Be stars in open clusters - II. Balmer line spectroscopy

Astronomy and Astrophysics Supplement Series, Aug 1997

We present Hα line profiles (, at continuum level, for some objects) for 58 stars in 8 northern open clusters and associations, complemented with Hβ and Hγ profiles for 36 of them as well as Hδ and Hϵ for 16 programme stars. Our observations, though lower in resolution, yield results which are in good agreement with those obtained in earlier works based mainly on field Be stars. Our data suggest a similar spectroscopic behavior for both field and cluster Be stars. The line parameters (EW, FWHM, ) have been obtained for each line. Nearly the whole set of stars have previously published quasi-simultaneous photometry, so a study of reliable correlations between photometric and spectroscopic data can be undertaken.

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Be stars in open clusters - II. Balmer line spectroscopy

Astron. Astrophys. Suppl. Ser. Be stars in open clusters J.M. Torrejon 1 2 J. Fabregat 1 2 G. Bernabeu 0 1 S. Alba 1 2 0 Departamento de Ingenier a de Sistemas y Comunicaciones, Universidad de Alicante, Apdo. 99, 03080 Alicante , Spain 1 Send o print requests to: J.M. Torrejon 2 Departamento de Astronom a y Astrof sica, Universidad de Valencia , 46100 Burjassot, Valencia , Spain We present H line pro les (R ' 3 103, the photospheric absorption lines are lled-in with an unS=N ' 102 at continuum level, ' 50 for some objects) known amount of circumstellar emission in such a way for 58 stars in 8 northern open clusters and associations, that it is very di cult to deduce either the spectral type complemented with H and Hγ pro les for 36 of them as of the underlying star or the actual amount of the emitwell as H and H for 16 programme stars. Our observa- ting material, the line pro les themselves are distorted and tions, though lower in resolution, yield results which are the photometric indices contaminated. As a consequence in good agreement with those obtained in earlier works we cannot apply the usual photometric and spectroscopic based mainly on eld Be stars. Our data suggest a similar techniques to derive the astrophysical parameters of the spectroscopic behavior for both eld and cluster Be stars. underlying star. lines; pro les | stars; emission line; be | open clusters and associations; general 1. Introduction Be objects are de ned as stars of spectral type B, and luminosity classes III to V, whose spectra show, or have shown at some time, Balmer (and sometimes other) lines in emission. The origin of this emission is atomic transitions in a circumstellar envelope, relatively cool ( 104 K) and relatively dense ( 1011 cm−3) with a composition similar to that of the stellar photosphere. The contribution of the circumstellar envelope has dramatic e ects upon the observables: the slope of the continuum is distorted, Our goal is to elaborate a method to determine the intrinsic parameters of Be stars from uvby photometry and Balmer line spectroscopy. A rst attempt to develop such a method was the work of Fabregat & Reglero (1990) based on published studies of emission properties of circumstellar disks around Be stars. However the photometry and the spectroscopy were not simultaneous. Since Be stars are known to be highly variable some uncertainty is expected in the derived relationships. Thus, the latter authors consider their calibration to be preliminary. With this in mind we planned to build a set of stars with high quality photometry and spectroscopy, as simultaneous as possible and with accurate values of reddening and distance modulus. This latter requirement was ful lled by observing Be stars with well established membership of open clusters so that both interstellar reddening and distance modulus can be inferred from that of the cluster (Mermilliod 1982; Slettebak 1985) . In Fabregat et al. (1996) , hereafter called Paper I, we presented the photometric measurements along with a careful analysis of the data. One of the main results obtained was the observation of a clearly di erent photometric behavior between Be stars earlier than B5V and later ones. This fact, of course, is expected to contribute to the uncertainty of the above mentioned calibration. In the present paper we present the spectroscopy and analyze the data as far as the available resolution makes it possible, mainly in order to guarantee the homogeneity of the sample. Further work will include the elaboration of a calibration valid for Be type stars, the determination of astrophysical parameters for circumstellar envelopes and their modeling. 2. The observations and reduction procedure We carried out two observing runs during the months of December 1992 and August 1993, both with the Cassegrain spectrograph attached to the 2.2 m telescope at the Calar Alto Observatory, Almer a, Spain. The detector used in both runs was the GEC#15 CCD. The two instrumental arrangements were di erent, giving slightly di erent resolutions. For the rst con guration, we used the f =3 camera, at 0.66 A/pixel spectral dispersion, giving a resolution of about 1.5 A at 6000 A. For the latter arrangement we used the f =1:5 camera which produces a spectral dispersion 1.36 A pixel−1 giving a resolution of about 2.7 A, at 6000 A. The red spectra were centered on the H line at 6563 A. The blue spectra included only H (4861 A) and Hγ (4340 A) in the rst run, while in the second one we extended the range from H to the Balmer discontinuity ( 3647 A). The spectra were reduced by means of the Starlink FIGARO software package (Shortridge 1991). Several bias frames were taken during the night, their stability was checked and then they were averaged, before subtraction from the program frames. No drift in dark current was found after several hours of operation, and at di erent time exposures. The flat elds were recorded at the end of the night. A smooth functio (...truncated)


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J. M. Torrejón, J. Fabregat, G. Bernabeu, S. Alba. Be stars in open clusters - II. Balmer line spectroscopy, Astronomy and Astrophysics Supplement Series, 1997, pp. 329-347, Volume 124, Issue 2, DOI: 10.1051/aas:1997359