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Astron. Astrophys. 346, L41-L44 (1999)

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2. Observations

2.1. X-ray properties

The X-ray data were obtained from the ROSAT all-sky survey in [FORMULA] fields centered on each one of the known TTS TW Hya, CoD-29o8887 and Hen(3)-600. The three fields slightly overlap to cover the region between the three T Tauri stars. The reduction of the X-ray data was performed following the pipelines of the Extended Scientific Analysis System (EXSAS) as described in Alcalá et al. (1995). Some 50 X-ray sources were detected. The three T Tauri stars TW Hya, CoD-29o8887 and Hen(3)-600, were all detected with a high confidence level. In order to search for candidate TTS of the X-ray sources detected, intermediate resolution spectroscopic observations were carried out using the Boller & Chivens spectrograph attached to the ESO 1.5m telescope at La Silla, Chile. Only four stars out of the 71 investigated showed Lithium absorption, late spectral type and/or H[FORMULA] emission, and hence, could be classified as new wTTS candidates. The rest of the X-ray sources can be identified with unrelated active stars. The X-ray properties of the wTTS candidates are listed in Table 1, as well as their maximum likelihood of detection [FORMULA], hardness ratios (HR1 and HR2), and X-ray fluxes, derived using an energy conversion factor of [FORMULA] ergs cm-2 cnts-1 (Schmitt et al. 1995). The optical counterpart of RXJ1121.3-3447 turned out to be a visual binary (see below). Spectra of both components could be obtained.


Table 1. X-ray properties of the new wTTS candidates.

2.2. High resolution spectroscopy

High resolution spectroscopic observations of the four wTTS candidates were performed using the Cassegrain Echelle Spectrograph (CASPEC) attached to the ESO 3.6m telescope in February 1998. The spectral range of the CASPEC spectra is from 5350 to 7720 Å  and their nominal resolving power, resulting from the measurements of the FWHM of several well isolated lines in the ThAr comparison spectrum, is [FORMULA] 22,000.

The CASPEC spectra in the Sodium and in the H[FORMULA] - Li ranges are shown in Fig. 1. With the same instrumental set-up we have also observed TW Hya, and display its spectrum as comparison. The spectral types assigned from the mid-resolution spectra and the lithium equivalent widths as well as radial and projected rotational velocities are also indicated in the lower panel. The latter three quantities were derived from the CASPEC spectra using the procedures described by Covino et al. (1997). Note that the intensity of the lithium absorption line is stronger than that of the Calcium line in the four stars. Except for the G3 type star, the other three show a typical chromospheric H[FORMULA] emission. The M type stars show evidence of He I ([FORMULA]5876 Å) in emission, as well as of the reversal emission in the Na I D lines, which are characteristics of an active chromosphere, similar to those identified in TW Hya itself.

[FIGURE] Fig. 1. High resolution spectra of the four new wTTS candidates (and TW Hydra itself as comparison) in the Sodium (upper panel) and the H[FORMULA] - Lithium range (lower panel). The spectra of the lower panel are plotted in the same order as those in the upper panel in which the RXJ names are indicated.

An estimate of the effective temperature of these stars has been performed using the calibrations between the Na I D lines equivalent width and log[FORMULA] for the G type star (Tripicchio et al. 1997) and the K I ([FORMULA]7699Å) line equivalent width versus log[FORMULA] for the M stars (Tripicchio et al. 1999).

In Table 2, the spectral types and the quantities derived from the high-resolution spectra are summarised. The spectral types and the derived effective temperatures are consistent, within the errors, when using the calibration between spectral types and effective temperatures (de Jager & Nieuwenhuijzen 1987). Lithium abundances in the log[FORMULA] scale were derived for the four wTTS candidates and TW Hya from the W(Li) and [FORMULA] values using the non-LTE curves of growth given by Pavlenko & Magazzù (1996), conservatively assuming log[FORMULA].


Table 2. Optical properties of the new wTTS candidates and, for comparison, of TW Hya.

2.3. Adaptive optics imaging

In order to study the stellar environment of the new wTTS candidates on spatial scales down to [FORMULA] AU ([FORMULA] at 50 pc distance) we have performed adaptive optics imaging using ADONIS at the ESO 3.6 m telescope on 8. and 9. Jan. 1999. We used a [FORMULA]-filter ([FORMULA]m) to search for close companions to the new TTS, where in case of a positive detection we performed additional [FORMULA]m) and [FORMULA]m) band imaging. The companion to RXJ1121.3-3447 was easily detected on our near-infrared images. It is not a really close system, but separated by [FORMULA], having a measured brightness difference of [FORMULA], [FORMULA], and [FORMULA]. The position angle determined from the images is [FORMULA] degree. No companions were detected for the other two targets. For RXJ1109.7-3907 we estimate that potential companions separated from the primary by [FORMULA] or more have to be fainter than [FORMULA], while at separations of [FORMULA] and [FORMULA] potential companions must be fainter than [FORMULA], or [FORMULA] respectively, to be undetected in our images. Due to the faintness of star RXJ1121.1-3845 in the optical, wavefront sensing was difficult, i.e. only very low order adaptive optics corrections could be applied, and thus the sensitivity to close companions is worse than for the other objects. Essentially, companions to RXJ1121.1-3845 separated by less than [FORMULA] remain undetected. At a separation of [FORMULA] ([FORMULA]) our detection limit is [FORMULA] (1.0).

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© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999