arXiv subjects

Kurtis A. Williams

Publications and source records attributed to Kurtis A. Williams.

40 records · Page 3Linked to original sources

arXiv2004

Spectroscopic Identification of Faint White Dwarf Candidates in the Praesepe Open Star Cluster

We present spectroscopic observations of the remaining four candidate white dwarfs in Praesepe. All four candidates are quasars with redshifts between 0.8 and 2.8. One quasar, LB 6072, is observed to have a strong metal-line absorption system blueward of the quasar redshift. The lack of additional white dwarfs in Praesepe leaves the total known white-dwarf population of the cluster at five, well below the number expected from commonly-assumed initial mass functions, though several undiscovered cluster WDs may lie in the outer regions of the cluster. All known Praesepe member white dwarfs are concentrated within 0.6 deg of the cluster center, and the radial profile of cluster white dwarfs is quite similar to the profile of massive cluster stars. This profile is mildly inconsistent with that of ~1Mo cluster stars and suggests that the white dwarfs did not receive a velocity kick during the progenitor star's mass loss phases. If complete, the observed Praesepe white dwarf population is consistent with a steeper high-end initial-mass function than commonly assumed, though the calculated slopes are inconsistent with the present-day mass function of Praesepe. Searches for white dwarfs outside the core of Praesepe and further study of the white dwarf populations of additional open clusters is necessary to constrain further the underlying cause of the white dwarf deficit.

astro-ph↗

arXiv2003

The impact of unresolved binaries on searches for white dwarfs in open clusters

Many open clusters have a deficit of observed white dwarfs (WDs) compared with predictions of the number of stars to have evolved into WDs. We evaluate the number of WDs produced in open clusters and the number of those WDS detectable using photometric selection techniques. This calculation includes the effects of varying the initial-mass function (IMF), the maximum progenitor masses of WDs, and the binary fraction. Differences between the calculated number of observable WDs and the actual number of WDs observed in a specific cluster then indicate the true deficit of WDs that must be explained through effects such as dynamical evolution of the cluster or close binary evolution. Observations of WDs in three open clusters, the Hyades, Pleiades, and Praesepe, are compared to the calculated observable populations in those clusters. The results suggest that a large portion of the white dwarf deficit may be explained by the presence of WDs in unresolved binary systems. However, the calculated WD populations still over-predict the number of observable WDs in each cluster. While these calculations cannot determine the cause of this residual white dwarf deficit, potential explanations include a steep high-mass IMF, dynamical evolution of the cluster, or an increased likelihood of equal-mass components in a binary system. Observations of complete WD samples in open clusters covering a range of ages and mass can help to distinguish between these possibilities.

astro-ph↗

arXiv2001

Serendipitous discovery of a cluster of galaxies with a peculiar central galaxy

We report the serendipitous discovery of a cluster of galaxies at z=0.369. Thirty-eight candidate members were identified based on rough broad-band photometric redshifts, and three members were confirmed spectroscopically. The brightest cluster galaxy (BCG) is exceptionally blue, with B-V=0.12 and V-I=1.02. The surface-brightness profile of the BCG follows an r^(1/4)-law profile out to 3" in all three bands. The effective radius is significantly smaller in bluer bandpasses, resulting in a blue core and a color gradient opposite to the metallicity-induced color gradient observed in typical elliptical galaxies. Beyond 3" an extended envelope of emission in excess of the r^(1/4)-law profile is observed, the position angle of which coincides with the major axis of the galaxy cluster. The spectrum of the BCG contains strong Balmer absorption, a minimal 4000 A break, and a broad Mg II emission line, suggesting that the galaxy has undergone recent star formation and may harbor a central AGN. The presence of numerous nearby bright stars makes this cluster an interesting target for next-generation adaptive optics using natural guide stars.

astro-ph↗

arXiv1999

The Velocity Function of Galaxies

We present a galaxy circular velocity function, Psi(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 LCDM model (corrected for baryonic infall) in order to demonstrate both the utility and 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 that of observed galaxies. The primary sources of uncertainty in construction of Psi(log v) from observations and N-body simulations are discussed and explanations are suggected to account for these discrepancies.

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