74 research outputs found
A dedicated photometric system for the detection of Wolf-Rayet stars
We present here tests of a five-filter photometric system aimed at WR
classification. In addition to the well-known easy separation between the WN
and WC spectral types, these tests indicate interesting potentialities in the
discrimination of subgroups among the WN and the WC which look well related to
the classical subtypes. The proposed combinations of filters (or derived ones)
bear enough discriminating power to satisfy some evolutionary studies in
crowded fields where spectroscopic follow-up is not possible.Comment: 10 pages, 7 figures, uses l-aa.sty and psfig for figure inclusion.
Also available at http://vela.astro.ulg.ac.be/preprint/P10 Accepted for
publication in Astronomy & Astrophysics Supp
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
VLT observations of the highly ionized nebula around Brey2
We present the first high resolution HeII 4686 images of the high excitation
nebula around the WR star Brey 2 in the LMC. This nebula presents a striking
morphology: a small arc-like feature some 3.6pc in radius is particularly
prominent in the HeII 4686 line. We further discover a previously unknown faint
HeII emission that extends over an area of 22*17 pc^2. An even fainter HeII
emission is apparently associated with the interstellar bubble blown by the
progenitor of Brey2. The total HeII flux corresponds to an ionizing flux of
4*10^{47} photons/s. Halpha, [OIII], and HeI 5876 images and long-slit spectra
are also examined in this letter, enabling us to investigate the detailed
physical properties at various locations of the nebula.Comment: 4 pages, 3 figures (2 in jpg), accepted by A&A Letters, also
available from http://vela.astro.ulg.ac.be/Preprints/P80/index.htm
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
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
A multiwavelength investigation of the massive eclipsing binary Cyg OB2 #5
The properties of the early-type binary Cyg OB2 #5 have been debated for many
years and spectroscopic and photometric investigations yielded conflicting
results. We have attempted to constrain the physical properties of the binary
by collecting new optical and X-ray observations. We find that the orbital
period of the system slowly changes though we are unable to discriminate
between several possible explanations of this trend. The best fit solution of
the continuum light curve reveals a contact configuration with the secondary
star being significantly brighter and hotter on its leading side facing the
primary. The mean temperature of the secondary star turns out to be only
slightly lower than that of the primary, whilst the bolometric luminosity ratio
is found to be 3.1. The solution of the light curve yields a distance of 925
+/- 25 pc much lower than the usually assumed distance of the Cyg OB2
association. Whilst we confirm the existence of episodes of higher X-ray
fluxes, the data reveal no phase-locked modulation with the 6.6 day period of
the eclipsing binary nor any clear relation between the X-ray flux and the 6.7
yr radio cycle. The bright region of the secondary star is probably heated by
energy transfer in a common envelope in this contact binary system as well as
by the collision with the primary's wind. The existence of a common photosphere
probably also explains the odd mass-luminosity relation of the stars in this
system. Most of the X-ray, non-thermal radio, and possibly gamma-ray emission
of Cyg OB2 #5 is likely to arise from the interaction of the combined wind of
the eclipsing binary with at least one additional star of this multiple system
Optical Spectroscopy of X-Mega targets in Carina Nebula : III. The multiple system Tr 16-104 (≡CPD -59° 2603)
We discuss the orbital elements of the multiple system Tr 16-104 which is usually believed to be a member of the open cluster Trumpler 16 in the Carina complex. We show that Tr 16-104 could be a hierarchical triple system consisting of a short-period (2.15 d) eclipsing O7 V+O9.5 V binary bound to a B0.2 IV star. Our preliminary orbital solution of the third body indicates that the B star most probably describes an eccentric orbit with a period of ∼285 or ∼1341 d around the close binary. Folding photometric data from the literature with our new ephemerides, we find that the light curve of the close binary exhibits rather narrow eclipses indicating that the two O stars must be well inside their Roche lobes. Our analysis of the photometric data yields a lower limit on the inclination of the orbit of the close binary of i≥77°. The stellar radii and luminosities of the O7 V and O9.5 V stars are significantly smaller than expected for stars of this spectral type. Our results suggest that Tr 16-104 lies at a distance of the order of 2.5 kpc and support a fainter absolute magnitude for zero-age main-sequence O stars than usually adopted. We find that the dynamical configuration of Tr 16-104 corresponds to a hierarchical system that should remain stable provided that it suffers no strong perturbation. Finally, we also report long-term temporal variations of high-velocity interstellar Ca ii absorptions in the line of sight towards Tr 16-104.Facultad de Ciencias Astronómicas y Geofísica
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