Tomoya Hirota

Astrometry of H2O Masers in Nearby Star-Forming Regions with VERA II SVS 13 in NGC 1333

We report on the results of multi-epoch VLBI observations with VERA (VLBI Exploration of Radio Astrometry) of the 22 GHz H2O masers associated with the young stellar object SVS 13 in the NGC 1333 region. We have carried out phase-referencing VLBI astrometry and measured an annual parallax of the maser features in SVS 13 of 4.25+/-0.32 mas, corresponding to the distance of 235+/-18 pc from the Sun. Our result is consistent with a photometric distance of 220 pc previously reported. Even though the maser features were detectable only for 6 months, the present results provide the distance to NGC 1333 with much higher accuracy than photometric methods. The absolute positions and proper motions have been derived, revealing that the H2O masers with the LSR (local standard of rest) velocities of 7-8 km s-1 are most likely associated with VLA4A, which is a radio counterpart of SVS 13. The origin of the observed proper motions of the maser features are currently difficult to attribute to either the jet or the rotating circumstellar disk associated with VLA4A, which should be investigated through future high-resolution astrometric observations of VLA4A and other radio sources in NGC 1333.

Abundant Carbon-Chain Molecules toward the Low-Mass Protostar IRAS 04368+2557 in L1527

We have detected the high-excitation lines of carbon-chain molecules such as C4H2 (J ¼ 100;10Y90;9), C4 H( N ¼ 9Y8, F1, F2), l-C3H2 (41,3Y31,2), and CH3CCH (J ¼ 5Y4, K ¼ 2) toward a low-mass star-forming region, L1527. In particular, the F1 line of C4 Hi s as strong as 1.7 K( TMB). The rotational temperature of C4H2 is determined to be 12:3 � 0:8 K, which is higher than that in TMC-1 (3.8 K). Furthermore, the column density of C4H2 is derived to be about 1/4 of that in TMC-1, indicating that carbon-chain molecules are abundant in L1527 for a star-forming region. Small mapping observations show that the C4H, C4H2, and c-C3H2 emissions are distributed from the infalling envelope to the inner part. Furthermore, we have detected the lines of C5H, HC7N, and HC9N in the 20 GHz region. Sincethecarbon-chainmolecules aregenerallydeficientinstar-forming cores, theaboveresultscannot simply beexplained by the existing chemical models. The following hypothesis is proposed. If the timescale of the prestellar collapse in L1527 were shorter than those of the other star-forming cores, the carbon-chain molecules could survive in thecentralpartofthecore.Inaddition,regenerationprocessesofthecarbon-chainmoleculesduetostarformationactivities would play an important role. Evaporation of CH4 from the grain mantles would drive the regeneration processes.Thepresentobservationsshow newchemistryinawarmanddenseregionneartheprotostars,whichisnamed ‘‘warm carbon-chain chemistry (WCCC).’’ Subject headingg ISM: abundances — ISM: individual (L1527) — ISM: molecules — stars: formation

Distance to Orion KL Measured with VERA

We present the initial results of multiepoch VLBI observations of 22 GHz H2O masers in the Orion KL region with VERA (VLBI Exploration of Radio Astrometry). With the VERA dual-beam receiving system, we carried out phase-referencing VLBI astrometry, and successfully detected the annual parallax of Orion KL to be 2.29 ˙ 0.10 mas, corresponding to a distance of 437 ˙ 19 pc from the Sun. The distance to Orion KL was determined for the first time with the trigonometric parallax method in these observations. Although this value is consistent with that previously reported, 480 ˙ 80 pc, which was estimated from a statistical parallax method using the proper motions and radial velocities of the H2O maser features, our new results provide a much more accurate value with an uncertainty of only 4%. In addition to the annual parallax, we detected an absolute proper motion of the maser feature, suggesting an outflow motion powered by the radio source I along with the systematic motion of source I itself.

Astrometry of Galactic Star-Forming Region Sharpless 269 with VERA: Parallax Measurements and Constraint on Outer Rotation Curve

We have performed high-precision astrometry of H2O maser sources in the Galactic star-forming region Sharpless 269 (S269) with VERA. We successfully detected a trigonometric parallax of 189 ˙ 8� as, corresponding to a source distance of 5:28 +0:24 � 0:22 kpc. This is the smallest parallax ever measured, and the first one detected beyond 5 kpc. The source distance as well as the proper motions were used to constrain the outer rotation curve of the Galaxy, demonstrating that the difference of rotation velocities at the Sun and at S269 (which is 13.1 kpc away from the Galaxy’s center) is less than 3%. This gives the strongest constraint on the flatness of the outer rotation curve, and provides a direct confirmation of the existence of a large amount of dark matter in the Galaxy’s outer disk.

Change in the chemical composition of infalling gas forming a disk around a protostar.

IRAS 04368+2557 is a solar-type (low-mass) protostar embedded in a protostellar core (L1527) in the Taurus molecular cloud, which is only 140 parsecs away from Earth, making it the closest large star-forming region. The protostellar envelope has a flattened shape with a diameter of a thousand astronomical units (1 au is the distance from Earth to the Sun), and is infalling and rotating. It also has a protostellar disk with a radius of 90 au (ref. 6), from which a planetary system is expected to form. The interstellar gas, mainly consisting of hydrogen molecules, undergoes a change in density of about three orders of magnitude as it collapses from the envelope into the disk, while being heated from 10 kelvin to over 100 kelvin in the mid-plane, but it has hitherto not been possible to explore changes in chemical composition associated with this collapse. Here we report that the unsaturated hydrocarbon molecule cyclic-C3H2 resides in the infalling rotating envelope, whereas sulphur monoxide (SO) is enhanced in the transition zone at the radius of the centrifugal barrier (100 ± 20 au), which is the radius at which the kinetic energy of the infalling gas is converted to rotational energy. Such a drastic change in chemistry at the centrifugal barrier was not anticipated, but is probably caused by the discontinuous infalling motion at the centrifugal barrier and local heating processes there.

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 R0, the angular rotation velocity at the LSR Ω0, the mean peculiar motion of the sources with respect to Galactic rotation (Usrc, Vsrc, Wsrc), the rotation-curve shape index, and the V component of the Solar peculiar motions, Vˇ. Based on a Markov chain Monte-Carlo method, we find that the Galaxy center distance is constrained at a 5% level to be R0 = 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 Usrc and Wsrc are fairly small compared to the Galactic rotation velocity, being Usrc = 1.0 ˙ 1.5 km s � 1 and Wsrc = � 1.4 ˙ 1.2 km s � 1 . Also, the rotation curve shape is found to be basically flat between Galacto-centric radii of 4 and 13 kpc. On the other hand,

DISCOVERY OF THE SECOND WARM CARBON-CHAIN-CHEMISTRY SOURCE, IRAS15398 – 3359 IN LUPUS

We have conducted a search for carbon-chain molecules toward 16 protostars with the Mopra 22 m and Nobeyama 45 m telescopes, and have detected high excitation lines from several species, such as C4H (N = 9-8), C4H2(J = 100,10-90,9), CH3CCH(J = 5-4, K = 2), and HC5N(J = 32-31), toward the low-mass protostar, IRAS15398 – 3359 in Lupus. The C4H line is as bright as 2.4 K measured with the Nobeyama 45 m telescope. The kinetic temperature is derived to be 12.6 ± 1.5 K from the K = 1 and K = 2 lines of CH3CCH. These results indicate that the carbon-chain molecules exist in a region of warm and dense gas near the protostar. The observed features are similar to those found toward IRAS04368+2557 in L1527, which shows warm carbon-chain chemistry (WCCC). In WCCC, carbon-chain molecules are produced efficiently by the evaporation of CH4 from the grain mantles in a lukewarm region near the protostar. Our data clearly indicate that WCCC is no longer specific to L1527, but occurs in IRAS15398 – 3359. In addition, we draw attention to a remarkable contrast between WCCC and hot corino chemistry in low-mass star-forming regions. Carbon-chain molecules are deficient in hot corino sources like NGC1333 IRAS4B, whereas complex organic molecules seem to be less abundant in the WCCC sources. A possible origin for such source-to-source chemical variations is suggested to arise from the timescale of the starless-core phase in each source. If this is the case, the chemical composition provides an important clue to explore the variation of star formation processes between sources and/or molecular clouds.

A Molecular Line Observation toward Massive Clumps Associated with Infrared Dark Clouds

We have surveyed the N2H+ -->J = 1?0, HC3N -->J = 5?4, CCS -->JN = 43?32, NH3 (J, K) = (1, 1), (2, 2), (3, 3), and CH3OH -->J = 7?6 lines toward the 55 massive clumps associated with infrared dark clouds by using the Nobeyama Radio Observatory 45 m telescope and the Atacama Submillimeter Telescope Experiment 10 m telescope. The N2H+, HC3N, and NH3 lines are detected toward most of the objects. On the other hand, the CCS emission is detected toward none of the objects. The [CCS]/[N2H+] ratios are found to be mostly lower than unity even in the Spitzer 24 ?m dark objects. This suggests that most of the massive clumps are chemically more evolved than the low-mass starless cores. The CH3OH emission is detected toward 18 out of 55 objects. All the CH3OH-detected objects are associated with the Spitzer 24 ?m sources, suggesting that star formation has already started in all the CH3OH-detected objects. The velocity widths of the CH3OH -->JK = 70?60 -->A+ and -->7?1?6?1 E lines are broader than those of N2H+ -->J = 1?0. The CH3OH -->JK = 70?60 -->A+ and -->7?1?6?1 E lines tend to have broader line width in the MSX dark objects than in the others, the former being younger or less luminous than the latter. The origin of the broad emission is discussed in terms of the interaction between an outflow and an ambient cloud.

Astrometry of H

We have carried out multi-epoch VLBI observations with VERA (VLBI Exploration of Radio Astrometry) of the 22~GHz H

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