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Delorme P.,University of St. Andrews | Cameron A.C.,University of St. Andrews | Hebb L.,Vanderbilt University | Rostron J.,University of St. Andrews | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

We present the results of photometric surveys for stellar rotation in the Hyades and in Praesepe, using data obtained as part of the SuperWASP exoplanetary transit-search programme. We determined accurate rotation periods for more than 120 sources whose cluster membership was confirmed by common proper motion and colour-magnitude fits to the clusters' isochrones. This allowed us to determine the effect of magnetic braking on a wide range of spectral types for expected ages of ~600Myr for the Hyades and Praesepe. Both clusters show a tight and nearly linear relation between J-Ks colour and rotation period in the F, G and K spectral range. This confirms that loss of angular momentum was significant enough that stars with strongly different initial rotation rates have converged to the same rotation period for a given mass, by the ages of Hyades and Praesepe. In the case of the Hyades, our colour-period sequence extends well into the M dwarf regime and shows a steep increase in the scatter of the colour-period relation, with identification of numerous rapid rotators from ~0.5M⊙ down to the lowest masses probed by our survey (~0.25M⊙). This provides crucial constraints on the rotational braking time-scales and further clears the way to use gyrochronology as an accurate age measurement tool for main-sequence stars. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.


News Article | December 12, 2016
Site: www.eurekalert.org

Back in 2015 when astronomers discovered an intense flare in a distant galaxy, they considered it the brightest supernova ever observed. Now, UC Santa Barbara astrophysicists and a group of international colleagues offer an entirely different interpretation based on new astronomical observation data from the Las Cumbres Observatory (LCO), a global robotic telescope network, and the Hubble Space Telescope. The new information indicates that the event, called ASASSN-15lh, is actually a tidal disruption event (TDE) -- the destruction of a star by a supermassive black hole. The findings appear in the inaugural issue of the journal Nature Astronomy. "Years ago we just wouldn't have been able to follow an event like this," said co-author Andy Howell, leader of the supernova group at the Goleta, California-based LCO and an adjunct professor in UCSB's Department of Physics. "This study shows that large-area surveys, a global robotic telescope network and a NASA satellite can come together to reveal dramatic new discoveries that wouldn't be possible without each piece of that puzzle." Using images from the Hubble Space Telescope that were not available when ASASSN-15lh was discovered, the scientists found that the event occurred at the center of the galaxy where the supermassive black hole resides. The black hole inferred to lie in this galaxy is more than 100 million times the mass of the sun. For a star to be tidally disrupted by such a massive black hole -- rather than swallowed whole -- the black hole must be spinning very rapidly. This discovery marks the first time that a TDE has been used to probe the spin of a black hole, a property that is very difficult to measure and is used to infer the existence of so-called Kerr black holes. ASASSN-15lh occurred when the star strayed too close to the supermassive black hole and was torn apart by the tides generated by the extreme gravity. The stellar material orbited around the black hole, collided with itself at high velocity and started falling into the black hole. This released copious amounts of energy and generated the bright flare astronomers observed as ASASSN-15lh. "We've only been studying the optical flares of tidal disruptions for the last few years," said co-author Iair Arcavi, principal investigator of the LCO program that's used to observe ASASSN-15lh and an Einstein postdoctoral fellow at UCSB. "ASASSN-15lh is similar in some ways to the other events we've been seeing but is different in ways we didn't expect. It turns out that these events -- and the black holes that make them -- are more diverse than we had previously imagined." "This is like discovering a new kind of dinosaur," Howell said. "Now that we have the right tools and know what to look for, we're going to find more and get a better sense of the population. It's exciting to have new ways of learning about black holes and stellar death."


News Article | December 12, 2016
Site: www.rdmag.com

Back in 2015 when astronomers discovered an intense flare in a distant galaxy, they considered it the brightest supernova ever observed. Now, UC Santa Barbara astrophysicists and a group of international colleagues offer an entirely different interpretation based on new astronomical observation data from the Las Cumbres Observatory (LCO), a global robotic telescope network, and the Hubble Space Telescope. The new information indicates that the event, called ASASSN-15lh, is actually a tidal disruption event (TDE) -- the destruction of a star by a supermassive black hole. The findings appear in the inaugural issue of the journal Nature Astronomy. "Years ago we just wouldn't have been able to follow an event like this," said co-author Andy Howell, leader of the supernova group at the Goleta, California-based LCO and an adjunct professor in UCSB's Department of Physics. "This study shows that large-area surveys, a global robotic telescope network and a NASA satellite can come together to reveal dramatic new discoveries that wouldn't be possible without each piece of that puzzle." Using images from the Hubble Space Telescope that were not available when ASASSN-15lh was discovered, the scientists found that the event occurred at the center of the galaxy where the supermassive black hole resides. The black hole inferred to lie in this galaxy is more than 100 million times the mass of the sun. For a star to be tidally disrupted by such a massive black hole -- rather than swallowed whole -- the black hole must be spinning very rapidly. This discovery marks the first time that a TDE has been used to probe the spin of a black hole, a property that is very difficult to measure and is used to infer the existence of so-called Kerr black holes. ASASSN-15lh occurred when the star strayed too close to the supermassive black hole and was torn apart by the tides generated by the extreme gravity. The stellar material orbited around the black hole, collided with itself at high velocity and started falling into the black hole. This released copious amounts of energy and generated the bright flare astronomers observed as ASASSN-15lh. "We've only been studying the optical flares of tidal disruptions for the last few years," said co-author Iair Arcavi, principal investigator of the LCO program that's used to observe ASASSN-15lh and an Einstein postdoctoral fellow at UCSB. "ASASSN-15lh is similar in some ways to the other events we've been seeing but is different in ways we didn't expect. It turns out that these events -- and the black holes that make them -- are more diverse than we had previously imagined." "This is like discovering a new kind of dinosaur," Howell said. "Now that we have the right tools and know what to look for, we're going to find more and get a better sense of the population. It's exciting to have new ways of learning about black holes and stellar death."


A flash in the sky that earlier this year was called the brightest supernova ever detected—tens of times brighter than our entire Milky Way galaxy—may be something much more exotic: a supermassive black hole tearing apart—and consuming—a star that strayed too close, according to a new study. The flash was first spotted in 2015 by the All-Sky Automated Survey for SuperNovae (ASAS-SN), a network of small telescopes in Chile and Hawaii that monitors the sky for fast-changing objects. Astronomers assumed it was a superluminous supernova (SLSN), which occurs when a massive star collapses under its own gravity at the end of its life, spewing out a fireball of hot dust and gas that glows brightly for a short time before gradually fading. At the time, the event was twice as bright as the previous record holder. But ASASSN-15lh, as the event was named, was in the wrong sort of galaxy for an SLSN. The right sort is generally a young dwarf galaxy full of gas and dust where huge stars can form rapidly, burn brightly, and explode in a blaze of supernova glory. ASASSN-15lh, however, was in an old, burned-out galaxy with little evidence of star formation. “The minute they told me about this event, I was suspicious. It just didn’t seem right,” says Giorgos Leloudas, an astronomer at the Weizmann Institute of Science in Rehovot, Israel, who was not a member of the original team. Leloudas and his colleagues began gathering more data from a variety of sources, including the Swift gamma-ray satellite, the Las Cumbres Observatory global telescope network, the Hubble Space Telescope, and the European Southern Observatory’s Very Large Telescope and the New Technology Telescope, both based in Chile. Hubble data showed that the source of the flash was close to the center of its galaxy, whereas the rapid star formation that produces SLSNs typically happens farther out. Also, unlike a normal SLSN, ASASSN-15lh seemed to fade before getting brighter again weeks later, indicating a rise in temperature maintained for about 100 days, Leloudas says. A spectrum of the ultraviolet light from the event, recorded by Hubble, suggested a low-mass star in the prime of life, not on its deathbed. As the team reports today in , all these signs pointed to the idea that ASASSN-15lh might, in fact, be the dying gasp of a star that strayed too close to the supermassive black hole at the center of its galaxy and was ripped apart by the extreme gravitational field, a so-called tidal disruption event (TDE). TDEs are very rare—there are only about 10 such events currently suspected by astronomers. But, Leloudas says, the changes of output from ASASSN-15lh suggested a TDE: the initial flash from gravity tearing the star apart and heating its remains to high temperatures; the later burst from those remains being heated again as they were accreted onto the surface of the black hole. The one flaw in this argument is that the galaxy in question is thought to have a very massive black hole at its heart: more than 100 million times the mass of our sun. Theorists predict that such a leviathan would more likely swallow a star whole and only tear it up once it’s below the event horizon, where it can’t be seen. But the team realized there was a scenario in which the black hole would chew first and swallow later—if it were spinning. The gravitational field around a rotating black hole is different from a nonrotating one and would allow a visible TDE to occur. If it is confirmed that this was the fate of ASASSN-15lh, it will be the first verified rotating black hole at the center of a quiescent galaxy. The team will continue to observe ASASSN-15lh, hoping to learn more as its dazzle ceases to illuminate the rest of the galaxy. And because other candidate TDEs all occur around smaller black holes, ASASSN-15lh broadens the range of places TDEs may occur. “By adding to the diversity, we will learn more about the physics that happens during a disruption,” Leloudas says. “This whole new phenomenon of tidal disruption events gives us a unique opportunity to learn about supermassive black holes during their quiescent phase,” says astronomer Benny Trakhtenbrot of the Swiss Federal Institute of Technology in Zurich, who was not involved in the study. If you can determine how close the disrupted star passed to the black hole, he says, “that can directly tell us how fast the black hole is spinning.” And spin can reveal something of the formation history of otherwise inscrutable black holes, he says.


Littlefair S.P.,University of Sheffield | Naylor T.,University of Exeter | Mayne N.J.,University of Exeter | Saunders E.,Las Cumbres Observatory | Jeffries R.D.,Keele University
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2011

We present an analysis of the rotation of young stars in the associations Cepheus OB3b, NGC 2264, 2362 and the Orion Nebula Cluster (ONC). We discover a correlation between rotation rate and position in a colour-magnitude diagram (CMD) such that stars which lie above an empirically determined median pre-main sequence rotate more rapidly than stars which lie below this sequence. The same correlation is seen, with a high degree of statistical significance, in each association studied here. If position within the CMD is interpreted as being due to genuine age spreads within a cluster, then the stars above the median pre-main sequence would be the youngest stars. This would in turn imply that the most rapidly rotating stars in an association are the youngest, and hence those with the largest moments of inertia and highest likelihood of ongoing accretion. Such a result does not fit naturally into the existing picture of angular momentum evolution in young stars, where the stars are braked effectively by their accretion discs until the disc disperses. Instead, we argue that, for a given association of young stars, position within the CMD is not primarily a function of age, but of accretion history. We show that this hypothesis could explain the correlation we observe between rotation rate and position within the CMD. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Littlefair S.P.,University of Sheffield | Naylor T.,University of Exeter | Mayne N.J.,University of Exeter | Saunders E.S.,Las Cumbres Observatory | Jeffries R.D.,Keele University
Monthly Notices of the Royal Astronomical Society | Year: 2010

We present a photometric study of I-band variability in the young association Cepheus OB3b. The study is sensitive to periodic variability on time-scales of less than a day, to more than 20 d. After rejection of contaminating objects using V, I, R and narrow-band Hα photometry, we find 475 objects with measured rotation periods, which are very likely pre-main-sequence members of the Cep OB3b star-forming region.We revise the distance and age to Cep OB3b, putting it on the self-consistent age and distance ladder of Mayne & Naylor. This yields a distance modulus of 8.8 ± 0.2 mag, corresponding to a distance of 580 ± 60 pc, and an age of 4-5 Myr.The rotation period distribution confirms the general picture of rotational evolution in young stars, exhibiting both the correlation between accretion (determined in this case through narrow-band Hα photometry) and rotation expected from disc locking, and the dependence of rotation upon mass that is seen in other star-forming regions. However, this mass dependence is much weaker in our data than found in other studies. Comparison to the similarly aged NGC 2362 shows that the low-mass stars in Cep OB3b are rotating much more slowly. This points to a possible link between star-forming environment and rotation properties. Such a link would call into question models of stellar angular momentum evolution, which assume that the rotational period distributions of young clusters and associations can be assembled into an evolutionary sequence, thus ignoring environmental effects. © 2010 The Authors. Journal compilation © 2010 RAS.


Barnes J.W.,University of Idaho | Linscott E.,Oklahoma Baptist University | Shporer A.,University of California at Santa Barbara | Shporer A.,Las Cumbres Observatory
Astrophysical Journal, Supplement Series | Year: 2011

We model the asymmetry of the KOI-13.01 transit lightcurve assuming a gravity-darkened rapidly rotating host star in order to constrain the system's spin-orbit alignment and transit parameters. We find that our model can reproduce the Kepler lightcurve for KOI-13.01 with a sky-projected alignment of λ = 23° 4° and with the star's north pole tilted away from the observer by 48° 4° (assuming M * = 2.05 M ⊙). With both these determinations, we calculate that the net misalignment between this planet's orbit normal and its star's rotational pole is 56° ± 4°. Degeneracies in our geometric interpretation also allow a retrograde spin-orbit angle of 124° ± 4°. This is the first spin-orbit measurement to come from gravity darkening and is one of only a few measurements of the full (not just the sky-projected) spin-orbit misalignment of an extrasolar planet. We also measure accurate transit parameters incorporating stellar oblateness and gravity darkening: R * = 1.756 ± 0.014 R⊙, R p = 1.445 ± 0.016 RJup, and i = 859 04. The new lower planetary radius falls within the planetary mass regime for plausible interior models for the transiting body. A simple initial calculation shows that KOI-13.01's circular orbit is apparently inconsistent with the Kozai mechanism having driven its spin-orbit misalignment; planet-planet scattering and stellar spin migration remain viable mechanisms. Future Kepler data will improve the precision of the KOI-13.01 transit lightcurve, allowing more precise determination of transit parameters and the opportunity to use the Photometric Rossiter-McLaughlin effect to resolve the prograde/retrograde orbit determination degeneracy. © 2011. The American Astronomical Society. All rights reserved.


News Article | December 13, 2016
Site: www.gizmag.com

In 2015, astronomers detected what they believed to be the most luminous supernova ever recorded, which shone 20 times brighter than the light output of the entire Milky Way. However, fresh research suggests that the event, which has since been named ASASSN–15lh, might not have been a supernova at all, but instead the light signature of a vast, fast-spinning black hole devouring a Sun-like star. For the new study, an international team of astronomers made use of a number of famous ground and space-based observatories including the Hubble Space Telescope, and the European Southern Observatory's Very Large Telescope, located at the Paranal Observatory, Chile. The telescope data revealed that in the ten months following the supposed supernova event, ASASSN-15lh experienced a significant increase in both temperature and the emission of ultraviolet light. According to the researchers, these characteristics make it unlikely that the 2015 event took the form of a superluminous supernova. Instead the team suggest that the light signature detected in 2015 was created when a Sun-like star orbiting in close proximity to the supermassive black hole located at the centre of ASASSN -15lh's host galaxy was torn apart, or "spaghettified" by the gravity of the black hole. For the black hole to rip the star apart as it orbited beyond its event horizon, it must have been spinning incredibly fast. The extreme gravitational influence of such a fast spinning black hole, known as a Kerr black hole, could have created a tidal disruption event capable of destroying a star orbiting beyond its event horizon. Only 10 tidal disruption events have been observed to date, making it a rare occurrence. The researchers believe that the 2015 light signature was created when the remnants of a star destroyed by a tidal disruption event collided as they fell towards the event horizon of the supermassive black hole, which has at least 100 times the mass of our own sun. This process would have led to the release of an enormous amount of energy, a portion of which was released in the form of ASASSN-15lh. The nature of the galaxy in which ASASSN-15lh was discovered, and the close proximity of the event to the supermassive black hole's event horizon further supports the tidal disruption theory. Superluminous supernovae are typically discovered in galaxies that are alight with the activity of star creation. However, ASASSN-15lh was found in a passive, red galaxy, which exhibits a relatively low-level of star formation. "We've only been studying the optical flares of tidal disruptions for the last few years," said co-author Iair Arcavi, principal investigator of the Las Cumbres Observatory program used to observe ASASSN-15lh. "ASASSN-15lh is similar in some ways to the other events we've been seeing but is different in ways we didn't expect. It turns out that these events — and the black holes that make them — are more diverse than we had previously imagined." Scroll down to see a simulation of the supermassive black hole thought to reside at the centre of ASASSN-15lh's host galaxy destroying a Sun-like star via spaghettification.


Volgenau N.,Las Cumbres Observatory | Boroson T.,Las Cumbres Observatory
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2016

With 18 telescopes distributed over 6 sites, and more telescopes being added in 2016, Las Cumbres Observatory Global Telescope Network is a unique resource for timedomain astronomy. The Network's continuous coverage of the night sky, and the optimization of the observing schedule over all sites simultaneously, have enabled LCOGTusers to produce significant science results. However, practical challenges to maximizing the Network's science output remain. The Network began providing observations for members of its Science Collaboration and other partners in May 2014. In the two years since then, LCOGT has made a number of improvements to increase the Network's science yield. We also now have two years' experience monitoring observatory performanceÍ3/4 effective monitoring of an observatory that spans the globe is a complex enterprise. Here, we describe some of LCOGT's efforts to monitor the Network, assess the quality of science data, and improve communication with our users. © 2016 SPIE.


Trimble V.,University of California at Irvine | Trimble V.,Las Cumbres Observatory
Scientometrics | Year: 2011

It has been shown that papers in stem cell research submitted from institutions in the USA are accepted faster than those submitted from elsewhere and that the cause might at least partly be some bias in the refereeing process. We investigate whether there is a similar difference in time scale for papers in astronomy, astrophysics, and cosmology and look briefly at some of the possible causes. We find a publication time lag of 3.8 days (out of a median time of 105 days) while in the stem cell case it is 24 days out of a median of 83 days. One of many possible causes is a difference in how useful the papers are to the community, and we will assess this in a second paper making use of citation analysis. © 2011 Akadémiai Kiadó, Budapest, Hungary.

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