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See detailThe stellar population of the Rosat North Ecliptic Pole survey. II. Spectral analysis
Affer, L.; Micela, G.; Morel, Thierry ULg

in Astronomy and Astrophysics (2008), 483

Context: X-ray surveys allow to identify young, main-sequence stars in the solar neighborhood. Young, stellar samples, selected according to their activity, can be used to determine the stellar birthrate ... [more ▼]

Context: X-ray surveys allow to identify young, main-sequence stars in the solar neighborhood. Young, stellar samples, selected according to their activity, can be used to determine the stellar birthrate in the last billion years. The ROSAT North Ecliptic Pole survey (NEP), with its moderately deep sensitivity (fluxes â 10[SUP]-14[/SUP] erg cm[SUP]-2[/SUP] s[SUP]-1[/SUP]), is the best survey, to date, able to sample the intermediate-age (10^8{-}10[SUP]9[/SUP] years) nearby population. The identification process of NEP X-ray sources resulted in 144 X-ray sources having a normal stellar counterpart, with an excess of yellow stars with respect to model predictions. <BR />Aims: We want to determine if these X-ray active stars are young or intermediate-age stars, or active binaries. <BR />Methods: We acquired high-resolution, high signal-to-noise ratio optical spectra, to determine the age and physical properties of the NEP X-ray-detected stellar sources. We measure the (i) lithium abundance using the Li I 6707.8 à line, which is an excellent, youth indicator for our age range of interest; (ii) rotational and radial velocities (through cross-correlation methods); and (iii) chromospheric emission (from Hα and Na I D{_1} and D{_2} lines). <BR />Results: The radial velocities distribution is consistent with that of a young field star population of age 4à 10{^9} yrs, or younger. Rotational velocity measurements imply that our sample is dominated by relatively young or intermediate-age stars, as confirmed by our lithium measurements. <BR />Conclusions: Most of the detected stars probably belong to a young or intermediate-age population. Our measurements suggest that a burst in the stellar birthrate of a factor of four occurred in the last 10[SUP]8[/SUP] years. We cannot, however, exclude the possibility that a small fraction of sources, amongst the fastest of the K-rotators, are old binary systems with tidally-locked rotation. [less ▲]

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See detailA Comparison of Methods for Photospheric Abundance Determinations in K-Type Stars
Affer, L.; Micela, G.; Morel, Thierry ULg et al

in Chemical Abundances and Mixing in Stars in the Milky Way and its Satellites, ESO ASTROPHYSICS SYMPOSIA. ISBN 978-3-540-34135-2. Springer-Verlag, 2006, p. 31 (2006)

We have performed a detailed abundance analysis of six inactive K-type stars using high-resolution optical spectra. We have used three different techniques and compared the results obtained in order to ... [more ▼]

We have performed a detailed abundance analysis of six inactive K-type stars using high-resolution optical spectra. We have used three different techniques and compared the results obtained in order to establish their respective merits and faults. The two spectroscopic methods give consistent results suggesting that non- LTE effects are small, whereas the â mixedâ spectroscopic-photometric method leads to photospheric parameters and abundances systematically lower than those obtained with the other two. We have also determined the starsâ positions in H-R diagrams and made a comparison between the gravities derived from the ionization equilibrium of the iron lines and from the evolutionary tracks: the agreement is reasonably good. [less ▲]

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See detailSpectroscopic determination of photospheric parameters and chemical abundances of 6 K-type stars
Affer, L.; Micela, G.; Morel, Thierry ULg et al

in Astronomy and Astrophysics (2005), 433

High resolution, high -S/N- ratio optical spectra have been obtained for a sample of 6 K-type dwarf and subgiant stars, and have been analysed with three different LTE methods in order to derive detailed ... [more ▼]

High resolution, high -S/N- ratio optical spectra have been obtained for a sample of 6 K-type dwarf and subgiant stars, and have been analysed with three different LTE methods in order to derive detailed photospheric parameters and abundances and to compare the characteristics of analysis techniques. The results have been compared with the aim of determining the most robust method to perform complete spectroscopic analyses of K-type stars, and in this perspective the present work must be considered as a pilot study. In this context we have determined the abundance ratios with respect to iron of several elements. In the first method the photospheric parameters (T_eff, log g, and ξ) and metal abundances are derived using measured equivalent widths and Kurucz LTE model atmospheres as input for the MOOG software code. The analysis proceeds in an iterative way, and relies on the excitation equilibrium of the ion{Fe}{i} lines for determining the effective temperature and microturbulence, and on the ionization equilibrium of the ion{Fe}{i} and ion{Fe}{ii} lines for determining the surface gravity and the metallicity. The second method follows a similar approach, but discards the ion{Fe}{i} low excitation potential transitions (which are potentially affected by non-LTE effects) from the initial line list, and relies on the B-V colour index to determine the temperature. The third method relies on the detailed fitting of the 6162 à ion{Ca}{i} line to derive the surface gravity, using the same restricted line list as the second method. Methods 1 and 3 give consistent results for the program stars; in particular the comparison between the results obtained shows that the ion{Fe}{i} low-excitation potential transitions do not appear significantly affected by non-LTE effects (at least for the subgiant stars), as suggested by the good agreement of the atmospheric parameters and chemical abundances derived. The second method leads to systematically lower T_eff and log g values with respect to the first one, and a similar trend is shown by the chemical abundances (with the exception of the oxygen abundance). These differences, apart from residual non-LTE effects, may be a consequence of the colour-T_eff scale used. The α-elements have abundance ratios consistent with the solar values for all the program stars, as expected for â normalâ disk stars. The first method appears to be the most reliable one, as it is self-consistent, it always leads to convergent solutions and the results obtained are in good agreement with previous determinations in the literature. Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roche de los Muchachos of the Instituto de Astrofisica de Canarias. Table 6 is only available in electronic form at http://www.edpsciences.org [less ▲]

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