We present new observations of the white dwarf G29-38 with the camera (4.5 and 8 mm), photometer (24 mm), and spectrograph (5.5-14 mm) of the Spitzer Space Telescope. This star has an exceptionally large infrared excess, amounting to 3% of the bolometric luminosity. The spectral energy distribution (SED) has a continuum peak around 4.5 mm and a 9-11 mm emission feature 1.25 times brighter than the continuum. A mixture of amorphous olivine and a small amount of forsterite in an emitting region 1-5 from the star can reproduce the shape of the 9-11 mm R , feature. The SED also appears to require amorphous carbon to explain the hot continuum. Our new measurements support the idea that a relatively recent disruption of a comet or asteroid created the cloud.

We present near-infrared spectroscopic observations of 20 previously known DAZ white dwarfs obtained at the NASA Infrared Telescope Facility. Two of these white dwarfs (G29-38 and GD362) are known to display significant Kband excesses due to circumstellar debris disks. Here we report the discovery of excess K-band radiation from another DAZ white dwarf, WD0408-041 (GD56). Using spectroscopic observations, we show that the excess radiation cannot be explained by a stellar or substellar companion, and is likely to be caused by a warm debris disk. Our observations strengthen the connection between the debris disk phenomena and the observed metal abundances in cool DAZ white dwarfs. However, we do not find any excess infrared emission from the most metal rich DAZs with T eff = 16000 -20000 K. This suggests that the metal abundances in warmer DAZ white dwarfs may require another explanation.

We present 35 new pulsating DA (hydrogen atmosphere) white dwarf stars discovered from the Sloan Digital Sky Survey (SDSS) and the Hamburg Quasar Survey (HQS). We have acquired high-speed time series photometry of preselected DA white dwarfs with a prime focus CCD photometer on the 2.1 m telescope at McDonald Observatory over 15 months. We selected these stars on the basis of prior photometric and spectroscopic observations by the SDSS and HQS. For the homogeneous SDSS sample, we achieve a success rate of 80% for finding new variables at a detection threshold of 0.1%-0.3%. With 35 newly discovered DA variable white dwarfs, we almost double the current sample of 39.

We present high-precision UBV RI CCD photometry of the old open cluster NGC 188. Our colormagnitude diagram extends from near the red giant branch tip to as faint as D5 mag below the mainsequence turno †. From an analysis of these data along with published photometry for M67, we draw the following conclusions : (1) From the UBV two-color diagram, we Ðnd a reddening of E(B[V) \ 0.04^0.02 for M67 and E(B[V) \ 0.09^0.02 for NGC 188. (2) Based on main-sequence Ðtting to solar abundance isochrones, the distance moduli turn out to be for (m [ M) v \ 9.69^0.11 M67 and for NGC 188. (3) The comparison of the CMDs to theoretical iso-(m [ M) v \ 11.44^0.08 chrones indicates that an amount of core convective overshoot equivalent to 0.10 of a pressure scale height is appropriate for M67, while no overshoot is required to Ðt the CMD of NGC 188. These isochrones suggest that NGC 188 is 3.0^0.7 Gyr older than M67. (4) There is a clear indication of mass segregation in both M67 and NGC 188, with the most massive stars being more centrally (M/M _ [ 1.1) concentrated than those that are the least massive (0.8 º M/M _ [ 0.65).

A reduced proper motion diagram utilizing Sloan Digital Sky Survey (SDSS) photometry and astrometry and USNO-B plate astrometry is used to separate cool white dwarf candidates from metal-weak, high-velocity main sequence Population II stars (subdwarfs) in the SDSS Data Release 2 imaging area. Follow-up spectroscopy using the Hobby-Eberly Telescope, the MMT, and the McDonald 2.7m Telescope is used to demonstrate that the white dwarf and subdwarf loci separate cleanly in the reduced proper motion diagram, and that the contamination by subdwarfs is small near the cool white dwarf locus. This enables large statistically complete samples of white dwarfs, particularly the poorly understood cool white dwarfs, to be created from the SDSS imaging survey, with important implications for white dwarf luminosity function studies. SDSS photometry for our sample of cool white dwarfs is compared to current white dwarf models.Comment: AJ accepted, 36 pages, 11 figures (Figure 1 low-rez version

Our mid-infrared survey of 124 white dwarfs with the Spitzer Space Telescope and the IRAC imager has revealed an infrared excess associated with the white dwarf WD 2115-560 naturally explained by circumstellar dust. This object is the fourth white dwarf observed to have circumstellar dust. All four are DAZ white dwarfs, i.e. they have both photospheric Balmer lines and photospheric metal lines. We discuss these four objects as a class, which we abbreviate "DAZd", where the "d" stands for "dust". Using an optically-thick, geometrically-thin disk model analogous to Saturn's rings, we find that the inner disk edges are at >~0.1 to 0.2 Ro and that the outer disk edges are ~0.3 to 0.6 Ro. This model naturally explains the accretion rates and lifetimes of the detected WD disks and the accretion rates inferred from photospheric metal abundances.Comment: 27 pages, 7 figures, ApJ accepte

We have used the Hubble Space Telescope's Advanced Camera for Surveys (ACS) to detect and measure ~5300 stars in a single intracluster field in the Virgo Cluster. By performing F606W and F814W photometry on these stars, we have determined their metallicity distribution function, and constrained the types of stars present in this portion of Virgo's intracluster space. Based on the small number of stars detected brighter than the red giant branch (RGB) tip, we suggest that in this region, Virgo's intracluster stars are mostly old (>~10 Gyr). Through analysis of the RGB stars themselves, we determine that the population contains the full range of metallicities probed (-2.3<[M/H]<0.0). We also present evidence that the younger (<10 Gyr) component of the population is more metal-rich, with [M/H]>-0.5. The spatial distribution of the most metal-poor stars in the field shows significantly more structure than that of the metal-rich stars, indicating that the intracluster population is not well-mixed. We discuss the implications these observations have for the production of intracluster stars and the dynamical evolution of the Virgo Cluster.Comment: 36 pages, 1 table, 13 figures (resolution lowered for Figs. 4, 6, and 7), accepted for publication in Ap

It has been suspected for nearly 50 years that clusters of galaxies contain a population of intergalactic stars, ripped from galaxies during cluster formation or when the galaxies' orbits take them through the cluster center. Support for the existence of such a population of free-floating stars comes from measurements of the diffuse light in clusters, and from recent detections of planetary nebulae with positions and/or velocities far removed from any observed cluster galaxy. But estimates for the mass of the diffuse population and its distribution relative to the galaxies are still highly uncertain. Here we report the direct detection of intergalactic stars in deep images of a blank field in the Virgo Cluster. The data suggest that approximately 10% of the stellar mass of the cluster is in intergalactic stars. We observe a relatively homogeneous distribution of stars, with evidence of a slight gradient toward M87.Comment: Accepted for publication in Nature. 10 pages, 2 postscript figures included. Uses nature.sty and astrobib.sty. (Astrobib is available from http://www.stsci.edu/software/TeX.html.