shock-cooling emission lines and using the HST's high ultraviolet spatial resolu- 

 tion, one might hope to trace details of the HH flows into the immediate stellar 

 vicinity using their characteristic shock lines. These observations might bear at 

 least indirectly on the issue of confinement of HH flows by circumstellar disks. 

 Examination of the "base" of an HH flow might serve to determine whether a 

 mechanism similar to a solar coronal hole is operating near the protostellar pole 

 or whether the confinement and orientation of these flows arises some distance 

 above the stellar surface. 



A crucial question, of course, relates to the nature of those stars that are 

 known to drive HH flows or jets. It is believed that they are the precursors of 

 even T Tauri stars, but they are often so deeply embedded in dark clouds that 

 they are not directly visible, optically. One technique for studying their photo- 

 spheres is to investigate the spectrum of scattered starlight from circumstellar 

 clouds and nebulae. This method has begun to bear fruit, optically, and it could 

 certainly be extended into the ultraviolet with the HST. 



Polarization studies on this scale may also yield important information on 

 the structure of the circumstellar disk, through the asymmetrical scattering of 

 starlight by grains in the disk. 



Recent work with the IUE has demonstrated that variable ultraviolet absorp- 

 tion lines occur in the spectrum of Beta Pictoris. These lines are thought to arise 

 in the circumstellar disk detected in the infrared around this star. Such spectra 

 could be obtained with the HST for a much larger sample of candidate stars 

 around which IRAS observations indicate the potential presence of dusty disks. 



SI RTF and ISO will be able to detect the infrared continuum emission from 

 low-mass protostars throughout the galaxy, although their relatively large beam 

 size in the far-infrared (~30 arcsec), where most of the luminosity is emitted, 

 will present some confusion problems for distances of about 10 kpc or larger. 

 LDR will be able to detect low-mass protostars to a distance of 100 kpc. This 

 includes, besides our own galaxy, the Large and Small Magellanic Clouds. 



SIRTF and ISO could provide diffraction-limited, two-dimensional imaging of 

 near-infrared starlight scattered by disk particles. Because of its larger aperture 

 size, SOFIA could improve upon these telescopes for far-infrared studies of the 

 collapsing envelope. In particular, because of the increased spatial resolution, 

 one might hope to resolve all dimensions of the collapsing envelope. Such studies 

 will also be helped by the advent of new detector technology (e.g., the develop- 

 ment of linear and fully two-dimensional detector arrays designed to oversample 

 the Airy disk, thereby producing the spatial resolution close to the theoretical 

 limit). 



Suggestions for Further Reading 



Black, D. C; and Matthews, M. S., eds.: Protostars and Protoplanets II. 

 University of Arizona Press, Tucson, 1985. 

 Rev. Mexicana Astron. Astrofis., vol. 7, 1983. 



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