The spectra are shown in Fig. 1 and the corresponding results are summarized in Table 2, with the relative photometry given as . For some objects, we could also detect H, H, [OI ] and the [SII ]6716,31 doublet in emission (see Appendix A:).
Table 2. Photometric and spectroscopic results for stars in our sample. "-" means that the line is undetected in our spectra. The relative photometry given in parentheses was obtained from our spectra (see Sect. 5). No emission line has been measured in the spectrum of FX Tau at wavelengths shorter than 6500 Å. The last column present our classification of the stars: "W" for WTTS and "C" for CTTS (see text for details).
3.1. Comments on individual binaries
The spectral type of GK Tau B could not be determined due to a poor signal-to-noise ratio but its spectrum does not show strong TiO absorption bands. The spectral type of RW Aur A is undetermined from our spectra because the star is heavily veiled by a hot continuum and does not show any photospheric feature; higher resolution spectra are needed to assess its spectral type, see Basri & Batalha (1990) or Chen et al. (1995).
UY Aur is one of the closest binary in our sample, leading to a possible contamination of the spectrum of the secondary by that of the primary. We have checked this point by performing careful cuts through the UY Aur spectrum perpendicular to the dispersion axis. These cuts show a systematic asymmetry, which position does not change with wavelength and is not observed on the primary of any other system, even if it is observed with the same position of the slit. Furthermore, the separation we infer from the spectra () is fully consistent with the result of near-infrared imaging (Close et al. 1998) and the resulting spectrum of the secondary displays different spectral features than the primary.
In the case of FX Tau, the raw spectrum clearly shows two separated peaks, but they are very close (the seeing was about FWHM). The double gaussian fitting procedure was unsuccessful, and we had to apply a line by line deconvolution process. The seeing is slightly better at longer wavelength and we could retrieve both components in this part of the spectra only. They show significantly different features, so we believe that we have resolved the system. This is enough to measure the H emission and to estimate the spectral type, though with a larger uncertainty (2 subclasses).
Optical spectra of the GG Tau/c binary were obtained by White et al. (1999), who found spectral types M5 and M7 for the primary and secondary, respectively. This is in agreement with our findings for both components, although we could not determine accurately the spectral type of the secondary.
We have also determined an accurate estimate of the separation of HBC 356-357: . Walter et al. (1988) reported a somewhat larger separation (2"). However, these authors did not publish the uncertainty on their result, and we believe that this discrepancy is unlikely to be due to orbital motion.
In order to study the relative accretion activity of the individual components of the binaries of our sample, we first determined which stars actually accrete, i.e. the respective classification of the observed stars as CTTS or WTTS. In the following, we use every available piece of information to establish this classification.
3.2. TTS classification criteria
The first large scale surveys for TTS were objective prism surveys and the "historical" criterion to detect a CTTS used the H EW by checking whether it was larger than 10 Å or not (e.g. Strom et al. 1989). The stars identified as TTS from their photometry, but with smaller H EWs were classified as WTTS, i.e. non-active PMS stars. However, this threshold is not a sharp edge, and a more physically meaningful diagnosis would be the H flux (Cohen & Kuhi 1979). Moreover, Martín (1998) discussed the possibility that the H EW threshold varies with spectral type, later spectral types stars having a higher threshold. He proposed a 5 Å EW limit for K stars and 10 Å for early M stars. We adopt this criterion in our classification.
We have also checked this classification against other criteria such as [OI ] emission line and infrared excess. Edwards et al. (1993) have found that all stars with detectable [OI ] emission or excess () systematically have H EWs larger than 10 Å.
However, in order to compare our newly classified TTS with previously known field TTS, the use of different criteria may lead to confusion and unexpected biases. This point will be carefully examined below (see Sect. 4.2), but we stress that only one star out of the 31 listed in Table 2 has a discrepant classification when using different criteria (see also Sect. 4.2.1).
3.3. Classification of individual stars
NTTS 040012+2545, 040047+2603, LkCa 7 and J 4872: no component in any of these systems shows evidence of accretion activity, as they all exhibit only low H emission and no other emission line, to the exception of HBC 358. These four systems are thus considered as WW binaries.
GK Tau, IT Tau, UY Aur, and RW Aur: all of these systems contain stars with moderate to strong H and H emission and, for some of the stars, metallic and forbidden lines. All the stars in these systems can thus be safely classifed as CTTS.
FX Tau: the secondary shows a very low H emission and is probably a WTTS. On the other hand, the primary shows a moderate emission in this line, as well as H emission (Cohen & Kuhi 1979). Furthermore, Strom et al. (1989) and Skrutskie et al. (1990) reported moderate and excesses for the system. All these evidences support the idea that the primary is a CTTS.
UX Tau and HK Tau: UX Tau A was observed in Paper I and classified as a WTTS from its H EW of Å. In such a star apparently "at the border" between C and WTTS, we reexamined this classification and propose to (re)classify it as a CTTS, because of the large H emission flux (for a spectral type K4, the CTTS threshold is only about Å). Another clue is its significant excess (Skrutskie et al. 1990). This post facto reconsideration of classification criteria can be misleading at first sight, but we stress that it is only done here to define a more accurate picture of an accreting TTauri star. The classification of all other stars is identical to Paper I. In particular, our previous classification of HK Tau B as a CTTS (H EW of 12.5 Å) has been recently confirmed by an HST image of this star showing a remarkable edge-on circumstellar disk (Stapelfeldt et al. 1998).
© European Southern Observatory (ESO) 1999
Online publication: November 16, 1999