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Astron. Astrophys. 320, 185-195 (1997)

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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 [FORMULA] is the dereddened color of the star and [FORMULA] the intrinsic color corresponding to its spectral type. Edwards et al. (1993 ) find [FORMULA] 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 [FORMULA] 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 [FORMULA]. One of them, RXJ 1514.8-4220, is a K1 star which shows H [FORMULA] 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 [FORMULA] emission ([FORMULA] equivalent width), thus the presence of an accretion disk is not unexpected. The third star, RXJ1608.6-3922, also shows H [FORMULA] in emission, with an equivalent width of [FORMULA]. This value is below the standard limit of [FORMULA] for CTTS, but the H [FORMULA] 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 [FORMULA] equivalent width alone.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998