24 research outputs found
Analysis of Spatial Structure of the SPica H II Region
Far ultraviolet (FUV) spectral images of the Spica H II region are first
presented here for the Si II* 1533.4A and Al II 1670.8A lines and then compared
with the optical Halpha image. The H alpha and Si II* images show enhanced
emissions in the southern part of the H II region where H I density increases
outwards. This high density region, which we identify as part of the
"interaction ring" of the Loop I superbubble and the Local Bubble, seems to
bound the southern H II region. On the other hand, the observed profile of Al
II shows a broad central peak, without much difference between the northern and
southern parts, which we suspect results from multiple resonant scattering. The
extended tails seen in the radial profiles of the FUV intensities suggest that
the nebula may be embedded in a warm ionized gas. Simulation with a spectral
synthesis code yields the values of the Lyman continuum luminosity and the
effective temperature of the central star similar to previous estimates with
10^46.2 photons s^-1 and 26,000 K, respectively, but the density of the
northern H II region, 0.22 cm^-3, is much smaller than previous estimates for
the H alpha brightest region.Comment: 15 pages, 5 figures, accepted for Ap
Nitrogen Abundances in Damped Ly alpha Galaxies
Nitrogen abundances have been derived in Damped Ly alpha (DLA) galaxies at
Zabs = 2.309, 2.827 and 3.025 toward the QSOs 0100+1300, 1425+6039 and
0347-3819 respectively. The behaviour of nitrogen relative to iron-peak and
alpha-elements has been investigated by considering all the extant NI
determinations for a total of 9 DLA galaxies. We have estimated the fraction of
iron locked into dust grains to convert the observed [N/Fe] ratios into overall
(dust plus gas) relative abundances, [N/Fe]corr. The ratios [N/alpha] have been
mostly determined by using sulphur as a tracer of alpha-elements which is
unaffected by dust. The [N/Fe] and [N/alpha] ratios show high dispersions, of
one order of magnitude or more, which have no equivalent in other
element-to-element ratios in DLAs. The lowest values of the [N/Fe]corr and
[N/alpha] ratios are at variance with the values measured in Galactic halo
stars of similar metallicity suggesting that part of the DLA galaxies do not
follow the chemical evolution of the Milky Way. The DLA nitrogen abundances and
their dispersion show some similarities with those observed in dwarf galaxies.
The behaviour of nitrogen abundance ratios can be ascribed, in general to the
delayed release of nitrogen in the course of evolution. However it is difficult
to conciliate this interpretation with the lowest [N/alpha] values measured,
since an expected enhancement of alpha-elements respect to the iron-peak
elements is not observed simultaneously in these DLA galaxies. In two cases,
relatively high [N/alpha] values are observed which require also a more complex
chemical evolution to be explained.Comment: 29 pages including 5 tables and figure captions,LaTeX, 8 figures, ApJ
accepte
The Definitive Abundance of Interstellar Oxygen
Using the Goddard High Resolution Spectrograph (GHRS) onboard HST, we have
obtained high S/N echelle observations of the weak interstellar O I 1356 A
absorption toward the stars Gamma Cas, Epsilon Per, Delta Ori, Epsilon Ori, 15
Mon, Tau CMa, and Gamma Ara. In combination with previous GHRS measurements in
six other sightlines (Zeta Per, Xi Per, Lambda Ori, Iota Ori, Kappa Ori, and
Zeta Oph), these new observations yield a mean interstellar gas-phase oxygen
abundance (per 10 H atoms) of 10 O/H = 319 +/- 14. The largest
deviation from the mean is less than 18%, and there are no statistically
significant variations in the measured O abundances from sightline to sightline
and no evidence of density-dependent oxygen depletion from the gas phase.
Assuming various mixtures of silicates and oxides, the abundance of
interstellar oxygen tied up in dust grains is unlikely to surpass 10 O/H
180. Consequently, the GHRS observations imply that the total
abundance of interstellar oxygen (gas plus grains) is homogeneous in the
vicinity of the Sun and about 2/3 of the solar value of 10 O/H = 741 +/-
130. This oxygen deficit is consistent with that observed in nearby B stars and
similar to that recently found for interstellar krypton with GHRS. Possible
explanations for this deficit include: (1) early solar system enrichment by a
local supernova, (2) a recent infall of metal-poor gas in the local Milky Way,
or (3) an outward diffusion of the Sun from a smaller galactocentric distance.Comment: 23 pages, LaTeX, 5 Postscript figures; ApJ, in pres
The Abundance of Interstellar Boron
We use new Space Telescope Imaging Spectrograph (STIS) and archival Goddard
High Resolution Spectrograph (GHRS) observations to study interstellar B II
1362 and O I 1355 absorption along seven sightlines. Our new column density
measurements, combined with measurements of four sightlines from the
literature, allow us to study the relative B/O abundances over a wide range of
interstellar environments. We measure sightline-integrated relative gas-phase
abundances in the range [B/O] = -1.00 to -0.17, and our data show the B/O
abundances are anticorrelated with average sightline densities over the range
log ~ -1.3 to +0.7. Detailed comparisons of the B II and O I line shapes
show that the B/O ratio is significantly higher in warm interstellar clouds
than in cool clouds. These results are consistent with the incorporation of
boron into dust grains in the diffuse ISM. Since boron is likely incorporated
into grains, we derive a lower limit to the present-day total (gas+dust)
interstellar boron abundance of B/H > (2.5+/-0.9)x10^-10. The effects of dust
depletion and ionization differences from element to element will make it very
difficult to reliably determine 11B/10B along most interstellar sightlines.Comment: Accepted for publication in The Astrophysical Journal. 13 pages,
including 2 tables and 4 figures. Also available at
http://fuse.pha.jhu.edu/~howk/Papers
Using Cepheids to determine the galactic abundance gradient I. The solar neighbourhood
A number of studies of abundance gradients in the galactic disk have been
performed in recent years. The results obtained are rather disparate: from no
detectable gradient to a rather significant slope of about -0.1 dex kpc -1. The
present study concerns the abundance gradient based on the spectroscopic
analysis of a sample of classical Cepheids. These stars enable one to obtain
reliable abundances of a variety of chemical elements. Additionally, they have
well determined distances which allow an accurate determination of abundance
distributions in the galactic disc. Using 236 high resolution spectra of 77
galactic Cepheids, the radial elemental distribution in the galactic disc
between galactocentric distances in the range 6-11 kpc has been investigated.
Gradients for 25 chemical elements (from carbon to gadolinium) are derived...Comment: 28 pages, 14 postscript figures, LaTeX, uses Astronomy and
Astrophysics macro aa.cls, graphicx package, to be published in Astronomy and
Astrophysics (2002) also available at
http://www.iagusp.usp.br/~maciel/index.htm
Nitrogen enrichment, boron depletion and magnetic fields in slowly-rotating B-type dwarfs
Evolutionary models for massive stars, accounting for rotational mixing
effects, do not predict any core-processed material at the surface of B dwarfs
with low rotational velocities. Contrary to theoretical expectations, we
present a detailed and fully-homogeneous, NLTE abundance analysis of 20 early
B-type dwarfs and (sub)giants that reveals the existence of a population of
nitrogen-rich and boron-depleted, yet intrinsically slowly-rotating objects.
The low-rotation rate of several of these stars is firmly established, either
from the occurrence of phase-locked UV wind line-profile variations, which can
be ascribed to rotational modulation, or from theoretical modelling in the
pulsating variables. The observational data presently available suggest a
higher incidence of chemical peculiarities in stars with a (weak) detected
magnetic field. This opens the possibility that magnetic phenomena are
important in altering the photospheric abundances of early B dwarfs, even for
surface field strengths at the one hundred Gauss level. However, further
spectropolarimetric observations are needed to assess the validity of this
hypothesis.Comment: Accepted by A&A. 11 pages, 4 figures (some in colour
Abundances and Physical Conditions in the Warm Neutral Medium Towards mu Columbae
We present ultraviolet interstellar absorption line measurements for the
sightline towards the O9.5 V star mu Columbae obtained with the Goddard High
Resolution Spectrograph (GHRS) on board the Hubble Space Telescope. These
archival data represent the most complete GHRS interstellar absorption line
measurements for any line of sight towards an early-type star. The 3.5 km/s
resolution of the instrument allow us to accurately derive the gas-phase column
densities of many important ionic species in the diffuse warm neutral medium
using a combination of apparent column density and component fitting
techniques, and we study in detail the contamination from ionized gas along
this sightline. The low-velocity material shows gas-phase abundance patterns
similar to the warm cloud (cloud A) towards the disk star zeta Oph, while the
component at v = +20.1 km/s shows gas-phase abundances similar to those found
in warm halo clouds. We find the velocity-integrated gas-phase abundances of
Zn, P, and S relative to H along this sightline are indistinguishable from
solar system abundances. We discuss the implications of our gas-phase abundance
measurements for the composition of interstellar dust. The relative ionic
column density ratios of the intermediate velocity components show the imprint
both of elemental incorporation into grains and (photo)ionization. The
components at v = -30 and -48 km/s along this sightline likely trace shocked
gas with very low hydrogen column densities. Appendices include a new
derivation of the GHRS instrumental line spread function, and a new very
accurate determination of the total H I column along this sightline. (Abridged)Comment: Accepted for publication in the Astrophysical Journal. 80 pages
including 19 embedded figures and 12 embedded tables. Version with higher
resolution figures can be downloaded from
http://fuse.pha.jhu.edu/~howk/Papers/papers.htm
The neon content of nearby B-type stars and its implications for the solar model problem
The recent downward revision of the solar photospheric abundances now leads
to severe inconsistencies between the theoretical predictions for the internal
structure of the Sun and the results of helioseismology. There have been claims
that the solar neon abundance may be underestimated and that an increase in
this poorly-known quantity could alleviate (or even completely solve) this
problem. Early-type stars in the solar neighbourhood are well-suited to testing
this hypothesis because they are the only stellar objects whose absolute neon
abundance can be derived from the direct analysis of photospheric lines. Here
we present a fully homogeneous NLTE abundance study of the optical Ne I and Ne
II lines in a sample of 18 nearby, early B-type stars, which suggests log
epsilon(Ne)=7.97+/-0.07 dex (on the scale in which log epsilon[H]=12) for the
present-day neon abundance of the local ISM. Chemical evolution models of the
Galaxy only predict a very small enrichment of the nearby interstellar gas in
neon over the past 4.6 Gyr, implying that our estimate should be representative
of the Sun at birth. Although higher by about 35% than the new recommended
solar abundance, such a value appears insufficient by itself to restore the
past agreement between the solar models and the helioseismological constraints.Comment: Accepted by A&A. 14 pages, contains colour figure
VLT multi-object spectroscopy of 33 eclipsing binaries in the Small Magellanic Cloud. New distance and depth of the SMC, and a record-breaking apsidal motion
Aim: Our purpose is to provide reliable stellar parameters for a significant
sample of eclipsing binaries, which are representative of a whole dwarf and
metal-poor galaxy. We also aim at providing a new estimate of the mean distance
to the SMC and of its depth along the line of sight for the observed field of
view. Method: We use radial velocity curves obtained with the ESO FLAMES
facility at the VLT and light curves from the OGLE-II photometric survey. The
radial velocities were obtained by least-squares fits of the observed spectra
to synthetic ones, excluding the hydrogen Balmer lines. Results: Our sample
contains 23 detached, 9 semi-detached and 1 overcontact systems. Most detached
systems have properties consistent with stellar evolution calculations from
single-star models at the standard SMC metallicity Z = 0.004, though they tend
to be slightly overluminous. The few exceptions are probably due to third light
contribution or insufficient signal-to-noise ratio. The mass ratios are
consistent with a flat distribution, both for detached and
semi-detached/contact binaries. A mass-luminosity relation valid from ~4 to ~18
Msol is derived. The uncertainties are in the +-2 to +-11% range for the
masses, in the +-2 to +-5% range for the radii and in the +-1 to +-6% range for
the effective temperatures. The average distance modulus is 19.11+-0.03
(66.4+-0.9 kpc). The moduli derived from the V and from the I data are
consistent within 0.01 mag. The 2-sigma depth of the SMC is, for our field, of
0.25 mag or 7.6 kpc under the assumption of a gaussian distribution of stars
along the line of sight. Three systems show significant apsidal motion, one of
them with an apsidal period of 7.6 years, the shortest known to date for a
detached system with main sequence stars.Comment: 61 pages, 41 figures; accepted for publication in Astronomy &
Astrophysic
New insights into the nature of the peculiar star theta Carinae
We acquired high resolution spectroscopic and low resolution
spectropolarimetric observations to achieve the following goals: a) to improve
the orbital parameters to allow a more in-depth discussion on the possibility
of mass transfer in the binary system, b) to carry out a non-local
thermodynamic equilibrium (NLTE) abundance analysis, and c) to search for the
presence of a magnetic field.
The study of the radial velocities using CORALIE spectra allowed us to
significantly improve the orbital parameters. A comparative NLTE abundance
analysis was undertaken for theta Car and two other early B-type stars with
recently detected magnetic fields, tau Sco and xi^1 CMa. The analysis revealed
significantly different abundance patterns: a one-order-of-magnitude nitrogen
overabundance and carbon depletion was found in theta Car, while the oxygen
abundance is roughly solar. For the stars xi^1 CMa and tau Sco the carbon
abundance is solar and, while an N excess is also detected, it is of much
smaller amplitude (0.4-0.6dex). Such an N overabundance is typical of the
values already found for other slowly-rotating (magnetic) B-type dwarfs. For
theta Car, we attribute instead the chemical peculiarities to a past episode of
mass transfer between the two binary components. The results of the search for
a magnetic field using FORS1 at the VLT consisting of 26 measurements over a
time span of ~1.2h are rather inconclusive: only few measurements have a
significance level of 3sigma. Although we detect a periodicity of the order of
~8.8min in the dataset involving the measurements on all hydrogen Balmer lines
with the exception of the Halpha and Hbeta lines, these results have to be
confirmed by additional time-resolved magnetic field observations.Comment: 10 pages, 9 figures, 6 tables, accepted for publication in A&