6,367 research outputs found
The Initial Mass Function: Now and Then
We examine whether existing data in clusters, both old and young, and in the
field of the Galactic disk and halo is consistent with a universal slope for
the initial mass function (IMF). The most reasonable statement that can be made
at the current time is that there is no strong evidence to support a claim of
any real variations in this slope. If the IMF slope is universal then this in
itself is remarkable implying that variations in metallicity, gas density or
other environmental factors in the star formation process play no part in
determining the slope of the mass function.Comment: 10 pages, postscript. To appear in "Proceedings 7th Annual
Astrophysics Conference in Maryland-STAR FORMATION NEAR AND FAR
The stratified evolution of a cool star
A low mass star usually experiences stratification and abundance anomalies
during its evolution. A 0.95 solar mass star with a metallicity Z = 0.004 is
followed from the main-sequence to the Horizontal Branch (HB). On the
main-sequence the larger effects of stratification may come from accretion as
was suggested in relation to metallicity and planet formation. As it evolves
through the giant branch, stratification appears around the hydrogen burning
shell. It may create hydrodynamic instabilities and be related to abundance
anomalies on the giant branch. After the He flash the star evolves to the HB.
If it loses enough mass, it ends up a hot HB star (or in the field an sdB star)
with effective temperatures larger than 11000 K. All sdB stars are observed to
have an approximately solar iron abundance whatever their original metallicity,
implying overabundances by factors of up to 100. So should the 0.95 solar mass
star. How its internal hydrodynamic properties on the main sequence may
influence its fate on the HB is currently uncertain.Comment: Astronomische Nachrichten - Astronomical Notes (AN) papers presented
at the Cool Stars 17 conference 2012 (AN 334, issue 1-2
Implications of WMAP observations on Li abundance and stellar evolution models
The WMAP determination of the baryon-to-photon ratio implies, through Big
Bang nucleosynthesis, a cosmological Li abundance larger, by a factor of 2 to
3, than the Li abundance plateau observed in the oldest Pop II stars. It is
however inescapable that there be a reduction by a factor of at least 1.6 to
2.0 of the surface Li abundance during the evolution of Pop II field stars with
[Fe/H] < -1.5. That the observed Li be lower than cosmologically produced Li is
expected from stellar evolution models. Since at turnoff most of the Li
abundance reduction is caused by gravitational settling, the presence of
Lithium 6 in some turnoff stars is also understood. Given that the WMAP
implications for Li cosmological abundance and the Li Spite plateau can be
naturally explained by gravitational settling in the presence of weak
turbulence, there appears little need for exotic physics as suggested by some
authors. Instead, there is a need for a better understanding of turbulent
transport in the radiative zones of stars. This requires simulations from first
principles. Rather strict upper limits to turbulent transport are determined
for the Sun and Pop II stars.Comment: The Astrophysical Journal, in pres
The Double Cluster G185 in M31
We have identified a small globular cluster in M31 located approximately 4
arcseconds northwest of the M31 globular cluster G185. While several multiple
globular clusters have been observed in the Magellanic Clouds none have been
found in the Galaxy or in M31. We estimate the probability of such a chance
line-of-sight alignment occuring near the nucleus of M31 to be 0.09 +/- 0.03
and find no obvious indication of any tidal deformation in either cluster, as
would be expected if the clusters were interacting.
Two-dimensional modelling suggests G185 has a King (1966) [AJ, 71, 64]
concentration of c = 1.11 +/- 0.08 while the companion has c = 0.67 +/- 0.17
and is physically smaller than G185. Both objects have integrated dereddened
colors similar to those of Galactic globular clusters.Comment: 22 pages, ~1Mb postscript file
http://www.astro.ubc.ca/~holland/bib.html/
ftp://nukta.astro.ubc.ca/pub/holland/G185_preprint.ps.
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