Spectral and luminosity classification for the cool components in symbiotic stars

Z.X. Zhu^1,4, M. Friedjung², G. Zhao^3,4, H.R. Hang^1,4, and C.C. Huang^1,4

1 - Purple Mountain Observatory, 210008 Nanjing, PR China
e-mail: zxzhu@mail.pmo.ac.cn
2 - Institute d'Astrophysique, 98 bis Boulevard, 75014 Paris, France
e-mail: fried@iap.fr
3 - Beijing Astronomical Observatory, Datun Road 20A, 100012, PR China
4 - National Astronomical Observatories, Chinese Academy of Sciences, PR China

Abstract:

The near infrared spectra of 12 S-type symbiotic stars and 78 comparison stars have been observed with moderate dispersion in five runs from 1992 to 1997, the resolving power being $R= \frac{\lambda}{\Delta\lambda}>2000$ , with a signal to noise ratio S/N>100. The triple-headed absorption band of TiO ( $\lambda\lambda$ 8432, 8422 and 8452 Å) emerges when a star is later than M2, and the depth of the TiO absorption band is very sensitive to the spectral type (ST) and insensitive to the luminosity class of the star. We fit a curve of spectral type against the index of the absorption depth of this band with a standard deviation $\sigma=0.37$ of a subdivision of one spectral type. The IR CaII triplet ( $\lambda\lambda$ 8498, 8542, 8662 Å), Fe I 8689 Å, and Fe I 8675 Å are good luminosity indicators although the equivalent widths (EWs) of these lines clearly decrease for a star later than M3. When the star is a supergiant, the lines have a smaller central residual intensity and broader wings than in the case of a normal giant. The Ca II 8662 Å/Fe I 8675 Å and Fe I 8689 Å/Fe I 8675 Å ratios are also good luminosity indicators for K-type giants. The latter is particularly useful when there are abundance anomalies. The metal-poor symbiotic star AG Dra is classified as a Ib or II giant, as is TX CVn, on the basis of Fe I 8689 Å/Fe I 8675 Å. 9 other symbiotic stars containing M-type cool components are classified as giants by direct comparison and quantitative analysis. Due to there being no known good ratio indicator of luminosity for M-type stars in the band studied and because there is no metal abundance data for the symbiotic stars studied by us except for AG Dra, the results for these 9 symbiotic stars are only preliminary. The infrared Ca II triplet of most symbiotic stars clearly varies between the different observing runs. The different luminosity classes given to the same symbiotic star are probably caused by the variability of the lines of ionized elements, while in some cases they are affected by a low metal abundance.

Key words: stars: binaries: symbiotic -- stars: fundamental parameters -- stars: individual: TX CVn, T CrB, AG Dra -- infrared: stars