3 Color-color diagram

Figure 1: Color-color diagram for the components of weak-lined T Tauri stars (WTTS, left panel) and classical T Tauri stars (CTTS, right panel). The almost vertical solid line denotes the main sequence, with the giant branch to the right (Bessell & Brett 1988). The dotted lines are parallel to the reddening vector (indicated for AV = 5 in the lower right of both panels). The cross gives the typical error bars of our observations (see Sect. 4.2). Only in the region labeled "A'' and on the main sequence the observed colors are consistent with photospheric emission, whereas for objects located right to this region circumstellar excess emission is present. The numbers in the right panel denote Haro6-28B (1), CZTauB (2), the infrared companion of XZTau (3), the components of FSTau (4, 5), HNTauA (6) and the infrared companion of UYAur (7).

   
3.1 Presence of circumstellar excess emission

We correct the resolved JHK-photometry for interstellar extinction using the reddening law of Rieke & Lebovsky (1985) and applying the A_V given in Table A.2 to all components of one system.

The resulting dereddened colors are not necessarily stellar colors, because infrared excess emission caused by circumstellar disks is a common phenomenon in T Tauri stars (e.g. Beckwith et al. 1990). For this reason we first consider a subsample of systems that consist of weak-lined T Tauri stars (WTTS). The adopted classification criterion for WTTS is that their H$\alpha$ equivalent width is less than $10~{{\rm\AA}}$ (Herbig & Bell 1988). WTTS are not expected to have prominent disks and their mean excess emission in J, H and K is zero (Hartigan et al. 1995). Almost all components of these systems have colors comparable to main sequence stars (Fig. 1, left panel), as is expected. The only exceptions are the two components of UXTauB and the companion of V773Tau. For components of systems with classical T Tauri stars (CTTS) where significant circumstellar excess emission is expected, the positions in the color-color diagram are much more spread around (Fig. 1, right panel). These colors cannot be referred to stellar photospheres in a simple way.

   
3.2 Infrared companion candidates

It is interesting to look for companions with extreme red colors, because these can be candidates for infrared companions (IRCs). IRCs are objects that are very weak or have even not been detected at optical wavelengths, but dominate the system's brightness in the infrared. They are somewhat puzzling for star formation theory, because some of them appear to be more massive than the optical "primary'' but are at the same time more embedded and less evolved.

At this time 8 IRCs are known (Koresko et al. 1997; Ressler & Barsony 2001). Two of these objects - XZTauB and UYAurB - do indeed show unusual red colors in the color-color diagram (Fig. 1, right panel). Two other known IRCs that belong to our sample - TTauB and Haro6-10 B - are not discussed here, because we could not detect those objects in the J-band. In Fig. 1 we have also indicated CZTauB, Haro6-28B, both components of FSTau and HNTauA as unsually red objects. For these systems additional spatially resolved observations at longer and shorter wavelengths will be necessary to decide if they really contain IRCs.

The best candidate for a new IRC is the companion of FVTau/c that we have observed in H and K, but failed to detect in the J-band. Ghez et al. (1997a) have proposed HBC603B and VWChaC to be IRCs, because these objects were found by their K-band survey, but missed at $\lambda = 0.9\,\mu{\rm m}$ by Reipurth & Zinnecker (1993). We have observed these systems in J and H and did not detect any companion either, which calls for additional observations at longer wavelengths.

Extinction by circumstellar envelopes or edge-on disks is not the only possible explanation for extremely red colors. The objects mentioned in this section may also have a very late spectral type and may even be young brown dwarfs. We will discuss the topic of possible substellar companions in Sect. 6.1.

In any case extremely red objects are not frequent among the companions detected by Leinert et al. (1993) in Taurus-Auriga. This indicates that the observed overabundance of binaries in this SFR compared to nearby main sequence stars is real and not the result of using infrared wavelengths for multiplicity surveys among young stars.