3 H-R diagrams

In order to visualize the age and mass distribution of our stars we have placed the subset of TTS observed with the PSPC and with known bolometric luminosity $L_{\rm bol}$ and effective temperature $T_{\rm eff}$ in the Hertzsprung-Russell diagram (H-R diagram). We dispense with H-R diagrams for Pleiades and Hyades stars because most of the stars in these two clusters are well known to lie on the MS (see previous discussion). The H-R diagram for the TTS in Taurus-Auriga is shown in Fig. 2.

Figure 2: H-R diagram of TTS observed with the ROSAT PSPC during pointed observations. Note, that the stars on display represent only a fraction of all X-ray observed TTS because $L_{\rm bol}$ and $T_{\rm eff}$ are not known in all cases. The data are compared to three different theoretical calculations for the PMS evolution: top - D'Antona & Mazzitelli (1994), middle - Baraffe et al. (1998), and bottom - Palla & Stahler (1999). The masses are given in solar units and the isochrones represent $\log{\rm age}$ except for Palla & Stahler (1999) where the ages are given in Myrs.

We have used the $L_{\rm bol}$ values given by Kenyon & Hartmann (1995), and $T_{\rm eff}$ was obtained from the spectral types using the conversion by Schmidt-Kaler (1982). The location of the stars is compared to different models of evolutionary PMS tracks: D'Antona & Mazzitelli (1994), Baraffe et al. (1998), and Palla & Stahler (1999). All diagrams are drawn with the same scale to facilitate the perception of differences between the model calculations. The computation by Baraffe et al. (1998) does not represent a useful description of the complete TTS sample due to its restriction to masses below $\sim$ $1~{M_\odot}$. Furthermore, the lines of equal mass show significant deviations from the other calculations. A closer look reveals that there are also significant differences between the models of D'Antona & Mazzitelli (1994) and Palla & Stahler (1999).

It would be highly desirable to use the theoretical calculations to assign ages and masses to the individual TTS. However, from the comparison provided in Fig. 2 it is obvious that the calibration of the models is uncertain, i.e. tracks computed by different groups would lead to controversial results on the masses and ages of the stars.

Despite such uncertainties the H-R diagram can be used to demonstrate the average distribution of the cTTS and wTTS. Although the stars closest to the birthline tend to be cTTS, and those nearest to the MS are wTTS, the overall distribution of cTTS and wTTS is mixed. This indicates that individual wTTS are not always older than cTTS despite the fact that they represent a later evolutionary stage. This is known since the discovery of many wTTS by the EO (Walter et al. 1988), and the situation is similar in other star forming regions.