Publications (21)32.59 Total impact
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Article: EROS and MACHO Combined Limits on Planetary-Mass Dark Matter in the Galactic Halo
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ABSTRACT: The EROS and MACHO collaborations have each published upper limits on the amount of planetary-mass dark matter in the Galactic halo obtained from gravitational microlensing searches. In this Letter, the two limits are combined to give a much stronger constraint on the abundance of low-mass MACHOs. Specifically, objects with masses 10−7 Mm10−3 M make up less than 25% of the halo dark matter for most models considered, and less than 10% of a standard spherical halo is made of MACHOs in the 3.5×10−7 M<m<4.5×10−5 M mass range.The Astrophysical Journal 01/2009; 499(1):L9. · 6.02 Impact Factor -
Article: AGAPEROS: searches for microlensing in the LMC with the pixel method. II. Selection of possible microlensing events
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ABSTRACT: We apply the pixel method of analysis (sometimes called ``pixel lensing'') to a small subset of the EROS-1\ microlensing observations of the bar of the Large Magellanic Cloud (LMC). The pixel method is designed to find microlensing events of unresolved source stars and had heretofore been applied only to M31 where essentially all sources are unresolved. With our analysis optimised for the detection of long-duration microlensing events due to 0.01-1 M_sun Machos, we detect no microlensing events and compute the corresponding detection efficiencies. We show that the pixel method, applied to crowded fields, should detect 10 to 20 times more microlensing events for M>0.05 M_sun Machos compared to a classical analysis of the same data which latter monitors only resolved stars. In particular, we show that for a full halo of Machos in the mass range 0.1 - 0.5 M_sun, a pixel analysis of the three-year EROS-1 data set covering 0.39 deg(2) would yield =~ 4 events.Astronomy and Astrophysics 10/1998; 339:658-670. · 4.59 Impact Factor -
Article: EROS variable stars: discovery of a slow nova in the SMC
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ABSTRACT: We report the discovery of a slow nova found in the core of the Small Magellanic Cloud by the EROS microlensing survey. Nova SMC 1994 is a classical nova with a DQ Her type lightcurve characterized by a deep minimum. Low amplitude variations occuring on time-scales of hours and days are also detected at maximum light. Spectra collected during the nebular phase indicate that Nova SMC 1994 is similar to Galactic novae of the same class. Large helium enhancement in the shell is found and O and N enrichments are suspected. Based on observations collected at the European Southern Observatory, Chile.Astronomy and Astrophysics 06/1998; 335:L93-L96. · 4.59 Impact Factor -
Article: Search for planetary mass objects in the Galactic halo through microlensing
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ABSTRACT: We present final results from a search for gravitational microlensing of stars in the Magellanic Clouds by very light machos (MAssive Compact Halo Objects) in the Galactic Halo. The EROS programme used a CCD camera devoted to the detection of events with a duration ranging from 15 minutes to a few days; none of the 350,000 light curves exhibits a form which is consistent with a microlensing event. Using our detection efficiency, largely affected by the finite size of the source and blending effects at the lowest durations, and different models of the Galaxy, we can exclude planetary mass objects as a major constituent of the dark Halo. Based on observations made at the European Southern Observatory, La Silla, Chile.Astronomy and Astrophysics 12/1997; 329:522-537. · 4.59 Impact Factor -
Article: EROS Variable Stars : Discovery of Beat Cepheids in the Small Magellanic Cloud and the effect of metallicity on pulsation
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ABSTRACT: We report the discovery of eleven beat Cepheids in the Small Magellanic Cloud, using data obtained by the EROS microlensing survey. Four stars are beating in the fundamental and first overtone mode (F/1OT), seven are beating in the first and second overtone (1OT/2OT). The SMC F/1OT ratio is systematically higher than the LMC F/1OT, while the 1OT/2OT period ratio in the SMC Cepheids is the same as the LMC one.07/1997; -
Article: The effect of metallicity on the Cepheid distance scale and its implications for the Hubble constant ($H_0$) determination
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ABSTRACT: Recent HST determinations of the expansion's rate of the Universe (the Hubble constant, H_0) assumed that the Cepheid Period-Luminosity relation at V and I are independent of metallicity (Freedman, et al., 1996, Saha et al., 1996, Tanvir et al., 1995). The three groups obtain different vales for H_0. We note that most of this discrepancy stems from the asumption (by both groups) that the Period-Luminosity relation is independent of metallicity. We come to this conclusion as a result of our study of the Period-Luminosity relation of 481 Cepheids with 3 millions two colour measurements in the Large Magellanic Cloud and the Small Magellanic Cloud obtained as a by-product of the EROS microlensing survey. We find that the derived interstellar absorption corrections are particularly sensitive to the metallicity and when our result is applied to recent estimates based on HST Cepheids observations it makes the low-H_0 values higher and the high-H_0 value lower, bringing those discrepant estimates into agrement around $H_0 \approx 70 km/s Mpc^{-1}$.01/1997; -
Article: Metallicity Effects on the Cepheid Extragalactic Distance Scale from EROS photometry in LMC and SMC
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ABSTRACT: This is an investigation of the period-luminosity relation of classical Cepheids in samples of different metallicity. It is based on 481 Cepheids in the Large and Small Magellanic Clouds from the blue and red filter CCD observations (most similar to V_J & R_J) of the French EROS microlensing project. The data-set is complete and provides an excellent basis for a differential analysis between LMC and SMC. In comparison to previous studies of effects on the PL-relation, the EROS data-set offers extremely well-sampled light curves and well-filled instability strips. This allows reliable separation of Cepheids pulsating in the fundamental and the first overtone mode and derivation of differential reddening. Our main result concerns the determination of distances to galaxies which are inferred by using the LMC as a base and using two color photometry to establish the amount of reddening. We find a zero-point offset between SMC and LMC which amounts to a difference between inferred and true distance modulus of 0.14 \pm 0.06 mag in the VI_c system. The offset is exactly the same in both sets of PL-relations - of the fundamental and of the first overtone mode Cepheids. No effect is seen on the slopes of the PL-relations, although the fundamental and the first overtone mode Cepheids have different PL slopes. We attribute the color and the zero-point offset to the difference in metallicity between the SMC and LMC Cepheids. A metallicity effect of that small magnitude still has important consequencies for the inferred Cepheid distances and the determination of H_0. When applied to recent estimates based on HST Cepheid observations, our metallicity dependence makes the low-H_0 values (Sandage et al. 1994) higher and the high-H_0 values (Freedman et al. 1994b) lower, thus bringing those01/1997; -
Article: Observational Limits on Machos in the Galactic Halo
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ABSTRACT: We present final results from the first phase of the EROS search for gravitational microlensing of stars in the Magellanic Clouds by unseen deflectors (machos: MAssive Compact Halo Objects). The search is sensitive to events with time scales between 15 minutes and 200 days corresponding to deflector masses in the range 1.e-7 to a few solar masses. Two events were observed that are compatible with microlensing by objects of mass of about 0.1 Mo. By comparing the results with the expected number of events for various models of the Galaxy, we conclude that machos in the mass range [1.e-7, 0.02] Mo make up less than 20% (95% C.L.) of the Halo dark matter.01/1997; -
Article: Observational limits on the contribution of sub-stellar and stellar objects to the galactic halo.
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ABSTRACT: EROS (Experience de Recherche d'Objets Sombres) has been monitoring the luminosity of 4 million stars in the Large Magellanic Cloud in order to search for gravitational microlensing by unseen objects in the galactic halo. We present here the results from 3 years of EROS Schmidt plates data. Two stars exhibit light curves that are consistent with a sizeable microlensing effect. CCD data obtained later on revealed that one of these stars is an eclipsing binary system. Combining Schmidt plates data and the published results from our 16 CCD camera, we set upper limits on the number of unseen objects in the halo in the mass range [10^-7^,1]Msun_.Astronomy and Astrophysics 09/1996; 314:94-103. · 4.59 Impact Factor -
Article: Variability of the EROS no. 2 microlensing candidate.
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ABSTRACT: In this letter we report the analysis of data concerning the EROS no. 2 microlensing candidate. We have performed photometric measurements on this star using three different telescopes. It was found to exhibit a periodic variability. This source could be an eclipsing binary system, of period 2.8 days.Astronomy and Astrophysics 06/1995; 299:L21. · 4.59 Impact Factor -
Article: EROS VARIABLE STARS : FUNDAMENTAL-MODE AND FIRST OVERTONE CEPHEIDS IN THE BAR OF THE LARGE MAGELLANIC CLOUD
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ABSTRACT: We present CCD phase-binned light curves at 490 nm for 97 Cepheid variable stars in the bar of the LMC. The photometry was obtained as part of the French EROS project and has excellent phase coverage, permitting accurate decomposition into Fourier components. We identify as `sinusoidal' or s-Cepheids those stars with periods less than 5.5 d and small second-harmonic components. These stars comprise $\sim$30% of our sample and most form a sequence $\sim$1 mag brighter than the LMC classical Cepheids in the period-luminosity diagram. They are also generally bluer and have lower-amplitude light curves. We infer that the s-Cepheids are first-overtone pulsators because, when their periods are converted to expected fundamental-mode values, they obey a common period-luminosity-colour relation with classical Cepheids. This also confirms the reality of the colour term in the Cepheid period-luminosity-colour relation. Further, the blue edge of the classical Cepheid instability strip agrees well with the theoretical calculations for the fundamental mode made by Chiosi et al. (1993) for the Hertzsprung-Russell and period-luminosity diagrams, but we find that our observed s-Cepheids are $>0.2$ mag brighter and bluer than the Chiosi et al.\ predictions for the first-overtone. We identify a number of features in plots of our stars' Fourier-component amplitude ratios and phase differences. These features have been identified with resonances between different pulsation modes. In the LMC we find these features seem to occur at periods very similar to Galactic ones for classical Cepheids, but at different periods for s-Cepheids. We discover a double-mode Cepheid in the LMC, for which $P({\rm first overtone})/P({\rm fundamental}) = 0.710 \pm 0.001$, very similar to observed ratios for Galactic double-mode Cepheids.06/1995; -
Article: SEARCH FOR VERY LOW MASS OBJECTS IN THE GALACTIC HALO
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ABSTRACT: We present results from a search for gravitational microlensing of stars in the Large Magellanic Cloud by low mass objects in the Galactic Halo. The search uses the CCD light curves of about 82,000 stars with up to 46 measurements per night over a period of 10 months. No light curve exhibits a form that is consistent with a microlensing event of maximum amplification greater than 1.2. This null result makes it unlikely that the Halo is dominated by objects in the mass range $5 10^{-8}M_{\odot} < M <5 10^{-4}M_{\odot} $. keywords{Galaxy : Halo, kinematics and dynamics, stellar content -- Cosmology : dark matter, gravitational lensing}03/1995; -
Article: EROS catalogue of eclipsing binary stars in the bar of the Large Magellanic Cloud.
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ABSTRACT: We present a catalogue of 79 certain or probable eclipsing binary stars located in the bar of the Large Magellanic Cloud. The stars were identified from the French EROS microlensing project's 1991-92 season of CCD observations using Grison's (1994) period-searching procedure, which incorporates higher harmonics into the periodogram search of photometric time sequences. Only one of the systems was previously identified and our catalogue doubles the number of known eclipsing binaries in the LMC. The systems mostly range from 15^th^ to 18^th^ mag., exhibit blue colours and have periods less than 10 days. At least ten systems are eccentric. Coordinates and ephemerides are presented, as well as identification charts and light curves at 490 and 670nm. The catalogue and light curves are available electronically from the CDS, Strasbourg.Astronomy and Astrophysics Supplement Series 02/1995; 109:447-469. -
Article: Spectroscopic Studies of the Two Eros Candidate Microlensed Stars
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ABSTRACT: Low resolution spectroscopy, and UBVRI photometry, have been obtained for the two EROS microlensing candidates. Radial velocities indicate that both stars are members of the Large Magellanic Cloud. The spectrum and the colours of EROS1, the first candidate, reveal that it is a moderately reddened main-sequence B star with H emission lines. The presence of H absorption lines seems to be more the signature of a normal star than that of a cataclysmic variable. As to EROS2, the second candidate, its spectrum and photometry are those of an unreddened normal main-sequence A star, but it cannot be totally excluded that they represent those of a nova in the pre-outburst phase. Although it is not yet possible to exclude intrinsic stellar variations, the interpretation in terms of microlensing effects remain the most natural one.12/1994; -
Article: A 20cm2 CCD mosaic camera for a dark matter search part II : Electronics, data acquisition and performance
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ABSTRACT: To summarize, the readout and the control system of the CCD mosaic camera are running since December 1991 at the La Silla Observatory (ESO). The overall performance of the camera has been good. About 12000 pictures (data and flat-fields) have been successfully registered up to now. We will report in the near future preliminary scientific results of the EROS experiment.Experimental Astronomy 08/1994; 4(3):279-296. · 1.82 Impact Factor -
Article: A 20 cm2 CCD mosaic camera for a dark matter search part I : Mechanics, optics and cryogeny
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ABSTRACT: A 20cm 2 CCD mosaic camera has been especially built to search for dark galactic halo objects by the gravitational microlensing effect. The sensitive area is made of 16 edge-buttable CCDs developped by Thomson-CTS, with 2323 m 2 pixels. The 35 kg camera housing and mechanical equipment is presented. The associated electronics and data acquisition system are described in a separate paper. The camera resides at the focal plane of a 40 cm, f/10, Ferson reflector. The instrument has been in operation since December 1991 at the La Silla Observatory (ESO).Experimental Astronomy 08/1994; 4(3):265-278. · 1.82 Impact Factor -
Article: Evidence for gravitational microlensing by dark objects in the Galactic halo
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ABSTRACT: THE flat rotation curves of spiral galaxies, including our own, indicate that they are surrounded by unseen haloes of ‘dark matter’1,2. In the absence of a massive halo, stars and gas in the outer portions of a galaxy would orbit the centre more slowly, just as the outer planets in the Solar System circle the Sun more slowly than the inner ones. So far, however, there has been no direct observational evidence for the dark matter, or its characteristics. Paczyński3suggested that dark bodies in the halo of our Galaxy can be detected when they act as gravitational ‘microlenses’, amplifying the light from stars in nearby galaxies. The duration of such an event depends on the mass, distance and velocity of the dark object. We have been monitoring the brightness of three million stars in the Large Magellanic Cloud for over three years, and here report the detection of two possible microlensing events. The brightening of the stars was symmetrical in time, achromatic and not repeated during the monitoring period. The timescales of the two events are about thirty days and imply that the masses of the lensing objects lie between a few hundredths and one solar mass. The number of events observed is consistent with the number expected if the halo is dominated by objects with masses in this range.10/1993; 365(6447):623-625. -
Article: The EROS Search for Dark Halo Objects
The Messenger. 05/1993; 72:20-27. -
Article: AGAPEROS: Searching for microlensing in the LMC with the pixel method
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ABSTRACT: Recent surveys monitoring millions of light curves of resolved stars in the LMC have discovered several microlensing events. Unresolved stars could however significantly contribute to the microlensing rate towards the LMC. Monitoring pixels, as opposed to individual stars, should be able to detect stellar variability as a variation of the pixel flux. We present a first application of this new type of analysis (Pixel Method) to the LMC Bar. We describe the complete procedure applied to the EROS 91-92 data (one tenth of the existing CCD data set) in order to monitor pixel fluxes. First, geometric and photometric alignments are applied to each image. Averaging the images of each night reduces significantly the noise level. Second, one light curve for each of the $2.1~10^6$ pixels is built and pixels are lumped into 3.6$\arcsec \times$ 3.6$\arcsec$ super-pixels, one for each elementary pixel. An empirical correction is then applied to account for seeing variations. We find that the final super-pixel light curves fluctuate at a level of 1.8% of the flux in blue and 1.3% in red. We show that this noise level corresponds to about twice the expected photon noise and confirms previous assumptions used for the estimation of the contribution of unresolved stars. We also demonstrate our ability to correct very efficiently for seeing variations affecting each pixel flux. The technical results emphasised here show the efficacy of the Pixel Method and allow us to study luminosity variations due to possible microlensing events and variable stars in two companion papers.http://dx.doi.org/10.1051/aas:1999439. -
Article: Status of EROS (presented by M. MONIEZ)
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2009
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Commissariat à l'énergie atomique et aux énergies alternatives
- Centre d'Etudes de Saclay
Gif-sur-Yvette, Ile-de-France, France
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