arXiv subjects

Masao Saito

Publications and source records attributed to Masao Saito.

At least 37 records · Page 2Linked to original sources

arXiv2021

Clump-scale chemistry in the NGC2264-D cluster-forming region

We have conducted mapping observations toward the n3 and n5 positions in the NGC\,2264-D cluster-forming region with the Atacama Compact Array (ACA) of the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 3. Observations with 10000 au scale beam reveal the chemical composition at the clump scale. The spatial distributions of the observed low upper-state-energy lines of CH$_{3}$OH are similar to those of CS and SO, and the HC$_{3}$N emission seems to be predominantly associated with clumps containing young stellar objects. The turbulent gas induced by the star formation activities produces large-scale shock regions in NGC\,2264-D, which are traced by the CH$_{3}$OH, CS and SO emissions. We derive the HC$_{3}$N, CH$_{3}$CN, and CH$_{3}$CHO abundances with respect to CH$_{3}$OH. Compared to the n5 field, the n3 field is farther (in projected apparent distance) from the neighboring NGC\,2264-C, yet the chemical composition in the n3 field tends to be similar to that of the protostellar candidate CMM3 in NGC\,2264-C. The HC$_{3}$N/CH$_{3}$OH ratios in the n3 field are higher than those in the n5 field. We find an anti-correlation between the HC$_{3}$N/CH$_{3}$OH ratio and their excitation temperatures. The low HC$_{3}$N/CH$_{3}$OH abundance ratio at the n5 field implies that the n5 field is an environment with more active star formation compared with the n3 field.

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arXiv2021

Carbon-Chain Chemistry vs. Complex-Organic-Molecule Chemistry in Envelopes around Three Low-Mass Young Stellar Objects in the Perseus Region

We have analyzed ALMA Cycle 5 data in Band 4 toward three low-mass young stellar objects (YSOs), IRAS 03235+3004 (hereafter IRAS 03235), IRAS 03245+3002 (IRAS 03245), and IRAS 03271+3013 (IRAS 03271), in the Perseus region. The HC$_{3}$N ($J=16-15$; $E_{\rm {up}}/k = 59.4$ K) line has been detected in all of the target sources, while four CH$_{3}$OH lines ($E_{\rm {up}}/k = 15.4-36.3$ K) have been detected only in IRAS 03245. Sizes of the HC$_{3}$N distributions ($\sim 2930-3230$ au) in IRAS 03235 and IRAS 03245 are similar to those of the carbon-chain species in the warm carbon chain chemistry (WCCC) source L1527. The size of the CH$_{3}$OH emission in IRAS 03245 is $\sim 1760$ au, which is slightly smaller than that of HC$_{3}$N in this source. We compare the CH$_{3}$OH/HC$_{3}$N abundance ratios observed in these sources with predictions of chemical models. We confirm that the observed ratio in IRAS 03245 agrees with the modeled values at temperatures around 30--35 K, which supports the HC$_{3}$N formation by the WCCC mechanism. In this temperature range, CH$_{3}$OH does not thermally desorb from dust grains. Non-thermal desorption mechanisms or gas-phase formation of CH$_{3}$OH seem to work efficiently around IRAS 03245. The fact that IRAS 03245 has the highest bolometric luminosity among the target sources seems to support these mechanisms, in particular the non-thermal desorption mechanisms.

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arXiv2021

Low-metallicity Young Clusters in the Outer Galaxy. III. Sh 2-127

In deep near-infrared imaging of the low-metallicity (${\rm [O/H]}=-0.7$ dex) H II region Sh 2-127 (S127) with Subaru/MOIRCS, we detected two young clusters with 413 members (S127A) in a slightly extended H II region and another with 338 members (S127B) in a compact H II region. The limiting magnitude was $K=21.3$ mag (10$σ$), corresponding to a mass detection limit of $\sim$0.2 $M_\odot$. These clusters are an order of magnitude larger than previously studied young low-metallicity clusters and larger than the majority of solar neighborhood young clusters. Fits to the K-band luminosity functions indicate very young cluster ages of 0.5 Myr for S127A and 0.1-0.5 Myr for S127B, consistent with the large extinction (up to $A_V\simeq20$ mag) from thick molecular clouds and the presence of a compact H II region and class I source candidates, and suggest that the initial mass function (IMF) of the low-metallicity clusters is indistinguishable from typical solar neighborhood IMFs. Disk fractions of $28\% \pm 3\%$ for S127A and $40\% \pm 4\%$ for S127B are significantly lower than those of similarly aged solar neighborhood clusters ($\sim$50$\%$-60$\%$). The disk fraction for S127B is higher than those of previously studied low-metallicity clusters ($<$30 $\%$), probably due to S127B's age. This suggests that a large fraction of very young stars in low-metallicity environments have disks, but the disks are lost on a very short timescale. These results are consistent with our previous studies of low-metallicity star-forming regions, suggesting that a solar neighborhood IMF and low disk fraction are typical characteristics for low-metallicity regions, regardless of cluster scales.

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arXiv2020

Carbon Chain Chemistry in Hot-Core Regions around Three Massive Young Stellar Objects Associated with 6.7 GHz Methanol Masers

We have carried out observations of CCH ($N=1-0$), CH$_{3}$CN ($J=5-4$), and three $^{13}$C isotopologues of HC$_{3}$N ($J=10-9$) toward three massive young stellar objects (MYSOs), G12.89+0.49, G16.86--2.16, and G28.28--0.36, with the Nobeyama 45-m radio telescope. Combined with previous results on HC$_{5}$N, the column density ratios of $N$(CCH)/$N$(HC$_{5}$N), hereafter the CCH/HC$_{5}$N ratios, in the MYSOs are derived to be $\sim 15$. This value is lower than that in a low-mass warm carbon chain chemistry (WCCC) source by more than one order of magnitude. We compare the observed CCH/HC$_{5}$N ratios with hot-core model calculations (Taniguchi et al. 2019). The observed ratios in the MYSOs can be best reproduced by models when the gas temperature is $\sim 85$ K, which is higher than in L1527, a low-mass WCCC source ($\sim 35$ K). These results suggest that carbon-chain molecules detected around the MYSOs exist at least partially in higher temperature regions than those in low-mass WCCC sources. There is no significant difference in column density among the three $^{13}$C isotopologues of HC$_{3}$N in G12.89+0.49 and G16.86-2.16, while HCC$^{13}$CN is more abundant than the others in G28.28--0.36. We discuss carbon-chain chemistry around the three MYSOs based on the CCH/HC$_{5}$N ratio and the $^{13}$C isotopic fractionation of HC$_{3}$N.

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arXiv2020

Confirming the Explosive Outflow in G5.89 with ALMA

The explosive molecular outflow detected decades ago in the Orion BN/KL region of massive star formation was considered to be a bizarre event. This belief was strengthened by the non detection of similar cases over the years with the only exception of the marginal case of DR21. Here, we confim a similar explosive outflow associated with the UCH$_{\rm II}$ region G5.89$-$0.39 that indicates that this phenomenon is not unique to Orion or DR21. Sensitive and high angular resolution ($\sim$ 0.1$''$) ALMA CO(2$-$1) and SiO(5$-$4) observations show that the molecular outflow in the massive star forming region G5.89$-$0.39 is indeed an explosive outflow with an age of about 1000 yrs and a liberated kinetic energy of 10$^{46-49}$ erg. Our new CO(2$-$1) ALMA observations revealed over 30 molecular filaments, with Hubble-like expansion motions, pointing to the center of UCH$_{\rm II}$ region. In addition, the SiO(5$-$4) observations reveal warmer and strong shocks very close to the origin of the explosion, confirming the true nature of the flow. A simple estimation for the occurrence of these explosive events during the formation of the massive stars indicates an event rate of once every $\sim$100 yrs, which is close to the supernovae rate.

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arXiv2020

Distortion of Magnetic Fields in the Dense Core SL42 (CrA-E) in the Corona Australis Molecular Cloud Complex

Detailed magnetic field structure of the dense core SL42 (CrA-E) in the Corona Australis molecular cloud complex was investigated based on near-infrared polarimetric observations of background stars to measure dichroically polarized light produced by magnetically aligned dust grains. The magnetic fields in and around SL42 were mapped using 206 stars and curved magnetic fields were identified. On the basis of simple hourglass (parabolic) magnetic field modeling, the magnetic axis of the core on the plane of sky was estimated to be $40^{\circ} \pm 3^{\circ}$. The plane-of-sky magnetic field strength of SL42 was found to be $22.4 \pm 13.9$ $μ$G. Taking into account the effects of thermal/turbulent pressure and the plane-of-sky magnetic field component, the critical mass of SL42 was obtained to be $M_{\rm cr} = 21.2 \pm 6.6$ M$_{\odot}$, which is close to the observed core mass of $M_{\rm core} \approx 20$ M$_{\odot}$. We thus conclude that SL42 is in a condition close to the critical state if the magnetic fields lie near the plane of the sky. Since there is a very low luminosity object (VeLLO) toward the center of SL42, it is unlikely this core is in a highly subcritical condition (i.e., magnetic inclination angle significantly deviated from the plane of sky). The core probably started to collapse from a nearly kinematically critical state. In addition to the hourglass magnetic field modeling, the Inoue \& Fukui (2013) mechanism may explain the origin of the curved magnetic fields in the SL42 region.

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arXiv2020

Chemical composition in the IRAS 16562--3959 high-mass star-forming region

We have analyzed the Atacama Large Millimeter/submillimeter Array (ALMA) cycle 2 data of band 6 toward the G345.4938+01.4677 massive young protostellar object (G345.5+1.47 MYSO) in the IRAS 16562--3959 high-mass star-forming region with an angular resolution of $\sim 0.3"$, corresponding to $\sim 760$ au. We spatially resolve the central region which consists of three prominent molecular emission cores. A hypercompact (HC) H$_{\rm {II}}$ region (Core A) and two molecule-rich cores (Core B and Core C) are identified using the moment zero images of the H30$α$ line and a CH$_{3}$OH line, respectively. Various oxygen-bearing complex organic molecules (COMs), such as (CH$_{3}$)$_{2}$CO and CH$_{3}$OCHO, have been detected toward the positions of Core B and Core C, while nitrogen-bearing species, CH$_{3}$CN, HC$_{3}$N and its $^{13}$C isotopologues, have been detected toward all of the cores. We discuss the formation mechanisms of H$_{2}$CO by comparing the spatial distribution of C$^{18}$O with that of H$_{2}$CO. The $^{33}$SO emission, on the other hand, shows a ring-like structure surrounding Core A, and it peaks on the outer edge of the H30$α$ emission region. These results imply that SO is enhanced in a shock produced by the expanding motion of the ionized region.

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arXiv2020

Circumbinary Disks of the Protostellar Binary Systems in the L1551 Region

We report ALMA Cycle 4 observations of the Class I binary protostellar system L1551 IRS 5 in the 0.9-mm continuum emission, C18O (J=3-2), OCS (J=28-27), and four other Band 7 lines. At ~0.07" (= 10 au) resolution in the 0.9 mm emission, two circumstellar disks (CSDs) associated with the binary protostars are separated from the circumbinary disk (CBD). The CBD is resolved into two spiral arms, one connecting to the CSD around the northern binary source, Source N, and the other to Source S. As compared to the CBD in the neighboring protobinary system L1551 NE, the CBD in L1551 IRS 5 is more compact (r ~150 au) and the m=1 mode of the spirals found in L1551 NE is less obvious in L1551 IRS 5. Furthermore, the dust and molecular-line brightness temperatures of CSDs and CBD reach >260 K and >100 K, respectively, in L1551 IRS 5, much hotter than those in L1551 NE. The gas motions in the spiral arms are characterized by rotation and expansion. Furthermore, the transitions from the CBD to the CSD rotations at around the L2 and L3 Lagrangian points and gas motions around the L1 point are identified. Our numerical simulations reproduce the observed two spiral arms and expanding gas motion as a result of gravitational torques from the binary, transitions from the CBD to the CSD rotations, and the gas motion around the L1 point. The higher temperature in L1551 IRS 5 likely reflects the inferred FU-Ori event.

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arXiv2020

Distortion of Magnetic Fields in BHR 71

The magnetic field structure of a star-forming Bok globule BHR 71 was determined based on near-infrared polarimetric observations of background stars. The magnetic field in BHR 71 was mapped from 25 stars. By using a simple 2D parabolic function, the plane-of-sky magnetic axis of the core was found to be $θ_{\rm mag} = 125^{\circ} \pm 11^{\circ}$. The plane-of-sky mean magnetic field strength of BHR 71 was found to be $B_{\rm pos} = 8.8 - 15.0$ $μ$G, indicating that the BHR 71 core is magnetically supercritical with $λ= 1.44 - 2.43$. Taking into account the effect of thermal/turbulent pressure and the plane-of-sky magnetic field component, the critical mass of BHR 71 was $M_{\rm cr} = 14.5-18.7$ M$_{\odot}$, which is consistent with the observed core mass of $M_{\rm core} \approx 14.7$ M$_{\odot}$ (Yang et al. 2017). We conclude that BHR 71 is in a condition close to a kinematically critical state, and the magnetic field direction lies close to the plane of sky. Since BHR 71 is a star-forming core, a significantly subcritical condition (i.e., the magnetic field direction deviating from the plane of sky) is unlikely, and collapsed from a condition close to a kinematically critical state. There are two possible scenarios to explain the curved magnetic fields of BHR 71, one is an hourglass-like field structure due to mass accumulation and the other is the Inoue \& Fukui (2013) mechanism, which proposes the interaction of the core with a shock wave to create curved magnetic fields wrapping around the core.

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arXiv2020

Distortion of Magnetic Fields in Barnard 335

In this study, the detailed magnetic field structure of the dense protostellar core Barnard 335 (B335) was revealed based on near-infrared polarimetric observations of background stars to measure dichroically polarized light produced by magnetically aligned dust grains in the core. Magnetic fields pervading B335 were mapped using 24 stars after subtracting unrelated ambient polarization components, for the first time revealing that they have an axisymmetrically distorted hourglass-shaped structure toward the protostellar core. On the basis of simple two- and three-dimensional magnetic field modeling, magnetic inclination angles in the plane-of-sky and line-of-sight directions were determined to be $90^{\circ} \pm 7^{\circ}$ and $50^{\circ} \pm 10^{\circ}$, respectively. The total magnetic field strength of B335 was determined to be $30.2 \pm 17.7$ $μ{\rm G}$. The critical mass of B335, evaluated using both magnetic and thermal/turbulent support against collapse, was determined to be $M_{\rm cr} = 3.37 \pm 0.94$ ${\rm M}_{\odot}$, which is identical to the observed core mass of $M_{\rm core}=3.67$ M$_{\odot}$. We thus concluded that B335 started its contraction from a condition near equilibrium. We found a linear relationship in the polarization versus extinction diagram, up to $A_V \sim 15$ mag toward the stars with the greatest obscuration, which verified that our observations and analysis provide an accurate depiction of the core.

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arXiv2019

Investigation of Chemical Differentiation among the NGC2264 Cluster-Forming Clumps

We have carried out mapping observations of molecular emission lines of HC$_{3}$N and CH$_{3}$OH toward two massive cluster-forming clumps, NGC2264-C and NGC2264-D, using the Nobeyama 45-m radio telescope. We derive an $I$(HC$_{3}$N)/$I$(CH$_{3}$OH) integrated intensity ratio map, showing a higher value at clumps including 2MASS point sources at the northern part of NGC2264-D. Possible interpretations of the $I$(HC$_{3}$N)/$I$(CH$_{3}$OH) ratio are discussed. We have also observed molecular emission lines from CCS and N$_{2}$H$^{+}$ toward five positions in each clump. We investigate the $N$(N$_{2}$H$^{+}$)/$N$(CCS) and $N$(N$_{2}$H$^{+}$)/$N$(HC$_{3}$N) column density ratios among the ten positions in order to test whether they can be used as chemical evolutionary indicators in these clumps. The $N$(N$_{2}$H$^{+}$)/$N$(CCS) ratio shows a very high value toward a bright embedded IR source (IRS1), whereas the $N$(N$_{2}$H$^{+}$)/$N$(HC$_{3}$N) ratio at IRS1 is comparable with those at the other positions. These results suggest that UV radiation affects the chemistry around IRS1. We find that there are positive correlations between these column density ratios and the excitation temperatures of N$_{2}$H$^{+}$, which implies the chemical evolution of clumps. These chemical evolutionary indicators likely reflect the combination of evolution along the filamentary structure and evolution of each clump.

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arXiv2019

Distortion of Magnetic Fields in the Dense Core CB81 (L1774, Pipe 42) in the Pipe Nebula

The detailed magnetic field structure of the starless dense core CB81 (L1774, Pipe 42) in the Pipe Nebula was determined based on near-infrared polarimetric observations of background stars to measure dichroically polarized light produced by magnetically aligned dust grains in the core. The magnetic fields pervading CB81 were mapped using 147 stars and axisymmetrically distorted hourglass-like fields were identified. On the basis of simple 2D and 3D magnetic field modeling, the magnetic inclination angles in the plane-of-sky and line-of-sight directions were determined to be $4^{\circ} \pm 8^{\circ}$ and $20^{\circ} \pm 20^{\circ}$, respectively. The total magnetic field strength of CB81 was found to be $7.2 \pm 2.3$ $μ{\rm G}$. Taking into account the effects of thermal/turbulent pressure and magnetic fields, the critical mass of CB81 was calculated to be $M_{\rm cr}=4.03 \pm 0.40$ M$_{\odot}$, which is close to the observed core mass of $M_{\rm core}=3.37 \pm 0.51$ M$_{\odot}$. We thus conclude that CB81 is in a condition close to the critical state. In addition, a spatial offset of $92''$ was found between the center of magnetic field geometry and the dust extinction distribution; this offset structure could not have been produced by self-gravity. The data also indicate a linear relationship between polarization and extinction up to $A_V \sim 30$ mag going toward the core center. This result confirms that near-infrared polarization can accurately trace the overall magnetic field structure of the core.

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arXiv2019

Distortion of Magnetic Fields in a Starless Core VI: Application of Flux Freezing Model and Core Formation of FeSt 1-457

Observational data for the hourglass-like magnetic field toward the starless dense core FeSt 1-457 were compared with a flux freezing magnetic field model (Myers et al. 2018). Fitting of the observed plane-of-sky magnetic field using the flux freezing model gave a residual angle dispersion comparable with the results based on a simple three-dimensional parabolic model. The best-fit parameters for the flux freezing model were a line-of-sight magnetic inclination angle of $γ_{\rm mag} = 35^{\circ} \pm 15^{\circ}$ and a core center to ambient (background) density contrast of $ρ_{\rm c} / ρ_{\rm bkg} = 75$. The initial density for core formation ($ρ_0$) was estimated to be $ρ_{\rm c} / 75 = 4670$ cm$^{-3}$, which is about one order of magnitude higher than the expected density ($\sim 300$ cm$^{-3}$) for the inter-clump medium of the Pipe Nebula. FeSt 1-457 is likely to have been formed from the accumulation of relatively dense gas, and the relatively dense background column density of $A_V \simeq 5$ mag supports this scenario. The initial radius (core formation radius) $R_0$ and the initial magnetic field strength $B_0$ were obtained to be 0.15 pc ($1.64 R$) and $10.8-14.6$ $μ$G, respectively. We found that the initial density $ρ_0$ is consistent with the mean density of the nearly critical magnetized filament with magnetic field strength $B_0$ and radius $R_0$. The relatively dense initial condition for core formation can be naturally understood if the origin of the core is the fragmentation of magnetized filaments.

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arXiv2019

Protostellar Evolution in Serpens Main: Possible Origin of Disk-Size Diversity

We have observed the submillimeter continuum condensations SMM2, SMM4, SMM9, and SMM11 in the star forming cluster Serpens Main using the Atacama Large Millimeter/submillimeter Array during Cycle 3 in the 1.3 mm continuum, 12CO J=2-1, SO J_N=6_5-5_4, and C18O J=2-1 lines at an angular resolution of ~0.55 (240 au). Sixteen sources have been detected in the 1.3 mm continuum, which can be classified into three groups. Group 1 consists of six sources showing extended continuum emission and bipolar/monopolar 12CO outflows. Although all the Group 1 members are classified as Class 0 protostars, our observations suggest evolutionary trends among them in terms of 12CO outflow dynamical time, SO emission distribution, C18O fractional abundance, and continuum morphology. Group 2 consists of four sources associated with a continuum filamentary structure and no 12CO outflows. Central densities estimated from the 1.3 mm continuum intensity suggest that they are prestellar sources in a marginally Jeans unstable state. Group 3 consists of six Spitzer sources showing point-like 1.3 mm continuum emission and clumpy 12CO outflows. These features of Group 3 suggest envelope dissipation, preventing disk growth from the present size, r <~ 60 au. The Group 3 members are protostars that may be precursors to the T Tauri stars associated with small disks at tens-au radii identified in recent surveys.

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arXiv2019

Distortion of Magnetic Fields in Barnard 68

The magnetic field structure, kinematical stability, and evolutionary status of the starless dense core Barnard 68 (B68) are revealed based on the near-infrared polarimetric observations of background stars, measuring the dichroically polarized light produced by aligned dust grains in the core. After subtracting unrelated ambient polarization components, the magnetic fields pervading B68 are mapped using 38 stars and axisymmetrically distorted hourglass-like magnetic fields are obtained, although the evidence for the hourglass field is not very strong. On the basis of simple 2D and 3D magnetic field modeling, the magnetic inclination angles on the plane-of-sky and in the line-of-sight direction are determined to be $47^{\circ} \pm 5^{\circ}$ and $20^{\circ} \pm 10^{\circ}$, respectively. The total magnetic field strength of B68 is obtained to be $26.1 \pm 8.7$ $μ{\rm G}$. The critical mass of B68, evaluated using both magnetic and thermal/turbulent support, is $M_{\rm cr} = 2.30 \pm 0.20$ ${\rm M}_{\odot}$, which is consistent with the observed core mass of $M_{\rm core}=2.1$ M$_{\odot}$, suggesting nearly critical state. We found a relatively linear relationship between polarization and extinction up to $A_V \sim 30$ mag toward the stars with deepest obscuration. Further theoretical and observational studies are required to explain the dust alignment in cold and dense regions in the core.

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arXiv2019

The ALMA Survey of 70 $μ$m dark High-mass clumps in Early Stages (ASHES). I. Pilot Survey: Clump Fragmentation

(Abridged) ASHES has been designed to systematically characterize the earliest stages and to constrain theories of high-mass star formation. A total of 12 massive (>500 $M_{\odot}$), cold (<15 K), 3.6-70 $μ$m dark prestellar clump candidates, embedded in IRDCs, were carefully selected in the pilot survey to be observed with ALMA. We mosaiced each clump (~1 arcmin^2) in dust and line emission with the 12m/7m/TP arrays at 224 GHz, resulting in ~1.2" resolution (~4800 AU). As the first paper of the series, we concentrate on the dust emission to reveal the clump fragmentation. We detect 294 cores, from which 84 (29%) are categorized as protostellar based on outflow activity or 'warm core' line emission. The remaining 210 (71%) are considered prestellar core candidates. The number of detected cores is independent of the mass sensitivity range of the observations. On average, more massive clumps tend to form more cores. We find a large population of low-mass (<1 M) cores and no high-mass (>30 $M_{\odot}$) prestellar cores. The most massive prestellar core has a mass of 11 $M_{\odot}$. From the prestellar CMF, we derive a power law index of 1.17+-0.1, slightly shallower than Salpeter (1.35). We use the MST technique to characterize the separation between cores and their spatial distribution, and derive mass segregation ratios. While there is a range of core masses and separations detected in the sample, the mean separation and mass of cores are well explained by thermal fragmentation and are inconsistent with turbulent Jeans fragmentation. The core spatial distribution is well described by hierarchical subclustering rather than centrally peaked clustering. There is no conclusive evidence of mass segregation. We test several theoretical conditions, and conclude that overall, competitive accretion and global hierarchical collapse scenarios are favored over the turbulent core accretion scenario.

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arXiv2019

Survey Observation of CH$_3$NH$_2$ and Its Formation Process

We present the observational result of a glycine precursor, methylamine (CH$_3$NH$_2$), together with methanol (CH$_3$OH) and methanimine (CH$_2$NH) towards high-mass star-forming regions, NGC6334I, G10.47+0.03, G31.41+0.3, and W51~e1/e2 using ALMA. The molecular abundances toward these sources were derived using the rotational diagram method and compared with our state-of-the-art chemical model. We found that the observed ratio of "CH$_3$NH$_2$/CH$_3$OH" is in between 0.11 and 2.2. We also found that the observed "CH$_3$NH$_2$/CH$_3$OH" ratio agrees well with our chemical model by considering the formation of CH$_3$NH$_2$ on the grain surface via hydrogenation process to HCN. This result clearly shows the importance of hydrogenation processes to form CH$_3$NH$_2$. NGC63343I MM3, where CH$_3$NH$_2$ was not detected in this study and showed "CH$_3$NH$_2$/CH$_3$OH" ratio of less than 0.02, is clearly distinguished from the other cores.

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arXiv2019

$^{13}$C Isotopic Fractionation of CCH in Two Starless Cores: L1521B and L134N

We have carried out observations of CCH and its two $^{13}$C isotopologues, $^{13}$CCH and C$^{13}$CH, in the 84 - 88 GHz band toward two starless cores, L1521B and L134N (L183), using the Nobeyama 45 m radio telescope. We have detected C$^{13}$CH with a signal-to-noise (S/N) ratio of 4, whereas no line of $^{13}$CCH was detected in either the dark clouds. The column densities of the normal species were derived to be ($1.66 \pm 0.18$)$\times 10^{14}$ cm$^{-2}$ and ($7.3 \pm 0.9$)$\times 10^{13}$ cm$^{-2}$ ($1 σ$) in L1521B and L134N, respectively. The column density ratios of $N$(C$^{13}$CH)/$N$($^{13}$CCH) were calculated to be $>1.1$ and $>1.4$ in L1521B and L134N, respectively. The characteristic that $^{13}$CCH is less abundant than C$^{13}$CH is likely common for dark clouds. Moreover, we find that the $^{12}$C/$^{13}$C ratios of CCH are much higher than those of HC$_{3}$N in L1521B by more than a factor of 2, as well as in Taurus Molecular Cloud-1 (TMC-1). In L134N, the differences in the $^{12}$C/$^{13}$C ratios between CCH and HC$_{3}$N seem to be smaller than those in L1521B and TMC-1. We discuss the origins of the $^{13}$C isotopic fractionation of CCH and investigate possible routes that cause the significantly high $^{12}$C/$^{13}$C ratio of CCH especially in young dark clouds, with the help of chemical simulations. The high $^{12}$C/$^{13}$C ratios of CCH seem to be caused by reactions between hydrocarbons (e.g., CCH, C$_{2}$H$_{2}$, $l,c$-C$_{3}$H) and C$^{+}$.

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