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US Space Telescope Science Institute

Baltimore, MD, United States
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Kalirai J.S.,US Space Telescope Science Institute | Kalirai J.S.,Johns Hopkins University
Nature | Year: 2012

The Milky Way galaxy has several components, such as the bulge, disk and halo. Unravelling the assembly history of these stellar populations is often restricted because of difficulties in measuring accurate ages for low-mass, hydrogen-burning stars.1,2 Unlike these progenitors, white dwarf stars3, the 'cinders' of stellar evolution, are remarkably simple objects and their fundamental properties can be measured with little ambiguity. Here I report observations of newly formed white dwarf stars in the halo of the Milky Way, and a separate analysis of archival data in the well studied 12.5-billion-year-old globular cluster Messier 4. I measure the mass distribution of the remnant stars and invert the stellar evolution process to develop a mathematical relation that links this final stellar mass to the mass of their immediate progenitors, and therefore to the age of the parent population. By applying this technique to a small sample of four nearby and kinematically confirmed halo white dwarf stars, I calculate the age of local field halo stars to be 11.4 ± 0.7 billion years. The oldest globular clusters formed 13.5 billion years ago. Future observations of newly formed white dwarf stars in the halo could be used to reduce the uncertainty, and to probe relative differences between the formation times of the youngest globular clusters and the inner halo. © 2012 Macmillan Publishers Limited.

Bond H.E.,US Space Telescope Science Institute
Astrophysical Journal | Year: 2011

M31 RV is a red transient that erupted in 1988 in the Andromeda bulge, reaching a luminosity intermediate between novae and supernovae. It was cool throughout its outburst, unlike a normal classical nova. In 2006, Bond & Siegel examined archival Hubble Space Telescope (HST) optical images of the M31 RV site, obtained in 1999. We found only old red giants at the site and no stars of unusual color. However, Shara et al. recently claimed to have detected (1) a bright UV source within the error box in HST UV images taken in 1995, (2) a hot (T eff > 40, 000 K) optical source in the same 1999 images that we examined, and (3) cooling of this source from 1999 to 2008. Shara et al. argue that this source's behavior is consistent with a classical-nova outburst occurring on a low-mass white dwarf. I have re-examined all of the HST frames, including new ones obtained in 2009-2010. I find that (1) the bright 1995 UV source reported by Shara et al. was actually due to cosmic rays striking the same pixel in two successive exposures; (2) the claim that an optically bright star in the error box is very hot is actually due to misinterpretation of red-giant colors in the STmagnitude system; (3) there is no evidence for variability of any source within the error box from 1999 to 2010; and (4) there are no stars of unusually blue or red color in the error box. Our 2006 conclusions remain valid: either M31 RV had faded below HST detectability by 1999, or its remnant is an unresolved companion of a red giant in the field, or the remnant is one of the red giants. © 2011. The American Astronomical Society. All rights reserved..

Soderblom D.R.,US Space Telescope Science Institute
Annual Review of Astronomy and Astrophysics | Year: 2010

The age of an individual star cannot be measured, only estimated through mostly model-dependent or empirical methods, and no single method works well for a broad range of stellar types or for a full range in age. This review presents a summary of the available techniques for age-dating stars and ensembles of stars, their realms of applicability, and their strengths and weaknesses. My emphasis is on low-mass stars because they are present from all epochs of star formation in the Galaxy and because they present both special opportunities and problems. The ages of open clusters are important for understanding the limitations of stellar models and for calibrating empirical age indicators. For individual stars, a hierarchy of quality for the available age-dating methods is described. Although our present ability to determine the ages of even the nearest stars is mediocre, the next few years hold great promise as asteroseismology probes beyond stellar surfaces and starts to provide precise interior properties of stars and as models continue to improve when stressed by better observations. © 2010 by Annual Reviews.

Williams R.,US Space Telescope Science Institute
Astronomical Journal | Year: 2012

The spectra of postoutburst novae display either He+N or Fe II lines as the most prominent non-Balmer lines at maximum light. Spectral diagnostics indicate physical conditions for "He/N" spectra that are consistent with their origin in the white dwarf (WD) ejecta, whereas "Fe II" spectra point to their formation in a large circumbinary envelope of gas whose origin is the secondary star. A determining parameter for which of the two types of spectra predominate may be the binary mass ratio M sec/M WD. The increasing fraction of novae that are observed to be "hybrid" objects, where both classes of spectra appear sequentially, is explained by changing parameters in the two emitting regions during the postoutburst decline. We argue that most novae may be hybrids that show both types of spectra during decline. The emission line intensity ratio O I λ8446/λ7773 is suggested as a good density diagnostic for the ejecta, and a finding list of emission lines identified in recent spectroscopic surveys of novae is presented as an aid to future line identification work. © 2012. The American Astronomical Society. All rights reserved.

Bohlin R.C.,US Space Telescope Science Institute
Astronomical Journal | Year: 2014

The Space Telescope Imaging Spectrograph (STIS) has measured the flux for Sirius from 0.17 to 1.01 μm on the Hubble Space Telescope (HST) White Dwarf scale. Because of the cool debris disk around Vega, Sirius is commonly recommended as the primary IR flux standard. The measured STIS flux agrees well with predictions of a special Kurucz model atmosphere, adding confidence to the modeled IR flux predictions. The IR flux agrees to 2%-3% with respect to the standard template of Cohen and to 2% with the Midcourse Space Experiment absolute flux measurements in the mid-IR. A weighted average of the independent visible and mid-IR absolute flux measures implies that the monochromatic flux at 5557.5 Å (5556 Å in air) for Sirius and Vega, respectively, is 1.35 × 10-8 and 3.44 × 10-9 erg cm-2 s-1 Å-1 with formal uncertainties of 0.5%. Contrary to previously published conclusions, the Hipparcos photometry offers no support for the variability of Vega. Pulse pileup severely affects the Hp photometry for the brightest stars. © 2014. The American Astronomical Society. All rights reserved..

Brown R.A.,US Space Telescope Science Institute
Astrophysical Journal | Year: 2015

We use Nt, the number of exoplanets observed in time t, as a science metric to study direct-search missions like Terrestrial Planet Finder. In our model, N has 27 parameters, divided into three categories: 2 astronomical, 7 instrumental, and 18 science-operational. For various "27-vectors" of those parameters chosen to explore parameter space, we compute design reference missions to estimate Nt. Our treatment includes the recovery of completeness c after a search observation, for revisits, solar and antisolar avoidance, observational overhead, and follow- on spectroscopy. Our baseline 27-vector has aperture D = 16 m, inner working angle IWA = 0.039″, mission time t = 0-5 yr, occurrence probability for Earth-like exoplanets η = 0.2, and typical values for the remaining 23 parameters. For the baseline case, a typical five-year design reference mission has an input catalog of ∼4700 stars with nonzero completeness, ∼1300 unique stars observed in ∼2600 observations, of which ∼1300 are revisits, and it produces N1 ∼ 50 exoplanets after one year and N5 ∼ 130 after five years. We explore offsets from the baseline for 10 parameters. We find that N depends strongly on IWA and only weakly on D. It also depends only weakly on zodiacal light for Z < 50 zodis, end-to-end efficiency for h > 0.2, and scattered starlight for ζ < 10-10. We find that observational overheads, completeness recovery and revisits, solar and antisolar avoidance, and follow-on spectroscopy are all important factors in estimating N. © 2015. The American Astronomical Society. All rights reserved.

We study mass functions of globular clusters derived from Hubble Space Telescope/Advanced Camera for Surveys images of the early-type merger remnant galaxy NGC 1316, which hosts a significant population of metal-rich globular clusters of intermediate age (3Gyr). For the old, metal-poor ("blue") clusters, the peak mass of the mass function increases with internal half-mass density ρh as , whereas it stays approximately constant with galactocentric distance R gal. The mass functions of these clusters are consistent with a simple scenario in which they formed with a Schechter initial mass function and evolved subsequently by internal two-body relaxation. For the intermediate-age population of metal-rich ("red") clusters, the faint end of the previously reported power-law luminosity function of the clusters with R gal > 9kpc is due to many of those clusters having radii larger than the theoretical maximum value imposed by the tidal field of NGC 1316 at their R gal. This renders disruption by two-body relaxation ineffective. Only a few such diffuse clusters are found in the inner regions of NGC 1316. Completeness tests indicate that this is a physical effect. Using comparisons with star clusters in other galaxies and cluster disruption calculations using published models, we hypothesize that most red clusters in the low-ρh tail of the initial distribution have already been destroyed in the inner regions of NGC 1316 by tidal shocking, and that several remaining low-ρh clusters will evolve dynamically to become similar to "faint fuzzies" that exist in several lenticular galaxies. Finally, we discuss the nature of diffuse red clusters in early-type galaxies. © 2012. The American Astronomical Society. All rights reserved.

Mason E.,US Space Telescope Science Institute
Astronomy and Astrophysics | Year: 2011

This paper presents U Sco nebular spectra collected in the period March-May 2010 after the binary outburst on Jan. 28, 2010. The spectra display strong [Nev] and [Neiii] lines that can be used to compute the relative abundance of [Ne/O]. The value obtained ([Ne/O] = 1.69) is higher than the typical [Ne/O] abundance found in classical novae from CO progenitors and suggests that U Sco has a ONeMg white-dwarf progenitor. It follows that U Sco will not explode as a SN Ia but rather collapse to become a neutron star or a millisecond pulsar. © 2011 ESO.

This paper presents theoretical star formation and chemical enrichment histories for the stellar halo of the Milky Way (MW) based on new chemodynamical modeling. The goal of this study is to assess the extent to which metal-poor stars in the halo reflect the star formation conditions that occurred in halo progenitor galaxies at high redshift, before, and during the epoch of re-ionization. Simple prescriptions that translate dark-matter (DM) halo mass into baryonic gas budgets and star formation histories yield models that resemble the observed MW halo in its total stellar mass, metallicity distribution, and the luminosity function and chemical enrichment of dwarf satellite galaxies. These model halos in turn allow an exploration of how the populations of interest for probing the epoch of re-ionization are distributed in physical and phase space, and of how they are related to lower-redshift populations of the same metallicity. The fraction of stars dating from before a particular time or redshift depends strongly on radius within the galaxy, reflecting the "inside-out" growth of cold DM halos, and on metallicity, reflecting the general trend toward higher metallicity at later times. These results suggest that efforts to discover stars from z>6-10 should select for stars with [Fe/H] ≲-3 and favor stars on more tightly bound orbits in the stellar halo, where the majority are from z>10 and 15%-40% are from z>15. The oldest, most metal-poor stars - those most likely to reveal the chemical abundances of the first stars - are most common in the very center of the Galaxy's halo: they are in the bulge, but not of the bulge. These models have several implications for the larger project of constraining the properties of the first stars and galaxies using data from the local universe. © 2010. The American Astronomical Society.

Williams R.,US Space Telescope Science Institute
Astronomical Journal | Year: 2013

Available data for novae show that the X-ray and visible spectral regions correlate with each other as they evolve. Large differences in ionization exist simultaneously in the two wavelength regimes, and a straightforward model is proposed that explains the characteristics observed in both spectral regimes. Its key features are (1) ejected blobs of very high density gas from the white dwarf (WD) that expand to create within each clump a wide range of emitting density, ionization, and velocity, and (2) a more homogeneous circumbinary envelope of gas that is produced by secondary star mass loss. The relative mass loss rates from the two stars determine whether the He/N or the Fe II visible spectrum predominates during decline, when hard X-rays are detected, and when the WD can be detected as a super soft X-ray source. © 2013. The American Astronomical Society. All rights reserved.

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