23 research outputs found
Stellar adiabatic mass loss model and applications
Roche-lobe overflow and common envelope evolution are very important in
binary evolution, which is believed to be the main evolutionary channel to hot
subdwarf stars. The details of these processes are difficult to model, but
adiabatic expansion provides an excellent approximation to the structure of a
donor star undergoing dynamical time scale mass transfer. We can use this model
to study the responses of stars of various masses and evolutionary stages as
potential donor stars, with the urgent goal of obtaining more accurate
stability criteria for dynamical mass transfer in binary population synthesis
studies. As examples, we describe here several models with the initial masses
equal to 1 Msun and 10 Msun, and identify potential limitations to the use of
our results for giant-branch stars.Comment: 7 pages, 5 figures,Accepted for publication in AP&SS, Special issue
Hot Sub-dwarf Stars, in Han Z., Jeffery S., Podsiadlowski Ph. ed
High-resolution spectroscopy of the R Coronae Borealis and Other Hydrogen Deficient Stars
High-resolution spectroscopy is a very important tool for studying stellar
physics, perhaps, particularly so for such enigmatic objects like the R Coronae
Borealis and related Hydrogen deficient stars that produce carbon dust in
addition to their peculiar abundances.
Examples of how high-resolution spectroscopy is used in the study of these
stars to address the two major puzzles are presented: (i) How are such rare
H-deficient stars created? and (ii) How and where are the obscuring soot clouds
produced around the R Coronae Borealis stars?Comment: 16 pages, 9 figures, Astrophysics and Space Science Proceedings,
Springer-Verlag, Berlin, 201
Graviton Mass from Close White Dwarf Binaries Detectable with LISA
The arrival times of gravitational waves and optical light from orbiting
binaries provide a mechanism to understand the propagation speed of gravity
when compared to that of light or electromagnetic radiation. This is achieved
with a measurement of any offset between optically derived orbital phase
related to that derived from gravitational wave data, at a specified location
of one binary component with respect to the other. Using a sample of close
white dwarf binaries (CWDBs) detectable with the Laser Interferometer Space
Antenna (LISA) and optical light curve data related to binary eclipses from
meter-class telescopes for the same sample, we determine the accuracy to which
orbital phase differences can be extracted. We consider an application of these
measurements involving a variation to the speed of gravity, when compared to
the speed of light, due to a massive graviton. For a subsample of 400
CWDBs with high signal-to-noise gravitational wave and optical data with
magnitudes brighter than 25, the combined upper limit on the graviton mass is
at the level of eV. This limit is two orders of
magnitude better than the present limit derived by Yukawa-correction arguments
related to the Newtonian potential and applied to the Solar-system.Comment: revised version, 8 pages, 5 figures, to appear in PR
The Blue Stragglers of the Old Open Cluster NGC 188
The old (7 Gyr) open cluster NGC 188 has yielded a wealth of astrophysical
insight into its rich blue straggler population. Specifically, the NGC 188 blue
stragglers are characterized by: A binary frequency of 80% for orbital periods
less than days;Typical orbital periods around 1000 days;Typical
secondary star masses of 0.5 M; At least some white dwarf companion
stars; Modestly rapid rotation; A bimodal radial spatial distribution;
Dynamical masses greater than standard stellar evolution masses (based on
short-period binaries); Under-luminosity for dynamical masses (short-period
binaries). Extensive -body modeling of NGC 188 with empirical initial
conditions reproduces the properties of the cluster, and in particular the
main-sequence solar-type binary population. The current models also reproduce
well the binary orbital properties of the blue stragglers, but fall well short
of producing the observed number of blue stragglers. This deficit could be
resolved by reducing the frequency of common-envelope evolution during Roche
lobe overflow. Both the observations and the -body models strongly indicate
that the long-period blue-straggler binaries - which dominate the NGC 188 blue
straggler population - are formed by asymptotic-giant (primarily) and red-giant
mass transfer onto main sequence stars. The models suggest that the few
non-velocity-variable blue stragglers formed from mergers or collisions.
Several remarkable short-period double-lined binaries point to the importance
of subsequent dynamical exchange encounters, and provide at least one example
of a likely collisional origin for a blue straggler.Comment: Chapter 3, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Population synthesis of Galactic subdwarf B stars
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84185.pdf (preprint version ) (Open Access