This Letter reports on the photometric detection of transits of the Neptune-mass planet orbiting the nearby M-dwarf star GJ 436. It is by far the closest, smallest, and least massive transiting planet detected so far. Its mass is slightly larger than Neptune's at M = 22.6 ± 1.9 M ⊕ . The shape and depth of the transit lightcurves show that it is crossing the host star disc near its limb (impact parameter 0.84 ± 0.03) and that the planet size is comparable to that of Uranus and Neptune, R = 25 200 ± 2200 km = 3.95 ± 0.35 R ⊕ . Its main constituant is therefore very likely to be water ice. If the current planet structure models are correct, an outer layer of H/He constituting up to ten percent in mass is probably needed on top of the ice to account for the observed radius.

Abstract. We present new yields of 26 Al from Wolf-Rayet stellar winds based on rotating stellar models which account well for numerous observed properties of massive stars. We study the impacts on the yields of a change of initial mass, metallicity and initial rotation velocity. We also consider the effects of a change of mass loss rates during the Wolf-Rayet phase. We show that for surface rotation velocities during the core H-burning phase matching to the observed ones, the quantity of 26 Al ejected by a star of a given initial mass and metallicity is roughly doubled when the effects of rotation are taken into account. The metallicity dependence of the yield is, on the other hand, very similar to that obtained from non-rotating models. We estimate that at least about 20% to 50% (e.g. ∼0.6−1.4 M ) of the live 26 Al detected in the Milky-Way originates from Wolf-Rayet stellar winds. We show the importance of a good knowledge of the present metallicity gradient and star formation rate in our galaxy for modeling both the variation of the 26 Al surface density with the galactocentric distance and the global contribution of the Wolf-Rayet stellar winds to the present galactic mass of 26 Al.

Gravitationally lensed quasars can be used to map the mass distribution in lensing galaxies and to estimate the Hubble constant H 0 by measuring the time delays between the quasar images. Here we report the measurement of two independent time delays in the quadruply imaged quasar WFI J2033−4723 (z = 1.66). Our data consist of R-band images obtained with the Swiss 1.2 m EULER telescope located at La Silla and with the 1.3 m SMARTS telescope located at Cerro Tololo. The light curves have 218 independent epochs spanning 3 full years of monitoring between March 2004 and May 2007, with a mean temporal sampling of one observation every 4th day. We measure the time delays using three different techniques, and we obtain Δt B−A = 35.5 ± 1.4 days (3.8%) and Δt B−C = 62.6 + 4.1 − 2.3 days ( + 6.5% − 3.7% ), where A is a composite of the close, merging image pair. After correcting for the time delays, we find R-band flux ratios of F A /F B = 2.88 ± 0.04, F A /F C = 3.38 ± 0.06, and F A1 /F A2 = 1.37 ± 0.05 with no evidence for microlensing variability over a time scale of three years. However, these flux ratios do not agree with those measured in the quasar emission lines, suggesting that longer term microlensing is present. Our estimate of H 0 agrees with the concordance value: non-parametric modeling of the lensing galaxy predicts H 0 = 67 (68% confidence level). More complex lens models using a composite de Vaucouleurs plus NFW galaxy mass profile show twisting of the mass isocontours in the lensing galaxy, as do the non-parametric models. As all models also require a significant external shear, this suggests that the lens is a member of the group of galaxies seen in field of view of WFI J2033−4723.

Abstract. We use numerical simulations to test a broad range of plausible observational strategies designed to measure the time delay between the images of gravitationally lensed quasars. Artificial quasar light curves are created along with Monte-Carlo simulations in order to determine the best temporal sampling to adopt when monitoring the photometric variations of systems with time delays between 5 and 120 days, i.e., always shorter than the visibility window across the year. Few and realistic assumptions are necessary on the quasar photometric variations (peak-to-peak amplitude and time-scale of the variations) and on the accuracy of the individual photometric points. The output of the simulations is the (statistical) relative error made on the time delay measurement, as a function of 1-the object visibility over the year; 2-the temporal sampling of the light curves; and 3-the time delay. Also investigated is the effect of long term microlensing variations which must be below the 5% level (either intrinsically or by subtraction) if the goal is to measure time delays with an accuracy of 1-2%. However, while microlensing increases the random error on the time delay, it does not significantly increase the systematic error, which is always a factor 5 to 10 smaller than the random error. Finally, it is shown that, when the time delay is comparable to the visibility window of the object, a logarithmic sampling can significantly improve the time delay determination. All results are presented in the form of compact plots to be used to optimize the observational strategy of future monitoring programs.Key words. gravitational lensing -cosmological parameters -cosmology: observations Measuring time delaysMeasuring time delays in gravitationally lensed quasars is difficult, but not as difficult as it first appeared in the late eighties when the first monitoring programs were started. Obtaining regular observing time on telescopes in good sites was (and is still) not easy and the small angular separations between the quasar images require to perform accurate photometry of blended objects, sometimes with several quasar images plus the lensing galaxy within the seeing disk. COSMOGRAILThe COSMOGRAIL project (COSmological MOnitoring of GRAvItational Lenses), started in April 2004, addresses both issues of carrying out photometry of faint blended sources and of obtaining well sampled light curves of lensed quasars.The project involves 5 telescopes: (1) the Swiss 1.2 m Euler telescope located at La Silla, Chile; (2) the Swiss-Belgian 1.2 m Mercator telescope, located in the Canaria islands (La Palma, Spain); (3) the 2 m robotic telescope of the Liverpool University (UK), also located at La Palma; (4) the 1.5 m telescope of Maidanak observatory in Uzbekistan; and (5) the 2 m Himalayan Chandra Telescope (HCT).All 5 telescopes, and others that will join the collaboration, are used in order to follow the photometric variations of most known gravitationally lensed quasars that are suitable for a determination of H 0 . The sample of...

Aims. To provide the observational constraints required to use the gravitationally lensed quasar SDSS J0924+0219 for the determination of H 0 from the time delay method. We measure here the redshift of the lensing galaxy, we show the spectral variability of the source, and we resolve the lensed host galaxy of the source. Methods. We present our VLT/FORS1 deep spectroscopic observations of the lensed quasar SDSS J0924+0219, as well as archival HST/NICMOS and ACS images of the same object. The two-epoch spectra, obtained in the Multi Object Spectroscopy (MOS) mode, allow for very accurate flux calibration and spatial deconvolution. This strategy provides spectra for the lensing galaxy and for the quasar images A and B, free of any mutual light contamination. We deconvolve the HST images as well, which reveal a double Einstein ring. The mass distributions in the lens, reconstructed in several ways, are compared. Results. We determine the redshift of the lensing galaxy in SDSS J0924+0219: z lens = 0.394 ± 0.001. Only slight spectral variability is seen in the continuum of quasar images A and B, while the C III], Mg II and Fe II emission lines display obvious changes. The flux ratio between the quasar images A and B is the same in the emission lines and in the continuum. One of the Einstein rings found using deconvolution corresponds to the lensed quasar host galaxy at z = 1.524 and a second bluer one, is the image either of a star-forming region in the host galaxy, or of another unrelated lower redshift object. A broad range of lens models give a satisfactory fit to the data. However, they predict very different time delays, making SDSS J0924+0219 an object of particular interest for photometric monitoring. In addition, the lens models reconstructed using exclusively the constraints from the Einstein rings, or using exclusively the astrometry of the quasar images, are not compatible. This suggests that multipole-like structures play an important role in SDSS J0924+0219.