arXiv subjects

Yoshinori Yonekura

Publications and source records attributed to Yoshinori Yonekura.

At least 55 records · Page 3Linked to original sources

arXiv2014

Observations of the Bursting Activity of the 6.7GHz Methanol Maser in G33.641-0.228

We have observed bursting variability of the 6.7 GHz methanol maser of G33.641-0.228. Five bursts were detected in the observation period of 294 days from 2009 to 2012. The typical burst is a large flux density rise in about one day followed by a slow fall. A non-typical burst observed in 2010 showed a large and rapid flux density enhancement from the stable state, but the rise and fall of the flux density were temporally symmetric and a fast fluctuation continued 12 days. On average, the bursts occurred once every 59 days, although bursting was not periodic. Since the average power required for causing the burst of order of 10^21 Js^-1 is far smaller than the luminosity of G33.641-0.228, a very small fraction of the source's power would be sufficient to cause the burst occasionally. The burst can be explained as a solar-flare like event in which the energy is accumulated in the magnetic field of the circumstellar disk, and is released for a short time. However, the mechanism of the energy release and the dust heating process are still unknown.

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arXiv2014

VERA and ALMA Observations of the H2O Supermaser Burst in Orion KL

The 22 GHz H2O maser in Orion KL has shown extraordinary burst events in 1979-1985 and 1998-1999, sometimes called supermaser. We have conducted monitoring observations of the supermaser in Orion KL using VERA, VLBI Exploration of Radio Astrometry, in the current third burst since 2011 March. Three flux maxima are detected in 2011 and 2012 with rising and falling timescales of 2-7 months. Time variations of the supermaser seem symmetric for all of the active phases. The maximum total flux density of 135000 Jy is observed in 2012 June while it is still one order of magnitude lower than those in previous bursts. The supermaser consists of two spatially different components at different velocities. They are elongated along a northwest-southeast direction perpendicular to the low-velocity outflow driven by Source I. Proper motions of the supermaser features with respect to Source I are measured toward west and southwest directions, almost parallel to the low-velocity outflow. The flux density and linewidth show an anti-correlation as expected for an unsaturated maser emission. The supermaser is located close to the methylformate (HCOOCH3) line and continuum emission peaks in the Orion Compact Ridge detected by ALMA. The broader velocity range of the weak HCOOCH3 emission at the supermaser position would be an evidence of a shock front. On the other hand, the 321 GHz H2O line is not detected at the position of the supermaser. It can be explained qualitatively by one of the theoretical H2O excitation models without extraordinary conditions. Our results support a scenario that the supermaser is excited in the dense gas interacting with the low-velocity outflow in the Compact Ridge. The extremely high flux density and its symmetric time variation for rising and falling phases could be explained by a beaming effect during the amplification process rather than changes in physical conditions.

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arXiv2014

Cluster Formation Triggered by Filament Collisions in Serpens South

The Serpens South infrared dark cloud consists of several filamentary ridges, some of which fragment into dense clumps. On the basis of CCS ($J_N=4_3-3_2$), HC$_3$N ($J=5-4$), N$_2$H$^+$ ($J=1-0$), and SiO ($J=2-1, v=0$) observations, we investigated the kinematics and chemical evolution of these filamentary ridges. We find that CCS is extremely abundant along the main filament in the protocluster clump. We emphasize that Serpens South is the first cluster-forming region where extremely-strong CCS emission is detected. The CCS-to-N$_2$H$^+$ abundance ratio is estimated to be about 0.5 toward the protocluster clump, whereas it is about 3 in the other parts of the main filament. We identify six dense ridges with different $V_{\rm LSR}$. These ridges appear to converge toward the protocluster clump, suggesting that the collisions of these ridges may have triggered cluster formation. The collisions presumably happened within a few $\times \ 10^5$ yr because CCS is abundant only in such a short time. The short lifetime agrees with the fact that the number fraction of Class I objects, whose typical lifetime is $0.4 \times \ 10^5$ yr, is extremely high as about 70 percent in the protocluster clump. In the northern part, two ridges appear to have partially collided, forming a V-shape clump. In addition, we detected strong bipolar SiO emission that is due to the molecular outflow blowing out of the protostellar clump, as well as extended weak SiO emission that may originate from the filament collisions.

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arXiv2014

Periodic Flare of the 6.7 GHz Methanol Maser in IRAS22198+6336

We have detected periodic flares of the 6.7 GHz methanol maser from an intermediate-mass star-forming region IRAS22198+6336. The maser was monitored daily in 2011, 2012, and 2013. Six flares were observed with a period of 34.6 days. The variation pattern is intermittent, and the flux ratio of the flaring and the quiescent states exceeds 30. Such intermittent variation with the short period uniquely characterizes the variation of the IRAS22198+6336 maser. At least five spectral components were identified. The spectral components varied almost synchronously, but their peak times differed by 1.8 days. These characteristics can be explained by the colliding-wind binary model.

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arXiv2013

Short-Term Radio Variability and Parsec-Scale Structure in a Gamma-Ray Narrow-Line Seyfert 1 Galaxy 1H 0323+342

We made simultaneous single-dish and very long baseline interferometer (VLBI) observations of a narrow-line Seyfert 1 galaxy (NLS1) 1H 0323+342, showing gamma-ray activity revealed by Fermi/LAT observations. We found significant variation of the total flux density at 8 GHz on the time scale of one month by the single-dish monitoring. The total flux density varied by 5.5% in 32 days, which is comparable to the gamma-ray variability time scale, corresponding to the variability brightness temperature of $7.0 \times 10^{11}$ K. The source consists of central and southeastern components on the parsec (pc) scale. The flux of only the central component decreased in the same way as the total flux density, indicating that the short-term radio variability, and probably the gamma-ray emitting region, is associated with this component. From the VLBI observations we obtained the brightness temperatures of greater than $(5.2 \pm 0.3) \times 10^{10}$ K, and derived the equipartition Doppler factor of greater than 1.7, the variability Doppler factor of 2.2, and the 8 GHz radio power of $10^{24.6}$ W Hz$^{-1}$. Combining them we conclude that acceleration of radio jets and creation of high-energy particles are ongoing in the central engine, and that the apparent very radio-loud feature of the source is due to the Doppler-boosting effect, resulting in the intrinsic radio loudness to be an order of magnitude smaller than the observed values. We also conclude that the pc-scale jet represents recurrent activity from the the spectral fitting and the estimated kinematic age of pc- and kpc-scale extended components with different position angle.

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arXiv2013

Observations of 6.7 GHz Methanol Masers with EAVN I: VLBI Images of the first Epoch of Observations

Very long baseline interferometry (VLBI) monitoring of the 6.7 GHz methanol maser allows us to measure the internal proper motions of the maser spots and therefore study the gas motion around high-mass young stellar objects. To this end, we have begun monitoring observations with the East-Asian VLBI Network. In this paper we present the results of the first epoch observation for 36 sources, including 35 VLBI images of the methanol maser. Since two independent sources were found in three images, respectively, images of 38 sources were obtained. In 34 sources, more than or equal to 10 spots were detected. The observed spatial scale of the maser distribution was from 9 to 4900 astronomical units, and the following morphological categories were observed: elliptical, arched, linear, paired, and complex. The position of the maser spot was determined to an accuracy of approximately 0.1 mas, sufficiently high to measure the internal proper motion from two years of monitoring observations. The VLBI observation, however, detected only approximately 20% of all maser emission, suggesting that the remaining 80% of the total flux was spread into an undetectable extended distribution. Therefore, in addition to high-resolution observations, it is important to observe the whole structure of the maser emission including extended low-brightness structures, to reveal the associated site of the maser and gas motion.

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arXiv2013

Dense optical and near-infrared monitoring of CTA 102 during high state in 2012 with OISTER: Detection of intra-night "orphan polarized flux flare"

CTA 102, classified as a flat spectrum radio quasar at z=1.037, produced exceptionally bright optical flare in 2012 September. Following Fermi-LAT detection of enhanced gamma-ray activity, we densely monitored this source in the optical and near-infrared bands for the subsequent ten nights using twelve telescopes in Japan and South-Africa. On MJD 56197 (2012 September 27, 4-5 days after the peak of bright gamma-ray flare), polarized flux showed a transient increase, while total flux and polarization angle remained almost constant during the "orphan polarized-flux flare". We also detected an intra-night and prominent flare on MJD 56202. The total and polarized fluxes showed quite similar temporal variations, but PA again remained constant during the flare. Interestingly, the polarization angles during the two flares were significantly different from the jet direction. Emergence of a new emission component with high polarization degree (PD) up to 40% would be responsible for the observed two flares, and such a high PD indicates a presence of highly ordered magnetic field at the emission site. We discuss that the well-ordered magnetic field and even the observed directions of polarization angle which is grossly perpendicular to the jet are reasonably accounted for by transverse shock(s) propagating down the jet.

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arXiv2011

Bursting Activity in a High-Mass Star-Forming Region G33.64-0.21 Observed with the 6.7 GHz Methanol Maser

We report the detection of bursts of 6.7 GHz methanol maser emission in a high-mass star-forming region, G33.64-0.21. One of the spectral components of the maser in this source changed its flux density by 7 times that of the previous day, and it decayed with a timescale of 5 days. The burst occurred repeatedly in the spectral component, and no other components showed such variability. A VLBI observation with the Japanese VLBI Network (JVN) showed that the burst location was at the southwest edge of a spatial distribution, and the bursting phenomenon occurred in a region much smaller than 70 AU. We suggest an impulsive energy release like a stellar flare as a possible mechanism for the burst. These results imply that 6.7 GHz methanol masers could be a useful new probe for studying bursting activity in the process of star formation of high-mass YSOs with a high-resolution of AU scale.

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arXiv2011

Identification of Bursting Water Maser Features in Orion KL

In February 2011, a burst event of the H$_{2}$O maser in Orion KL (Kleinmann-Low object) has started after 13-year silence. This is the third time to detect such phenomena in Orion KL, followed by those in 1979-1985 and 1998. We have carried out astrometric observations of the bursting H$_{2}$O maser features in Orion KL with VERA (VLBI Exploration of Radio Astrometry), a Japanese VLBI network dedicated for astrometry. The total flux of the bursting feature at the LSR velocity of 7.58 km s$^{-1}$ reaches 4.4$\times10^{4}$ Jy in March 2011. The intensity of the bursting feature is three orders of magnitudes larger than that of the same velocity feature in the quiescent phase in 2006. Two months later, another new feature appears at the LSR velocity of 6.95 km s$^{-1}$ in May 2011, separated by 12 mas north of the 7.58 km s$^{-1}$ feature. Thus, the current burst occurs at two spatially different features. The bursting masers are elongated along the northwest-southeast direction as reported in the previous burst in 1998. We determine the absolute positions of the bursting features for the first time ever with a submilli-arcsecond (mas) accuracy. Their positions are coincident with the shocked molecular gas called the Orion Compact Ridge. We tentatively detect the absolute proper motions of the bursting features toward southwest direction. It is most likely that the outflow from the radio source I or another young stellar object interacting with the Compact Ridge is a possible origin of the H$_{2}$O maser burst.

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arXiv2011

The Galactic Census of High- and Medium-mass Protostars. I. Catalogues and First Results from Mopra HCO+ Maps

The Census of High- and Medium-mass Protostars (CHaMP) is the first large-scale, unbiased, uniform mapping survey at sub-parsec scale resolution of 90 GHz line emission from massive molecular clumps in the Milky Way. We present the first Mopra (ATNF) maps of the CHaMP survey region (300°>l>280°) in the HCO+ J=1-0 line, which is usually thought to trace gas at densities up to 10^11 m-3. In this paper we introduce the survey and its strategy, describe the observational and data reduction procedures, and give a complete catalogue of moment maps of the HCO+ J=1-0 emission from the ensemble of 301 massive molecular clumps. From these maps we also derive the physical parameters of the clumps, using standard molecular spectral-line analysis techniques. This analysis yields the following range of properties: integrated line intensity 1-30 K km s-1, peak line brightness 1-7 K, linewidth 1-10 km s-1, integrated line luminosity 0.5-200 K km s-1 pc^2, FWHM size 0.2-2.5 pc, mean projected axial ratio 2, optical depth 0.08-2, total surface density 30-3000 M{\sun} pc-2, number density 0.2-30 x 10^9 m-3, mass 15-8000 M{\sun}, virial parameter 1-55, and total gas pressure 0.3-700 pPa. We find that the CHaMP clumps do not obey a Larson-type size-linewidth relation. Among the clumps, there exists a large population of subthermally excited, weakly-emitting (but easily detectable) dense molecular clumps, confirming the prediction of Narayanan et al. (2008). These weakly-emitting clumps comprise 95% of all massive clumps by number, and 87% of the molecular mass, in this portion of the Galaxy; their properties are distinct from the brighter massive star-forming regions that are more typically studied. If the clumps evolve by slow contraction, the 95% of fainter clumps may represent a long-lived stage of pressure-confined, gravitationally stable massive clump evolution, while the CHaMP ... (abridged)

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arXiv2011

A Pan-Carina YSO Catalog: Intermediate-Mass Young Stellar Objects in the Carina Nebula Identified Via Mid-Infrared Excess Emission

We present a catalog of 1439 young stellar objects (YSOs) spanning the 1.42 deg^2 field surveyed by the Chandra Carina Complex Project (CCCP), which includes the major ionizing clusters and the most active sites of ongoing star formation within the Great Nebula in Carina. Candidate YSOs were identified via infrared (IR) excess emission from dusty circumstellar disks and envelopes, using data from the Spitzer Space Telescope Vela--Carina survey and the Two-Micron All Sky Survey. We model the 1--24 /mu m IR spectral energy distributions of the YSOs to constrain physical properties. Our Pan-Carina YSO Catalog (PCYC) is dominated by intermediate-mass (2 Msun < m < 10 Msun) objects with disks, including Herbig Ae/Be stars and their less evolved progenitors. The PCYC provides a valuable complementary dataset to the CCCP X-ray source catalogs, identifying 1029 YSOs in Carina with no X-ray detection. We also catalog 410 YSOs with X-ray counterparts, including 62 candidate protostars. Candidate protostars with X-ray detections tend to be more evolved than those without. In most cases, X-ray emission apparently originating from intermediate-mass, disk-dominated YSOs is consistent with the presence of low-mass companions, but we also find that X-ray emission correlates with cooler stellar photospheres and higher disk masses. We suggest that intermediate-mass YSOs produce X-rays during their early pre-main sequence evolution, perhaps driven by magnetic dynamo activity during the convective atmosphere phase, but this emission dies off as the stars approach the main sequence. Extrapolating over the stellar initial mass function scaled to the PCYC population, we predict a total population of >2x10^4 YSOs and a present-day star formation rate (SFR) of >0.008 Msun/yr. The global SFR in the Carina Nebula, averaged over the past ~5 Myr, has been approximately constant.

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arXiv2011

Source Contamination in X-ray Studies of Star-Forming Regions: Application to the Chandra Carina Complex Project

We describe detailed simulations of X-ray-emitting populations to evaluate the levels of contamination by both Galactic and extragalactic X-ray sources unrelated to a star-forming region under study. For Galactic contaminations, we consider contribution from main-sequence stars and giants (not including cataclysmic variables and other classes of accretion-driven X-ray binary systems) as they make the dominant contribution at the position of the Carina Nebula. The simulations take into consideration a variety of technical factors involving a Galactic population synthesis model, stellar X-ray luminosity functions, Chandra telescope response, source detection methodology, and possible spatial variations in the X-ray background and absorption through molecular clouds. When applied to the 1.42 square-degree field of the Chandra Carina Complex Project (CCCP), the simulations predict ~5000 contaminating sources (1 source per square arcminute of the survey), evenly distributed across the field. The results of the simulations are further employed in a companion CCCP study to assign membership probabilities to individual sources.

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arXiv2010

The AMANOGAWA-2SB Galactic Plane Survey I: Data on the Galactic Equator

Using a waveguide-type sideband-separating receiver (2SB receiver) on the Tokyo-NRO 60-cm telescope (renamed the AMANOGAWA telescope), we carried out simultaneous observations in the 12CO(J = 2-1) and 13CO(J = 2-1) lines over the Galactic plane l = 10-245 deg along b = 0 deg with a 3.75 arcmin grid. Using the 12CO(J = 1-0) data of Dame et al. (2001), who used a beam size almost the same as ours, we show 12CO(J = 2-1)/12CO(J = 1-0) and 13CO(J = 2-1)/12CO(J = 2-1) intensity ratios on the l-v map and the intensity correlations among the three lines. As a result, a linear correlation between 12CO(J = 1-0) and 12CO(J = 2-1) and a curve correlation between 12CO(J = 2-1) and 13CO(J = 2-1), as produced by most of the data, are found. We investigate these correlations with simple radiative transfer equations to ascertain a number of restrictions on the physical quantities of molecular gas on a galactic scale.

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arXiv2009

Discovery of Large-Scale Gravitational Infall in a Massive Protostellar Cluster

We report Mopra (ATNF), Anglo-Australian Telescope, and Atacama Submillimeter Telescope Experiment observations of a molecular clump in Carina, BYF73 = G286.21+0.17, which give evidence of large-scale gravitational infall in the dense gas. From the millimetre and far-infrared data, the clump has mass ~ 2 x 10^4 Msun, luminosity ~ 2-3 x 10^4 Lsun, and diameter ~ 0.9 pc. From radiative transfer modelling, we derive a mass infall rate ~ 3.4 x 10^-2 Msun yr-1. If confirmed, this rate for gravitational infall in a molecular core or clump may be the highest yet seen. The near-infrared K-band imaging shows an adjacent compact HII region and IR cluster surrounded by a shell-like photodissociation region showing H2 emission. At the molecular infall peak, the K imaging also reveals a deeply embedded group of stars with associated H2 emission. The combination of these features is very unusual and we suggest they indicate the ongoing formation of a massive star cluster. We discuss the implications of these data for competing theories of massive star formation.

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arXiv2008

High-Resolution Studies of the Multiple-Core Systems toward Cluster-Forming Regions Including Massive Stars

We present the results of C18O observations by the Nobeyama Millimeter Array toward dense clumps with radii of ~ 0.3 pc in six cluster-forming regions including massive (proto)stars. We identified 171 cores, whose radius, line width, and molecular mass range from 0.01 to 0.09 pc, 0.43 to 3.33 km/s, and 0.5 to 54.1 Mo, respectively. Many cores with various line widths exist in one clump, and the index of the line width-radius relationship of the cores and the parental clump differs from core to core in the clump. This indicates that the degree of dissipation of the turbulent motion varies for each core in one clump. Although the mass of the cores increases with the line width, most cores are gravitationally bound by the external pressure. In addition, the line width and the external pressure of the cores tend to decrease with the distance from the center of the clump, and these dependencies may be caused by the inner H2 density structure of the clump that affects the physical properties of the cores. Moreover, the number density of the cores and the number density of young (proto)stars have a similar relationship to the average H2 density of the clumps. Thus, our findings suggest that the cluster is formed in the clump through the formation of such multiple cores, whose physical properties would have been strongly related to the H2 density structure and the turbulent motion of the clump.

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arXiv2008

The Second Survey of the Molecular Clouds in the Large Magellanic Cloud by NANTEN I: Catalog of Molecular Clouds

The second survey of the molecular clouds in 12CO (J = 1-0) was carried out in the Large Magellanic Cloud by NANTEN. The sensitivity of this survey is twice as high as that of the previous NANTEN survey, leading to a detection of molecular clouds with M_CO > 2 x 10^4 M_sun. We identified 272 molecular clouds, 230 of which are detected at three or more observed positions. We derived the physical properties, such as size, line width, virial mass, of the 164 GMCs which have an extent more than the beam size of NANTEN in both the major and minor axes. The CO luminosity and virial mass of the clouds show a good correlation of M_VIR propto L_CO^{1.1 +- 0.1} with a Spearman rank correlation of 0.8 suggesting that the clouds are in nearly virial equilibrium. Assuming the clouds are in virial equilibrium, we derived an X_CO-factor to be ~ 7 x 10^20 cm^-2 (K km s^-1)^-1. The mass spectrum of the clouds is fitted well by a power law of N_cloud(>M_CO) proportional to M_CO^{-0.75 +- 0.06} above the completeness limit of 5 x 10^4 M_sun. The slope of the mass spectrum becomes steeper if we fit only the massive clouds; e.g., N_cloud (>M_CO) is proportional to M_CO^{-1.2 +- 0.2} for M_CO > 3 x 10^5 M_sun.

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arXiv2008

A New 100-GHz Band Front-End System with a Waveguide-Type Dual-Polarization Sideband-Separating SIS Receiver for the NRO 45-m Radio Telescope

We developed a waveguide-type dual-polarization sideband-separating SIS receiver system of the 100-GHz band for the 45-m radio telescope at the Nobeyama Radio Observatory, Japan. This receiver is composed of an ortho-mode transducer and two sideband-separating SIS mixers, which are both based on the waveguide technique. The receiver has four intermediate frequency bands of 4.0--8.0 GHz. Over the radio frequency range of 80--120 GHz, the single-sideband receiver noise temperatures are 50--100 K and the image rejection ratios are greater than 10 dB. We developed new matching optics for the telescope beam as well as new IF chains for the four IF signals. The new receiver system was installed in the telescope, and we successfully observed the 12CO, 13CO and C18O emission lines simultaneously toward the Sagittarius B2 region to confirm the performance of the receiver system. The SSB noise temperature of the system, including the atmosphere, became approximately half of that of the previous receiver system. The Image Rejection Ratios (IRRs) of the two 2SB mixers were calculated from the 12CO and HCO+ spectra from the W51 giant molecular cloud, resulting in > 20 dB for one polarization and > 12 dB for the other polarization.

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arXiv2007

New 60-cm Radio Survey Telescope with the Sideband-Separating SIS Receiver for the 200 GHz Band

We have upgraded the 60-cm radio survey telescope located in Nobeyama, Japan. We developed a new waveguide-type sideband-separating SIS mixer for the telescope, which enables the simultaneous detection of distinct molecular emission lines both in the upper and lower sidebands. Over the RF frequency range of 205-240 GHz, the single-sideband receiver noise temperatures of the new mixer are 40-100 K for the 4.0-8.0 GHz IF frequency band. The image rejection ratios are greater than 10 dB over the same range. For the dual IF signals obtained by the receiver, we have developed two sets of acousto-optical spectrometers and a telescope control system. Using the new telescope system, we successfully detected the 12CO (J=2-1) and 13CO (J=2-1) emission lines simultaneously toward Orion KL in 2005 March. Using the waveguide-type sideband-separating SIS mixer for the 200 GHz band, we have initiated the first simultaneous 12CO (J=2-1) and 13CO (J=2-1) survey of the galactic plane as well as large-scale mapping observations of nearby molecular clouds.

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