arXiv subjects

Yoshimasa Watanabe

Publications and source records attributed to Yoshimasa Watanabe.

63 records · Page 4Linked to original sources

arXiv2015

Probing the effects of external irradiation on low-mass protostars through unbiased line surveys

(abridged) Context: The envelopes of molecular gas around embedded low-mass protostars show different chemistries, which can be used to trace their formation history and physical conditions. The excitation of some molecular species can also be used to trace these physical conditions, making it possible to constrain e.g. sources of heating and excitation. Aims: To study the range of influence of an intermediate-mass Herbig Be protostar, and to find what chemical and physical impact feedback effects from the environment may have on embedded protostars. Methods: We follow up on an earlier line survey of the Class 0/I source R CrA IRS7B in the 0.8 mm window with an unbiased line survey of the same source in the 1.3 mm window using the APEX telescope. We also study the excitation of the key species H2CO, CH3OH, and c-C3H2 in a complete sample of the 18 embedded protostars in the Corona Australis star-forming region. Radiative transfer models are used to establish abundances of the molecular species. Results: We detect line emission from 20 molecular species (32 including isotopologues) in the two surveys. The most complex species detected are CH3OH, CH3CCH, CH3CHO, and CH3CN. Several complex organics are significantly under-abundant in comparison with "hot corino" protostars. The H2CO temperatures of the sources in the region decrease with the distance to the Herbig Be star R CrA, whereas the c-C3H2 temperatures remain constant across the star-forming region. Conclusions: The high H2CO temperatures observed towards objects close to R CrA suggest that this star has a sphere of influence of several 10000 AU in which it increases the temperature of the molecular gas to 30-50 K through irradiation. The chemistry in the IRS7B envelope differs significantly from many other embedded protostars, which could be an effect of the external irradiation from R CrA.

astro-ph.SR↗

arXiv2015

Spectral Line Survey toward Young Massive Protostar NGC 2264 CMM3 in the 4 mm, 3 mm, and 0.8 mm Bands

Spectral line survey observations are conducted toward the high-mass protostar candidate NGC 2264 CMM3 in the 4 mm, 3 mm, and 0.8 mm bands with the Nobeyama 45 m telescope and the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope. In total, 265 emission lines are detected in the 4 mm and 3 mm bands, and 74 emission lines in the 0.8 mm band. As a result, 36 molecular species and 30 isotopologues are identified. In addition to the fundamental molecular species, many emission lines of carbon-chain molecules such as HC5N, C4H, CCS, and C3S are detected in the 4 mm and 3 mm bands. Deuterated molecular species are also detected with relatively strong intensities. On the other hand, emission lines of complex organic molecules such as HCOOCH3, and CH3OCH3 are found to be weak. For the molecules for which multiple transitions are detected, rotation temperatures are derived to be 7-33 K except for CH3OH. Emission lines with high upper-state energies (Eu > 150 K) are detected for CH3OH, indicating existence of a hot core. In comparison with the chemical composition of the Orion KL, carbon-chain molecules and deuterated molecules are found to be abundant in NGC 2264 CMM3, while sulfur-bearing species and complex organic molecules are deficient. These characteristics indicate chemical youth of NGC 2264 CMM3 in spite of its location at the center of the cluster forming core, NGC 2264 C.

astro-ph.GA↗

arXiv2015

Abundance Anomaly of the $^{13}$C Isotopic Species of c-C$_3$H$_2$ in the Low-Mass Star Formation Region L1527

The rotational spectral lines of c-C$_3$H$_2$ and two kinds of the $^{13}$C isotopic species, c-$^{13}$CCCH$_2$ ($C_{2v}$ symmetry) and c-CC$^{13}$CH$_2$ ($C_s$ symmetry) have been observed in the 1-3 mm band toward the low-mass star-forming region L1527. We have detected 7, 3, and 6 lines of c-C$_3$H$_2$, c-$^{13}$CCCH$_2$ , and c-CC$^{13}$CH$_2$, respectively, with the Nobeyama 45 m telescope, and 34, 6, and 13 lines, respectively, with the IRAM 30 m telescope, where 7, 2, and 2 transitions, respectively, are observed with the both telescopes. With these data, we have evaluated the column densities of the normal and $^{13}$C isotopic species. The [c-C$_3$H$_2$]/[c-$^{13}$CCCH$_2$] ratio is determined to be $310\pm80$, while the [c-C$_3$H$_2$]/[c-CC$^{13}$CH$_2$] ratio is determined to be $61\pm11$. The [c-C$_3$H$_2$]/[c-$^{13}$CCCH$_2$] and [c-C$_3$H$_2$]/[c-CC$^{13}$CH$_2$] ratios expected from the elemental $^{12}$C/$^{13}$C ratio are 60-70 and 30-35, respectively, where the latter takes into account the statistical factor of 2 for the two equivalent carbon atoms in c-C$_3$H$_2$. Hence, this observation further confirms the dilution of the $^{13}$C species in carbon-chain molecules and their related molecules, which are thought to originate from the dilution of $^{13}$C$^+$ in the gas-phase C$^+$ due to the isotope exchange reaction: $\mathrm{^{13}C^++CO\rightarrow{}^{13}CO+C^+}$. Moreover, the abundances of the two $^{13}$C isotopic species are different from each other. The ratio of c-$\mathrm{^{13}CCCH_2}$ species relative to c-$\mathrm{CC^{13}CH_2}$ is determined to be $0.20\pm0.05$. If $^{13}$C were randomly substituted for the three carbon atoms, the [c-$\mathrm{^{13}CCCH_2}$]/[c-$\mathrm{CC^{13}CH_2}$] ratio would be 0.5. Hence, the observed ratio indicates that c-$\mathrm{CC^{13}CH_2}$ exists more favorably. Possible origins of the different abundances are discussed.

astro-ph.SR↗

arXiv2014

A Substellar-Mass Protostar and its Outflow of IRAS 15398-3359 Revealed by Subarcsecond-Resolution Observations of H$_2$CO and CCH

Sub-arcsecond ($0.^{\prime\prime}5$) images of H$_2$CO and CCH line emission have been obtained in the $0.8$ mm band toward the low-mass protostar IRAS 15398-3359 in the Lupus 1 cloud as one of the Cycle 0 projects of the Atacama Large Millimeter/Submillimeter Array. We have detected a compact component concentrated in the vicinity of the protostar and a well-collimated outflow cavity extending along the northeast-southwest axis. The inclination angle of the outflow is found to be about $20^\circ$, or almost edge-on, based on the kinematic structure of the outflow cavity. This is in contrast to previous suggestions of a more pole-on geometry. The centrally concentrated component is interpreted by use of a model of the infalling rotating envelope with the estimated inclination angle, and the mass of the protostar is estimated to be less than $0.09\ M_\odot$. Higher spatial resolution data are needed to infer the presence of a rotationally supported disk for this source, hinted at by a weak high-velocity H$_2$CO emission associated with the protostar.

astro-ph.SR↗

arXiv2014

Stacking Analysis of 12CO and 13CO Spectra of NGC3627: Existence of non-optically thick 12CO emission?

We stacked 12CO and 13CO spectra of NGC 3627 after redefining the velocity axis of each spectrum of the mapping data so that the zero corresponds to the local mean velocity of 12CO spectra. The signal-to-noise ratios of the resulting spectra are improved by a factor of up to 3.2 compared to those obtained with normal stacking analysis. We successfully detect a weak 13CO emission from the interarm region where the emission was not detected in the individual pointings. We compare the integrated intensity ratios I12 CO/I13 CO among six characteristic regions (center, bar, bar-end, offset, arm, and interarm). We find that I12CO/I13CO in the bar and interarm are higher than those in the other regions by a factor of ~2 and I12CO/I13CO in the center is moderately high. These high I12CO/I13CO ratios in the bar and center are attributed to a high intensity ratio (T12CO/T13CO) and one in the interarm is attributed to a high ratio of the full width at half maximum of spectra (FWHM12CO/FWHM13CO). The difference between FWHM12CO and FWHM13CO of the interarm indicates the existence of two components, one with a narrow line width (~FWHM13CO) and the other with a broad line width (~FWHM12CO). Additionally, the T12CO/T13CO ratio in the broad-line-width component of the interarm is higher than the other regions. The high T12CO/T13CO in the center and bar and of the broad-line-width component in the interarm suggest the existence of non-optically thick 12CO components. We find that more than half of the 12CO emissions of the interarm are likely to be radiated from the diffuse component. Our result suggests that the use of a universal CO-to-H2 conversion factor might lead to an overestimation of molecular gas mass and underestimation of star-formation efficiency in the interarm by a factor of a few.

astro-ph.GA↗

arXiv2014

Spectral Line Survey toward Spiral Arm of M51 in the 3 mm and 2 mm Bands

We have conducted a spectral line survey in the 3 mm and 2 mm bands toward two positions in a spiral arm of M51 (NGC 5194) with the IRAM 30 m telescope. In this survey, we have identified 13 molecular species, including CN, CCH, N2H+, HNCO, and CH3OH. Furthermore, 6 isotopologues of the major species have been detected. On the other hand, SiO, HC3N, CH3CN, and the deuterated species such as DCN and DCO+ are not detected. The deuterium fractionation ratios are evaluated to be less than 0.8 % and 1.2 % for DCN/HCN and DCO+/HCO+, respectively. By comparing the results of the two positions with different star formation activities, we have found that the observed chemical compositions do not strongly depend on star formation activities. They seem to reflect a chemical composition averaged over the 1-kpc scale region including many giant molecular clouds. Among the detected molecules CN, CCH, and CH3OH are found to be abundant. High abundances of CN, and CCH are consistent with the above picture of a wide spread distribution of molecules, because they can be produced by photodissociation. On the other hand, it seems likely that CH3OH is liberated into the gas phase by shocks associated with large scale phenomena such as cloud-cloud collisions and/or by non-thermal desorption processes such as photoevaporation due to cosmic-ray induced UV photons. The present result demonstrates a characteristic chemical composition of a giant molecular cloud complex in the spiral arm, which can be used as a standard reference for studying chemistry in AGNs and starbursts.

astro-ph.GA↗

arXiv2012

Tentative Detection of Deuterated Methane toward the Low-Mass Protostar IRAS 04368+2557 in L1527

The millimeter-wave rotational transition line ($J_K = 1_0 - 0_0$) of deuterated methane CH$_3$D has tentatively been detected toward the low-mass Class 0 protostar IRAS 04368+2557 in L1527 with the Heinrich Hertz Submillimeter Telescope. This is the first detection of CH$_3$D in interstellar clouds, if confirmed. The column density and fractional abundance of CH$_3$D are determined to be $(9.1\pm3.4) \times 10^{15}$ cm$^{-2}$ and $(3.0\pm1.1) \times 10^{-7}$, respectively, where we assume the rotational temperature of 25 K. The column density and fractional abundance of the gaseous CH$_4$ are estimated to be $(1.3-4.6) \times 10^{17}$ cm$^{-2}$ and $(4.3-15.2) \times 10^{-6}$, respectively, by adopting the molecular D/H ratios of 2--7% reported for various molecules in L1527. The fractional abundance of CH$_4$ is higher than or comparable to that found in high-mass star-forming cores by infrared observations. It is sufficiently high for triggering the efficient production of various carbon-chain molecules in a lukewarm region near the protostar, which supports the picture of the warm carbon-chain chemistry.

astro-ph.SR↗

arXiv2011

Detection of CO(J=1-0) Emission from Barred Spiral Galaxies at z~0.1

We present the results of CO (J=1-0) observations towards nine barred spiral galaxies at z=0.08-0.25 using the 45-m telescope at Nobeyama Radio Observatory (NRO). This survey is the first one specialized for barred spiral galaxies in this redshift range. We detected CO emission from six out of nine galaxies, whose CO luminosity (L_CO') ranges (1.09-10.8)\times10^9 K km s^{-1} pc^2. These are the infrared (IR) dimmest galaxies that have ever been detected in CO at z~0.1 to date. They follow the L_CO'-L_IR relation among local spiral galaxies, Luminous Infrared Galaxies (LIRGs), Ultra-Luminous Infrared Galaxies (ULIRGs) and Sub-millimeter Galaxies (SMGs). Their L_CO' and L_IR are higher than that of local spiral galaxies which have been detected in CO so far, and L_IR/L'_CO, which is a measure of star formation efficiency, is comparable to or slightly higher than that of local ones. This result suggests that these galaxies are forming stars more actively than local spirals galaxies simply because they have more fuel.

astro-ph.CO↗

arXiv2011

An Unbiased Spectral Line Survey toward R CrA IRS7B in the 345 GHz Window with ASTE

We have conducted a spectral line survey in the 332 - 364 GHz region with the ASTE 10 m telescope toward R CrA IRS7B, a low-mass protostar in the Class 0 or Class 0/I transitional stage. We have also performed some supplementary observations in the 450 GHz band. In total, 16 molecular species are identified in the 332 - 364 GHz region. Strong emission lines of CN and CCH are observed, whereas complex organic molecules and long carbon-chain molecules which are characteristics of hot corino and warm carbon-chain chemistry (WCCC) source, respectively, are not detected. The rotation temperature of CH3OH is evaluated to be 31 K, which is significantly lower than that reported for the prototypical hot corino IRAS 16293-2422 (~85 K). The deuterium fractionation ratios for CCH and H2CO are obtained to be 0.038 and 0.050, respectively, which are much lower than those in the hot corino. These results suggest a weak hot corino activity in R CrA IRS7B. On the other hand, the carbon-chain related molecules, CCH and c-C3H2, are found to be abundant. However, this source cannot be classified as a WCCC source, since long carbon-chain molecules are not detected. If WCCC and hot corino chemistry represent the two extremes in chemical compositions of low-mass Class 0 sources, R CrA IRS7B would be a source with a mixture of these two chemical characteristics. The UV radiation from the nearby Herbig Ae star R CrA may also affect the chemical composition. The present line survey demonstrates further chemical diversity in low-mass star-forming regions.

astro-ph.GA↗