11. Discussion and conclusions
We have obtained optical and IR photometric measurements and high- and low-resolution spectra of the very active young star P1724 in Orion with the following results:
From optical follow-up observations of unidentified X-ray sources found in the RASS, large numbers of wTTS have been discovered in and around Orion and other SFRs (Alcalá et al. 1996; see also the recent review by Neuhäuser (1997) with more references therein). One possible explanation for the presence of young wTTS located several degrees from active star forming clouds is the run-away TTS (raTTS) hypothesis. According to Sterzik & Durisen (1995), young stars can be ejected when formed in multiple protostellar systems, so that they would move with high velocities and could be found far away from their birth places. Such raTTS should preferentially be either single TTS or close binaries, since tight binaries do not easily break apart in typical encounters. Depending on the ejection geometry, raTTS may show a radial velocity and/or proper motion different from the mean value of their parent association, i.e., may seem to have kinematics inconsistent with membership. The evidence discussed above supports the conclusion that P1724 is indeed a raTTS.
Ejected TTS may lose at least part of their circumstellar material during the encounter (e.g., Brandl & Sterzik 1997), consistent with the fact that we detect no IR excess emission in the SED of P1724. However, we also note that Hillenbrand et al. (1998) found a gradient in the fraction of stars which show near-IR excess as a function of projected radius from the center of the Trapezium, even after correcting for the falloff in membership probability with increasing cluster radius. Furthermore, there appears to be a gradient in the mean accretion rates with cluster radius, where they are higher in the center than in the outer parts of the cluster. Their interpretation is that the higher stellar density in the interior causes enhanced disk accretion rates and, consequently, that more stars with near-IR excess are detected than one would detect if the mean accretion rates were lower (Hillenbrand et al. 1998). At a projected radius of from Ori, they found a minimum disk fraction of , and more disks further inwards. We can speculate that the lower disk fraction and the smaller near-IR excesses found with increasing distance from the Trapezium center may partly be due to a some contribution of raTTS, which have lost their circumstellar material during the encounter.
In many regards, P1724 appears to be similar to P1540, studied in detail by Marschall & Mathieu (1988): Both stars show RV consistent with membership, but inconsistent proper motion; they are located just outside the Trapezium cluster and show 3D space motion pointing away from the Trapezium. Both might have been ejected from the cluster years ago, in both cases easily within the respective ages of the stars. P1540 is a double-lined spectroscopic binary, but its separation is small enough, so that the encounter would not disrupt the binary. These two stars appear to be very good examples for the raTTS hypothesis.
Kroupa (1995) has suggested that low-mass stars can be ejected dynamically from very dense clusters such as the Trapezium by mutual interactions. According to both Kroupa (1995) and Sterzik & Durisen (1995), there is in fact some chance that even stars as massive as P1724 can be ejected. Although it is much more likely for lower-mass stars to be ejected, more massive stars can be more easily detected because they are much more luminous.
The rather large projected rotational velocity and the rather long rotational period imply that P1724 is seen not far from equator-on. We derived above an inclination angle of , from which . Given the observational constraints, the minimum possible value of the radius assuming is . This in turn would be consistent with a distance of , which we consider as a lower limit. Although P1724 does not show any IR excess, its apparent extinction of mag is rather large for a naked wTTS. However, on optical images, one can see a small nebulosity (cloudlet) at the location of P1724, and it is very likely that this nebula is the reason for the large extinction. P1724 is located inside or behind the nebula.
According to our data, the local-standard-of-rest (LSR) radial velocity of P1724 is , while the LSR velocity of the nebula at its location is slightly different, namely (Maddalena et al. 1986), computed for the material at the location of P1724 using the original Maddalena et al. data by T. Dame (priv. comm.), but the Maddalena et al. beam was not directly pointed towards P1724. However, the RV of P1724 is not different from the RV obtained (above) for the [NII] line, which probably originated in this nebula. Hence, we cannot conclusively confirm, whether or not P1724 and the nebula are co-moving in the radial direction. Yet, from the proper motion of P1724 and the Trapezium stars, we know that P1724 is moving with a large velocity relative to the Trapezium and the Orion clouds. Hence, we conclude that P1724 is currently crossing through this nebula, although they may have the same RV. Unfortunatelly, the proper motion of the nebula is unknown and difficult to measure.
It is also possible that the nebula itself, or even both the nebula and the star together, were ejected from the Trapezium. Gorti & Bhatt (1996) argue that small neabulae can be ejected from molecular cloud regions by cloud-cloud interaction. This might have happened to the nebula we see near P1724, but we cannot measure the proper motion of the cloud to check this. P1724 could have formed in the nebula some time after the ejection of the nebula, or the nebula and the star could have been ejected together, or the cloud and the star just happen to be located at the same position. However, all these hypotheses appear to be very unlikely.
Because this small nebula is located close to the Trapazium and because its RV is not different from the mean RV of the Orion clouds and stars, this nebula is most likely part of the Orion molecular cloud complex, which can be taken as another indication for the fact that the distance of P1724 cannot be much smaller than the distance towards the Orion SFR. We also stress again that both a distance-dependent gravity measurement (namely the location in the H-R diagram) and a distance-independent gravity measurement (namely fitting our high-resolution spectra) yield the same surface gravity, so that the distance assumed cannot be far off.
We conclude that P1724 is a very young, relatively massive weak-line T Tauri star without UV or IR excess emission located at the distance of the Orion clouds, and it may well be a good example of a run-away T Tauri star ejected from the Trapezium cluster years ago.
© European Southern Observatory (ESO) 1998
Online publication: June 2, 1998