231 research outputs found
Populations of OB-type stars in galaxies
One of the challenges for stellar astrophysics is to reach the point at which
we can undertake reliable spectral synthesis of unresolved populations in
young, star-forming galaxies at high redshift. Here I summarise recent studies
of massive stars in the Galaxy and Magellanic Clouds, which span a range of
metallicities commensurate with those in high-redshift systems, thus providing
an excellent laboratory in which to study the role of environment on stellar
evolution. I also give an overview of observations of luminous supergiants in
external galaxies out to a remarkable 6.7 Mpc, in which we can exploit our
understanding of stellar evolution to study the chemistry and dynamics of the
host systems.Comment: 9 pages, 3 figures, invited review at IAU Symposium 272 (eds Neiner,
Wade, Meynet & Peters
Modeling the near-infrared lines of O-type stars
We use a grid of 30 line-blanketed unified stellar photosphere and wind
models for O-type stars; computed with the code CMFGEN in order to evaluate its
potential in the near-infrared spectral domain. The grid includes dwarfs,
giants and supergiants. We analyse the equivalent width behaviour of the 20
strongest lines of hydrogen and helium in spectral windows that can be observed
using ground-based instrumentation and compare the results with observations.
Our main findings are that: i) HeI/HeII line ratios in the J, H and K bands
correlate well with the optical ratio employed in spectral classification, and
can therefore be used to determine the spectral type; ii) in supergiant stars
the transition from the stellar photosphere to the wind follows a shallower
density gradient than the standard approach followed in our models, which can
be mimicked by adopting a lower gravity in our prescription of the density
stratification. iii) the Brackett gamma line poses a number of peculiar
problems which partly might be related to wind clumping, and iv) the Brackett
alpha line is an excellent mass-loss indicator. For the first and last item we
provide quantitative calibrations.Comment: 14 pages, 7 figures, accepted by A&
The star formation process in the Magellanic Clouds
The Magellanic Clouds offer unique opportunities to study star formation both
on the global scales of an interacting system of gas-rich galaxies, as well as
on the scales of individual star-forming clouds. The interstellar media of the
Small and Large Magellanic Clouds and their connecting bridge, span a range in
(low) metallicities and gas density. This allows us to study star formation
near the critical density and gain an understanding of how tidal dwarfs might
form; the low metallicity of the SMC in particular is typical of galaxies
during the early phases of their assembly, and studies of star formation in the
SMC provide a stepping stone to understand star formation at high redshift
where these processes can not be directly observed. In this review, I introduce
the different environments encountered in the Magellanic System and compare
these with the Schmidt-Kennicutt law and the predicted efficiencies of various
chemo-physical processes. I then concentrate on three aspects that are of
particular importance: the chemistry of the embedded stages of star formation,
the Initial Mass Function, and feedback effects from massive stars and its
ability to trigger further star formation.Comment: 12pages, 5figures, invited review at the IAUS 256, The Magellanic
System: Stars, Gas, and Galaxies, eds. Jacco van Loon, Joana Oliveir
The VLT-FLAMES survey of massive stars: Nitrogen abundances for Be-type stars in the Magellanic Clouds
Aims. We compare the predictions of evolutionary models for early-type stars
with atmospheric parameters, projected rotational velocities and nitrogen
abundances estimated for a sample of Be-type stars. Our targets are located in
4 fields centred on the Large Magellanic Cloud cluster: NGC 2004 and the N 11
region as well as the Small Magellanic Cloud clusters: NGC 330 and NGC 346.
Methods. Atmospheric parameters and photospheric abundances have been
determined using the non-LTE atmosphere code tlusty. Effective temperature
estimates were deduced using three different methodologies depending on the
spectral features observed; in general they were found to yield consistent
estimates. Gravities were deduced from Balmer line profiles and
microturbulences from the Si iii spectrum. Additionally the contributions of
continuum emission from circumstellar discs were estimated. Given its
importance in constraining stellar evolutionary models, nitrogen abundances (or
upper limits) were deduced for all the stars analysed. Results. Our nitrogen
abundances are inconsistent with those predicted for targets spending most of
their main sequence life rotating near to the critical velocity. This is
consistent with the results we obtain from modelling the inferred rotational
velocity distribution of our sample and of other investigators. We consider a
number of possibilities to explain the nitrogen abundances and rotational
velocities of our Be-type sample.Comment: 14 pages, 9 figures, submitted to A&
UV Spectroscopy of Metal-Poor Massive Stars in the Small Magellanic Cloud
The Hubble Space Telescope has provided the first clear evidence for weaker
winds of metal-poor massive stars in the Small Magellanic Cloud, confirming
theoretical predictions of the metallicity dependence of mass-loss rates and
wind terminal velocities. For lower luminosity O-type stars however, derived
mass-loss rates are orders of magnitude lower than predicted, and are at
present unexplained.Comment: 4 pages, 3 figures. To appear in 'The Impact of HST on European
Astronomy', Eds., G. De Marchi & F.D. Macchetto, Astrophysics & Space
Science, Springe
First VLT/X-shooter spectroscopy of early-type stars outside the Local Group
As part of the VLT/X-shooter science verification, we obtained the first
optical medium-resolution spectrum of a previously identified bright O-type
object in NGC55, an LMC-like galaxy at a distance of \sim2.0 Mpc. Based on the
stellar and nebular spectrum, we investigate the nature and evolutionary status
of the central object(s) and its influence on the surrounding interstellar
medium. We conclude that the source, NGC55_C1_31, is a composite object, likely
a stellar cluster, which contains one or several hot (T_eff \simeq 50000 K) WN
stars with a high mass-loss rate (\sim3 \times 10^{-5} M_\odot yr^{-1}) and a
helium-rich composition (N_He/N_H = 0.8). The visual flux is dominated by
OB-type (super)giant stars with T_eff \sim< 35000 K, solar helium abundance
(N_He/N_H = 0.1), and mass-loss rate \sim2 \times 10^{-6} M_\odot yr^{-1}. The
surrounding H II region has an electron density n_e < 10^2 cm^{-3} and an
electron temperature T(OIII) \simeq 11500 \pm 600 K. The oxygen abundance of
this region is [O/H] = 8.18 \pm 0.03 which corresponds to Z = 0.31 \pm 0.04
Z_\odot. We observed no significant gradients in T(OIII), n_e or [O/H] on a
scale of 73 pc extending in four directions from the ionising source. The
properties of the HII region can be reproduced by a CLOUDY model which uses the
central cluster as ionising source, thus providing a self-consistent
interpretation of the data. We also report on the serendipitous discovery of
HeII nebular emission associated with the nearby source NGC55_C2_35, a feature
usually associated with strong X-ray sources.Comment: 12 pages, 10 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society; the definitive version will be available at
wwww.blackwell-synergy.co
Hydrogen and helium line formation in OB dwarfs and giants. A hybrid non-LTE approach
Aims: Hydrogen and helium line spectra are crucial diagnostic features for
the quantitative analysis of OB stars. We compute synthetic spectra based on a
hybrid non-LTE approach in order to test the ability of these models to
reproduce high-resolution and high-S/N spectra of dwarf and giant stars and
also to compare them with published grids of non-LTE (OSTAR2002) and LTE
(Padova) models. Methods: Our approach solves the restricted non-LTE problem
based on classical line-blanketed LTE model atmospheres. State-of-the-art model
atoms and line-broadening theories are employed to model the H and He I/II
spectra over the entire optical range and in the near-IR. Results: The
synthetic spectra match almost all measurable hydrogen and helium lines
observed in six test stars over a wide spectral range from the Balmer limit to
the NIR, except for only a few well-understood cases. Our approach reproduces
other published non-LTE calculations, however avoids inconsistencies with the
modelling of the He I singlets recently discussed in the literature. It
improves on the published LTE models in many aspects: non-LTE strengthening and
the use of improved line-broadening data result in overall significant
differences in the line profiles and equivalent widths of the Balmer and helium
lines. Where possible, systematic effects on the stellar parameter
determination are quantified, e.g. gravities derived from the Hgamma wings may
be overestimated by up to ~0.2 dex at our upper temperature boundary in LTE.
(abridged)Comment: 25 pages, 19 figures. Modified according to suggestions of the
referee. Accepted for publication in A&A. Several figures in low resolution.
A high-resolution pdf version of the preprint can be downloaded from
http://www.sternwarte.uni-erlangen.de/~ai97/preprints/HHe_nieva.pd
Evolution of progenitor stars of Type Ibc supernovae and long gamma-ray bursts
We discuss how rotation and binary interactions may be related to the
diversity of type Ibc supernovae and long gamma-ray bursts. After presenting
recent evolutionary models of massive single and binary stars including
rotation, the Tayler-Spruit dynamo and binary interactions, we argue that the
nature of SNe Ibc progenitors from binary systems may not significantly differ
from that of single star progenitors in terms of rotation, and that most long
GRB progenitors may be produced via the quasi-chemically homogeneous evolution
at sub-solar metallicity. We also briefly discuss the possible role of magnetic
fields generated in the convective core of a massive star for the transport of
angular momentum, which is potentially important for future stellar evolution
models of supernova and GRB progenitors.Comment: 6 pages, 4 figures, to appear in IAU Symp. 250, Massive Stars as
Cosmic Engines, Kauai (HI), 12/2007, ed. F. Bresolin, P. Crowther, & J. Pul
The ARAUCARIA project: Grid-Based Quantitative Spectroscopic Study of Massive Blue Stars in NGC55
The quantitative study of the physical properties and chemical abundances of
large samples of massive blue stars at different metallicities is a powerful
tool to understand the nature and evolution of these objects. Their analysis
beyond the Milky Way is challenging, nonetheless it is doable and the best way
to investigate their behavior in different environments. Fulfilling this task
in an objective way requires the implementation of automatic analysis
techniques that can perform the analyses systematically, minimizing at the same
time any possible bias.
As part of the ARAUCARIA project we carry out the first quantitative
spectroscopic analysis of a sample of 12 B-type supergiants in the galaxy NGC55
at 1.94 Mpc away. By applying the methodology developed in this work, we derive
their stellar parameters, chemical abundances and provide a characterization of
the present-day metallicity of their host galaxy.
Based on the characteristics of the stellar atmosphere/line formation code
FASTWIND, we designed and created a grid of models for the analysis of massive
blue supergiant stars. Along with this new grid, we implemented a spectral
analysis algorithm. Both tools were specially developed to perform fully
consistent quantitative spectroscopic analyses of low spectral resolution of
B-type supergiants in a fast and objective way.
We present the main characteristics of our FASTWIND model grid and perform a
number of tests to investigate the reliability of our methodology. The
automatic tool is applied afterward to a sample of 12 B-type supergiant stars
in NGC55, deriving the stellar parameters and abundances. The results indicate
that our stars are part of a young population evolving towards a red supergiant
phase. The derived chemical composition hints to an average metallicity similar
to the one of the Large Magellanic Cloud, with no indication of a spatial trend
across the galaxy.Comment: 19 pages, 12 figures and 9 tables. Accpeted for publication in A&
The VLT-FLAMES survey of massive stars: Wind properties and evolution of hot massive stars in the LMC
[Abridged] We have studied the optical spectra of 28 O- and early B-type
stars in the Large Magellanic Cloud, 22 of which are associated with the young
star-forming region N11. Stellar parameters are determined using an automated
fitting method, combining the stellar atmosphere code FASTWIND with the
genetic-algorithm optimisation routine PIKAIA. Results for stars in the LH9 and
LH10 associations of N11 are consistent with a sequential star formation
scenario, in which activity in LH9 triggered the formation of LH10. Our sample
contains four stars of spectral type O2, of which the hottest is found to be
~49-54 kK (cf. ~45-46 kK for O3 stars). The masses of helium-enriched dwarfs
and giants are systematically lower than those implied by non-rotating
evolutionary tracks. We interpret this as evidence for efficient
rotationally-enhanced mixing, leading to the surfacing of primary helium and to
an increase of the stellar luminosity. This result is consistent with findings
for SMC stars by Mokiem et al. For bright giants and supergiants no such
mass-discrepancy is found, implying that these stars follow tracks of modestly
(or non-)rotating objects. Stellar mass-loss properties were found to be
intermediate to those found in massive stars in the Galaxy and the SMC, and
comparisons with theoretical predictions at LMC metallicity yielded good
agreement over the luminosity range of our targets, i.e. 5.0 < log L/L(sun) <
6.1
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