5. Differential accretion in CC binaries
For all binaries where both stars have active accretion disks (CC pairs), we have used the available spectra to compare the accretion activity of each component, using their H flux as an accretion diagnosis. The H EW has already been shown to correlate well with the infrared excess in CTTS (eg. Edwards et al., 1993). Moreover, recent studies in the near infrared where the extinction is about ten times smaller than at H wavelengths, have revealed tight correlations between the accretion luminosity and the Pa and Br emission fluxes (Muzerolle et al. 1998). We will then hereafter use the ratio of H fluxes in binaries, assuming that this flux is proportional to the energy dissipated in the accretion process, i.e. to the accretion luminosity.
We also assume that the extinction toward both components of the binary is the same, based on the tight correlation observed in the data of H94 between toward the primary and the secondary. We checked that this correlation is still valid at smaller separations: we evaluated a rough photometry from our spectra and compared the results to dwarfs colors. Due to observational uncertainties (mag and 1 subclass for the spectral type), the final accuracy of the extinction is rather poor (typically, mag). However, we did not find any evidence that the correlation is modified. This correlation is likely due to the fact that both components of a binary system are equally embedded in the Taurus molecular cloud, but other explanations include the existence of a common circumbinary envelope and/or of circumstellar disks with similar orientations. Brandner & Zinnecker (1997) reported a similar correlation for close (AU) PMS binaries in southern SFRs.
For each binary, the ratio of the H luminosities is computed as follows:
where is the nearby continuum flux estimated from our spectra when available and from H94's R photometry otherwise.
Fig. 2 shows a clear trend for the primaries to have a higher H flux than the secondaries thus a higher accretion luminosity. It is unlikely that this result is the consequence of a systematic bias introduced by the assumption that both extinctions are the same. This would imply that we systematically overestimated the H luminosity ratios by a factor of 4, which requires that the extinctions toward the secondaries are larger by about 1.5 mag. Such a systematic trend would have been detected in H94 (the authors quote mag), as well as in our data. This suggests that the accretion rate is larger on the more massive component of the system.
In a few cases, clues exist that the photosphere is not seen directly, and that we only detect scattered light. This is the case for UY Aur B (Close et al. 1998) and HK Tau B (Stapelfeldt et al. 1998) where an edge-on disk has been recently detected. For these stars, the observed photometry is therefore only a lower limit to their actual flux, and we consequently underestimated their H flux. Arrows have been accordingly added in Fig. 2. No nebular structure is seen at high angular resolution around any other star so far and we assume that there is no object with strong scattering in our sample appart from these two stars.
© European Southern Observatory (ESO) 1999
Online publication: November 16, 1999