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Register a new userConstraining the Dust Opacity Law in Three Small and Isolated Molecular Clouds
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Density profiles of isolated cores derived from thermal dust continuum emission rely on models of dust properties, such as mass opacity, which are poorly constrained. With complementary measures from near-infrared extinction maps, we can assess the reliability of commonly-used dust models. In this work, we compare Herschel-derived maps of the optical depth with equivalent maps derived from CFHT WIRCAM near-infrared observations for three isolated cores: CB68, L429, and L1552. We assess the dust opacities provided from four models: OH1a, OH5a, Orm1, and Orm4. Although the consistency of the models differs between the three sources, the results suggest that the optical properties of dust in the envelopes of the cores are best described by either silicate and bare graphite grains (e.g., Orm1) or carbonaceous grains with some coagulation and either thin or no ice mantles (e.g., OH5a). None of the models, however, individually produced the most consistent optical depth maps for every source. The results suggest that either the dust in the cores is not well described by any one dust property model, the application of the dust models cannot be extended beyond the very center of the cores, or more complex SED fitting functions are necessary.
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We present radiation-magnetohydrodynamic simulations aimed at studying evolutionary properties of H,{ ormalsize II} regions in turbulent, magnetised, and collapsing molecular clouds formed by converging flows in the warm neutral medium. We focus on the structure, dynamics and expansion laws of these regions. Once a massive star forms in our highly structured clouds, its ionising radiation eventually stops the accretion (through filaments) toward the massive star-forming regions. The new over-pressured H,{ ormalsize II} regions push away the dense gas, thus disrupting the more massive collapse centres. Also, because of the complex density structure in the cloud, the H,{ ormalsize II} regions expand in a hybrid manner: they virtually do not expand toward the densest regions (cores), while they expand according to the classical analytical result towards the rest of the cloud, and in an accelerated way, as a blister region, towards the diffuse medium. Thus, the ionised regions grow anisotropically, and the ionising stars generally appear off-centre of the regions. Finally, we find that the hypotheses assumed in standard H,{ ormalsize II}-region expansion models (fully embedded region, blister-type, or expansion in a density gradient) apply simultaneously in different parts of our simulated H,{ ormalsize II} regions, producing a net expansion law ($R propto t^alpha$, with $alpha$ in the range of 0.93-1.47 and a mean value of $1.2 pm 0.17$) that differs from any of those of the standard models.
We argue that impact velocities between dust grains with sizes less than $sim 0.1$ $mu m$ in molecular cloud cores are dominated by drift arising from ambipolar diffusion. This effect is due to the size dependence of the dust coupling to the magnetic field and the neutral gas. Assuming perfect sticking in collisions up to $approx 50$ m/s, we show that this effect causes rapid depletion of small grains - consistent with starlight extinction and IR/microwave emission measurements, both in the core center ($n sim 10^{6}$ cm$^{-3}$) and envelope ($n sim 10^{4}$ cm$^{-3}$). The upper end of the size distribution does not change significantly if only velocities arising from this effect are considered. We consider the impact of an evolved dust size distribution on the gas temperature, and argue that if the depletion of small dust grains occurs as would be expected from our model, then the cosmic ray ionization rate must be well below $10^{-16}$ s$^{-1}$ at a number density of $10^{5}$ cm$^{-3}$.
We perform numerical simulations of dusty, supersonic turbulence in molecular clouds. We model 0.1, 1 and 10 {mu}m sized dust grains at an initial dust-to-gas mass ratio of 1:100, solving the equations of combined gas and dust dynamics where the dust is coupled to the gas through a drag term. We show that, for 0.1 and 1 {mu}m grains, the dust-to-gas ratio deviates by typically 10-20% from the mean, since the stopping time of the dust due to gas drag is short compared to the dynamical time. Contrary to previous findings, we find no evidence for orders of magnitude fluctuation in the dust-to-gas ratio for 0.1 {mu}m grains. Larger, 10 {mu}m dust grains may have dust-to-gas ratios increased by up to an order of magnitude locally. Both small (0.1 {mu}m) and large ($gtrsim$ 1 {mu}m) grains trace the large-scale morphology of the gas, however we find evidence for size-sorting of grains, where turbulence preferentially concentrates larger grains into dense regions. Size-sorting may help to explain observations of coreshine from dark clouds, and why extinction laws differ along lines of sight through molecular clouds in the Milky Way compared to the diffuse interstellar medium.
Many galaxies host pronounced circumnuclear starbursts, fuelled by infalling gas. Such activity is expected to drive the secular evolution of the nucleus and generate super winds, while the intense radiation fields and extreme gas and cosmic ray densities present may act to modify the outcome of star formation with respect to more quiescent galactic regions. The centre of the Milky Way is the only example of this phenomenon where, by virtue of its proximity, individual stars may be resolved. Previous studies have revealed that it hosts a rich population of massive stars; these are located within three clusters, with an additional contingent dispersed throughout the Central Molecular Zone (CMZ). We employed VLT+KMOS to obtain homogeneous, high S/N spectroscopy of the later cohort for classification and quantitative analysis. Including previously identified examples, we found a total of 83 isolated massive stars within the Galactic Centre, which are biased towards objects supporting powerful stellar winds and/or extensive circumstellar envelopes. No further stellar clusters, or their tidally stripped remnants, were identified, although an apparent stellar overdensity was found to be coincident with the Sgr B1 star forming region. The cohort of isolated massive stars within the CMZ is comparable in size to that of the known clusters but, due to observational biases, is likely highly incomplete at this time. Combining both populations yields over 320 spectroscopically classified stars that are expected to undergo core collapse within the next 20Myr. Given that this is presumably an underestimate of the true number, the population of massive stars associated with the CMZ appears unprecedented amongst star formation complexes within the Milky Way, and one might anticipate that they play a substantial role in the energetics and evolution of the nuclear region.
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).
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