Astron. Astrophys. 356, 585-589 (2000)

4. Analysis

4.1. Spectral analysis

The 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. Extinction

Using 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_v3.00.480.06. Schweizer & Middleditch measured the V magnitude from photoelectric photometry as 16.740.02. Therefore, we take the redening corrected magnitude as V₀16.260.07.

4.3. Distance

Since 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_V5.4. However, the error on log g is large (0.3 dex), translating to d1050pc for log g6.48, or d2100pc for log g5.88.

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_V6.4 and d940pc (assuming log g6.18). If M0.1, then M_V7.2 and d650pc.

Fig. 4. NLTE model fit to the H Balmer lines and He lines detected in the SM Star's optical spectrum.

Fig. 5. Position of the SM star in the /log g plane (large cross). The zero-age extended horizontal branch (ZAEHB) and He main sequence are marked. Loci showing how stars of various masses evolve away from the ZAEHB are also shown.

© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000
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