envelope of the protostar. The mass in these regions is difficult to estimate from 

 the far-infrared observations because the dust properties are essentially 

 unknown. Moreover, these observations probe only the dust and not the gas, 

 which makes up most of the mass. Ground-based aperture-synthesis observations 

 of the gas through molecular line transitions in the millimeter range will be 

 forthcoming. Although such observations can yield much insight into the 

 dynamics of these systems, mass estimates will still be difficult, because the 

 dominant component of the gas, molecular hydrogen (H 2 ), cannot be observed 

 this way. 



T Tauri stars are newly formed stars as evidenced by their high lithium abun- 

 dance, their high rotation velocities, and their general association with dark 

 clouds. Their ages, as estimated from their position in the Hertzsprung-Russell 

 (HR) diagram, range from about 10 5 to 10 7 years. T Tauri stars are often sur- 

 rounded by a circumstellar disk. As inferred from statistics, these disks are 

 presumed to have a large radius-to-thickness ratio and they give the impression 

 of a continuous transition in the morphology of protostellar nebulae from the 

 very earliest protostellar stages (the HH-exciting star), through the pre-main- 

 sequence stars (the T Tauri stars), to the main sequence, where protoplanetary 

 "disks" may be detected. Recent studies indicate that T Tauri stars are very 

 active and exert a significant influence on their environment. Besides the strong 

 stellar wind mentioned before, X-ray and chromospheric emission have been 

 detected, indicating strong flare activity on the protostellar surface. All T Tauri 

 stars may undergo the FU Orionis phenomenon (a major stellar eruption) during 

 their evolution. 



Among the T Tauri stars, the object HL Tau is a keystone. It is currently the 

 only T Tauri star to show ice absorption in its circumstellar shell. It also exhibits 

 the strongest silicate absorption feature at 10 /jm and the greatest degree of 

 visible linear polarization. Polarimetric, near-infrared images indicate a flattened, 

 extended object on a scale of 10 2 AU. The star shows HH object nebulosity as 

 well as an extended streamer or jet. All of these characteristics point toward the 

 presence of a flattened dust distribution close to the exciting star, possibly a 

 dusty disk. In the HR diagram, HL Tau is located at the top of the convective 

 track for a 1-M^ star, with a stellar age of about 10 s years. HL Tau may there- 

 fore provide an ideal analog for the protosun at age 1 5 years. 



Estimates of the column density in small particles provide a lower limit to 

 the mass of dust that is consistent with the far-infrared estimates of 

 1-6X10" 5 Mq. Much more material might be hidden at the much smaller scale 

 size than presently resolvable, or the gas in the immediate vicinity of HL Tau 

 may have already condensed into solid grains and accreted into undetectable 

 large (centimeter-sized) particles. 



Understanding the formation of stars and planets and the distribution and 

 composition of matter (particularly the biogenic compounds) in protostellar 

 nebulae is a topic for exobiology. This includes studies of the physical and 



49 



