Synthetic Mid‐UV Spectroscopic Indices of Stars

Abstract

Using the UVBLUE library of synthetic stellar spectra we have computed a set of mid-UV line and continuum spectroscopic indices. We explore their behavior in terms of the leading stellar parameters (T eff , log g). The overall result is that synthetic indices follow the general trends depicted by those computed from empirical databases. Separately we also examine the index sensitivity to changes in chemical composition, an analysis only feasible under a theoretical approach. In this respect, lines indices Fe i 3000, BL 3096 and Mg i 2852 and the continuum index 2828/2921 are the least sensitive features, an important characteristic to be taken into account for the analyses of integrated spectra of stellar systems. We also quantify the effects of instrumental resolution on the indices and find that indices display variations up to 0.1 mag in the resolution interval between 6-10Å of FWHM. We discuss the extent to which synthetic indices are compatible with indices measured in spectra collected by the International Ultraviolet Explorer (IUE). Five line and continuum indices (Fe i 3000, 2110/2570, 2828/2921, S2850, and S2850L) display a remarkable good correlation with observations. The rest of the indices are either underestimated or overestimated, however, two of them, Mg Wide and BL 3096, display only marginal discrepancies. For 11 indices we give the coefficients to convert synthetic indices to the IUE system. This work represents the first attempt to synthesize mid-UV indices from high resolution theoretical spectra and foresees important applications for the study of the ultraviolet morphology of old stellar aggregates.

Statistical properties of the GALEX spectroscopic stellar sample

“…7. The indices that measure common spectral features with those of Fanelli et al (1992) show similar trends, with respect to T eff , log g, and [M/H], as those calculated with the Fanelli's passbands definition (Rodriguez-Merino 2004;Chavez et al 2007). Apart from the two bluest indices, the values are larger for lower gravities.…”

“…We performed this calibration, as in Chavez et al (2007), by means of a sample of stars, which were observed by GALEX and whose complete set of atmospheric parameters (T eff ,log g,[M/H]) is given in the PASTEL catalog (Soubiran et al 2010). Since the total number of these stars is not large enough to provide a good calibration, we added a sample of IUE star from the Wu et al (1983) catalog.…”

Absorption line indices in theUV

“…Some of the mid-UV indices by Fanelli et al have also been studied in theoretical Kurucz stellar spectra by Chavez et al (2007).…”

Open issues on the synthesis of evolved stellar populations at ultraviolet wavelengths

“…The overall project consists in four main steps, namely a)-the creation of a theoretical stellar database that we have called UVBLUE 3 , b)-the comparison of such data base with observational stellar data, c)-the calculation of a set of synthetic SEDs of SSPs and their validation through a comparison with observations of a sample of Galactic globular clusters, d)-construction of models for dating local ellipticals and distant red galaxies. In Chavez (2009), we presented a summary of the results obtained in steps (a) and (b) and the reader is referred to that paper and the original references for a detailed description of the project (Rodriguez-Merino et al 2005;Chavez et al 2007). In what follows, we elaborate on the third step.…”

“…As quoted by Chavez et al (2007) and Maraston et al (2009), the IUE stellar library has prevailed as the most complete for the analyses of Cimatti et al (2008). Different colors stand for five different metallicities, as described in the figure label.…”