1,526 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
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.
Planetary nebulae in M32 and the bulge of M31: Line intensities and oxygen abundances
We present spectroscopy of planetary nebulae in M32 and in the bulge of M31
that we obtained with the MOS spectrograph at the Canada-France-Hawaii
Telescope. Our sample includes 30 planetary nebulae in M31 and 9 planetary
nebulae in M32. We also observed one H II region in the disk of M31. We
detected [O III]4363 in 18 of the planetary nebulae, 4 in M32 and 14
in the bulge of M31. We use our line intensities to derive electron
temperatures and oxygen abundances for the planetary nebulae.Comment: 17 pages, 12 figures, accepted by Astronomy & Astrophysics Supplement
Serie
Deep HST V- and I-Band Observations of Two Globular Clusters in the Halo of M31
We present deep (V ~= 27) V- and I-band stellar photometry of G302 and G312,
two globular star clusters in the halo of M31. These data were obtained using
the Hubble Space Telescope's Wide Field/Planetary Camera 2. We find iron
abundances of [Fe/H] = -1.85 +/- 0.12 for G302 and [Fe/H] = -0.56 +/- 0.03 for
G312, consistent with spectroscopic measurements. The color-magnitude diagrams
for each cluster show no evidence for an intermediate-aged population of stars,
or a second parameter effect in the morphology of the horizontal branch. G302
shows no evidence for a color gradient but the inner regions of G312 are bluer
than the outer regions. G312 shows no evidence of ellipticity or an extended
halo of unbound stars. G302 has a projected ellipticity of 0.195 +/- 0.012 with
the projected major axis oriented towards the center of M31. G302 also shows
evidence of an extended asymmetric stellar halo extending to at least twice the
fitted Michie-King tidal radius. The amount of mass beyond the tidal radius of
G302 is consistent with the stellar escape rates which have been predicted by
N-body simulations of globular clusters in the Galactic tidal field.Comment: 29 pages, 21 Postscript figures, uses aaspp4.sty, to be published in
the October 1997 A
Abundance anomalies in pre-main-sequence stars: Stellar evolution models with mass loss
The effects of atomic diffusion on internal and surface abundances of A and F
pre-main-sequence stars with mass loss are studied in order to determine at
what age the effects materialize, as well as to further understand the
processes at play in HAeBe and young ApBp stars. Self-consistent stellar
evolution models of 1.5 to 2.8Msun with atomic diffusion (including radiative
accelerations) for all species within the OPAL opacity database were computed
and compared to observations of HAeBe stars. Atomic diffusion in the presence
of weak mass loss can explain the observed abundance anomalies of
pre-main-sequence stars, as well as the presence of binary systems with metal
rich primaries and chemically normal secondaries such as V380 Ori and HD72106.
This is in contrast to turbulence models which do not allow for abundance
anomalies to develop on the pre-main-sequence. The age at which anomalies can
appear depends on stellar mass. For A and F stars, the effects of atomic
diffusion can modify both the internal and surface abundances before the onset
of the MS. The appearance of important surface abundance anomalies on the
pre-main-sequence does not require mass loss, though the mass loss rate affects
their amplitude. Observational tests are suggested to decipher the effects of
mass loss from those of turbulent mixing. If abundance anomalies are confirmed
in pre-main-sequence stars they would severely limit the role of turbulence in
these stars.Comment: 9 pages, 6 figures, accepeted for publicatio
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