The Effects of Radiation Feedback on Early Fragmentation and Multiplicity

Forming stars emit a significant amount of radiation into their natal environment. While the importance of radiation feedback from high-mass stars is widely accepted, radiation has generally been ignored in simulations of low-mass star formation. I use ORION, an adaptive mesh refinement (AMR) three-dimensional gravito-radiation-hydrodynamics code, to model low-mass star formation in a turbulent molecular cloud. I demonstrate that including radiation feedback has a profound effect on fragmentation and protostellar multiplicity. Although heating is mainly confined within the core envelope, it is sufficient to suppress disk fragmentation that would otherwise result in low-mass companions or brown dwarfs. As a consequence, turbulent fragmentation, not disk fragmentation, is likely the origin of low-mass binaries.Comment: 4 pages, 2 figures, to appear in the Proceedings of IAU Symposium 270: Computational Star Formatio

SABOCA 350-micron and LABOCA 870-micron dust continuum imaging of IRAS 05399-0121: mapping the dust properties of a pre- and protostellar core system

We present a 350 micron APEX/SABOCA map of IRAS 05399-0121/SMM 1, which is a dense double-core system in Orion B9. We combined these data with our previous LABOCA 870-micron data. The spatial resolution of the new SABOCA image, ~3400 AU, is about 2.6 times better than provided by LABOCA. We also make use of Spitzer infrared observations to characterise the star-formation activity in the source. The source is filamentary and remains a double-core system on the 3400 AU scale probed here, where the projected separation between IRAS 05399 and SMM 1 is 0.14 pc. The broadband spectral energy distribution of IRAS 05399 suggests that it is near the Stage 0/I borderline. A visual inspection of the Spitzer/IRAC images provides hints of a quadrupolar-like jet morphology around IRAS 05399, supporting the possibility that it is a binary system. The temperature map reveals warm spots towards IRAS 05399 and the southeastern tip of the source. These features are likely to be imprints of protostellar or shock heating, while external heating could be provided by the nearby high-mass star-forming region NGC 2024. A simple analysis suggests that the density profile at the position of SMM 1 has the form ~r^-(2.3_{-0.9}^{+2.2}). The source splitting into two subcores along the long axis can be explained by cylindrical Jeans-type fragmentation but the steepness of the density profile is shallower than what is expected for an isothermal cylinder. The difference between the evolutionary stages of IRAS 05399 (protostellar) and SMM 1 (starless) suggests that the former has experienced a phase of rapid mass accretion, supported by the very long outflow it drives. The protostellar jet from IRAS 05399 might have influenced the nearby core SMM 1.Comment: A&A, in press; 14 pages, 7 figures, 3 tables; very minor language corrections+revised arXiv abstrac

An Alternative Accurate Tracer of Molecular Clouds: The "XCIX_{\rm CI}-Factor"

We explore the utility of CI as an alternative high-fidelity gas mass tracer for Galactic molecular clouds. We evaluate the X$_{\rm CI}$-factor for the 609 $\mu$m carbon line, the analog of the CO X-factor, which is the ratio of the H$_2$ column density to the integrated $^{12}$CO(1-0) line intensity. We use 3D-PDR to post-process hydrodynamic simulations of turbulent, star-forming clouds. We compare the emission of CI and CO for model clouds irradiated by 1 and 10 times the average background and demonstrate that CI is a comparable or superior tracer of the molecular gas distribution for column densities up to $6 \times 10^{23}$ cm$^{-2}$. Our results hold for both reduced and full chemical networks. For our fiducial Galactic cloud we derive an average $X_{\rm CO}$ of $3.0\times 10^{20}$ cm$^{-2}$K$^{-1}$km$^{-1}$s and $X_{\rm CI}$ of $1.1\times 10^{21}$ cm$^{-2}$K$^{-1}$km$^{-1}$s.Comment: 5 pages, 4 figures, 1 table, accepted to MNRAS Letter

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 C$^{18}$O$(2\to 1)$, NH$_3(1,1)$, and N$_2$H$^+(1\to 0)$ lines, and from the simulated observations we measure the linewidths of individual cores, the linewidths 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 $\rho$ 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 linewidths through protostellar cores in both simulations are too large compared to the observations. The disagreement is noticably 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.Comment: 18 pages, 12 figures, accepted to A