Papers by Yoshinori Yonekura
Symposium - International Astronomical Union
We have made 12CO(J=1−0) observations of the LMC with NANTEN. We report the results of a comparis... more We have made 12CO(J=1−0) observations of the LMC with NANTEN. We report the results of a comparison between CO clouds and SNRs in the LMC. Among the 35 known SNRs, only 10 are possibly associated with CO clouds. These 10 CO clouds and SNRs deserve follow-up studies for possible interactions. We present overlays of CO clouds on the optical images of some of these SNRs.
Symposium - International Astronomical Union
We have made 12CO(J=1-0) observations in the LMC with NANTEN, and compared the detected giant mol... more We have made 12CO(J=1-0) observations in the LMC with NANTEN, and compared the detected giant molecular clouds (GMCs) with HII regions and stellar clusters. It is found that ~ 80% of the GMCs are associated with HII regions. The results of comparisons of the GMCs with the HII regions and the stellar clusters are presented.
Symposium - International Astronomical Union
We have made a 12CO(J = 1−0) survey of the LMC with NANTEN. A sample of 55 giant molecular clouds... more We have made a 12CO(J = 1−0) survey of the LMC with NANTEN. A sample of 55 giant molecular clouds has been identified and comparisons with stellar clusters, HII regions and SNRs are presented. The connection between the clouds and cluster formation is discussed.
Symposium - International Astronomical Union
Fully sampled 12CO(J=1−0) observations of the whole extent of the LMC have been made with a linea... more Fully sampled 12CO(J=1−0) observations of the whole extent of the LMC have been made with a linear resolution of ~ 30 pc at a detection limit of N(H2) = 2 × 1021 cm−2. In addition, several selected regions have been mapped with higher sensitivity corresponding to a detection limit of 1 × 1021 cm−2. Based on these results, a new estimate of the molecular mass in the LMC is presented.
The Astrophysical Journal
We report on interferometric observations of a face-on accretion system around the high-mass youn... more We report on interferometric observations of a face-on accretion system around the high-mass young stellar object, G353.273+0.641. The innermost accretion system of 100 au radius was resolved in a 45 GHz continuum image taken with the Jansky-Very Large Array. Our spectral energy distribution analysis indicated that the continuum could be explained by optically thick dust emission. The total mass of the dusty system is ∼0.2 M ☉ at minimum and up to a few M ☉ depending on the dust parameters. 6.7 GHz CH 3 OH masers associated with the same system were also observed with the Australia Telescope Compact Array. The masers showed a spiral-like, nonaxisymmetric distribution with a systematic velocity gradient. The line-of-sight velocity field is explained by an infall motion along a parabolic streamline that falls onto the equatorial plane of the face-on system. The streamline is quasi-radial and reaches the equatorial plane at a radius of 16 au. This is clearly smaller than that of typical accretion disks in high-mass star formation, indicating that the initial angular momentum was very small, or the CH 3 OH masers selectively trace accreting material that has small angular momentum. In the former case, the initial specific angular momentum is estimated to be 8×10 20 (* M 10 M ☉) 0.5 cm 2 s −1 , or a significant fraction of the initial angular momentum was removed outside of 100 au. The physical origin of such a streamline is still an open question and will be constrained by the higher-resolution (∼10 mas) thermal continuum and line observations with ALMA long baselines.
Proceedings of the International Astronomical Union
We report on interferometric observations of a face-on accretion system around the high mass youn... more We report on interferometric observations of a face-on accretion system around the high mass young stellar object, G353.273+0.641. The innermost accretion system of 100-au radius was resolved in a 45-GHz continuum image taken with the Jansky Very Large Array. Our SED analysis indicated that the continuum could be explained by optically-thick dust emission. 6.7 GHz CH3OH masers associated with the same system were also observed with the Australia Telescope Compact Array. The masers showed a spiral-like, non-axisymmetric distribution with a systematic velocity gradient. The line-of-sight velocity field is explained by an infall motion along a parabolic streamline that falls onto the equatorial plane of the face-on system. The streamline is quasi-radial and reaches the equatorial plane at a radius of 16 au. The physical origin of such a streamline is still an open question and will be constrained by the higher-resolution thermal continuum and line observations with ALMA long baselines.
Proceedings of the International Astronomical Union
For the high-sensitivity 6.7 GHz methanol maser observations, we developed a new technology for c... more For the high-sensitivity 6.7 GHz methanol maser observations, we developed a new technology for coherently combining the two signals from the Hitachi 32 m radio telescope and the Takahagi 32 m radio telescope of the Japanese VLBI Network. Furthermore, we compared the SNRs of the 6.7 GHz maser spectra for two methods. One is a VLBI method and the other is the newly developed digital position switching, which is a similar technology to that used in noise-cancelling headphones. We report the phase-up technique and the observation.
Publications of the Astronomical Society of Japan
We detected flaring flux variability that regularly occurred with a period of 23.9 d on a 6.7 GHz... more We detected flaring flux variability that regularly occurred with a period of 23.9 d on a 6.7 GHz methanol maser emission at V lsr = 25.30 km s −1 in G 014.23−00.50 through highly frequent monitoring using the Hitachi 32 m radio telescope. Analyzing data from 2013 January 5 to 2016 January 21, the periodic variability has persisted in at least 47 cycles, corresponding to ∼1100 d. The period of 23.9 d is the shortest one observed in masers around high-mass young stellar objects so far. The flaring component normally falls below the detection limit (3 σ) of ∼0.9 Jy. In the flaring periods, the component rises above the detection limit with a ratio of the peak flux density more than 180 in comparison with the quiescent phase, showing intermittent periodic variability. The timescale of the flux rise was typically two days or shorter, and both symmetric and asymmetric profiles of flux variability were observed through intraday monitoring. These characteristics might be explained by a change in the flux of seed photons in a colliding-wind binary (CWB) system, or a variation of the dust temperature by the extra heating source of a shock formed by a CWB system within a gap region in a circumbinary disk, in which the orbital semi-major axes of the binary are 0.26-0.34 au.
Publications of The Korean Astronomical Society, 2015
VLBI experiments have been conducted by radio telescopes in the East Asia VLBI Network (EAVN) in ... more VLBI experiments have been conducted by radio telescopes in the East Asia VLBI Network (EAVN) in which 14 telescopes in China, Japan, and Korea participated. One of the aims of the EAVN is to obtain higher angular resolution that is provided by the 6,000 km baseline between China and Japan and better sensitivity by adding large telescopes. Data were recorded at 1 a Gbps recording rate at all stations and processed on the Korea-Japan Joint VLBI Correlator (KJJVC) at the Korea-Japan Correlation Center (KJCC) after experiments. Fringes were obtained from these experiments conducted at 8 GHz and 22 GHz and post-correlation data analysis of the experiments is undergoing. The outcomes of these experiments open the possibility of conducting EAVN observations with global VLBI networks. In this presentation, the recent status of these experiments and future prospects are presented.
Publications of The Korean Astronomical Society, 2015
We have initiated single-dish monitoring observations of ∼400 methanol maser sources at 6.7 GHz u... more We have initiated single-dish monitoring observations of ∼400 methanol maser sources at 6.7 GHz using the Hitachi 32-m radio telescope from December 2012 to systematically research periodic flux variations, which are observed in some methanol maser sources associated with high-mass (proto-)stars. In our monitoring, we have made daily monitoring, so that each source has been observed every nine days with an integration time of 5 min (typical 3 σ detection sensitivities of 0.9 Jy). The monitoring observations help us statistically understand periodic flux variations with a period longer than 50 days. As an initial result, we present a new detection of periodic flux variations in the 6.7 GHz methanol maser source G 036.70+00.09. The period of the flux variations is ∼53 days (∼0.019 cycles day −1), and seems to be stable over 9 cycles, at least until the middle of August 2014.
Proceedings of the International Astronomical Union, 2005
Pasj Publications of the Astronomical Society of Japan, Dec 1, 1999
Journal of Geomagnetism and Geoelectricity, 1994
The J = 61 ,5-60,6 (110.8359 GHz) line of mesospheric and stratospheric ozone has been observed b... more The J = 61 ,5-60,6 (110.8359 GHz) line of mesospheric and stratospheric ozone has been observed by employing a newly developed microwave ozone sensor equipped with a Nb SIS mixer receiver in a temporal manner. The SIS mixer has a receiver noise of 34 K (SSB) and the spectrometer covers >60 MHz bandwidth with a frequency resolution of 35 kHz. Altitude profiles of ozone concentration retrieved from observed line profiles are compared with those in Keating et al. (1990) and in Parrish et al. (1992) for inter-comparison and obtained agreement with an accuracy of about 10% in the altitude range between 40 and 50 km. From these observations, the following are concluded. (1) Time profiles of relative variations of mesospheric 03 concentration at dawn and dusk are obtained with a good time resolution. (2) Diurnal variations of ozone mixing ratio at various altitudes from 36 km through 76 km appear qualitatively in agreement with theoretical calculations and earlier observations except nighttime variations near the top of the mesosphere. (3) Ozone mixing ratio above 70 km shows nighttime variations variable from day to day with an amplitude of about 20% of the midnight values.
Atnf Proposal, Apr 1, 2007
ABSTRACT We plan to search for dense cores embedded in the filamentary cloud of Musca for the fir... more ABSTRACT We plan to search for dense cores embedded in the filamentary cloud of Musca for the first time in order to investigate how filamentary cloud evolves by gravitational instability and fragmentation to form stars. The remarkably elongated structure of the Musca cloud has penetrating magnetic field lines perpendicular to the long axis without surrounding source of external pressure. As indicated by IRAS 100 micron dust emission and 13CO map by NANTEN, it is supposed to have dense molecular cloud cores embedded in the filament as results of gravitational fragmentation. Our near-IR survey found 2 young stars in the filament, and submm observations revealed that one of them is embedded in a dense core and driving a bipolar molecular outflow. These show that star formation is ongoing. It is likely that the Musca cloud is dynamically contracting and has formed dense cores in a quiescent condition under the influence of magnetic field. We aim to investigate the cloud and core dynamics by estimating the physical conditions of the cores (mass, density and gravitational stability) by mappings in optically thin molecular lines which traces densities from 10^4 to >10^5 cm^-3.
Proceedings of the International Astronomical Union, 2003
ABSTRACT
ABSTRACT We propose to search for dense (n(H_2) 10^5 cm^{-3}) and compact cores embedded in C^{18... more ABSTRACT We propose to search for dense (n(H_2) 10^5 cm^{-3}) and compact cores embedded in C^{18}O cores that we identified in the last season. In this remarkably filamentary cloud, we will investigate the star-formation activities under the influence of magnetic field and turbulence through physical properties of the dense cores. In our previous statistical studies, cloud cores with smaller linewidth tend to be gravitationally bound and more evolved. The C^{18}O result shows it the Musca cloud has typically small linewidth, namely little turbulence, while star formation is not as active as for the Taurus and Chamaeleon I clouds. The complex velocity and morphological structures indicate that the cloud has been gravitationally fragmented into even denser cores. Optically thin high density tracers of H^{13}CO^{+} and N_{2}H^{+} lines will be used for the dense core census and their linewwidth estimation, while optically thick HCO^{+} line will reveal indications of gravitational collapse, if exist.
Proceedings of the International Astronomical Union, 2003
The CCCP catalog (Broos et al., 2011, Cat. J/ApJS/194/2) contains over 14000 point sources detect... more The CCCP catalog (Broos et al., 2011, Cat. J/ApJS/194/2) contains over 14000 point sources detected by the Advanced CCD Imaging Spectrometer (ACIS) of the Chandra X-ray Observatory. Broos et al. matched the Vela-Carina Archive to the CCCP X-ray catalog and found 6543 counterparts to CCCP sources. The Vela-Carina Catalog is a subset of the Archive and provides mid-IR counterparts for
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Papers by Yoshinori Yonekura