198 research outputs found
R Coronae Borealis stars in the Galactic Bulge discovered by EROS-2
Rare types of variable star may give unique insight into short-lived stages
of stellar evolution. The systematic monitoring of millions of stars and
advanced light curve analysis techniques of microlensing surveys make them
ideal for discovering also such rare variable stars. One example is the R
Coronae Borealis (RCB) stars, a rare type of evolved carbon-rich supergiant. We
have conducted a systematic search of the EROS-2 database for the Galactic
catalogue Bulge and spiral arms to find Galactic RCB stars. The light curves of
100 million stars, monitored for 6.7 years (from July 1996 to February
2003), have been analysed to search for the main signature of RCB stars, large
and rapid drops in luminosity. Follow-up spectroscopy has been used to confirm
the photometric candidates. We have discovered 14 new RCB stars, all in the
direction of the Galactic Bulge, bringing the total number of confirmed
Galactic RCB stars to about 51. After reddening correction, the colours and
absolute magnitudes of at least 9 of the stars are similar to those of
Magellanic RCB stars. This suggests that these stars are in fact located in the
Galactic Bulge, making them the first RCB stars discovered in the Bulge. The
localisation of the 5 remaining RCBs is more uncertain: 4 are either located
behind the Bulge at an estimated maximum distance of 14 kpc or have an unusual
thick circumstellar shell; the other is a DY Per RCB which may be located in
the Bulge, even if it is fainter than the known Magellanic DY Per. From the
small scale height found using the 9 new Bulge RCBs,
pc (95% C.L.), we conclude that the RCB stars follow a disk-like distribution
inside the Bulge.Comment: 20 pages, 26 figures, Accepted in A&
New Magellanic Cloud R Coronae Borealis and DY Per type stars from the EROS-2 database: the connection between RCBs, DYPers and ordinary carbon stars
R Coronae Borealis stars (RCB) are a rare type of evolved carbon-rich
supergiant stars that are increasingly thought to result from the merger of two
white dwarfs, called the Double degenerate scenario. This scenario is also
studied as a source, at higher mass, of type Ia Supernovae (SnIa) explosions.
Therefore a better understanding of RCBs composition would help to constrain
simulations of such events. We searched for and studied RCB stars in the EROS
Magellanic Clouds database. We also extended our research to DY Per type stars
(DYPers) that are expected to be cooler RCBs (T~3500 K) and much more numerous
than their hotter counterparts. The light curves of ~70 millions stars have
been analysed to search for the main signature of RCBs and DYPers: a large drop
in luminosity. Follow-up optical spectroscopy was used to confirm each
photometric candidate found. We have discovered and confirmed 6 new Magellanic
Cloud RCB stars and 7 new DYPers, but also listed new candidates: 3 RCBs and 14
DYPers. We estimated a range of Magellanic RCB shell temperatures between 360
and 600 K. We confirm the wide range of absolute luminosity known for RCB
stars, M_V~-5.2 to -2.6. Our study further shows that mid-infrared surveys are
ideal to search for RCB stars, since they have thinner and cooler circumstellar
shells than classical post-AGB stars. In addition, by increasing the number of
known DYPers by ~400%, we have been able to shed light on the similarities in
the spectral energy distribution between DYPers and ordinary carbon stars. We
also observed that DYPer circumstellar shells are fainter and hotter than those
of RCBs. This suggests that DYPers may simply be ordinary carbon stars with
ejection events, but more abundance analysis is necessary to give a status on a
possible evolutionnary connexion between RCBs and DYPers.Comment: 22 pages, 38 figures, Accepted for publication in A&
A Nearby Supernovae Search: Eros2
Type Ia supernovae (SNIa) have been used as approximate standard candles to
measure cosmological parameters such as the Hubble constant and the
deceleration parameter. These measurements rely on empirical correlations
between peak luminosities and other features that can be observed in the
supernovae spectra and their light curves. Such correlations deserve further
study since they have been established from small samples of nearby SNIa. Two
years ago, the EROS2 collaboration launched an automated search for supernovae
with the 1m Marly telescope operating at La Silla. In all, 57 SNe have been
discovered in this EROS2 search and spectra have been obtained for 26 of them.
We found that 75% were of type Ia and 25% of type II. Using this sample, a
preliminary SN explosion rate has been obtained. Our most recent observation
campaign took place in February and March 99. It was performed in the framework
of a large consortium led by the {\em Supernova Cosmology Project}. The aim of
this intensive campaign was to provide an independent set of high quality light
curves and spectra to study systematic effects in the measurement of
cosmological parameters. We will briefly describe our search procedure and
present the status of our ongoing analysis.Comment: 5 page
Not enough stellar mass Machos in the Galactic halo
We present an update of results from the search for microlensing towards the
Large Magellanic Cloud (LMC) by EROS (Experience de Recherche d'Objets
Sombres). We have now monitored 25 million stars over three years. Because of
the small number of observed microlensing candidates (four), our results are
best presented as upper limits on the amount of dark compact objects in the
halo of our Galaxy. We discuss critically the candidates and the possible
location of the lenses, halo or LMC . We compare our results to those of the
MACHO group. Finally, we combine these new results with those from our search
towards the Small Magellanic Cloud as well as earlier ones from the EROS1 phase
of our survey. The combined data is sensitive to compact objects in the broad
mass range solar masses. The derived upper limit on the
abundance of stellar mass MACHOs rules out such objects as the dominant
component of the Galactic halo if their mass is smaller than 2 solar masses.Comment: 7 pages, 4 figures, presented at the XIX International Conference on
Neutrino Physics and Astrophysics, Sudbury, Canada, June 200
Observation of Microlensing towards the Galactic Spiral Arms. EROS II 2 year survey
We present the analysis of the light curves of 8.5 million stars observed
during two seasons by EROS (Experience de Recherche d'Objets Sombres), in the
galactic plane away from the bulge. Three stars have been found that exhibit
luminosity variations compatible with gravitational microlensing effects due to
unseen objects. The corresponding optical depth, averaged over four directions,
is 0.38 (+0.53, -0.15) 10^{-6}. All three candidates have long Einstein radius
crossing times ( 70 to 100 days). For one of them, the lack of evidence
for a parallax or a source size effect enabled us to constrain the lens-source
% geometric configuration. Another candidate displays a modulation of the
magnification, which is compatible with the lensing of a binary source.
The interpretation of the optical depths inferred from these observations is
hindered by the imperfect knowledge of the distance to the target stars. Our
measurements are compatible with expectations from simple galactic models under
reasonable assumptions on the target distances.Comment: 11 pages, 13 figures, accepted by A&A in Aug 9
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