Where are the Old-Population High Velocity Stars?

To date, all of the reported high velocity stars (HVSs), which are believed to be ejected from the Galactic center, are blue and therefore almost certainly young. Old-population HVSs could be much more numerous than the young ones that have been discovered, but still have escaped detection because they are hidden in a much denser background of Galactic halo stars. Discovery of these stars would shed light on star formation at the Galactic center, the mechanism by which they are ejected from it, and, if they prove numerous, enable detailed studies of the structure of the dark halo. We analyze the problem of finding these stars and show that the search should be concentrated around the main-sequence turnoff (0.3<g-i<1.1) at relatively faint magnitudes (19.5<g<21.5). If the ratio of turnoff stars to B stars is the same for HVSs as it is in the local disk, such a search would yield about 1 old-population HVS per 10 deg^2. A telescope similar to the Sloan 2.5m could search about 20 deg^2 per night, implying that in short order such a population, should it exist, would show up in interesting numbers.Comment: Submitted to ApJ. 7 pages including 2 figure

Calcium-rich Gap Transients: Solving the Calcium Conundrum in the Intracluster Medium

X-ray measurements suggest the abundance of Calcium in the intracluster medium is higher than can be explained using favored models for core-collapse and Type Ia supernovae alone. We investigate whether the Calcium conundrum in the intracluster medium can be alleviated by including a contribution from the recently discovered subclass of supernovae known as Calcium-rich gap transients. Although the Calcium-rich gap transients make up only a small fraction of all supernovae events, we find that their high Calcium yields are sufficient to reproduce the X-ray measurements found for nearby rich clusters. We find the $\chi^{2}$ goodness-of-fit metric improves from 84 to 2 by including this new class. Moreover, Calcium-rich supernovae preferentially occur in the outskirts of galaxies making it easier for the nucleosynthesis products of these events to be incorporated in the intracluster medium via ram-pressure stripping. The discovery of a Calcium-rich gap transients in clusters and groups far from any individual galaxy suggests supernovae associated with intracluster stars may play an important role in enriching the intracluster medium. Calcium-rich gap transients may also help explain anomalous Calcium abundances in many other astrophysical systems including individual stars in the Milky Way, the halos of nearby galaxies and the circumgalactic medium. Our work highlights the importance of considering the diversity of supernovae types and corresponding yields when modeling the abundance of the intracluster medium and other gas reservoirs

The star capture model for fueling quasar accretion disks

Although the powering mechanism for quasars is now widely recognized to be the accretion of matter in a geometrically thin disk, the transport of matter to the inner region of the disk where luminosity is emitted remains an unsolved question. Miralda-Escud\'e & Kollmeier (2005) proposed a model whereby quasars are fuelled when stars are captured by the accretion disk as they plunge through the gas. Such plunging stars can then be destroyed and deliver their mass to the accretion disk. Here we present the first detailed calculations for the capture of stars originating far from the accretion disk near the zone of influence of the central black hole. In particular we examine the effect of adding a perturbing mass to a fixed stellar cusp potential on bringing stars into the accretion disk where they can be captured. The work presented here will be discussed in detail in an upcoming publication Kennedy et al. (2010).Comment: 2 pages, 1 figure, to be published in Proceedings of IAU Symp. 271, Astrophysical Dynamics: from Stars to Galaxies, ed. N. Brummell &amp; A.S. Brun, Cambrige Univ Pres