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Astron. Astrophys. 354, L9-L12 (2000)

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2. The star 1RXSJ104230.3-334014 = TWA-7

With follow-up observations of unidentified ROSAT X-ray sources, many new low-mass pre-main sequence stars were discovered (Neuhäuser 1997 and references therein). One of the regions, where new, young stars were found among ROSAT sources is the TW Hya association (TWA), a group of 14 young stars (Rucinski & Krautter 1983, de la Reza et al. 1989, Gregorio-Hetem et al. 1992, Kastner et al. 1997, Jensen et al. 1998, Webb et al. 1999, Hoff 1999, Sterzik et al. 1999) that share the same proper motion and radial velocity. Many of these stars are multiple systems. Four TWA members were observed with Hipparcos, and the weighted mean distance is 55 pc. Hence, TWA is the nearest association of young stars. Lowrance et al. (1999) and Webb et al. (1999) found a faint object [FORMULA] north of the TWA member CoD[FORMULA]. From its magnitudes and colors, they concluded that the faint object may be a [FORMULA] M[FORMULA] mass companion, but it is still unclear whether this visual pair forms a proper motion pair.

The apparent angular diameter of this association as measured from the largest distance between two members is [FORMULA], which corresponds to a projected extent of 31 pc at the mean Hipparcos distance of 55 pc. If the association has the same extent in radial direction, the distances of the stars can range from 40 to 71 pc, i.e. slightly more than the range in distances of the four stars observed by Hipparcos. Thus, we can assume [FORMULA] pc as distance of TWA members not observed by Hipparcos.

One of the new members of this association is 1RXSJ104230.3-334014 (Voges et al. 1999), also called TWA-7 (Webb et al. 1999). Using the fibre-fed spectrograph FEROS on the ESO 1.52m telescope, we obtained a spectrum of TWA-7 on 1 June 1999 (3500Å to 9200Å with [FORMULA]), see Fig. 1. TWA-7 is clearly a pre-main sequence star, as the equivalent width of the Li 6707Å line of this M1 star is [FORMULA]Å. The barycentric radial velocity of TWA-7 is [FORMULA] km/s, consistent with the other TWA members. Furthermore, we find W[FORMULA](H[FORMULA])=[FORMULA]Å, i.e. TWA-7 is a weak-line T Tauri star; its rotational velocity of v[FORMULA] i =[FORMULA] km/s is relativelly small.

[FIGURE] Fig. 1. Our high-resolution spectrum of TWA-7 shows strong lithium absorption and H[FORMULA] emission, typical for young stars

TWA-7 is listed in the USNO-A2.0 (Monet et al. 1998) at [FORMULA] and [FORMULA] for epoch 1982.235, with B [FORMULA], and R [FORMULA] mag, and also in the Hubble Space Telescope (HST) Guide Star Catalog (GSC1.2, Lasker et al. 1996) with B=[FORMULA] mag. In the STARNET catalog of proper motions and positions (Röser 1996), the proper motion of TWA-7 on the Hipparcos system is given as [FORMULA] and [FORMULA] mas/yr (S. Frink, priv. comm.). In the SPM catalog, Webb et al. (1999) found [FORMULA] and [FORMULA] mas/yr. Because TWA-7 shares the radial velocity and proper motion of the other TWA members, it is most certainly also a member.

From the spectral type M1 and the JHK photometry (Webb et al. 1999), we can conclude that the absorption is very small. Then, we obtain the bolometric luminosity at [FORMULA] pc to be [FORMULA] L[FORMULA]/L[FORMULA]. A spectral type of M1 ([FORMULA] one sub-type) corresponds to T[FORMULA] K according to Luhman (1999) for young M-dwarfs with surface gravities intermediate between giants and dwarfs. The location of TWA-7 in the H-R diagram compared to evolutionary tracks and isochrones by D`Antona & Mazzitelli (1994) and Baraffe et al. (1998) yields an age of 1 to 6 Myrs, i.e. co-eval with the other TWA stars, and a mass of [FORMULA] M[FORMULA].

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000