3. Accretion disks
The presence of optically thick accretion disks can be inferred from the observation of near-infrared excess emission above the photospheric continuum. This excess emission can be measured by
where is the dereddened color of the star and the intrinsic color corresponding to its spectral type. Edwards et al. (1993 ) find for a sample of TTS with spectral types of K7-M1 that show spectroscopic evidence of accretion.
Using the spectral types given by Krautter et al. (1996 ), we have computed for all the new TTS of their study for which near-infrared photometry was available, and present the results in Table 7. Only 3 out of 42 stars showed a value of . One of them, RXJ 1514.8-4220, is a K1 star which shows H in absorption. As this star is quite faint in the infrared, the photometric errors are very large and we doubt that this star really has an optically thick accretion disk.
RXJ1556.1-3655 (= Th 11) shows strong H emission ( equivalent width), thus the presence of an accretion disk is not unexpected. The third star, RXJ1608.6-3922, also shows H in emission, with an equivalent width of . This value is below the standard limit of for CTTS, but the H emission of CTTS is known to be variable, thus this star might well be a CTTS.
However, nearly all of the stars discovered by Krautter et al. (1996 ) for which infrared photometry is available, do not show any evidence for optically thick accretion disks. This confirms their classification of these stars as WTTS, which was based on the H equivalent width alone.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998