arXiv subjects

Masao Saito

Publications and source records attributed to Masao Saito.

At least 73 records · Page 4Linked to original sources

arXiv2017

Detection of New Methylamine (CH$_3$NH$_2$) Sources: Candidates for Future Glycine Surveys

It has been a long-standing problem to detect interstellar glycine (NH$_2$CH$_2$COOH), the simplest amino acid, in studying a possible relation between the Universe and origin of life. In the last about 40 years all surveys of glycine failed, and it would be an alternative strategy to search for precursor(s) to glycine. Such studies of precursors would be crucial prior to conducting sensitive surveys by ALMA. Laboratory studies have suggested that CH$_3$NH$_2$ is a possible precursor to glycine. Further theoretical study also suggested that the CH$_2$NH$_2$ radical that can be formed from CH$_3$NH$_2$ through photodissociation can be a good precursor to glycine. Thus we observed CH$_3$NH$_2$ towards several hot core sources by using the Nobeyama 45m radio telescope, and succeeded in finding a new CH$_3$NH$_2$ source, G10.47+0.03, with its fractional abundance of 2.4+-0.6x10^{-8}; at the time of writing, this source is the most abundant source of CH$_3$NH$_2$ ever known. We also found another new source of CH$_3$NH$_2$, NGC6334F, however, the detection may be tentative since the signal-to-noise ratios are not sufficient to claim secure detection. Detailed analysis of the detected data revealed the detection of the hyperfine components of CH$_3$NH$_2$ for the first time. We found that the observed abundance of CH$_3$NH$_2$ agrees fairly well with the theoretically predicted value by Garrod (2013). Detectability of interstellar glycine is discussed.

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arXiv2017

ALMA Observations of the Protostar L1527 IRS: Probing Details of the Disk and the Envelope Structures

We have newly observed the Class 0/I protostar L1527 IRS using the Atacama Large Millimeter/submillimeter Array (ALMA) during its Cycle 1 in 220 GHz dust continuum and C18O (J=2-1) line emissions with a ~2 times higher angular resolution (~0.5") and ~4 times better sensitivity than our ALMA Cycle 0 observations. Continuum emission shows elongation perpendicular to the associated outflow, with a deconvolved size of 0.53"x0.15". C18O emission shows similar elongation, indicating that both emissions trace the disk and the flattened envelope surrounding the protostar. The velocity gradient of the C18O emission along the elongation due to rotation of the disk/envelope system is re-analyzed, identifying Keplerian rotation proportional to r^-0.5 more clearly than the Cycle 0 observations. The Keplerian-disk radius and the dynamical stellar mass are kinematically estimated to be ~74 AU and ~0.45 Mo, respectively. The continuum visibility is fitted by models without any annulus averaging, revealing that the disk is in hydrostatic equilibrium. The best-fit model also suggests a density jump by a factor of ~5 between the disk and the envelope, suggesting that disks around protostars can be geometrically distinguishable from the envelope from a viewpoint of density contrast. Importantly, the disk radius geometrically identified with the density jump is consistent with the radius kinematically estimated. Possible origin of the density jump due to the mass accretion from the envelope to the disk is discussed. C18O observations can be reproduced by the same geometrical structures derived from the dust observations, with possible C18O freeze-out and localized C18O desorption.

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arXiv2017

$^{13}$C isotopic fractionation of HC$_{3}$N in two starless cores: L1521B and L134N (L183)

We observed the $J=5-4$ rotational lines of the normal species and three $^{13}$C isotopologues of HC$_{3}$N at the 45 GHz band toward two low-mass starless cores, L1521B and L134N (L183), using the Nobeyama 45 m radio telescope in order to study the main formation pathways of HC$_{3}$N in each core. The abundance ratios of the three $^{13}$C isotopologues in L1521B are derived to be [H$^{13}$CCCN]:[HC$^{13}$CCN]:[HCC$^{13}$CN] = 0.98 (+- 0.14) : 1.00 : 1.52 (+- 0.16) (1 sigma). The fractionation pattern is consistent with that at the cyanopolyyne peak in Taurus Molecular Cloud-1. This fractionation pattern suggests that the main formation pathway of HC$_{3}$N is the neutral-neutral reaction between C$_{2}$H$_{2}$ and CN. On the other hand, their abundance ratios in L134N are found to be [H$^{13}$CCCN]:[HC$^{13}$CCN]:[HCC$^{13}$CN]= 1.5 (+- 0.2) : 1.0 : 2.1 (+- 0.4) (1 sigma), which are different from those in L1521B. From this fractionation pattern, we propose that the reaction between HNC and CCH is a possible main formation pathway of HC$_{3}$N in L134N. We find out that the main formation pathways of the same molecule are not common even in the similar physical conditions. We discuss the possible factors to make a difference in fractionation pattern between L134N and L1521B/TMC-1.

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arXiv2017

First Detection of HC$_{5}$$^{15}$N in the Interstellar Medium

We report the first detection of HC$_{5}$$^{15}$N with the $J=9-8$ rotational line from the cyanopolyyne peak in Taurus Molecular Cloud-1 (TMC-1 CP) using the 45-m radio telescope of the Nobeyama Radio Observatory. The column density of HC$_{5}$$^{15}$N is derived to be (1.9 +- 0.5)*$10^{11}$ cm$^{-2}$ (1 sigma). We apply the double isotope method to derive the $^{14}$N/$^{15}$N ratios of HC$_{5}$N and HC$_{3}$N in TMC-1 CP. The $^{14}$N/$^{15}$N ratios are calculated to be 344 +- 53 and 257 +- 54 for HC$_{5}$N and HC$_{3}$N, respectively. The $^{14}$N/$^{15}$N ratio of HC$_{5}$N is lower than the elemental ratio in the local interstellar medium (~440) and slightly higher than that of HC$_{3}$N in TMC-1 CP. Since HC$_{3}$N is formed via the neutral-neutral reaction between C$_{2}$H$_{2}$ and CN, the slightly higher $^{14}$N/$^{15}$N ratio of HC$_{5}$N may support our previous suggestions that the main formation mechanism of HC$_{5}$N is the ion-molecule reactions between hydrocarbon ions (C$_{5}$H$_{n}^{+}$) and nitrogen atoms.

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arXiv2017

Observations of Cyanopolyynes toward Four High-Mass Star-Forming Regions Containing Hot Cores

We carried out line survey observations at the 26-30 GHz band toward the four high-mass star-forming regions containing hot cores, G10.30-0.15, G12.89+0.49, G16.86-2.16, and G28.28-0.36, with the Robert C. Byrd Green Bank Telescope. We have detected HC5N from all of the sources, and HC7N from the three sources, except for G10.30-0.15. We further conducted observations of HC5N at the 42-46 GHz and 82-103 GHz bands toward the three sources, G12.89+0.49, G16.86-2.16, and G28.28-0.36, with the Nobeyama 45 m radio telescope. The rotational lines of HC5N with the high excitation energies (Eu/k=63-100 K), which are hardly excited in the cold dark clouds, have been detected from the three sources. The rotational temperatures of HC5N are found to be 13-20 K in the three sources. The detection of the lines with the high excitation energies and the derived rotational temperatures indicate that HC5N exists in the warm gas within 0.07-0.1 pc radii around massive young stellar objects. The column densities of HC5N in the three sources are derived to be (2.0+-2.8)*10^(13) cm-2 . We compare the ratios between N(HC5N) the column density of HC5N and W(CH3OH) the integrated intensity of the thermal CH3OH emission line among the three high-mass star-forming regions. We found a possibility of the chemical differentiation in the three high-mass star-forming regions; G28.28-0.36 shows the largest N (HC5N)/W (CH3OH) ratio of > 8.0*10^(14) in units of (K km s-1 )-1 cm-2 , while G12.89+0.49 and G16.86-2.16 show the smaller values (~2*10^(13) ).

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arXiv2017

Discovery of a distant molecular cloud in the extreme outer Galaxy with the Nobeyama 45-m telescope

We report the discovery of the molecular cloud whose kinematic distance is the largest in the Galaxy at the present moment, named G213.042$+$0.003, at $l =$ 213.042$^\circ$ and $b =$ 0.003$^\circ$ in the $^{12}$CO($J =$ 1-0) line using the Nobeyama 45-m telescope and a multi-beam receiver BEARS. This molecular cloud is located at the heliocentric distance of 21$_{-7}^{+12}$ kpc and Galactocentric distance of 29$_{-7}^{+12}$ kpc, which are estimated as the kinematic distances with the Galactic parameters obtained by Reid et al. (2014, ApJ, 783, 130). Its major and minor diameters and line width were measured to be 4.0$_{-1.3}^{+2.3}$ pc, 3.0$_{-1.0}^{+1.7}$ pc, and 1.5 km s$^{-1}$, respectively. The cloud mass was estimated to be 2.5$_{-1.4}^{+3.7}$ $\times$ 10$^2$ $M_\odot$ using the CO-to-H$_2$ conversion factor of 5.6 $\times$ 10$^{20}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$ obtained in far outer Galaxy.

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arXiv2017

Spiral Arms, Infall, and Misalignment of the Circumbinary Disk from the Circumstellar Disks in the Protostellar Binary System L1551 NE

We report the ALMA Cycle 2 observations of the Class I binary protostellar system L1551 NE in the 0.9-mm continuum, C18O (3-2), 13CO (3-2), SO (7_8-6_7), and the CS (7-6) emission. At 0.18" (= 25 AU) resolution, ~4-times higher than that of our Cycle 0 observations, the circumbinary disk as seen in the 0.9-mm emission is shown to be comprised of a northern and a southern spiral arm, with the southern arm connecting to the circumstellar disk around Source B. The western parts of the spiral arms are brighter than the eastern parts, suggesting the presence of an m=1 spiral mode. In the C18O emission, the infall gas motions in the inter-arm regions and the outward gas motions in the arms are identified. These observed features are well reproduced with our numerical simulations, where gravitational torques from the binary system impart angular momenta to the spiral-arm regions and extract angular momenta from the inter-arm regions. Chemical differentiation of the circumbinary disk is seen in the four molecular species. Our Cycle 2 observations have also resolved the circumstellar disks around the individual protostars, and the beam-deconvolved sizes are 0.29" X 0.19" (= 40 X 26 AU) (P.A. = 144 deg) and 0.26" X 0.20" (= 36 X 27 AU) (P.A. = 147 deg) for Sources A and B, respectively. The position and inclination angles of these circumstellar disks are misaligned with that of the circumbinary disk. The C18O emission traces the Keplerian rotation of the misaligned disk around Source A.

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arXiv2017

Chemistry of Carbon-Chain Molecules in Star Forming Regions - Formation Pathway of HC3N in G28.28-0.36 Hot Core -

We aim to reveal the chemistry of carbon-chain molecules in high-mass star-forming regions from molecular level to chemical evolution, applying the methods established in low-mass star-forming regions. In this proceeding, we summarize the topic about the main formation pathway of HC3N. We carried out observations of three 13C isotopologues of HC3N toward a hot core G28.28-0.36 with the 45-m radio telescope of the Nobeyama Radio Observatory. From the observational results, we propose that the main formation pathway of HC3N is the neutral-neutral reaction between C2H2 and CN. We also compare the results among the different star-forming regions, from a low-mass starless core to a high-mass star-forming core.

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arXiv2016

The Brightness Temperature of the Quiet Solar Chromosphere at 2.6 mm

The absolute brightness temperature of the Sun at millimeter wavelengths is an important diagnostic of the solar chromosphere. Because the Sun is so bright, measurement of this property usually involves the operation of telescopes under extreme conditions and requires a rigorous performance assessment of the telescope. In this study, we establish solar observation and calibration techniques at 2.6-mm wavelength for the Nobeyama 45-m telescope and derive the absolute solar brightness temperature accurately. We tune the superconductor-insulator-superconductor (SIS) receiver by inducing different bias voltages onto the SIS mixer to prevent saturation. Then, we examine the linearity of the receiver system by comparing outputs derived from different tuning conditions. Further, we measure the lunar filled beam efficiency of the telescope using the New Moon, and then derive the absolute brightness temperature of the Sun. The derived solar brightness temperature is 7700+-310 K at 115 GHz. The telescope beam pattern is modeled as a summation of three Gaussian functions and derived using the solar limb. The real shape of the Sun is determined via deconvolution of the beam pattern from the observed map. Such well-calibrated single-dish observations are important for high-resolution chromospheric studies because they provide the absolute temperature scale missing from interferometer observations.

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arXiv2016

Astrochemical Properties of Planck Cold Clumps

We observed thirteen Planck cold clumps with the James Clerk Maxwell Telescope/SCUBA-2 and with the Nobeyama 45 m radio telescope. The N$_2$H$^+$ distribution obtained with the Nobeyama telescope is quite similar to SCUBA-2 dust distribution. The 82 GHz HC$_3$N, 82 GHz CCS, and 94 GHz CCS emission are often distributed differently with respect to the N$_2$H$^+$ emission. The CCS emission, which is known to be abundant in starless molecular cloud cores, is often very clumpy in the observed targets. We made deep single-pointing observations in DNC, HN$^{13}$C, N$_2$D$^+$, cyclic-C$_3$H$_2$ toward nine clumps. The detection rate of N$_2$D$^+$ is 50\%. Furthermore, we observed the NH$_3$ emission toward 15 Planck cold clumps to estimate the kinetic temperature, and confirmed that most of targets are cold ($\lesssim$ 20 K). In two of the starless clumps observe, the CCS emission is distributed as it surrounds the N$_2$H$^+$ core (chemically evolved gas), which resembles the case of L1544, a prestellar core showing collapse. In addition, we detected both DNC and N$_2$D$^+$. These two clumps are most likely on the verge of star formation. We introduce the Chemical Evolution Factor (CEF) for starless cores to describe the chemical evolutionary stage, and analyze the observed Planck cold clumps.

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arXiv2016

13C Isotopic Fractionation of HC3N in Star-Forming Regions -Low-Mass Star Forming Region L1527 and High-Mass Star Forming Region G28.28-0.36-

We observed the J=9-8 and 10-9 rotational lines of three13C isotopologues of HC3N in L1527 and G28.28-0.36 with the 45-m radio telescope of the Nobeyama Radio Observatory in order to constrain the main formation mechanisms of HC3N in each source. The abundance ratios of the three 13C isotopologues of HC3N are found to be 0.9 (0.2) : 1.00 : 1.29 (0.19) (1sigma) and 1.0 (0.2) : 1.00 : 1.47 (0.17) (1sigma) for [H13CCCN]: [HC13CCN]: [HCC13CN] in L1527 and G28.28-0.36, respectively. We recognize a similar 13C isotopic fractionation pattern that the abundances of H13CCCN and HC13CCN are comparable, and HCC13CN is more abundant than the others. Based on the results, we discuss the main formation pathway of HC3N. The 13C isotopic fractionation pattern derived from our observations can be explained by the neutral-neutral reaction between C2H2 and CN in both the low-mass (L1527) and high-mass (28.28-0.36) star forming regions.

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arXiv2016

Survey Observations of a Possible Glycine Precursor, Methanimine (CH$_2$NH)

We conducted survey observations of a glycine precursor, methanimine or methylenimine (CH$_2$NH), with the NRO 45 m telescope and the SMT telescope towards 12 high-mass and two low-mass star-forming regions in order to increase number of CH$_2$NH sources and to better understand the characteristics of CH2NH sources. As a result of our survey, CH$_2$NH was detected in eight sources, including four new sources. The estimated fractional abundances were ~10$^8$ in Orion KL and G10.47+0.03, while they were ~10$^9$ towards the other sources. Our hydrogen recombination line and past studies suggest that CH$_2$NH-rich sources have less evolved HII regions. The less destruction rates by UV flux from the central star would be contributed to the high CH$_2$NH abundances towards CH$_2$NH-rich sources. Our gas-grain chemical simulations suggest that CH$_2$NH is mostly formed in the gas-phase by neutral-neutral reactions rather than the product of thermal evaporation from the dust surfaces.

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arXiv2016

Low-metallicity Young Clusters in the Outer Galaxy. II. Sh 2-208

We obtained deep near-infrared images of Sh 2-208, one of the lowest-metallicity HII regions in the Galaxy, [O/H] = -0.8 dex. We detected a young cluster in the center of the HII region with a limiting magnitude of K = 18.0 mag (10sigma), which corresponds to a mass detection limit of ~0.2 M_sun. This enables the comparison of star-forming properties under low metallicity with those of the solar neighborhood. We identified 89 cluster members. From the fitting of the K-band luminosity function (KLF), the age and distance of the cluster are estimated to be ~0.5 Myr and ~4 kpc, respectively. The estimated young age is consistent with the detection of strong CO emission in the cluster region and the estimated large extinction of cluster members (Av ~ 4--25 mag). The observed KLF suggests that the underlying initial mass function (IMF) of the low-metallicity cluster is not significantly different from canonical IMFs in the solar neighborhood in terms of both high-mass slope and IMF peak (characteristic mass). Despite the very young age, the disk fraction of the cluster is estimated at only 27pm6 %, which is significantly lower than those in the solar metallicity. Those results are similar to Sh 2-207, which is another low-metallicity star-forming region close to Sh 2-208 with a separation of 12 pc,suggesting that their star-forming activities in low-metallicity environments are essentially identical to those in the solar neighborhood, except for the disk dispersal timescale. From large-scale mid-infrared images, we suggest that sequential star formation is taking place in Sh 2-207, Sh 2-208 and the surrounding region, triggered by an expanding bubble with a ~30 pc radius.

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arXiv2016

Herbig Ae/Be candidate stars in the innermost Galactic disk: Quartet cluster

In order to investigate the Galactic-scale environmental effects on the evolution of protoplanetary disks, we explored the near-infrared (NIR) disk fraction of the Quartet cluster, which is a young cluster in the innermost Galactic disk at the Galactocentric radius Rg ~ 4 kpc. Because this cluster has a typical cluster mass of ~10^3 M_sun as opposed to very massive clusters, which have been observed in previous studies (>10^4 M_sun), we can avoid intra-cluster effects such as strong UV field from OB stars. Although the age of the Quartet is previously estimated to be 3-8 Myr old, we find that it is most likely ~3-4.5 Myr old. In moderately deep JHK images from the UKIDSS survey, we found eight HAeBe candidates in the cluster, and performed K-band medium-resolution ($R \equiv Δλ/ λ~ 800$) spectroscopy for three of them with the Subaru 8.2 m telescope. These are found to have both Brγabsorption lines as well as CO bandhead emission, suggesting that they are HAeBe stars with protoplanetary disks. We estimated the intermediate-mass disk fraction (IMDF) to be ~25 % for the cluster, suggesting slightly higher IMDF compared to those for young clusters in the solar neighborhood with similar cluster age, although such conclusion should await future spectroscopic study of all candidates of cluster members.

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arXiv2015

Low-metallicity Young Clusters in the Outer Galaxy I. Sh 2-207

To study star formation in low metallicity environments ([M/H] ~ -1 ,dex), we obtained deep near-infrared (NIR) images of Sh 2-207 (S207), which is an HII region in the outer Galaxy with spectroscopically determined metallicity of [O/H] ~= -0.8 dex. We identified a young cluster in the western region of S207 with a limiting magnitude of Ks =19.0 mag (10 sigma) that corresponds to a mass detection limit of <~0.1 M_sun and enables the comparison of star-forming properties under low metallicity with those of the solar neighborhood. From the fitting of the K-band luminosity function (KLF), the age and distance of S207 cluster are estimated at 2-3Myr and ~4 kpc, respectively. The estimated age is consistent with the suggestion of small extinctions of stars in the cluster (Av ~ 3 mag) and the non-detection of molecular clouds. The reasonably good fit between observed KLF and model KLF suggests that the underlying initial mass function (IMF) of the cluster down to the detection limit is not significantly different from the typical IMFs in the solar metallicity. From the fraction of stars with NIR excesses, a low disk fraction (<10 %) in the cluster with relatively young age is suggested, as we had previously proposed.

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arXiv2015

Implication of Formation Mechanisms of HC5N in TMC-1 as Studied by 13C Isotopic Fractionation

We observed the J = 9-8 and 16-15 rotational transitions of the normal species and five 13C isotopologues of HC5N to study its formation mechanisms toward the cyanopolyyne peak in Taurus Molecular Cloud-1, with the 45-m radio telescope of Nobeyama Radio Observatory. We detected the five 13C isotopologues with high signal-to-noise ratios between 12 and 20, as well as the normal species. The abundance ratios of the five 13C isotopologues of HC5N are found to be 1.00:0.97:1.03:1.05:1.16 (0.19) for [H13CCCCCN]:[HC13CCCCN]:[HCC13CCCN]:[HCCC13CCN]:[HCCCC13CN]. We do not find any significant differences among the five 13C isotopologues. The averaged [HC5N]/[13C isotopologues] abundance ratio is determined to be 94 (6), which is slightly higher than the local interstellar elemental 12C/13C ratio of 60-70. Possible formation pathways are discussed on the basis of these results.

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arXiv2015

Spectral-Line Survey at Millimeter and Submillimeter Wavelengths toward an Outflow-Shocked Region, OMC 2-FIR 4

We performed the first spectral-line survey at 82--106 GHz and 335--355 GHz toward the outflow-shocked region, OMC 2-FIR 4, the outflow driving source, FIR 3, and the northern outflow lobe, FIR 3N. We detected 120 lines of 20 molecular species. The line profiles are found to be classifiable into two types: one is a single Gaussian component with a narrow ($<$ 3 km s$^{-1}$) width and another is two Gaussian components with narrow and wide ($>$ 3km s$^{-1}$) widths. The narrow components for the most of the lines are detected at all positions, suggesting that they trace the ambient dense gas. For CO, CS, HCN, and HCO$^{+}$, the wide components are detected at all positions, suggesting the outflow origin. The wide components of C$^{34}$S, SO, SiO, H$^{13}$CN, HC$^{15}$N, H$_2^{13}$CO, H$_2$CS, HC$_3$N, and CH$_3$OH are detected only at FIR 4, suggesting the outflow-shocked gas origin. The rotation diagram analysis revealed that the narrow components of C$_2$H and H$^{13}$CO$^+$ show low temperatures of 12.5$\pm$1.4 K, while the wide components show high temperatures of 20--70 K. This supports our interpretation that the wide components trace the outflow and/or outflow-shocked gas. We compared observed molecular abundances relative to H$^{13}$CO$^+$ with those of the outflow-shocked region, L1157 B1, and the hot corino, IRAS 16293-2422. Although we cannot exclude a possibility that the chemical enrichment in FIR 4 is caused by the hot core chemistry, the chemical compositions in FIR 4 are more similar to those in L1157 B1 than those in IRAS 16293-2422.

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arXiv2015

Dispersing Envelope around the Keplerian Circumbinary Disk in L1551 NE and its Implications for the Binary Growth

We performed mapping observations of the Class I protostellar binary system L1551 NE in the C$^{18}$O ($J$=3-2), $^{13}$CO ($J$=3-2), CS ($J$=7-6), and SO ($J_N$=7$_8$-6$_7$) lines with Atacama Submillimeter Telescope Experiment (ASTE). The ASTE C$^{18}$O data are combined with our previous SMA C$^{18}$O data, which show a $r \sim$300-AU scale Keplerian disk around the protostellar binary system. The C$^{18}$O maps show a $\sim$20000-AU scale protostellar envelope surrounding the central Keplerian circumbinary disk. The envelope exhibits a northeast (blue) - southwest (red) velocity gradient along the minor axis, which can be interpreted as a dispersing gas motion with an outward velocity of 0.3 km s$^{-1}$, while no rotational motion in the envelope is seen. In addition to the envelope, two $\lesssim$4000 AU scale, high-velocity ($\gtrsim$1.3 km s$^{-1}$) redshifted $^{13}$CO and CS emission components are found to $\sim$40$^{\prime\prime}$ southwest and $\sim$20$^{\prime\prime}$ west of the protostellar binary. These redshifted components are most likely outflow components driven from the neighboring protostellar source L1551 IRS 5, and are colliding with the envelope in L1551 NE. The net momentum, kinetic and internal energies of the L1551 IRS 5 outflow components are comparable to those of the L1551 NE envelope, and the interactions between the outflows and the envelope are likely to cause the dissipation of the envelope and thus suppression of the further growth of the mass and mass ratio of the central protostellar binary in L1551 NE.

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