We are engaged in a program to survey the circumprotostellar environments of a complete sample of low-mass protostars in Taurus, whose infrared (Tamura et al 1991) and mm/sub-mm properties (Moriarty-Schieven et al. 1992, 1994) indicate them to be younger than T Tauri stars. We have recently completed a sub-mm photometric survey of our sources at the JCMT, ranging in wavelength from 450 mm to 2mm. Beckwith & Sargent (1991) have observed the mm/sub-mm emission from a selection of T Tauri stars in Taurus, which have similar luminosities to our younger sources. The range of flux densities seen for the two types of object were similar, suggesting that the masses of the circumstellar envelopes/disks were also similar. Let a be the slope in the relation S(n) ~ l-a. Then, when we compare the slopes of the spectral energy distributions (SED), we found that there was a significant difference between embedded sources and optically visible T Tauri stars. The older, more evolved T Tauri stars had a significantly shallower SED at mm/sub-mm wavelengths than the younger, embedded sources. This is clearly shown in Figure 1.

This dichotomy is clearly a sign of the evolution of the circumstellar envelope/disk. As the protostar evolves, one expects changes in the properties of the disk/envelope. The envelope presumably shrinks as it accretes onto the circumstellar disk, while the disk should become more optically thick as it builds up mass. One might also expect that the dust properties themselves will change, since the standard model for planet formation starts with the growth of dust particles. Both effects can be expected to affect the SED of the disk/envelope at long wavelengths. For example, the more optically thick disk in the evolved object may tend to dominate the SED at longer wavelengths, making the spectral index shallower (Butner, Natta, & Evans 1994). Also, in a denser disk environment, dust particles should be growing "fluffier", changing the dust emissivity law (index b). Spherical dielectric grains have b = 2 (Draine & Lee 1984), while "fluffy" fractal particles may have b <=1 (Wright 1987). Miyake & Nakagawa (1993) have indeed shown that as spherical dust grains grow to sizes comparable to sub-millimeter and millimeter wavelengths, their emissivity laws become much smaller. The much shallower SED for the more evolved objects is thus probably due either to the growth of the circumstellar disk at the expense of the envelope, or to the growth of the dust particles themselves, or perhaps is due to both effects.

References:

Beckwith, S. V. W., & Sargent, A. I. 1991, ApJ, 381, 250

Butner, H. M., Natta, A., & Evans, N., J. 1994, ApJ, 400, 326

Draine, B. T., & Lee, H. M. 1984, ApJ, 285, 89

Miyake, K., & Nakagawa, Y. 1993, Icarus, 106, 20

Moriarty-Schieven, G. H., Wannier, P. G., Tamura, M., & Keene, J. 1992, ApJ, 400, 260

Moriarty-Schieven, G. H., Wannier, P. G., Keene, J., & Tamura, M. 1994, ApJ, in press

Tamura, M., Gatley, I., Waller, W., & Werner, M. W. 1991, ApJ, 374, L25

Wright, E. L. 1987, ApJ, 320, 818

G.H. Moriarty-Schieven (DRAO, Penticton),

P.G. Wannier (JPL, Pasadena),

Jocelyn Keene (Caltech, Pasadena),

M. Tamura (NAO)

& H.M. Butner (DTM-CIW)