980 research outputs found
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 kinematics of the most oxygen-poor planetary nebula PN G135.9+55.9
PN G135.9+55.9 is a compact, high excitation nebula that has been identified
recently as the most oxygen-poor halo planetary nebula. Given its very peculiar
characteristics and potential implications in the realms of stellar and
Galactic evolution, additional data are needed to firmly establish its true
nature and evolutionary history. Here we present the first long-slit, high
spectral resolution observations of this object in the lines of H and
He II 4686. The position-velocity data are shown to be compatible with the
interpretation of PN G135.9+55.9 being a halo planetary nebula. In both
emission lines, we find the same two velocity components that characterize the
kinematics as that of an expanding elliptical envelope. The kinematics is
consistent with a prolate ellipsoidal model with axis ratio about 2:1, a
radially decreasing emissivity distribution, a velocity distribution that is
radial, and an expansion velocity of 30 km/s for the bulk of the material. To
fit the observed line profiles, this model requires an asymmetric matter
distribution, with the blue-shifted emission considerably stronger than the
red-shifted emission. We find that the widths of the two velocity components
are substantially wider than those expected due to thermal motions, but
kinematic structure in the projected area covered by the slit appears to be
sufficient to explain the line widths. The present data also rule out the
possible presence of an accretion disk in the system that could have been
responsible for a fraction of the H flux, further supporting the
planetary nebula nature of PN G135.9+55.9.Comment: accepted by Astronomy & Astrophysic
Sirius A: turbulence or mass loss?
Context. Abundance anomalies observed in a fraction of A and B stars of both
Pop I and II are apparently related to internal particle transport. Aims. Using
available constraints from Sirius A, we wish to determine how well evolutionary
models including atomic diffusion can explain observed abundance anomalies when
either turbulence or mass loss is used as the main competitor to atomic
diffusion. Methods. Complete stellar evolution models, including the effects of
atomic diffusion and radiative accelerations, have been computed from the zero
age main-sequence of 2.1M\odot stars for metallicities of Z0 = 0.01 \pm 0.001
and shown to agree with the observed parameters of Sirius A. Surface abundances
were predicted for three values of the mass loss rate and for four values of
the mixed surface zone. Results. A mixed mass of ~ 10^-6 M\odot or a mass loss
rate of 10^-13 M\odot/yr were determined through comparison with observations.
Of the 17 abundances determined observationally which are included in our
calculations, up to 15 can be predicted within 2 sigmas and 3 of the 4
determined upper limits are compatible. Conclusions. While the abundance
anomalies can be reproduced slightly better using turbulence as the process
competing with atomic diffusion, mass loss probably ought to be preferred since
the mass loss rate required to fit abundance anomalies is compatible with the
observationally determined rate. A mass loss rate within a factor of 2 of
10^-13 M\odot/yr is preferred. This restricts the range of the directly
observed mass loss rate.Comment: Accepted by A&A, 25/07/201
The white dwarf cooling sequence in the old open cluster NGC 188
We develop the white dwarf luminosity function (LF) of the old open cluster
NGC 188 in order to determine a lower limit to the age of the cluster by using
the faint end of the cooling sequence.
To produce an extensive sequence of the cooling white dwarfs we imaged four
contiguous HST-WFPC2 fields in the center of the cluster in the F555W and F814W
filters. After imposing selection criteria on the detected objects we found a
white dwarf cooling sequence (down to V ~26.5) including 28 candidate white
dwarfs in the cluster. The exposures are not deep enough to reach the end of
this sequence, but the results of our analysis allow us to establish a lower
limit to the age of the cluster independently of the isochrone fit to the
cluster turnoff.
The most ancient white dwarfs found are ~4 Gyr old, an age that is set solely
by the photometric limit of our data. Classical methods provide an estimate of
\~7 Gyr (Sarajedini et al., 1999).Comment: 6 pages, 5 figures, accepted on Astronomy and Astrophysic
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