13 research outputs found
Common Envelope Evolution Redux
Common envelopes form in dynamical time scale mass exchange, when the
envelope of a donor star engulfs a much denser companion, and the core of the
donor plus the dense companion star spiral inward through this dissipative
envelope. As conceived by Paczynski and Ostriker, this process must be
responsible for the creation of short-period binaries with degenerate
components, and, indeed, it has proven capable of accounting for short-period
binaries containing one white dwarf component. However, attempts to reconstruct
the evolutionary histories of close double white dwarfs have proven more
problematic, and point to the need for enhanced systemic mass loss, either
during the close of the first, slow episode of mass transfer that produced the
first white dwarf, or during the detached phase preceding the final, common
envelope episode. The survival of long-period interacting binaries with massive
white dwarfs, such as the recurrent novae T CrB and RS Oph, also presents
interpretative difficulties for simple energetic treatments of common envelope
evolution. Their existence implies that major terms are missing from usual
formulations of the energy budget for common envelope evolution. The most
plausible missing energy term is the energy released by recombination in the
common envelope, and, indeed, a simple reformulation the energy budget
explicitly including recombination resolves this issue.Comment: 25 pages, 6 figures. To appear in "Short Period Binary Stars", ed.
E.F. Milone, D.A. Leahy, & D.W. Hobill (Springer
Effects of helium enrichment in globular clusters
Recently, the study of globular cluster (GC) CMDs has shown that some of them
harbor multiple populations with different chemical compositions and/or ages.
In the first case, the most common candidate is a spread in the initial helium
abundance, but this quantity is difficult to determine spectroscopically due to
the fact that helium absorption lines are not present in cooler stars, whereas
for hotter GC stars gravitational settling of helium becomes important. As a
consequence, indirect methods to determine the initial Y among populations are
necessary. For that reason, in this series of papers, we investigate the
effects of a Y enrichment in populations covering the range of GC
metallicities. In this first paper, we present the theoretical evolutionary
tracks, isochrones, and ZAHB loci calculated with the Princeton-Goddard-PUC
(PGPUC) stellar evolutionary code, which has been updated with the most recent
input physics and compared with other theoretical databases. The chemical
composition grid covers 9 Z ranging from Z=1.60x10^-4 to 1.57x10^-2, 7 Y from
Y=0.230 to 0.370, and an alpha-element enhancement of [alpha/Fe]=0.3. The
effects of different helium abundances that can be observed in isochrones are:
splits in the MS, differences in the L and Teff of the turn off point, splits
in the SGB being more prominent for lower ages or higher metallicities, splits
in the lower red giant branch being more prominent for higher ages or higher
metallicities, differences in L of the RGB bump (with small changes in Teff),
and differences in L at the RGB tip. At the ZAHB, when Y is increased there is
an increase of L for low Teff, which is affected in different degrees depending
on the age of the GC being studied. Finally, the ZAHB morphology distribution
depending on the age explains how for higher GC metallicities a population with
higher helium abundance could be hidden at the red ZAHB locus.Comment: 18 pages, 15 figure