1,756 research outputs found
The multiplicity of massive stars
Binaries are excellent astrophysical laboratories that provide us with direct
measurements of fundamental stellar parameters. Compared to single isolated
star, multiplicity induces new processes, offering the opportunity to confront
our understanding of a broad range of physics under the extreme conditions
found in, and close to, astrophysical objects. In this contribution, we will
discuss the parameter space occupied by massive binaries, and the observational
means to investigate it. We will review the multiplicity fraction of OB stars
within each regime, and in different astrophysical environments. In particular
we will compare the O star spectroscopic binary fraction in nearby open
clusters and we will show that the current data are adequately described by an
homogeneous fraction of f~0.44. We will also summarize our current
understanding of the observed parameter distributions of O+OB spectroscopic
binaries. We will show that the period distribution is overabundant in short
period binaries and that it can be described by a bi-modal Oepik law with a
break point around P~10d. The distribution of the mass-ratios shows no
indication for a twin population of equal mass binaries and seems rather
uniform in the range 0.2< q=M_2/M_1<1.0.Comment: 12 pages, 4 figures, IAU272: Active OB stars: structure, evolution,
mass los
The nature of the line profile variability in the spectrum of the massive binary HD 152219
HD 152219 is a massive binary system with O9.5 III + B1-2 V/III components
and a short orbital period of 4.2 d. Its primary component further displays
clear line profile variability (LPV). The primary component being located
within the pulsational instability domain predicted for high-luminosity stars,
we previously suggested that the observed LPV could be associated with
non-radial pulsations. The aim of the present work is to determine the nature
of the observed LPV in the spectrum of the primary component of HD
152219.During a 4-night FEROS monitoring campaign, we collected a new set of
134 high signal-to-noise spectra. These new observations were then used to
re-investigate the variability of different line profiles in the spectrum of HD
152219. Based on the present analysis, we discard the non-radial pulsations and
point out the Rossiter-McLaughlin effect as the cause of the LPV in HD 152219.
The upper limit on the amplitude of possible weak pulsations is set at a few
parts per thousand of the continuum level.Comment: Manuscript accepted for publication in A&A, 5p, 4 figure
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
An exceptional X-ray view of the young open cluster NGC 6231: what XMM-Newton has taught us
Considered as the core of the Sco OB1 association, the young open cluster NGC
6231 harbours a rich O-type star population. In 2001, the XMM-Newton satellite
targeted the cluster for a nominal duration of about 180 ks. Thanks to the
detector sensitivity, the EPIC cameras provided an unprecedented X-ray view of
NGC 6231, revealing about 600 point-like sources. In this contribution, we
review the main results that have been obtained thanks to this unprecedented
data set. Concerning the O-type stars, we present the latest developments
related to the so-called 'canonical' Lx-Lbol relation. The dispersion around
this relation might actually be much smaller than previously thought. In our
data set, the sole mechanism that yields a significant deviation from this
scheme is wind interaction. It is also the sole mechanism that induces a
significant variation of the early-type star X-ray flux. In a second part of
this contribution, we probe the properties of the optically faint X-ray
sources. Most of them are believed to be low mass pre-main sequence stars.
Their analysis provides direct insight into the star formation history of the
cluster.Comment: 6 pages, 5 figures, to appear in "The X-Ray Universe 2005", ESA
Symposium held at El Escorial, Madrid (Spain), 26-30 Sep 200
The properties of ten O-type stars in the low-metallicity galaxies IC 1613, WLM and NGC 3109
Massive stars likely played an important role in the reionization of the
Universe, and the formation of the first black holes. Massive stars in
low-metallicity environments in the local Universe are reminiscent of their
high redshift counterparts. In a previous paper, we reported on indications
that the stellar winds of low-metallicity O stars may be stronger than
predicted, which would challenge the current paradigm of massive star
evolution. In this paper, we aim to extend our initial sample of six O stars in
low-metallicity environments by four. We aim to derive their stellar and wind
parameters, and compare these to radiation-driven wind theory and stellar
evolution models. We have obtained intermediate-resolution VLT/X-Shooter
spectra of our sample of stars. We derive the stellar parameters by fitting
synthetic fastwind line profiles to the VLT/X-Shooter spectra using a genetic
fitting algoritm. We compare our parameters to evolutionary tracks and obtain
evolutionary masses and ages. We also investigate the effective temperature
versus spectral type calibration for SMC and lower metallicities. Finally, we
reassess the wind momentum versus luminosity diagram. The derived parameters of
our target stars indicate stellar masses that reach values of up to 50
. The wind strengths of our stars are, on average, stronger than
predicted from radiation-driven wind theory and reminiscent of stars with an
LMC metallicity. We discuss indications that the iron content of the host
galaxies is higher than originally thought and is instead SMC-like. We find
that the discrepancy with theory is lessened, but remains significant for this
higher metallicity. This may imply that our current understanding of the wind
properties of massive stars, both in the local universe as well as at cosmic
distances, remains incomplete.Comment: Accepted for publication in Astronomy and Astrophysics. 10 pages, 8
figure
The massive star binary fraction in young open clusters I. NGC 6231 revisited
We present the results of a long-term high-resolution spectroscopy campaign
on the O-type stars in NGC 6231. We revise the spectral classification and
multiplicity of these objects and we constrain the fundamental properties of
the O-star population. Almost three quarters of the O-type stars in the cluster
are members of a binary system. The minimum binary fraction is 0.63, with half
the O-type binaries having an orbital period of the order of a few days. The
eccentricities of all the short-period binaries are revised downward, and
henceforth match a normal period-eccentricity distribution. The mass-ratio
distribution shows a large preference for O+OB binaries, ruling out the
possibility that, in NGC 6231, the companion of an O-type star is randomly
drawn from a standard IMF. Obtained from a complete and homogeneous population
of O-type stars, our conclusions provide interesting observational constraints
to be confronted with the formation and early-evolution theories of O stars.Comment: 16 pages, 14 figures. Accepted by MNRA
The Struve-Sahade effect in the optical spectra of O-type binaries I. Main-sequence systems
We present a spectroscopic analysis of four massive binary systems that are
known or are good candidates to display the Struve-Sahade effect (defined as
the apparent strengthening of the secondary spectrum of the binary when the
star is approaching, and the corresponding weakening of the lines when it is
receding).
We use high resolution optical spectra to determine new orbital solutions and
spectral types of HD 165052, HD 100213, HD 159176 and DH Cep. As good knowledge
of the fundamental parameters of the considered systems is necessary to examine
the Struve-Sahade effect. We then study equivalent width variations in the
lines of both components of these binaries during their orbital cycle.
In the case of these four systems, variations appear in the equivalent widths
of some lines during the orbital cycle, but the definition given above can any
longer be valid, since it is now clear that the effect modifies the primary
spectrum as much as the secondary spectrum. Furthermore, the lines affected,
and the way in which they are affected, depend on the considered system. For at
least two of them (HD 100213 and HD 159176) these variations probably reflect
the ellipsoidal variable nature of the system.Comment: 12 pages, 20 figures, in press A&
The massive star binary fraction in young open clusters - II. NGC 6611 (Eagle Nebula)
Based on a set of over 100 medium- to high-resolution optical spectra
collected from 2003 to 2009, we investigate the properties of the O-type star
population in NGC6611 in the core of the Eagle Nebula (M16). Using a much more
extended data set than previously available, we revise the spectral
classification and multiplicity status of the nine O-type stars in our sample.
We confirm two suspected binaries and derive the first SB2 orbital solutions
for two systems. We further report that two other objects are displaying a
composite spectrum, suggesting possible long-period binaries. Our analysis is
supported by a set of Monte-Carlo simulations, allowing us to estimate the
detection biases of our campaign and showing that the latter do not affect our
conclusions. The absolute minimal binary fraction in our sample is f_min=0.44
but could be as high as 0.67 if all the binary candidates are confirmed. As in
NGC6231 (see Paper I), up to 75% of the O star population in NGC6611 are found
in an O+OB system, thus implicitly excluding random pairing from a classical
IMF as a process to describe the companion association in massive binaries. No
statistical difference could be further identified in the binary fraction,
mass-ratio and period distributions between NGC6231 and NGC6611, despite the
difference in age and environment of the two clusters.Comment: Accepted by MNRAS; 15 pages, 17 fi
An XMM-Newton view of the young open cluster NGC 6231 -- II. The OB star population
In this second paper, we pursue the analysis of the 180 ks XMM-Newton
campaign towards the young open cluster NGC 6231 and we focus on its rich OB
star population. We present a literature-based census of the OB stars in the
field of view with more than one hundred objects, among which 30% can be
associated with an X-ray source. All the O-type stars are detected in the X-ray
domain as soft and reasonably strong emitters. In the 0.5-10.0 keV band, their
X-ray luminosities scale with their bolometric luminosities as . Such a scaling law holds in
the soft (0.5-1.0 keV) and intermediate (1.0-2.5 keV) bands but breaks down in
the hard band. While the two colliding wind binaries in our sample clearly
deviate from this scheme, the remaining O-type objects show a very limited
dispersion (40% or 20% according to whether `cool' dwarfs are included or not),
much smaller than that obtained from previous studies. At our detection
threshold and within our sample, the sole identified mechanism that produces
significant modulations in the O star X-ray emission is related to wind
interaction. The intrinsic X-ray emission of non-peculiar O-type stars seems
thus constant for a given star and the level of its X-ray emission is
accurately related to the its luminosity or, equivalently, to its wind
properties. Among B-type stars, the detection rate is only about 25% in the
sub-type range B0-B4 and remains mostly uniform throughout the different
sub-populations while it drops significantly at later sub-types. The associated
X-ray spectra are harder than those of O-type stars. Our analysis points
towards the detected emission being associated with a physical PMS companion
>... [see paper for the complete abstract]Comment: 21 pages, 14 figures, Table 2 and Figs 2 to 5 will be available
through the CDS only, accepted for publication by MNRAS, Fig 1 not included
in the present preprint because of size limitation
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