David B. Reitzel

The Initial-Final Mass Relation: Direct Constraints at the Low-Mass End* **

The initial-final mass relation represents a mapping between the mass of a white dwarf remnant and the mass that the hydrogen-burning main-sequence star that created it once had. The empirical relation thus far has been constrained using a sample of ~40 stars in young open clusters, ranging in initial mass from ~2.75 to 7 -->M?, and shows a general trend that connects higher mass main-sequence stars with higher mass white dwarfs. In this paper, we present CFHT CFH12K photometric and Keck LRIS multiobject spectroscopic observations of a sample of 22 white dwarfs in two older open clusters, NGC 7789 ( -->t = 1.4 Gyr) and NGC 6819 ( -->t = 2.5 Gyr). We measure masses for the highest signal-to-noise ratio spectra by fitting the Balmer lines to atmosphere models and place the first direct constraints on the low-mass end of the initial-final mass relation. Our results indicate that the observed general trend at higher masses continues down to low masses, with -->Minitial = 1.6 M? main-sequence stars forming -->Mfinal = 0.54 M? white dwarfs. When added to our new data from the very old cluster NGC 6791, the relation is extended down to -->Minitial = 1.16 M? (corresponding to -->Mfinal = 0.53 M?). This extension of the relation represents a fourfold increase in the total number of hydrogen-burning stars for which the integrated mass loss can now be calculated from empirical data, assuming a Salpeter initial mass function. The new leverage at the low-mass end is used to derive a purely empirical initial-final mass relation. The sample of white dwarfs in these clusters also shows several interesting systems that we discuss further: a DB (helium) white dwarf, a magnetic white dwarf, a DAB (mixed hydrogen/helium atmosphere or a double degenerate DA+DB) white dwarf(s), and two possible equal-mass DA double degenerate binary systems.

The Metal-poor Halo of the Andromeda Spiral Galaxy (M31)

We present spectroscopic observations of red giant branch (RGB) stars over a large expanse in the halo of the Andromeda spiral galaxy (M31), acquired with the DEIMOS instrument on the Keck II 10 m telescope. Using a combination of five photometric/spectroscopic diagnostics?(1) radial velocity, (2) intermediate-width DDO51 photometry, (3) Na I equivalent width (surface gravity-sensitive), (4) position in the color-magnitude diagram, and (5) comparison between photometric and spectroscopic [Fe/H] estimates?we isolate over 250 bona fide M31 bulge and halo RGB stars located in 12 fields ranging from R = 12 to 165 kpc from the center of M31 (47 of these stars are halo members with R > 60 kpc). We derive the M31 spheroid (bulge and halo) metallicity distribution function and find it to be systematically more metal-poor with increasing radius, shifting from [Fe/H] = -0.47 ? 0.03 (? = 0.39) at R 60 kpc, assuming [?/Fe] = 0.0. These results indicate the presence of a metal-poor RGB population at large radial distances out to at least R = 160 kpc, thereby supporting our recent discovery of a stellar halo in M31 (structural component with an R-2 power-law surface brightness profile). This component has a distinct metallicity distribution from M31s bulge. If we assume an ?-enhancement of [?/Fe] = +0.3 for M31s halo, we derive [Fe/H] = -1.5 ? 0.1 (? = 0.7). Therefore, the mean metallicity and metallicity spread of this newly found remote M31 RGB population are similar to those of the Milky Way halo.

The Bulge Radial Velocity Assay (BRAVA). II. Complete Sample And Data Release

We present new radial velocity measurements from the Bulge Radial Velocity Assay, a large-scale spectroscopic survey of M-type giants in the Galactic bulge/bar region. The sample of ~4500 new radial velocities, mostly in the region –10° < l < +10° and b ≈ –6°, more than doubles the existent published data set. Our new data extend our rotation curve and velocity dispersion profile to +20°, which is ~2.8 kpc from the Galactic center. The new data confirm the cylindrical rotation observed at –6° and –8° and are an excellent fit to the Shen et al. N-body bar model. We measure the strength of the TiOe molecular band as a first step toward a metallicity ranking of the stellar sample, from which we confirm the presence of a vertical abundance gradient. Our survey finds no strong evidence of previously unknown kinematic streams. We also publish our complete catalog of radial velocities, photometry, TiO band strengths, and spectra, which is available at the Infrared Science Archive as well as at UCLA.

The Golden Standard Type Ia Supernova 2005cf: Observations from the Ultraviolet to the Near-Infrared Wavebands

We present extensive photometry at ultraviolet (UV), optical, and near-infrared (NIR) wavelengths, as well as dense sampling of optical spectra, for the normal Type Ia supernova (SN Ia) 2005cf. The optical photometry, performed at eight different telescopes, shows a 1σ scatter of ≾0.03 mag after proper corrections for the instrument responses. From the well-sampled light curves, we find that SN 2005cf reached a B-band maximum at 13.63 ± 0.02 mag, with an observed luminosity decline rate Δm _(15)(B) = 1.05 ± 0.03 mag. The correlations between the decline rate and various color indexes, recalibrated on the basis of an expanded SN Ia sample, yield a consistent estimate for the host-galaxy reddening of SN 2005cf, E(B – V)_(host) = 0.10 ± 0.03 mag. The UV photometry was obtained with the Hubble Space Telescope and the Swift Ultraviolet/Optical Telescope, and the results match each other to within 0.1-0.2 mag. The UV light curves show similar evolution to the broadband U, with an exception in the 2000-2500 A spectral range (corresponding to the F220W/uvm2 filters), where the light curve appears broader and much fainter than that on either side (likely owing to the intrinsic spectral evolution). Combining the UV data with the ground-based optical and NIR data, we establish the generic UV-optical-NIR bolometric light curve for SN 2005cf and derive the bolometric corrections in the absence of UV and/or NIR data. The overall spectral evolution of SN 2005cf is similar to that of a normal SN Ia, but with variety in the strength and profile of the main feature lines. The spectra at early times displayed strong, high-velocity (HV) features in the Ca II H&K doublet and NIR triplet, which were distinctly detached from the photosphere (v ≈ 10,000 km s^(–1)) at a velocity ranging from 20,000 to 25,000 km s^(–1). One interesting feature is the flat-bottomed absorption observed near 6000 A in the earliest spectrum, which rapidly evolved into a triangular shape and then became a normal Si II λ6355 absorption profile at about one week before maximum brightness. This premaximum spectral evolution is perhaps due to the blending of the Si IIλ6355 at photospheric velocity and another HV absorption component (e.g., an Si II shell at a velocity ~18,000 km s^(–1)) in the outer ejecta, and may be common in other normal SNe Ia. The possible origin of the HV absorption features is briefly discussed.

Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low-Mass White Dwarfs* **

We present the first detailed study of the properties (temperatures, gravities, and masses) of the NGC 6791 white dwarf population. This unique stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~ +0.4) open clusters in our Galaxy and has a color-magnitude diagram (CMD) that exhibits both a red giant clump and a much hotter extreme horizontal branch. Fitting the Balmer lines of the white dwarfs in the cluster using Keck/LRIS spectra suggests that most of these stars are undermassive, M = 0.43 ± 0.06 M☉, and therefore could not have formed from canonical stellar evolution involving the helium flash at the tip of the red giant branch. We show that at least 40% of NGC 6791s evolved stars must have lost enough mass on the red giant branch to avoid the flash and therefore did not convert helium into carbon-oxygen in their core. Such increased mass loss in the evolution of the progenitors of these stars is consistent with the presence of the extreme horizontal branch in the CMD. This unique stellar evolutionary channel also naturally explains the recent finding of a very young age (2.4 Gyr) for NGC 6791 from white dwarf cooling theory; helium-core white dwarfs in this cluster will cool ~3 times slower than carbon-oxygen-core stars, and therefore the corrected white dwarf cooling age is in fact 7 Gyr, consistent with the well-measured main-sequence turnoff age. These results provide direct empirical evidence that mass loss is much more efficient in high-metallicity environments and therefore may be critical in interpreting the ultraviolet upturn in elliptical galaxies.

The Bulge Radial Velocity Assay (BRAVA): I. Sample Selection and a Rotation Curve

Results from the ongoing Bulge Radial Velocity Assay (BRAVA) are presented. BRAVA uses M red giant stars, selected from the 2MASS catalog to lie within a bound of reddening-corrected color and luminosity, as targets for the Cerro Tololo Inter-American Observatory 4 m Hydra multiobject spectrograph. Three years of observations investigate the kinematics of the Galactic bulge major (–10° < l < + 10°, b = − 4°) and minor (–6° < b < + 5°, -0.4° < l < 0.0°) axes with ~3300 radial velocities from 32 bulge fields and one disk field. We construct a longitude-velocity plot for the bulge stars and find that, contrary to previous studies, the bulge does not rotate as a solid body; from –4° < l < + 4° the rotation curve has a slope of roughly 100 km s−1 kpc−1 and flattens considerably at greater l, reaching a maximum rotation of 75 km s−1. We compare our rotation curve and velocity dispersion profile both to the self-consistent model of Zhao and to N-body models; neither fits both our observed rotation curve and velocity dispersion profile. We place the bulge on the plot of Vmax/σ vs. epsilon and find that the bulge lies near the oblate rotator line and very close to the parameters of NGC 4565, an edge-on spiral galaxy with a bulge similar to that of the Milky Way. We find that our summed velocity distribution of bulge stars appears to be sampled from a Gaussian distribution, with σ = 116 ± 2 km s−1 for our full data set. Two candidate cold streams are not confirmed with additional data.

An age difference of two billion years between a metal-rich and a metal-poor globular cluster

Globular clusters trace the formation history of the spheroidal components of our Galaxy and other galaxies, which represent the bulk of star formation over the history of the Universe. The clusters exhibit a range of metallicities (abundances of elements heavier than helium), with metal-poor clusters dominating the stellar halo of the Galaxy, and higher-metallicity clusters found within the inner Galaxy, associated with the stellar bulge, or the thick disk. Age differences between these clusters can indicate the sequence in which the components of the Galaxy formed, and in particular which clusters were formed outside the Galaxy and were later engulfed along with their original host galaxies, and which were formed within it. Here we report an absolute age of 9.9 ± 0.7 billion years (at 95 per cent confidence) for the metal-rich globular cluster 47 Tucanae, determined by modelling the properties of the cluster’s white-dwarf cooling sequence. This is about two billion years younger than has been inferred for the metal-poor cluster NGC 6397 from the same models, and provides quantitative evidence that metal-rich clusters like 47 Tucanae formed later than metal-poor halo clusters like NGC 6397.

A New Method for Isolating M31 Red Giant Stars: The Discovery of Stars out to a Radial Distance of 165 kpc

We present a method for isolating a clean sample of red giant branch stars in the outer regions of M31. Our study is based on an ongoing spectroscopic survey using the DEIMOS instrument on the Keck II 10 m telescope. The survey aims to study the kinematics, (sub)structure, and metallicity of M31s halo. Although most of our spectroscopic targets were photometrically screened to reject foreground Milky Way dwarf star contaminants, dwarf stars still constitute a substantial fraction of the observed spectra in the sparse outer halo. Our likelihood-based method for isolating M31 red giants uses five criteria: (1) radial velocity, (2) photometry in the intermediate-width DDO51 band to measure the strength of the MgH/Mg b absorption features, (3) strength of the Na I λ8190 absorption line doublet, (4) location within an (I, V - I) color-magnitude diagram, and (5) comparison of photometric (color-magnitude diagram based) versus spectroscopic (Ca II λ8500 triplet based) metallicity estimates. We also discuss other potential giant/dwarf separation criteria: the strength of the K I absorption lines at 7665 and 7699 A and the TiO bands at 7100, 7600, and 8500 A. Training sets consisting of definite M31 red giants and Galactic dwarf stars are used to derive empirical probability distribution functions for each diagnostic. These functions are used to calculate the likelihood that a given star is a red giant in M31 versus a Milky Way dwarf star. Using our diagnostic method, we isolate 40 M31 red giants beyond a projected distance of R = 60 kpc from the galaxys center, including three red giants at R ~ 165 kpc. The ability to identify individual M31 red giant stars gives us an unprecedented level of sensitivity in studying the properties of the galaxys outer halo.

Detection of the main-sequence turnoff of a newly discovered milky way halo structure in the triangulum-andromeda region

An upper main sequence (MS) and main-sequence turnoff (MSTO) feature appears in the color-magnitude diagram (CMD) of a large-area photometric survey of the southern half of M31 stretching to M33. Imaging in the Washington M, T2, DDO51 system allows us to remove the background M31/M33 giants from our CMD and more clearly see the dwarf star feature, which has an MSTO near M ~ 20.5. The corresponding stellar population shows little density variation over the 12° × 6° area of the sky sampled and is of very low surface brightness, Σ > 32 mag arcsec-2. We show that this feature is not the same as a previously identified MS+MSTO in the foreground of the Andromeda galaxy that has been associated with the tidal stream ringing the Milky Way disk at less than half the distance. Thus, the new stellar system is a separate, more distant entity, perhaps a segment of tidal debris from a disrupted satellite galaxy. It is most likely related to the structure with similar distance, location, and density uniformity seen as an excess of K and M giants in the Two Micron All Sky Survey reported in the companion paper by Rocha-Pinto and coworkers.

KINEMATICS AT THE EDGE OF THE GALACTIC BULGE: EVIDENCE FOR CYLINDRICAL ROTATION

We present new results from BRAVA, a large-scale radial velocity survey of the Galactic bulge, using M giant stars selected from the Two Micron All Sky Survey catalog as targets for the Cerro Tololo Inter-American Observatory 4 m Hydra multi-object spectrograph. The purpose of this survey is to construct a new generation of self-consistent bar models that conform to these observations. We report the dynamics for fields at the edge of the Galactic bulge at latitudes b = –8° and compare to the dynamics at b = –4°. We find that the rotation curve V(r) is the same at b = –8° as at b = –4°. That is, the Galactic boxy bulge rotates cylindrically, as do boxy bulges of other galaxies. The summed line-of-sight velocity distribution at b = –8° is Gaussian, and the binned longitude-velocity plot shows no evidence for either a (disk) population with cold dynamics or for a (classical bulge) population with hot dynamics. The observed kinematics are well modeled by an edge-on N-body bar, in agreement with published structural evidence. Our kinematic observations indicate that the Galactic bulge is a prototypical product of secular evolution in galaxy disks, in contrast with stellar population results that are most easily understood if major mergers were the dominant formation process.