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Register a new userCarbon Monoxide Depletion in Orion B Molecular Cloud Cores
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D. Savva
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We have observed several cloud cores in the Orion B (L1630) molecular cloud in the 2-1 transitions of C18O, C17O and 13C18O. We use these data to show that a model where the cores consist of very optically thick C18O clumps cannot explain their relative intensities. There is strong evidence that the C18O is not very optically thick. The CO emission is compared to previous observations of dust continuum emission to deduce apparent molecular abundances. The abundance values depend somewhat on the temperature but relative to `normal abundance values, the CO appears to be depleted by about a factor of 10 at the core positions. CO condensation on dust grains provides a natural explanation for the apparent depletion both through gas-phase depletion of CO, and through a possible increase in dust emissivity in the cores. The high brightness of HCO+ relative to CO is then naturally accounted for by time-dependent interstellar chemistry starting from `evolved initial conditions. Theoretical work has shown that condensation of H2O, which destroys HCO+, would allow the HCO+ abundance to increase while that of CO is falling.
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We have mapped six molecular cloud cores in the Orion A giant molecular cloud (GMC), whose kinetic temperatures range from 10 to 30 K, in CCS and N2H+ with Nobeyama 45 m radio telescope to study their chemical characteristics. We identified 31 intensity peaks in the CCS and N2H+ emission in these molecular cloud cores. It is found for cores with temperatures lower than ~ 25 K that the column density ratio of N(N2H+)/N(CCS) is low toward starless core regions while it is high toward star-forming core regions, in case that we detected both of the CCS and N2H+ emission. This is very similar to the tendency found in dark clouds (kinetic temperature ~ 10 K). The criterion found in the Orion A GMC is N(N2H+)/N(CCS) ~ 2-3. In some cases, the CCS emission is detected toward protostars as well as the N2H+ emission. Secondary late-stage CCS peak in the chemical evolution caused by CO depletion may be a possible explanation for this. We found that the chemical variation of CCS and N2H+ can also be used as a tracer of evolution in warm (10-25 K) GMC cores. On the other hand, some protostars do not accompany N2H+ intensity peaks but are associated with dust continuum emitting regions, suggesting that the N2H+ abundance might be decreased due to CO evaporation in warmer star-forming sites.
Observations towards L1630 in the Orion B molecular cloud, comprising the iconic Horsehead Nebula, allow us to study the interplay between stellar radiation and a molecular cloud under relatively benign conditions, that is, intermediate densities and an intermediate UV radiation field. Contrary to the well-studied Orion Molecular Cloud 1 (OMC1), which hosts much harsher conditions, L1630 has little star formation. We aim to relate the [CII] fine-structure line emission to the physical conditions predominant in L1630 and compare it to studies of OMC1. The [CII] $158,mumathrm{m}$ emission from an area of $12 times 17$ in L1630 was observed using the upGREAT instrument onboard SOFIA. Of the [CII] emission from the mapped area 95%, $13,L_{odot}$, originates from the molecular cloud; the adjacent HII region contributes only 5%, that is, $1,L_{odot}$. From comparison with other data (CO (1-0)-line emission, far-infrared (FIR) continuum studies, emission from polycyclic aromatic hydrocarbons (PAHs)), we infer a gas density of the molecular cloud of $n_{mathrm{H}}sim 3cdot 10^3,mathrm{cm^{-3}}$, with surface layers, including the Horsehead Nebula, having a density of up to $n_{mathrm{H}}sim 4cdot 10^4,mathrm{cm^{-3}}$. The temperature of the surface gas is $Tsim 100,mathrm{K}$. The average [CII] cooling efficiency within the molecular cloud is $1.3cdot 10^{-2}$. The fraction of the mass of the molecular cloud within the studied area that is traced by [CII] is only $8%$. Our PDR models are able to reproduce the FIR-[CII] correlations and also the CO (1-0)-[CII] correlations. Finally, we compare our results on the heating efficiency of the gas with theoretical studies of photoelectric heating by PAHs, clusters of PAHs, and very small grains, and find the heating efficiency to be lower than theoretically predicted, a continuation of the trend set by other observations.
The ratio of mass and magnetic flux determines the relative importance of magnetic and gravitational forces in the evolution of molecular clouds and their cores. Its measurement is thus central in discriminating between different theories of core formation and evolution. Here we discuss the effect of chemical depletion on measurements of the mass-to-flux ratio using the same molecule (OH) both for Zeeman measurements of the magnetic field and the determination of the mass of the region. The uncertainties entering through the OH abundance in determining separately the magnetic field and the mass of a region have been recognized in the literature. It has been proposed however that, when comparing two regions of the same cloud, the abundance will in both cases be the same. We show that this assumption is invalid. We demonstrate that when comparing regions with different densities, the effect of OH depletion in measuring changes of the mass-to-flux ratio between different parts of the same cloud can even reverse the direction of the underlying trends (for example, the mass-to-flux ratio may appear to decrease as we move to higher density regions). The systematic errors enter primarily through the inadequate estimation of the mass of the region.
We performed new comprehensive $^{13}$CO($J$=2--1) observations toward NGC 2024, the most active star forming region in Orion B, with an angular resolution of $sim$100 obtained with NANTEN2. We found that the associated cloud consists of two independent velocity components. The components are physically connected to the H{sc ii} region as evidenced by their close correlation with the dark lanes and the emission nebulosity. The two components show complementary distribution with a displacement of $sim$0.6 pc. Such complementary distribution is typical to colliding clouds discovered in regions of high-mass star formation. We hypothesize that a cloud-cloud collision between the two components triggered the formation of the late O-type stars and early B stars localized within 0.3 pc of the cloud peak. The duration time of the collision is estimated to be 0.3 million years from a ratio of the displacement and the relative velocity $sim$3 km s$^{-1}$ corrected for probable projection. The high column density of the colliding cloud $sim$10$^{23}$ cm$^{-2}$ is similar to those in the other high-mass star clusters in RCW 38, Westerlund 2, NGC 3603, and M42, which are likely formed under trigger by cloud-cloud collision. The present results provide an additional piece of evidence favorable to high-mass star formation by a major cloud-cloud collision in Orion.
We quantify the spatial distributions of dense cores in three spatially distinct areas of the Orion B star-forming region. For L1622, NGC2068/NGC2071 and NGC2023/NGC2024 we measure the amount of spatial substructure using the $mathcal{Q}$-parameter and find all three regions to be spatially substructured ($mathcal{Q} < 0.8$). We quantify the amount of mass segregation using $Lambda_{rm MSR}$ and find that the most massive cores are mildly mass segregated in NGC2068/NGC2071 ($Lambda_{rm MSR} sim 2$), and very mass segregated in NGC2023/NGC2024 ($Lambda_{rm MSR} = 28^{+13}_{-10}$ for the four most massive cores). Whereas the most massive cores in L1622 are not in areas of relatively high surface density, or deeper gravitational potentials, the massive cores in NGC2068/NGC2071 and NGC2023/NGC2024 are significantly so. Given the low density (10 cores pc$^{-2}$) and spatial substructure of cores in Orion B, the mass segregation cannot be dynamical. Our results are also inconsistent with simulations in which the most massive stars form via competitive accretion, and instead hint that magnetic fields may be important in influencing the primordial spatial distributions of gas and stars in star-forming regions.
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