Whether or not molecular clouds and embedded cloud fragments are stable against collapse is of utmost importance for the study of the star formation process. Only supercritical cloud fragments are able to collapse and form stars. The virial parameter
, alpha=M_vir/M, which compares the virial to the actual mass, provides one way to gauge stability against collapse. Supercritical cloud fragments are characterized by alpha<2, as indicated by a comprehensive stability analysis considering perturbations in pressure and density gradients. Past research has suggested that virial parameters alpha>2 prevail in clouds. This would suggest that collapse towards star formation is a gradual and relatively slow process, and that magnetic fields are not needed to explain the observed cloud structure. Here, we review a range of very recent observational studies that derive virial parameters <<2 and compile a catalogue of 1325 virial parameter estimates. Low values of alpha are in particular observed for regions of high mass star formation (HMSF). These observations may argue for a more rapid and violent evolution during collapse. This would enable competitive accretion in HMSF, constrain some models of monolithic collapse, and might explain the absence of high--mass starless cores. Alternatively, the data could point at the presence of significant magnetic fields ~1 mG at high gas densities. We examine to what extent the derived observational properties might be biased by observational or theoretical uncertainties. For a wide range of reasonable parameters, our conclusions appear to be robust with respect to such biases.
We present the first interferometric molecular line and dust emission maps for the Galactic Center (GC) cloud G0.253+0.016, observed using the Combined Array for Research in Millimeter--wave Astronomy (CARMA) and the Submillimeter Array (SMA). This c
loud is very dense, and concentrates a mass exceeding the Orion Molecular Cloud Complex (2x10^5 M_sun) into a radius of only 3pc, but it is essentially starless. G0.253+0.016 therefore violates star formation laws presently used to explain trends in galactic and extragalactic star formation by a factor ~45. Our observations show a lack of dense cores of significant mass and density, thus explaining the low star formation activity. Instead, cores with low densities and line widths 1km/s---probably the narrowest lines reported for the GC region to date---are found. Evolution over several 10^5 yr is needed before more massive cores, and possibly an Arches--like stellar cluster, could form. Given the disruptive dynamics of the GC region, and the potentially unbound nature of G0.253+0.016, it is not clear that this evolution will happen.
We report the detection of a compact (of order 5 arcsec; about 1800 AU projected size) CO outflow from L1148-IRS. This confirms that this Spitzer source is physically associated with the nearby (about 325 pc) L1148 dense core. Radiative transfer mode
ling suggests an internal luminosity of 0.08 to 0.13 L_sun. This validates L1148-IRS as a Very Low Luminosity Object (VeLLO; L < 0.1 L_sun). The L1148 dense core has unusually low densities and column densities for a star-forming core. It is difficult to understand how L1148-IRS might have formed under these conditions. Independent of the exact final mass of this VeLLO (which is likely < 0.24 M_sun), L1148-IRS and similar VeLLOs might hold some clues about the isolated formation of brown dwarfs.
We present a new high-resolution study of pre-protocluster regions in tracers exclusively probing the coldest and dense gas (NH_2D). The data are used to constrain the chemical, thermal, kinematic, and physical conditions (i.e., densities) in G29.96e
and G35.20w. NH_3, NH_2D, and continuum emission were mapped using the VLA, and PdBI. In particular, NH_2D is a unique tracer of cold, precluster gas at high densities, while NH_3 traces both the cold and warm gas of modest-to-high densities. In G29.96e, Spitzer images reveal two massive filaments, one of them in extinction (infrared dark cloud). We observe very low line widths in NH_3 (FWHM <1km/s). These multi-wavelength, high-resolution observations of high-mass pre-protocluster regions show that the target regions are characterized by (i) turbulent Jeans fragmentation of massive clumps into cores (from a Jeans analysis); (ii) cores and clumps that are over-bound/subvirial, i.e. turbulence is too weak to support them against collapse, meaning that (iii) some models of monolithic cloud collapse are quantitatively inconsistent with data; (iv) accretion from the core onto a massive star, which can (for observed core sizes and velocities) be sustained by accretion of envelope material onto the core, suggesting that (similar to competitive accretion scenarios) the mass reservoir for star formation is not necessarily limited to the natal core; (v) high deuteration ratios ([NH_2D/NH_3]>6%), which make the above discoveries possible; (vi) and the destruction of NH_2D toward embedded stars. [abridged]
AIMS: To study the structure of nearby (< 500 pc) dense starless and star-forming cores with the particular goal to identify and understand evolutionary trends in core properties, and to explore the nature of Very Low Luminosity Objects (< 0.1 L_sun;
VeLLOs). METHODS: Using the MAMBO bolometer array, we create maps unusually sensitive to faint (few mJy per beam) extended (approx. 5 arcmin) thermal dust continuum emission at 1.2 mm wavelength. Complementary information on embedded stars is obtained from Spitzer, IRAS, and 2MASS. RESULTS: Our maps are very rich in structure, and we characterize extended emission features (``subcores) and compact intensity peaks in our data separately to pay attention to this complexity. We derive, e.g., sizes, masses, and aspect ratios for the subcores, as well as column densities and related properties for the peaks. Combination with archival infrared data then enables the derivation of bolometric luminosities and temperatures, as well as envelope masses, for the young embedded stars. CONCLUSIONS: (abridged) Starless and star-forming cores occupy the same parameter space in many core properties; a picture of dense core evolution in which any dense core begins to actively form stars once it exceeds some fixed limit in, e.g., mass, density, or both, is inconsistent with our data. Comparison of various evolutionary indicators for young stellar objects in our sample (e.g., bolometric temperatures) reveals inconsistencies between some of them, possibly suggesting a revision of some of these indicators.
