Aims. The EROS-2 project was designed to test the hypothesis that massive compact halo objects (the so-called "machos") could be a major component of the dark matter halo of the Milky Way galaxy. To ...this end, EROS- 2 monitored over 6.7 years 33\times10 similar to stars in the Magellanic clouds for microlensing events caused by such objects. Methods. In this work, we use only a subsample of 7\times10 similar to bright stars spread over 84\,\rm deg arrow up of the LMC and 9\,\rm deg arrow up of the SMC. The strategy of using only bright stars helps to discriminate against background events due to variable stars and allows a simple determination of the effects of source confusion (blending). The use of a large solid angle makes the survey relatively insensitive to effects that could make the optical depth strongly direction dependent. Results. Using this sample of bright stars, only one candidate event was found, whereas similar to 39 events would have been expected if the Halo were entirely populated by objects of mass M\sim0.4 similar to M_{\odot}. Combined with the results of EROS-1, this implies that the optical depth toward the Large Magellanic Cloud ( LMC) due to such lenses is \tau<0.36\times10 (95% CL), corresponding to a fraction of the halo mass of less than 8%. This optical depth is considerably less than that measured by the MACHO collaboration in the central region of the LMC. More generally, machos in the mass range 0.6\times10 contains as a subset _\odot<M<15 similar to M_{\odot} are ruled out as the primary occupants of the Milky Way Halo.
Context. 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. Aims. 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 ($T \sim 3500$ K) and much more numerous than their hotter counterparts. With the aim of studying possible evolutionary connections between RCBs and DYPers, and also ordinary carbon stars, we compared their publically available broad band photometry in the optical, near, and mid-infrared. Methods. The light curves of ~70 millions stars, monitored for 6.7 years (from July 1996 to February 2003), have been analysed to search for the main signature of RCBs and DYPers: a large (up to 9 mag) drop in luminosity. Carbon stars with fading episodes were also found by inspecting numerous light curves of objects that presented an infrared excess in the 2MASS and Spitzer- SAGE and S3MC databases. Follow-up optical spectroscopy was used to confirm each photometric candidate found. Results. 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. Optical and infrared colour magnitude diagrams that give new insights into these two sets of stars are discussed. We estimated a range of Magellanic RCB shell temperatures between 360 and 600 K. Conclusions. We confirm the wide range of absolute luminosity known for RCB stars, $M_V \sim -5.2$ 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.
Context. Rare types of variable stars may provide 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 such rare variable stars. One example is the R Coronae Borealis (RCB) stars, a rare type of evolved carbon-rich supergiant. Aims. We have conducted a systematic search of the EROS-2 database for the Galactic catalogue Bulge and spiral arms to find Galactic RCB stars. Methods. 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. Results. 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. Conclusions. 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, $61<h^{\rm RCB}_{\rm Bulge}<246$ pc (95% C.L.), we conclude that the RCB stars follow a disk-like distribution inside the Bulge.
We present the results of a massive variability search based on a photometric survey of a six square degree region along the Galactic plane at ($l = 305^\circ$, $b = -0.8^\circ$) and ($l = ...330^\circ$, $b = -2.5^\circ$). This survey was performed in the framework of the EROS II (Expérience de Recherche d'Objets Sombres) microlensing program. The variable stars were found among 1 913 576 stars that were monitored between April and June 1998 in two passbands, with an average of 60 measurements. A new period-search technique is proposed which makes use of a statistical variable that characterizes the overall regularity of the flux versus phase diagram. This method is well suited when the photometric data are unevenly distributed in time, as is our case. 1362 objects whose luminosity varies were selected. Among them we identified 9 Cepheids, 19 RR Lyræ, 34 Miras, 176 eclipsing binaries and 266 Semi-Regular stars. Most of them are newly identified objects. The cross-identification with known catalogues has been performed. The mean distance of the RR Lyræ is estimated to be ~$ 4.9 \pm 0.3$ kpc undergoing an average absorption of ~$ 3.4 \pm 0.2$ mag. This distance is in good agreement with that of disc stars that contribute to the microlensing source star population. Our catalogue and light curves are available electronically from the CDS, Strasbourg and from our Web site.
Aims.We present a new EROS-2 measurement of the microlensing optical depth toward the Galactic Bulge. Methods.Light curves of $5.6\times 10^{6}$ clump-giant stars distributed over $66\,\rm deg^2$ of ...the Bulge were monitored during seven Bulge seasons. 120 events were found with apparent amplifications greater than 1.6 and Einstein radius crossing times in the range $5\,{\rm d}<t_{\rm E}<400\,{\rm d}$. This is the largest existing sample of clump-giant events and the first to include northern Galactic fields. Results.In the Galactic latitude range $1.4\degr<|b|<7.0\degr$, we find $\tau/10^{-6}=(1.62\,\pm 0.23)\exp\,-a(|b|-3 \,{\rm deg})$ with $a=(0.43\,\pm0.16)~\rm deg^{-1}$. These results are in good agreement with our previous measurement, with recent measurements of the MACHO and OGLE-II groups, and with predictions of Bulge models.
Aims. The EROS-2 project has been designed to search for microlensing events towards any dense stellar field. The densest parts of the Galactic spiral arms have been monitored to maximize the ...microlensing signal expected from the stars of the Galactic disk and bulge. Methods. 12.9 million stars have been monitored during 7 seasons towards 4 directions in the Galactic plane, away from the Galactic center. Results. A total of 27 microlensing event candidates have been found. Estimates of the optical depths from the 22 best events are provided. A first order interpretation shows that simple Galactic models with a standard disk and an elongated bulge are in agreement with our observations. We find that the average microlensing optical depth towards the complete EROS-cataloged stars of the spiral arms is $\bar{\tau}$ = 0.51±.13$\times$10-6, a number that is stable when the selection criteria are moderately varied. As the EROS catalog is almost complete up to IC = 18.5, the optical depth estimated for the sub-sample of bright target stars with $I_C < 18.5$ ($\bar{\tau}$ = 0.39±.11$\times$10-6) is easier to interpret. Conclusions. The set of microlensing events that we have observed is consistent with a simple Galactic model. A more precise interpretation would require either a better knowledge of the distance distribution of the target stars, or a simulation based on a Galactic model. For this purpose, we define and discuss the concept of optical depth for a given catalog or for a limiting magnitude.
Five years of eros data towards the Small Magellanic Cloud have been searched for gravitational microlensing events, using a new, more accurate method to assess the impact of stellar blending on the ...efficiency. Four long-duration candidates have been found which, if they are microlensing events, hint at a non-halo population of lenses. Combined with results from other eros observation programs, this analysis yields strong limits on the amount of Galactic dark matter made of compact objects. Less than 25% of a standard halo can be composed of objects with a mass between $2\times10^{-7}~M_\odot$ and 1 $M_\odot$ at the 95% C.L.
We present a measurement of the microlensing optical depth toward the Galactic bulge based on the analysis of 15 contiguous $1\,\rm deg^2$ fields centered on $(l=2.\hskip-2pt^\circ 5, ...b=-4.\hskip-2pt^\circ 0)$ and containing $N_*=1.42\times 10^{6}$ clump-giant stars (belonging to the extended clump area) monitored during almost three bulge seasons by EROS (Expérience de Recherche d'Objets Sombres). We find $\tau_{\rm bulge}=0.94\pm 0.29\times 10^{-6}$ averaged over all fields, based on 16 microlensing events with clump giants as sources. This value is substantially below several other determinations by the MACHO and OGLE groups and is more in agreement with what is expected from axisymmetric and non-axisymmetric bulge models.
Context. A number of microlensing dark-matter surveys have produced tens of millions of light curves of individual background stars. These data provide an unprecedented opportunity for systematic ...studies of whole classes of variable stars and their host galaxies. Aims. We aim to use the EROS-2 survey of the Magellanic Clouds to detect and study the population of beat Cepheids (BCs) in both Clouds. BCs pulsating simultaneously in the first overtone and fundamental modes (FO/F) or in the second and first overtone modes (SO/FO) are of particular interest. Methods. Using special software designed to search for periodic variables, we have scanned the EROS-2 data base for variables in the typical period range of Cepheids. Metallicities of FO/F objects were then calculated from linear nonadiabatic convective stellar models. Results. We identify 74 FO/F BCs in the LMC and 41 in the SMC, and 173 and 129 SO/FO pulsators in the LMC and SMC, respectively; 185 of these stars are new discoveries. For nearly all the FO/F objects we determine minimum, mean, and maximum values of the metallicity. Conclusions. The EROS data have expanded the samples of known BCs in the LMC by 31%, in the SMC by 110%. The FO/F objects provide independent measures of metallicities in these galaxies. The mean value of metallicity is 0.0045 in the LMC and 0.0018 in the SMC.
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