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Astron. Astrophys. 325, 613-622 (1997)

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2. Data

To cover the whole region a deep all-sky catalogue of proper motions is required. The proper motions we present in the following sections are taken from STARNET (Röser 1996), which is a large proper motion catalogue well suited for our purposes. It contains about 4.3 million stars (most stars down to V=11.5 mag and some even fainter) with proper motions derived from plates with nearly 100 years epoch difference (Astrographic Catalogue (AC) and HST Guide Star Catalogue (GSC 1.2, Röser et al. 1996) ). The average accuracy of the proper motions is about 5 mas/y, which corresponds to an error of about 3 km s-1 at the distance of Tau-Aur.

For brighter stars the PPM catalogue is a suitable source of proper motions. It contains some 400 000 stars down to about V=10.0 mag. On the northern hemisphere the measurements used in PPM have a smaller epoch difference than GSC and AC used for STARNET, but PPM contains more observations per star. In general, we took the proper motions from STARNET except in the following two cases: (1) There was no entry in STARNET, maybe because the star was too bright for the GSC or for any other reason. In our sample this was only the case for SAO 76411 A, BD +17 724 B and BD +12 511. (2) The proper motions from STARNET and PPM deviated significantly from each other. This happened for the stars HD 283798 and BD +07 582(B), for which STARNET gives very high proper motions. We assume that this is due to a wrong identification of the star on the plates. This is a consequence of the very large epoch difference, which on the one hand reduces errors in the proper motions, but on the other hand makes the identification of high proper motion stars more difficult and sometimes erroneous. In these cases, PPM is the more reliable source, as it contains more observations per star. Stars with large proper motions are discussed in Sect. 4in more detail.

The positions in all tables are valid for equinox and epoch J2000.0. Note that the mean errors in [FORMULA] are multiplied by [FORMULA] (whereas the proper motions themselves are not).

2.1. Proper motions of PMS stars known prior to ROSAT

In STARNET or in the PPM we could identify 34 PMS stars known prior to the ROSAT mission. Their proper motions are given in Table 1. Of these, 15 stars had no proper motion measurement so far. Previous determinations of proper motions were performed in the surveys of Jones & Herbig (1979) (which was the most extensive one, including 80 suspected or confirmed T Tauri stars together with 241 stars possibly associated with Tau-Aur) and in the spatially more restricted surveys by Hartmann et al. (1991) and Gomez et al. (1992). These previous papers publish relative proper motions of stars on pairs of photographic plates. This is not an obstacle to local kinematics. However, it prevents us from a comparison of the individual accuracy in these previous papers and in the present one, because the number of stars in common is so small.


[TABLE]

Table 1. STARNET proper motions for 34 stars which were known to be of PMS nature before ROSAT. In the second column references for previous measured proper motions are given: (1) Jones & Herbig 1979, (2) Walter et al. 1987, (3) Hartmann et al. 1991, (4) Gomez et al. 1992. The last column indicates whether it is a classical (c) or a weak-line (w) T Tauri star. Note that the values for SAO 76411 A are quoted from the PPM as there is no corresponding entry in STARNET. Remarks after the name of a star refer to stars not regarded as members of the Tau-Aur association and discussed separately in Sect. 4([FORMULA] Pleiades candidates, [FORMULA] other high proper motion stars).


2.2. Proper motions of PMS stars discovered by ROSAT

2.2.1. The central region of Tau-Aur

ROSAT observations in a region including the centre of Tau-Aur in combination with optical follow-up spectroscopy have revealed 4 new classical and 72 new weak-line T Tauri stars (Wichmann et al. 1996). Of these, 38 stars (all WTTS) have proper motions either in STARNET or in the PPM which are given in Table 2. Additional pointed ROSAT observations led to the discovery of 8 confirmed (Strom & Strom 1994), 2 likely and 5 possible TTS (Carkner et al. 1996), but none of them could be found in STARNET because they are too faint.


[TABLE]

Table 2. All new ROSAT-discovered PMS stars (all WTTS) in the central region of Tau-Aur which we could identify in STARNET. The entries quoted for HD 283798 are from the PPM as the proper motions from PPM and STARNET deviate significantly for this star, probably due to a misidentification. Because BD +17 724 B is not present in STARNET we added its proper motion from the PPM. Remarks after the name of a star refer to stars with proper motions investigated in Table 4.


2.2.2. Region south of Tau-Aur

Recently Neuhäuser et al. (1995b, 1997) and Magazzù et al. (1997) have studied a sample of 111 stars in a region located just south of the Taurus molecular clouds. The field boundaries were chosen by these authors in such a way that the Orion field (in the lower left in Fig. 1) was excluded. The strip in the lower right of Fig. 1, which is perpendicular to the galactic plane, was included in order to search for a gradient in the space density of TTS as a function of distance from the Taurus clouds.

[FIGURE] Fig. 1. Positions of the stars in Tables 1 - 3. Stars of Table 1 (TTS known prior to ROSAT) are marked with triangles (CTTS filled, WTTS open), stars of Table 2 (WTTS studied by Wichmann et al. 1996) with filled hexagons and the youngest stars of Table 3 (stars studied by Neuhäuser et al. 1995b, 1997 and Magazzù et al. 1997) with open stars. [FORMULA] yrs old stars and stars older than [FORMULA] yrs of Table 3 are marked with centered stars and crosses, respectively. A circle around a symbol indicates possible Pleiades members, see Sect. 4. The regions studied by those authors are indicated, too.

Proper motions of 58 stars in the sample of these authors are available, 55 from STARNET, supplemented by 3 from PPM. The data are given in Table 3. Of these, 18 stars are classified as very young stars with ages [FORMULA] yrs (with lithium excess above Pleiades level), 12 as stars with ages around [FORMULA] yrs (with lithium, but no excess) and 28 stars as even older with ages [FORMULA] yrs (without any detected lithium) by Neuhäuser et al. (1997). The youngest stars with spectral types F or G are just reaching the main-sequence, whereas stars with spectral type K are pre-main sequence stars.


[TABLE]

Table 3. STARNET proper motions for stars not older than [FORMULA] yrs, [FORMULA] yrs old stars and stars older than [FORMULA] yrs in a region located somewhat south of the Taurus molecular clouds. The entries for the stars BD +07 582 and BD +07 582 B are from PPM as there is only one entry in STARNET for both components with a quite different proper motion. Remarks after a star's name refer again to stars discussed in Sect. 4.



[TABLE]

Table 3. (continued)


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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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