P. Fouque

Discovery of a Jupiter/Saturn Analog with Gravitational Microlensing

Searches for extrasolar planets have uncovered an astonishing diversity of planetary systems, yet the frequency of solar system analogs remains unknown. The gravitational microlensing planet search method is potentially sensitive to multiple-planet systems containing analogs of all the solar system planets except Mercury. We report the detection of a multiple-planet system with microlensing. We identify two planets with masses of ∼0.71 and ∼0.27 times the mass of Jupiter and orbital separations of ∼2.3 and ∼4.6 astronomical units orbiting a primary star of mass ∼0.50 solar mass at a distance of ∼1.5 kiloparsecs. This system resembles a scaled version of our solar system in that the mass ratio, separation ratio, and equilibrium temperatures of the planets are similar to those of Jupiter and Saturn. These planets could not have been detected with other techniques; their discovery from only six confirmed microlensing planet detections suggests that solar system analogs may be common.

A New Calibration Of Galactic Cepheid Period-Luminosity Relations From B To K Bands, And A Comparison To LMC Relations

Context. The universality of the Cepheid Period-Luminosity relations has been under discussion since metallicity effects have been assumed to play a role in the value of the intercept and, more recently, of the slope of these relations. Aims. The goal of the present study is to calibrate the Galactic PL relations in various photometric bands (from B to K) and to compare the results to the well-established PL relations in the LMC. Methods. We use a set of 59 calibrating stars, the distances of which are measured using five different distance indicators: Hubble Space Telescope and revised Hipparcos parallaxes, infrared surface brightness and interferometric Baade-Wesselink parallaxes, and classical Zero-Age-Main-Sequence-fitting parallaxes for Cepheids belonging to open clusters or OB stars associations. A detailed discussion of absorption corrections and projection facto r to be used is given. Results. We find no significant di fference in the slopes of the PL relations between LMC and our Galaxy. Conclusions. We conclude that the Cepheid PL relations have universal slopes in all photometric bands, n ot depending on the galaxy under study (at least for LMC and Milky Way). The possible zero-point variation with metal content is not discussed in the present work, but an upper limit of 18.50 for the LMC distance modulus can be deduced from our data.

Cepheid period-radius and period-luminosity relations and the distance to the large magellanic cloud

We have used the infrared Barnes-Evans surface brightness technique to derive the radii and distances of 34 Galactic Cepheid variables. Radius and distance results obtained from both versions of the technique are in excellent agreement. The radii of 28 variables are used to determine the period-radius (PR) relation. This relation is found to have a smaller dispersion than in previous studies, and is identical to the PR relation found by Laney & Stobie from a completely independent method, a fact which provides persuasive evidence that the Cepheid PR relation is now determined at a very high confidence level. We use the accurate infrared distances to determine period-luminosity (PL) relations in the V, I, J, H, and K passbands from the Galactic sample of Cepheids. We derive improved slopes of these relations from updated LMC Cepheid samples and adopt these slopes to obtain accurate absolute calibrations of the PL relation. By comparing these relations to the ones defined by the LMC Cepheids, we derive strikingly consistent and precise values for the LMC distance modulus in each of the passbands that yield a mean value of μ0(LMC) = 18.46 ± 0.02. By analyzing the observed dispersions of the PL relations defined by the LMC and Galactic samples of Cepheids, we disentangle the contributions due to uncertainties in the reddenings, in distance measurement, and due to metallicity effects, and we estimate the intrinsic dispersion of the PL relation with the Wesenheit function. Assuming that the Galactic Cepheid distances are typically accurate to ±3% (as shown in a previous paper), and assuming an intrinsic spread in [Fe/H] of ~0.4 dex among the Cepheids of our sample as obtained by Fry & Carney, the observed dispersion of the Galactic Cepheid PL relation suggests a metallicity dependence of Δμ/Δ[Fe/H] ≈ 0.2, about half the value suggested by Sasselov et al. from EROS data. Since this estimate of the metallicity dependence of the PL (V) relation is rather uncertain, however, we prefer to retain μ0(LMC) = 18.46 as our best value, but with an increased uncertainty of ±0.06, most of which is due to the uncertainty in the appropriate metallicity correction. Our results show that the infrared Barnes-Evans technique is very insensitive to both Cepheid metallicity and adopted reddening, and therefore is a very powerful tool to derive accurate distances to nearby galaxies by a direct application of the technique to their Cepheid variables, rather than by comparing PL relations of different galaxies, which introduces much more sensitivity to metallicity and absorption corrections that are usually difficult to determine.

Direct Distances to Cepheids in the Large Magellanic Cloud: Evidence for a Universal Slope of the Period-Luminosity Relation up to Solar Abundance

We have applied the infrared surface brightness (ISB) technique to derive distances to 13 Cepheid variables in the LMC that span a period range from 3 to 42 days. From the absolute magnitudes of the variables calculated from these distances, we find that the LMC Cepheids define tight period-luminosity (PL) relations in the V, I, W, J, and K bands that agree exceedingly well with the corresponding Galactic PL relations derived from the same technique and are significantly steeper than the LMC PL relations in these bands observed by the OGLE-II Project in V, I, and W and by Persson and coworkers in J and K. We find that the LMC Cepheid distance moduli we derive, after correcting them for the tilt of the LMC bar, depend significantly on the period of the stars, in the sense that the shortest period Cepheids have distance moduli near 18.3, whereas the longest period Cepheids are found to lie near 18.6. Since such a period dependence of the tilt-corrected LMC distance moduli should not exist, there must be a systematic, period-dependent error in the ISB technique not discovered in previous work. We identify as the most likely culprit the p-factor, which is used to convert the observed Cepheid radial velocities into their pulsational velocities. By demanding (1) a zero slope on the distance modulus versus period diagram and (2) a zero mean difference between the ISB and ZAMS fitting distance moduli of a sample of well-established Galactic cluster Cepheids, we find that p = 1.58(±0.02) - 0.15(±0.05) log P, with the p-factor depending more strongly on Cepheid period (and thus luminosity) than indicated by past theoretical calculations. When we recalculate the distances of the LMC Cepheids with the revised p-factor law suggested by our data, we not only obtain consistent distance moduli for all stars but also decrease the slopes in the various LMC PL relations (and particularly in the reddening-independent K and W bands) to values that are consistent with the values observed by OGLE-II and Persson and coworkers. From our 13 Cepheids, we determine the LMC distance modulus to be 18.56 ± 0.04 mag, with an additional estimated systematic uncertainty of ~0.1 mag. Using the same corrected p-factor law to redetermine the distances of the Galactic Cepheids, the new Galactic PL relations are also found consistent with the observed optical and near-infrared PL relations in the LMC. Our main conclusion from the ISB analysis of the LMC Cepheid sample is that, within current uncertainties, there seems to be no significant difference between the slopes of the PL relations in the Milky Way and LMC. With literature data on more metal-poor systems, it seems now possible to conclude that the slope of the Cepheid PL relation is independent of metallicity in the broad range in [Fe/H] from -1.0 dex to solar abundance, within a small uncertainty. The new evidence from the first ISB analysis of a sizable sample of LMC Cepheids suggests that the previous, steeper Galactic PL relations obtained from this technique were caused by an underestimation of the period dependence in the model-based p-factor law used in the previous work. We emphasize, however, that our current results must be substantiated by new theoretical models capable of explaining the steeper period dependence of the p-factor law, and we will also need data on more LMC field Cepheids to rule out remaining concerns about the validity of our current interpretation.

The effect of metallicity on the Cepheid Period-Luminosity relation from a Baade-Wesselink analysis of Cepheids in the Galaxy and in the Small Magellanic Cloud

We have applied the near-IR Barnes-Evans realization of the Baade-Wesselink method as calibrated by Fouque& Gieren (1997) to five metal-poor Cepheids with periods between 13 and 17 days in the Small Magellanic Cloud as well as to a sample of 34 Galactic Cepheids to determine the effect of metallicity on the period-luminosity (P-L) relation. For ten of the Galactic Cepheids we present new accurate and well sampled radial-velocity curves. The Baade-Wesselink analysis provides accurate individual distances and luminosities for the Cepheids in the two samples, allowing us to constrain directly, in a purely differential way, the metallicity effect on the Cepheid P-L relation. For the Galactic Cepheids we provide a new set of P-L relations which have zero-points in excellent agreement with astrometric and interferometric determinations. These relations can be used directly for the determination of distances to solar-metallicity samples of Cepheids in distant galaxies, circumventing any corrections for metallicity effects on the zero-point and slope of the P-L relation. We find evidence for both such metallicity effects in our data. Comparing our two samples of Cepheids at a mean period of about 15 days, we find a weak effect of metallicity on the luminosity similar to that adopted by the HST Key Project on the Extragalactic Distance Scale. The effect is smaller for the V band, where we find ∆MV /∆(Fe/H) = −0.21 ± 0.19, and larger for the Wesenheit index W, where we find ∆MW/∆(Fe/H) = −0.29 ± 0.19. For the I and K bands we find ∆MI/∆(Fe/H) = −0.23 ± 0.19 and ∆MK /∆(Fe/H) = −0.21 ± 0.19, respectively. The error estimates are 1 σ statistical errors. It seems now well established that metal-poor Cepheids with periods longer than about 10 days are intrinsically fainter in all these bands than their metal-rich counterparts of identical period. Correcting the LMC distance estimate of Fouque et al. (2003) for this metallicity effect leads to a revised LMC distance modulus of (m − M)0 = 18.48 ± 0.07, which is also in excellent agreement with the value of (m − M)0 = 18.50 ± 0.10 adopted by the Key Project. From our SMC Cepheid distances we determine the SMC distance to be 18.88 ± 0.13 mag irrespective of metallicity.

Limits on Galactic dark matter with 5 years of EROS SMC data

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 10^-7 Msol and 1 Msol at the 95% C.L.

DENIS: A DEEP NEAR-INFRARED SURVEY OF THE SOUTHERN SKY

The DENIS project is the first attempt to carry out a complete digitized survey of the southern sky in the range 1–2.5µm. The main objectives of the programme and the main specifications of the camera and of the data processing stream are briefly outlined

Cepheid distances from infrared long-baseline interferometry III. Calibration of the surface brightness-color relations

The recent VINCI/VLTI observations presented in Paper I have nearly doubled the total number of available angular diameter measurements of Cepheids. Taking advantage of the significantly larger color range covered by these ob- servations, we derive in the present paper high precision calibrations of the surface brightness-color relations using exclu- sively Cepheid observations. These empirical laws make it possible to determine the distance to Cepheids through a Baade- Wesselink type technique. The least dispersed relations are based on visible-infrared colors, for instance FV (V − K) = −0.1336±0.0008 (V − K) + 3.9530±0.0006. The convergence of the Cepheid (this work) and dwarf star (Kervella et al. 2004c) visible-infrared surface brightness-color relations is strikingly good. The astrophysical dispersion of these relations appears to be very small, and below the present detection sensitivity.

Structure, mass and distance of the Virgo cluster from a Tolman-Bondi model

We have applied a relativistic Tolman-Bondi model of the Virgo cluster to a sample of 183 galaxies with measured distances within a radius of 8 degrees from M 87. We nd that the sample is signicantly contaminated by background galaxies which lead to too large a cluster mean distance if not excluded. The Tolman-Bondi model predictions, together with the HI deciency of spiral galaxies, allows one to identify these background galaxies. One such galaxy is clearly identied among the 6 calibrating galaxies with Cepheid distances. As the Tolman- Bondi model predicts the expected distance ratio to the Virgo distance, this galaxy can still be used to estimate the Virgo distance, and the average value over the 6 galaxies is 15:4 0:5 Mpc. Well-known background groups of galaxies are clearly recovered, together with laments of galaxies which link these groups to the main cluster, and are falling into it. No foreground galaxy is clearly detected in our sample. Applying the B-band Tully-Fisher method to a sample of 51 true members of the Virgo cluster according to our classication gives a cluster distance of 18:0 1:2 Mpc, larger than the mean Cepheid distance. Finally, the same model is used to estimate the Virgo cluster mass, which is M =1 :2 10 15 M within 8 degrees from the cluster center (2.2 Mpc radius), and amounts to 1.7 virial mass.

Galactic Bulge microlensing optical depth from EROS-2

We present a new EROS-2 measurement of the microlensing optical depth toward the Galactic Bulge. Light curves of