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Subsections

   
7 Kinematics

Because CrA is not part of the Gould Belt, the young stars surrounding the CrA dark cloud most certainly are associated with the CrA association. They could have formed near the present locations, where the gas has dispersed since then. In that case, these young stars should be somewhat older than the on-cloud TTS. Alternatively, if the CrA cloud has not shrunk, the off-cloud young stars could have dispersed out of the cloud, either slowly (then, the outermost stars would again be older than the on-cloud stars) or, at least in some cases, have higher velocities. In the latter case, one would expect at least a few young run-away TTS ejected from the dark cloud. For checking these possibilities, we need to investigate proper motions and radial velocities.

7.1 Proper motions

In order to examine the kinematical state of the stars in the R CrA association, we searched for proper motions for all the stars in Tables 1 and 3 in the Hipparcos (ESA 1997), PPM (Röser & Bastian 1991; Bastian et al. 1993; Röser et al. 1994), ACT (Urban et al. 1997); TRC (Høg et al. 1998), Tycho2 (Høg et al. 2000), and STARNET (Röser 1996) proper motion catalogs. Altogether we could identify 24 stars in these catalogs, including nine stars known already before the ROSAT mission, nine TTS newly identified here, as well as four ZAMS and two dKe/dMe stars. Note that R CrA itself is not listed; it is included in the Hipparcos catalog (HIP 93449), but no meaningful solution for its astrometric parameters could be derived. Proper motions are given in Table 6. For stars present in more than one catalog we usually adopted the most precise proper motion determination, unless it was in conflict with values from other catalogs. All proper motions were transformed to the Hipparcos astrometric system (ICRS) before comparison.


 \begin{figure}
\par\includegraphics[angle=-90,width=18cm,clip]{ds1875_f6.ps}
\end{figure} Figure 6: Lithium versus effective temperature. We plot the lithium equivalent width $W_{\lambda }$(Li) versus the effective temperature $T_{\rm eff}$ for the previously known TTS in CrA (Table 1, open squares), the newly identified PMS stars (Table 4, full squares), bona-fide Taurus TTS (crosses and arrows, see Neuhäuser et al. 1997 for references), and the Pleiades as dots (Soderblom et al. 1993; García López et al. 1994). We converted spectral types to $T_{\rm eff}$ following Bessell (1979, 1991). Also shown are lithium iso-abundance lines (as dashed lines) for $\log~g~=~4.5$ from Pavlenko & Magazzù (1996). Stars with more lithium than ZAMS stars of the same spectral type, i.e. stars which lie above the upper envelope (solid line) of the Pleiades lithium data, are younger than ZAMS, i.e. PMS stars. Because late F- and G-type stars do not significantly burn lithium during the ZAMS phase, the three full squares in the lower left could be either PMS or ZAMS stars


  
Table 6: Proper motions for CrA TTS. Listed are objects from Tables 1 to 4, which are included in at least one of the proper motion catalogues Hipparcos (HIP), PPM, ACT, TRC (T), STARNET (S) or Tycho2 (T2). All proper motions refer to the Hipparcos astrometric system

\begin{displaymath}\begin{tabular}{@{}llr@{$~\pm~$}lr@{$~\pm~$}l@{}}
\hline\noal...
...
\noalign{\smallskip }\hline\noalign{\smallskip }
\end{tabular}\end{displaymath}

(a) The visible double star HR 7169/HR 7170 is represented by two entries in the Hipparcos Catalogue (HIP 93368 and HIP 93371) with two individual component solutions. The solution quality is classified as fair; however the proper motion errors are much smaller in TRC, which we adopt here.
(b) Hipparcos distance is
$65\pm 5$pc, i.e. foreground to the cloud.
(c) RXJ1921.4-3459 has an acceleration solution in the Hipparcos Catalogue.

Positions and proper motion diagrams for stars in Table 6 are shown in Fig. 8. The mean proper motion of the R CrA member stars seems to be very well defined. All except maybe one (HR 7170) of the member stars known before ROSAT (including the late B-type stars listed at the end of Table 1, whose membership was not clear before) show very similar proper motions, and all except two of the newly identified TTS nicely follow this trend. In contrast to this, the stars classified as ZAMS or dKe/dMe form a kinematically much more inhomogeneous distribution. The mean proper motion for all 15 likely members of the association is ( $\mu_{\alpha}\cos \delta,\mu_{\delta}$) = (5.5,-27.0)masyr-1. The largest part of this motion is simply the reflex of the solar motion, which is ( $\mu_{\alpha}\cos \delta,\mu_{\delta}$) $\approx$ (7.1,-23.7)masyr-1at 19$^{\rm h}$ 00$^{\rm m}$, -37 $^{\mbox{\scriptsize o}}$ 00$^\prime$ and a distance of 130pc, and ( $\mu_{\alpha}\cos \delta,\mu_{\delta}$) $\approx$ (5.6,-24.0)masyr-1for 18$^{\rm h}$ 45$^{\rm m}$ and the same declination and distance values as above. Thus the slight difference between the mean proper motion in right ascension of the eight R CrA member stars known before ROSAT [( $\mu_{\alpha}\cos \delta,\mu_{\delta}$) = (7.0,-27.5)masyr-1] and the seven newly identified member TTS [( $\mu_{\alpha}\cos \delta,\mu_{\delta}$) = (3.9,-26.4)masyr-1] is partly a projection effect reflecting the fact that the new TTS are located at slightly lower right ascension.

7.2 Space velocities

We calculated space velocities for those stars with measured radial velocities (taken from Table 1 for six stars and from Table 4 for another six stars) and corrected them for the influence of galactic rotation (Fig. 9). The solar motion has also been subtracted, although this does not change the relative space velocities between stars, in contrast to galactic rotation or projection effects.

With the exception of HD 176386, which shows a discordant motion in the U-direction, all calculated space velocities are very similar. The mean (U,V,W) values are ( 4.8,-2.1,-2.7)kms-1for the stars known before ROSAT (excluding HD 176386 for taking the mean of the U-velocities) and ( 4.4,-2.2,-0.7)kms-1for the new TTS, i.e. no systematic differences seem to be present. There could still be a small difference in the W-velocities, but the number of stars is so small that this is maybe not significant.

The velocity dispersion is indeed very low, ( $\sigma_U,\sigma_V,\sigma_W$)= ( 1.4,1.2,1.8)kms-1 for the whole sample, which excludes the ejection mechanism (so-called run-away TTS) as major source for off-cloud TTS, because they should have discrepant velocities. The velocity dispersion is highest in the W-direction, and taking also into account that the positions of the stars form a broader distribution in the Z-direction ( $\sigma_Z=4.9$pc, or $\sigma_Z=4.0$ pc if the far off lying RXJ1917.4-3756 is excluded) than in the Y-direction ( $\sigma_Y=1.9$pc)[*] this could be interpreted in terms of stars oscillating around the galactic plane. It simultaneously would explain why no PMS stars were found in the opposite direction of the R CrA cloud: the stars already reached the largest distance from the galactic plane and are currently near their turning point, consistent with their W-velocities being close to zero. One complete oscillation around the galactic plane would last $\sim 10^{8}$ yrs, but need not be finished by now for this scenario to be true. The off-cloud stars are currently located on average at z = -35 pc, while the cloud is at z = -39 pc. The stars which now appear to be off-cloud and the cloud itself, given their current locations and velocities and tracing their paths back in time, would have been at the same location $\sim 3 \ 10^{7}$ yrs ago. The stars located outside of the current cloud borders are on average $\sim 10$ Myrs old, i.e. older than those inside the cloud, which supports the cloud oszillation scenario. However, the details and exact time-scales depend on the unknown total cloud mass. A very similar interpretation could also explain the positions and the motions of the stars found south of the Taurus clouds (Neuhäuser et al. 1997; Frink et al. 1997).


  \begin{figure}
\par\includegraphics[angle=-90,width=8.8cm,clip]{ds1875_f7.ps}
\end{figure} Figure 7: H-R diagram with the newly found young stars. Bolometric luminosity versus effective temperature for all newly found Li-rich ROSAT counterparts (from Table 4, all filled symbols) and for Li-rich EO counterparts (Walter et al. 1997, open symbols), compared to tracks and isochrones from D'Antona & Mazzitelli (1994) for ages (given on a log scale) and masses (given in $M_{\odot }$). The two newly found cTTS are marked by a plus sign; surprisingly, they are relatively old, namely $\sim 10$ Myrs old (at the assumed distance of 130 pc)

Lépine & Duvert (1994) suggested that high velocity cloud impacts could trigger star formation, and that subsequently to the impact clouds and stars could be separated from each other due to different friction during the passage through the galactic plane. If this is true, the new TTS found outside the dark cloud today could very well have been born inside the molecular cloud. The fact that the observed velocity dispersions are so low supports this scenario. A typical proper motion uncertainty of 3masyr-1 and an assumed distance uncertainty of 15pc translate into a combined uncertainty of $\approx$3kms-1 for the V and W components of the space velocity. The observed velocity dispersions are found to be even lower, so that this is consistent with a very small intrinsic dispersion of the velocities.

   
7.3 Stars with discordant proper motions

There are three stars among the sample of pre-ROSAT R CrA members and newly identified TTS with discordant proper motions from the mean.

HR 7170 belongs to a possibly quadruple system, and therefore the proper motion determination is highly problematic (see footnote to Table 6). Given the fact that the TRC proper motion of HR 7169 is consistent with kinematical membership to the R CrA association and that HR 7170 presumably belongs to the same system, it is likely that the different TRC proper motion of HR 7170 reflects orbital motions within the system. Although the kinematical membership of HR 7170 could not be proven directly, it still should be considered an R CrA association member. The stars HR 7169 and HR 7170 lie in a cavity in the CO distribution (Loren 1979). This suggests similar distances to the stars and cloud. Additionally, optical images show an extensive reflection nebulosity surrounding the pair.


  \begin{figure}
\par\includegraphics[width=18cm,clip]{ds1875_f8.eps}
\end{figure} Figure 8: Positions (left panel) and proper motions (right panel) of all the stars in Table 6, i.e. with proper motions available. The mean proper motion of R CrA member stars is well defined by most of the previously known stars (including early-type as well as late-type stars) and newly identified young stars in this region. Note that the proper motions of the stars classified as TTS are much more uniformly distributed than the proper motions of the stars classified as ZAMS or dKe/dMe


  \begin{figure}
\par\includegraphics[width=18cm,clip]{ds1875_f9.eps}
\end{figure} Figure 9: Space velocities for all likely R CrA member stars (six stars known before ROSAT and six newly identified TTS shown hatched) with known proper motions and radial velocities. The effects of differential galactic rotation and the reflex of the solar motion were eliminated using IAU standard values for all constants. A distance of 130 pc has been assumed for all stars. U points in the direction of the galactic center, V in the direction of galactic rotation and W towards the north galactic pole. The velocity dispersions are very small

The two other stars are the newly identified TTS RXJ1844.3-3541 and RXJ1901.4-3422. The case for RXJ1844.3-3541 is not clear. According to its spectral type and lithium line strength, it is clearly a young TTS, and also its radial velocity is consistent with membership. Either the proper motion from STARNET maybe in error or the star could have been ejected from the cloud. Comparing its proper motion with the association mean, it would move towards the south-west relative to the cloud, but it is located north-west of the cloud, so that it probably was not ejected from the R CrA cloud.

For RXJ1901.4-3422 the classification as TTS is doubtful. At a late-F spectral type, even Pleiades ZAMS stars have not yet burned significantly lithium, so that classification as either pre-MS or ZAMS stars is difficult. Because its lithium line is stronger than its nearby calcium line, we classify the star as TTS. The Hipparcos parallax of $65\pm 5$pc clearly places it foreground to the R CrA cloud, which explains its large proper motion, and it is the only star located clearly north of the R CrA cloud. We conclude that RXJ1901.4-3422 is most likely not an association member, but young.


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