We present a suite of cosmological radiation-hydrodynamical simulations of the assembly of galaxies driving the reionization of the intergalactic medium (IGM) at z ≳ 6. The simulations account for the hydrodynamical feedback from photoionization heating and the explosion of massive stars as supernovae (SNe). Our reference simulation, which was

We present results from a Chandra observation of Ross 154, a nearby (2.93 pc) flaring M dwarf (M4.5Ve). During the 57-ksec ACIS-S exposure, a very large flare occurred in which the counting rate increased by over a factor of 100, with an even larger increase in total X-ray power. For the earlier part of the lightcurve, we analyze photon arrival

On the basis of the Galactic O Star Spectroscopic Survey (GOSSS), we present a detailed systematic investigation of the O Vz stars. The currently used spectral classification criteria are rediscussed, and the Vz phenomenon is recalibrated through the addition of a quantitative criterion based on the equivalent widths of the He i λ4471, He ii λ4542

The way in which the effect of the instrumental polarization of a spectrograph can distort the shape of the intensity profiles in sunspots is shown empirically. A spectral region around a magnetically sensitive line and a sunspot away from the disk center were chosen. The resulting limitations which arise when trying to derive physically meaningful

We review recent advances in our understanding of massive stars through the analysis of their spectra. Improvements in model atmospheres and analysis methods are briefly discussed. Results obtained for stars in the Local Group are compared and the present status of different open questions, like the temperature scale of OB stars, the Wind Momentum

Understanding the peculiar properties of ultra-diffuse galaxies (UDGs) via spectroscopic analysis is a challenging task requiring very deep observations and exquisite data reduction. In this work, we perform one of the most complete characterizations of the stellar component of UDGs to date using deep optical spectroscopic data from OSIRIS at Gran