Regions Of Protostellar Disks Research Articles

EMBEDDED PROTOSTELLAR DISKS AROUND (SUB-)SOLAR STARS. II. DISK MASSES, SIZES, DENSITIES, TEMPERATURES, AND THE PLANET FORMATION PERSPECTIVE

We present basic properties of protostellar disks in the embedded phase of star formation (EPSF), which is difficult to probe observationally using available observational facilities. We use numerical hydrodynamics simulations of cloud core collapse and focus on disks formed around stars in the 0.03-1.0 Msun mass range. Our obtained disk masses scale near-linearly with the stellar mass. The mean and median disk masses in the Class 0 and I phases (M_{d,C0}^{mean}=0.12 Msun, M_{d,C0}^{mdn}=0.09 Msun and M_{d,CI}^{mean}=0.18 Msun, M_{d,CI}^{mdn}=0.15 Msun, respectively) are greater than those inferred from observations by (at least) a factor of 2--3. We demonstrate that this disagreement may (in part) be caused by the optically thick inner regions of protostellar disks, which do not contribute to millimeter dust flux. We find that disk masses and surface densities start to systematically exceed that of the minimum mass solar nebular for objects with stellar mass as low as M_st=0.05-0.1 Msun. Concurrently, disk radii start to grow beyond 100 AU, making gravitational fragmentation in the disk outer regions possible. Large disk masses, surface densities, and sizes suggest that giant planets may start forming as early as in the EPSF, either by means of core accretion (inner disk regions) or direct gravitational instability (outer disk regions), thus breaking a longstanding stereotype that the planet formation process begins in the Class II phase.

DEUTERIUM CHEMISTRY IN PROTOPLANETARY DISKS. II. THE INNER 30 AU

We present the results of models of the chemistry, including deuterium, in the inner regions of protostellar disks. We find good agreement with recent gas phase observations of several (non--deuterated) species. We also compare our results with observations of comets and find that in the absence of other processing e.g. in the accretion shock at the surface of the disk, or by mixing in the disk, the calculated D/H ratios in ices are higher than measured and reflect the D/H ratio set in the molecular cloud phase. Our models give quite different abundances and molecular distributions to other inner disk models because of the differences in physical conditions in the model disk. This emphasizes how changes in the assumptions about the density and temperature distribution can radically affect the results of chemical models.

Benzene Formation in the Inner Regions of Protostellar Disks

Benzene (c-C6H6) formation in the inner 3 AU of a protostellar disk can be efficient, resulting in high abundances of benzene in the midplane region. The formation mechanism is different to that found in interstellar clouds and in protoplanetary nebulae, and proceeds mainly through the reaction between allene (C3H4) and its ion. This has implications for PAH formation, in that some fraction of PAHs seen in the solar system could be native rather than inherited from the interstellar medium.

Molecular distributions in the inner regions of protostellar disks

The distributions of molecules in the inner regions of a protostellar disk are presented. These were calculated using an uncoupled chemical/dynamical model, with a numerical integration of the vertical disk struc- ture. A comparison between models with and without the eects of X-ray ionisation is made, and molecules are identied which are good tracers of the ionisation level in this part of the disk, notably CN and C2H. In the re- gion considered in this paper (r 10 AU), the chemistry is dominated by the thermal desorption of species from grains. This shows that a critically important detail in this region of the disk, as far as molecular distributions are concerned, is the temperature prole. We nd that not all of the gaseous material is frozen onto grain surfaces at 10 AU, and we identify species, including some organic molecules, which should exist in observable quantities in the inner regions of protostellar disks.