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Title: The VLA Nascent Disk and Multiplicity Survey of Perseus Protostars (VANDAM). IV. Free–Free Emission from Protostars: Links to Infrared Properties, Outflow Tracers, and Protostellar Disk Masses
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Journal Name:
The Astrophysical Journal Supplement Series
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Medium: X
Sponsoring Org:
National Science Foundation
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  1. Abstract

    Massive protostars launch accretion-powered, magnetically collimated outflows, which play crucial roles in the dynamics and diagnostics of the star formation process. Here we calculate the shock heating and resulting free–free radio emission in numerical models of outflows of massive star formation within the framework of the Turbulent Core Accretion model. We postprocess 3D magnetohydrodynamic simulation snapshots of a protostellar disk wind interacting with an infalling core envelope, and calculate shock temperatures, ionization fractions, and radio free–free emission. We find heating up to ∼107K and near-complete ionization in shocks at the interface between the outflow cavity and infalling envelope. However, line-of-sight averaged ionization fractions peak around ∼10%, in agreement with values reported from observations of massive protostar G35.20-0.74N. By calculating radio-continuum fluxes and spectra, we compare our models with observed samples of massive protostars. We find our fiducial models produce radio luminosities similar to those seen from low- and intermediate-mass protostars that are thought to be powered by shock ionization. Comparing to more massive protostars, we find our model radio luminosities are ∼10–100 times less luminous. We discuss how this apparent discrepancy either reflects aspects of our modeling related to the treatment of cooling of the post-shock gas or a dominant contribution in the observed systems from photoionization. Finally, our models exhibit 10 yr radio flux variability of ∼5%, especially in the inner 1000 au region, comparable to observed levels in some hypercompact Hiiregions.

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  2. Abstract

    Star formation is ubiquitously associated with the ejection of accretion-powered outflows that carve bipolar cavities through the infalling envelope. This feedback is expected to be important for regulating the efficiency of star formation from a natal prestellar core. These low-extinction outflow cavities greatly affect the appearance of a protostar by allowing the escape of shorter-wavelength photons. Doppler-shifted CO line emission from outflows is also often the most prominent manifestation of deeply embedded early-stage star formation. Here, we present 3D magnetohydrodynamic simulations of a disk wind outflow from a protostar forming from an initially 60Mcore embedded in a high-pressure environment typical of massive star-forming regions. We simulate the growth of the protostar fromm*= 1Mto 26Mover a period of ∼100,000 yr. The outflow quickly excavates a cavity with a half opening angle of ∼10° through the core. This angle remains relatively constant until the star reaches 4M. It then grows steadily in time, reaching a value of ∼50° by the end of the simulation. We estimate a lower limit to the star formation efficiency (SFE) of 0.43. However, accounting for continued accretion from a massive disk and residual infall envelope, we estimate that the final SFE may be as high as ∼0.7. We examine observable properties of the outflow, especially the evolution of the cavity's opening angle, total mass, and momentum flux, and the velocity distributions of the outflowing gas, and compare with the massive protostars G35.20-0.74N and G339.88-1.26 observed by the Atacama Large Millimeter/submillimeter Array (ALMA), yielding constraints on their intrinsic properties.

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  3. Abstract

    To test theoretical models of massive star formation it is important to compare their predictions with observed systems. To this end, we conduct CO molecular line radiative transfer post-processing of 3D magnetohydrodynamic simulations of various stages in the evolutionary sequence of a massive protostellar core, including its infall envelope and disk wind outflow. Synthetic position–position–velocity cubes of various transitions of12CO,13CO, and C18O emission are generated. We also carry out simulated Atacama Large Millimeter/submillimeter Array (ALMA) observations of this emission. We compare the mass, momentum, and kinetic energy estimates obtained from molecular lines to the true values, finding that the mass and momentum estimates can have uncertainties of up to a factor of 4. However, the kinetic energy estimated from molecular lines is more significantly underestimated. Additionally, we compare the mass outflow rate and momentum outflow rate obtained from the synthetic spectra with the true values. Finally, we compare the synthetic spectra with real examples of ALMA-observed protostars and determine the best-fitting protostellar masses and outflow inclination angles. We then calculate the mass outflow rate and momentum outflow rate for these sources, finding that both rates agree with theoretical protostellar evolutionary tracks.

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