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This study constitutes one part of our multi-disciplinary approach to the evolution of planet-forming disks. The goal is to establish the disks' thermal and mechanical properties as they grow by the infall of their parent interstellar clouds. Thus far, significant advances toward establishing the evolving surface density of such disks was made.
We have studied the protostellar system in S140 at 2.2, 3.1 and 3.45 microns using a 128x128 InSb array at the Lick Observatory 3m telescope. Besides the protostellar sources, the data reveal a bright infrared reflection nebula. We have developed a simple model of this region and derived the physical conditions. IRSI is surrounded by a dense dusty disk viewed almost edge-on. Photons leaking out through the poles illuminate almost directly north and south the inner edge of a surrounding shell of molecular gas, Analysis of the observed colors and intensities of the NIR light, using Mie scattering theory, reveal that the dust grains in the molecular cloud are somewhat larger than in the general diffuse interstellar medium. Moreover, the incident light has a "cool" color temperature, approximately equals 800K, and likely originates from a dust photosphere close to the protostar. Finally, we find little H2O ice associated with the dusty disk around IRSI. Most of the 3.1 micron ice extinction arises instead from cool intervening molecular cloud material. We have compared our infrared dust observations with millimeter and radio observations of molecular gas associated with this region. The large scale structure observable in the molecular gas is indicative of the interaction between the protostellar wind and the surrounding molecular cloud rather than the geometry of the protostellar disk. We conclude that S140 is a young blister formed by this outflow on the side of a molecular cloud and viewed edge-on.
The class 0 sources form an interesting new category of protostellar objects. Many have strong millimeter continuum emission and exhibit jetlike outflows. There are suggestions that these objects are systematically younger than typical embedded (class I) sources. We are investigating the properties of class 0 sources to determine whether they are indeed very young or perhaps represent extreme physical conditions, such as rapid rotation. We present millimeter interferometric data for the class 0 object known as L1448 IRS3. This young low-mass star has extremely strong millimeter continuum emission. The interferometer data show the emission is resolved on a scale of a few arcseconds (1000 AU) This suggests the bulk of the dust continuum emission originates in an 'infall' envelope rather than a protostar disk. In addition, the C180 line data display a strong velocity gradiant which indicates the dense core is rapidly rotating. We compare our data with the predictions of protostellar collapse models.
A semiquantitative description of the origin of interstellar grains in a protostellar environment is given. Grains grow in the protostellar collapse phase and can be expelled by radiation pressure from stars of at least about 5 solar masses. A convective shell of grains forms in which shattering produces a power-law spectrum of fragments. A bimodal distribution of grain sizes, peaking at approximately 0.1 and 0.01 micron results. The effects of protostellar winds and rotation are considered.
Recent radio observations have indicated that appreciable, continuing mass loss is occurring from regions of star formation. The conditions under which massive protostellar objects may exhibit strong, rotationally driven winds are investigated. For plausible rotational velocities and magnetic field strengths, ejection of about 10,000 solar masses a year at speeds of 10-100 km/s can be maintained for time scales of about 0.0001 yr; these values are insensitive to the protostellar luminosity. An evolutionary scenario is sketched which suggests that protostellar clouds may be expected to be rapidly rotating and to possess substantial magnetic fields when the clouds have contracted to radii of about 10 to the 14th cm. The centrifugally driven winds proposed present an alternative to fragmentation for providing the angular momentum loss required in the phase of rapid cloud contraction.
The formation of rapidly rotating protostellar objects in turbulent and clumpy molecular clouds is analyzed. It is shown that the early dissipation of the protostellar rotational energy via a rotationally driven wind can keep their parent molecular clouds at the observed temperatures and in the observed turbulent state. The model requires a low space density of protostellar shells in order to provide the energy requirements to stabilize molecular clouds against gravitational collapse in regions of star formation. The dependence of this mechanism on the star formation rate suggests that the star formation is self-regulated.
The status of theoretical work on protostellar disks is reviewed. Accretion disk theory and its application to models of the solar nebula and protostellar disks are discussed. A unified view of the process of star formation is presented, starting from the evolution of molecular clouds, and leading naturally to the formation of protostellar disks. The models used to describe this process are idealized, but are believed to provide good prototypes that well represent the essential hydromagnetic phenomena involved in star and disk formation. Several possible evolutionary paths and final configurations are qualitatively discussed, showing how the outcomes depend on the relative efficiencies of various angular momentum transport processes.
This paper investigates the effect of variations in the Rosseland mean opacity of dust grains on numerical models of three-dimensional protostellar collapse and fragmentation. In particular, it is found that increasing the dust grain opacity by factors of three to four has little effect upon the gross characteristics of protostellar fragmentation. Consequently, theoretical quantities such as the estimated minimum protostellar mass for Population I star formation are insensitive to the precise value of the opacity.
The production and destruction processes for molecules in very fast protostellar winds are analyzed and modeled with a one-dimensional chemical kinetics code. Radial density and temperature distributions suggested by protostellar theory are explored as are a range of mass-loss rates. The efficiency of in situ formation of heavy molecules is found to be high if the wind temperature falls sufficiently rapidly, as indicated by theory. The degree of molecular conversion is a strong function of the mass-loss rate and of density gradients associated with the acceleration and collimation of the wind. Even in cases where essentially all of the heavy atoms are processed into molecules, a significant fraction of atomic hydrogen remains so that hghly molecular, protostellar winds are able to emit the 21-cm line. Although CO has a substantial abundance in most models relevant to very young protostars, high abundances of other molecules such as SiO and H2O signify more complete association characteristic of winds containing regions of very high density. Although the models apply only to regions close to the protostar, they are in qualitative accord with recent observations at much larger distances of both atomic and molecular emission from extremely high-velocity flow.
In Boss & Myhill (1992) we described the derivation and testing of a spherical coordinate-based scheme for solving the hydrodynamic equations governing the gravitational collapse of nonisothermal, nonmagnetic, inviscid, radiative, three-dimensional protostellar clouds. Here we discuss a Cartesian coordinate-based scheme based on the same set of hydrodynamic equations. As with the spherical coorrdinate-based code, the Cartesian coordinate-based scheme employs explicit Eulerian methods which are both spatially and temporally second-order accurate. We begin by describing the hydrodynamic equations in Cartesian coordinates and the numerical methods used in this particular code. Following Finn & Hawley (1989), we pay special attention to the proper implementations of high-order accuracy, finite difference methods. We evaluate the ability of the Cartesian scheme to handle shock propagation problems, and through convergence testing, we show that the code is indeed second-order accurate. To compare the Cartesian scheme discussed here with the spherical coordinate-based scheme discussed in Boss & Myhill (1992), the two codes are used to calculate the standard isothermal collapse test case described by Bodenheimer & Boss (1981). We find that with the improved codes, the intermediate bar-configuration found previously disappears, and the cloud fragments directly into a binary protostellar system. Finally, we present the results from both codes of a new test for nonisothermal protostellar collapse.
This program implements a spectral line, radiative transfer tool for interpreting Spitzer Space Telescope observations by matching them with models of protostellar disks for improved understanding of planet and star formation. The Spitzer Space Telescope detects gas phase molecules in the infrared spectra of protostellar disks, with spectral lines carrying information on the chemical composition of the material from which planets form. Input to the software includes chemical models developed at JPL. The products are synthetic images and spectra for comparison with Spitzer measurements. Radiative transfer in a protostellar disk is primarily affected by absorption and emission processes in the dust and in molecular gases such as H2, CO, and HCO. The magnitude of the optical absorption and emission is determined by the population of the electronic, vibrational, and rotational energy levels. The population of the molecular level is in turn determined by the intensity of the radiation field. Therefore, the intensity of the radiation field and the population of the molecular levels are inter-dependent quantities. To meet the computational challenges of solving for the coupled radiation field and electronic level populations in disks having wide ranges of optical depths and spatial scales, the tool runs in parallel on the JPL Dell Cluster supercomputer with C++ and Fortran compiler with a Message Passing Interface. Because this software has been developed on a distributed computing platform, the modeling of systems previously beyond the reach of available computational resources is possible.
During the formation of stars, the accretion of surrounding material toward the central object is thought to undergo strong luminosity outbursts followed by long periods of relative quiescence, even at the early stages of star formation when the protostar is still embedded in a large envelope. We investigated the gas-phase formation and recondensation of the complex organic molecules (COMs) di-methyl ether and methyl formate, induced by sudden ice evaporation processes occurring during luminosity outbursts of different amplitudes in protostellar envelopes. For this purpose, we updated a gas-phase chemical network forming COMs in which ammonia plays a key role. The model calculations presented here demonstrate that ion-molecule reactions alone could account for the observed presence of di-methyl ether and methyl formate in a large fraction of protostellar cores without recourse to grain-surface chemistry, although they depend on uncertain ice abundances and gas-phase reaction branching ratios. In spite of the short outburst timescales of about 100 years, abundance ratios of the considered species higher than 10% with respect to methanol are predicted during outbursts due to their low binding energies relative to water and methanol which delay their recondensation during cooling. Although the current luminosity of most embedded protostars would be too low to produce complex organics in the hot-core regions that are observable with current sub-millimetric interferometers, previous luminosity outburst events would induce the formation of COMs in extended regions of protostellar envelopes with sizes increasing by up to one order of magnitude.
The paper discusses a full three spatial-dimension gravitational hydrodynamic code used to follow the collapse of isothermal rotating clouds subjected to various nonaxially symmetric perturbations (NAP). An initially axially symmetric cloud collapsed to form a ring which then fragmented into a binary protostellar system; a low thermal energy cloud with a large bar-shaped NAP collapsed and fragmented into a binary, and higher thermal energy clouds damp out such NAPs while higher rotational energy clouds produce binaries with wider separations. The three-dimensional calculations indicate that isothermal interstellar clouds may fragment into protostellar objects while still in the isothermal regime. Interstellar clouds and their fragments may pass through collapse phases with fragmentation and reduction of spin angular momentum terminating in the formation of pre-main-sequence stars with the observed pre-main-sequence rotation rates.
Aspects of the current theory of molecular-cloud (MC) evolution and star formation are reviewed, with consideration of recent star and MC observations. The problem of MC collapse is discussed in terms of fragmentation in spherically symmetric collapse, the role of magnetic fields and density fluctuations, anisotropic collapse, and nonlinear interactions between fragments. Protostellar winds are identified as the source of the energy supporting MC against collapse, sweeping shells of mass into MC to produce low-mass stars continuously, and sustaining the production of massive stars once it is triggered by the collision of MCs, as in the spiral density-wave peaks. The clumping and coagulation processes are considered for the case of the MC in Orion. The protostellar-wind mass input required to inhibit systematic MC-core collapse is estimated at 10 to the -6th solar mass/cu pc year.
The application of rotational stability criteria to a specific model of star formation leads to the conclusion that the growth of stellar angular momentum is limited by its transfer to the disk. Excess accreted angular momentum can be transferred by torques connected with spiral density waves induced by even a slight protostellar triaxiality. In addition, viscous damping of the density waves is likely to cause the excess angular momentum to be deposited within a small region close to the protostar. Thus, it would be appropriate to treat that part of the growing protostellar disk beyond the outer Lindblad resonance as an accretion disk with a torque applied to its inner edge. It is noted that this situation is directly relevant to certain models of the evolution of the protosun and solar nebula.
Transmission electron microscopy of a hydrated interplanetary dust particle (IDP) indicates that it contains abundant magnesium-iron carbonates, primarily breunnerite and magnesian siderite. This IDP displays a strong absorption band at 6.8 micrometers in its infrared spectrum, similar to that in certain protostellar spectra. The carbonates probably account for the 6.8-micrometer band in the IDP spectrum, suggesting that carbonate also may occur in interstellar dust and be the source of the controversial 6.8-micrometer feature from the protostellar spectra.
Numerical calculations are presented for rigorous models spanning a four-dimensional parameter space of initial conditions of the three-dimensional collapse of rotating protostellar clouds, encompassing radiative transfer in the Eddington approximation and detailed thermodynamical relations. It is found that protostellar formation may involve a few stages of hierarchical fragmentation terminated by increased thermal pressure in the nonisothermal regime, that high thermal energy clouds remain nearly axisymmetric during the first dynamic collapse phase, and that very slowly rotating clouds can fragment. The presolar nebula was probably formed from a cloud with very little initial rotation.
The collapse and fragmentation of rotating protostellar clouds is explored, starting from nonuniform density and nonuniform rotation initial conditions. Whether binary fragmentation occurs during the first dynamic collapse phase depends strongly on the initial density profile. Exponential clouds are only somewhat more resistant to fragmentation than uniform-density clouds, but power-law clouds do not undergo fragmentation for likely values of a relevant parameter. Because binary fragments start from profiles intermediate between uniform density and exponential clouds, minimum protostellar mass for population I stars should be increased to approximately 0.02 solar mass. The axisymmetric Terey et al. (1984) model should be stable with respect to nonaxisymmetric perturbations. Considering the observed binary frequency, collapse from power-law initial conditions appears to be less common than collapse from more uniform initial conditions.