5 Extinction and stellar parameters

Most of the time RW Aur A is probably seen through foreground circumstellar dust, and it is difficult to derive any precise values of the interstellar extinction to the star. We have integrated all FUV spectra obtained with the International Ultraviolet Explorer (IUE) of RW Aur A covering the spectral region with the interstellar 2200Å signature. The integrated spectrum shows a number of blended emission lines, and we can only set a rather high upper limit of the average extinction of $A_V \leq 0\hbox{$.\!\!^{\rm m}$ }7$. Ghez et al. (1997) give $A_V = 0\hbox{$.\!\!^{\rm m}$ }3$ as a possible upper limit. We have taken one spectrum of RW Aur B, $1\hbox{$.\!\!^{\prime\prime}$ }4$ from A, showing only weak traces of interstellar Na D absorption. The total equivalent width of these lines amount to 0.2Å, also consistent with a low interstellar extinction to RW Aur. Hence, we assume that the interstellar extinction to RW Aur is low, which is consistent with its location outside any molecular cloud boundary.

When the star is brightest and bluest, we expect the circumstellar extinction to be minimal. A minimum of $B-V=0\hbox{$.\!\!^{\rm m}$ }45$ was observed on HJD 2450386 (see Table 1). The colour is too blue for a K dwarf, indicating the presence of a hotter continuum source also responsible for the veiling. On this occasion, the star had $V=9\hbox{$.\!\!^{\rm m}$ }95$ and $\rm veiling=1.5$. Taking into account the correction for the contribution of emission lines ( $0\hbox{$.\!\!^{\rm m}$ }2$ in V) and the correction for the veiling ( $2.5\,\log(1+{\rm veiling})$), we find a corrected $V=11\hbox{$.\!\!^{\rm m}$ }14$. With the distance of 140pc (Elias 1978) and $A_V = 0\hbox{$.\!\!^{\rm m}$ }3$, the absolute magnitude is $M_V= 5\hbox{$.\!\!^{\rm m}$ }11$. Using a bolometric correction appropriate for the spectral types K1-K4, which is BC  $= 0\hbox{$.\!\!^{\rm m}$ }2 \ldots 0\hbox{$.\!\!^{\rm m}$ }5$, we get $M_{\rm bol}=4\hbox{$.\!\!^{\rm m}$ }9...4\hbox{$.\!\!^{\rm m}$ }6$, which corresponds to $L/L_{\odot}=0.85 \ldots 1.1$. The derived luminosity represents a lower limit, since circumstellar extinction can be present.

From the spectral type range and the luminosity we can estimate the radius of the star as $1.3 \ldots 1.5\,R_{\odot}$. Since the luminosity is a lower limit, the radius is also a lower limit.

With the lower limit of the luminosity, and with the range in spectral type, we obtain a lower limit for the mass of the star of about 1.1$M_{\odot}$according to evolutionary model tracks (e.g. Palla & Stahler 1999). Notice, that with these values of mass and radius, the free-fall velocity (starting from a distance of 10 stellar radii) is about 500kms^-1 at the stellar surface. This is in a good agreement with the observed maximum accretion velocity of 400kms^-1.