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

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1. Introduction

The classification of TW Hydrae as a classical T Tauri star (cTTS) by Rucinski & Krautter (1983) has challenged the conventional picture of star formation, because (i) TW Hydrae is located far from any dark cloud or concentration of molecular material and (ii) appears to be far from other pre-main sequence (PMS) stars. Dubbed as isolated TTS the origin of TW Hya remained puzzling, even after de la Reza et al. (1989) and Gregorio-Hetem et al. (1992) identified four other T Tauri systems in the same region of the sky on the basis of the IRAS point source catalog. However their physical association remained unclear until recently, when results of the HIPPARCOS and the ROSAT mission shed new light on their nature.

HIPPARCOS distances towards HD 98800 and TW Hya of [FORMULA] 50-60 pc each confirmed earlier speculations that the "isolated" T Tauri stars around TW Hya are actually the closest PMS stars known with ages between [FORMULA] 10-20 Myrs based on evolutionary tracks in the HR diagram (Wichmann et al. 1998). Selection within the HIPPARCOS catalog in a search area of about 10 degrees around TW Hya also revealed the existence of one additional PMS star, namely CoD[FORMULA]7429 (Jensen et al. 1998). More evidence for the physical association of PMS stars around TW Hya came from X-ray observations with the ROSAT satellite. Strong X-ray emission from all postulated PMS stars around TW Hya was detected, similar to those TTS found in other star forming regions (Kastner et al. 1997). However a detailled analysis of ROSAT pointed observations around TW Hya and CoD[FORMULA]8887, another association member, did not find evidence for more TTS within 1 degree radius around each target position. The relative looseness around TW Hya is plausible in terms of member proximity to Earth and older ages, which could imply larger mean nearest-neighbourhood distances (Hoff et al. 1998). For even larger scales, TTS candidates can be selected from the ROSAT All-Sky Survey (RASS) (see e.g. Sterzik et al. 1995). And indeed, seven new TTS and one brown dwarf candidate could be discovered in extended areas around the previously postulated TW Hya association members, based on a search within a sample of X-ray bright RASS sources (Webb et al. 1999).

The relevance of this group of TTS in the vicinity of the sun is manifold. Their origin, and star formation scenarios can be studied without obscuration. The exceptional proximity of this group allows study of their immediate circumstellar environment with high spatial resolution. And their evolutionary status at the end of the TTS phase will help to understand circumstellar disk dispersal, and possible planetary system formation.

In this Letter we report additional TTS in the general direction of TW Hya. Our original candidate sample is based on RASS sources, but considers fainter X-ray sources than Webb et al. (1999). We describe our X-ray selection and detection procedure, together with the X-ray properties of the new TTS. The identification as TTS is based on high resolution echelle spectroscopy which enables measurement of Lithium abundances and radial velocities. Furthermore, we performed adaptive optics near-infrared imaging to search for close companions. We interpret our findings in terms of mounting evidence for the physical association of this group of young stars.

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© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999