622 research outputs found
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
On the Absorption of X-rays in the Interstellar Medium
We present an improved model for the absorption of X-rays in the ISM intended
for use with data from future X-ray missions with larger effective areas and
increased energy resolution such as Chandra and XMM, in the energy range above
100eV. Compared to previous work, our formalism includes recent updates to the
photoionization cross section and revised abundances of the interstellar
medium, as well as a treatment of interstellar grains and the H2molecule. We
review the theoretical and observational motivations behind these updates and
provide a subroutine for the X-ray spectral analysis program XSPEC that
incorporates our model.Comment: ApJ, in press, for associated software see
http://astro.uni-tuebingen.de/nh
Chemical Composition of Extremely Metal-Poor Stars in the Sextans Dwarf Spheroidal Galaxy
Chemical abundances of six extremely metal-poor ([Fe/H]<-2.5) stars in the
Sextans dwarf spheroidal galaxy are determined based on high resolution
spectroscopy (R=40,000) with the Subaru Telescope High Dispersion Spectrograph.
(1) The Fe abundances derived from the high resolution spectra are in good
agreement with the metallicity estimated from the Ca triplet lines in low
resolution spectra. The lack of stars with [Fe/H]=<-3 in Sextans, found by
previous estimates from the Ca triplet, is confirmed by our measurements,
although we note that high resolution spectroscopy for a larger sample of stars
will be necessary to estimate the true fraction of stars with such low
metallicity. (2) While one object shows an overabundance of Mg (similar to
Galactic halo stars), the Mg/Fe ratios of the remaining five stars are similar
to the solar value. This is the first time that low Mg/Fe ratios at such low
metallicities have been found in a dwarf spheroidal galaxy. No evidence for
over-abundances of Ca and Ti are found in these five stars, though the
measurements for these elements are less certain. Possible mechanisms to
produce low Mg/Fe ratios, with respect to that of Galactic halo stars, are
discussed. (3) Ba is under-abundant in four objects, while the remaining two
stars exhibit large and moderate excesses of this element. The abundance
distribution of Ba in this galaxy is similar to that in the Galactic halo,
indicating that the enrichment of heavy elements, probably by the r-process,
started at metallicities [Fe/H] < -2.5, as found in the Galactic halo.Comment: 15 pages, 6 figures, 6 tables, A&A, in pres
The cool atmospheres of the binary brown dwarf eps Indi B
We have imaged Indi B, the closest brown dwarf binary known, with
VISIR at the VLT in three narrow-band mid-infrared bandpasses located around
8.6m, 10.5m and 11.3m. We are able to spatially resolve both
components, and determine accurate mid-infrared photometry for both components
independently. In particular, our VISIR observations probe the NH feature
in the atmospheres of the cooler and warmer brown dwarfs. For the first time,
we can disentangle the contributions of the two components, and find that % our
photometry of IndiBb is in good agreement with recent ``cloud-free''
atmosphere models having an effective temperature of K.
With an assumed age of 1 Gyr for the Indi system, component Ba
agrees more with K rather than with
K, as suggested by SPITZER spectroscopic observations of
the combined Indi B system (Roellig et al., 2004). Even higher
effective temperatures appear inconsistent with our absolute photometry, as
they would imply an unphysical small size of the brown dwarf IndiBa.Comment: 4 pages, 2 figure
The Rigidly Rotating Magnetosphere of Sigma Ori E
We attempt to characterize the observed variability of the magnetic
helium-strong star sigma Ori E in terms of a recently developed rigidly
rotating magnetosphere model. This model predicts the accumulation of
circumstellar plasma in two co-rotating clouds, situated in magnetohydrostatic
equilibrium at the intersection between magnetic and rotational equators. We
find that the model can reproduce well the periodic modulations observed in the
star's light curve, H alpha emission-line profile, and longitudinal field
strength, confirming that it furnishes an essentially correct, quantitative
description of the star's magnetically controlled circumstellar environment.Comment: 4 pages, 3 figures, accepted by Ap
The VLT-FLAMES survey of massive stars: observations in the Galactic clusters NGC3293, NGC4755 and NGC6611
We introduce a new survey of massive stars in the Galaxy and the Magellanic
Clouds using the Fibre Large Array Multi-Element Spectrograph (FLAMES)
instrument at the Very Large Telescope (VLT). Here we present observations of
269 Galactic stars with the FLAMES-Giraffe Spectrograph (R ~ 25,000), in fields
centered on the open clusters NGC 3293, NGC 4755 and NGC 6611. These data are
supplemented by a further 50 targets observed with the Fibre-Fed Extended Range
Optical Spectrograph (FEROS, R = 48,000). Following a description of our
scientific motivations and target selection criteria, the data reduction
methods are described; of critical importance the FLAMES reduction pipeline is
found to yield spectra that are in excellent agreement with less automated
methods. Spectral classifications and radial velocity measurements are
presented for each star, with particular attention paid to morphological
peculiarities and evidence of binarity. These observations represent a
significant increase in the known spectral content of NGC 3293 and NGC 4755,
and will serve as standards against which our subsequent FLAMES observations in
the Magellanic Clouds will be compared.Comment: 26 pages, 9 figures (reduced size). Accepted for publication in A&A.
A copy with full res. figures is available from
http://www.ing.iac.es/~cje/flames_mw.ps.gz. Minor changes following
correction of proof
Two distinct ancient components in the Sculptor Dwarf Spheroidal Galaxy: First Results from DART
We have found evidence for the presence of two distinct ancient stellar
components (both geq 10 Gyr old) in the Sculptor dwarf spheroidal galaxy. We
used the ESO Wide Field Imager (WFI) in conjunction with the VLT/FLAMES
spectrograph to study the properties of the resolved stellar population of
Sculptor out to and beyond the tidal radius. We find that two components are
discernible in the spatial distribution of Horizontal Branch stars in our
imaging, and in the [Fe/H] and v_hel distributions for our large sample of
spectroscopic measurements. They can be generally described as a ``metal-poor''
component ([Fe/H] -1.7). The
metal-poor stars are more spatially extended than the metal-rich stars, and
they also appear to be kinematically distinct. These results provide an
important insight into the formation processes of small systems in the early
universe and the conditions found there. Even this simplest of galaxies appears
to have had a surprisingly complex early evolution.Comment: accepted for publication in ApJL 12 pages, 4 figure
Connecting RS OPh to [some] Type Ia Supernovae
Aims: Recurrent nova systems like RS Oph have been proposed as a possible
channel to Type Ia Supernova explosions, based on the high mass of the
accreting white dwarf. Additional support to this hypothesis has been recently
provided by the detection of circumstellar material around SN2006X and
SN2007le, showing a structure compatible with that expected for recurrent nova
outbursts.In this paper we investigate the circumstellar environment of RS Oph
and its structure, with the aim of establishing a firmer and independent link
between this class of objects and Type Ia SN progenitors.
Methods: We study the time evolution of CaII, NaI and KI absorption features
in RS Oph, before, during, and after the last outburst, using multi-epoch,
high-resolution spectroscopy, and applying the same method adopted for SN2006X
and SN2007le.
Results: A number of components, blue-shifted with respect to the systemic
velocity of RS Oph, are detected. In particular, one feature strongly weakens
in the first two weeks after the outburst, simultaneously with the
disappearance of very narrow P-Cyg profiles overimposed on the much wider nova
emission lines of H, He, FeII and other elements.
Conclusions: We interpret these facts as the signature of density
enhancements in the circumstellar material, suggesting that the recurrent
eruptions might indeed create complex structures within the material lost by
the donor star. This establishes a strong link between RS Oph and the
progenitor system of the Type Ia SN2006X, for which similar features have been
detected.Comment: 8 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysic
SINFONI in the Galactic Center: young stars and IR flares in the central light month
We report 75 milli-arcsec resolution, near-IR imaging spectroscopy within the
central 30 light days of the Galactic Center [...]. To a limiting magnitude of
K~16, 9 of 10 stars in the central 0.4 arcsec, and 13 of 17 stars out to 0.7
arcsec from the central black hole have spectral properties of B0-B9, main
sequence stars. [...] all brighter early type stars have normal rotation
velocities, similar to solar neighborhood stars. We [...] derive improved 3d
stellar orbits for six of these S-stars in the central 0.5 arcsec. Their
orientations in space appear random. Their orbital planes are not co-aligned
with those of the two disks of massive young stars 1-10 arcsec from SgrA*. We
can thus exclude [...] that the S-stars as a group inhabit the inner regions of
these disks. They also cannot have been located/formed in these disks [...].
[...] we conclude that the S-stars were most likely brought into the central
light month by strong individual scattering events. The updated estimate of
distance to the Galactic center from the S2 orbit fit is Ro = 7.62 +/- 0.32
kpc, resulting in a central mass value of 3.61 +/- 0.32 x 10^6 Msun. We
happened to catch two smaller flaring events from SgrA* [...]. The 1.7-2.45 mum
spectral energy distributions of these flares are fit by a featureless, red
power law [...]. The observed spectral slope is in good agreement with
synchrotron models in which the infrared emission comes from [...] radiative
inefficient accretion flow in the central R~10 Rs region.Comment: 50 pages, 10 figures, 2 tables, submitted to ApJ, February 6th, 2005,
abstract abridge
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