arXiv subjects

Satoshi Ohashi

Publications and source records attributed to Satoshi Ohashi.

39 records · Page 3Linked to original sources

arXiv2015

Chemical evolution of the HC3N and N2H+ molecules in dense cores of the Vela C giant molecular cloud complex

We have observed the HC3N (J=10-9) and N2H+ (J=1-0) lines toward the Vela C molecular clouds with the Mopra 22 m telescope to study chemical characteristics of dense cores. The intensity distributions of these molecules are similar to each other at an angular resolution of 53", corresponding to 0.19 pc suggesting that these molecules trace the same dense cores. We identified 25 local peaks in the velocity-integrated intensity maps of the HC3N and/or N2H+ emission. Assuming LTE conditions, we calculated the column densities of these molecules and found a tendency that N2H+/HC3N abundance ratio seems to be low in starless regions while it seems to be high in star-forming regions, similar to the tendencies in the NH3/CCS, NH3/HC3N, and N2H+/CCS abundance ratios found in previous studies of dark clouds and the Orion A GMC. We suggest that carbon chain molecules, including HC3N, may trace chemically young molecular gas and N-bearing molecules, such as N2H+, may trace later stages of chemical evolution in the Vela C molecular clouds. It may be possible that the N2H+/HC3N abundance ratio of ~ 1.4 divides the star-forming and starless peaks in the Vela C, although it is not as clear as those in NH3/CCS, NH3/HC3N, and N2H+/CCS for the Orion A GMC. This less clear separation may be caused by our lower spatial resolution or the misclassification of star-forming and starless peaks due to the larger distance of the Vela C. It might be also possible that the HC3N (J=10-9) transition is not a good chemical evolution tracer compared with CCS (J=4-3 and 7-6) transitions.

astro-ph.GA↗

arXiv2014

Thermal Starless Ammonia Core Surrounded by CCS in the Orion A Cloud

We imaged two starless molecular cloud cores, TUKH083 and TUKH122, in the Orion A giant molecular cloud in the CCS and ammonia (NH$_3$) emission with the Very Large Array. TUKH122 contains one NH$_3$ core "TUKH122-n," which is elongated and has a smooth oval boundary. Where observed, the CCS emission surrounds the NH$_3$ core. This configuration resembles that of the N$_2$H$^+$ and CCS distribution in the Taurus starless core L1544, a well-studied example of a dense prestellar core exhibiting infall motions. The linewidth of TUKH122-n is narrow (0.20 km s$^{-1}$) in the NH$_3$ emission line and therefore dominated by thermal motions. The smooth oval shape of the core boundary and narrow linewidth in NH$_3$ seem to imply that TUKH122-n is dynamically relaxed and quiescent. TUKH122-n is similar to L1544 in the kinetic temperature (10 K), linear size (0.03 pc), and virial mass ($\sim$ 2 $M_{\odot}$). Our results strongly suggest that TUKH122-n is on the verge of star formation. TUKH122-n is embedded in the 0.2 pc massive (virial mass $\sim$ 30 $M_{\odot}$) turbulent parent core, while the L1544 NH$_3$ core is embedded in the 0.2 pc less-massive (virial mass $\sim$ 10 $M_{\odot}$) thermal parent core. TUKH083 shows complicated distribution in NH$_3$, but was not detected in CCS. The CCS emission toward TUKH083 appears to be extended, and is resolved out in our interferometric observations.

astro-ph.GA↗

arXiv2013

Chemical Variation in Molecular Cloud Cores in the Orion A Cloud. II

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.

astro-ph.GA↗