Adrienne Margaret Cool

The Multiplicity of the Subgiant Branch of ω Centauri: Evidence for Prolonged Star Formation

We combine spectroscopic and photometric data for subgiant stars of ω Cen to extract results that neither data set could have provided on its own. GIRAFFE@VLT spectra of 80 stars at R = 6400 give metallicities for all of them and abundances of C, N, Ca, Ti, and Ba for a subset of them. The photometric data, which have unusually high accuracy, come from a ~10 × 10 arcmin2 mosaic of HST ACS images centered on the cluster center and on multicolor images of a ~34 × 33 arcmin2 field, taken with the WFI@ESO2.2m camera. Stars with [Fe/H] < -1.25 have a large magnitude spread on the flat part of the SGB. We interpret this as empirical evidence for an age spread, and from theoretical isochrones we derive a relative age for each star. Within the SGB region we identify four distinct stellar groups: (1) an old, metal-poor group ([Fe/H] ~ -1.7); (2) an old, metal-rich group ([Fe/H] ~ -1.1); (3) a young (up to 4-5 Gyr younger than the old component) metal-poor group ([Fe/H] ~ -1.7); (4) a young, intermediate-metallicity ([Fe/H] ~ -1.4) group, on average 1-2 Gyr younger than the old metal-poor population, and with an age spread that we cannot properly quantify with the present sample. In addition, many SGB stars are spread between the intermediate-metallicity and metal-rich branches. We tentatively propose connections between the SGB stars and both the multiple main-sequence and the red giant branches. Finally, we discuss the implications of the multiple stellar populations on the formation and evolution of ω Cen. The spread in age within each population establishes that the original system must have had a composite nature.

Analysis of the Quiescent Low-Mass X-Ray Binary Population in Galactic Globular Clusters

Quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars have been identified in several globular clusters using Chandra or XMM X-ray observations, via their distinctive soft thermal spectra. We report a complete census of the qLMXB population in these clusters, identifying three additional probable qLMXBs in NGC 6440. We conduct several analyses of the qLMXB population and compare it with the harder, primarily cataclysmic variable (CV), population of low-luminosity X-ray sources with 1031 ergs s-1 < LX < 1032.5 ergs s-1. The radial distribution of our qLMXB sample suggests an average system mass of 1.5 M☉, consistent with a neutron star and low-mass companion. Spectral analysis reveals that no globular cluster qLMXBs, other than the transient in NGC 6440, require an additional hard power-law component, as often observed in field qLMXBs. We identify an empirical lower luminosity limit of ~1032 ergs s-1 among globular cluster qLMXBs. The bolometric luminosity range of qLMXBs implies (in the deep crustal heating model of Brown and collaborators) low time-averaged mass-transfer rates, below the disk stability criterion. The X-ray luminosity functions of the CV populations alone in NGC 6397 and 47 Tuc are shown to differ. The distribution of qLMXBs among globular clusters is consistent with their dynamical formation by either tidal capture or exchange encounters, allowing us to estimate that 7 times more qLMXBs than bright LMXBs reside in globular clusters. The distribution of harder sources (primarily CVs) has a weaker dependence on density than that of the qLMXBs. Finally, we discuss possible effects of core collapse and globular cluster destruction on X-ray source populations.

HUBBLE SPACE TELESCOPE PROPER MOTION (HSTPROMO) CATALOGS OF GALACTIC GLOBULAR CLUSTERS. I. SAMPLE SELECTION, DATA REDUCTION, AND NGC 7078 RESULTS

We present the first study of high-precision internal proper motions (PMs) in a large sample of globular clusters, based on Hubble Space Telescope (HST) data obtained over the past decade with the ACS/WFC, ACS/HRC, and WFC3/UVIS instruments. We determine PMs for over 1.3 million stars in the central regions of 22 clusters, with a median number of ~60,000 stars per cluster. These PMs have the potential to significantly advance our understanding of the internal kinematics of globular clusters by extending past line-of-sight (LOS) velocity measurements to two- or three-dimensional velocities, lower stellar masses, and larger sample sizes. We describe the reduction pipeline that we developed to derive homogeneous PMs from the very heterogeneous archival data. We demonstrate the quality of the measurements through extensive Monte-Carlo simulations. We also discuss the PM errors introduced by various systematic effects, and the techniques that we have developed to correct or remove them to the extent possible. We provide in electronic form the catalog for NGC 7078 (M 15), which consists of 77,837 stars in the central 2.4 arcmin. We validate the catalog by comparison with existing PM measurements and LOS velocities, and use it to study the dependence of the velocity dispersion on radius, stellar magnitude (or mass) along the main sequence, and direction in the plane of the sky (radial/tangential). Subsequent papers in this series will explore a range of applications in globular-cluster science, and will also present the PM catalogs for the other sample clusters.

The Luminosity Function of the Globular Cluster NGC 6397 near the Limit of Hydrogen Burning

Second-epoch Hubble Space Telescope observations of NGC 6397 have led to the measurement of proper motions accurate enough to separate the faintest cluster stars from the field, thus extending the luminosity function of this globular cluster far enough to approach the limit of hydrogen burning on the main sequence. We isolate a sample of 1385 main-sequence stars, from just below the turnoff down to I = 24.5 (MI 12.5), which corresponds to a mass of less than 0.1 m☉ for the metallicity of this cluster. Below I = 22 (MI 10), the main-sequence luminosity function drops rapidly, in a manner similar to that predicted by theoretical models of low-mass stars near the hydrogen-burning limit.

Improved mass and radius constraints for quiescent neutron stars in ω Cen and NGC 6397

We use Chandra and XMM-Newton observations of the globular clusters omega Cen and NGC 6397 to measure the spectrum of their quiescent neutron stars (NSs), and thus to constrain the allowed ranges of mass and radius for each. We also use Hubble Space Telescope photometry of NGC 6397 to identify a potential optical companion to the quiescent NS, and find evidence that the companion lacks hydrogen. We carefully consider a number of systematic problems, and show that the choices of atmospheric composition, interstellar medium abundances, and cluster distances can have important effects on the inferred NS mass and radius. We find that for typical NS masses, the radii of both NSs are consistent with the 10-13 km range favoured by recent nuclear physics experiments. This removes the evidence suggested by Guillot and collaborators for an unusually small NS radius, which relied upon the small inferred radius of the NGC 6397 NS.

Discovery of candidate cataclysmic variables in the post-core-collapse globular cluster NGC 6397

A photometric search for objects with H alpha emission in the globular cluster NGC 6397 has been carried out with the Hubble Space Telescope (HST) Planetary Camera. Images were obtained through the F675W (similar to Johnson R) and F656N (H alpha) filters, and photometry carried out on approximately 900 stars detected in the central approximately 38 x 35 arcseconds of the cluster. Limiting magnitudes of R approximately 21 and 19.5 were reached in the F675W and F656N images, respectively. Three H alpha-bright stars are found which have apparent magnitudes in the range R = 17.8-19.5. The corresponding absolute magnitudes (M(sub R) = 5.6-7.3) and inferred H alpha emission-line strengths (EW(H alpha) greater than or approximately equal to 15-26 A) make these objects good candidates to be cataclysmic variables (CVs) in this nearby, post-collapse cluster. All three H alpha-bright objects are found to be UV-bright in a photometric comparison of the F675W images with archival HST Faint Object Camera (FOC) images of the cluster center through the F220W and F346M filters. Such UV excesses further support the identification of these objects as candidates to be CVs in the cluster. The H alpha-bright stars are within the error circles of three X-ray sources previously found with the ROSAT High Resoltion Imager (HRI) and are likley to be the dominant source of the X-ray emission. The implied X-Ray to optical flux ratios (f(sub x)/f(sub v) approximately 0.8-4.2) are consistent with CVs known elsewhere in the Galaxy. Two additional stars with possible H alpha emission are also discussed, along the stars identified on the basis of UV emisssion. Preliminary completeness estimates suggest that these observations should be sensitive to approximately one-third to two-thirds of most varieties of CVs in the cluster, and that therefore greater than or approximately 5-10 CVs are likely to be present in the observed section of NGC 6397. Scaling these numbers to the cluster as a whole would increase these numbers by up to a factor of 2. A total CV population on the order of 5-20 is in reasonable with previous estimates made on the basis of X-ray observations, and with theoretical predictions scaled to this cluster.

The Discovery of an Anomalous Subgiant Branch in the Color-Magnitude Diagram of ω Centauri

Using deep high-resolution multiband images taken with the Very Large Telescope and the Hubble Space Telescope, we discovered a new anomalous sequence in the color-magnitude diagram of ω Cen. This feature appears as a narrow well-defined subgiant branch (SGB-a), which merges into the main sequence of the dominant cluster population at a magnitude significantly fainter than the cluster turnoff (TO). The simplest hypothesis assumes that the new feature is the extension of the anomalous red giant branch (RGB-a) metal-rich population discovered by Lee et al. and Pancino et al. However, under this assumption the interpretation of the SGB-a does not easily fit into the context of a self-enrichment scenario within ω Cen. In fact, its TO magnitude, shape, and extension are not compatible with a young metal-rich population, as required by the self-enrichment process. The TO level of the SGB-a suggests indeed an age as old as the main cluster population, further supporting the extracluster origin of the most metal-rich stars, as suggested by Ferraro, Bellazzini, & Pancino. Only accurate measurements of radial velocities and metal abundances for a representative sample of stars will firmly establish whether or not the SGB-a is actually related to the RGB-a and will finally shed light on the origin of the metal-rich population of ω Cen.

Cataclysmic Variables and a Candidate Helium White Dwarf in the Globular Cluster NGC 6397

We have used the Hubble Space Telescope (HST ) and the Faint Object Spectrograph (FOS) to study faint UV stars in the core of the nearby globular cluster NGC 6397. We con—rm the presence of a fourth cataclysmic variable (CV) in NGC 6397 (hereafter CV 4), and we use the photometry of Cool et al. to present evidence that CVs 1¨4 all have faint disks and probably low accretion rates. By combining these results with new UV spectra of CV 1 and the published spectra of Grindlay et al., we present new evi- dence that CVs 1¨3 may be DQ Her systems, as originally suggested by Grindlay et al., and we show that CV 4 may either be a dwarf nova or another magnetic system. Another possibility is that the CVs could be old novae in hibernation between nova eruptions. We also present the —rst spectrum of a member of a new class of UV bright stars in NGC 6397. These faint, hot stars do not vary, unlike the CVs, and are thus denoted as ii non—ickerers ˇˇ (NFs). Like the CVs, their spatial concentration is strongly concentrated toward the cluster center. Using detailed comparisons with stellar atmosphere models, we have determined log g \ 6.25 ^ 1.0 and K for this NF. Using these line parameters T eff \ 17,500 ^ 5,000 and the luminosity of the NF, we show that the NF spectrum is consistent with a helium WD having a mass of D0.25 and an age between 0.1 and 0.5 Gyr (depending on the models used). The NF spec- M _ trum appears to be signi—cantly Doppler shifted from the expected wavelength, suggesting the presence of a dark, massive companion, probably a carbon-oxygen (CO) WD. Subject headings: globular clusters: individual (NGC 6397) ¨ novae, cataclysmic variables

Cataclysmic Variables and a New Class of Faint Ultraviolet Stars in the Globular Cluster NGC 6397

We present evidence that the globular cluster NGC 6397 contains two distinct classes of centrally concentrated UV-bright stars. Color-magnitude diagrams constructed from U -->336, B -->439, V -->555, and I -->814 data obtained with the Hubble Space Telescope Wide Field Planetary Camera 2 reveal seven UV-bright stars fainter than the main-sequence turnoff, three of which had previously been identified as cataclysmic variables (CVs). Light curves of these stars show the characteristic flicker of CVs as well as longer term variability. A fourth star is identified as a CV candidate on the basis of its variability and UV excess. Three additional UV-bright stars show no photometric variability and have broadband colors characteristic of B stars. These nonflickering UV stars are too faint to be extended horizontal-branch stars. We suggest that they could be low-mass helium white dwarfs, formed when the evolution of a red giant is interrupted, due either to Roche lobe overflow onto a binary companion or to envelope ejection following a common-envelope phase in a tidal-capture binary. Alternatively, they could be very low mass core-He-burning stars. Both the CVs and the new class of faint UV stars are strongly concentrated toward the cluster center, to the extent that mass segregation from two-body relaxation alone may be unable to explain their distribution.

Identification of Faint Chandra X-ray Sources in the Core-collapsed Globular Cluster NGC 6397: Evidence for a Bimodal Cataclysmic Variable Population

We have searched for optical identifications for 79 Chandra X-ray sources that lie within the half-mass radius of the nearby, core-collapsed globular cluster NGC 6397, using deep Hubble Space Telescope Advanced Camera for Surveys Wide Field Channel imaging in Hα, R, and B. Photometry of these images allows us to classify candidate counterparts based on color-magnitude diagram location. In addition to recovering nine previously detected cataclysmic variables (CVs), we have identified six additional faint CV candidates, a total of 42 active binaries (ABs), two millisecond pulsars, one candidate active galactic nucleus, and one candidate interacting galaxy pair. Of the 79 sources, 69 have a plausible optical counterpart. The 15 likely and possible CVs in NGC 6397 mostly fall into two groups: a brighter group of six for which the optical emission is dominated by contributions from the secondary and accretion disk and a fainter group of seven for which the white dwarf dominates the optical emission. There are two possible transitional objects that lie between these groups. The faintest CVs likely lie near the minimum of the CV period distribution, where an accumulation is expected. The spatial distribution of the brighter CVs is much more centrally concentrated than those of the fainter CVs and the ABs. This may represent the result of an evolutionary process in which CVs are produced by dynamical interactions, such as exchange reactions, near the cluster center and are scattered to larger orbital radii, over their lifetimes, as they age and become fainter.