## 4. Analysis## 4.1. Spectral analysisThe H Balmer series is visible in the calibrated optical spectrum (Fig. 1) to H11. HeI is detected at 4026Å, 4144Å, 4472Å and marginally at 4922Å. There is also a marginal detection of HeII at 4686Å. A grid of synthetic spectra derived from H & He line blanketed NLTE model atmospheres (Napiwotzki 1997) was matched to the data to simultaneously determine the effective temperature, surface gravity and He abundance (see Heber et al. 1999). We find 32,900K, log g6.18 and log (N(He)N(H))-1.7. While formal statistical errors from the fitting procedure are relatively small (1: ()=340K, (log g)(log(He/H))0.1dex), systematics dominate the error budget and are estimated from varying the spectral windows for the profile fitting and the continuum setting to be ()1500K, (log g)0.3 dex and (log (N(He)N(H))0.3 dex. These best-fit parameters are unchanged if is omitted from the fit (since it might be contaminated by CaII). A more precise error estimate would, however, require repeat observations. Therefore, we find that both the temperature and gravity are at the low end of the large range estimated by Schweizer & Middleditch (1980). With these parameters the SM star resembles an ordinary subdwarf B star close to the zero-age extended horizontal branch (ZAEHB). ## 4.2. ExtinctionUsing the Matthews & Sandage (1963) calibration, combined with
our model fit parameters, we estimate the colour excess
0.160.02.
From Whitford (1958) we then estimate the visual extinction
A ## 4.3. DistanceSince bolometric corrections for hot subluminous stars are large
and somewhat uncertain, we prefer not to make use of them for the
distance determination. Instead we calculate the angular radius from
the ratio of the observed (dereddened) flux at the effective
wavelength of the V filter and the corresponding model flux. Assuming
the canonical mass for hot subdwarf stars,
M0.5,
we determine the stellar radius from the gravity and finally derive
the distance from the angular diameter and the stellar radius. We
obtain a distance of d1485pc which
corresponds to an absolute magnitude of
M If the SM star has a much lower mass than usually assumed for these
objects, as suggested by Wellstein et al. (1999), then the
absolute magnitude will be lower and hence the star will be much
closer to us. For example, if
M0.2
then we find M
© European Southern Observatory (ESO) 2000 Online publication: April 10, 2000 |