1,451 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
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&
Over-parameterisation, a major obstacle to the use of artificial neural networks in hydrology?
International audienceRecently Feed-Forward Artificial Neural Networks (FNN) have been gaining popularity for stream flow forecasting. However, despite the promising results presented in recent papers, their use is questionable. In theory, their "universal approximator? property guarantees that, if a sufficient number of neurons is selected, good performance of the models for interpolation purposes can be achieved. But the choice of a more complex model does not ensure a better prediction. Models with many parameters have a high capacity to fit the noise and the particularities of the calibration dataset, at the cost of diminishing their generalisation capacity. In support of the principle of model parsimony, a model selection method based on the validation performance of the models, "traditionally" used in the context of conceptual rainfall-runoff modelling, was adapted to the choice of a FFN structure. This method was applied to two different case studies: river flow prediction based on knowledge of upstream flows, and rainfall-runoff modelling. The predictive powers of the neural networks selected are compared to the results obtained with a linear model and a conceptual model (GR4j). In both case studies, the method leads to the selection of neural network structures with a limited number of neurons in the hidden layer (two or three). Moreover, the validation results of the selected FNN and of the linear model are very close. The conceptual model, specifically dedicated to rainfall-runoff modelling, appears to outperform the other two approaches. These conclusions, drawn on specific case studies using a particular evaluation method, add to the debate on the usefulness of Artificial Neural Networks in hydrology. Keywords: forecasting; stream-flow; rainfall-runoff; Artificial Neural Network
The atypical emission-line star Hen3-209
We analyse observations, spanning 15 years, dedicated to the extreme
emission-line object Hen3-209. Our photometric data indicate that the
luminosity of the star undergoes marked variations with a peak-to-peak
amplitude of 0.65mag. These variations are recurrent, with a period of
16.093+-0.005d. The spectrum of Hen3-209 is peculiar with many different lines
(HI, HeI, FeII,...) showing P Cygni profiles. The line profiles are apparently
changing in harmony with the photometry. The spectrum also contains [OIII]
lines that display a saddle profile topped by three peaks, with a maximum
separation of about 600km/s. Hen3-209 is most likely an evolved luminous object
suffering from mass ejection events and maybe belonging to a binary system.Comment: 6p, 5 fig, accepted for publication in MNRAS
(www.blackwell-synergy.com
Observational signatures of past mass-exchange episodes in massive binaries: The case of LSS 3074
The role of mass and momentum exchanges in close massive binaries is very
important in the subsequent evolution of the components. Such exchanges produce
several observational signatures such as asynchronous rotation and altered
chemical compositions, that remain after the stars detach again. We
investigated these effects for the close O-star binary LSS 3074 (O4 f + O6-7
:(f):), which is a good candidate for a past Roche lobe overflow (RLOF) episode
because of its very short orbital period, P = 2.185 days, and the luminosity
classes of both components. We determined a new orbital solution for the
system. We studied the photometric light curves to determine the inclination of
the orbit and Roche lobe filling factors of both stars. Using phase-resolved
spectroscopy, we performed the disentangling of the optical spectra of the two
stars. We then analysed the reconstructed primary and secondary spectra with
the CMFGEN model atmosphere code to determine stellar parameters, such as the
effective temperatures and surface gravities, and to constrain the chemical
composition of the components. We confirm the apparent low stellar masses and
radii reported in previous studies. We also find a strong overabundance in
nitrogen and a strong carbon and oxygen depletion in both primary and secondary
atmospheres, together with a strong enrichment in helium of the primary star.
We propose several possible evolutionary pathways through a RLOF process to
explain the current parameters of the system. We confirm that the system is
apparently in overcontact configuration and has lost a significant portion of
its mass to its surroundings. We suggest that some of the discrepancies between
the spectroscopic and photometric properties of LSS 3074 could stem from the
impact of a strong radiation pressure of the primary
A modern study of HD166734: a massive supergiant system
Aims. HD166734 is an eccentric eclipsing binary system composed of two
supergiant O-type stars, orbiting with a 34.5-day period. In this rare
configuration for such stars, the two objects mainly evolve independently,
following single-star evolution so far. This system provides a chance to study
the individual parameters of two supergiant massive stars and to derive their
real masses. Methods. An intensive monitoring was dedicated to HD166734.We
analyzed mid- and high-resolution optical spectra to constrain the orbital
parameters of this system. We also studied its light curve for the first time,
obtained in the VRI filters. Finally, we disentangled the spectra of the two
stars and modeled them with the CMFGEN atmosphere code in order to determine
the individual physical parameters. Results. HD166734 is a O7.5If+O9I(f)
binary. We confirm its orbital period but we revise the other orbital
parameters. In comparison to what we found in the literature, the system is
more eccentric and, now, the hottest and the most luminous component is also
the most massive one. The light curve exhibits only one eclipse and its
analysis indicates an inclination of 63.0{\deg} 2.7{\deg}. The
photometric analysis provides us with a good estimation of the luminosities of
the stars, and therefore their exact positions in the Hertzsprung-Russell
diagram. The evolutionary and the spectroscopic masses show good agreement with
the dynamical masses of 39.5 Msun for the primary and 33.5 Msun for the
secondary, within the uncertainties. The two components are both enriched in
helium and in nitrogen and depleted in carbon. In addition, the primary also
shows a depletion in oxygen. Their surface abundances are however not different
from those derived from single supergiant stars, yielding, for both components,
an evolution similar to that of single stars.Comment: 13 pages, 13 figures, A&A accepte
Evidence for a physically bound third component in HD 150136
Context. HD150136 is one of the nearest systems harbouring an O3 star.
Although this system was for a long time considered as binary, more recent
investigations have suggested the possible existence of a third component.
Aims. We present a detailed analysis of HD 150136 to confirm the triple nature
of this system. In addition, we investigate the physical properties of the
individual components of this system. Methods. We analysed high-resolution,
high signal-to-noise data collected through multi-epoch runs spread over ten
years. We applied a disentangling program to refine the radial velocities and
to obtain the individual spectra of each star. With the radial velocities, we
computed the orbital solution of the inner system, and we describe the main
properties of the orbit of the outer star such as the preliminary mass ratio,
the eccentricity, and the orbital-period range. With the individual spectra, we
determined the stellar parameters of each star by means of the CMFGEN
atmosphere code. Results. We offer clear evidence that HD 150136 is a triple
system composed of an O3V((f\ast))-3.5V((f+)), an O5.5-6V((f)), and an
O6.5-7V((f)) star. The three stars are between 0-3 Myr old. We derive dynamical
masses of about 64, 40, and 35 Msun for the primary, the secondary and the
third components by assuming an inclination of 49{\deg}. It currently
corresponds to one of the most massive systems in our galaxy. The third star
moves with a period in the range of 2950 to 5500 d on an outer orbit with an
eccentricity of at least 0.3. This discovery makes HD 150136 the first
confirmed triple system with an O3 primary star. However, because of the long
orbital period, our dataset is not sufficient to constrain the orbital solution
of the tertiary component with high accuracy.Comment: 13 pages, 11 figures, accepted at A&
A multi-method approach to radial-velocity measurement for single-object spectra
The derivation of radial velocities from large numbers of spectra that
typically result from survey work, requires automation. However, except for the
classical cases of slowly rotating late-type spectra, existing methods of
measuring Doppler shifts require fine-tuning to avoid a loss of accuracy due to
the idiosyncrasies of individual spectra. The radial velocity spectrometer
(RVS) on the Gaia mission, which will start operating very soon, prompted a new
attempt at creating a measurement pipeline to handle a wide variety of spectral
types.
The present paper describes the theoretical background on which this software
is based. However, apart from the assumption that only synthetic templates are
used, we do not rely on any of the characteristics of this instrument, so our
results should be relevant for most telescope-detector combinations.
We propose an approach based on the simultaneous use of several alternative
measurement methods, each having its own merits and drawbacks, and conveying
the spectral information in a different way, leading to different values for
the measurement. A comparison or a combination of the various results either
leads to a "best estimate" or indicates to the user that the observed spectrum
is problematic and should be analysed manually.
We selected three methods and analysed the relationships and differences
between them from a unified point of view; with each method an appropriate
estimator for the individual random error is chosen. We also develop a
procedure for tackling the problem of template mismatch in a systematic way.
Furthermore, we propose several tests for studying and comparing the
performance of the various methods as a function of the atmospheric parameters
of the observed objects. Finally, we describe a procedure for obtaining a
knowledge-based combination of the various Doppler-shift measurements.Comment: 16 pages, 4 figure
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