The Na I D resonance lines in main sequence late-type stars

We study the sodium D lines (D1: 5895.92 \AA; D2: 5889.95 \AA) in late-type dwarf stars. The stars have spectral types between F6 and M5.5 (B-V between 0.457 and 1.807) and metallicity between [Fe/H] = -0.82 and 0.6. We obtained medium resolution echelle spectra using the 2.15-m telescope at the argentinian observatory CASLEO. The observations have been performed periodically since 1999. The spectra were calibrated in wavelength and in flux. A definition of the pseudo-continuum level is found for all our observations. We also define a continuum level for calibration purposes. The equivalent width of the D lines is computed in detail for all our spectra and related to the colour index (B-V) of the stars. When possible, we perform a careful comparison with previous studies. Finally, we construct a spectral index (R_D') as the ratio between the flux in the D lines, and the bolometric flux. We find that, once corrected for the photospheric contribution, this index can be used as a chromospheric activity indicator in stars with a high level of activity. Additionally, we find that combining some of our results, we obtain a method to calibrate in flux stars of unknown colour.Comment: 12 pages, including 14 figures and 4 tables. Accepted for publication in MNRA

Thermal Evolution of Neutron Stars in 2 Dimensions

There are many factors that contribute to the breaking of the spherical symmetry of a neutron star. Most notably is rotation, magnetic fields, and/or accretion of matter from companion stars. All these phenomena influence the macroscopic structures of neutron stars, but also impact their microscopic compositions. The purpose of this paper is to investigate the cooling of rotationally deformed, two-dimensional (2D) neutron stars in the framework of general relativity theory, with the ultimate goal of better understand the impact of 2D effects on the thermal evolution of such objects. The equations that govern the thermal evolution of rotating neutron stars are presented in this paper. The cooling of neutron stars with different frequencies is computed self-consistently by combining a fully general relativistic 2D rotation code with a general relativistic 2D cooling code. We show that rotation can significantly influence the thermal evolution of rotating neutron stars. Among the major new aspects are the appearances of hot spots on the poles, and an increase of the thermal coupling times between the core and the crust of rotating neutron stars. We show that this increase is independent of the microscopic properties of the stellar core, but depends only on the frequency of the star.Comment: 8 pages, 6 figures, revised versio

Chromospheric changes in K stars with activity

We study the differences in chromospheric structure induced in K stars by stellar activity, to expand our previous work for G stars, including the Sun as a star. We selected six stars of spectral type K with 0.82$<B-V<$0.90, including the widely studied Epsilon Eridani and a variety of magnetic activity levels. We computed chromospheric models for the stars in the sample, in most cases in two different moments of activity. The models were constructed to obtain the best possible match with the Ca II K and the H$\beta$ observed profiles. We also computed in detail the net radiative losses for each model to constrain the heating mechanism that can maintain the structure in the atmosphere. We find a strong correlation between these losses and \Sc, the index generally used as a proxy for activity, as we found for G stars

Decisão contrária à súmula vinculante : incabimento da suspensão de segurança

ACESSO RESTRITO: Em respeito à Lei de Direitos Autorais, trata-se de documento de uso interno do STJ