872 research outputs found
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&
On the Mass-Loss Rates of Massive Stars in the Low-Metallicity Galaxies IC 1613, WLM and NGC 3109
We present a spectroscopic analysis of VLT/X-Shooter observations of six
O-type stars in the low-metallicity (Z ~ 1/7 Z\odot) galaxies IC 1613, WLM and
NGC 3109. The stellar and wind parameters of these sources allow us, for the
first time, to probe the mass-loss versus metallicity dependence of stellar
winds below that of the Small Magellanic Cloud (at Z ~ 1/5Z\odot) by means of a
modified wind momentum versus luminosity diagram. The wind strengths that we
obtain for the objects in WLM and NGC 3109 are unexpectedly high and do not
agree with theoretical predictions. The objects in IC 1613 tend towards a
higher than expected mass-loss rate, but remain consistent with predictions
within their error bars. We discuss potential systematic uncertainties in the
mass-loss determinations to explain our results. However, if further
scrutinization of these findings point towards an intrinsic cause for this
unexpected sub-SMC mass-loss behavior, implications would include a higher than
anticipated number of Wolf-Rayet stars and Ib/Ic supernovae in low-metallicity
environments, but a reduced number of long-duration gamma-ray bursts produced
through a single-star evolutionary channel.Comment: 9 pages, 3 figures; accepted for publication in The Astrophysical
Journal Letter
Bottling the champagne: dynamics and radiation trapping of wind-driven bubbles around massive stars
In this paper we make predictions for the behaviour of wind bubbles around
young massive stars using analytic theory. We do this in order to determine why
there is a discrepancy between theoretical models that predict that winds
should play a secondary role to photoionisation in the dynamics of HII regions,
and observations of young HII regions that seem to suggest a driving role for
winds. In particular, regions such as M42 in Orion have neutral hydrogen
shells, suggesting that the ionising radiation is trapped closer to the star.
We first derive formulae for wind bubble evolution in non-uniform density
fields, focusing on singular isothermal sphere density fields with a power law
index of -2. We find that a classical "Weaver"-like expansion velocity becomes
constant in such a density distribution. We then calculate the structure of the
photoionised shell around such wind bubbles, and determine at what point the
mass in the shell cannot absorb all of the ionising photons emitted by the
star, causing an "overflow" of ionising radiation. We also estimate
perturbations from cooling, gravity, magnetic fields and instabilities, all of
which we argue are secondary effects for the conditions studied here. Our
wind-driven model provides a consistent explanation for the behaviour of M42 to
within the errors given by observational studies. We find that in relatively
denser molecular cloud environments \around single young stellar sources,
champagne flows are unlikely until the wind shell breaks up due to turbulence
or clumping in the cloud.Comment: 17 pages, 10 figures, published in MNRA
Wolf-Rayet nebulae as tracers of stellar ionizing fluxes: I. M1-67
We use WR124 (WN8h) and its associated nebula M1-67, to test theoretical
non-LTE models for Wolf-Rayet (WR) stars. Lyman continuum ionizing flux
distributions derived from a stellar analysis of WR124, are compared with
nebular properties via photo-ionization modelling. Our study demonstrates the
significant role that line blanketing plays in affecting the Lyman ionizing
energy distribution of WR stars, of particular relevance to the study of HII
regions containing young stellar populations.
We confirm previous results that non-line blanketed WR energy distributions
fail to explain the observed nebular properties of M1-67, such that the
predicted ionizing spectrum is too hard. A line blanketed analysis of WR124 is
carried out using the method of Hillier & Miller (1998), with stellar
properties in accord with previous results, except that the inclusion of
clumping in the stellar wind reduces its wind performance factor to only
approx2. The ionizing spectrum of the line blanketed model is much softer than
for a comparable temperature unblanketed case, such that negligible flux is
emitted with energy above the HeI 504 edge. Photo-ionization modelling,
incorporating the observed radial density distribution for M1-67 reveals
excellent agreement with the observed nebular electron temperature, ionization
balance and line strengths. An alternative stellar model of WR124 is
calculated, following the technique of de Koter et al. (1997), augmented to
include line blanketing following Schmutz et al. (1991). Good consistency is
reached regarding the stellar properties of WR124, but agreement with the
nebular properties of M1-67 is somewhat poorer than for the Hillier & Miller
code.Comment: 12 pages, 5 figures, latex2e style file, Astronomy & Astrophysics
(accepted
The rotation rates of massive stars: the role of binary interaction through tides, mass transfer and mergers
Rotation is thought to be a major factor in the evolution of massive stars,
especially at low metallicity, with consequences for their chemical yields,
ionizing flux and final fate. Determining the natal rotation-rate distribution
of stars is of high priority given its importance as a constraint on theories
of massive star formation and as input for models of stellar populations in the
local Universe and at high redshift. Recently, it has become clear that the
majority of massive stars interact with a binary companion before they die. We
investigate how this affects the distribution of rotation rates.
For this purpose, we simulate a massive binary-star population typical for
our Galaxy assuming continuous star formation. We find that, because of binary
interaction, 20^+5_-10% of all massive main-sequence stars have projected
rotational velocities in excess of 200km/s. We evaluate the effect of uncertain
input distributions and physical processes and conclude that the main
uncertainties are the mass transfer efficiency and the possible effect of
magnetic braking, especially if magnetic fields are generated or amplified
during mass accretion and stellar mergers.
The fraction of rapid rotators we derive is similar to that observed. If
indeed mass transfer and mergers are the main cause for rapid rotation in
massive stars, little room remains for rapidly rotating stars that are born
single. This implies that spin down during star formation is even more
efficient than previously thought. In addition, this raises questions about the
interpretation of the surface abundances of rapidly rotating stars as evidence
for rotational mixing. Furthermore, our results allow for the possibility that
all early-type Be stars result from binary interactions and suggest that
evidence for rotation in explosions, such as long gamma-ray bursts, points to a
binary origin.Comment: 14 pages, 5 figures, accepted for publication in ApJ., no changes
with v1 apart from fixed typos/ref
SwSt 1: an O-rich planetary nebula around a C-rich central star
The hydrogen-deficient carbon-rich [WCL] type central star HD167362 and its
oxygen-rich planetary nebula (PN) SwSt~1 are investigated. The nebular
chemistry might indicate a recent origin for the carbon-rich stellar spectrum.
Its stellar and nebular properties might therefore provide further
understanding of the origin of the [WCL] central star class. The UV-IR stellar
spectra are modelled with state of the codes and show ~40kK central star with a
wind and a C/O~3, indicative of efficient third dredge-up. The synthetic
stellar flux distribution is used to model the high density, compact PN, which
has a solar C/O ratio, is still enshrouded by 1200K and 230K dust shells and,
reported here for the first time, in molecular hydrogen. Although it appears
that the change in C/O ratio has been recent, the published spectroscopy since
1895 has been re-examined and no clear spectral change is seen. If an event
occurred that has turned it into a hydrogen-deficient central star, it did not
happen in the last 100 years.Comment: 31 pages, 19 figures (some are gif files), MNRAS in pres
A dearth of short-period massive binaries in the young massive star forming region M17: Evidence for a large orbital separation at birth?
The formation of massive stars remains poorly understood and little is known
about their birth multiplicity properties. Here, we investigate the strikingly
low radial-velocity dispersion measured for a sample of 11 massive pre- and
near-main-sequence stars (sigma_rv = 5.6 +/- 0.2 km/s) in the young massive
star forming region M17 to obtain first constraints on the multiplicity
properties of young massive stellar objects. Methods: We compute the RV
dispersion of synthetic populations of massive stars for various multiplicity
properties and we compare the simulated sigma_rv distributions to the observed
value. We specifically investigate two scenarios: a low binary fraction and a
dearth of short-period binary systems. Results: Simulated populations with low
binary fractions (f_bin = 0.12_{-0.09}^{+0.16}) or with truncated period
distributions (P_cutoff > 9 months) are able to reproduce the low sigma_rv
observed within their 68%-confidence intervals. Parent populations with f_bin >
0.42 or P_cutoff < 47 d can however be rejected at the 5%-significance level.
Both constraints are contrast with the high binary fraction and plethora of
short-period systems found in few Myr-old, OB-type populations. To explain the
difference, the first scenario requires a variation of the outcome of the
massive star formation process. In the the second scenario, compact binaries
must form later on, and the cut-off period may be related to physical
length-scales representative of the bloated pre-main-sequence stellar radii or
of their accretion disks. Conclusions: If the obtained constraints are
representative of the overall properties of massive young stellar objects, our
results may provide support to a formation process in which binaries are
initially formed at larger separations, then harden or migrate to produce the
typical (untruncated) power-law period distribution observed in few Myr-old OB
binaries.Comment: 5 pages; Accepted for publication in Astronomy and Astrophysics
Letter
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