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Register a new userCO Depletion and Deuterium Fractionation in Prestellar Cores
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Aurore Bacmann
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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.
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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.
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.
High levels of deuterium fraction in N$_2$H$^+$ are observed in some pre-stellar cores. Single-zone chemical models find that the timescale required to reach observed values ($D_{rm frac}^{{rm N}_2{rm H}^+} equiv {rm N}_2{rm D}^+/{rm N}_2{rm H}^+ gtrsim 0.1$) is longer than the free-fall time, possibly ten times longer. Here, we explore the deuteration of turbulent, magnetized cores with 3D magnetohydrodynamics simulations. We use an approximate chemical model to follow the growth in abundances of N$_2$H$^+$ and N$_2$D$^+$. We then examine the dynamics of the core using each tracer for comparison to observations. We find that the velocity dispersion of the core as traced by N$_2$D$^+$ appears slightly sub-virial compared to predictions of the Turbulent Core Model of McKee & Tan, except at late times just before the onset of protostar formation. By varying the initial mass surface density, the magnetic energy, the chemical age, and the ortho-to-para ratio of H$_2$, we also determine the physical and temporal properties required for high deuteration. We find that low initial ortho-to-para ratios ($lesssim 0.01$) and/or multiple free-fall times ($gtrsim 3$) of prior chemical evolution are necessary to reach the observed values of deuterium fraction in pre-stellar cores.
The deuterium fraction [N$_2$D$^+$]/[N$_2$H$^+$], may provide information about the ages of dense, cold gas structures, important to compare with dynamical models of cloud core formation and evolution. Here we introduce a complete chemical network with species containing up to three atoms, with the exception of the Oxygen chemistry, where reactions involving H$_3$O$^+$ and its deuterated forms have been added, significantly improving the consistency with comprehensive chemical networks. Deuterium chemistry and spin states of H$_2$ and H$_3^+$ isotopologues are included in this primarily gas-phase chemical model. We investigate dependence of deuterium chemistry on model parameters: density ($n_{rm H}$), temperature, cosmic ray ionization rate, and gas-phase depletion factor of heavy elements ($f_{rm D}$). We also explore the effects of time-dependent freeze-out of gas-phase species and dynamical evolution of density at various rates relative to free-fall collapse. For a broad range of model parameters, the timescales to reach large values of $D_{rm frac}^{rm N_2H^+} gtrsim 0.1$, observed in some low- and high-mass starless cores, are relatively long compared to the local free-fall timescale. These conclusions are unaffected by introducing time-dependent freeze-out and considering models with evolving density, unless the initial $f_{rm D} gtrsim$ 10. For fiducial model parameters, achieving $D_{rm frac}^{rm N_2H^+} gtrsim 0.1$ requires collapse to be proceeding at rates at least several times slower than that of free-fall collapse, perhaps indicating a dynamically important role for magnetic fields in the support of starless cores and thus the regulation of star formation.
Clouds of high infrared extinction are promising sites of massive star/cluster formation. A large number of cloud cores discovered in recent years allows investigation of possible evolutionary sequence among cores in early phases. We have conducted a survey of deuterium fractionation toward 15 dense cores in various evolutionary stages, from high-mass starless cores to ultracompact Hii regions, in the massive star-forming clouds of high extinction, G34.43+0.24, IRAS 18151-1208, and IRAS 18223-1243, with the Submillimeter Telescope (SMT). Spectra of N2H+ (3 - 2), N2D+ (3 - 2), and C18O (2 - 1) were observed to derive the deuterium fractionation of N2H+, Dfrac equiv N(N2D+)/N(N2H+), as well as the CO depletion factor for every selected core. Our results show a decreasing trend in Dfrac with both gas temperature and linewidth. Since colder and quiescent gas is likely to be associated with less evolved cores, larger Dfrac appears to correlate with early phases of core evolution. Such decreasing trend resembles the behavior of Dfrac in the low-mass protostellar cores and is consistent with several earlier studies in high-mass protostellar cores. We also find a moderate increasing trend of Dfrac with the CO depletion factor, suggesting that sublimation of ice mantles alters the competition in the chemical reactions and reduces Dfrac. Our findings suggest a general chemical behavior of deuterated species in both low- and high-mass proto-stellar candidates at early stages. In addition, upper limits to the ionization degree are estimated to be within 2 times 10^-7 and 5 times 10^-6. The four quiescent cores have marginal field-neutral coupling and perhaps favor turbulent cooling flows.
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