S. Yamamoto

The University of Tokyo, Tokyo, Tokyo-to, Japan

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Publications (73)118.06 Total impact

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    ABSTRACT: Superconducting hot electron bolometer (HEB) mixers have matured as the most sensitive heterodyne detectors for astronomy and atmospheric science in the THz region. The HEB mixer fabrication is based on an ultra thin superconducting film such as Nb, NbN, and NbTiN. In general, the critical temperature (T c) of such thin films is much lower than those of the corresponding bulk materials. In order to improve T c of the thin films, we have employed an AlN film as a buffer layer between a substrate and NbN/NbTiN films. The AlN film is deposited on the quartz or glass substrate in a mixture of Ar and N 2 gas by DC reactive magnetron sputtering. We optimize the Ar and N 2 flow rates to ensure that the X-ray diffraction from the 002 surface of Wurtzite type AlN is dominant. We measure the T c values of the NbN and NbTiN films with the AlN buffer layer (20 nm) deposited on quartz and glass substrates respectively. For the 8 nm NbTiN film on the grass substrate, T c is increased from 8.4 K to 11.0 K by insertion of the AlN buffer layer. For the 10 nm and 6 nm NbN films on the quartz substrate, the T c is increased from 6.9 K to 11.4 K and 6.1 K to 9.8 K respectively. The improvement in Tc would originate from better epitaxial growth of the NbN and NbTiN films on the AlN buffer layer than on the bare substrate. The HEB mixer fabrication using the NbN and NbTiN films with the AlN buffer layer is ongoing.
    12/2023;
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    ABSTRACT: Chemical models predict that the deuterated fraction (the column density ratio between a molecule containing D and its counterpart containing H) of N2H+, Dfrac(N2H+), is high in massive pre-protostellar cores and rapidly drops of an order of magnitude after the protostar birth, while that of HNC, Dfrac(HNC), remains constant for much longer. We tested these predictions by deriving Dfrac(HNC) in 22 high-mass star forming cores divided in three different evolutionary stages, from high-mass starless core candidates (HMSCs, 8) to high-mass protostellar objects (HMPOs, 7) to Ultracompact HII regions (UCHIIs, 7). For all of them, Dfrac (N2H+) was already determined through IRAM-30m Telescope observations, which confirmed the theoretical rapid decrease of Dfrac(N2H+) after protostar birth (Fontani et al. 2011). Therefore our comparative study is not affected by biases introduced by the source selection. We have found average Dfrac(HNC) of 0.012, 0.009 and 0.008 in HMSCs, HMPOs and UCHIIs, respectively, with no statistically significant differences among the three evolutionary groups. These findings confirm the predictions of the chemical models, and indicate that large values of Dfrac(N2H+) are more suitable than large values of Dfrac(HNC) to identify cores on the verge of forming high-mass stars, likewise what found in the low-mass regime.
    02/2014;
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    ABSTRACT: We have carried out survey of carbon-chain molecules and NH3 toward more than 100 nearby molecular cloud cores with the Nobeyama 45m telescope. As a result, variations in the NH3/CCS ratios, ranging from 1-500, are found both among dark cloud cores and molecular cloud complexes. It is most likely due to the difference in the contraction timescale and/or the evolutionary phase of the cores. We have identified dark cloud cores with remarkably low NH3/CCS ratios in the Taurus, Aquila, and Cepheus regions. They are possible candidates of chemically and dynamically less evolved cores.
    10/2013;
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    ABSTRACT: Stars form out of dense cores in turbulent, magnetized molecular clouds. However, the role of magnetic fields in star formation remains poorly understood both theoretically and observationally. This is in part due to the lack of the observational characterization of the magnetic fields that are associated with the dense cores prior to star formation, or pre-protostellar cores. To shed light on the issue of the magnetic field in the gravitational collapse of pre-protostellar cores, we have started a project of the magnetic field measurements toward pre-protostellar cores using the Zeeman splitting of CCS (JN = 43 - 32) line. We are now developing a new 45GHz band, dual polarization, reciever for the Nobeyama 45m telescope. In this contribution, we summarize a current status of our Zeeman project and future plan.
    10/2013;
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    ABSTRACT: We have conducted systematic surveys of several molecular lines toward massive clumps, which include infrared dark clouds (IRDCs) and high-mass protostellar objects (HMPOs), in order to investigate their evolutionary stages. From these surveys, we have found that most of the massive clumps are chemically more evolved than young low-mass starless cores, and that the shocks due to interaction between outflow and ambient dense gas are more significant in the earlier evolutionary stage of high-mass star formation. Furthermore, we suggest that the initial condition may be different between high- and low-mass star formation.
    10/2013;
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    ABSTRACT: We have recently observed four hyperfine components of the ground state Λ-doublet-type transition of OH toward several positions in the Taurus Molecular Cloud with the Effelsberg 100 m radio telescope. We have found that the 1612 MHz line appears in absorption toward the eastern position of HCL2, where the [CI](3P1-3P0) emission shows a local peak. Furthermore, the spectrum observed toward the other positions in HCL2 shows two velocity components. The higher velocity component is seen in absorption, whereas the lower velocity component is seen in emission. In order to understand the above results, we have made statistical equilibrium calculations of the OH molecule. It is found that the 1612 MHz line appears in absorption only when the gas kinetic temperature is higher than 40 K. Thus, intensities of the four hyperfine components of OH can be a good thermometer. The combination of emission and absorption observed in HCL2 would reflect a cloud structure like cold dense cores surrounded by a warm less-dense envelope.
    10/2013;
  • R. Kawabe, N. Kuno, S. Yamamoto
    10/2013;
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    ABSTRACT: We carried out a new line survey project between December 2007 and May 2012. The target sources are interesting low and high-mass star forming regions (L1527, L1157 B1, and G28.34+0.06), and nearby galaxies (NGC 1068, NGC 253, and IC 342). As a result, many molecular lines were detected: For example, various carbon-chain molecules in L1527, organic molecules and PN in L1157 B1, organic molecules and deuterated molecules in G28.34+0.06, and fundamental molecules including relatively strong HCN and CN in NGC 1068. For their details, see the related papers in this proceedings.
    10/2013;
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    ABSTRACT: We have conducted observations of the ground-state transition lines (J = 1-0) of the fundamental deuterated species DCO+, DNC, DCN, CCD and N2D+ as well as those of H13CO+, HN13C, H13CN, CCH and N2H+ with the Nobeyama 45 m telescope. The target sources are the cold starless cores, TMC-1 and Lupus-1A, and the low-mass star forming cores, L1527 and IRAS15398-3359. The excitation temperatures derived from intensities of resolved hyperfine components are systematically different between DNC and HN13C. On the other hand, the excitation temperatures of DCN and H13CN are comparable to each other. Although the origin of these results is puzzling, the present result indicates that accurate evaluation of the excitation temperature is essential for deriving deuterium fractionation ratios accurately.
    10/2013;
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    ABSTRACT: We have developed low-noise waveguide-type superconducting hot electron bolometer (HEB) mixers for astronomical observations in the 0.8-1.0 and 1.3-1.5 THz bands, by using a relatively thick NbTiN superconducting film (10.8 nm). The receiver noise temperature of 350 K (DSB) at 0.81 THz and 490 K at 1.475 THz has been achieved. We have built the 0.8-1.0/1.3-1.5 THz dual band heterodyne receiver using these low noise HEB mixers, and have installed it on the ASTE (Atacama Submillimeter Telescope Experiment) 10 m telescope in Chile in 2011. The 13CO emission (J = 8-7 : 0.8813 THz) has successfully been detected toward the Orion A molecular cloud with our HEB mixer receiver.
    10/2013;
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    ABSTRACT: A mapping observation of the J=1/2 Λ-type doubling transition (3.3GHz) of CH has been conducted toward Heiles Cloud 2 (HCL2) in the Taurus molecular cloud complex to reveal its molecular cloud-scale distribution. The observations were carried out with the Effelsberg 100m telescope. ************************************************************************** * * * The data cube will be available in February 2013 * * * **************************************************************************
    VizieR Online Data Catalog. 10/2012;
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    ABSTRACT: A mapping observation of the $J=1/2$ $\Lambda$-type doubling transition (3.3 GHz) of CH has been conducted toward Heiles Cloud 2 (HCL2) in the Taurus molecular cloud complex to reveal its molecular cloud-scale distribution. The observations were carried out with the Effelsberg 100 m telescope. The CH emission is found to be extended over the whole region of HCL2. It is brighter in the southeastern part, which encloses the TMC-1 cyanopolyyne peak than in the northwestern part. Its distribution extends continuously from the peak of the neutral carbon emission (CI peak) to the TMC-1 ridge, as if it were connecting the distributions of the [C I] and C$^{18}$O emissions. Since CH is an intermediate in gas-phase chemical reactions from C to CO, its emission should trace the transition region. The above distribution of the CH emission is consistent with this chemical behavior. Since the CH abundance is subject to the chemical evolutionary effect, the CH column density in HCL2 no longer follows a linear correlation wit the H$_2$ column density reported for diffuse and translucent clouds. More importantly, the CH line profile is found to be composed of the narrow and broad components. Although the broad component is dominant around the CI peak, the narrow component appears in the TMC-1 ridge and dense core regions such as L1527 and TMC-1A. This trend seems to reflect a narrowing of the line width during the formation of dense cores. These results suggest that the 3.3 GHz CH line is a useful tool for tracing the chemical and physical evolution of molecular clouds.
    Astronomy and Astrophysics 09/2012; · 5.08 Impact Factor
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    ABSTRACT: We present the initial results of a spectral line survey of L1157 B1 with the Nobeyama 45 m telescope. So far, we have covered the frequencey range of 13.7 GHz (82.0-94.5 GHz and 96.3-97.5 GHz), and have detected 22 species including CH3CHO, HCOOH, HCOOCH3, HNCO, NH2CHO, CH3CN, and CCS. We have also detected the line of CH2DOH. These results demonstrate rich chemistry in this shocked region, which would mainly originate from evaporation of ice mantles by means of shocks.
    EAS Publications Series 11/2011;
  • N. Sakai, T. Sakai, T. Hirota, S. Yamamoto
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    ABSTRACT: We have found low-mass star-forming regions with extremely high abundances of carbon-chain molecules. Those are L1527 in Taurus and IRAS 15398-3359 in Lupus. In these sources, carbon-chain molecules would be regenerated in a lukewarm region near the protostar, triggered by the evaporation of the CH4 ice. This is new carbon-chain chemistry (Warm Carbon-Chain Chemistry: WCCC) in contrast to the conventional one applied to cold starless cores. Our interferometric observation shows a steep abundance increase of the carbon-chain molecules inward of a radius of 500 - 1000 AU from the protostar. This size corresponds to the temperature range of 20-30 K, which is close to the evaporation temperature of CH4 (25 K). This result is therefore consistent with the WCCC. The discovery of the WCCC sources demonstrates chemical diversity of low-mass star-forming regions. In particular, a remarkable contrast can be seen between the WCCC and the hot corino chemistry known in IRAS 16293-2422. A possible origin for the diversity would be the time scale of the starless-core phase; a shorter contraction time would result in the WCCC. Relatively low deuterium fractionation ratios in L1527 also support this scenario. If so, chemical compositions will tell us an important clue to understand the source-to-source variation of star-formation processes.
    EAS Publications Series 11/2011;
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    ABSTRACT: We are developing quasi-optical superconducting hot-electron bolometer (HEB) mixer receivers for astronomical and atmospheric remote sensing applications. The microbridge of the HEB mixer was fabricated at room temperature from a 6.8-nm-thick niobium titanium nitride (NbTiN) film deposited on a 20-nm-thick aluminum nitride (AlN) buffer layer, using a helicon sputtering technique at a slow deposition rate. The mixer was cooled to 4.2 K using a closed-cycled mechanical 4 K pulse-tube cryocooler with a temperature fluctuation of ±1.6 mK. The stability of a large-volume NbTiN HEB mixer was studied at 1.47 THz by changing local oscillator (LO) power with the mixer bias voltage fixed. The intermediate frequency (IF) signal measured at 1.5 GHz had a maximum peak at a certain mixer bias current. The receiver noise temperature was lowest at around the IF maximum peak. It was also found that the IF signal was most stable at around the IF maximum peak under conditions in which the instability of LO pumping level, induced by small mechanical vibrations of the cryostat, remained in the optical system.
    IEEE Transactions on Applied Superconductivity 07/2011; · 1.20 Impact Factor
  • K. Furuya, Y. Aikawa, N. Sakai, S. Yamamoto
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    ABSTRACT: Observations of 13C species would be useful to investigate chemistry of carbon-bearing species. Recent observations in TMC-1 indicate that the abundances are different among carbon isotopomers of the same species. For instance, Takano et al. (1998) found that HCC13CN is more abundant than HC13CCN and H13CCCN, which indicates the three carbon atoms are not equivalent in HC_3N. Sakai et al. (2007; 2010) reported the abundance ratios of C13CS/13CCS and CCH/13CCH to be 4.2 and 1.6, respectively. Again, two carbon atoms are not equivalent in CCS and CCH. Sakai et al. (2007; 2010) discussed an origin of these anomalies and pointed out two possibilities: (i) fractionation during the formation of the species and (ii) rearrangements of the 13C position after the formation of molecules by isotopomer-exchange reactions. We construct a gas-grain chemical network model which includes carbon isotopes (12C and 13C) and isotopomers in order to investigate the evolution of molecular abundances, the carbon isotope ratios (12CX/13CX) and the isotopomer ratios (12C13CX/13C12CX) of CCH and CCS in dense molecular cores. We confirm that the isotope ratios of molecules, both in the gas phase and on grain surfaces, mostly depend on whether the species is formed from the carbon atom (ion) or the CO molecule; the isotope ratio is larger than the elemental abundance ratio of 12C/13C if the species is formed from the carbon atom, while the ratio is smaller if the species is formed from the CO molecule (cf. Langer et al. 1984). We successfully reproduce the observed C13CH/13CCH ratio in TMC-1 by considering the isotopomer-exchange reaction, 13CCH + H rightleftharpoons C13CH + H + 8.1 K. However, the C13CS/13CCS ratio remains lower than observed in TMC-1. We then assume the isotopomer-exchange reaction catalyzed by the H atom, 13CCS + H rightleftharpoons C13CS + H + 17.4 K. In the model with this reaction, the observed C13CS/13CCS, CCS/C13CS and CCS/13CCS ratios can be reproduced simultaneously.
    Proceedings of the International Astronomical Union 01/2011; 280.
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    ABSTRACT: To understand the initial conditions of star formation, it is useful to observe deuterated species, because the deuterium fractionation can be enhanced in cold starless phase. We have observed the HN13C J=1--0 and DNC J=1--0 lines toward 18 massive clumps, including infrared dark clouds (IRDCs) and high-mass protostellar objects (HMPOs), by using the Nobeyama Radio Observatory 45 m telescope. We have found that the HN13C emission is stronger than the DNC emission toward all the observed sources. The averaged DNC/HNC ratio of the observed sources is found to be 0.007, which is lower than that of the low-mass cores. The DNC/HNC ratio is found to be roughly anti-correlated with the kinetic temperature derived from NH_3 (J, K) = (1, 1) and (2, 2). We have also found that the DNC/HNC ratio of some IRDCs is lower than that of HMPOs, although the kinetic temperature of the IRDCs is lower than that of the HMPOs. With the aid of chemical model simulations, we discuss how the deuterium fractionation decreases after the onset of star formation. We suggest that the DNC/HNC ratio of star forming cores may reflect the timescale of starless phase. In addition to the above results, we report the current status of some instruments, which we have developed for observations of deuterated species. We have developed the 70 GHz receiver for the Nobeyama Radio Observatory (NRO) 45 m telescope. By using this receiver, we can observe the J=1-0 lines of various fundamental deuterated species such as DCN, DCO^+, and C_2D. For observations of the H_2D^+ line at 372 GHz, we have improved the 350 GHz receiver for the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope. We will also report the observation plans of deuterated species with these receivers.
    Proceedings of the International Astronomical Union 01/2011; 280.
  • 03/2010;
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    ABSTRACT: Aims: We have observed the N = 1-0 lines of CCH and its 13C isotopic species toward a cold dark cloud, TMC-1 and a star-forming region, L1527, to investigate the 13C abundances and formation pathways of CCH. Methods: The observations have been carried out with the IRAM 30 m telescope. Results: We have successfully detected the lines of 13CCH and C13CH toward the both sources and found a significant intensity difference between the two 13C isotopic species. The [C13CH] /[13CCH] abundance ratios are 1.6 ± 0.4 (3sigma) and 1.6 ± 0.1 (3sigma) for TMC-1 and L1527, respectively. The abundance difference between C13CH and 13CCH means that the two carbon atoms of CCH are not equivalent in the formation pathway. On the other hand, the [CCH]/[C13CH] and [CCH]/[13CCH] ratios are evaluated to be larger than 170 and 250 toward TMC-1, and to be larger than 80 and 135 toward L1527, respectively. Therefore, both of the 13C species are significantly diluted in comparison with the interstellar 12C/13C ratio of 60. The dilution is discussed in terms of a behavior of 13C in molecular clouds.
    Astronomy and Astrophysics 01/2010; 512. · 5.08 Impact Factor
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    ABSTRACT: In this paper, we present the development of the waveguide niobium titanium nitride (NbTiN) superconducting hot electron bolometer (HEB) mixers, cryogenically cooled by a 4 K close-cycled refrigerator. The NbTiN thin film is formed on a crystalline quartz substrate by sputtering an NbTi target with the Ar and N<sub>2</sub> gas at room temperature. The HEB mixer element is fabricated by using the 12 nm NbTiN film, and is mounted on a waveguide block. Measurement of a Fourier transform spectrometer shows that the response of the mixer is centered near 810 GHz with a bandwidth of about 500 GHz. The uncorrected DSB receiver noise temperature is measured to be 500 K, and the noise bandwidth is to be 1.4 GHz at 810 GHz. The present result shows that a good noise performance can be obtained for the NbTiN HEB mixer even with a relatively thick film (12 nm) fabricated at the room temperature.
    IEEE Transactions on Applied Superconductivity 07/2009; · 1.20 Impact Factor