arXiv subjects

Satoshi Yamamoto

Publications and source records attributed to Satoshi Yamamoto.

At least 55 records · Page 3Linked to original sources

arXiv2019

The Effect of Far-infrared Radiation on the Hyperfine Anomaly of the OH 18 cm Transition

We present observations of the four hyperfine structure components of the OH 18 cm transition (1612, 1665, 1667 and 1720 MHz) toward Heiles Cloud 2 (HCL2) in Taurus and the dense cores L183 and L169. Toward the peculiar straight structure in the northern part of HCL2, the 1720 MHz line shows absorption against the cosmic microwave background at a velocity of $\sim$ 5.5 km s$^{-1}$, whereas the 1612 MHz line shows brighter emission than expected in local thermodynamic equilibrium (LTE). Such an intensity anomaly is also observed toward TMC-1 (CP), L183, and L169, where the 1612 MHz line is stronger and the 1720 MHz line is weaker than those expected under LTE. We conduct statistical equilibrium calculations considering the effect of far-infrared (FIR) radiation from surrounding clouds and find that the absorption feature of the 1720 MHz line can be reproduced by the following conditions: gas kinetic temperature lower than about 30 K, OH column density higher than 10$^{15}$ cm$^{-2}$, H$_2$ density lower than 10$^3$ cm$^{-3}$, and an ortho-to-para ratio of H$_2$ lower than 1. Therefore, the absorption feature of the 1720 MHz line is considered to trace relatively cold and dense gas that is surrounded by a warm envelope consisting of CO-dark molecular gas.

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arXiv2019

An unbiased spectral line survey observation toward the low-mass star-forming region L1527

An unbiased spectral line survey toward a solar-type Class 0/I protostar, IRAS04368+2557, in L1527 has been carried out in the 3 mm band with the Nobeyama 45 m telescope. L1527 is known as a warm carbon-chain chemistry (WCCC) source, which harbors abundant unsaturated organic species such as C$_n$H ($n = 3,\ 4,\ 5,\ldots$) in a warm and dense region near the protostar. The observation covers the frequency range from 80 to 116 GHz. A supplementary observation has also been conducted in the 70 GHz band to observe fundamental transitions of deuterated species. In total, 69 molecular species are identified, among which 27 species are carbon-chain species and their isomers, including their minor isotopologues. This spectral line survey provides us with a good template of the chemical composition of the WCCC source.

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arXiv2018

Molecular-Cloud-Scale Chemical Composition III: Constraints of Average Physical Properties through Chemical Models

It is important to understand the origin of molecular line intensities and chemical composition in the molecular-cloud scale in the Galactic sources because it serves as a benchmark to compare with the chemical compositions of extragalactic sources. Recent observations of the 3-mm spectra averaged over the 10-pc scale show similar spectral pattern among sources for molecular lines HCN, HCO$^+$, CCH, HNC, HNCO, c-C$_3$H$_2$, CS, SO, N$_2$H$^+$, and CN. To constrain the average physical property emitting such spectral pattern, we model molecular spectra using a time-dependent gas-grain chemical model followed by a radiative transfer calculation. We use a grid of physical parameters such as the density $n=3 \times 10^2 - 3\times 10^4$ cm$^{-3}$, the temperature, $T=10-30$ K, the visual extinction $A_{\rm V} = 2,4,10$ mag, the cosmic-ray ionization rate $ζ= 10^{-17} - 10^{-16}$ s$^{-1}$, and the sulfur elemental abundance $S/H = 8\times 10^{-8} - 8\times 10^{-7}$. Comparison with the observed spectra indicates that spectra are well reproduced with the relatively low density of $n=(1-3) \times 10^3\,$cm$^{-3}$, $T=10\,$K, $ζ= 10^{-17}$ s$^{-1}$, and the short chemistry timescale of $10^5$ yrs. This short chemistry timescale may indicate that molecular clouds are constantly affected by the turbulence, and exposed to low-density, low $A_{\rm V}$ regions that "refreshes" the chemical clock by UV radiation. The relatively low density obtained is orders of magnitude lower than the commonly-quoted critical density in the optically thin case. Meanwhile, this range of density is consistent with results from recent observational analysis of molecular-cloud-scale mapping.

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arXiv2018

The Co-evolution of Disk and Star in Embedded Stages: The Case of the Very Low-mass Protostar

We have observed the CCH (N=3-2, J=7/2-5/2, F=4-3 and 3-2) and SO (6_7-5_6) emission at a 0"2 angular resolution toward the low-mass Class 0 protostellar source IRAS 15398-3359 with ALMA. The CCH emission traces the infalling-rotating envelope near the protostar with the outflow cavity extended along the northeast-southwest axis. On the other hand, the SO emission has a compact distribution around the protostar. The CCH emission is relatively weak at the continuum peak position, while the SO emission has a sharp peak there. Although the maximum velocity shift of the CCH emission is about 1 km s^-1 from the systemic velocity, a velocity shift higher than 2 km s^{-1} is seen for the SO emission. This high velocity component is most likely associated with the Keplerian rotation around the protostar. The protostellar mass is estimated to be 0.007^{+0.004}_{-0.003} from the velocity profile of the SO emission. With this protostellar mass, the velocity structure of the CCH emission can be explained by the model of the infalling-rotating envelope, where the radius of the centrifugal barrier is estimated to be 40 au from the comparison with the model. The disk mass evaluated from the dust continuum emission by assuming the dust temperature of 20 K-100 K is 0.1-0.9 times the stellar mass, resulting in the Toomre Q parameter of 0.4-5. Hence, the disk structure may be partly unstable. All these results suggest that a rotationally-supported disk can be formed in the earliest stages of the protostellar evolution.

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arXiv2018

Rotation in the NGC 1333 IRAS 4C Outflow

We report molecular line observations of the NGC 1333 IRAS 4C outflow in the Perseus Molecular Cloud with the Atacama Large Millimeter/Submillimeter Array. The CCH and CS emission reveal an outflow cavity structure with clear signatures of rotation with respect to the outflow axis. The rotation is detected from about 120 au up to about 1400 au above the envelope/disk mid-plane. As the distance to the central source increases, the rotation velocity of the outflow decreases while the outflow radius increases, which gives a flat specific angular momentum distribution along the outflow. The mean specific angular momentum of the outflow is about 100 au km/s. Based on reasonable assumptions on the outward velocity of the outflow and the protostar mass, we estimate the range of outflow launching radii to be 5-15 au. Such a launching radius rules out that this outflow is launched as an X-wind, but rather, it is more consistent to be a slow disk wind launched from relatively large radii on the disk. The radius of the centrifugal barrier is roughly estimated, and the role of the centrifugal barrier in the outflow launching is discussed.

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arXiv2018

Depletion of 15N in the center of L1544: Early transition from atomic to molecular nitrogen?

We performed sensitive observations of the N15ND+(1-0) and 15NND+(1-0) lines toward the prestellar core L1544 using the IRAM 30m telescope. The lines are not detected down to 3 sigma levels in 0.2 km/s channels of around 6 mK. The non-detection provides the lower limit of the 14N/15N ratio for N2D+ of ~700-800, which is much higher than the elemental abundance ratio in the local ISM of ~200-300. The result indicates that N2 is depleted in 15N in the central part of L1544, because N2D+ preferentially traces the cold dense gas, and because it is a daughter molecule of N2. In-situ chemistry is unlikely responsible for the 15N depletion in N2; neither low-temperature gas phase chemistry nor isotope selective photodissociation of N2 explains the 15N depletion; the former prefers transferring 15N to N2, while the latter requires the penetration of interstellar FUV photons into the core center. The most likely explanation is that 15N is preferentially partitioned into ices compared to 14N via the combination of isotope selective photodissociation of N2 and grain surface chemistry in the parent cloud of L1544 or in the outer regions of L1544 which are not fully shielded from the interstellar FUV radiation. The mechanism is the most efficient at the chemical transition from atomic to molecular nitrogen. In other words, our result suggests that the gas in the central part of L1544 already went trough the transition from atomic to molecular nitrogen in the earlier evolutionary stage, and that N2 is currently the primary form of gas-phase nitrogen.

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arXiv2018

A multiline study of a high-mass young stellar object in the Small Magellanic Cloud with ALMA: The detection of methanol gas at 0.2 solar metallicity

We report the results of subparsec-scale submillimeter observations towards an embedded high-mass young stellar object in the Small Magellanic Cloud (SMC) with ALMA. Complementary infrared data obtained with the AKARI satellite and the Gemini South telescope are also presented. The target infrared point source is spatially resolved into two dense molecular cloud cores; one is associated with a high-mass young stellar object (YSO core), while another is not associated with an infrared source (East core). The two cores are dynamically associated but show different chemical characteristics. Emission lines of CS, C33S, H2CS, SO, SO2, CH3OH, H13CO+, H13CN, SiO, and dust continuum are detected from the observed region. Tentative detection of HDS is also reported. The first detection of CH3OH in the SMC has a strong impact on our understanding of the formation of complex organic molecules in metal-poor environments. The gas temperature is estimated to be ~10 K based on the rotation analysis of CH3OH lines. The fractional abundance of CH3OH gas in the East core is estimated to be (0.5-1.5) x 10^(-8), which is comparable with or marginally higher than those of similar cold sources in our Galaxy despite a factor of five lower metallicity in the SMC. This work provides observational evidence that an organic molecule like CH3OH, which is largely formed on grain surfaces, can be produced even in a significantly lower metallicity environment compared to the solar neighborhood. A possible origin of cold CH3OH gas in the observed dense core is discussed.

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arXiv2018

Sub-arcsecond Kinematic Structure of the Outflow in the Vicinity of the Protostar in L483

The bipolar outflow associated with the Class 0 low-mass protostellar source (IRAS 18148-0440) in L483 has been studied in the CCH and CS line emission at 245 and 262 GHz, respectively. Sub-arcsecond resolution observations of these lines have been conducted with ALMA. Structures and kinematics of the outflow cavity wall are investigated in the CS line, and are analyzed by using a parabolic model of an outflow. We constrain the inclination angle of the outflow to be from 75 degree to 90 degree, i.e. the outflow is blowing almost perpendicular to the line of sight. Comparing the outflow parameters derived from the model analysis with those of other sources, we confirm that the opening angle of the outflow and the gas velocity on its cavity wall correlate with the dynamical timescale of the outflows. Moreover, a hint of a rotating motion of the outflow cavity wall is found. Although the rotation motion is marginal, the specific angular momentum of the gas on the outflow cavity wall is evaluated to be comparable to or twice that of the infalling-rotating envelope of L483.

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arXiv2018

Chemical survey toward young stellar objects in the Perseus molecular cloud complex

Chemical diversity of the gas in low-mass protostellar cores is widely recognized. In order to explore its origin, a survey of chemical composition toward 36 Class 0/I protostars in the Perseus molecular cloud complex, which are selected in an unbiased way under certain physical conditions, has been conducted with IRAM 30 m and NRO 45 m telescope. Multiple lines of C2H, c-C3H2 and CH3OH have been observed to characterize the chemical composition averaged over a 1000 au scale around the protostar. The derived beam-averaged column densities show significant chemical diversity among the sources, where the column density ratios of C2H/CH3OH are spread out by 2 orders of magnitude. From previous studies, the hot corino sources have abundant CH3OH but deficient C2H, their C2H/CH3OH column density ratios being relatively low. In contrast, the warm-carbon-chain chemistry (WCCC) sources are found to reveal the high C2H/CH3OH column density ratios. We find that the majority of the sources have intermediate characters between these two distinct chemistry types. A possible trend is seen between the C2H/CH3OH ratio and the distance of the source from the edge of a molecular cloud. The sources located near cloud edges or in isolated clouds tend to have a high C2H/CH3OH ratio. On the other hand, the sources having a low C2H/CH3OH ratio tend to be located in inner regions of the molecular cloud complex. This result gives an important clue to an understanding of the origin of the chemical diversity of protostellar cores in terms of environmental effects.

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arXiv2018

ALMA Observations of the IRDC Clump G34.43+00.24 MM3: Complex Organic and Deuterated Molecules

We have observed complex organic molecules (COMs) and deuterated species toward a hot core/corino (HC) associated with the infrared dark cloud (IRDC) clump G34.43+00.24 MM3 with the Atacama Large Millimeter/submillimeter Array (ALMA). We have detected six normal-COMs (CH3OH, CH3CHO, CH3CH2CN, CH3OCH3, HCOOCH3, and NH2CHO), one deuterated-COM (CH2DCN), and two deuterated fundamental molecules (D2CO and DNC) toward G34.43+00.24 MM3 HC. None of these lines, except for CH3OH, are detected toward the shocked regions in our data, which suggests that COMs do not originate in shocks. The abundance of the COMs relative to CH3OH in G34.43+00.24 MM3 HC is found to be similar to those in high-mass hot cores, rather than those in hot corinos in low-mass star-forming regions. This result suggests that the physical conditions of the warm-up phase of G34.43+00.24 MM3 HC are similar to those of high-mass sources. On the other hand, the D2CO abundance relative to CH$_3$OH in G34.43+00.24 MM3 HC is higher than that of other hot cores, and seems to be comparable to that of hot corinos. The relatively high D2CO/CH3OH ratio of G34.43+00.24 MM3 HC implies a long cold starless phase of G34.43+00.24 MM3 HC.

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arXiv2018

Seeds of Life in Space (SOLIS). III. Zooming into the methanol peak of the pre-stellar core L1544

Towards the pre-stellar core L1544, the methanol (CH$_3$OH) emission forms an asymmetric ring around the core centre, where CH$_3$OH is mostly in solid form, with a clear peak 4000~au to the north-east of the dust continuum peak. As part of the NOEMA Large Project SOLIS (Seeds of Life in Space), the CH$_3$OH peak has been spatially resolved to study its kinematics and physical structure and to investigate the cause behind the local enhancement. We find that methanol emission is distributed in a ridge parallel to the main axis of the dense core. The centroid velocity increases by about 0.2~km~s$^{-1}$ and the velocity dispersion increases from subsonic to transonic towards the central zone of the core, where the velocity field also shows complex structure. This could be indication of gentle accretion of material onto the core or interaction of two filaments, producing a slow shock. We measure the rotational temperature and show that methanol is in local thermodynamic equilibrium (LTE) only close to the dust peak, where it is significantly depleted. The CH$_3$OH column density, $N_{tot}({\rm CH_3OH})$, profile has been derived with non-LTE radiative transfer modelling and compared with chemical models of a static core. The measured $N_{tot}({\rm CH_3OH})$ profile is consistent with model predictions, but the total column densities are one order of magnitude lower than those predicted by models, suggesting that the efficiency of reactive desorption or atomic hydrogen tunnelling adopted in the model may be overestimated; or that an evolutionary model is needed to better reproduce methanol abundance.

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arXiv2018

Chemical and Physical Picture of IRAS 16293-2422 Source B at a Sub-arcsecond Scale Studied with ALMA

We have analyzed the OCS, H$_2$CS, CH$_3$OH, and HCOOCH$_3$ data observed toward the low-mass protostar IRAS 16293--2422 Source B at a sub-arcsecond resolution with ALMA. A clear chemical differentiation is seen in their distributions; OCS and H$_2$CS are extended with a slight rotation signature, while CH$_3$OH and HCOOCH$_3$ are concentrated near the protostar. Such a chemical change in the vicinity of the protostar is similar to the companion (Source A) case. The extended component is interpreted by the infalling-rotating envelope model with a nearly face-on configuration. The radius of the centrifugal barrier of the infalling-rotating envelope is roughly evaluated to be ($30-50$) au. The observed lines show the inverse P-Cygni profile, indicating the infall motion with in a few 10 au from the protostar. The nearly pole-on geometry of the outflow lobes is inferred from the SiO distribution, and thus, the infalling and outflowing motions should coexist along the line-of-sight to the protostar. This implies that the infalling gas is localized near the protostar and the current launching points of the outflow have an offset from the protostar. A possible mechanism for this configuration is discussed.

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arXiv2017

Molecular-Cloud-Scale Chemical Composition II: Mapping Spectral Line Survey toward W3(OH) in the 3 mm Band

In order to study a molecular-cloud-scale chemical composition, we have conducted a mapping spectral line survey toward the Galactic molecular cloud W3(OH), which is one of the most active star forming regions in the Perseus arm, with the NRO 45 m telescope. We have observed the area of 16' $\times$ 16', which corresponds to 9.0 pc $\times$ 9.0 pc. The observed frequency ranges are 87--91, 96--103, and 108--112 GHz. We have prepared the spectrum averaged over the observed area, in which 8 molecular species CCH, HCN, HCO$^+$, HNC, CS, SO, C$^{18}$O, and $^{13}$CO are identified. On the other hand, the spectrum of the W3(OH) hot core observed at a 0.17 pc resolution shows the lines of various molecules such as OCS, H$_2$CS CH$_3$CCH, and CH$_3$CN, in addition to the above species. In the spatially averaged spectrum, emission of the species concentrated just around the star-forming core such as CH$_3$OH and HC$_3$N is fainter than in the hot core spectrum, whereas emission of the species widely extended over the cloud such as CCH is relatively brighter. We have classified the observed area into 5 subregions according to the integrated intensity of $^{13}$CO, and have evaluated the contribution to the averaged spectrum from each subregion. The CCH, HCN, HCO$^+$, and CS lines can be seen even in the spectrum of the subregion with the lowest $^{13}$CO integrated intensity range ($< 10$ K km s$^{-1}$). Thus, the contributions of the spatially extended emission is confirmed to be dominant in the spatially averaged spectrum.

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arXiv2017

Discovery of Striking Difference of Molecular-Emission-Line Richness in the Potential Proto-Binary System NGC 2264 CMM3

We have conducted an interferometric line survey in the 0.8 mm band toward the young high-mass protostar candidate NGC 2264 CMM3 with ALMA. CMM3 is resolved into the two continuum peaks, CMM3A and CMM3B, at an angular separation of 0.9". Thus, CMM3 is found to be a binary system candidate. We have detected molecular outflows associated with CMM3A and CMM3B each, indicating active star formation. In addition to the two peaks, six faint continuum peaks are detected around CMM3A and CMM3B, most of which are thought to be evolved low-mass protostars. CMM3A is found to be rich in molecular line emission including complex organic molecules such as HCOOCH3 and CH3OCH3. The emission of complex organic molecules is distributed within a compact region around the continuum peak of CMM3A. Hence, CMM3A apparently harbors a hot core. On the other hand, CMM3B is deficient in molecular line emission, although its continuum flux is almost comparable to that of CMM3A. Possible origins of the striking difference between CMM3A and CMM3B are discussed.

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arXiv2017

Molecular-Cloud-Scale Chemical Composition I: Mapping Spectral Line Survey toward W51 in the 3 mm Band

We have conducted a mapping spectral line survey toward the Galactic giant molecular cloud W51 in the 3 mm band with the Mopra 22 m telescope in order to study an averaged chemical composition of the gas extended over a molecular cloud scale in our Galaxy. We have observed the area of $25' \times 30'$, which corresponds to 39 pc $\times$ 47 pc. The frequency ranges of the observation are 85.1 - 101.1 GHz and 107.0 - 114.9 GHz. In the spectrum spatially averaged over the observed area, spectral lines of 12 molecular species and 4 additional isotopologues are identified. An intensity pattern of the spatially-averaged spectrum is found to be similar to that of the spiral arm in the external galaxy M51, indicating that these two sources have similar chemical compositions. The observed area has been classified into 5 sub-regions according to the integrated intensity of $^{13}$CO($J=1-0$) ($I_{\rm ^{13}CO}$), and contributions of the fluxes of 11 molecular lines from each sub-region to the averaged spectrum have been evaluated. For most of molecular species, 50 % or more of the flux come from the sub-regions with $I_{\rm ^{13}CO}$ from 25 K km s$^{-1}$ to 100 K km s$^{-1}$, which does not involve active star forming regions. Therefore, the molecular-cloud-scale spectrum observed in the 3 mm band hardly represents the chemical composition of star forming cores, but mainly represents the chemical composition of an extended quiescent molecular gas. The present result constitutes a sound base for interpreting the spectra of external galaxies at a resolution of a molecular cloud scale ($\sim10$ pc) or larger.

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arXiv2017

Vertical Structure of the Transition Zone from Infalling Rotating Envelope to Disk in the Class 0 Protostar, IRAS04368+2557

We have resolved for the first time the radial and vertical structure of the almost edge-on envelope/disk system of the low-mass Class 0 protostar L1527. For that, we have used ALMA observations with a spatial resolution of 0.25$^{\prime\prime}$$\times$0.13$^{\prime\prime}$ and 0.37$^{\prime\prime}$$\times$0.23$^{\prime\prime}$ at 0.8 mm and 1.2 mm, respectively. The L1527 dust continuum emission has a deconvolved size of 78 au $\times$ 21 au, and shows a flared disk-like structure. A thin infalling-rotating envelope is seen in the CCH emission outward of about 150 au, and its thickness is increased by a factor of 2 inward of it. This radius lies between the centrifugal radius (200 au) and the centrifugal barrier of the infalling-rotating envelope (100 au). The gas stagnates in front of the centrifugal barrier and moves toward vertical directions. SO emission is concentrated around and inside the centrifugal barrier. The rotation speed of the SO emitting gas is found to be decelerated around the centrifugal barrier. A part of the angular momentum could be extracted by the gas which moves away from the mid-plane around the centrifugal barrier. If this is the case, the centrifugal barrier would be related to the launching mechanism of low velocity outflows, such as disk winds.

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arXiv2017

Detection of submillimeter-wave [C I] emission in gaseous debris disks of 49 Ceti and Beta Pictoris

We have detected [C I] 3P1-3P0 emissions in the gaseous debris disks of 49 Ceti and Beta Pictoris with the 10 m telescope of the Atacama Submillimeter Telescope Experiment, which is the first detection of such emissions. The line profiles of [C I] are found to resemble those of CO(J=3-2) observed with the same telescope and the Atacama Large Millimeter/submillimeter Array. This result suggests that atomic carbon (C) coexists with CO in the debris disks, and is likely formed by the photodissociation of CO. Assuming an optically thin [C I] emission with the excitation temperature ranging from 30 to 100 K, the column density of C is evaluated to be (2.2+-0.2)x10^17 and (2.5+-0.7)x10^16 cm^-2 for 49 Ceti and Beta Pictoris, respectively. The C/CO column density ratio is thus derived to be 54+-19 and 69+-42 for 49 Ceti and Beta Pictoris, respectively. These ratios are higher than those of molecular clouds and diffuse clouds by an order of magnitude. The unusually high ratios of C to CO are likely attributed to a lack of H2 molecules needed to reproduce CO molecules efficiently from C. This result implies a small number of H2 molecules in the gas disk; i.e., there is an appreciable contribution of secondary gas from dust grains.

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arXiv2017

L483: Warm Carbon-Chain Chemistry Source Harboring Hot Corino Activity

The Class 0 protostar, L483, has been observed in various molecular lines in the 1.2 mm band at a sub-arcsecond resolution with ALMA. An infalling-rotating envelope is traced by the CS line, while a very compact component with a broad velocity width is observed for the CS, SO, HNCO, NH$_2$CHO, and HCOOCH$_3$ lines. Although this source is regarded as the warm carbon-chain chemistry (WCCC) candidate source at a 1000 au scale, complex organic molecules characteristic of hot corinos such as NH$_2$CHO and HCOOCH$_3$ are detected in the vicinity of the protostar. Thus, both hot corino chemistry and WCCC are seen in L483. Although such a mixed chemical character source has been recognized as an intermediate source in previous single-dish observations, we here report the first spatially-resolved detection. A kinematic structure of the infalling-rotating envelope is roughly explained by a simple ballistic model with the protostellar mass of 0.1--0.2 $M_\odot$ and the radius of the centrifugal barrier (a half of the centrifugal radius) of 30--200 au, assuming the inclination angle of 80\degr\ (0\degr\ for a face-on). The broad line emission observed in the above molecules most likely comes from the disk component inside the centrifugal barrier. Thus, a drastic chemical change is seen around the centrifugal barrier.

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