## 2. Stellar parameters revisitedBefore moving on to the observational results of EW Lac, we will reconsider stellar parameters such as radius and mass for this rapidly rotating star. It is well known (Collins & Sonneborn 1977) that rapid rotators viewed edge-on are shifted in the HR diagram with respect to non-rotators so that they seem less massive and more evolved than they really are. The BCD stellar spectrophotometric parameters
( and
, respectively, the mean position and
value of the stellar Balmer discontinuity) are calibrated in terms of
fundamental stellar parameters (Divan & Zorec 1982, Zorec 1986)
which obviously correspond to the observed hemisphere-averaged
photospheric radiation field. For EW Lac
Å and
dex, which lead to
mag;
mag;
. From them we obtain a first
estimate of stellar radius and mass:
and , which imply a critical
rotational velocity
km s Due to its high , finding out the true nature of the central object of EW Lac means we have to interpret the observed apparent photospheric parameters by taking into account the effects of stellar rotation on spectra. First of all, we note that the Hipparcos distance d = pc, together with the apparent visual magnitude mag measured at the epoch when total Balmer discontinuity was (Moujtahid et al. 1999) and E(B-V) = 0.12 mag, produce a visual absolute magnitude (Hipp) = ] mag, where is the magnitude excess due to the circumstellar envelope. Analysis of EW Lac energy distribution from to (Moujtahid & Zorec 2000) shows that mag. Within the parallax uncertainty, this implies that (Hipp) and thus, that is a good approximation of the radiation field in the V-band emitted by the observed rotationally distorted stellar hemisphere. Knowing that as a function of rotational effects varies in the same way as , we can assume that the emitted bolometric luminosity of EW Lac is reliably given by = . Hence, to relate the observed quantities to those the star would have if it were not rotating, we adopt the following representations: where and
are functions of the angular
velocity ratio and the inclination
angle
Another estimate of as a function
of and
Another straightforward, though less consistent, way of deriving , uses the relation = that leads to an estimate of which gives combined with ). The masses thus obtained produce curves which are similar to the "vertical" ones shown in Fig. 1. The i-averaged parameters thus obtained are and . From these estimations we conclude that the central object of EW Lac most probably corresponds to a B1.5-B2 rotating main sequence star, which contrasts with the spectral type B3 III derived from the apparent photospheric parameters. For this star seen under , we
adopt ,
and
km s It should be kept in mind that the above error bars are obtained adopting Collins & Sonneborn (1977) and Collins et al. (1991) models and Maeder & Meynet (1988) evolutionary tracks. © European Southern Observatory (ESO) 2000 Online publication: October 30, 2000 |