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Register a new userChemical Variation in Molecular Cloud Cores in the Orion A Cloud. II
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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.
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We present the discovery of expanding spherical shells around low to intermediate-mass young stars in the Orion A giant molecular cloud using observations of $^{12}$CO (1-0) and $^{13}$CO (1-0) from the Nobeyama Radio Observatory 45-meter telescope. The shells have radii from 0.05 to 0.85 pc and expand outward at 0.8 to 5 km/s. The total energy in the expanding shells is comparable to protostellar outflows in the region. Together, shells and outflows inject enough energy and momentum to maintain the cloud turbulence. The mass-loss rates required to power the observed shells are two to three orders of magnitude higher than predicted for line-driven stellar winds from intermediate-mass stars. This discrepancy may be resolved by invoking accretion-driven wind variability. We describe in detail several shells in this paper and present the full sample in the online journal.
We aim to reveal the physical properties and chemical composition of the cores in the California molecular cloud (CMC), so as to better understand the initial conditions of star formation. We made a high-resolution column density map (18.2) with Herschel data, and extracted a complete sample of the cores in the CMC with the textsl{fellwalker} algorithm. We performed new single-pointing observations of molecular lines near 90 GHz with the IRAM 30m telescope along the main filament of the CMC. In addition, we also performed a numerical modeling of chemical evolution for the cores under the physical conditions. We extracted 300 cores, of which 33 are protostellar and 267 are starless cores. About 51% (137 of 267) of the starless cores are prestellar cores. Three cores have the potential to evolve into high-mass stars. The prestellar core mass function (CMF) can be well fit by a log-normal form. The high-mass end of the prestellar CMF shows a power-law form with an index $alpha=-0.9pm 0.1$ that is shallower than that of the Galactic field stellar mass function. Combining the mass transformation efficiency ($varepsilon$) from the prestellar core to the star of $15pm 1%$ and the core formation efficiency (CFE) of 5.5%, we suggest an overall star formation efficiency of about 1% in the CMC. In the single-pointing observations with the IRAM 30m telescope, we find that 6 cores show blue-skewed profile, while 4 cores show red-skewed profile. [$rm {HCO}^{+}$]/[HNC] and [$rm {HCO}^{+}$]/$rm [N_{2}H^{+}]$ in protostellar cores are higher than those in prestellar cores; this can be used as chemical clocks. The best-fit chemical age of the cores with line observations is $sim 5times 10^4$~years.
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.
The Galactic Center 50 km s$^{-1}$ Molecular Cloud (50MC) is the most remarkable molecular cloud in the Sagittarius A region. This cloud is a candidate for the massive star formation induced by cloud-cloud collision (CCC) with a collision velocity of $sim30rm~km~s^{-1}$ that is estimated from the velocity dispersion. We observed the whole of the 50MC with a high angular resolution ($sim2.0times1.4$) in ALMA cycle 1 in the H$^{13}$CO$^+~J=1-0$ and ${rm C^{34}S}~J=2-1$ emission lines. We identified 241 and 129 bound cores with a virial parameter of less than 2, which are thought to be gravitationally bound, in the H$^{13}$CO$^+$ and ${rm C^{34}S}$ maps using the clumpfind algorithm, respectively. In the CCC region, the bound ${rm H^{13}CO^+}$ and ${rm C^{34}S}$ cores are 119 and 82, whose masses are $68~%$ and $76~%$ of those in the whole 50MC, respectively. The distribution of the core number and column densities in the CCC are biased to larger densities than those in the non-CCC region. The distributions indicate that the CCC compresses the molecular gas and increases the number of the dense bound cores. Additionally, the massive bound cores with masses of $>3000~M_{odot}$ exist only in the CCC region, although the slope of the core mass function (CMF) in the CCC region is not different from that in the non-CCC region. We conclude that the compression by the CCC efficiently formed massive bound cores even if the slope of the CMF is not changed so much by the CCC.
We analyzed the NANTEN2 13CO (J=2-1 and 1-0) datasets in NGC 2024. We found that the cloud consists of two velocity components, whereas the cloud shows mostly single-peaked CO profiles. The two components are physically connected to the HII 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 0.4 pc. Such complementary distribution is typical to colliding clouds discovered in regions of high-mass star formation. We hypothesize that cloud-cloud collision between the two components triggered the formation of the late O stars and early B stars localized within 0.3 pc of the cloud peak. The collision timescale is estimated to be ~ 10^5 yrs from a ratio of the displacement and the relative velocity 3-4 km s-1 corrected for probable projection. The high column density of the colliding cloud 1023 cm-2 is similar to those in the other massive 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.
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