced SKOO-tee) or Triple Mode HADS(B), for short. "The discovery of this object helps to flesh out the characteristics of this unique type of variable star. These and further measurements can be used to probe the way the pulsations happen," said SMU's Robert Kehoe, a professor in the Department of Physics who leads the SMU astronomy team. "Pulsating stars have also been important to improving our understanding of the expansion of the universe and its origins, which is another exciting piece of this puzzle." The star doesn't yet have a common name, only an official designation based on the telescope that recorded it and its celestial coordinates. The star can be observed through a telescope, but identifying it was much more complicated. A high school student in an SMU summer astronomy program made the initial discovery upon culling through archived star observation data recorded by the small but powerful ROTSE-I telescope formerly at Los Alamos National Laboratory in New Mexico. Upon verification, the star was logged into the International Variable Star Index as ROTSE1 J232056.45+345150.9 by the American Association of Variable Star Observers at https:/ . SMU's astrophysicists discovered the variable star by analyzing light curve shape, a key identifier of star type. Light curves were created from archived data procured by ROTSE-I during multiple nights in September 2000. The telescope generates images of optical light from electrical signals based on the intensity of the source. Data representing light intensity versus time is plotted on a scale to create the light curves. Plano Senior High School student Derek Hornung first discovered the object in the ROTSE-I data and prepared the initial light curves. From the light curves, the astronomers knew they had something special. It became even more challenging to determine the specific kind of variable star. Then Eric Guzman, a physics graduate from the University of Texas at Dallas, who is entering SMU's graduate program, solved the puzzle, identifying the star as pulsating. "Light curve patterns are well established, and these standard shapes correspond to different types of stars," Ferrante said. "In a particular field of the night sky under observation there may have been hundreds or even thousands of stars. So the software we use generates a light curve for each one, for one night. Then -- and here's the human part -- we use our brain's capacity for pattern recognition to find something that looks interesting and that has a variation. This allows the initial variable star candidate to be identified. From there, you look at data from several other nights. We combine all of those into one plot, as well as add data sets from other telescopes, and that's the evidence for discerning what kind of variable star it is." That was accomplished conclusively during the referee process with the Variable Star Index moderator. The work to discover and analyze this rare variable star was carried out in conjunction with analyses by eight other high school students and two other undergraduates working on other variable candidates. The high school students were supported by SMU's chapter of the Department of Energy/National Science Foundation QuarkNet program. Of the stars that vary in brightness intrinsically, a large number exhibit amazingly regular oscillations in their brightness which is a sign of some pulsation phenomenon in the star, Ferrante said. Pulsation results from expanding and contracting as the star ages and exhausts the hydrogen fuel at its core. As the hydrogen fuel burns hotter, the star expands, then cools, then gravity shrinks it back, and contraction heats it back up. "I'm speaking very generally, because there's a lot of nuance, but there's this continual struggle between thermal expansion and gravitational contraction," Ferrante said. "The star oscillates like a spring, but it always overshoots its equilibrium, doing that for many millions of years until it evolves into the next phase, where it burns helium in its core. And if it's about the size and mass of the sun -- then helium fusion and carbon is the end stage. And when helium is used up, we're left with a dying ember called a white dwarf." Within the pulsating category is a class of stars called delta Scuti, of which there are thousands. They are named for a prototype star whose characteristic features -- including short periods of pulsating on the scale of a few hours -- are typical of the entire class. Within delta Scuti is a subtype of which hundreds have been identified, called high amplitude delta Scuti, or HADS. Their brightness varies to a particularly large degree, registering more than 10 percent difference between their minimum and maximum brightness, indicating larger pulsations. Common delta Scuti pulsate along the radius in a uniform contraction like blowing up a balloon. A smaller sub-category are the HADS, which show asymmetrical-like pulsating curves. Within HADS, there's the relatively rare subtype called HADS(B) , of which there are only 114 identified. A HADS(B) is distinguished by its two modes of oscillation -- different parts of the star expanding at different rates in different directions but the ratio of those two periods is always the same. For the SMU star, two modes of oscillation weren't immediately obvious in its light curve. "But we knew there was something going on because the light curve didn't quite match known light curves of other delta Scuti's and HADS' objects we had studied. The light curves -- when laid on top of each other -- presented an asymmetry," Ferrante said. "Ultimately the HADS(B) we discovered is even more unique than that though -- it's a Triple Mode HADS(B) and there were previously only six identified in the Milky Way. So it has three modes of oscillation, all three with a distinct period, overlapping, and happening simultaneously." So rare, in fact, there's no name yet for this new category nor a separate registry designation for it. Guzman, the student researcher who analyzed and categorized the object, recalled how the mystery unfolded. "When I began the analysis of the object, we had an initial idea of what type it could be," Guzman said. "My task was to take the data and try to confirm the type by finding a second period that matched a known constant period ratio. After successfully finding the second mode, I noticed a third signal. After checking the results, I discovered the third signal coincided with what is predicted of a third pulsation mode." The SMU Triple Mode HADS(B) oscillates on a scale of 2.5 hours, so it will expand and contract 10 times in one Earth day. It and the other known six HADS(B)'s are in the same general region of the Milky Way galaxy, within a few thousand light years of one another. "I'm sure there are more out there," Ferrante said, "but they're still rare, a small fraction." SMU's Triple Mode HADS(B) is unstable and further along in its stellar evolution than our sun, which is about middle-aged and whose pulsating variations occur over a much longer period of time. SMU's Triple Mode HADS(B) core temperature, heated from the burning of hydrogen fuel, is about 15 million Kelvin or 28 million degrees Fahrenheit. Someday, millions of years from now, SMU's Triple Mode HADS(B) will deplete the hydrogen fuel at its core, and expand into a red giant. "Our sun might eventually experience this as well," Ferrante said. "But Earth will be inhospitable long before then. We won't be here to see it." Funding was through the Texas Space Grant Consortium, an affiliate of NASA; SMU Dedman College; and the Department of Energy/National Science Foundation QuarkNet program. ROTSE-I began operating in late 1997, surveying the sky all night, every clear night of the year for three years. It was decommissioned in 2001 and replaced by ROTSE-III. SMU owns the ROTSE-IIIb telescope at McDonald Observatory, Fort Davis, Texas.
News Article | April 20, 2016
In the first section below, we describe spectroscopic data for NGC 1600 and our procedures for measuring stellar kinematics. In the second section, we describe photometric data and the surface brightness profile of NGC 1600. In the third section, we describe our stellar orbit modelling procedures. In the fourth section we compare the stellar mass-to-light ratio obtained from the dynamical models with independent constraints from a stellar population analysis. The implied distribution of stellar orbits in NGC 1600 is described in the fifth section. Finally, in the last section we present the black-hole scaling relations for core radii, determined from an alternative fit to the light profiles. We obtained high-spatial-resolution spectroscopic data from GMOS-N, an IFS on the 8-metre Gemini North Telescope. We observed the central region of NGC 1600 with GMOS-N, which provides continuous two-dimensional coverage of a 5 arcsec × 7 arcsec science field and simultaneously covers a sky field offset by 60 arcsec. Our spectra were centred on the triplet of calcium absorption lines from 8,480 Å to 8,680 Å, a well studied region frequently used for stellar kinematic measurements31, 32. We obtained nine 1,230-second exposures of NGC 1600 over three nights of queue-mode observations in November 2014. GMOS-N is seeing-limited, with spatial sampling of 0.2 arcsec on the IFS. We estimated the point-spread function (PSF) on each observing night by measuring the width of foreground stars in the acquisition images of NGC 1600. Our average PSF for GMOS-N has a full width at half-maximum (FWHM) of 0.6 arcsec. A Gaussian model of the PSF is included in our orbit superposition models. We used the image reduction and analysis facility (IRAF) software package supplied by the Gemini Observatory to flat-field and wavelength-calibrate the GMOS-N data, and to extract a one-dimensional spectrum for each IFS lenslet. We developed custom routines to construct collapsed images of the galaxy and record the position of each one-dimensional spectrum with respect to the galaxy centre. The spectra were then resampled on a two-dimensional grid and binned to a consistent signal-to-noise ratio (of about 100 per pixel) using Voronoi tessellation33. Our binning implementation imposed symmetry over four projected quadrants of the galaxy, so that the kinematic measurements could be folded into a single quadrant before orbit modelling. We obtained wide-field spectroscopic data from the Mitchell Spectrograph34 on the 2.7-metre Harlan J. Smith Telescope at the McDonald Observatory. The Mitchell Spectrograph is an optical IFS with a 107 arcsec × 107 arcsec field of view and 246 fibres, each of 4.1-arcsec diameter. The low-resolution blue setting (R ≈ 850) was used, providing wavelength coverage from 3,650 Å to 5,850 Å, including the Ca H+K region, the G-band region, Hβ, the Mgb region, and several Fe absorption features. The spectral resolution varied spatially and with wavelength, with an average of 5 Å FWHM, corresponding to a dispersion of ~1.1 Å pixel−1 and σ ≈ 100 km s−1 in the red and σ ≈ 150 km s−1 in the blue part of the spectrum (where σ is the instrumental resolution). Data reduction was performed using the Vaccine package35. We fit Mitchell spectra with the MILES library of 985 stellar spectra36 and determined the best-fit line-of-sight velocity distribution (LOSVD) for each of the 58 spatial bins6, 37. Our GMOS and Mitchell data are the first IFS observations of NGC 1600 and reveal a stellar velocity distribution that is well aligned with the galaxy’s light distribution, indicating that NGC 1600 is axisymmetric. We used a high-resolution image taken with the near-infrared camera and multi-object spectrometer (NICMOS) instrument on the HST to measure the central light distribution of NGC 1600. The observation (from General Observer Program number 7886) consisted of four dithered exposures of NGC 1600 taken with NICMOS camera 2 in the F160W bandpass. We downloaded the calibrated, combined image from the Hubble Legacy Archive. The image had a pixel scale of 0.05 arcsec per pixel and total exposure time of 460 seconds. We combined the HST observations with ground-based photometric data at large radii taken from the literature7. The NICMOS data were calibrated to the R-band of the ground-based data by minimizing the squared magnitude differences between the two surface brightness profiles in the radial region where both data sets overlap and PSF effects are negligible (r = 2–10 arcsec). Single one-dimensional profiles of the surface brightness, the ellipticity and the isophotal shape parameters38 a and a were then constructed by using the NICMOS data at r < 8 arcsec and the ground-based data at r ≥ 8 arcsec. The position angle (PA) of the isophotes is constant with radius7 (ΔPA < 2°), consistent with an axisymmetric stellar distribution. The resulting circularized surface brightness distribution of NGC 1600 is well described by a core-Sérsic function with a core radius of r = 2.15 arcsec (Extended Data Fig. 2). Over several orders of magnitude in radius, the light profiles of lower-luminosity elliptical galaxies follow a single Sérsic function characterized by the Sérsic index n, the half-light radius r and a surface brightness scale μ = μ(r ). The core-Sérsic function combines a Sérsic profile at r > r and a power-law distribution with slope at r < r . The transition is controlled by a smoothness parameter α and the surface brightness scale is μ = μ(r ). Inside r < 5 arcsec, the inward extrapolation of the outer Sérsic component overpredicts the central surface brightness of NGC 1600 by about three magnitudes. From the difference between the integrated light of the Sérsic component and the actual core-Sérsic fit, we derive a ‘light deficit’ of ΔL = 9.47 × 109L in the centre of NGC 1600. We use the isophotal model of NGC 1600 to compute the galaxy’s intrinsic luminosity density distribution. The deprojection is nonparametric39 and accounts for the observed ellipticity profile and boxy shape of NGC 1600’s isophotes7. The same technique has been used for the dynamical modelling of other galaxies12, 40. We generate dynamical models of NGC 1600 using Schwarzschild’s orbit superposition method41. Because the two-body relaxation time of stars in massive elliptical galaxies exceeds the age of the Universe, their dynamics is governed by the collisionless Boltzmann equation. Any steady-state equilibrium solution of the Boltzmann equation can be written as a sum over single-orbit distribution functions, where the phase-space density of stars along each trajectory is constant. The total number of stars on each orbit—that is, the orbital luminosity or orbital occupation number—can take arbitrary (positive) values. For the modelling, we assume that NGC 1600 is axisymmetric and that the stellar mass profile follows the observed light distribution with a constant stellar mass-to-light ratio, M /L. The models also assume a central black hole of mass M and a cored isothermal dark-matter halo with core radius r and asymptotic circular velocity v . The four parameters of the mass model are constrained by the photometrically derived luminosity density and by 1,978 LOSVD data points measured from the galaxy spectra between 0.4 arcsec and ~45 arcsec. Given some specific values for M /L, M , r and v , the Poisson equation is solved for the gravitational potential generated by the respective mass model and the luminosity densities, and the LOSVDs of ~29,000 representative stellar orbits are computed9. The orbits are convolved with the PSF of the observations and integrated over the respective areas on the sky. We use a maximum entropy technique42 to determine the orbital occupation numbers that minimize the χ2 difference among the observational data and the orbit superposition model. Thousands of different mass distributions are compared to the data by systematically varying M /L, M , r and v . We repeat the computation of the stellar orbits and the phase-space optimization independently for each model. The best-fit values and confidence intervals in M and M /L are determined by evaluating the relative likelihood43 for all models with different assumed values of M , M /L and dark halo parameters (Extended Data Fig. 3). The same modelling technique to determine the mass of the stars, the central black hole and the dark matter halo was first applied to the central galaxy of the Virgo Cluster, M87 (ref. 44). Previous models of NGC 1600, based on stellar velocity data only along the major and minor axes of the galaxy and with a lower spatial resolution, did not include all three mass components45, 46, 47, 48. We have tested that models without a dark-matter halo and/or without a central black hole cannot reproduce the full set of our new observations. Extended Data Fig. 4 shows our best-fit orbit model together with the velocity data for NGC 1600. The stellar mass-to-light ratio derived from our dynamical modelling is M /L = (4.0 ± 0.15)M /L . The stellar mass deficit in the core of NGC 1600 is thus ΔM = 3.8 × 1010M . In other core galaxies, mass deficits have been reported to range from one to ten times the mass of the central black hole15, 17, 49. For NGC 1600 we find a mass deficit, ΔM , of 2.2 × M . Results from numerical simulations of mergers of galaxies with central black holes50 suggest mass deficits of ~N × 0.5M , where N is the number of mergers. We measure a stellar population age of τ ≈ 10 Gyr, a total metallicity of [Z/H] = 0.03 and an iron abundance of [Fe/H] = −0.15 for NGC 1600, from the absorption-line strengths of hydrogen, iron and other metallicity-dependent stellar lines in the 1-kpc galaxy core. It has been reported that the fraction of low-luminosity dwarf stars in elliptical galaxies less luminous than NGC 1600 is larger than that in the Milky Way51, 52, 53. Extrapolating correlations with the galaxy velocity dispersion obtained for these smaller elliptical galaxies would yield a higher stellar mass of M /L ≈ 6.0 for NGC 1600. We find the galaxy-wide contribution of dwarf stars to the stellar mass in NGC 1600 to be consistent with observations of the Milky Way. Dynamical constraints on the fractional mass of dwarf stars depend on the assumed dark-matter halo profile54 and on the assumed shape of the stellar initial mass function55. Some nearby massive galaxies have, like NGC 1600, dwarf-star fractions consistent with that of the Milky Way56. The spectroscopic analysis of the age and chemical composition of the stars in NGC 1600 does not provide evidence for a notable change in the stellar population with radius. In the dynamical modelling we assume a constant M /L ratio throughout the galaxy. Individual massive galaxies have been reported to host extreme populations of dwarf stars at their centres57 that would not be detectable in our optical spectra. The respective dwarf stars can increase the central M /L by up to a factor of three. If the assumed constant M /L does not account for all of the stellar mass at the centre of NGC 1600, then the dynamical models may compensate for the missing stellar mass by overestimating M . Extended Data Fig. 5 shows the enclosed mass distribution of NGC 1600 over the region for which we have obtained stellar velocity data. For a constant M /L, we find the enclosed stellar mass at the smallest observed radius (r ≈ 0.2 kpc) to be 100 times smaller than M . A central increase of M /L by a factor as extreme as ten would imply an unaccounted-for extra stellar mass of 10% of M (dotted lines in Extended Data Fig. 5). Even in this unrealistic case, we would overestimate M only by its one sigma measurement error. Because there is so little stellar light in the core of NGC 1600, uncertainties in the central stellar population have a negligible effect on our black-hole mass measurement. The stars in a galaxy are collisionless and their velocity distribution can be anisotropic. We compute the intrinsic velocity dispersions of the stars along the radial and the two angular directions of a polar coordinate system—σ , σ , and σ , respectively—from the orbital occupation numbers of our best-fit dynamical model in 20 spherical shells centred on NGC 1600’s black hole. The classic measure for the anisotropy of the stellar velocities is , where the tangential velocity dispersion is the average of the motions in the two angular directions. Stellar orbits in core galaxies have been reported to be very uniform18. In NGC 1600 and similar galaxies with a flat central surface brightness, most of the stars inside the diffuse core region (r < r ) are moving along tangential directions. With increasing distance from the centre, more and more stars are found on radially elongated orbits (Extended Data Fig. 6). In the black-hole-binary model, the observed stellar motions are naturally explained as the leftover of the core scouring process (shaded regions in Extended Data Fig. 6). Central stars originally on radial trajectories are subject to interactions with the black-hole binary as they frequently pass the galaxy centre. Eventually, these stars get ejected to larger radii via gravitational slingshot. The stars that we observe today in the centres of core galaxies remained there because they moved (and still move) on tangential orbits that avoid the centre58. It has not yet been tested whether other black-hole activities—such as their feedback processes on ambient accreting gas—can produce the tight relations between black-hole mass, core radius, sphere of influence and mass deficit together with the observed orbital structure. While the core-Sérsic function describes galaxy light profiles from the core region out to large radii30, 59, the Nuker function60 has been widely used to fit the central light profiles of galaxies observed with HST. Fifteen out of the 21 core galaxies discussed in the text also have core radii measured from Nuker fits13. For the Nuker r , we obtain log (r /kpc) = (−0.18 ± 0.21) + (1.00 ± 0.09)log (r /kpc) with an intrinsic scatter of ϵ = 0.16, and log (M /M ) = (10.06 ± 0.45) + (1.25 ± 0.17)log (r /kpc) with an intrinsic scatter of ϵ = 0.31. The Nuker r values were measured along the major axis of the galaxies, while the core-Sérsic r values discussed in the text come from the galaxies’ circularized light profiles15. Galaxy core sizes have also been quantified by the cusp radius61, r —that is, the radius at which the negative logarithmic slope of the surface brightness profile equals 1/2. We obtained the cusp radii of the galaxies that are shown in Figs 3 and 4 from their core-Sérsic models. Using r as a measure of the core size, we find log (r /kpc) = (0.06 ± 0.28) + (0.94 ± 0.09)log (r /kpc) (intrinsic scatter ϵ = 0.16) and log (M /M ) = (10.37 ± 0.60) + (1.20 ± 0.19)log (r /kpc), ϵ = 0.33. Note that the core of NGC 1550 has a slope15 of γ = 0.52 ± 0.05. The scaling relations with r have been computed without including NGC 1550. The slope and the scatter in the above correlations are consistent with the results for the core-Sérsic r .
News Article | November 4, 2015
After several years and a massive team effort, one of the world's largest telescopes has opened its giant eye again. The Hobby-Eberly Telescope (HET) at The University of Texas at Austin's McDonald Observatory has completed a $25 million upgrade and, now using more of its primary mirror, has achieved "first light" as the world's third-largest optical telescope.
News Article | October 26, 2016
Insufficient instrument thermo-mechanical stability is one of the many roadblocks for achieving 10cm/s Doppler radial velocity (RV) precision, the precision needed to detect Earth-twins orbiting Solar-type stars. Highly temperature and pressure stabilized spectrographs allow us to better calibrate out instrumental drifts, thereby helping in distinguishing instrumental noise from astrophysical stellar signals. We present the design and performance of the Environmental Control System (ECS) for the Habitable-zone Planet Finder (HPF), a high-resolution (R=50,000) fiber-fed near infrared (NIR) spectrograph for the 10m Hobby Eberly Telescope at McDonald Observatory. HPF will operate at 180K, driven by the choice of an H2RG NIR detector array with a 1.7micron cutoff. This ECS has demonstrated 0.6mK RMS stability over 15 days at both 180K and 300K, and maintained high quality vacuum ( A similar ECS is being implemented to stabilize NEID, the NASA/NSF NN-EXPLORE spectrograph for the 3.5m WIYN telescope at Kitt Peak, operating at 300K. A full SolidWorks 3D-CAD model and a comprehensive parts list of the HPF ECS are included with this manuscript to facilitate the adaptation of this versatile environmental control scheme in the broader astronomical community. A Versatile Technique to Enable sub-milli-Kelvin Instrument Stability for Precise Radial Velocity Measurements: Tests with the Habitable-zone Planet Finder Comments: Accepted for publication in ApJ. 16 pages, 10 figures. For a publicly available SolidWorks model of the HPF ECS, see this https URL Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Earth and Planetary Astrophysics (astro-ph.EP) Cite as: arXiv:1610.06216 [astro-ph.IM] (or arXiv:1610.06216v1 [astro-ph.IM] for this version) Submission history From: Gudmundur Stefansson [v1] Wed, 19 Oct 2016 20:59:41 GMT (11416kb,D) https://arxiv.org/abs/1610.06216
News Article | March 22, 2016
The blue-white dot at the center of this image is supernova 2012cg, seen by the 1.2-meter telescope at Fred Lawrence Whipple Observatory. At 50 million light-years away, this supernova is so distant that its host galaxy, the edge-on spiral NGC 4424, appears here as only an extended smear of purple light. Credit: Peter Challis/Harvard-Smithsonian CfA A team of astronomers including Harvard's Robert Kirshner and Peter Challis has detected a flash of light from the companion to an exploding star. This is the first time astronomers have witnessed the impact of an exploding star on its neighbor. It provides the best evidence on the type of binary star system that leads to Type Ia supernovae. This study reveals the circumstances for the violent death of some white dwarf stars and provides deeper understanding for their use as tools to trace the history of the expansion of the universe. These types of stellar explosions enabled the discovery of dark energy, the universe's accelerating expansion that is one of the top problems in science today. The subject of how Type Ia supernovae arise has long been a topic of debate among astronomers. "We think that Type Ia supernovae come from exploding white dwarfs with a binary companion," said Howie Marion of The University of Texas at Austin (UT Austin), the study's lead author. "The theory goes back 50 years or so, but there hasn't been any concrete evidence for a companion star before now." Astronomers have battled over competing ideas, debating whether the companion was a normal star or another white dwarf. "This is the first time a normal Type Ia has been associated with a binary companion star," said team member and professor of astronomy J. Craig Wheeler (UT Austin). "This is a big deal." The binary star progenitor theory for Type Ia supernovae starts with a burnt-out star called a white dwarf. Mass must be added to that white dwarf to trigger its explosion - mass that the dwarf pulls off of a companion star. When the influx of mass reaches the point that the dwarf is hot enough and dense enough to ignite the carbon and oxygen in its interior, a thermonuclear reaction starts that causes the dwarf to explode as a Type Ia supernova. For a long time, the leading theory was that the companion was an old red giant star that swelled up and lost matter to the dwarf, but recent observations have virtually ruled out that notion. No red giant is seen. The new work presents evidence that the star providing the mass is still burning hydrogen at its center, that is, that this companion star is still in the prime of life. According to team member Robert P. Kirshner of the Harvard-Smithsonian Center for Astrophysics, "If a white dwarf explodes next to an ordinary star, you ought to see a pulse of blue light that results from heating that companion. That's what theorists predicted and that's what we saw. "Supernova 2012cg is the smoking—actually glowing—gun: some Type Ia supernovae come from white dwarfs doing a do-si-do with ordinary stars." Located 50 million light-years away in the constellation Virgo, Supernova 2012cg was discovered on May 17, 2012 by the Lick Observatory Supernova Search. Marion's team began studying it the next day with the telescopes of the Harvard-Smithsonian Center for Astrophysics. "It's important to get very early observations," Marion said, "because the interaction with the companion occurs very soon after the explosion." The team continued to observe the supernova's brightening for several weeks using many different telescopes, including the 1.2-meter telescope at Fred Lawrence Whipple Observatory and its KeplerCam instrument, the Swift gamma-ray space telescope, the Hobby-Eberly Telescope at McDonald Observatory, and about half a dozen others. "This is a global enterprise," Wheeler said. Team members hail from about a dozen U.S. universities, as well as institutions in Chile, Hungary, Denmark, and Japan. What the team found was evidence in the characteristics of the light from the supernova that indicated it could be caused by a binary companion. Specifically, they found an excess of blue light coming from the explosion. This excess matches with the widely accepted models created by U.C. Berkeley astronomer Dan Kasen for what astronomers expect to see when a star explodes in a binary system. "The supernova is blowing up next to a companion star, and the explosion impacts the companion star," Wheeler explained. "The side of that companion star that's hit gets hot and bright. The excess blue light is coming from the side of the companion star that gets heated up." Combined with the models, the observations indicate that the binary companion star has a minimum mass of six suns. "This is an interpretation that is consistent with the data," said team member Jeffrey Silverman, stressing that it is not concrete proof of the exact size of the companion, like would come from a photograph of the binary star system. Silverman is a postdoctoral researcher at UT Austin. Only a few other Type Ia supernovae have been observed as early as this one, Marion said, but they have not shown an excess of blue light. More examples are needed. "We need to study a hundred events like this and then we'll be able to know what the statistics are," Wheeler said. The work is published today in The Astrophysical Journal. More information: "SN 2012cg: Evidence for Interaction Between a Normal Type Ia Supernova and a Non-Degenerate Binary Companion," G. H. Marion et al., 2016, Astrophysical Journal, Preprint: arxiv.org/abs/1507.07261
News Article | April 13, 2016
An international team of astronomers, led by Marshall C. Johnson of the University of Texas at Austin, has used the data from K2's Campaign 4, which lasted from February 7 to April 23, 2015, to search for possible transiting planets. They found two periodic transit-like signals associated with two targets designated EPIC 211089792b (K2-29b) and EPIC 210957318b (K2-30b). While K2-30b was confirmed as a "hot Jupiter" exoplanet during previous observations, K2-29b is a new addition to the long list of Kepler's confirmed extrasolar worlds. The astronomers also used three different ground-based spectrographs to conduct high-resolution spectroscopic observations of K2-29b, in order to definitely verify it as a "hot Jupiter." The Robert G. Tull Coudé spectrograph, mounted on the 2.7m Harlan J. Smith Telescope at the McDonald Observatory, Texas, allowed the scientists to obtain both reconnaissance spectroscopy and radial velocity measurements. Similar observations were conducted using the Fiber-fed Échelle spectrograph (FIES) on the 2.56m Nordic Optical Telescope at the Observatorio del Roque de los Muchachos, La Palma (Spain) and the HARPS-N spectrograph on the 3.58m Telescopio Nazionale Galileo, also at La Palma. "Here, we present K2 photometry for two late-type dwarf stars, EPIC 211089792 (K2-29) and EPIC 210957318 (K2-30), for which we identified periodic transit signals, and our follow-up spectroscopic observations. These have allowed us to confirm both transiting objects as bona fide hot Jupiters, and to measure the stellar and planetary parameters," Johnson and his colleagues wrote in a paper. Hot Jupiters are gas giant planets, similar in characteristics to the solar system's biggest planet, with orbital periods of less than 10 days. They have high surface temperatures as they orbit their parent stars very closely—between 0.015 and 0.5 AU. While the newly discovered K2-29b exoplanet has a radius that is about the same as Jupiter's, it's less massive (0.6 Jupiter masses) than our solar system's biggest planet. It has an orbital period of 3.26 days and an equilibrium temperature of approximately 800 degrees Celsius, making it a textbook example of a hot Jupiter. The planet's parent star K2-29 is slightly smaller than our sun, with 0.75 solar radii and 0.86 solar masses. The star is about 2.6 billion years old and is located some 545 light years from the Earth. The researchers also found that the orbit of K2-29b is slightly eccentric. This suggests that either the planet migrated to its current location via high-eccentricity migration, or that there is an additional planet in the system exciting the eccentricity. "In general, eccentric orbits of hot Jupiters might be generated in two different manners: Either the eccentricity is primordial, a relic of high-eccentricity migration that emplaced the planet on a short-period orbit, or the eccentricity is being excited by an external perturber," the paper reads. However, to investigate these possibilities, future observations using long-term radial velocity and transit timing variation methods are required. "These possibilities could be distinguished using long-term radial velocity and transit timing variation monitoring to detect an additional companion," the team concluded. More information: Two Hot Jupiters from K2 Campaign 4, arXiv:1601.07844 [astro-ph.EP] arxiv.org/abs/1601.07844 Abstract We confirm the planetary nature of two transiting hot Jupiters discovered by the Kepler spacecraft's K2 extended mission in its Campaign 4, using precise radial velocity measurements from FIES@NOT, HARPS-N@TNG, and the coud'e spectrograph on the McDonald Observatory 2.7 m telescope. K2-29 b (EPIC 211089792 b) transits a K1V star with a period of 3.2589263±0.0000015 days; its orbit is slightly eccentric (e=0.084+0.032−0.023). It has a radius of RP=1.000+0.071−0.067 RJ and a mass of MP=0.613+0.027−0.026 MJ. Its host star exhibits significant rotational variability, and we measure a rotation period of Prot=10.777±0.031 days. K2-30 b (EPIC 210957318 b) transits a G6V star with a period of 4.098503±0.000011 days. It has a radius of RP=1.039+0.050−0.051 RJ and a mass of MP=0.579+0.028−0.027 MJ. The star has a low metallicity for a hot Jupiter host, [Fe/H]=−0.15±0.05.
News Article | January 6, 2016
Immediately after the detection by Swift/BAT on June 15.77197 ut, the VSNET collaboration team31 started a worldwide photometric campaign of V404 Cyg. There was also an independent detection by CCD (charge coupled device) photometry on June 16.169 ut32. Time-resolved CCD photometry was carried out at 27 sites using 36 telescopes with apertures of dozens of centimetres (Extended Data Table 2). We also used the public AAVSO data33. We corrected for bias and flat-fielding in the usual manner, and performed standard aperture photometry. The observers, except for TAOS34, used standard filters (B, V, R , I ; we write R and I for R and I in the main text and figures for brevity) and measured magnitudes of V404 Cyg relative to local comparison stars whose magnitudes were measured by A. Henden (sequence 15167RN) from the AAVSO Variable Star Database35. We applied small zero-point corrections to some observers’ measurements. When filtered observations were unavailable, we used unfiltered data to construct the light curve. The exposure times were mostly 2–30 s, with some exceptional cases of 120 s in B band, giving typical time resolution of a few seconds. All of the observation times were converted to BJD. For the Swift/XRT light curves (Fig. 3 and Extended Data Fig. 2), we extracted source events from a region with a 30-pixel radius centred on V404 Cyg. To avoid pile-up effects, we further excluded an inner circular region if the maximum count rate of the XRT raw light curves, binned in 10 s intervals, exceeded 200 counts s−1. The inner radii are set to be 10 and 20 pixels at the maximum raw rate of 1,000 counts s−1 and 2,000 counts s−1, respectively, and those for intermediate count rates were determined via linear interpolation between the two points. The presented light curves were corrected for photon losses due to this exclusion by using the xrtlccorr tool. In addition, from Fig. 3a, c and d, we can see a time delay in the start of a dip in optical light, relative to that in X-rays. The delay time was ~1 min, which is similar to the reported value of 0–50 s (ref. 36). This was determined by cross-correlating the U-band and X-ray (0.3–10 keV) light curves obtained with Swift/UltraViolet and Optical Telescope (UVOT) and Swift/XRT on ut 2015 June 2136. The observations were carried out when the source showed little rapid optical flickering and no extreme flares, and thus the nature of the lag may be different from that in our observations. We also note that the apparent difference between the Swift/UVOT and the ground-based times36 is caused by the drift of the clock on board the satellite, to which we have applied the necessary corrections. In order to examine the possibility that absorption by gas in the line-of-sight causes the observed violent flux variations in the optical and X-ray bands (Fig. 3), we studied intensity-sliced X-ray spectra. A striking example is shown in Extended Data Fig. 3a. The period shown corresponds to that in Fig. 3a when both the X-ray and optical fluxes exhibited a sudden intensity drop towards the latter part of the period. We divided it into five intervals (T1 to T5; Extended Data Fig. 3a), and generated spectra through the tools xrtpipeline and xrtproducts in standard pipeline processing. We excluded the central 60-arcsecond strip from this Windowed Timing (WT) mode data, to avoid the heavy pile-up effect when the raw count rate exceeds ~150 counts s−1. We compared the vF spectra of the five intervals, where the spectra are fitted by a single power-law model multiplied by photoelectric absorption (phabs × pegpwrlw; in the standard X-ray spectral fitting package XSPEC). The absorbed X-ray flux ranges by two orders of magnitude, from 2.1 × 10−9 erg s−1 cm−2 in T5 to 3.0 × 10−7 erg s−1 cm−2 in T3. However, the best-fit column density and photon index were relatively stable over the five intervals, ~(2–6) × 10−21 cm−2 and ~1.0–1.5, respectively. Since the X-ray spectrum does not show a noticeable rise in column density when the X-ray flux sharply dropped, and since there is no stronger iron edge in the latter part of the observation, absorption cannot be the primary cause of the time variation in our data sets that cover the X-ray and optical bands simultaneously. In Extended Data Table 3 we show the list of X-ray binaries that have shown violent short-term variations either in X-rays or in optical wavelengths. IGR J17091−3624 is known as the second black hole X-ray binary whose X-ray light curves showed a variety of patterns, resembling those of GRS 1915 + 10518. The variations observed in the 2011 outburst of this object were classified as ρ (‘heartbeat’), ν (similar to class ρ but with secondary peak after the dips), α (‘rounded-bumps’), β/λ (repetitive short-term oscillations after low-quiet period) and μ (ref. 18). The Rapid Burster (RB or MXB 1730−335), a low-mass X-ray binary (LMXB) containing a neutron star (NS), was discovered by Small Astronomy Satellite (SAS-3) observations37. This object has been recently reported to show cyclic long X-ray bursts with periods of a few seconds resembling class ρ (‘heartbeat’) variations and those with periods of 100–200 s resembling class θ (“M”-shaped light curves) variations of GRS 1915 + 10524. The emission of the Rapid Burster did not reach the Eddington luminosity during these variations38. V4641 Sgr was originally discovered as a variable star39 and was long confused with a different variable star, GM Sgr40. V4641 Sgr is famous for its short and bright outburst in 1999, which reached a optical magnitude of at least 8.8 mag (refs 41, 42, 43, 44). V4641 Sgr showed short-term variations in optical wavelengths during the 2002, 2003 and 2004 outbursts14, 45, 46, 47. It was the first case in which short-term and large-amplitude variations in the optical range during an outburst were detected. V4641 Sgr is classified as a LMXB, and has a long orbital period. Its mass-accretion rate is less than the Eddington rate (except for the 1999 outburst44, 48). These properties are similar to those of V404 Cyg. However, while the short-term variations of V4641 Sgr seemed to be random, those of V404 Cyg showed repetitive patterns; this is the greatest difference between these two objects. There has been a suggestion that V4641 Sgr is a ‘microblazar’49 because the jets observed during the outburst in 1999 were proposed to have the largest bulk Lorentz factor among known galactic sources43. There are also other X-ray transients showing short-term optical variations (for example, XTE J1118+480 and GX 339−4). However, these two sources are quasi-periodic oscillations (QPOs), characterized by very short periods. The periods are much shorter than those of repetitive patterns (tens of seconds to a few hours) that we discuss in this Letter. Furthermore, the amplitudes of their variations are significantly smaller than those observed in V4641 Sgr4, 50 on timescales longer than tens of seconds. Following the method in ref. 15, we estimated the mass stored in the disk at the onset of the outburst. By integrating the X-ray light curve of Swift/BAT and assuming the spectral model C in table 1 in ref. 15, we obtained a value of 5.0 × 1025 g assuming a radiative efficiency of 10% and a distance of 2.4 ± 0.2 kpc (ref. 8). The mass during the 1989 outburst has been updated to 3.0 × 1025 g by using this updated distance. The stored mass in the 2015 outburst was approximately the same as that in the 1989 one. As discussed in ref. 15, these masses are far smaller than the mass of a fully built-up disk, estimated to be 2.0 × 1028 g, if these outbursts were starting at the outermost region. We compare the published optical light curves of the 1989 and 1938 outbursts51, 52 with our data from the 2015 outburst (Extended Data Fig. 4). We can see that these outbursts have different durations. The 1938 outburst was apparently longer than the others, and it may have had different properties from the 1989 and 2015 ones. The fading rates of the 1989 and 2015 outbursts are significantly larger than those of classical X-ray transients6, or of FRED (fast rise and exponential decline)-type outbursts, such as 0.028 mag d−1 in V518 Per = GRO J0422+32 (ref. 53) and 0.015 mag d−1 in V616 Mon = A0620−00 (ref. 54). This supports the hypothesis that the outbursts in 1989 and 2015 are different from typical outbursts of classical X-ray transients and that the stored disk mass was a factor of ~103 smaller in the 1989 and 2015 outbursts than the mass of a fully built up disk. We performed power spectral analyses on BJD 2,457,193, BJD 2,457,196 and BJD 2,457,200. We used the continuous and regularly sampled high-cadence data set obtained by LCO (Extended Data Table 1) with exposure times of 5 s (on BJD 2,457,193) and 2 s (others). The durations of these observations are 1.4, 3.1 and 2.2 h, respectively. Considering the read-out times of 1 s, the Nyquist frequencies of these observations are 0.08 and 0.17 Hz, respectively. The power spectral densities (PSDs) were calculated using powspec software in the FTOOLS Xronos package on magnitude measurements. We did not apply de-trending of the light curve since the durations of the individual observations were shorter than the timescale of the global variation of the outburst. The power spectra are well expressed by a power law (P ∝ f −Γ ) with an index Γ of 1.9 ± 0.1, 1.8 ± 0.1, and 2.3 ± 0.1 on BJD 2,457,193, 2,457,196 and 2,457,200, respectively (Extended Data Fig. 5). Interpretation of the physical origins on the basis of these variations is difficult, because a power law index of ~2 in the PSDs is often observed in natural phenomena. In this region (f < 0.01 Hz), the power originating in the optical variations of V404 Cyg is significantly higher than that of white noise estimated from the observations. We next summarize the other reports on short-term variations of V404 Cyg during the present outburst. On BJD 2,457,191, this object was observed using the Argos photometer on the 2.1m Otto Struve Telescope at McDonald Observatory with an exposure time of 2 s55. They reported that the power spectrum was dominated by steep red noise. Observations on BJD 2,457,193 and BJD 2,457,194 were also performed using the ULTRACAM attached with the 4.2m William Herschel Telescope on La Palma observatory with a high time resolution (466.8 ms)56. They reported that the variations were dominated by timescales longer than tens of seconds. Although large amplitude flares (0.3–0.4 mag) on timescales shorter than 1 s were reported57, these flares may be of different origin. For the variations with timescales longer than 100 s, our results agree with these reports55, 56. The timescale τ of heating/cooling waves in dwarf novae and X-ray transients58 is a function of the mass of the central object (M ) and radius (r) with the form , where α is the viscosity parameter59. Here, we estimate the disk radius of V404 Cyg assuming that the timescale of the final fading reflected a dwarf nova-type cooling wave. Using the Kepler data of V344 Lyr and V1504 Cyg, we measured a fading rate of 1.5 mag d−1 of the normal outbursts immediately preceding superoutbursts. During the outbursts in V344 Lyr and V1504 Cyg60, the disk radius is expected to be very close to the 3:1 resonance radius. Adopting a typical mass of a white dwarf in a cataclysmic variable (M = 0.83M ; ref. 61), we estimated the disk radius of V404 Cyg to be 7.8 × 1010 cm for a black hole mass of 9M . This is much smaller than the radius (1.2 × 1012 cm) expected for a fully built-up disk15. Extended Data Fig. 6a shows the multi-wavelength SED on BJD 2,457,199.431 to 2,457,199.446, when the source was simultaneously observed in the X-ray, ultraviolet (UV) and optical bands. The optical fluxes in the V and I bands are taken from our photometric data averaged over the period. Note that R -band data are also available but not used here, because of the contamination by the continuum strong Hα line62, 63, 64. The X-ray spectrum is extracted from simultaneous Swift/XRT data (ObsID 00031403058) which were taken in the WT mode. The data are processed through the pipeline processing tool xrtpipeline. The events detected within 20 pixels around the source position are removed to mitigate pile-up effects. The U-band flux is obtained from the Swift/UVOT images with the same ObsID as the XRT, through the standard tool uvot2pha provided by the Swift team. A circular region centred at the source position with a radius of 5 arcsec is adopted as the source extraction region of the UVOT data. The optical, UV and X-ray data are corrected for interstellar extinction/absorption by assuming A (interstellar extinction in the V band) = 4 (ref. 65) and using the extinction curve in ref. 66 and the N (hydrogen column density) versus E(B−V) relation in ref. 67. Radio data are from the RATAN-600 observation performed in the same period68. The multi-wavelength SED can be reproduced with the diskir model69, 70, which accounts for the emission from the accretion disk, including the effects of Comptonization in the inner disk and reprocessing in the outer disk. We find that partial covering X-ray absorption (using the pcfabs model implemented in the spectral analysis software XSPEC) improves the quality of the fit significantly. The inner-disk temperature is estimated to be 0.12 ± 0.01 keV, and the electron temperature and photon index of the Comptonization component, the ratio between the luminosity of the Compton tail and disk blackbody (L /L ), and the fraction of the bolometric flux thermalized in the outer disk (f ), are 17.5 ± 0.8 keV, 1.78 ± 0.03, 1.17 ± 0.03, and , respectively (the errors in this section represent 90% confidence ranges for one parameter). The inner radius (R ) is estimated to be (1.5–5.4) × 108 cm, and the outer radius (R ) is (2.5 ± 0.3) × 1012 cm. The derived value of R is comparable to or even larger than the binary separation (~2.2 × 1012 cm). However, it could be smaller due to uncertainties in interstellar/circumbinary extinction71 and/or the contribution of jet emission. For instance, if A is 0.4 mag larger than the assumed value (4.0), R becomes (1.9 ± 0.2) × 1012 cm. The maximum achievable radius of a stable disk for a q (mass ratio) = 0.06 object (Extended Data Table 3) is around 0.62A (radius of the 2:1 resonance) to ~0.7A (tidal limit), where A is the binary separation72. Considering the uncertainties, the result of our analysis (>~ 0.77A) is compatible with this maximum radius. Our result appears to favour a large A value. For the partial covering absorber, the best-fit value of the column density is cm−2 and that of the covering fraction is 64 ± 4%. The radio SED can be approximated by a power-law with a photon index of ~1, as in other black hole binaries in the low/hard state73. This profile is likely to be generated by the optically-thick synchrotron emission from compact jets74. Because an optically-thick synchrotron spectrum often extends up to the millimetre to near-infrared bands75, 76, 77, it may contribute to the optical fluxes, in particular at longer wavelengths. The blackbody emission from the companion, a K3III-type star7 with a radius of ~3 R and a temperature of ~4,320 K, contributes to the SED negligibly. Extended Data Figure 6b plots the simultaneous SED on BJD 2,457,191.519 to 2,457,191.524, which is ~2 orders of magnitude fainter in the X-ray band than that shown in the left panel. The X-ray, UV and optical data are taken from the Swift data (ObsID 00031403038) and our photometric measurements in the same manner as described above. This SED can be reproduced with the irradiated disk model as well, with somewhat smaller photon index and inner-disk temperature (<0.07 keV), and a larger than those on BJD 2,457,199.431 to 2,457,199.446. The bolometric luminosity L of V404 Cyg is evaluated based on the hard X-rays above ~15 keV where the intrinsic spectrum is less affected by an absorption. We processed the Swift/BAT archival survey data via batsurvey in the HEAsoft package to derive count rates with individual exposures of ~300 s. Even within this short exposure, photon statistics are good during bright states (>0.05 counts s−1). Assuming a Crab-like spectrum (1 Crab ≈ 0.039 counts s−1), the BAT count rates R (counts s−1) are then converted into 15–50 keV flux (F ) and luminosity (L ) using F = 3.6 × 10−7R (erg s−1 cm−1) and a fiducial distance of 2.4 kpc, respectively. In Fig. 4, we show L after multiplying by a conversion factor L /L = 7 determined from SED modelling (previous section). We find that this bolometric correction factor lies within the range 2.5–10 by fitting 19 X-ray(XRT)-optical simultaneous SED in different periods between BJD 2,457,192.019 and 2,457,201.011. Since the BAT survey data are rather sparse, in order to catch shorter-term variations, we further overlaid the INTEGRAL IBIS/ISGRI monitoring in the 25–60 keV band available at ref. 78, assuming a conversion parameter of 1 Crab rate to be 172.1 counts s−1 and a bolometric correction factor of L /L = 9.97. The luminosity was highly variable during the outburst, changing by five orders of magnitude. While V404 Cyg sometimes reaches the Eddington luminosity (L ) at the peak of multiple sporadic flares, it also repeatedly dropped below 1–10% of L (Fig. 4). At earlier phases of this outburst, the characteristic oscillation already occurred during a lower luminosity state, as discussed in the main text. No statistical methods were used to predetermine sample size.
News Article | December 8, 2015
As a star approaches the end of its life, its internal nuclear fires become chaotic—switching between intense burning and calmer states. The resulting pulsations cause the stare to shed its outer layer. For a star the size of the sun, the shedding of outer layers continues until nothing is left but the stellar core, called a white dwarf. But in 1953, astronomers discovered blue stragglers, stars that were hotter and bluer than they were supposed to be for their advanced age. Curiously, many blue stragglers existed in binaries, a pair of stars that orbit one another. The find led astronomers to speculate that blue stragglers were siphoning gas from their more massive companion stars. Astronomers from the Univ. of Wisconsin-Madison and the Univ. of Texas published a study in The Astrophysical Journal confirming the postulated theory. Prof. Robert Mathieu, of the Univ. of Wisconsin-Madison, and Natalie Gosnell, a former student of Mathieu’s who know is an astronomer at the Univ. of Texas, previously confirmed that three-quarters of blue stragglers have stellar companions. Now, the two have used the Hubble Space Telescope’s Advanced Camera for Surveys to study an open star cluster 5,500 light-years away. Fourteen of the 21 blue stragglers in open star cluster NGC 188 showed signs of mass transfer from one star to the other. “Until now there was no concrete observational proof, only suggestive results,” said Gosnell. “It’s the first time we can place limits on the fraction of blue stragglers formed through mass transfer.” According to the researchers, the white dwarfs are around 300 million years old, still young enough to give off noticeable ultraviolet light. “These stars are not just an afterthought, a contaminant in our neat picture,” said Gosnell. “We need to bring this 25% of all stars into the fold, so we can say we really understand how stars evolve.” “For the evolution of single stars like our sun, by and large, we got it right, from birth to death,” said Mathieu. “Now, we’re starting to do the same thing for the on-quarter of stars that are close-orbiting binaries. This work allows us to talk not about points of light, but about the evolution of galaxies, including our Milky Way.” Gosnell plans to continue studying the stars with the 2.7-m Harlan J. Smith Telescope at the Univ. of Texas’ McDonald Observatory.
News Article | December 8, 2015
Though blue stragglers were first identified 62 years ago, astronomers have yet to converge on a solution for their odd appearance. The most popular explanation among several competing theories is that an aging star spills material onto a smaller companion star. The small star bulks up on mass to become hotter and bluer while the aging companion burns out and collapses to a white dwarf – a burned out cinder. To test this theory Gosnell's team conducted a survey of the open star cluster NGC 188 that has 21 blue stragglers. Of those, she found that seven had white dwarf companions, by identifying their ultraviolet glow that is detectable by Hubble. Of the remaining 14 of the 21 blue stragglers, a further seven show evidence of so-called mass transfer between stars in other ways. Gosnell said she believes these are older white dwarf-blue straggler binaries, and indicate two-thirds of blue stragglers form through mass transfer. "This was really great," Gosnell says. "Until now there was no concrete observational proof, only suggestive results," Gosnell said. "It's the first time we can place limits on the fraction of blue stragglers formed through mass transfer." This discovery sheds light on the physical processes responsible for changing the appearance of 25 percent of evolved stars. Gosnell's work, which closes gaps in our understanding of how stars age, is published in the current issue of the Astrophysical Journal. The problem came to light because in recent years, astronomers have been able to make a complete and accurate census of stars in a number of open star clusters, Gosnell said. "Open clusters really are the best laboratory for the study of stellar evolution," Gosnell said. "They have a simple stellar population." The stars in a cluster form at the same time and from the same materials, she explained. The cluster population studies revealed that up to a quarter of the oldest stars "are not evolving like we think they're supposed to," Gosnell said. Stars that astronomers expected to become red giants (like Aldebaran, the eye of Taurus, the bull) instead became "blue stragglers," unexpectedly bright, blue stars with a host of strange characteristics. Gosnell wanted to find out what happened to them. So she, along with Bob Mathieu at the University of Wisconsin-Madison and their collaborators, designed a study using Hubble Space Telescope's Advanced Camera for Surveys to try to differentiate between three theories of how these stars became blue stragglers. The theories included: collisions between stars in the cluster (with debris coalescing to form a blue straggler), the merger of two of the stars in a triple star system, or mass transfer between two stars in a binary pair. In a binary pair of stars, the larger star will evolve faster, Gosnell said. That star becomes a red giant. A red giant is so bloated that the outermost layers of gas on its surface are only tenuously held by the star's gravity. They can be pulled off by the gravity of the companion star. This is mass transfer. As the gas is siphoned off by the partner, the red giant is left with only its core, making it into a white dwarf. The partner—initially the less massive of the pair, but now the heavier one—becomes a blue straggler. Gosnell's method is limited by the fact that it will not detect white dwarfs that have cooled down enough so that they don't glow in UV light detectable by Hubble, she said. That means that only those white dwarfs formed in the last 250 million years (youngsters, astronomically speaking) are detectable. Knowing more about how these stars form is important because astronomers use their assumptions to model the stellar populations of distant galaxies (where the light from all the stars blends together). "You don't want to be ignoring 25 percent of the evolved stars" in those galaxies, Gosnell said. Such models are important because distant galaxies figure into many different types of cosmological studies. Right now, Gosnell said, "the models have a lot of room for improvement." "If we tweak the way models treat mass transfer, that would bring the observations and theory together," Gosnell said. "They would agree. And we can use this to inform our understanding of unresolved stellar populations"—that is, those stars in galaxies so far away that all their light is blended together. Gosnell plans to continue studying these stars using the 2.7-meter Harlan J. Smith Telescope at McDonald Observatory and its IGRINS spectrograph to constrain the number of blue stragglers that could form through mergers in triple systems. Explore further: Even stars get fat -- And 'stellar cannibalism' is the reason More information: "Implications for the Formation of Blue Straggler Stars from HST Ultraviolet Observations of NGC 188," Natalie M. Gosnell et al., 2015 Dec. 1, Astrophysical Journal, Vol. 814, No. 2 iopscience.iop.org/article/10.1088/0004-637X/814/2/163 , Arxiv: arxiv.org/abs/1510.04290, PDF: hubblesite.org/pubinfo/pdf/2015/43/pdf.pdf
News Article | November 21, 2016
Scientists pride themselves on their objectivity. It’s a fundamental tenet of scientific logic, that whatever we study, it must be carried out in a matter of fact way, immune to our own human flaws. This has lead to the myth of the “meritocracy”: that the best scientists are simply the folks who design the best experiments, who accomplish the most, and those are openly celebrated by the scientific community for their ingenuity. But most scientists are white. Most are men. This isn’t by chance. It is one piece of evidence pointing to a fundamental flaw in the process of science that systematically excludes people of color and women, even in the 21st century. Now, there has been some improvement: the proportion of white women scientists is creeping up decade by decade, but the contrast with more minoritized groups is striking. Representation from Black Americans, Latinx Americans and Native Americans is almost non-existent in the upper echelons of academic research, and it hasn’t grown in the past twenty years. It isn’t a coincidence that these are some of the most traditionally marginalized groups in the history of our society. In fact, physics and astronomy have some of the highest proportions of white representation compared to other STEM fields, and time has proven unhelpful in the battle for equal representation for people of color in the lab. With support from the University of Texas at Austin’s Department of Astronomy and McDonald Observatory, we’ve created a program designed specifically to address the retention of undergraduate students of color in STEM. The Texas Astronomy Undergraduate Research Experience for Under-represented Students (TAURUS) is a 9-week summer research experience for highly-motivated, excelling students from traditionally marginalized groups from all over the country. TAURUS Scholars come to UT to work one-on-one with a professional astronomer on a research project—from discovering new planets around other stars to investigating galaxies at the edge of the Universe. The highlight of the entire TAURUS summer experience is a group trip to McDonald Observatory in west Texas. For most TAURUS Scholars, it will be their first trip to a professional observatory and their first trip to one of the darkest night sky sites in the world. Between taking turns steering the 2.7m Harlan J. Smith Telescope to scientific targets related to their summer research, the scholars sneak out to the observatory catwalk and let their eyes adjust to the dark skies. The ambient light from the Milky Way galaxy pops against the darkness, bright enough to cast a shadow. For TAURUS Scholars, this is, without a doubt, a key moment that fuels their motivation for becoming astrophysicists. While their are many flavors of undergraduate research experiences for students of all STEM fields around the country, TAURUS builds in an extra component that makes this program unique. Scholars have a network of mentors from graduate students through their research advisors that keep tabs on their progress. Together with bi-weekly seminars on topics ranging from the latest in astrophysics to social justice in STEM, combined with a slew of social events and mentor check-in’s, TAURUS relies on community to ensure scholars reach their goals and feel encouraged and supported along the way. They remain a part of the UT community even after they have departed at the end of the summer, with guidance on graduate school applications and extended research projects. In additions to the benefits of this structured program for TAURUS Scholars, our graduate students and postdocs gain important experience in mentoring: a skill that is rarely practiced mindfully in STEM. Our program’s number one goal is to be a constant resource for TAURUS Scholars to reach their career aspirations in STEM, whether that is going to graduate school in astrophysics, physics, or entering industry. Six months after their time at UT, all of the scholars are reunited at the annual meeting of the American Astronomical Society held in January, where they network with prospective graduate schools, employers, and other undergraduate students—including scholars from two other programs focused on elevating students of color, Harvard’s Banneker Institute and Aztlan Institute. TAURUS, together with Banneker and Aztlan, hope to transform one tiny sector of STEM by propelling these highly-motivated students of color towards the astrophysics careers to which they’ve always aspired, but which they might not have had access to, or not received the right kind of career “boost” needed to flourish. These “boosts” are the type of career advancement many of us in the majority take for granted—conducting cutting-edge research, having a well-connected supervisor, and having a slew of great role models. Many scientists aren’t very cognizant of discrimination and unconscious bias in STEM, including many women. Sometimes it takes tangible setbacks to open our eyes to the challenges of professional advancement in a field where you aren’t proportionally represented. It’s important to recognize that it’s not just under-representation that’s the issue, though: it’s actively harmful stereotypes about intelligence and a fixed-mindset notion of what makes someone “brilliant” that seep into our sub-conscious after years of social conditioning. Human scientists are not immune from the social world in which they live, and the sooner we can get more scientists to acknowledge their flaws and their privilege then the more welcoming STEM will be to a broader base of participants. Broadening participation is key: not only does it level the playing field and allow all young folks to aspire to their dreams, but it can also have lasting positive effects on the quality of our science, too. TAURUS is fundraising for the 2017 class of scholars throughout the month of November and early December. Like the TAURUS facebook page, follow us on Twitter, or visit our website to learn more.