arXiv subjects

Tomoya Hirota

Publications and source records attributed to Tomoya Hirota.

At least 127 records · Page 7Linked to original sources

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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arXiv2014

Distribution of CCS and HC3N in L1147, an Early Phase Dark Cloud

We used the Nobeyama 45 m radio telescope to reveal spatial distributions of CCS and HC$_3$N in L1147, one of carbon-chain producing regions (CCPRs) candidates, where carbon-chain molecules are dominant rather than NH$_3$. We found that three cores (two CCS cores and one HC$_3$N core) exist along the NE-SW filament traced by the 850 micron dust continuum, which are away from a Very Low Luminosity Object (VeLLO - a source that may turn into sub-stellar mass brown dwarf). The column densities of CCS are 3-7x10$^{12}$ cm$^{-2}$ and those of HC$_3$N are 2-6x10$^{12}$ cm$^{-2}$, respectively, much lower than those previously reported towards other CCPRs. We also found that two CCS peaks are displaced from that of HC$_3$N. In order to interpret such interleaved distributions, we conducted chemical reaction network simulations, and found that slightly different gas densities could lead to large variation of CCS-to-HC$_3$N ratio in the early phase of dark cloud evolution. Such a chemical "variation" may be seen in other CCPRs. Finally we were able to confirm that the L1147 filament can be regarded as a CCPR.

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arXiv2014

Hot Molecular Circumstellar Disk around Massive Protostar Orion Source I

We report new Atacama Large Millimeter/Submillimeter Array (ALMA) observations of a circumstellar disk around Source I in Orion KL, an archetype of massive protostar candidate. We detected two ortho-H$_{2}$O lines at 321 GHz ($10_{2,9}$-$9_{3,6}$) and 336 GHz ($ν_{2}=1, 5_{2,3}$-$6_{1,6}$) for the first time in Source I. The latter one is in vibrationally excited state at the lower state energy of 2939 K, suggesting an evidence of hot molecular gas close to Source I. The integrated intensity map of the 321 GHz line is elongated along the bipolar outflow while the 336 GHz line map is unresolved with a beam size of 0.4\arcsec. Both of these maps show velocity gradient perpendicular to the bipolar outflow. The velocity centroid map of the 321 GHz line implies spatial and velocity structure similar to that of vibrationally-excited SiO masers tracing the root of the outflow emanating from the disk surface. In contrast, the 336 GHz line is most likely emitted from the disk midplane with a diameter of 0.2\arcsec (84 AU) as traced by a radio continuum emission and a dark lane devoid of the vibrationally-excited SiO maser emission. The observed velocity gradient and the spectral profile of the 336 GHz H$_{2}$O line can be reconciled with a model of an edge-on ring-like structure with an enclosed mass of $>$7$M_{\odot}$ and an excitation temperature of $>$3000 K. The present results provide a further evidence of hot and neutral circumstellar disk rotating around Source I with a diameter of $\sim$100 AU scale.

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arXiv2014

Accurate Parallax Measurement toward the Symbiotic Star R Aquarii

Multi-epoch phase-referencing VLBI (Very Long Baseline Interferometry) observations with VERA (VLBI Exploration of Radio Astrometry) were performed for the symbiotic star R Aquarii (R Aqr) from September 2005 to Oct 2006. Tracing one of the $v=2$, $J=1-0$ SiO maser spots, we measured an annual parallax of $π= 4.59\pm0.24$ mas, corresponding to a distance of $218_{-11}^{+12}$ pc. Our result is consistent with earlier distance measurements, but yields the highest accuracy of about $5\%$ level. Applying our distance, we derived an absolute K-band magnitude of $M_{\mathrm{K}} = -7.71 \pm 0.11$, which is consistent with the recent Period-Luminosity relation by VLBI parallax measurements for 5 OH-Mira variables. In addition, the expansion age of an inner nebulae around R Aqr is found to be about 240 years, corresponds to about the year 1773.

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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

Chemical Variation in Molecular Cloud Cores in the Orion A Cloud. II

We have mapped six molecular cloud cores in the Orion A giant molecular cloud (GMC), whose kinetic temperatures range from 10 to 30 K, in CCS and N2H+ with Nobeyama 45 m radio telescope to study their chemical characteristics. We identified 31 intensity peaks in the CCS and N2H+ emission in these molecular cloud cores. It is found for cores with temperatures lower than ~ 25 K that the column density ratio of N(N2H+)/N(CCS) is low toward starless core regions while it is high toward star-forming core regions, in case that we detected both of the CCS and N2H+ emission. This is very similar to the tendency found in dark clouds (kinetic temperature ~ 10 K). The criterion found in the Orion A GMC is N(N2H+)/N(CCS) ~ 2-3. In some cases, the CCS emission is detected toward protostars as well as the N2H+ emission. Secondary late-stage CCS peak in the chemical evolution caused by CO depletion may be a possible explanation for this. We found that the chemical variation of CCS and N2H+ can also be used as a tracer of evolution in warm (10-25 K) GMC cores. On the other hand, some protostars do not accompany N2H+ intensity peaks but are associated with dust continuum emitting regions, suggesting that the N2H+ abundance might be decreased due to CO evaporation in warmer star-forming sites.

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arXiv2013

ALMA Observations of the IRDC Clump G34.43+00.24 MM3: Hot Core and Molecular Outflows

We have observed a cluster forming clump (MM3) associated with the infrared dark cloud G34.43+00.24 in the 1.3 mm continuum and the CH3OH, CS, 13CS, SiO, CH3CH2CN, and HCOOCH3 lines with the Atacama Large Millimeter/submillimeter Array and in K-band with the Keck telescope. We have found a young outflow toward the center of this clump in the SiO, CS, and CH3OH lines. This outflow is likely driven by a protostar embedded in a hot core, which is traced by the CH3CH2CN, HCOOCH3, 13CS, and high excitation CH3OH lines. The size of the hot core is about 800 x 300 AU in spite of its low mass (<1.1 M_sun), suggesting a high accretion rate or the presence of multiple star system harboring a few hot corinos. The outflow is highly collimated, and the dynamical timescale is estimated to be less than 740 yr. In addition, we have also detected extended emission of SiO, CS, and CH3OH, which is not associated with the hot core and the outflow. This emission may be related to past star formation activity in the clump. Although G34.43+00.24 MM3 is surrounded by a dark feature in infrared, it has already experienced active formation of low-mass stars in an early stage of clump evolution.

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arXiv2012

Fundamental Parameters of the Milky Way Galaxy Based on VLBI astrometry

We present analyses to determine the fundamental parameters of the Galaxy based on VLBI astrometry of 52 Galactic maser sources obtained with VERA, VLBA and EVN. We model the Galaxy's structure with a set of parameters including the Galaxy center distance R_0, the angular rotation velocity at the LSR Omega_0, mean peculiar motion of the sources with respect to Galactic rotation (U_src, V_src, W_src), rotation-curve shape index, and the V component of the Solar peculiar motions V_sun. Based on a Markov chain Monte Carlo method, we find that the Galaxy center distance is constrained at a 5% level to be R_0 = 8.05 +/- 0.45 kpc, where the error bar includes both statistical and systematic errors. We also find that the two components of the source peculiar motion U_src and W_src are fairly small compared to the Galactic rotation velocity, being U_src = 1.0 +/- 1.5 km/s and W_src = -1.4 +/- 1.2 km/s. Also, the rotation curve shape is found to be basically flat between Galacto-centric radii of 4 and 13 kpc. On the other hand, we find a linear relation between V_src and V_sun as V_src = V_sun -19 (+/- 2) km/s, suggesting that the value of V_src is fully dependent on the adopted value of V_sun. Regarding the rotation speed in the vicinity of the Sun, we also find a strong correlation between Omega_0 and V_sun. We find that the angular velocity of the Sun, Omega_sun, which is defined as Omega_sun = Omega_0 + V_sun/R_0, can be well constrained with the best estimate of Omega_sun = 31.09 +/- 0.78 km/s/kpc. This corresponds to Theta_0 = 238 +/- 14 km/s if one adopts the above value of R_0 and recent determination of V_sun ~ 12 km/s.

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arXiv2012

Tentative Detection of Deuterated Methane toward the Low-Mass Protostar IRAS 04368+2557 in L1527

The millimeter-wave rotational transition line ($J_K = 1_0 - 0_0$) of deuterated methane CH$_3$D has tentatively been detected toward the low-mass Class 0 protostar IRAS 04368+2557 in L1527 with the Heinrich Hertz Submillimeter Telescope. This is the first detection of CH$_3$D in interstellar clouds, if confirmed. The column density and fractional abundance of CH$_3$D are determined to be $(9.1\pm3.4) \times 10^{15}$ cm$^{-2}$ and $(3.0\pm1.1) \times 10^{-7}$, respectively, where we assume the rotational temperature of 25 K. The column density and fractional abundance of the gaseous CH$_4$ are estimated to be $(1.3-4.6) \times 10^{17}$ cm$^{-2}$ and $(4.3-15.2) \times 10^{-6}$, respectively, by adopting the molecular D/H ratios of 2--7% reported for various molecules in L1527. The fractional abundance of CH$_4$ is higher than or comparable to that found in high-mass star-forming cores by infrared observations. It is sufficiently high for triggering the efficient production of various carbon-chain molecules in a lukewarm region near the protostar, which supports the picture of the warm carbon-chain chemistry.

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arXiv2012

The first detection of the 232 GHz vibrationally excited H2O maser in Orion KL with ALMA

We investigated the ALMA science verification data of Orion KL and found a spectral signature of the vibrationally excited H2O maser line at 232.68670 GHz (nu2=1, 5,5,0-6,4,3). This line has been detected in circumstellar envelopes of late-type stars so far but not in young stellar objects including Orion KL. Thus, this is the first detection of the 232 GHz vibrationally excited H2O maser in star-forming regions. The distribution of the 232 GHz maser is concentrated at the position of the radio Source I, which is remarkably different from other molecular lines. The spectrum shows a double-peak structure at the peak velocities of -2.1 and 13.3 km s-1. It appears to be consistent with the 22 GHz H2O masers and 43 GHz SiO masers observed around Source I. Thus, the 232 GHz H2O maser around Source I would be excited by the internal heating by an embedded protostar, being associated with either the root of the outflows/jets or the circumstellar disk around Source I, as traced by the 22 GHz H2O masers or 43 GHz SiO masers, respectively.

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arXiv2012

A Highly collimated Water Maser Bipolar Outflow in the Cepheus A HW3d Massive Young Stellar Object

We present the results of multi-epoch very long baseline interferometry (VLBI) water (H2O) maser observations carried out with the VLBI Exploration of Radio Astrometry (VERA) toward the Cepheus A HW3d object. We measured for the first time relative proper motions of the H2O maser features, whose spatio-kinematics traces a compact bipolar outflow. This outflow looks highly collimated and expanding through ~ 280 AU (400 mas) at a mean velocity of ~ 21 km/s (~ 6 mas/yr) without taking into account the turbulent central maser cluster. The opening angle of the outflow is estimated to be ~ 30{\circ}. The dynamical time-scale of the outflow is estimated to be ~ 100 years. Our results provide strong support that HW3d harbors an internal massive young star, and the observed outflow could be tracing a very early phase of star formation. We also have analyzed Very Large Array (VLA) archive data of 1.3 cm continuum emission obtained in 1995 and 2006 toward Cepheus A. The comparative result of the HW3d continuum emission suggests the possibility of the existence of distinct young stellar objects (YSOs) in HW3d and/or strong variability in one of their radio continuum emission components.

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arXiv2012

DNC/HNC Ratio of Massive Clumps in Early Evolutionary Stages of High-Mass Star Formation

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 is indeed lower toward the observed high-mass sources (0.009\pm0.005) than toward the low-mass starless and star-forming cores (0.06). The kinetic temperature derived from the NH3 (J, K) = (1, 1) and (2, 2) line intensities is higher toward the observed high-mass sources than toward the low-mass cores. However the DNC/HNC ratio of some IRDCs involving the Spitzer 24 μm sources is found to be lower than that of HMPOs, although the kinetic temperature of the IRDCs is lower than that of the HMPOs. This implies that the DNC/HNC ratio does not depend only on the current kinetic temperature. With the aid of chemical model simulations, we discuss how the DNC/HNC ratio decreases after the birth of protostars. We suggest that the DNC/HNC ratio in star-forming cores depends on the physical conditions and history in their starless-core phase, such as its duration time and the gas kinetic temperature.

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arXiv2011

H2O maser motions and the distance of the star forming region G192.16-3.84

We present the results of astrometic observations of H2O masers associated with the star forming region G192.16-3.84 with the VLBI Exploration of Radio Astrometry (VERA). The H2O masers seem to be associated with two young stellar objects (YSOs) separated by \sim1200 AU as reported in previous observations. In the present observations, we successfully detected an annual parallax of 0.66 \pm 0.04 mas for the H2 O masers, which corresponds to a distance to G192.16-3.84 of D = 1.52 \pm 0.08 kpc from the Sun. The determined distance is shorter than the estimated kinematic distance. Using the annual parallax distance and the estimated parameters of the millimeter continuum emission, we estimate the mass of the disk plus circumstellar cloud in the southern young stellar object to be 10.0+4.3M\cdot. We also estimate the galactocentric distance and the peculiar motion -3.6 of G192.16-3.84, relative to a circular Galactic rotation: R\star = 9.99 \pm 0.08 kpc, Z\star = -0.10 \pm 0.01 kpc, and (U\star,V\star,W\star)=(-2.8\pm1.0,-10.5\pm0.3,4.9\pm2.7)[kms-1]respectively. The peculiar motion of G192.16-3.84 is within that typically found in recent VLBI astrometric results. The angular distribution and three-dimensional velocity field of H2O maser features associated with the northern YSO indicate the existence of a bipolar outflow with a major axis along the northeast-southwest direction.

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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

Detection of Two Carbon-Chain-Rich Cores; CB130-3 and L673-SMM4

We have found two dense cores, CB130-3 and L673-SMM4, where the carbon-chain molecules are extremely abundant relative to NH$_{3}$, during a survey observation of radio emission lines of CCS, HC$_{3}$N, HC$_{5}$N, and NH$_{3}$ toward dark cloud cores. Judging from the low NH$_{3}$/CCS ratios, they are possible candidates for "Carbon-Chain--Producing Regions (CCPRs)" recognized as chemically young dark cloud cores. The deuterium fractionation ratios DNC/HN$^{13}$C in CB130-3 and L673-SMM4 are found to be 1.28$^{+0.27}_{-0.05}$ and 1.96$^{+0.32}_{-0.01}$, respectively, which are comparable to or slightly higher than those in CCPRs found previously. We suggest that the dense cores of CB130-3 and L673-SMM4 are analogous to CCPRs but their chemical evolutionary phase would be slightly older than those of the dense cores in the Taurus region.

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arXiv2011

Annual Parallax Measurements of an Infrared Dark Cloud MSXDC G034.43+00.24 with VERA

We have measured the annual parallax of the H2O maser source associated with an infrared dark cloud MSXDC G034.43+00.24 from the observations with VERA (VLBI Exploration of Radio Astrometry). The parallax is 0.643 +/- 0.049 mas, corresponding to the distance of 1.56 +0.12/-0.11 kpc. This value is less than the half of the previous kinematic distance of 3.7 kpc. We revise the core mass estimates of MSXDC G034.43+00.24, based on virial masses, LTE masses and dust masses and show that the core masses decrease from the previous estimations of ~1000 Mo to hundreds of Mo. The spectral type derived from the luminosity also changes from O9.5 to B1 in the case of MM1. This spectral type is still consistent with that of the massive star. The radial velocity derived from the flat rotation model is smaller than the observed velocity, which corresponds to the peculiar motion of ~40 km/s in the line-of-sight direction.

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arXiv2011

Astrometry of Star Forming Region IRAS 05137+3919 in the far outer Galaxy

We present the results of astrometric observations with VERA toward the H2O maser sources in IRAS 05137+3919, which is thought to be located in the far outer Galaxy. We have derived the parallax of π= 0.086 +/- 0.027 mas, which corresponds to the source distance of D=11.6+5.3-2.8 kpc. Although the parallax measurement is only 3-sigma level and thus the distance uncertainty is considerably large, we can strongly constrain the minimum distance to this source, locating the source at the distance from the Sun greater than 8.3 kpc (or 16.7 kpc from the Galaxy's center) at 90% confidence level. Our results provide an astrometric confirmation that this source is located in the far outer Galaxy beyond 15 kpc from the Galaxy center, indicating that IRAS 05137+3919 is one of the most distant star-forming regions from the Galaxy center.

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