6. Discussion

From Fig. 1 (click here) we see that many of our newly discovered low-mass PMS stars are located up to several tens of degrees away from regions of ongoing star formation. In fact, the area of on-going star formation in Taurus, as defined by the CO contours (Ungerechts & Thaddeus 1987) and also the area populated by the TTS known before ROSAT or newly discovered by Wichmann et al. (1996), is at . If the distance of our objects is the same as the Taurus-Auriga clouds (140 pc), these objects will lie up to several tens of parsecs from the clouds. In any case, they are located far from any known star forming region. Despite their location, the lithium equivalent width in most of these stars is indicative of young ages, just typical for wTTS.

 

(Å) (Å)

RXJ 0209.1-1536 INT -1.2 no K4 dKe

RXJ 0210.4-1308SW INT -0.12 no K3 dKe

RXJ 0210.4-1308NE INT 3.5 no G5 non-PMS

RXJ 0212.3-1330 INT 1.2 no K1 non-PMS

RXJ 0215.0-1402 INT 1.15 no K4 non-PMS maybe K3 III

RXJ 0218.6-1004 INT 1.6 no G8 non-PMS SB

RXJ 0219.4-1321C ESO abs no G0 non-PMS

RXJ 0219.4-1321B ESO 0.60 f no M0 dMe

RXJ 0219.4-1321A ESO -4.30 no M3 dMe

RXJ 0219.7-1026 INT -0.25 0.1: K4 PMS? (N95c), (1)

RXJ 0220.4-1305 INT 5.7 no F0? non-PMS

RXJ 0223.3-1615SW ESO -0.8 no K7 dKe

RXJ 0223.3-1615NE ESO 2.1 no G7 non-PMS

RXJ 0229.5-1224 ESO 2.6 0.28 G8 PMS HD 15526

RXJ 0237.3-0527 INT -0.30 no K5 dKe (N95c)

RXJ 0239.1-1028 INT -0.35 no K7-M0 dKe SB

RXJ 0243.9-0850 ESO 0.50 f no M2 dMe

RXJ 0248.3-1117 INT 2.1 no G7 non-PMS

RXJ 0251.8-0203 INT -0.60 no K6 dKe (N95c)

RXJ 0254.8-0709NW INT -4.2 no M5 dMe

RXJ 0254.8-0709SE INT -2.2 no M3 dMe

RXJ 0255.8-0750N INT -0.7 0.2: M5 PMS?

RXJ 0255.8-0750S INT -0.2 no K7-M0 dKe

RXJ 0309.1+0324N INT 3.6 no F7 non-PMS

RXJ 0309.1+0324S INT 1.7 no G6 non-PMS

RXJ 0312.8-0414NW ESO 3.5 0.2 G0 PMS

RXJ 0312.8-0414SE ESO 2.5 0.3 G8 PMS

RXJ 0314.8-0406 INT 5.5 no late A non-PMS

RXJ 0317.9+0231 INT 1.4 no G6 non-PMS

RXJ 0319.3+0003 ESO 4.4 no G5 non-PMS SAO 130417

RXJ 0324.4+0231 INT -0.40 0.33 K5 PMS (N95c)

RXJ 0329.1+0118 INT 4.0 0.13 G0 PMS? (N95c)

RXJ 0330.7+0306N INT 1.1 f no K5 dKe SB, (2)

RXJ 0330.7+0306S INT 1.0 no K7 non-PMS

RXJ 0333.0+0354 INT -2.4 0.2: K7-M0 PMS? (N95c)

RXJ 0333.1+1036 INT -0.8 0.32 K3 PMS (N95c)

RXJ 0336.0+0846 INT -0.1 no M3 dMe

RXJ 0338.1+1224 ESO 1.5 no K0 non-PMS

RXJ 0338.3+1020 INT 2.0 0.25 G9 PMS

RXJ 0339.6+0624 INT -0.1 0.13 G9 PMS? (N95c)

RXJ 0340.3+1220 INT -1.1 no K5 dKe

RXJ 0340.5+0639 ESO 1.6 no K2 non-PMS (3)

RXJ 0341.2+0453 INT 3.2 no G9 non-PMS

RXJ 0341.2+0759 ESO 3.2 no K0 non-PMS

RXJ 0343.6+1039 INT 1.5 0.1 K0 PMS? SB, (4)

RXJ 0344.8+0359 INT 0.3 f 0.30 K3 PMS? (N95c)

RXJ 0347.2+0933SW ESO -0.40 0.4 K4 PMS

RXJ 0347.2+0933NE ESO 2.00 0.1 G9 PMS?

RXJ 0347.9+0616 INT 2.6 0.2 G2 PMS

   

(Å) (Å)

RXJ 0348.5+0832 INT -0.1 0.26 G7 PMS (5)

RXJ 0349.4+1255N INT 2.5 no G0 non-PMS

RXJ 0349.4+1255S INT 2.0 no G7 non-PMS

RXJ 0350.2+0849 INT -0.2 no K5 dKe SB

RXJ 0350.6+0454 INT -0.15 no K7 dKe SB

RXJ 0350.6+1033 ESO 2.4 no K0 III non-PMS

RXJ 0351.4+0953W INT 0.5 f 0.3 K1 PMS?

RXJ 0351.4+0953E INT 2.0 no F0 non-PMS maybe F0 III

RXJ 0351.8+0413 ESO 2.00 0.12 G6 PMS? (3), (6)

RXJ 0352.4+1223 ESO 3.7 0.10 G2 PMS? BD+11 533

RXJ 0354.1+0528 ESO 2.8 0.24 G8 PMS (3)

RXJ 0354.3+0535 INT 3.5 0.2 G1 PMS (N95c)

RXJ 0354.4+1204 ESO 3.9 no G5 non-PMS

RXJ 0354.8+1232 INT -1.7 no K7 dKe

RXJ 0356.7+0943 INT -2.7 no M3 dMe

RXJ 0357.3+1258 INT 1.8 0.25 G0: PMS

RXJ 0358.1+0932 INT -0.10 f 0.2 K3 PMS? (N95c)

RXJ 0400.0+0730 INT 1.3 no G3 non-PMS

RXJ 0400.1+0818S ESO 2.5 0.24 K0 PMS?

RXJ 0400.1+0818N ESO -0.05 0.40 K2 PMS

RXJ 0400.8+1116 INT 1.8 no K0 non-PMS

RXJ 0402.5+0552 INT 1.7 no G4 non-PMS

RXJ 0403.5+0837 INT 1.3 no K0 non-PMS

RXJ 0404.4+0519 INT 1.0 f 0.25 K0 PMS?

RXJ 0405.5+0324 INT -0.4 no K4 dKe

RXJ 0407.2+0113N INT 3.3 0.2 G4 PMS

RXJ 0407.2+0113S INT 0.5 f 0.35 K3 PMS

RXJ 0407.6+0638 INT 2.0 no G0 non-PMS

RXJ 0408.6+1017 INT 1.8 no G7 non-PMS

RXJ 0408.8+1028 ESO 3.2 no G5 non-PMS HD 26172

RXJ 0409.8+1209 INT 3.3 0.10 F9 PMS? HD 286556

RXJ 0410.6+0608 INT -0.05 0.11 K4 PMS? SB?

RXJ 0413.2+1028 INT 3.0 no G0 non-PMS

RXJ 0418.6+0143 INT 0.4 f no K4 dKe

RXJ 0419.8+0214 INT 4.5 no F5 non-PMS

RXJ 0419.9+0231 INT 3.6 no F9 non-PMS

RXJ 0422.9+0141 INT 1.5 yes F8 PMS? SB, (7)

RXJ 0423.5+0955 INT -0.1 0.27 K4 PMS?

RXJ 0425.5+1210 INT 4.2 0.1 F9 PMS? HD 286753

RXJ 0426.4+0957W INT 7.5 no late A non-PMS

RXJ 0426.4+0957E INT 2.9 0.14 G2 PMS?

RXJ 0427.4+1039 INT 1.3 f 0.35 G0: PMS

RXJ 0427.5+0616 INT 1.5 0.25 G4 PMS

RXJ 0427.8+0049 INT 3.2 no G3 non-PMS BD+00 760, SB

RXJ 0429.9+0155 INT 1.5 no K3 non-PMS maybe K3 III

RXJ 0433.7+0522 INT 4.0 no F8 non-PMS

RXJ 0434.3+0226 INT -0.4 0.3 K4 PMS

RXJ 0435.5+0455 INT 1.2 no K3 III non-PMS

RXJ 0441.9+0537 INT abs no G5 non-PMS BD+05 706, (8)

RXJ 0442.3+0118 INT -1.0 no K2 dKe

RXJ 0442.5+0906 INT 1.4 0.25 G7 PMS BD+08 742

RXJ 0442.6+1018 INT 1.2 no K3 non-PMS maybe K3 III

RXJ 0442.9+0400 INT 1.1 0.22 K0 PMS?

 

(Å) (Å)

RXJ 0444.4+0725 INT -0.3 0.12 K5 PMS?

RXJ 0444.7+0814 INT -0.80 0.28 K3 PMS? (N95c)

RXJ 0445.2+0729 INT 2.6 0.25 G0 PMS

RXJ 0445.3+0914 INT 3.8 no G0 non-PMS

RXJ 0445.5+1207 INT -2.0 0.35 K7 PMS

RXJ 0448.0+0738 INT -0.1 f no K1 dKe (N95c)

RXJ 0450.0+0151 INT 0.5 f 0.35 K3 PMS

RXJ 0451.6+0619 INT 1.3 f no K2 dKe SB?

RXJ 0459.9+1017 INT 3.0 no F5 non-PMS SB

RXJ 0511.2+1031 INT -2.8 0.65 K7 PMS

RXJ 0511.9+1112 INT 1.2 0.25 G4 PMS

RXJ 0512.0+1020 INT -0.1 0.4 K2 PMS

RXJ 0513.6+0955 INT 1.4 no G6 non-PMS

RXJ 0515.3+1221 INT 1.2 f no K0 dKe SB, (9)

RXJ 0516.3+1148 INT 0.1 f 0.5 K4 PMS (N95c)

RXJ 0523.0+0934 INT 4.2 no F8 non-PMS

RXJ 0523.5+1005 INT -0.5 no K3 dKe SB, (9)

RXJ 0523.9+1101 INT 3.7 no G0 non-PMS

RXJ 0525.7+1205NW INT 1.4 no G8 non-PMS maybe G8 III

RXJ 0525.7+1205SE INT 1.4 no G8 non-PMS maybe G8 III

RXJ 0528.4+1213 INT 2.5 no K2 non-PMS

RXJ 0528.5+1219 INT 0.7 f 0.35 K3 PMS

RXJ 0528.9+1046 INT 0.1 f 0.4 K3 PMS

RXJ 0529.3+1210 INT -2.0 0.35 K7-M0 PMS

RXJ 0530.9+1227 INT 1.3 no K0 non-PMS

RXJ 0531.8+1218 INT -0.74 0.5 K4 PMS (N95c)

Remarks: (1) Spectrum blue-shifted by ; (2) Star A itself is SB, with the secondary having almost the same spectral type as the primary; (3) Also observed at INT; (4) Spectrum blue-shifted by , the line shows a P Cyg profile; (5) line shows an inverse P Cyg profile; (6) Spectrum red-shifted by ; (7) SB: the primary is F8 with in absorption and , the secondary has , thus the system may by a close PMS binary; line shows a P Cyg profile; (8) Very noisy spectrum; (9) Secondary seems to have a slightly earlier spectral type.

Radial velocities for some of them have been presented in Neuhäuser et al. (1995c) and indicate that about half of their 15 objects are kinematic members of the Taurus-Auriga T association. A complete analysis of the kinematic status of all stars studied here will be given in Neuhäuser et al. (1997) together with radial velocities (for almost all stars studied here) and proper motions for several stars identified here as new PMS stars.

In any SFR studied the RASS has revealed hundreds of new wTTS, which have been discovered even outside regions of ongoing star formation. However, only few cTTS have been discovered, either by EO or ROSAT. As the RASS is flux-limited and ROSAT pointed observations are spatially biased towards "interesting'' regions, many wTTS have not been discovered yet. We find 30 new PMS stars among 115 previously unidentified sources (i.e. 26%) selected by hardness ratios and V magnitude of the possible counterpart. Although our investigation has been carried out outside molecular gas regions, this percentage of new PMS stars is within the range expected in Neuhäuser et al.\ (1995a), who predict at least 286 new wTTS among 1143 unidentified pre-selected RASS sources (i.e. at least 25%). As far as the wTTS/cTTS ratio is concerned, we note that in the area studied in this paper no cTTS are known. In the complete area studied by Neuhäuser et al.\ (1995a) this ratio is about 8:1 or larger, while, considering only the dark cloud cores, the ratio is about 1:1 (see Neuhäuser et al.\ 1995a for a discussion).

  
Figure 5: Spectra of our newly discovered PMS stars

 
Figure 5: continued

 
Figure 5: continued

 
Figure 5: continued

Most of the stars classified here as PMS? and some of those labelled as PMS are probably not very young (as indicated by their relatively shallow Li line) and may be close to the Main Sequence. Hence, they may well be the long sought post-TTS which have been dispersed out of the region of ongoing star formation. If star formation has been going on in Taurus-Auriga for years, there should be numerous post-TTS all around the CO clouds, even if the velocity dispersion in Taurus-Auriga is only (Jones & Herbig 1979). On the other hand, considering the strength of the LiI doublet as a youth indicator, there seem to be very young new wTTS in our sample. If they are less than 10⁶ years old, it appears to be impossible for them to have moved the distance between central Taurus-Auriga and their present location by slow isotropic drifting. If they have been formed in central Taurus-Auriga, they must have moved to their present location with high velocities.

Two mechanisms have been proposed to explain the existence of very young objects so far away from the SFRs. According to the first one, these objects, called "run-away'' TTS (RATTS: Sterzik et al.\ 1995; Neuhäuser et al. 1995c), can be ejected by three-body encounters in multiple protostellar systems (Sterzik & Durisen 1995). Many RATTS are expected in the vicinity of other SFRs as the pre-selection of TTS candidates indicates (e.g. Sterzik et al.\ 1995). A different explanation has been proposed by Feigelson (1996), who argues that TTS can form in small, high-velocity, short-lived cloudlets within and around a turbulent giant molecular cloud complex (like Taurus-Auriga and Chamaeleon).

Our sample of new low-mass PMS stars found south of the Taurus-Auriga dark cloud complex may contain relatively old dispersed post-TTS, young wTTS formed locally in small short-lived cloudlets, and young wTTS ejected from the central areas of ongoing star formation. With kinematic data (both radial velocities and proper motions) and age estimates (e.g. from placing the stars into the HR-diagram and from precise Li abundances) one may be able to distinguish between these different contributions in the future. The presence of lithium in a large number of stars in the general direction of any known SFR studied is an observational fact that certainly needs further explanation.

Acknowledgements

We would like to thank Guillermo Torres for fruitful discussions. Many thanks also to the EXSAS and ROSAT teams at MPE as well as to the support teams at the different telescope sites used in this research. This work has made use of the SIMBAD database operated at CDS, Strasbourg. The ROSAT project has been supported by the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBW/DARA) and the Max-Planck-Gesellschaft.