1,455 research outputs found
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
Apsidal motion in the massive binary HD152218
Massive binary systems are important laboratories in which to probe the
properties of massive stars and stellar physics in general. In this context, we
analysed optical spectroscopy and photometry of the eccentric short-period
early-type binary HD 152218 in the young open cluster NGC 6231. We
reconstructed the spectra of the individual stars using a separating code. The
individual spectra were then compared with synthetic spectra obtained with the
CMFGEN model atmosphere code. We furthermore analysed the light curve of the
binary and used it to constrain the orbital inclination and to derive absolute
masses of 19.8 +/- 1.5 and 15.0 +/- 1.1 solar masses. Combining radial velocity
measurements from over 60 years, we show that the system displays apsidal
motion at a rate of (2.04^{+.23}_{-.24}) degree/year. Solving the
Clairaut-Radau equation, we used stellar evolution models, obtained with the
CLES code, to compute the internal structure constants and to evaluate the
theoretically predicted rate of apsidal motion as a function of stellar age and
primary mass. In this way, we determine an age of 5.8 +/- 0.6 Myr for HD
152218, which is towards the higher end of, but compatible with, the range of
ages of the massive star population of NGC 6231 as determined from isochrone
fitting.Comment: Accepted for publication in Astronomy & Astrophysic
Simultaneous X-ray and optical spectroscopy of the Oef supergiant lambda Cep
Probing the structures of stellar winds is of prime importance for the
understanding of massive stars. Based on their optical spectral morphology and
variability, the stars of the Oef class have been suggested to feature
large-scale structures in their wind. High-resolution X-ray spectroscopy and
time-series of X-ray observations of presumably-single O-type stars can help us
understand the physics of their stellar winds. We have collected XMM-Newton
observations and coordinated optical spectroscopy of the O6Ief star lambda Cep
to study its X-ray and optical variability and to analyse its high-resolution
X-ray spectrum. We investigate the line profile variability of the He II 4686
and H-alpha emission lines in our time series of optical spectra, including a
search for periodicities. We further discuss the variability of the broadband
X-ray flux and analyse the high-resolution spectrum of lambda Cep using
line-by-line fits as well as a code designed to fit the full high-resolution
X-ray spectrum consistently. During our observing campaign, the He II 4686 line
varies on a timescale of ~18 hours. On the contrary, the H-alpha line profile
displays a modulation on a timescale of 4.1 days which is likely the rotation
period of the star. The X-ray flux varies on time-scales of days and could in
fact be modulated by the same 4.1 days period as H-alpha, although both
variations are shifted in phase. The high-resolution X-ray spectrum reveals
broad and skewed emission lines as expected for the X-ray emission from a
distribution of wind-embedded shocks. Most of the X-ray emission arises within
less than 2R* above the photosphere.Comment: Accepted for publication in Astronomy & Astrophysic
Quaterionic Construction of the W(F_4) Polytopes with Their Dual Polytopes and Branching under the Subgroups B(B_4) and W(B_3)*W(A_1)
4-dimensional polytopes and their dual polytopes have been
constructed as the orbits of the Coxeter-Weyl group where the group
elements and the vertices of the polytopes are represented by quaternions.
Branchings of an arbitrary \textbf{} orbit under the Coxeter groups
and have been presented. The role of
group theoretical technique and the use of quaternions have been emphasizedComment: 26 pages, 10 figure
An XMM-Newton view of the young open cluster NGC 6231 III. Optically faint X-ray sources
We discuss the properties of the X-ray sources with faint optical
counterparts in the very young open cluster NGC 6231. From their positions in
the H-R diagram, we find that the bulk of these objects probably consists of
low-mass pre-main sequence stars with masses in the range 0.3 to 3.0
M. The age distribution of these objects indicates that low-mass star
formation in NGC 6231 started more than 10 Myr ago and culminated in a
starburst-like event about 1 to 4 Myr ago when the bulk of the low-mass PMS
stars as well as the massive cluster members formed. We find no evidence for a
spatial age gradient that could point towards a sequential star formation
process. Only a few X-ray sources have counterparts with a reddening exceeding
the average value of the cluster or with infrared colours indicating the
presence of a moderate near-IR excess. The X-ray spectra of the brightest PMS
sources are best fitted by rather hard thermal plasma models and a significant
fraction of these sources display flares in their light curve. The X-ray
brightest flaring sources have decay times between 2 and 16 ks. The X-ray
selected PMS stars in NGC 6231 have values that
increase strongly with decreasing bolometric luminosity and can reach a
saturation level () for non-flaring
sources and even more extreme values during flares.Comment: Accepted by MNRAS, 14pages, 19 figure
Multiplicity of massive O stars and evolutionary implications
Nearby companions alter the evolution of massive stars in binary systems.
Using a sample of Galactic massive stars in nearby young clusters, we
simultaneously measure all intrinsic binary characteristics relevant to
quantify the frequency and nature of binary interactions. We find a large
intrinsic binary fraction, a strong preference for short orbital periods and a
flat distribution for the mass-ratios. Our results do not support the presence
of a significant peak of equal-mass `twin' binaries. As a result of the
measured distributions, we find that over seventy per cent of all massive stars
exchange mass with a companion. Such a rate greatly exceeds previous estimates
and implies that the majority of massive stars have their evolution strongly
affected by interaction with a nearby companion.Comment: 4 pages, 2 figures. Conference proceedings to appear in "370 years of
astronomy in Utrecht
The massive binary CPD-41degr7742 II. Optical light curve and X-ray observations
In the first paper of this series, we presented a detailed high-resolution
spectroscopic study of CPD - 41degr7742, deriving for the first time an orbital
solution for both components of the system. In this second paper, we focus on
the analysis of the optical light curve and on recent XMM-Newton X-ray
observations. In the optical, the system presents two eclipses, yielding an
inclination i ~ 77degr. Combining the constraints from the photometry with the
results of our previous work, we derive the absolute parameters of the system.
We confirm that the two components of CPD - 41degr7742 are main sequence stars
(O9 V + B1-1.5 V) with masses (M_1 ~ 18 Msol and M_2 ~ 10 Msol) and respective
radii (R_1 ~ 7.5 Rsol and R_2 ~ 5.4 Rsol) close to the typical values expected
for such stars. We also report an unprecedented set of X-ray observations that
almost uniformly cover the 2.44-day orbital cycle. The X-ray emission from CPD
- 41degr7742 is well described by a two-temperature thermal plasma model with
energies close to 0.6 and 1.0 keV, thus slightly harder than typical early-type
emission. The X-ray light curve shows clear signs of variability. The emission
level is higher when the primary is in front of the secondary. During the high
emission state, the system shows a drop of its X-ray emission that almost
exactly matches the optical eclipse. We interpret the main features of the
X-ray light curve as the signature of a wind-photosphere interaction, in which
the overwhelming primary O9 star wind crashes into the secondary surface.
Alternatively the light curve could result from a wind-wind interaction zone
located near the secondary star surface. As a support to our interpretation, we
provide a phenomenological geometric model that qualitatively reproduces the
observed modulations of the X-ray emission.Comment: 18 pages, 16 figures, published in Astronomy & Astrophysic
Binary interaction dominates the evolution of massive stars
The presence of a nearby companion alters the evolution of massive stars in
binary systems, leading to phenomena such as stellar mergers, X-ray binaries
and gamma-ray bursts. Unambiguous constraints on the fraction of massive stars
affected by binary interaction were lacking. We simultaneously measured all
relevant binary characteristics in a sample of Galactic massive O stars and
quantified the frequency and nature of binary interactions. Over seventy per
cent of all massive stars will exchange mass with a companion, leading to a
binary merger in one third of the cases. These numbers greatly exceed previous
estimates and imply that binary interaction dominates the evolution of massive
stars, with implications for populations of massive stars and their supernovae.Comment: 9 page, 2 figures. This is the authors' version. Final version and
supplementary materials available at http://www.sciencemag.or
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