If not stated otherwise we take the systems' magnitudes, interstellar extinction coefficients AV and spectral types from the literature. The references are Kenyon & Hartmann (1995) for Taurus-Auriga, Walter et al. (1994) and also Köhler et al. (2000) for Upper Scorpius, Gauvin & Strom (1992) for Chamaeleon I and Hughes et al. (1994) for Lupus. Possible effects of variability are discussed in Sect. 4.2.2.
Data for the objects in Taurus-Auriga were obtained during several observing runs with the NIR camera MAGIC at the 3.5 m-telescope on Calar Alto from 1993 to 1998. Measurements at this telescope before September 1993 were done with a device for one-dimensional speckle-interferometry that has been described by Leinert & Haas (1989). Observations of multiple systems in southern SFRs were carried out in May 1998 at the ESO New Technology Telescope (NTT) on La Silla that is also a 3.5 m-telescope, using the SHARP camera of the Max-Planck-Institute for Extraterrestrial Physics (Hofmann et al. 1992).
Since most binaries of our sample have projected separations of less than
,
a high angular resolution technique is needed to overcome the
effects of atmospheric turbulence and to reach the diffraction limit which is
for a 3.5 m-telescope at
.
We have mostly used two-dimensional speckle interferometry.
Sequences of typically 1000 images with exposure times of
were taken for the object and a
nearby reference star. After background subtraction, flatfielding and badpixel
correction these data cubes are Fourier-transformed.
We determine the modulus of the complex visibility (i.e. the Fourier transform of the object brightness distribution) from power spectrum analysis. The phase is recursively reconstructed using two different methods: The Knox-Thompson algorithm (Knox & Thompson 1974) and the bispectrum analysis (Lohmann et al. 1983). A detailed description of this data reduction process has been given by Köhler et al. (2000, Appendix A). Modulus and phase show characteristic strip patterns for a binary. In the case of a triple or quadruple star these patterns will be overlayed by similar structures that belong to the additional companion(s).
Fitting a binary model to the complex visibility yields the binary parameters position angle, projected separation and flux ratio. The errors of these parameters are estimated by doing this fit for different subsets of the data. The comparison of our position angles and projected separations with those obtained by other authors that has been done in another paper (Woitas et al. 2001) has shown that systematic differences in relative astrometry are negligible. Differences in resolved K-band photometry have an order of magnitude that can be explained by the variability of T Tauri stars.
Together with the system brightness that is taken from the literature, in most cases the flux ratio determines the components' magnitudes. We present the results of our individual measurements in Table A.1. To reduce the effect of the variability of T Tauri stars we calculate the mean of all resolved photometric observations in one filter obtained by us and other authors (see Appendix A).
Our sample grew out of surveys for multiplicity in star forming regions. With a total of 119 individual components it is of reasonable size. By construction it is largely independent of biases due to duplicity. This makes it well suited for statistical discussions.
From the resolved photometry alone we can check for circumstellar excess emission, search for possible infrared companions, detect contamination by background stars and have some check on whether the components of a binary system are coeval. In the last point we encounter the limitations of our method: the large uncertainty in color, resulting mainly from variability, seriously degrades possible age determinations.
Therefore we use in the HRD known spectral types to derive masses for the components dominating the visual region, and derived masses for the companions on the assumption of coevality. The reliability of these mass determinations profit from our explicit knowledge of duplicity. The presentation of the results starts with those resulting from resolved photometry alone.
Copyright ESO 2001