Spectral shifting strongly constrains molecular cloud disruption by radiation pressure on dust

${\bf Aim:}$ To test the hypothesis that radiation pressure from star clusters acting on dust is the dominant feedback agent disrupting the largest star-forming molecular clouds and thus regulating the star-formation process. ${\bf Methods:}$ We perform multi-frequency, 3D, RT calculations including scattering, absorption, and re-emission to longer wavelengths for clouds with masses of $10^4$-$10^7\,$M$_{\odot}$, with embedded clusters and a star formation efficiencies of 0.009%-91%, and varying maximum grain sizes up to 200$\,\mu$m. We calculate the ratio between radiative force and gravity to determine whether radiation pressure can disrupt clouds. ${\bf Results:}$ We find that radiation acting on dust almost never disrupts star-forming clouds. UV and optical photons to which the cloud is optically thick do not scatter much. Instead, they quickly get absorbed and re-emitted by at thermal wavelengths. As the cloud is typically optically thin to far-IR radiation, it promptly escapes, depositing little momentum. The resulting spectrum is more narrowly peaked than the corresponding Planck function with an extended tail at longer wavelengths. As the opacity drops significantly across the sub-mm and mm, the resulting radiative force is even smaller than for the corresponding single-temperature black body. The force from radiation pressure falls below the strength of gravitational attraction by an order of magnitude or more for either Milky Way or starbust conditions. For unrealistically large maximum grain sizes, and star formation efficiencies far exceeding 50% do we find that the strength of radiation pressure can exceed gravity. ${\bf Conclusions:}$ We conclude that radiation pressure acting on dust does not disrupt star-forming molecular clouds in any Local Group galaxies. Radiation pressure thus appears unlikely to regulate the star-formation process on either local or global scales.Comment: 20 pages, 17 figure

Magnetic fields in star forming systems (I): Idealized synthetic signatures of dust polarization and Zeeman splitting in filaments

We use the POLARIS radiative transport code to generate predictions of the two main observables directly sensitive to the magnetic field morphology and strength in filaments: dust polarization and gas Zeeman line splitting. We simulate generic gas filaments with power-law density profiles assuming two density-field strength dependencies, six different filament inclinations, and nine distinct magnetic field morphologies, including helical, toroidal, and warped magnetic field geometries. We present idealized spatially resolved dust polarization and Zeeman-derived field strengths and directions maps. Under the assumption that dust grains are aligned by radiative torques (RATs), dust polarization traces the projected plane-of-the-sky magnetic field morphology. Zeeman line splitting delivers simultaneously the intensity-weighted line-of-sight field strength and direction. We show that linear dust polarization alone is unable to uniquely constrain the 3D field morphology. We demonstrate that these ambiguities are ameliorated or resolved with the addition of the Zeeman directional information. Thus, observations of both the dust polarization and Zeeman splitting together provide the most promising means for obtaining constraints of the 3D magnetic field configuration. We find that the Zeeman-derived field strengths are at least a factor of a few below the input field strengths due to line-of-sight averaging through the filament density gradient. Future observations of both dust polarization and Zeeman splitting are essential for gaining insights into the role of magnetic fields in star and cluster forming filaments.Comment: 16 pages, 11 figures, 1 tabl

The Relationship Between Molecular Gas, HI, and Star Formation in the Low-Mass, Low-Metallicity Magellanic Clouds

The Magellanic Clouds provide the only laboratory to study the effect of metallicity and galaxy mass on molecular gas and star formation at high (~20 pc) resolution. We use the dust emission from HERITAGE Herschel data to map the molecular gas in the Magellanic Clouds, avoiding the known biases of CO emission as a tracer of H$_{2}$. Using our dust-based molecular gas estimates, we find molecular gas depletion times of ~0.4 Gyr in the LMC and ~0.6 SMC at 1 kpc scales. These depletion times fall within the range found for normal disk galaxies, but are shorter than the average value, which could be due to recent bursts in star formation. We find no evidence for a strong intrinsic dependence of the molecular gas depletion time on metallicity. We study the relationship between gas and star formation rate across a range in size scales from 20 pc to ~1 kpc, including how the scatter in molecular gas depletion time changes with size scale, and discuss the physical mechanisms driving the relationships. We compare the metallicity-dependent star formation models of Ostriker, McKee, and Leroy (2010) and Krumholz (2013) to our observations and find that they both predict the trend in the data, suggesting that the inclusion of a diffuse neutral medium is important at lower metallicity.Comment: 24 pages, 14 figures, accepted for publication in ApJ. FITS files of the dust-based estimates of the H2 column densities for the LMC and SMC (shown in Figures 2 and 3) will be available online through Ap