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Register a new userEvolution of the density PDF in star forming clouds: the role of gravity
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We derive an analytical theory of the PDF of density fluctuations in supersonic turbulence in the presence of gravity in star-forming clouds. The theory is based on a rigorous derivation of a combination of the Navier-Stokes continuity equations for the fluid motions and the Poisson equation for the gravity. It extends upon previous approaches first by including gravity, second by considering the PDF as a dynamical system, not a stationary one. We derive the transport equations of the density PDF, characterize its evolution and determine the density threshold above which gravity strongly affects and eventually dominates the dynamics of turbulence. We demonstrate the occurence of {it two} power law tails in the PDF, with two characteristic exponents, corresponding to two different stages in the balance between turbulence and gravity. Another important result of this study is to provide a procedure to relate the observed {it column density} PDFs to the corresponding {it volume density} PDFs. This allows to infer, from the observation of column-densities, various physical parameters characterizing molecular clouds, notably the virial parameter. Furthermore, the theory offers the possibility to date the clouds in units of ${t}_{rm coll}$, the time since a statistically significant fraction of the cloud started to collapse. The theoretical results and diagnostics reproduce very well numerical simulations and observations of star-forming clouds. The theory provides a sound theoretical foundation and quantitative diagnostics to analyze observations or numerical simulations of star-forming regions and to characterize the evolution of the density PDF in various regions of molecular clouds.
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We study the star formation (SF) law in 12 Galactic molecular clouds with ongoing high-mass star formation (HMSF) activity, as traced by the presence of a bright IRAS source and other HMSF tracers. We define the molecular cloud (MC) associated to each IRAS source using 13CO line emission, and count the young stellar objects (YSOs) within these clouds using GLIMPSE and MIPSGAL 24 micron Spitzer databases.The masses for high luminosity YSOs (Lbol>10~Lsun) are determined individually using Pre Main Sequence evolutionary tracks and the evolutionary stages of the sources, whereas a mean mass of 0.5 Msun was adopted to determine the masses in the low luminosity YSO population. The star formation rate surface density (sigsfr) corresponding to a gas surface density (siggas) in each MC is obtained by counting the number of the YSOs within successive contours of 13CO line emission. We find a break in the relation between sigsfr and siggas, with the relation being power-law (sigsfr ~ siggas^N) with the index N varying between 1.4 and 3.6 above the break. The siggas at the break is between 150-360 Msun/pc^2 for the sample clouds, which compares well with the threshold gas density found in recent studies of Galactic star-forming regions. Our clouds treated as a whole lie between the Kennicutt (1998) relation and the linear relation for Galactic and extra-galactic dense star-forming regions. We find a tendency for the high-mass YSOs to be found preferentially in dense regions at densities higher than 1200 Msun/pc^2 (~0.25 g/cm^2).
We present a new approach to extract the power-law part of a density/column-density probability density function (rho-pdf/N-pdf) in star-forming clouds. It is based on the mathematical method bPLFIT of Virkar & Clauset (2014) and assesses the power-law part of an arbitrary distribution, without any assumptions about the other part of this distribution. The slope and deviation point are derived as averaged values as the number of bins is varied. Neither parameter is sensitive to spikes and other local features of the tail. This adapted bPLFIT method is applied to two different sets of data from numerical simulations of star-forming clouds at scales 0.5 and 500 pc and displays rho-pdf and N-pdf evolution in agreement with a number of numerical and theoretical studies. Applied to Herschel data on the regions Aquila and Rosette, the method extracts pronounced power-law tails, consistent with those seen in simulations of evolved clouds.
We have conducted a large-field simultaneous survey of $^{12}$CO, $^{13}$CO, and C$^{18}$O $J=1-0$ emission toward the Orion A giant molecular cloud (GMC) with a sky coverage of $sim$ 4.4 deg$^2$ using the PMO-13.7 m millimeter-wavelength telescope. We use the probability distribution function of the column density (N-PDF) to investigate the distribution of molecular hydrogen in the Orion A GMC. The H$_2$ column density, derived from the $^{13}$CO emission, of the GMC is dominated by log-normal distribution in the range from $sim$4$times10^{21}$ to $sim$1.5$times10^{23}$ cm$^{-2}$ with excesses both at the low-density and high-density ends. The excess of the low-density end is possibly caused by an extended and low-temperature ($sim$10 K) component with velocities in the range of 5$-$8 km s$^{-1}$. Compared with the northern sub-regions, the southern sub-regions of the Orion A GMC contain less gas with column density in $N_{H_2} > 1.25times 10^{22} rm{cm}^{-2}$. The dispersions of the N-PDFs of the sub-regions are found to correlate with the evolutionary stages of the clouds across the Orion A GMC. The structure hierarchy of Orion A GMC is explored with the DENDROGRAM algorithm, and it is found that the GMC is composed of two branches. All structures except one in the tree have virial parameters less than 2, indicating self-gravity is important on the spatial scales from $sim$0.3 to $sim$4 pc. Although power-laws and departures from log-normal distributions are found at the high-density end of N-PDFs of active star-forming regions, the N-PDFs of structures in the Orion A GMC are predominantly log-normal on scales from R$sim$0.4 to 4 pc.
We investigate the influence of Wolf-Rayet (W-R) stars on their surrounding star-forming molecular clouds. We study five regions containing W-R stars in the inner Galactic plane ($lsim$[14$^circ$-52$^circ$]), using multi-wavelength data from near-infrared to radio wavelengths. Analysis of $^{13}$CO line data reveals that these W-R stars have developed gas-deficient cavities in addition to molecular shells with expansion velocities of a few km s$^{-1}$. The pressure owing to stellar winds primarily drives these expanding shells and sweeps up the surrounding matter to distances of a few pc. The column densities of shells are enhanced by a minimum of 14% for one region to a maximum of 88% for another region with respect to the column densities within their central cavities. No active star formation - including molecular condensations, protostars, or ionized gas - is found inside the cavities, whereas such features are observed around the molecular shells. Although the expansion of ionized gas is considered an effective mechanism to trigger star formation, the dynamical ages of the HII regions in our sample are generally not sufficiently long to do so efficiently. Overall, our results hint at the possible importance of negative W-R wind-driven feedback on the gas-deficient cavities, where star formation is quenched as a consequence. In addition, the presence of active star formation around the molecular shells indicates that W-R stars may also assist in accumulating molecular gas, and that they could initiate star formation around those shells.
A number of ultracompact H II regions in Galactic star forming environments have been observed to vary significantly in radio flux density on timescales of 10-20 years. Theory predicted that such variations should occur when the accretion flow that feeds a young massive star becomes unstable and clumpy. We have targeted the massive star-forming region W49A with the Karl G. Jansky Very Large Array (VLA) for observations at 3.6 cm with the B-configuration at 0.8 resolution, to compare to nearly identical observations taken almost 21 years earlier (February 2015 and August 1994). Most of the sources in the crowded field of ultracompact and hypercompact H II regions exhibit no significant changes over this time period. However, one source, W49A/G2, decreased by 20% in peak intensity (from 71+/-4 mJy/beam to 57+/-3 mJy/beam), and 40% in integrated flux (from 0.109+/-0.011 Jy to 0.067+/-0.007 Jy), where we cite 5 sigma errors in peak intensity, and 10% errors in integrated flux. We present the radio images of the W49A region at the two epochs, the difference image that indicates the location of the flux density decrease, and discuss explanations for the flux density decrease near the position of W49A/G2.
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