Stella S. R. Offner

The formation of massive star systems by accretion.

Massive stars produce so much light that the radiation pressure they exert on the gas and dust around them is stronger than their gravitational attraction, a condition that has long been expected to prevent them from growing by accretion. We present three-dimensional radiation-hydrodynamic simulations of the collapse of a massive prestellar core and find that radiation pressure does not halt accretion. Instead, gravitational and Rayleigh-Taylor instabilities channel gas onto the star system through nonaxisymmetric disks and filaments that self-shield against radiation while allowing radiation to escape through optically thin bubbles. Gravitational instabilities cause the disk to fragment and form a massive companion to the primary star. Radiation pressure does not limit stellar masses, but the instabilities that allow accretion to continue lead to small multiple systems.

THE EFFECTS OF RADIATIVE TRANSFER ON LOW-MASS STAR FORMATION

Forming stars emit a substantial amount of radiation into their natal environment. We use ORION, an adaptive mesh refinement (AMR) three-dimensional gravito-radiation-hydrodyanics code, to simulate low-mass star formation in a turbulent molecular cloud. We compare the distributions of stellar masses, accretion rates, and temperatures in the cases with and without radiative transfer, and we demonstrate that radiative feedback has a profound effect on accretion, multiplicity, and mass by reducing the number of stars formed and the total rate at which gas turns into stars. We also show that once the star formation reaches a steady state, protostellar radiation is by far the dominant source of energy in the simulation, exceeding viscous dissipation and compressional heating by at least an order of magnitude. Calculations that omit radiative feedback from protstars significantly underestimate the gas temperature and the strength of this effect. Although heating from protostars is mainly confined to the protostellar cores, we find that it is sufficient to suppress disk fragmentation that would otherwise result in very low-mass companions or brown dwarfs. We demonstrate that the mean protostellar accretion rate increases with the final stellar mass so that the star formation time is only a weak function of mass.

Driven and Decaying Turbulence Simulations of Low-Mass Star Formation: From Clumps to Cores to Protostars

Molecular clouds are observed to be turbulent, but the origin of this turbulence is not well understood. As a result, there are two different approaches to simulating molecular clouds, one in which the turbulence is allowed to decay after it is initialized, and one in which it is driven. We use the adaptive mesh refinement (AMR) code, Orion, to perform high-resolution simulations of molecular cloud cores and protostars in environments with both driven and decaying turbulence. We include self-gravity, use a barotropic equation of state, and represent regions exceeding the maximum grid resolution with sink particles. We analyze the properties of bound cores such as size, shape, line width, and rotational energy, and we find reasonable agreement with observation. At high resolution the different rates of core accretion in the two cases have a significant effect on protostellar system development. Clumps forming in a decaying turbulence environment produce high-multiplicity protostellar systems with Toomre -->Q unstable disks that exhibit characteristics of the competitive accretion model for star formation. In contrast, cores forming in the context of continuously driven turbulence and virial equilibrium form smaller protostellar systems with fewer low-mass members. Our simulations of driven and decaying turbulence show some statistically significant differences, particularly in the production of brown dwarfs and core rotation, but the uncertainties are large enough that we are not able to conclude whether observations favor one or the other.

STELLAR KINEMATICS OF YOUNG CLUSTERS IN TURBULENT HYDRODYNAMIC SIMULATIONS

The kinematics of newly formed star clusters are interesting both as a probe of the state of the gas clouds from which the stars form, and because they influence planet formation, stellar mass segregation, cluster disruption, and other processes controlled in part by dynamical interactions in young clusters. However, to date there have been no attempts to use simulations of star cluster formation to investigate how the kinematics of young stars change in response to variations in the properties of their parent molecular clouds. In this Letter, we report the results of turbulent self-gravitating simulations of cluster formation in which we consider both clouds in virial balance and those undergoing global collapse. We find that stars in these simulations generally have velocity dispersions smaller than that of the gas by a factor of ~5, independent of the dynamical state of the parent cloud, so that subvirial stellar velocity dispersions arise naturally even in virialized molecular clouds. The simulated clusters also show large-scale stellar velocity gradients of ~0.2-2 km s–1 pc–1 and strong correlations between the centroid velocities of stars and gas, both of which are observed in young clusters. We conclude that star clusters should display subvirial velocity dispersions, large-scale velocity gradients, and strong gas-star velocity correlations regardless of whether their parent clouds are in virial balance, and, conversely, that observations of these features cannot be used to infer the dynamical state of the parent gas clouds.

OBSERVING SIMULATED PROTOSTARS WITH OUTFLOWS: HOW ACCURATE ARE PROTOSTELLAR PROPERTIES INFERRED FROM SEDs?

The properties of unresolved protostars and their local environment are frequently inferred from spectral energy distributions (SEDs) using radiative transfer modeling. In this paper, we use synthetic observations of realistic star formation simulations to evaluate the accuracy of properties inferred from fitting model SEDs to observations. We use ORION, an adaptive mesh refinement (AMR) three-dimensional gravito-radiation-hydrodynamics code, to simulate low-mass star formation in a turbulent molecular cloud including the effects of protostellar outflows. To obtain the dust temperature distribution and SEDs of the forming protostars, we post-process the simulations using HYPERION, a state-of-the-art Monte Carlo radiative transfer code. We find that the ORION and HYPERION dust temperatures typically agree within a factor of two. We compare synthetic SEDs of embedded protostars for a range of evolutionary times, simulation resolutions, aperture sizes, and viewing angles. We demonstrate that complex, asymmetric gas morphology leads to a variety of classifications for individual objects as a function of viewing angle. We derive best-fit source parameters for each SED through comparison with a pre-computed grid of radiative transfer models. While the SED models correctly identify the evolutionary stage of the synthetic sources as embedded protostars, we show that the disk and stellar parameters can be very discrepant from the simulated values, which is expected since the disk and central source are obscured by the protostellar envelope. Parameters such as the stellar accretion rate, stellar mass, and disk mass show better agreement, but can still deviate significantly, and the agreement may in some cases be artificially good due to the limited range of parameters in the set of model SEDs. Lack of correlation between the model and simulation properties in many individual instances cautions against overinterpreting properties inferred from SEDs for unresolved protostellar sources.

RADIATION-HYDRODYNAMIC SIMULATIONS OF PROTOSTELLAR OUTFLOWS: SYNTHETIC OBSERVATIONS AND DATA COMPARISONS

We present results from three-dimensional, self-gravitating, radiation-hydrodynamic simulations of low-mass protostellar outflows. We construct synthetic observations in 12CO in order to compare with observed outflows and evaluate the effects of beam resolution and outflow orientation on inferred outflow properties. To facilitate the comparison, we develop a quantitative prescription for measuring outflow opening angles. Using this prescription, we demonstrate that, in both simulations and synthetic observations, outflow opening angles broaden with time similarly to observed outflows. However, the interaction between the outflowing gas and the turbulent core envelope produces significant asymmetry between the redshifted and blueshifted outflow lobes. We find that applying a velocity cutoff may result in outflow masses that are underestimated by a factor five or more, and masses derived from optically thick CO emission further underpredict the mass of the high-velocity gas by a factor of 5-10. Derived excitation temperatures indicate that outflowing gas is hotter than the ambient gas with temperature rising over time, which is in agreement with the simulation gas temperatures. However, excitation temperatures are otherwise not well correlated with the actual gas temperature.

THE PROTOSTELLAR MASS FUNCTION

The protostellar mass function (PMF) is the present-day mass function of the protostars in a region of star formation. It is determined by the initial mass function weighted by the accretion time. The PMF thus depends on the accretion history of protostars and in principle provides a powerful tool for observationally distinguishing different protostellar accretion models. We consider three basic models here: the isothermal sphere model, the turbulent core model, and an approximate representation of the competitive accretion model. We also consider modified versions of these accretion models, in which the accretion rate tapers off linearly in time. Finally, we allow for an overall acceleration in the rate of star formation. At present, it is not possible to directly determine the PMF since protostellar masses are not currently measurable. We carry out an approximate comparison of predicted PMFs with observation by using the theory to infer the conditions in the ambient medium in several star-forming regions. Tapered and accelerating models generally agree better with observed star formation times than models without tapering or acceleration, but uncertainties in the accretion models and in the observations do not allow one to rule out any of the proposed models at present. The PMF is essential for the calculation of the protostellar luminosity function, however, and this enables stronger conclusions to be drawn.

The Kinematics of Molecular Cloud Cores in the Presence of Driven and Decaying Turbulence: Comparisons with Observations

In this study, we investigate the formation and properties of prestellar and protostellar cores using hydrodynamic self-gravitating adaptive mesh refinement simulations, comparing the cases where turbulence is continually driven and where it is allowed to decay. We model observations of these cores in the C18O(2 → 1), NH3(1, 1), and N2H+(1 → 0) lines, and from the simulated observations we measure the line widths of individual cores, the line widths of the surrounding gas, and the motions of the cores relative to one another. Some of these distributions are significantly different in the driven and decaying runs, making them potential diagnostics for determining whether the turbulence in observed star-forming clouds is driven or decaying. Comparing our simulations with observed cores in the Perseus and ρ Ophiuchus clouds shows reasonably good agreement between the observed and simulated core-to-core velocity dispersions for both the driven and decaying cases. However, we find that the line widths through protostellar cores in both simulations are too large compared to the observations. The disagreement is noticeably worse for the decaying simulation, in which cores show highly supersonic infall signatures in their centers that decrease toward their edges, a pattern not seen in the observed regions. This result gives some support to the use of driven turbulence for modeling regions of star formation, but reaching a firm conclusion on the relative merits of driven or decaying turbulence will require more complete data on a larger sample of clouds as well as simulations that include magnetic fields, outflows, and thermal feedback from the protostars.

An alternative accurate tracer of molecular clouds: the ‘XCi-factor’

We explore the utility of CI as an alternative high-fidelity gas mass tracer for Galactic molecular clouds. We evaluate the X

MODELING THE ATOMIC-TO-MOLECULAR TRANSITION AND CHEMICAL DISTRIBUTIONS OF TURBULENT STAR-FORMING CLOUDS

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