25 research outputs found
A WFPC2 Study of the Resolved Stellar Population of the Pegasus Dwarf Irregular Galaxy (DDO 216)
The stellar population of the Pegasus dwarf irregular galaxy is investigated
in images taken in the F439W (B), F555W (V), and F814W (I) bands with WFPC2.
These and ground-based data are combined to produce color-magnitude diagrams
which show the complex nature of the stellar population in this small galaxy. A
young (< 0.5 Gyr) main sequence stellar component is present and clustered in
two centrally-located clumps, while older stars form a more extended disk or
halo. The colors of the main sequence require a relatively large extinction of
A_V = 0.47 mag. The mean color of the well-populated red giant branch is
relatively blue, consistent with a moderate metallicity young, or older,
metal-poor stellar population. The red giant branch also has significant width
in color, implying a range of stellar ages and/or metallicities. A small number
of extended asymptotic giant branch stars are found beyond the red giant branch
tip. Near the faint limits of our data is a populous red clump superimposed on
the red giant branch. Efforts to fit self-consistent stellar population models
based on the Geneva stellar evolution tracks yield a revised distance of 760
kpc. Quantitative fits to the stellar population are explored as a means to
constrain the star formation history. The numbers of main sequence and core
helium-burning blue loop stars require that the star formation rate was higher
in the recent past, by a factor of 3-4 about 1 Gyr ago. Unique results cannot
be obtained for the star formation history over longer time baselines without
better information on stellar metallicities and deeper photometry. The youngest
model consistent with the data contains stars with constant metallicity of Z =
0.001 which mainly formed 2-4 Gyr ago. Even at its peak of star forming
activity, the Pegasus dwarf most likely remained relatively dim with M_V ~ -14.Comment: 46 pages, 16 figures, 1 tabl
Dynamical Processes in Globular Clusters
Globular clusters are among the most congested stellar systems in the
Universe. Internal dynamical evolution drives them toward states of high
central density, while simultaneously concentrating the most massive stars and
binary systems in their cores. As a result, these clusters are expected to be
sites of frequent close encounters and physical collisions between stars and
binaries, making them efficient factories for the production of interesting and
observable astrophysical exotica. I describe some elements of the competition
among stellar dynamics, stellar evolution, and other processes that control
globular cluster dynamics, with particular emphasis on pathways that may lead
to the formation of blue stragglers.Comment: Chapter 10, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Relativistic Binaries in Globular Clusters
Galactic globular clusters are old, dense star systems typically containing
10\super{4}--10\super{7} stars. As an old population of stars, globular
clusters contain many collapsed and degenerate objects. As a dense population
of stars, globular clusters are the scene of many interesting close dynamical
interactions between stars. These dynamical interactions can alter the
evolution of individual stars and can produce tight binary systems containing
one or two compact objects. In this review, we discuss theoretical models of
globular cluster evolution and binary evolution, techniques for simulating this
evolution that leads to relativistic binaries, and current and possible future
observational evidence for this population. Our discussion of globular cluster
evolution will focus on the processes that boost the production of hard binary
systems and the subsequent interaction of these binaries that can alter the
properties of both bodies and can lead to exotic objects. Direct {\it N}-body
integrations and Fokker--Planck simulations of the evolution of globular
clusters that incorporate tidal interactions and lead to predictions of
relativistic binary populations are also discussed. We discuss the current
observational evidence for cataclysmic variables, millisecond pulsars, and
low-mass X-ray binaries as well as possible future detection of relativistic
binaries with gravitational radiation.Comment: 88 pages, 13 figures. Submitted update of Living Reviews articl