AGB stars in Gaia DR2

Gaia Data Release 2 (DR2; April 25, 2018) provides astrometric and photometric data for more than a billion stars - among them many AGB stars. As part of DR2 the light curves of several hundreds of thousand variable stars, including many long-period variable (LPV) candidates, are made available. The publication of the light curves and LPV-specific attributes in addition to the standard DR2 products offers a unique opportunity to study AGB stars. In this contribution, we present the first results for AGB stars based on the analysis of the Gaia data performed after their release. As an immediate result of the Gaia DR2 LPV database we introduce a new photometric index capable of efficiently distinguishing AGB stars of different masses and chemical properties.

Determination of Teff for Candidate Standard Stars Based on Comparison with Model Atmoshperes

A systematic analysis, as complete as possible, of early type standard stars has been undertaken at the Astronomical Observatory of Trieste, in order to obtain a sequence of carefully determined spectroscopic data to be used in the comparison of “normal” and “chemically peculiar” stars. As an intermediate and necessary step, effective temperatures are derived. The determination of Teff is performed either by comparing observed and computed flux distributions, or by using a calibration of UV-visual photometric index versus Teff. The results for a sample of stars in the spectral type range B2-F8 are presented, and an analysis of the influence of the adopted value of log g on the derived Teff values is reported. As a check on the validity of the results, the Teff and log g values are used to construct a synthetic spectrum which is compared with the IUE high resolution observations of the stars in our list.

An Effective Temperature Calibration of A UV-Visual Photometric Index for “Normal” Non-Supergiant Stars

The determination of the effective temperature of each individual star requires observations of the flux distribution in the largest possible wavelength range. Unfortunately, owing to observational constraints, this requirement severely limits the number of stars for which the direct determination of Teff can be achieved. However, an estimate of Teff for a very large number of stars can be obtained through a calibration of a photometric index versus the stellar effective temperature. The UV-Visual dereddened index, R=log(F 1965/F 5445), has been shown to be suitable for “normal” non-supergiant stars in the spectral type range B5 - F7 (Malagnini et al. 1984). The analysis, with the method described in Malagnini et al. (1983), of stars in the spectral type range B5 - B0, enables us to extend and to refine the already proposed calibration of R versus Teff. By using the complete calibration, the determination of Teff for all the “normal” non-supergiant stars sampled from the 31215 objects in the Catalogue of Stellar Ultraviolet Fluxes (Thompson et al. 1978) is being carried out.

Behaviors of B Star Continua and Absorption Features Determined from the TD1 S2/68 “Ultraviolet Bright Star Spectrophotometric Catalogue” and from Copernicus Spectra

Ultraviolet low resolution spectra of most of the B stars observed with the S2/68 experiment onboard the TD1 satellite in the spectral range 1350 A - 2500 A are computer-investigated in order to define a grid of standard behaviors (J.M. Vreux and J.P. Swings, 1976 Astron. Astrophys., in press). The behavior of the relative intensities of the Balmer and Paschen continua is studied as a function of the spectral type and the visible photometric index Q: both relations are shown to be luminosity dependent. A comparison to theoretical predictions and to previous studies is also presented. A measure of the slope of the pseudo-continuum drawn between ~ λ 1660 A and ~ λ 2550 A is studied in the same way: the effect of luminosity is discussed in connection with recently proposed temperature scales for super-giants. The behavior of the depths of the absorption features at λλ 1550 A, 1940 A, 2000 A, 2055 A, 2105 A, 2340 A and 2395 A with respect to the spectral type, the Q index and the luminosity class is also briefly presented. Special emphasis is given to the absorption features at λ 1550 A in connection to the discussion of the Felll lines in the ultraviolet spectra of early B stars by J.P. Swings, M. Klutz, J.M. Vreux and E. Peytremann (1976, Astron. Astrophys. Suppl. 25, 193). Preliminary results of a study of high resolution Copernicus spectra in the A 1550 A region are also presented: it is shown that the role of Felll remains predominant for stars of spectral type as early as about BIII - BIIII (J.P. Swings and J.M. Vreux, to be published).