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B. Zuckerman, R. A. Webb (UCLA)
A ubiquitous signpost of newly formed stars has been a nearby interstellar molecular cloud. But no interstellar cloud has been found near the TW Hya Association (TWA) despite multiple searches. Therefore one wonders: Do additional unrealized associations far from molecular clouds exist near Earth? The answer is likely "yes". Based on the Hipparcos, PPM, ROSAT,and IRAS catalogs, Zuckerman and Webb (ApJ, in press) identified two regions in the southern hemisphere which appear to contain young (20 Myr old?) nearby (~50 pc from Earth) stellar associations whose members have similar proper motions and location in space. Probable members include, for example, PZ Tel and the IRAS excess stars HD 181327 and HR 7329. With NICMOS/HST, Lowrance et al (ApJ in press) found a likely brown dwarf companion to HR 7329. Yet more clusters may exist within 50 pc of Earth, but if their brightest members are too faint to appear in either the IRAS or Hipparcos catalogs, then their discovery will, likely, need to await deep proper motion catalogs currently in preparation.
In the coming decade, dust and gas in dissipating circumstellar disks near TWA members will be studied with unprecedented angular resolution with new ground, airborne, and space-based telescopes . With adaptive optics on large ground-based telescopes it should be possible to detect thermal emission at near IR wavelengths from cooling 1-5 Jupiter mass planets with semi-major axes of order tens of AU that orbit members of the TWA, that is, extrasolar giant planets located in the same general region as the giant planets of our solar system. The age of the TWA, ~ 10 Myrs, is comparable to the time it took Jupiter to form according to some models of the early solar system. Since late M-type objects have been detected already as companions to a few TWA members, future investigations may reveal free floating TWA members with masses near and below that of minimum mass stars.