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Astron. Astrophys. 339, 113-122 (1998)

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3. Observations and data reduction

3.1. The sample

Our sample is divided into two sets. A large separation set has been obtained from the literature and concerns binaries with separations larger than [FORMULA]. The second set is made of closer binaries, with separations less than [FORMULA], most of them being closer than [FORMULA].

To construct the large separation set we surveyed the literature, looking for binaries for which polarimetric data are available for each individual component. The lower limit on the separation ([FORMULA] or [FORMULA]AU at the distance of Taurus) is such that no contamination occurs from the other component, while the upper limit set at [FORMULA] ([FORMULA]AU) ensures that there is little contamination by (unbound) visual pairs.

We have selected only measurements taken with the same instrument through the same filters and at the same time for a given binary. This is important to avoid contamination by likely intrinsic polarization variations, which are quite common for TTS (Ménard & Bastien, 1992). Such variations affect P and [FORMULA], and most of the time, the polarization vector wanders back and forth about a given value with a limited amplitude. In the following, we will assume that the values presented in this paper are representative of an average position angle of the polarization. This large separation subsample of binaries is presented in Table 2.


[TABLE]

Table 2. The sample of wide TTS binaries (sep. [FORMULA]). The HBC numbers of the (brighter in V) primaries are boldfaced.


The second data set contains binaries with separation less than [FORMULA], except for one at [FORMULA], and have been chosen from a list published by Mathieu (1994). Stars were selected because of their large excess flux ratio over a photosphere at [FORMULA]m ([FORMULA], see Skrutskie et al., 1990) and/or their high flux at [FORMULA]m (IRAS) and 1.3 mm, suggesting their CTTS nature and indicating the presence of a large amount of circumstellar material, likely in the form of a disk. Table 3 presents the main characteristics of this subsample. The data given in this table were taken in Mathieu (1994) and references therein.


[TABLE]

Table 3. The sample of close TTS binaries (sep. [FORMULA]) with HBC number of the primaries boldfaced when available.
Note: The triple system of UX Tau has been treated as 2 separate binaries with the same primary HBC 43.


3.2. Spectroscopy

We have obtained spectrometric measurements of the close binaries listed in Table 3 (to the exception of V 710 Tau), in order to evaluate the spectral type of each star, assess their nature as CTTS or WTTS, and study the pairing statistics. The observations were performed during the nights of 1995 december 27 & 28 at the Canada-France-Hawaii 3.60m telescope. The Stabilized Image Spectrometer (SIS) was used with a [FORMULA] CCD. The plate scale was [FORMULA] per pixel. The starlight was dispersed from 4000 to [FORMULA]Å using a grism, with a final resolution of [FORMULA]Å/pixel. DK Tau has been measured during the first night with a [FORMULA] slit under subarcsecond seeing. All other sources were measured during the second night with a [FORMULA] slit under [FORMULA] seeing. Observations of the calibration stars Feige 25 and Feige 56 were performed every night. The integration time was 300 s in all cases except for DK Tau (500 s).

Data reduction was performed with the Longslit and Onedspec spectra reduction packages of NOAO/IRAF. The spectra have been corrected for transmission variations using the spectra of Feige 25 and Feige 56. In order to extract the spectrum of each component of the closer binaries, we have fitted two gaussian profiles on every line of the CCD, under the constraint that the width of each gaussian (selected to match the seeing value) must be the same for both components. The results of this extraction procedure are presented in Figs. 2 and 3.

[FIGURE] Fig. 2. Spectra of the sources with similar spectral types. The intensity range has been fixed (units in W.m-2-1), and some of the spectra have been scaled so that the spectral features are clearly visible. The wavelength units are expressed in nm.

[FIGURE] Fig. 3. Same as in Fig. 2 for HK Tau and UX Tau

3.3. Imaging polarimetry

In order to test the method on tighter binaries, we have obtained polarimetric images in the I-band for each star in Table 3, during the night of 1995 November 19, at the 2m Bernard-Lyot Telescope (TBL) of the Pic-du-Midi observatory. An additionnal star, V710 Tau, was also imaged. The camera used at the f/25 Cassegrain focus was equipped with a [FORMULA] TEK CCD. The readout noise was [FORMULA]electrons. The focal plate scale was [FORMULA] per pixel (square field-of-view of [FORMULA] per image). The seeing (FWHM as measured on the reduced images) was [FORMULA], stable over the night. Two filters were used. A plain I-band filter for direct imaging and an I + polaroid sheet sandwich for polarimetric imaging. Three images at 3 different position angles ([FORMULA], [FORMULA] and [FORMULA]) were obtained for polarimetry by rotating the complete filter+camera assembly. This procedure minimizes the intrumental polarization contamination. The instrumental observing conditions were therefore constant over the 3 images. Each binary was observed twice with a given polarization position angle in order to remove the cosmic rays and increase the S/N ratio without saturation. Flat fields were taken for each position angle of the polaroids against the sun-rising sky.

After correcting for bias, flat field and cosmic rays, the photometry of every star in the field for every position of the polaroids was done with the IRAF Daophot package. Then the P and [FORMULA] parameters were determined for every binary component, using the formulae given in Appendix A.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998
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