3. Effective temperatures, gravities and helium abundances
Detailed analyses of optical spectra are available for Barnard 29 (Conlon et al., 1994), whereas only an estimate of the effective temperature from low resolution IUE spectra (Cacciari et al., 1984, = 24000 K, ) exists for ROA 5701 . Therefore we determined the photospheric parameters of ROA 5701 by analysing the optical CASPEC spectra together with optical photometry (Norris 1974) and low resolution IUE spectrophotometry (SWP07849, LWR06845).
To derive a first estimate of the temperature we dereddened the optical photometry and the IUE data, using the interstellar extinction law of Savage & Mathis (1979). The UBV magnitudes were converted into fluxes, using the conversion given by Heber et al. (1984). The data were then fitted with ATLAS9 (Kurucz 1992) models for [M/H] = -1.5. For a reddening of (Norris, 1974) resp. (Djorgovski, 1993) the best fits were achieved for effective temperatures of 22000 K resp. 24000 K. These results compare very well to those of Cacciari et al. (1984).
For the temperatures given above we fitted ATLAS9 model spectra with solar helium abundances and [M/H] = -1.5 to the Balmer lines and obtained the surface gravities from the best fit. To verify our results we also derived effective temperature, surface gravity, and helium abundances from the Balmer and He I line profiles only. For this purpose we calculated model atmospheres using ATLAS9 (Kurucz 1991, priv. comm.) and used the LINFOR spectrum synthesis program (developed originally by Holweger, Steffen, and Steenbock at Kiel university) to compute a grid of theoretical spectra, which include the Balmer lines to H22 and He I lines. The grid covered the range 10000 ... 27500 K in , 2.5 ... 5.0 in and -2.0 ... -0.3 in at a metallicity of -1. For the actual fit we used the routines developed by Bergeron et al. (1992) and Saffer et al. (1994), which employ a test. The results of all procedures are listed in Table 1. It is clear that the agreement is rather good. For the further analysis we used the mean value of the IUE derived results, i.e. an effective temperature of 23000 K and a log g value of 3.3.
Table 1. Photospheric parameters of our programme stars.
© European Southern Observatory (ESO) 1998
Online publication: October 21, 1998