arXiv subjects

E. Aubourg

Publications and source records attributed to E. Aubourg.

51 records · Page 3Linked to original sources

arXiv2003

Bulge Microlensing Optical Depth from EROS 2 observations

We present a measurement of the microlensing optical depth toward the Galactic bulge based on the analysis of 15 contiguous1 square degrees fields centered on (l=2.5 deg, b=-4.0 deg) and containing 1.42 million clump-giant stars (belonging to the extended clump area) monitored during almost three bulge seasons by EROS (Experience de Recherche d'Objets Sombres). We find a microlensing optical depth towards the bulge tau_bulge=0.94 +/- 0.29 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.

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arXiv2003

Cosmological constraints from Archeops

We analyze the cosmological constraints that Archeops places on adiabatic cold dark matter models with passive power-law initial fluctuations. Because its angular power spectrum has small bins in l and large l coverage down to COBE scales, Archeops provides a precise determination of the first acoustic peak in terms of position at multipole l_peak=220 +- 6, height and width. An analysis of Archeops data in combination with other CMB datasets constrains the baryon content of the Universe, Omega(b)h^2 = 0.022 (+0.003,-0.004), compatible with Big-Bang nucleosynthesis and with a similar accuracy. Using cosmological priors obtainedfrom recent non-CMB data leads to yet tighter constraints on the total density, e.g. Omega(tot)=1.00 (+0.03,-0.02) using the HST determination of the Hubble constant. An excellent absolute calibration consistency is found between Archeops and other CMB experiments, as well as with the previously quoted best fit model.The spectral index n is measured to be 1.04 (+0.10,-0.12) when the optical depth to reionization, tau, is allowed to vary as a free parameter, and 0.96 (+0.03,-0.04) when tau is fixed to zero, both in good agreement with inflation.

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arXiv2003

The Cosmic Microwave Background Anisotropy Power Spectrum measured by Archeops

We present a determination by the Archeops experiment of the angular power spectrum of the cosmic microwave background anisotropy in 16 bins over the multipole range l=15-350. Archeops was conceived as a precursor of the Planck HFI instrument by using the same optical design and the same technology for the detectors and their cooling. Archeops is a balloon-borne instrument consisting of a 1.5 m aperture diameter telescope and an array of 21 photometers maintained at ~100 mK that are operating in 4 frequency bands centered at 143, 217, 353 and 545 GHz. The data were taken during the Arctic night of February 7, 2002 after the instrument was launched by CNES from Esrange base (Sweden). The entire data cover ~ 30% of the sky.This first analysis was obtained with a small subset of the dataset using the most sensitive photometer in each CMB band (143 and 217 GHz) and 12.6% of the sky at galactic latitudes above 30 degrees where the foreground contamination is measured to be negligible. The large sky coverage and medium resolution (better than 15 arcminutes) provide for the first time a high signal-to-noise ratio determination of the power spectrum over angular scales that include both the first acoustic peak and scales probed by COBE/DMR. With a binning of Delta(l)=7 to 25 the error bars are dominated by sample variance for l below 200. A companion paper details the cosmological implications.

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arXiv2002

EROS 2 proper motion survey: Constraints on the halo white dwarfs

We are conducting a 377-square-degree proper motion survey in the ~V and I bands in order to determine the cool white dwarf contribution to the Galactic dark matter. Using the 250 square degrees for which we possess three epochs, and applying selection criteria designed to isolate halo-type objects, we find no candidates in a 5500 pc^3 effective volume for old, fast M_V=17 white dwarfs. We check the detection efficiency by cross-matching our catalogue with Luyten's NLTT catalogue. The halo white dwarf contribution cannot exceed 5% (95% C.L.) for objects with M_V=17 and 1<V-I<1.5. The same conclusion applies to a 14Gyr halo composed of white dwarfs with hydrogen atmosphere, as modeled by Chabrier (99).

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arXiv2002

Observation of periodic variable stars towards the galactic spiral arms by EROS II

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. 1,362 objects whose luminosity varies were selected. Among them we identified 9 Cepheids, 19 RR Lyrae, 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 Lyrae is estimated to be $\sim 4.9 \pm 0.3$ kpc undergoing an average absorption of $\sim 3.4 \pm 0.2$ magnitudes. This distance is in good agreement with the one of disc stars which contribute to the microlensing source star population.Our catalogue and light curves are available electronically from the CDS, Strasbourg and from our Web site http://eros.in2p3.fr.

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arXiv2001

Archeops: A High Resolution, Large Sky Coverage Balloon Experiment for Mapping CMB Anisotropies

Archeops is a balloon-borne instrument dedicated to measuring cosmic microwave background (CMB) temperature anisotropies at high angular resolution (8 arcminutes) over a large fraction (25%) of the sky in the millimetre domain. Based on Planck High Frequency Instrument (HFI) technology, cooled bolometers (0.1 K) scan the sky in total power mode with large circles at constant elevation. During the course of a 24-hour Arctic-night balloon flight, Archeops will observe a complete annulus on the sky in four frequency bands centered at 143, 217, 353 and 545 GHz with an expected sensitivity to CMB fluctuations of \~100muK for each of the 90 thousand 20 arcminute average pixels. We describe the instrument and its performance obtained during a test flight from Trapani (Sicily) to Spain in July 1999.

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arXiv1999

Kinematics of LMC stellar populations and self-lensing optical depth

Recent observations give some clues that the lenses discovered by the microlensing experiments in the direction of the Magellanic Clouds may be located in these satellite galaxies. We re-examine the possibility that self-lensing alone may account for the optical depth measured towards the Large Magellanic Cloud (LMC). We present a self-consistent multi-component model of the LMC consisting of distinct stellar populations, each associated to a vertical velocity dispersion ranging from 10 to 60 km/s. The present work focuses on showing that such dispersions comply with current 20 - 30 km/s limits set by observation on specific LMC populations. We also show that this model reproduces both the 1-2 10^-7 observed optical depth and the event duration distribution.

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arXiv1999

Microlensing Optical Depth of the Large Magellanic Cloud

The observed microlensing events towards the LMC do not have yet a coherent explanation. If they are due to Galactic Halo objects, the nature of these objects is puzzling --- half the halo in dark 0.5 Msol objects. On the other hand, traditional models of the LMC predict a self-lensing optical depth about an order of magnitude too low, although characteristics of some of the observed events favor a self-lensing explanation. We present here two models of the LMC taking into account the correlation between the mass of the stars and their velocity dispersion: a thin Mestel disk, and an ellipsoidal model. Both yield optical depths, event rates, and event duration distributions compatible with the observations. The grounds for such models are discussed, as well as their observational consequences.

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arXiv1997

EROS Variable Stars : Discovery of Beat Cepheids in the Small Magellanic Cloud and the effect of metallicity on pulsation

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.

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arXiv1997

Observational Limits on Machos in the Galactic Halo

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.

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arXiv1996

The effect of metallicity on the Cepheid distance scale and its implications for the Hubble constant ($H_0$) determination

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 $H_0 \approx 70 km/s Mpc^{-1}$.

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arXiv1996

Metallicity Effects on the Cepheid Extragalactic Distance Scale from EROS photometry in LMC and SMC

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 those

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arXiv1995

EROS VARIABLE STARS : FUNDAMENTAL-MODE AND FIRST OVERTONE CEPHEIDS IN THE BAR OF THE LARGE MAGELLANIC CLOUD

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.

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arXiv1995

SEARCH FOR VERY LOW MASS OBJECTS IN THE GALACTIC HALO

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}

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arXiv1994

Spectroscopic Studies of the Two Eros Candidate Microlensed Stars

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.

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