4. CTTS - WTTS pairing within Taurus binaries
In the following, we call "twins" the systems where the TTS are of the same type (either CC or WW), and "mixed" the systems where the stars are different.
One of our objects contains stars physically associated in a multiple system (UX Tau). Although this system is not strongly hierarchical ( - ), we consider that it can be split into two "independent" binaries, leading to a total of 16 binaries in our sample. The validity of this assumption will be evaluated in Sect. 4.2.4.
4.1. Testing the random pairing hypothesis
The 16 binaries considered here can be divived into three categories: 9 binaries contain only CTTS (%), 4 are formed of two WTTS (25%), and 3 are mixed systems, all with a CTTS primary and two of them in the same triple system, UX Tau, representing less than 19% of our sample. Mixed systems appear to be rare in TTS binaries, and this is even more striking when we use the "historical" H Å EW criterion. Then only one mixed system remains among 16 binaries, and the proportion drops to about 6%. We use this sample to address the question: are binary components taken at random from the TTS population?
If we want to compare this result with a distribution randomly taken from a single TTS population, we need to know the ratio of WTTS-to-CTTS in Taurus. In a study limited to the central parts of the Tau-Aur dark cloud, Hartmann et al. (1991) found a ratio close to unity. Considering a larger sky area leads to an even larger WTTS-to-CTTS ratio, mainly because of the widespread ROSAT population (e.g. Wichmann et al. 1996). Since our sample mostly contains systems in the center of the molecular cloud, we conservatively adopt a W/C ratio .
Taking a fixed distribution of primaries (4 WTTS and 12 CTTS), the probability to get 3 mixed systems out of 16 binaries from randomly taken secondaries is . We therefore reject the hypothesis that components of TTS binaries are randomly associated from the distribution of single stars. In other words, the TTS types of Taurus binary components are significantly correlated.
4.2. Possible sources of bias
In this section, we discuss some possible sources of errors in our result on a preferential CC pairing in TTS binaries.
4.2.1. The use of different classification criteria
In Sect. 3.2, we have used complementary criteria to establish the C/W TTS nature of our sources. However, considering only the "historical" ÅH EW classification criterion does not only imply minor changes (1 star in UX Tau is modified upon 31), but makes mixed binaries even more rare: only one mixed pair (FX Tau) out of 16 remains. The probability that the observed C/W distribution in our 16 binaries results from random pairing then falls to .
4.2.2. The case of WW pairs
The evolutionary status of the WTTS population identified from the ROSAT All-Sky Survey is somewhat uncertain: some of these stars may be unrelated to the TTS population (e.g. Favata et al. 1997). If they are young main sequence stars, we expect that both components will mimic WTTS since they are too old to still be accreting. Then the observation of such binaries can lead to a bias towards WW pairs in our study. This can potentially affect 2 binaries in our sample, which were first detected by ROSAT (their names starts with "NTTS"). If we exclude all WW binaries for safety, we end with at most 3 mixed systems out of 12, yielding a proportion of 25% mixed systems in TTS binaries. Then the probability that this distribution results from random associations is only . We therefore conclude that the high proportion of twin binaries in our sample is not strongly affected by the presence of spurious WW binaries.
4.2.3. Time evolution
Since the proportion of stars surrounded by a circumstellar disk decreases with age, we inspect the possibility that our binary population is younger, on average, than the population of singles. In such a case, we would expect to find more CTTS ("young and active") than in the singles sample and, consequently, more CC binaries. In Simon & Prato's (1995) study, the median age of their single stars sample is . In our sample, we find that half of the primaries that have an age assigned by Simon & Prato are older than this value. We thus conclude that our study includes about as many young systems as old systems, and time evolution effects do not impinge our conclusion.
4.2.4. Close companions and hierarchical systems
The issue of how to treat known binaries which we do not resolve is not straightforward. Moreover, currently undetected companions may exist around some of the stars in our sample. These unresolved companions may strongly impact on the evolution and the accretion history of their associated star. Furthermore, considering a triple system as two independent binaries may not be a valid hypothesis.
To evaluate the impact of such multiple systems, we considered a subsample of binaries where no third component, either spectroscopic (Mathieu 1994), very tight visual (Simon et al. 1995) or wider, is known so far. To our knowledge, only 7 binaries in our overall sample match this criterion: LkCa 7, FX Tau, DK Tau, HK Tau, IT Tau, HN Tau and UY Aur. This subsample contains 6 twins and 1 mixed binaries. Once again, only about of these binaries are mixed, leading us to think that the possible existence of additional companions does not significantly modify the results.
4.3. Complementary results from the literature
We have considered previous results in the literature providing information on the classification of the components of more PMS binaries in the Taurus SFR. We complement our results with those of H94 and PS97 and obtain a sample that contains over 90% of all known binaries located in Taurus in the separation range -. The list of these supplementary objects is given in Table 3.
To classify the members of binaries studied by H94, we used their H EWs and spectral types together with the estimated near-infrared excesses. Both indicators agree well for all stars except for V710 Tau S. This star presents an H EW hardly above the classical limit (11 Å), with a spectral type M3, and no infrared excess. Moreover, Cohen & Kuhi (1979), measured an H EW of 3.3 Å and no emission in the forbidden lines, leading us to classify this star as a WTTS. V710 Tau consequently happens to be one of the few mixed pairs (CW) among TTS binaries.
For the two binaries studied by PS97 and included in our sample, all stars have mag. Such high values are strong evidences for the presence of an optically thick accretion disk in the inner 0.5 AU around each star (the upper limit for photospheric colors is mag, Edwards et al. 1993), so that these stars can be safely classified as CTTS. It is also worth mentionning that for all systems common to the PS97's sample and ours (DK Tau and UY Aur with KL photometry and Haro 6-37 with Br spectroscopy), their classification and ours are fully consistent.
If we take these complementary results into account, we obtain a sample of 26 binaries with 15 CC twins, 7 WW twins, and 4 mixed. The proportion of mixed systems is then , and even only if we adopt the H EW criterion, yielding similar results as in Sect. 4.1.
© European Southern Observatory (ESO) 1999
Online publication: November 16, 1999