62 research outputs found
The 'Forbidden' Abundance of Oxygen in the Sun
We reexamine closely the solar photospheric line at 6300 A, which is
attributed to a forbidden line of neutral oxygen, and is widely used in
analyses of other late-type stars.
We use a three-dimensional time-dependent hydrodynamical model solar
atmosphere which has been tested successfully against observed granulation
patterns and an array of absorption lines. We show that the solar line is a
blend with a Ni I line, as previously suggested but oftentimes neglected.
Thanks to accurate atomic data on the [O I] and Ni I lines we are able to
derive an accurate oxygen abundance for the Sun: log epsilon (O) = 8.69 +/-
0.05 dex, a value at the lower end of the distribution of previously published
abundances, but in good agreement with estimates for the local interstellar
medium and hot stars in the solar neighborhood. We conclude by discussing the
implication of the Ni I blend on oxygen abundances derived from the [O I] 6300
A line in disk and halo stars.Comment: 16 pages, 3 eps figures included; a more compact PostScript version
created using emulateapj.sty is available from
http://hebe.as.utexas.edu/recent_publi.html; to appear in ApJ
First Stellar Abundances in the Dwarf Irregular Galaxy Sextans A
We present the abundance analyses of three isolated A-type supergiant stars
in the dwarf irregular galaxy Sextans A from high-resolution spectra the UVES
spectrograph at the VLT. Detailed model atmosphere analyses have been used to
determine the stellar atmospheric parameters and the elemental abundances of
the stars. The mean iron group abundance was determined from these three stars
to be [(FeII,CrII)/H]=-0.99+/-0.04+/-0.06. This is the first determination of
the present-day iron group abundances in Sextans A. These three stars now
represent the most metal-poor massive stars for which detailed abundance
analyses have been carried out. The mean stellar alpha element abundance was
determined from the alpha element magnesium as
[alpha(MgI)/H]=-1.09+/-0.02+/-0.19. This is in excellent agreement with the
nebular alpha element abundances as determined from oxygen in the H II regions.
These results are consistent from star-to-star with no significant spatial
variations over a length of 0.8 kpc in Sextans A. This supports the nebular
abundance studies of dwarf irregular galaxies, where homogeneous oxygen
abundances are found throughout, and argues against in situ enrichment. The
alpha/Fe abundance ratio is [alpha(MgI)/FeII,CrII]=-0.11+/-0.02+/-0.10, which
is consistent with the solar ratio. This is consistent with the results from
A-supergiant analyses in other Local Group dwarf irregular galaxies but in
stark contrast with the high [alpha/Fe] results from metal-poor stars in the
Galaxy, and is most clearly seen from these three stars in Sextans A because of
their lower metallicities. The low [alpha/Fe] ratios are consistent with the
slow chemical evolution expected for dwarf galaxies from analyses of their
stellar populations.Comment: 40 pages, 8 figures, accepted for publication in A
Polarimetric Evidence of Non-Spherical Winds
Polarization observations yield otherwise unobtainable information about the
geometrical structure of unresolved objects. In this talk we review the
evidences for non-spherically symmetric structures around Luminous Hot Stars
from polarimetry and what we can learn with this technique. Polarimetry has
added a new dimension to the study of the envelopes of Luminous Blue Variables,
Wolf-Rayet stars and B[e] stars, all of which are discussed in some detail.Comment: 8 pages, 2 encapsulated Postscript figures, uses lamuphys.sty.
Invited review to appear in IAU Coll. 169, Variable and Non-Spherical Stellar
Winds in Luminous Hot Stars, eds. B. Wolf, A.Fullerton and O. Stahl
(Springer
Determining the Physical Properties of the B Stars I. Methodology and First Results
We describe a new approach to fitting the UV-to-optical spectra of B stars to
model atmospheres and present initial results. Using a sample of lightly
reddened stars, we demonstrate that the Kurucz model atmospheres can produce
excellent fits to either combined low dispersion IUE and optical photometry or
HST FOS spectrophotometry, as long as the following conditions are fulfilled:
1) an extended grid of Kurucz models is employed,
2) the IUE NEWSIPS data are placed on the FOS absolute flux system using the
Massa & Fitzpatrick (1999) transformation, and
3) all of the model parameters and the effects of interstellar extinction are
solved for simultaneously.
When these steps are taken, the temperatures, gravities, abundances and
microturbulence velocities of lightly reddened B0-A0 V stars are determined to
high precision. We also demonstrate that the same procedure can be used to fit
the energy distributions of stars which are reddened by any UV extinction curve
which can be expressed by the Fitzpatrick & Massa (1990) parameterization
scheme.
We present an initial set of results and verify our approach through
comparisons with angular diameter measurements and the parameters derived for
an eclipsing B star binary. We demonstrate that the metallicity derived from
the ATLAS 9 fits to main sequence B stars is essentially the Fe abundance. We
find that a near zero microturbulence velocity provides the best-fit to all but
the hottest or most luminous stars (where it may become a surrogate for
atmospheric expansion), and that the use of white dwarfs to calibrate UV
spectrophotometry is valid.Comment: 17 pages, including 2 pages of Tables and 6 pages of Figures.
Astrophysical Jounral, in pres
High-Resolution X-ray Spectroscopy of the Interstellar Medium
The interstellar medium (ISM) has a multiphase structure characterized by
gas, dust and molecules. The gas can be found in different charge states:
neutral, low-ionized (warm) and high-ionized (hot). It is possible to probe the
multiphase ISM through the observation of its absorption lines and edges in the
X-ray spectra of background sources. We present a high-quality RGS spectrum of
the low-mass X-ray binary GS 1826-238 with an unprecedent detailed treatment of
the absorption features due to the dust and both the neutral and ionized gas of
the ISM. We constrain the column density ratios within the different phases of
the ISM and measure the abundances of elements such as O, Ne, Fe and Mg. We
found significant deviations from the proto-Solar abundances: oxygen is
over-abundant by a factor 1.23 +/- 0.05, neon 1.75 +/- 0.11, iron 1.37 +/- 0.17
and magnesium 2.45 +/- 0.35. The abundances are consistent with the measured
metallicity gradient in our Galaxy: the ISM appears to be metal-rich in the
inner regions. The spectrum also shows the presence of warm/hot ionized gas.
The gas column has a total ionization degree less than 10%. We also show that
dust plays an important role as expected from the position of GS 1826-238: most
iron appears to be bound in dust grains, while 10-40% of oxygen consists of a
mixture of dust and molecules
Abundance Gradients and the Formation of the Milky Way
In this paper we adopt a chemical evolution model, which is an improved
version of the Chiappini, Matteucci and Gratton (1997) model, assuming two main
accretion episodes for the formation of the Galaxy. The present model takes
into account in more detail than previously the halo density distribution and
explores the effects of a threshold density in the star formation process,
during both the halo and disk phases. In the comparison between model
predictions and available data, we have focused our attention on abundance
gradients as well as gas, stellar and star formation rate distributions along
the disk. We suggest that the mechanism for the formation of the halo leaves
detectable imprints on the chemical properties of the outer regions of the
disk, whereas the evolution of the halo and the inner disk are almost
completely disentangled. This is due to the fact that the halo and disk
densities are comparable at large Galactocentric distances and therefore the
gas lost from the halo can substantially contribute to building up the outer
disk. We also show that the existence of a threshold density for the star
formation rate, both in the halo and disk phase, is necessary to reproduce the
majority of observational data in the solar vicinity and in the whole disk.
Moreover, we predict that the abundance gradients along the Galactic disk must
have increased with time and that the average [alpha/Fe] ratio in stars (halo
plus disk) slightly decrease going from 4 to 10 Kpcs from the Galactic center.
We also show that the same ratios increase substantially towards the outermost
disk regions and the expected scatter in the stellar ages decreases, because
the outermost regions are dominated by halo stars.Comment: 41 pages (including the figures), To be published in Ap
The origin of abundance gradients in the Milky Way: the predictions of different models
We aim at studying the abundance gradients along the Galactic disk and their
dependence upon several parameters: a threshold in the surface gas density
regulating star formation, the star formation efficiency, the timescale for the
formation of the thin disk and the total surface mass density of the stellar
halo. We test a model which considers a cosmological infall law. This law does
not predict an inside-out disk formation, but it allows to well fit the
properties of the solar vicinity. We study several cases. We find that to
reproduce at the same time the abundance, star formation rate and surface gas
density gradients along the Galactic disk it is necessary to assume an
inside-out formation for the disk. The threshold in the gas density is not
necessary and the same effect could be reached by assuming a variable star
formation efficiency. A cosmologically derived infall law with an inside-out
process for the disk formation and a variable star formation efficiency can
indeed well reproduce all the properties of the disk. However, the cosmological
model presented here does not have sufficient resolution to capture the
requested inside-out formation for the disk.Comment: 13 pages, 17 figures and 2 tables. Accepted for publication in
Astronomy & Astrophysic
Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19
young clusters by fitting the H-gamma profile in their spectra. We use
synthetic model profiles for rotating stars to develop a method to estimate the
polar gravity for these stars, which we argue is a useful indicator of their
evolutionary status. We combine these results with projected rotational
velocity measurements obtained in a previous paper on these same open clusters.
We find that the more massive B-stars experience a spin down as predicted by
the theories for the evolution of rotating stars. Furthermore, we find that the
members of binary stars also experience a marked spin down with advanced
evolutionary state due to tidal interactions. We also derive non-LTE-corrected
helium abundances for most of the sample by fitting the He I 4026, 4387, 4471
lines. A large number of helium peculiar stars are found among cooler stars
with Teff < 23000 K. The analysis of the high mass stars (8.5 solar masses < M
< 16 solar masses) shows that the helium enrichment process progresses through
the main sequence (MS) phase and is greater among the faster rotators. This
discovery supports the theoretical claim that rotationally induced internal
mixing is the main cause of surface chemical anomalies that appear during the
MS phase. The lower mass stars appear to have slower rotation rates among the
low gravity objects, and they have a large proportion of helium peculiar stars.
We suggest that both properties are due to their youth. The low gravity stars
are probably pre-main sequence objects that will spin up as they contract.
These young objects very likely host a remnant magnetic field from their natal
cloud, and these strong fields sculpt out surface regions with unusual chemical
abundances.Comment: 50 pages 18 figures, accepted by Ap
The Distribution Of Heavy Elements In Spiral And Elliptical Galaxies
This review recaps significant results as they apply to non-dwarf galaxies,
including the Milky Way, spiral disks and bulges, and elliptical and lenticular
galaxies. Conclusions that span the galaxy types treated here are as follows.
All galaxies, on average, have heavy element abundances (metallicities) that
systematically decrease outward from their galactic centers while their global
metallicities increase with galaxy mass. Abundance gradients are steepest in
normal spirals and are seen to be progressively flatter going in order from
barred spirals, lenticulars, and ellipticals. For spiral galaxies, local
metallicity appears to be correlated with total (disk plus bulge) surface
density. Observed abundance patterns indicate that N production is dominated by
primary processes at low metallicity and secondary processes at high
metallicity; C production increases with increasing metallicity; and O, Ne, S,
and Ar are produced in lockstep independent of metallicity. In elliptical
galaxies, nuclear abundances are in the range [Z/H] = 0.0 to 0.4, but the
element mixture is not scaled-solar. In large elliptical galaxies [Mg/Fe] is in
the range 0.3 to 0.5, decreasing to ~0 in smaller elliptical galaxies. Other
light elements track the Mg enhancement, but the heavier Ca tracks Fe. Velocity
dispersion appears to be a key parameter in the modulation of [Mg/Fe], but the
cause of the connection is unclear.Comment: 55-page manuscript plus 16 figures. Invited review to appear in the
Publications Of The Astronomical Society Of The Pacifi
- âŠ