Kurtis A. Williams

PROBING THE LOWER MASS LIMIT FOR SUPERNOVA PROGENITORS AND THE HIGH-MASS END OF THE INITIAL-FINAL MASS RELATION FROM WHITE DWARFS IN THE OPEN CLUSTER M35 (NGC 2168)*

We present a photometric and spectroscopic study of the white dwarf (WD) population of the populous, intermediate-age open cluster M35 (NGC 2168); this study expands upon our previous study of the WDs in this cluster. We spectroscopically confirm 14 WDs in the field of the cluster: 12 DAs, 1 hot DQ, and 1 DB star. For each DA, we determine the WD mass and cooling age, from which we derive each stars progenitor mass. These data are then added to the empirical initial-final mass relation (IFMR), where the M35 WDs contribute significantly to the high-mass end of the relation. The resulting points are consistent with previously published linear fits to the IFMR, modulo moderate systematics introduced by the uncertainty in the star cluster age. Based on this cluster alone, the observational lower limit on the maximum mass of WD progenitors is found to be ~5.1 M ? ? 5.2 M ? at the 95% confidence level; including data from other young open clusters raises this limit to as high as 7.1 M ?, depending on the cluster membership of three massive WDs and the core composition of the most massive WDs. We find that the apparent distance modulus and extinction derived solely from the cluster WDs ((m ? M) V = 10.45 ? 0.08 and E(B-V) = 0.185 ? 0.010, respectively) is fully consistent with that derived from main-sequence fitting techniques. Four M35 WDs may be massive enough to have oxygen-neon cores; the assumed core composition does not significantly affect the empirical IFMR. Finally, the two non-DA WDs in M35 are photometrically consistent with cluster membership; further analysis is required to determine their memberships.

THE WHITE DWARF LUMINOSITY FUNCTION FROM SLOAN DIGITAL SKY SURVEY IMAGING DATA

A sample of white dwarfs is selected from SDSS DR3 imaging data using their reduced proper motions, based on improved proper motions from SDSS plus USNO-B combined data. Numerous SDSS and followup spectra (Kilic et al. 2005) are used to quantify completeness and contamination of the sample; kinematic models are used to understand and correct for velocity-dependent selection biases. A luminosity function is constructed covering the range 7 < Mbol < 16, and its sensitivity to various assumptions and selection limits is discussed. The white dwarf luminosity function based on 6000 stars is remarkably smooth, and rises nearly monotonically to Mbol = 15.3. It then drops abruptly, although the small number of low-luminosity stars in the sample and their unknown atmospheric composition prevent quantitative conclusions about this decline. Stars are identified that may have high tangential velocities, and a preliminary luminosity function is constructed for them.

The open‐cluster initial–final mass relationship and the high‐mass tail of the white dwarf distribution

Recent studies of white dwarfs in open clusters have provided new constraints on the initial-final mass relationship (IFMR) for main-sequence stars with masses in the range 2.5-6.5 M ○. . We re-evaluate the ensemble of data that determines the IFMR and argue that the IFMR can be characterized by a mean IFMR about which there is an intrinsic scatter. We investigate the consequences of the IFMR for the observed mass distribution of field white dwarfs using population synthesis calculations. We show that while a linear IFMR predicts a mass distribution that is in reasonable agreement with the recent results from the Palomar-Green survey, the data are better fitted by an IFMR with some curvature. Our calculations indicate that a significant (∼28) percentage of white dwarfs originating from a single star evolution has masses in excess of ∼0.8 M ○. , obviating the necessity for postulating the existence of a dominant population of high-mass white dwarfs that arise from binary star mergers.

The Velocity Function of Galaxies

We present a galaxy circular velocity function, Ψ(log v), derived from existing luminosity functions and luminosity-velocity relations. Such a velocity function is desirable for several reasons. First, it enables an objective comparison of luminosity functions obtained in different bands and for different galaxy morphologies, with a statistical correction for dust extinction. In addition, the velocity function simplifies comparison of observations with predictions from high-resolution cosmological N-body simulations. We derive velocity functions from five different data sets and find rough agreement among them, but about a factor of 2 variation in amplitude. These velocity functions are then compared with N-body simulations of a ΛCDM model (corrected for baryonic infall) in order to demonstrate both the utility and the current limitations of this approach. The number density of dark matter halos and the slope of the velocity function near v*, the circular velocity corresponding to an ~L* spiral galaxy, are found to be comparable to those of observed galaxies. The primary sources of uncertainty in construction of Ψ(log v) from observations and N-body simulations are discussed, and explanations to account for discrepancies are suggested.

Cool White Dwarfs in the Sloan Digital Sky Survey

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 2.7 m 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.

First Results from a Photometric Survey of Strong Gravitational Lens Environments

Many strong gravitational lenses lie in complex environments, such as poor groups of galaxies, that significantly bias conclusions from lens analyses. We are undertaking a photometric survey of all known galaxy-mass strong lenses to characterize their environments and include them in careful lens modeling, as well as to build a large, uniform sample of galaxy groups at intermediate redshifts for evolutionary studies. In this paper we present wide-field photometry of the environments of 12 lens systems with 0.24 ≤ zlens ≤ 0.5. Using a red sequence identifying technique, we find that 8 of the 12 lenses lie in groups and that 10 group-like structures are projected along the line of sight toward 7 of these lenses. Follow-up spectroscopy of a subset of these fields confirms these results. For lenses in groups, the group centroid position is consistent with the direction of the external tidal shear required by lens models. Lens galaxies are not all super-L* elliptical galaxies; the median lens luminosity is L*, and the distribution of lens luminosities extends 3 mag below L* (in agreement with theoretical models). Only two of the lenses in groups are the brightest group galaxy, in qualitative agreement with theoretical predictions. As in the local universe, the highest velocity dispersion (σ) groups contain a brightest member spatially coincident with the group centroid, whereas lower σ groups tend to have an offset brightest group galaxy. This suggests that higher σ groups are more dynamically relaxed than lower σ groups and that at least some evolved groups exist by z ~ 0.5.

The age and progenitor mass of Sirius B

The Sirius AB binary system has masses that are well determined from many decades of astrometric measurements. Because of the well-measured radius and luminosity of Sirius A, we employed the TYCHO stellar evolution code to determine the age of the Sirius AB binary system accurately, at 225-250 Myr. Note that this fit requires the assumption of solar abundance and the use of the new Asplund et al. primordial solar metallicity. No fit to Sirius As position is possible using the old Grevesse & Sauval scale. Because the Sirius B white dwarf parameters have also been determined accurately from space observations, the cooling age could be determined from recent calculations by Fontaine et al. or Wood to be 124 ± 10 Myr. The difference in the two ages yields the nuclear lifetime and mass of the original primary star, 5.056 M☉. This result yields, in principle, the most accurate data point at relatively high masses for the initial-to-final mass relation. However, the analysis relies on the assumption that the primordial abundance of the Sirius stars was solar, based on membership in the Sirius supercluster. A recent study suggests that its membership in the group is by no means certain.

An Empirical Initial-Final Mass Relation from Hot, Massive White Dwarfs in NGC 2168 (M35)

The relation between the zero-age main-sequence mass of a star and its white dwarf remnant (the initial-final mass relation) is a powerful tool for the exploration of mass-loss processes during stellar evolution. We present an empirical derivation of the initial-final mass relation based on spectroscopic analysis of seven massive white dwarfs in NGC 2168 (M35). Using an internally consistent data set, we show that the resultant white dwarf mass increases monotonically with progenitor mass for masses greater than 4 M☉, one of the first open clusters to show this trend. We also find two massive white dwarfs foreground to the cluster that are otherwise consistent with cluster membership. These white dwarfs can be explained as former cluster members moving steadily away from the cluster at speeds of 0.5 km s-1 since their formation and may provide the first direct evidence of the loss of white dwarfs from open clusters. Based on these data alone, we constrain the upper mass limit of white dwarf progenitors to be 5.8 M☉ at the 90% confidence level for a cluster age of 150 Myr.

Ophiuchus 1622–2405: Not a Planetary-Mass Binary

We present an analysis of the mass and age of the young low-mass binary Oph 1622-2405. Using resolved optical spectroscopy of the binary, we measure spectral types of M7.25 ± 0.25 and M8.75 ± 0.25 for the A and B components, respectively. We show that our spectra are inconsistent with the spectral types of M9 and M9.5-L0 from Jayawardhana & Ivanov and M9 ± 0.5 and M9.5 ± 0.5 from Close and coworkers. Based on our spectral types and the theoretical evolutionary models of Chabrier and Baraffe, we estimate masses of ~0.055 and ~0.019 M_⊙ for Oph 1622-2405A and B, which are significantly higher than the values of 0.013 and 0.007 M_⊙ derived by Jayawardhana & Ivanov and above the range of masses observed for extrasolar planets (M ≾ 0.015 M_⊙). Planet-like mass estimates are further contradicted by our demonstration that Oph 1622-2405A is only slightly later (by 0.5 subclass) than the composite of the young eclipsing binary brown dwarf 2M 0535-0546, whose components have dynamical masses of 0.034 and 0.054 M_⊙. To constrain the age of Oph 1622-2405, we compare the strengths of gravity-sensitive absorption lines in optical and near-infrared spectra of the primary to lines in field dwarfs (τ > 1 Gyr) and members of Taurus (τ ~ 1 Myr) and Upper Scorpius (τ ~ 5 Myr). The line strengths for Oph 1622-2405A are inconsistent with membership in Ophiuchus (τ < 1 Myr) and instead indicate an age similar to that of Upper Sco, which is in agreement with a similar analysis performed by Close and coworkers. We conclude that Oph 1622-2405 is part of an older population in Sco-Cen, perhaps Upper Sco itself.

A new detailed examination of white dwarfs in NGC 3532 and NGC 2287

We present the results of a photometric and spectroscopic study of the white dwarf candidate members of the intermediate age open clusters NGC 3532 and NGC 2287. Of the nine objects investigated, it is determined that six are probable members of the clusters, four in NGC 3532 and two in NGC 2287. For these six white dwarfs, we use our estimates of their cooling times together with the cluster ages to constrain the lifetimes and masses of their progenitor stars. We examine the location of these objects in initial mass–final mass space and find that they now provide no evidence for substantial scatter in initial mass–final mass relation (IFMR) as suggested by previous investigations. Instead, we demonstrate that, when combined with current data from other solar metallicity open clusters and the Sirius binary system, they hint at an IFMR that is steeper in the initial mass range 3 M⊙ ≲ M init ≲ 4 M⊙ than at progenitor masses immediately lower and higher than this. This form is generally consistent with the predictions of stellar evolutionary models and can aid population synthesis models in reproducing the relatively sharp drop observed at the high mass end of the main peak in the mass distribution of white dwarfs.