1,943 research outputs found
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
On the Radial Distribution of White Dwarfs in the Globular Cluster NGC 6397
We have examined the radial distribution of white dwarfs over a single
HST/ACS field in the nearby globular cluster NGC 6397. In relaxed populations,
such as in a globular cluster, stellar velocity dispersion, and hence radial
distribution, is directly dependent on stellar masses. The progenitors of very
young cluster white dwarfs had a mass of ~0.8 solar masses, while the white
dwarfs themselves have a mass of ~0.5 solar masses. We thus expect young white
dwarfs to have a concentrated radial distribution (like that of their
progenitors) that becomes more extended over several relaxation times to mimic
that of ~0.5 solar mass main-sequence stars. However, we observe young white
dwarfs to have a significantly extended radial distribution compared to both
the most massive main sequence stars in the cluster and also to old white
dwarfs.Comment: 13 pages including 1 table and 3 figures. Accepted for publication in
the MNRAS Letter
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
The JCMT dense gas survey of the Perseus Molecular Cloud
We present the results of a large-scale survey of the very dense gas in the
Perseus molecular cloud using HCO+ and HCN (J = 4 - 3) transitions. We have
used this emission to trace the structure and kinematics of gas found in pre-
and protostellar cores, as well as in outflows. We compare the HCO+/HCN data,
highlighting regions where there is a marked discrepancy in the spectra of the
two emission lines. We use the HCO+ to identify positively protostellar
outflows and their driving sources, and present a statistical analysis of the
outflow properties that we derive from this tracer. We find that the relations
we calculate between the HCO+ outflow driving force and the Menv and Lbol of
the driving source are comparable to those obtained from similar outflow
analyses using 12CO, indicating that the two molecules give reliable estimates
of outflow properties. We also compare the HCO+ and the HCN in the outflows,
and find that the HCN traces only the most energetic outflows, the majority of
which are driven by young Class 0 sources. We analyse the abundances of HCN and
HCO+ in the particular case of the IRAS 2A outflows, and find that the HCN is
much more enhanced than the HCO+ in the outflow lobes. We suggest that this is
indicative of shock-enhancement of HCN along the length of the outflow; this
process is not so evident for HCO+, which is largely confined to the outflow
base.Comment: 25 pages, 14 figures, 9 table
A Cluster of Compact Radio Sources in NGC 2024 (Orion B)
We present deep 3.6 cm radio continuum observations of the H II region NGC
2024 in Orion B obtained using the Very Large Array in its A-configuration,
with angular resolution. We detect a total of 25 compact radio
sources in a region of . We discuss the nature of these sources
and its relation with the infrared and X-ray objects in the region. At least
two of the radio sources are obscured proplyds whose morphology can be used to
restrict the location of the main ionizing source of the region. This cluster
of radio sources is compared with others that have been found in regions of
recent star formation.Comment: 21 pages, 7 figure
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