195 research outputs found
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 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 & A.S.
Brun, Cambrige Univ Pres
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