119057925
The UV Upturn: From M32 to Distant Clusters
I review the observational constraints on the stars responsible for the upturn in the UV spectra of ellipticals, ranging from galaxies in the local Universe to distant clusters. In nearby galaxies, this UV upturn is produced by a minority population of extreme horizontal branch (EHB) stars, with the large variations observed in the UV-to-optical flux ratio driven by variations in the number of EHB stars, and not the type of UV-bright stars. Deep UV images of the nearest elliptical galaxy, M32, show that it has a well-populated EHB, even though it has the weakest UV upturn of any known elliptical galaxy. However, M32 suffers from a striking dearth of the hot post-HB stars expected from canonical evolutionary theory. As we observe to larger lookback times in more distant galaxy clusters, the UV upturn fades, as predicted by theories of stellar and galactic evolution, but does so gradually. Because the EHB stars do not appear suddenly in the Universe, their presence is likely driven by a large dispersion in the parameters that govern HB morphology.
67759178
THE FIRST DETECTION OF BLUE STRAGGLER STARS IN THE MILKY WAY BULGE
We report the first detections of Blue Straggler Stars (BSS) in the bulge of the Milky Way. Proper motions from extensive space-based observations along a single sight line allow us to separate a sufficiently clean and well-characterized bulge sample such that we are able to detect a small population of bulge objects in the region of the color-magnitude diagram commonly occupied by young objects and blue stragglers. Variability measurements of these objects clearly establish that a fraction of them are blue stragglers. Out of the 42 objects found in this region of the color-magnitude diagram, we estimate that at least 18 are genuine BSS. We normalize the BSS population by our estimate of the number of horizontal branch stars in the bulge in order to compare the bulge to other stellar systems. The BSS fraction is clearly discrepant from that found in stellar clusters. The blue straggler population of dwarf spheroidals remains a subject of debate; some authors claim an anticorrelation between the normalized blue straggler fraction and integrated light. If this trend is real, then the bulge may extend it by three orders of magnitude in mass. Conversely, we find that the genuinely young (<5 Gyr) population in the bulge, must be at most 3.4% under the most conservative scenario for the BSS population.
119330474
100 Times Faster and 3 Times Sharper: Background-Dominated Observations of Stellar Populations with an 8-meter Optical-UV Space Telescope
An 8 m successor to the Hubble Space Telescope (HST) would make incredible gains in the study of stellar populations, especially in the Local Group. If diffraction-limited at 0.5 microns, the "Next HST" could produce high-resolution imaging of faint sources over a wide field in 1 percent of the time needed with the HST. With these capabilities, photometry of the ancient main sequence could be obtained for many sight-lines through Local Group galaxies, thus determining directly the ages of their structures and providing a formation history for the Local Group populations.
14120815
The Star Formation History in the Andromeda Halo
I present the preliminary results of a program to measure the star formation history in the halo of the Andromeda galaxy. Using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope, we obtained the deepest optical images of the sky to date, in a field on the southeast minor axis of Andromeda, 51 arcmin (11 kpc) from the nucleus. The resulting color-magnitude diagram (CMD) contains approximately 300,000 stars and extends more than 1.5 mag below the main sequence turnoff, with 50% completeness at V=30.7 mag. We interpret this CMD using comparisons to ACS observations of five Galactic globular clusters through the same filters, and through chi-squared fitting to a finely-spaced grid of calibrated stellar population models. We find evidence for a major (approximately 30%) intermediate-age (6-8 Gyr) metal-rich ([Fe/H] > -0.5) population in the Andromeda halo, along with a significant old metal-poor population akin to that in the Milky Way halo. The large spread in ages suggests that the Andromeda halo formed as a result of a more violent merging history than that in our own Milky Way.
18506635
THE WFC3 GALACTIC BULGE TREASURY PROGRAM: A FIRST LOOK AT RESOLVED STELLAR POPULATION TOOLS
When the Wide Field Camera 3 (WFC3) is installed on the Hubble Space Telescope (HST), the astronomical community will have access to powerful new capabilities for investigating resolved stellar populations. The WFC3 Galactic Bulge Treasury program will obtain deep imaging in five photometric bands on four low-extinction fields. These data will have no proprietary period, and will enable a variety of science investigations not possible with previous data sets. To aid in planning for the use of these data and for future observing proposals, we provide an introduction to the Treasury program, its photometric system, and the associated calibration effort. The observing strategy is based upon a new photometric system employing five WFC3 bands spanning the UV, optical, and near-infrared: F390W, F555W, F814W, F110W, and F160W (analogous but not identical to the ground-based filters Washington C, V, I, J, and H). With these bands, one can construct reddening-free indices of temperature and metallicity. Using this photometric system, the program will target six fields in well studied star clusters, spanning a wide range of metallicity, and four fields in low-extinction windows of the Galactic bulge. The cluster data serve to calibrate the reddening-free indices, provide empirical population templates, and correct the transformation of theoretical isochrone libraries into the WFC3 photometric system. The bulge data will shed light on the bulge formation history, and will also serve as empirical population templates for other studies. One of the fields includes 12 candidate hosts of extrasolar planets. Color-magnitude diagrams (CMDs) are the most popular tool for analyzing resolved stellar populations. However, due to degeneracies among temperature, metallicity, and reddening in traditional CMDs, it can be difficult to draw robust conclusions from the data. The five-band system used for the bulge Treasury observations will provide reddening-free indices that are roughly orthogonal in temperature and metallicity, and we argue that model fitting in an index-index diagram will make better use of the information than fitting separate CMDs. We provide some results from simulations to show the expected quality of the data and their potential for differentiating between different star formation histories.
35491181
A SPECTROSCOPIC SEARCH FOR LEAKING LYMAN CONTINUUM AT z ∼ 0.7*
We present the results of rest-frame, UV slitless spectroscopic observations of a sample of 32 z ~ 0.7 Lyman break galaxy (LBG) analogs in the COSMOS field. The spectroscopic search was performed with the Solar Blind Channel on the Hubble Space Telescope. We report the detection of leaking Lyman continuum (LyC) radiation from an active galactic nucleus-starburst composite. While we find no direct detections of LyC emission in the remainder of our sample, we achieve individual lower limits (3σ) of the observed non-ionizing UV-to-LyC flux density ratios, f -ν (1500 A)/f _ν(830 A) of 20 to 204 (median of 73.5) and 378.7 for the stack. Assuming an intrinsic Lyman break of 3.4 and an intergalactic medium transmission of LyC photons along the line of sight to the galaxy of 85%, we report an upper limit for the relative escape fraction in individual galaxies of 0.02-0.19 and a stacked 3σ upper limit of 0.01. We find no indication of a relative escape fraction near unity as seen in some LBGs at z ~ 3. Our UV spectra achieve the deepest limits to date at any redshift for the escape fraction in individual sources. The contrast between these z ~ 0.7 low escape fraction LBG analogs with z ~ 3 LBGs suggests that either the processes conducive to high f esc are not being selected for in the z 1 samples or the average escape fraction is decreasing from z ~ 3 to z ~ 1. We discuss possible mechanisms that could affect the escape of LyC photons
18958265
Expected Detection and False Alarm Rates for Transiting Jovian Planets
Ground-based searches for transiting Jupiter-sized planets have so far produced few detections of planets but many of stellar systems with eclipse depths, durations, and orbital periods that resemble those expected from planets. The detection rates prove to be consistent with our present knowledge of binary and multiple-star systems and of Jovian-mass extrasolar planets. Space-based searches for transiting Earth-sized planets will be largely unaffected by the false alarm sources that afflict ground-based searches, except for distant eclipsing binaries whose light is strongly diluted by that of a foreground star. A by-product of the rate estimation is evidence that the period distribution of extrasolar planets is depressed for periods between 5 and 200 days.
200227432
Observations of Transiting Hot Compact Objects
119250819
Kepler Input Catalog: Photometric Calibration and Stellar Classification
We describe the photometric calibration and stellar classification methods used by the Stellar Classification Project to produce the Kepler Input Catalog (KIC). The KIC is a catalog containing photometric and physical data for sources in the Kepler mission field of view; it is used by the mission to select optimal targets. Four of the visible-light (g, r, i, z) magnitudes used in the KIC are tied to Sloan Digital Sky Survey magnitudes; the fifth (D51) is an AB magnitude calibrated to be consistent with Castelli & Kurucz (CK) model atmosphere fluxes. We derived atmospheric extinction corrections from hourly observations of secondary standard fields within the Kepler field of view. For these filters and extinction estimates, repeatability of absolute photometry for stars brighter than magnitude 15 is typically 2%. We estimated stellar parameters {T eff, log (g), log (Z), E B – V } using Bayesian posterior probability maximization to match observed colors to CK stellar atmosphere models. We applied Bayesian priors describing the distribution of solar-neighborhood stars in the color-magnitude diagram, in log (Z), and in height above the galactic plane. Several comparisons with samples of stars classified by other means indicate that for 4500 K ≤T eff ≤ 6500 K, our classifications are reliable within about ±200 K and 0.4 dex in log (g) for dwarfs, with somewhat larger log (g) uncertainties for giants. It is difficult to assess the reliability of our log (Z) estimates, but there is reason to suspect that it is poor, particularly at extreme T eff. Comparisons between the CK models and observed colors are generally satisfactory with some exceptions, notably for stars cooler than 4500 K. Of great importance for the Kepler mission, for T eff ≤ 5400 K, comparison with asteroseismic results shows that the distinction between main-sequence stars and giants is reliable with about 98% confidence. Larger errors in log (g) occur for warmer stars, for which our filter set provides inadequate gravity diagnostics. The KIC is available through the MAST data archive.
13379379
Far-UV Line Strengths in Elliptical Galaxies
Much of the far-UV emission from elliptical galaxies is thought to arise from extreme horizontal branch stars and related objects. Only about 10% of the stellar population needs to evolve through this phase even in galaxies with the strongest UV upturn. However it is not yet clear if this population represents the extreme low-metallicity or high-metallicty tail of the distribution, or rather arises from the overall population through some metallicity-insensitive mechanism that causes increased mass loss in a small fraction of RGB stars. We investigate the utility of far-UV line strengths for deciding between these possiblities. Complications include the fact that the line strengths reflect both the temperature distribution and the metallicity distribution of the stars, that there may be abundance anomalies introduced on the RGB, and that metals are likely to be redistributed by gravitational settling and radiative diffusion in the atmospheres of hot high-gravity stars. Line-strength measurements from Astro-2 HUT spectra are considered in this context.
119395717
Color-Luminosity Relations for the Resolved Hot Stellar Populations in the Centers of M31 and M32*
We present Faint Object Camera (FOC) ultraviolet images of the central 14'' × 14'' of Messier 31 and Messier 32. The hot stellar populations detected in the composite UV spectra of these nearby galaxies are partially resolved into individual stars, and their individual colors and apparent magnitudes are measured. We detect 433 stars in M31 and 138 stars in M32, down to detection limits mF275W = 25.5 mag and mF175W = 24.5 mag. We investigate the luminosity functions of the sources, their spatial distribution, their color-magnitude diagrams, and their total integrated far-UV flux. Comparison to IUE and Hopkins Ultraviolet Telescope (HUT) spectrophotometry and WFPC2 stellar photometry indicates consistency at the 0.3 mag level, with possible systematic offsets in the FOC photometry at a level less than this. Further calibrations or observations with the Space Telescope Imaging Spectrograph (STIS) will be necessary to resolve the discrepancies. Our interpretation rests on the assumption that the published FOC on-orbit calibration is correct. Although M32 has a weaker UV upturn than M31, the luminosity functions and color-magnitude diagrams (CMDs) of M31 and M32 are surprisingly similar and are inconsistent with a majority contribution from any of the following: post-asymptotic giant branch (PAGB) stars more massive than 0.56 M☉ (with or without associated planetary nebulae), main sequence stars, and blue stragglers. Both the luminosity functions and color-magnitude diagrams are consistent with a dominant population of stars that have evolved from the extreme horizontal branch (EHB) along tracks with masses between 0.47 and 0.53 M☉. These stars are well below the detection limits of our images while on the zero-age EHB but become detectable while in the more luminous (but shorter) AGB-manque and post-early asymptotic giant branch (PEAGB) phases. The FOC observations require that only very small fractions of the main-sequence populations (2% in M31 and 0.5% in M32) in these two galaxies evolve though the EHB and post-EHB phases, with the remainder evolving through bright PAGB evolution that is so rapid that few if any stars are expected in the small field of view covered by the FOC. A model with a flat EHB star mass distribution reproduces the HUT and IUE spectra of these two galaxies reasonably well, although there is some indication that an additional population of very hot (Teff > 25,000 K) EHB stars may be needed to reproduce the HUT spectrum of M31 near the Lyman limit and to bring integrated far-UV fluxes of M31 and M32 into agreement with IUE. In addition to the post-EHB population detected in the FOC, we find a minority population (~10%) of brighter stars that populate a region of the CMD that cannot be explained by canonical post-HB evolutionary tracks. The nature of these stars remains open to interpretation. The spatial distributions of the resolved UV-bright stars in both galaxies are more centrally concentrated than the underlying diffuse emission, implying that stellar populations of different ages and/or metallicities might be responsible for each component.