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Register a new userDynamics, CO depletion, and deuterium fractionation of the dense condensations within the fragmented prestellar core Orion B9-SMM 6
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Oskari Miettinen PhD
Publication date
2013
fields
Physics
and research's language is
English
Authors
Oskari Miettinen
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We present APEX observations of C17O(2-1), N2H+(3-2), and N2D+(3-2) towards the subfragments inside the prestellar core SMM 6 in Orion B9. We combined these spectral line data with our previous SABOCA 350-{mu}m dust continuum map of the source. The subfragments are characterised by subsonic internal non-thermal motions ({sigma}NT~0.5cs), and most of them appear to be gravitationally bound. The dispersion of the N2H+ velocity centroids among the condensations is very low (0.02 km/s). The CO depletion factors we derive, fD=0.8+/-0.4 - 3.6+/-1.5, do not suggest any significant CO freeze-out but this may be due to the canonical CO abundance we adopt. The fractional abundances of N2H+ and N2D+ with respect to H2 are found to be ~0.9-2.3x10^-9 and ~4.9-9.9x10^-10, respectively. The deuterium fractionation of N2H+, or the N2D+/N2H+ column density ratio, lies in the range 0.30+/-0.07 - 0.43+/-0.09. The detected substructure inside SMM 6 is likely the result of cylindrical Jeans-type gravitational fragmentation. We estimate the timescale for this fragmentation to be ~1.8x10^5 yr. The condensations are unlikely to be able to interact with one another and coalesce before local gravitational collapse ensues. Moreover, significant mass growth of the condensations via competitive-like accretion from the parent core seems unfeasible. The high level of molecular deuteration in the condensations suggests that gas-phase CO should be strongly depleted. It also points towards an advanced stage of chemical evolution. The subfragments of SMM 6 might therefore be near the onset of gravitational collapse, but whether they can form protostellar or substellar objects (brown dwarfs) depends on the local star formation efficiency and remains to be clarified.
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We report the detection of D2CO in a sample of starless dense cores, in which we previously measured the degree of CO depletion. The deuterium fractionation is found extremely high, [D2CO]/[H2CO] ~ 1-10 %, similar to that reported in low-mass protostars. This provides convincing evidence that D2CO is formed in the cold pre-stellar cores, and later desorbed when the gas warms up in protostars. We find that the cores with the highest CO depletions have also the largest [D2CO]/[H2CO] ratios, supporting the theoretical prediction that deuteration increases with increasing CO depletion.
Disentangling the different stages of the star-formation process, in particular in the high-mass regime, is a challenge in astrophysics. Chemical clocks could help alleviating this problem, but their evolution strongly depends on many parameters, leading to degeneracy in the interpretation of the observational data. One of these uncertainties is the degree of CO depletion. We present here the first self-consistent magneto-hydrodynamic simulations of high-mass star-forming regions at different scales, fully coupled with a non-equilibrium chemical network, which includes C-N-O bearing molecules. Depletion and desorption processes are treated time-dependently. The results show that full CO-depletion (i.e. all gas-phase CO frozen-out on the surface of dust grains), can be reached very quickly, in one third or even smaller fractions of the free-fall time, whether the collapse proceeds on slow or fast timescales. This leads to a high level of deuteration in a short time both for typical tracers like N$_2$H$^+$, as well as for the main ion H$_3^+$, the latter being in general larger and more extended. N$_2$ depletion is slightly less efficient, and no direct effects on N-bearing molecules and deuterium fractionation are observed. We show that CO depletion is not the only driver of deuteration, and that there is a strong impact on $D_{frac}$ when changing the grain-size. We finally apply a two-dimensional gaussian Point Spread Function to our results to mimic observations with single-dish and interferometers. Our findings suggest that the low-values observed in high-mass star-forming clumps are in reality masking a full-depletion stage in the inner 0.1 pc region.
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We present high-angular-resolution observations of the massive star forming core DR21(OH) at 880 mum using the Submillimeter Array (SMA). The dense core exhibits an overall velocity gradient in a Keplerian-like pattern, which breaks at the center of the core where SMA 6 and SMA 7 are located. The dust polarization shows a complex magnetic field, compatible with a toroidal configuration. This is in contrast with the large, parsec-scale filament that surrounds the core, where there is a smooth magnetic field. The total magnetic field strengths in the filament and in the core are 0.9 and 2.1 mG, respectively. We found evidence of magnetic field diffusion at the core scales, far beyond the expected value for ambipolar diffusion. It is possible that the diffusion arises from fast magnetic reconnection in the presence of turbulence. The dynamics of the DR 21(OH) core appear to be controlled energetically in equal parts by the magnetic field, magneto-hydrodynamic (MHD) turbulence and the angular momentum. The effect of the angular momentum (this is a fast rotating core) is probably causing the observed toroidal field configuration. Yet, gravitation overwhelms all the forces, making this a clear supercritical core with a mass-to-flux ratio of ~6 times the critical value. However, simulations show that this is not enough for the high level of fragmentation observed at 1000 AU scales. Thus, rotation and outflow feedback is probably the main cause of the observed fragmentation.
The CS molecule is known to be absorbed onto dust in the cold and dense conditions, causing it to get significantly depleted in the central region of cores. This study is aimed to investigate the depletion of the CS molecule using the optically thin C$^{34}$S molecular line observations. We mapped five prestellar cores, L1544, L1552, L1689B, L694-2, and L1197 using two molecular lines, C$^{34}$S $(J=2-1)$ and N$_2$H$^+$ $(J=1-0)$ with the NRO 45-m telescope, doubling the number of cores where the CS depletion was probed using C$^{34}$S. In most of our targets, the distribution of C$^{34}$S emission shows features that suggest that the CS molecule is generally depleted in the center of the prestellar cores. The radial profile of the CS abundance with respect to H$_2$ directly measured from the CS emission and the Herschel dust emission indicates that the CS molecule is depleted by a factor of $sim$3 toward the central regions of the cores with respect to their outer regions. The degree of the depletion is found to be even more enhanced by an order of magnitude when the contaminating effect introduced by the presence of CS molecules in the surrounding envelope that lie along the line-of-sight is removed. Except for L1197 which is classified as relatively the least evolved core in our targets based on its observed physical parameters, we found that the remaining four prestellar cores are suffering from significant CS depletion at their central region regardless of the relative difference in their evolutionary status.
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