We report differential I-band and J-band photometry of S Ori 45, a cool (spectral type M8.5), young (1-8 Myr) brown dwarf of the sigma Orionis cluster with a likely mass estimated at around 20 times the mass of Jupiter. We detect variability (amplitudes ranging from 34 to 81 mmag) and observe a modulation at a period of 2.5-3.6 h in both optical

We discuss the effects of collisional enhancement of Balmer lines on the determination of the primordial helium abundance. To this aim, we present a photoionization model of the metal-poor extragalactic H 0 region SBS 0335-052. We show that the derived helium abundance (Y) of this H 0 region depends on the amount of collisional excitation affecting

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We have used William Herschel Telescope/INGRID Ks images on two high-latitude fields, the Coppi and Groth strips, to obtain galaxy number counts over ~180 arcmin2, to a depth of Ks~21.0. Detection efficiency corrections as a function of object size have been calculated on each pointing. We have used a signal-to-noise threshold in two complementary

We use a high-resolution collisionless simulation of a Virgo-like cluster in a ΛCDM cosmology to determine the velocity and clustering properties of the diffuse stellar component in the intracluster region at the present epoch. The simulated cluster builds up hierarchically and tidal interactions between member galaxies and the cluster potential

In Tingley (cite{tingley}), all available transit detection algorithms were compared in a simple, rigorous test. However, the implementation of the Box-fitting Least Squares (BLS) approach of Kovács et al. (cite{kovacs}) used in that paper was not ideal for those purposes. This letter revisits the comparison, using a version of the BLS better