Faculty Profiles - TAKAKUWA Shigehisa

Papers - TAKAKUWA Shigehisa

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Lim, J., Yeung, P. K. H., Hanawa, T., Takakuwa, S., et al. . Rotationally driven Fragmentation in the Formation of the Binary Protostellar System L1551 IRS 5 . THE ASTROPHYSICAL JOURNAL826 ( 153 ) 20pp 2016.8Reviewed

Language：English Publishing type：Research paper (scientific journal) Publisher：The American Astronomical Society

Both bulk rotation and local turbulence have been widely suggested to drive the fragmentation in collapsing cores that produces multiple star systems. Even when the two mechanisms predict different alignments for stellar spins and orbits, subsequent internal or external interactions can drive multiple systems toward or away from alignment, thus masking their formation processes. Here, we demonstrate that the geometrical and dynamical relationship between a binary system and its surrounding bulk envelope provide the crucial distinction between fragmentation models. We find that the circumstellar disks of the binary protostellar system L1551 IRS 5 are closely parallel, not just with each other but also with their surrounding flattened envelope. Measurements of the relative proper motion of the binary components spanning nearly 30 years indicate an orbital motion related to that of the envelope rotation. Eliminating orbital solutions whereby the circumstellar disks would be tidally truncated to sizes smaller than observed, the remaining solutions favor a circular or low-eccentricity orbit tilted by up to ˜25° from the circumstellar disks. Turbulence-driven fragmentation can generate local angular momentum to produce a coplanar binary system, but this would have no particular relationship to the system’s surrounding envelope. Instead, the observed properties conform with predictions for rotationally driven fragmentation. If the fragments were produced at different heights or on opposite sides of the mid-plane in the flattened central region of a rotating core, the resulting protostars would then exhibit circumstellar disks parallel with the surrounding envelope but tilted from the orbital plane, as is observed.

DOI： 10.3847/0004-637X/826/2/153

arXiv

Hsi-Wei Yen, Hauyu Baobab Liu, Pin-Gao Gu, Naomi Hirano, Chin-Fei Lee, Evaria Puspitaningrum, & Shigehisa Takakuwa . Gas Gaps in the Protoplanetary Disk around the Young Protostar HL Tau . THE ASTROPHYSICAL JOURNAL LETTERS820 ( L25 ) 7pp 2016.4Reviewed

Language：English Publishing type：Research paper (scientific journal) Publisher：The American Astronomical Society

We have analyzed the HCO+ (1-0) data of the Class I-II protostar, HL Tau, obtained from the Atacama Large Millimeter/submillimeter Array long baseline campaign. We generated the HCO+ image cube at an angular resolution of ˜0.″07 (˜10 au) and performed azimuthal averaging on the image cube to enhance the signal-to-noise ratio and measure the radial profile of the HCO+ integrated intensity. Two gaps at radii of ˜28 and ˜69 au and a central cavity are identified in the radial intensity profile. The inner HCO+ gap is coincident with the millimeter continuum gap at a radius of 32 au. The outer HCO+ gap is located at the millimeter continuum bright ring at a radius of 69 au and overlaps with the two millimeter continuum gaps at radii of 64 and 74 au. On the contrary, the presence of the central cavity is likely due to the high optical depth of the 3 mm continuum emission and not the depletion of the HCO+ gas. We derived the HCO+ column density profile from its intensity profile. From the column density profile, the FWHM widths of the inner and outer HCO+ gaps are both estimated to be ˜14 au, and their depths are estimated to be ˜2.4 and ˜5.0. These results are consistent with the expectation from the gaps opened by forming (sub-)Jovian mass planets, while placing tight constraints on the theoretical models solely incorporating the variation of dust properties and grain sizes.

DOI： 10.3847/2041-8205/820/2/L25

arXiv

Hiroyuki HIRASHITA, Patrick M. KOCH, Satoki MATSUSHITA, Shigehisa TAKAKUWA, Masanori NAKAMURA, Keiichi ASADA, Hauyu Baobab LIU, Yuji URATA, Ming-Jye WANG, Wei-Hao WANG, Satoko TAKAHASHI, Ya-Wen TANG, Hsian-Hong CHANG, Kuiyun HUANG, Oscar MORATA, Masaaki OTSUKA, Kai-Yang LIN, An-Li TSAI, Yen-Ting LIN, Sundar SRINIVASAN, Pierre MARTIN -COCHER, Hung-Yi PU, Francisca KEMPER, Nimesh PATEL, Paul GRIMES, Yau-De HUANG, Chih-Chiang HAN, Yen-Ru HUANG, Hiroaki NISHIOKA, Lupin Chun-Che LIN, Qizhou ZHANG, Eric KETO, Roberto BURGOS, Ming-Tang CHEN, Makoto I NOUE, & Paul T. P. HO . First-generation science cases for ground-based terahertz telescopes . Publications of the Astronomical Society of Japan68 ( 1 ) 41pp 2016.2Reviewed

Language：English Publishing type：Research paper (scientific journal) Publisher：日本天文学会

Ground-based observations at terahertz (THz) frequencies are a newly explorable area of astronomy in the coming decades. We discuss science cases for a first-generation 10-m class THz telescope, focusing on the Greenland Telescope as an example of such a facility. We propose science cases and provide quantitative estimates for each case. The largest advantage of ground-based THz telescopes is their higher angular resolution (˜ 4″ for a 10-m dish), as compared to space or airborne THz telescopes. Thus, high-resolution mapping is an important scientific argument. In particular, we can isolate zones of interest for Galactic and extragalactic star-forming regions. The THz windows are suitable for observations of high-excitation CO lines and [N II] 205-μm lines, which are scientifically relevant tracers of star formation and stellar feedback. Those lines are the brightest lines in the THz windows, so they are suitable for the initiation of ground-based THz observations. THz polarization of star-forming regions can also be explored since it traces the dust population contributing to the THz spectral peak. For survey-type observations, we focus on "sub-THz" extragalactic surveys, the uniqueness of which is detecting galaxies at redshifts z ˜ 1-2, where the dust emission per comoving volume is the largest in the history of the Universe. Finally we explore possibilities of flexible time scheduling, which enables us to monitor active galactic nuclei, and to target gamma-ray burst afterglows. For these objects, THz and submillimeter wavelength ranges have not yet been explored.

DOI： 10.1093/pasj/psv115

arXiv

Shigehisa Takakuwa, Kazuhiro Kiyokane, Kazuya Saigo, & Masao Saito . Dispersing Envelope around the Keplerian Circumbinary Disk in L1551 NE and Its Implications for Binary Growth . The Astrophysical Journal814 ( 160 ) 18pp 2015.12Reviewed

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：The American Astronomical Society

We performed mapping observations of the Class I protostellar binary system L1551 NE in the C18O (J = 3-2), 13CO (J = 3-2), CS (J = 7-6), and SO (JN = 78-67) lines with the Atacama Submillimeter Telescope Experiment (ASTE). The ASTE C18O data were combined with our previous SMA C18O data, which show a r ˜ 300 AU scale Keplerian disk around the protostellar binary system. The C18O maps show a ˜20,000 AU scale protostellar envelope surrounding the central Keplerian circumbinary disk. The envelope exhibits a northeast (blue) to southwest (red) velocity gradient along the minor axis, which can be interpreted as a dispersing gas motion with an outward velocity of 0.3 km s-1, while no rotational motion in the envelope is seen. In addition to the envelope, two ≲4000 AU scale, high-velocity (≳1.3 km s-1) redshifted 13CO and CS emission components are found ˜40″ southwest and ˜20″ west of the protostellar binary. These redshifted components are most likely outflow components driven from the neighboring protostellar source L1551 IRS 5, and are colliding with the envelope in L1551 NE. The net momentum, kinetic, and internal energies of the L1551 IRS 5 outflow components are comparable to those of the L1551 NE envelope, and the interactions between the outflows and the envelope are likely to cause the dissipation of the envelope and thus suppression of further growth of the mass and mass ratio of the central protostellar binary in L1551 NE.

DOI： 10.1088/0004-637X/814/2/160

arXiv

Shigehisa Takakuwa, Masao Saito, Kazuya Saigo, Tomoaki Matsumoto, Jeremy Lim, Tomoyuki Hanawa, & Paul T. P. Ho . Angular Momentum Exchange by Gravitational Torques and Infall in the Circumbinary Disk of the Protostellar System L1551 NE . The Astrophysical Journal796 ( 1 ) 17pp 2014.11Reviewed

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：The American Astronomical Society

We report an ALMA observation of the Class I binary protostellar system L1551 NE in the 0.9 mm continuum, C18O (3-2), and 13CO (3-2) lines at a ~1.6 times higher resolution and a ~6 times higher sensitivity than those of our previous SubMillimeter Array (SMA) observations, which revealed a r ~ 300 AU scale circumbinary disk in Keplerian rotation. The 0.9 mm continuum shows two opposing U-shaped brightenings in the circumbinary disk and exhibits a depression between the circumbinary disk and the circumstellar disk of the primary protostar. The molecular lines trace non-axisymmetric deviations from Keplerian rotation in the circumbinary disk at higher velocities relative to the systemic velocity, where our previous SMA observations could not detect the lines. In addition, we detect inward motion along the minor axis of the circumbinary disk. To explain the newly observed features, we performed a numerical simulation of gas orbits in a Roche potential tailored to the inferred properties of L1551 NE. The observed U-shaped dust features coincide with locations where gravitational torques from the central binary system are predicted to impart angular momentum to the circumbinary disk, producing shocks and hence density enhancements seen as a pair of spiral arms. The observed inward gas motion coincides with locations where angular momentum is predicted to be lowered by the gravitational torques. The good agreement between our observation and model indicates that gravitational torques from the binary stars constitute the primary driver for exchanging angular momentum so as to permit infall through the circumbinary disk of L1551 NE.

DOI： 10.1088/0004-637X/796/1/1

arXiv

Takakuwa, S., Saito, M., Lim, J., & Saigo, K. . Evidence for Infalling Gas of Low Angular Momentum toward the L1551 NE Keplerian Circumbinary Disk . The Astrophysical Journal776 ( 51 ) 11pp 2013.10Reviewed

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：American Astronomical Society

We report follow-up C18O(3-2) line observations of the Class I binary protostellar system L1551 NE with the Submillimeter Array in its compact and subcompact configurations. Our previous observations at a higher angular resolution in the extended configuration revealed a circumbinary disk exhibiting Keplerian motion. The combined data, with more extensive spatial coverage (~140-2000 AU), verify the presence of a Keplerian circumbinary disk and reveal for the first time a distinct low-velocity (lsim ± 0.5 km s-1 from the systemic velocity) component that displays a velocity gradient along the minor axis of the circumbinary disk. Our simple model that reproduces the main features seen in the position-velocity diagrams comprises a circumbinary disk exhibiting Keplerian motion out to a radius of ~300 AU, beyond which the gas exhibits pure infall at a constant velocity of ~0.6 km s-1. This velocity is significantly smaller than the expected free-fall velocity of ~2.2 km s-1 onto the L1551 NE protostellar mass of ~0.8 M ⊙ at ~300 AU, suggesting that the infalling gas is decelerated as it moves into regions of high gas pressure in the circumbinary disk. The discontinuity in angular momenta between the outer infalling gas and the inner Keplerian circumbinary disk implies an abrupt transition in the effectiveness at which magnetic braking is able to transfer angular momentum outward, a result perhaps of the different plasma β values and the ionization fractions between the outer and inner regions of the circumbinary disk.

DOI： 10.1088/0004-637X/776/1/51

arXiv

Takakuwa, S., Saito, M., Lim, J., Saigo, K., Sridharan, T. K., & Patel, N. A. . A Keplerian Circumbinary Disk around the Protostellar System L1551 NE . The Astrophysical Journal754 ( 52 ) 12pp 2012.7Reviewed

Language：English Publishing type：Research paper (scientific journal) Publisher：American Astronomical Society

We present Submillimeter Array observations of a Keplerian disk around the Class I protobinary system L1551 NE in 335 GHz continuum emission and submillimeter line emission in 13CO (J = 3-2) and C18O (J = 3-2) at a resolution of ~120 × 80 AU. The 335 GHz dust-continuum image shows a strong central peak closely coincident with the binary protostars and likely corresponding to circumstellar disks, surrounded by a ~600 × 300 AU feature elongated approximately perpendicular to the [Fe II] jet from the southern protostellar component suggestive of a circumbinary disk. The 13CO and C18O images confirm that the circumbinary continuum feature is indeed a rotating disk; furthermore, the C18O channel maps can be well modeled by a geometrically thin disk exhibiting Keplerian rotation. We estimate a mass for the circumbinary disk of ~0.03-0.12 M ⊙, compared with an enclosed mass of ~0.8 M ⊙ that is dominated by the protobinary system. Compared with several other Class I protostars known to exhibit Keplerian disks, L1551 NE has the lowest bolometric temperature (~91 K), highest envelope mass (~0.39 M ⊙), and the lowest ratio in stellar mass to envelope + disk + stellar mass (~0.65). L1551 NE may therefore be the youngest protostellar object so far found to exhibit a Keplerian disk. Our observations present firm evidence that Keplerian disks around binary protostellar systems, "Keplerian circumbinary disks," can exist. We speculate that tidal effects from binary companions could transport angular momenta toward the inner edge of the circumbinary disk and create the Keplerian circumbinary disk.

DOI： 10.1088/0004-637X/754/1/52

arXiv

Takakuwa, S., & Kamazaki, T. . Skewed Distributions and Opposite Velocity Gradients of Submillimeter Molecular Lines in Low-Mass Protostellar Envelopes . Publications of the Astronomical Society of Japan63 ( 4 ) 921 - 939 2011.8Reviewed

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：日本天文学会

We have made mapping observations of L 1551 IRS 5, L 1551 NE, L 723, and L 43 and single-point observations of IRAS 16293-2422 in the submillimeter CS (J ="> 7-6) and HCN (J = ;4-3) lines with Atacama Submillimeter Telescope Experiment (ASTE). Using the present data and our previous ASTE observations of L 483 and B 335 together, we found a definite correlation between source bolometric luminosities and total integrated intensities of the submillimeter lines (ICS ∝ Lbol0.92). The combined ASTE + Submillimeter Array CS (7-6) image of L 1551 IRS 5 exhibits an extended (˜2000 AU) component tracing the associated reflection nebula at west and southwest, as well as a compact (≲ 500 AU) component centered on the protostellar position. The peaks of the CS and HCN emissions in L 1551 NE are not located at the protostellar position but offset (˜1400 AU) toward the associated reflection nebula at west. By statistical analyses, we confirmed the velocity gradients of the CS (7-6) emission that are opposite to those of the millimeter lines along the outflow direction, which we reported in our early paper. The magnitudes of the submillimeter velocity gradients are estimated to be (9.7±1.7) × 10-3 km s-1 arcsec-1 in L 1551 IRS 5 and (7.6±2.4) × 10-3 km s-1 arcsec-1 in L 483. We suggest that the ``skewed' submillimeter molecular emissions toward the associated reflection nebulae at a few thousand AU scale trace the warm (≳ 40 K) walls of the envelope cavities, excavated by the associated outflows and directly irradiated by the central protostars. The opposite velocity gradients along the outflow direction likely reflect the dispersing gas motion at the wall of the cavity in envelopes perpendicular to the outflow.

DOI： 10.1093/pasj/63.4.921

arXiv

Takakuwa, S., Ohashi, N., & Aikawa, Y. . Carbon-chain and Organic Molecules Around Very Low Luminosity Protostellar Objects of L1521F-IRS and IRAM 04191+1522 . The Astrophysical Journal728 ( 101 ) 10pp 2011.2Reviewed

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：American Astronomical Society

We have observed dense gas around the Very Low Luminosity Objects (VeLLOs) L1521F-IRS and IRAM 04191+1522 in carbon-chain and organic molecular lines with the Nobeyama 45 m telescope. Toward L1521F-IRS, carbon-chain lines of CH3CCH (50-40), C4H ({17/2}-{15/2}), and C3H2 (212-101) are 1.5-3.5 times stronger than those toward IRAM 04191+1522, and the abundances of the carbon-chain molecules toward L1521F-IRS are 2-5 times higher than those toward IRAM 04191+1522. Mapping observations of these carbon-chain molecular lines show that in L1521F the peak positions of these carbon-chain molecular lines are different from each other and there is no emission peak toward the VeLLO position, while in IRAM 04191+1522 these carbon-chain lines are as weak as the detection limits, except for the C3H2 line. The observed chemical differentiation between L1521F and IRAM 04191+1522 suggests that the evolutionary stage of L1521F-IRS is younger than that of IRAM 04191+1522, consistent with the extent of the associated outflows seen in the 13CO (1-0) line. The non-detection of the organic molecular lines of CH3OH (6-2-7-1 E) and CH3CN (60-50) implies that the warm (~100 K) molecular-desorbing region heated by the central protostar is smaller than ~100 AU toward L1521F-IRS and IRAM 04191+1522, suggesting the young age of these VeLLOs. We propose that the chemical status of surrounding dense gas can be used to trace the evolutionary stages of VeLLOs.

DOI： 10.1088/0004-637X/728/2/101

arXiv

Takakuwa, S., Iono, D., Vila-Vilaro, B., Sekiguchi, T., & Kawabe, R. . Scientific role of ACA for low-mass star-formation study . Astrophysics and Space Science313 ( 1-3 ) 169 - 173 2008.1Reviewed

Language：English Publishing type：Research paper (international conference proceedings) Publisher：SPRINGER

We discuss the scientific role of the Atacama Compact Array (ACA), the Japanese contribution to the ALMA project, for low-mass star-formation study. Our recent observations of several low-mass protostellar envelopes in the submillimeter CS ( J=7 6) and HCN ( J=4 3) lines with the SMA and ASTE have revealed that these submillimeter emissions are more extended than ˜2000 AU and show different velocity structures from those traced by millimeter lines. These results suggest the importance of taking short-spacing informations the ACA can offer. Our comprehensive imaging simulations of these protostellar envelopes, as well as prestellar cores and debris disks, unprecedentedly demonstrate the scientific importance of ACA.

DOI： 10.1007/s10509-007-9617-2

Takakuwa, S., Ohashi, N., Bourke, T. L., et al. . Arcsecond-Resolution Submillimeter HCN Imaging of the Binary Protostar IRAS 16293-2422 . The Astrophysical Journal662 ( 1 ) 431 - 442 2007.6Reviewed

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：The American Astronomical Society

With the Submillimeter Array (SMA) we have made high angular resolution (~1''=160 AU) observations of the protobinary system IRAS 16293-2422 in the HCN (4-3), HC15N (4-3), and 354.5 GHz continuum emission. The HCN (4-3) line was also observed using the JCMT to supply missing short-spacing information. The submillimeter continuum emission is detected from the individual binary components of source A in the southeast and source B in the northwest, with a separation of ~5". The optically thin HC15N (4-3) emission taken with the SMA has revealed a compact (~500 AU) flattened structure (P.A.=-16deg) at source A. This compact structure shows a velocity gradient along the projected minor axis, which can be interpreted as an infalling gas motion. Our HCN image including the short-spacing information shows an extended (~3000 AU) circumbinary envelope, as well as the compact structure at source A. A toy model consisting of a flattened structure with radial infall toward a 1 Msolar central star reproduces the HCN/HC15N position-velocity diagram along the minor axis of the HC15N emission. In the extended envelope there is also a northeast (blue) to southwest (red) velocity gradient across the binary alignment, which is likely to reflect gas motion in the swept-up dense gas associated with the molecular outflow from source A. Only a weak and narrow (~2 km s-1) compact HC15N emission is associated with source B, where no clear molecular outflow is identified, suggesting the different evolutionary starges between sources A and B. Our study demonstrates the importance of adding short-spacing data to interferometer data in order to probe the detailed structure and kinematics of low-mass protostellar envelopes.

DOI： 10.1086/513589

arXiv

Takakuwa, S., Kamazaki, T., Saito, M., Yamaguchi, N., & Kohno, K. . ASTE Observations of Warm Gas in Low-Mass Protostellar Envelopes: Different Kinematics between Submillimeter and Millimeter Lines . Publications of the Astronomical Society of Japan59 ( 1 ) 1 - 13 2007.2Reviewed

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：日本天文学会

With the ASTE telescope, we have observed three low-mass protostellar envelopes around L483, B335, and L723 in the submillimeter CS (J = 7-6) and HCN (J = 4-3) lines. We detected both the CS and HCN lines toward all targets, and the typical CS intensity (˜ 1.0 K in TB) was twice higher than that of the HCN line. Mapping observations of L483 in these lines have shown that the submillimeter emissions are resolved, exhibit a western extension from the central protostar, and that the deconvolved size is ˜ 5500 AU × 3700 AU (PA = 78°) in the HCN emission. The extent of the submillimeter emission in L483 implies the presence of higher temperature (/gtrsim 40 K) gas at 4000 AU away from the central protostar, which suggests that we need to take 2-dimensional radiative transfer models with a bipolar cavity into account. The position-velocity diagrams of these submillimeter lines along the axis of the associated molecular outflow exhibit that the sense of the submillimeter velocity gradient is opposite to that of the millimeter observation or the associated molecular outflow, both in L483 and in B335. We suggest that expanding gas motions at the surface of the flattened envelope, which is irradiated by the central protostar directly, are the origin of the observed submillimeter velocity structure.

DOI： 10.1093/pasj/59.1.1

arXiv

Takakuwa, S., Ohashi, N., Ho, P. T. P., et al. . Submillimeter Array Observations of L1551 IRS 5 in CS J = 7-6 . The Astrophysical Journal Letters616 ( 1 ) L15 - L18 2004.11Reviewed

Language：English Publishing type：Research paper (scientific journal) Publisher：The American Astronomical Society

We have imaged the circumstellar envelope around the binary protostar L1551 IRS 5 in CS J=7-6 and 343 GHz continuum emission at ~3" resolution using the Submillimeter Array. The continuum emission shows an elongated structure (~220×100 AU) around the binary perpendicular to the axis of the associated radio jet. The CS emission extends over ~400 AU, appears approximately circularly symmetric, and shows a velocity gradient from southeast (blueshifted) to northwest (redshifted). The direction of the velocity gradient is different from that observed in C18O J=1-0. This may be because rotation is more dominant in the CS envelope than the C18O envelope, in which both infall and rotation exist. The CS emission can be divided into two velocity components: (1) a ``high'-velocity disklike structure surrounding the protostar +/-1.0-1.5 km s-1 from the systemic velocity, and (2) a ``low'-velocity structure located southwest of the protostar less than 1.0 km s-1 from the systemic velocity. The high-velocity component traces warm and dense gas with kinematics consistent with rotation around the protostar. The low-velocity component may arise from dense gas entrained in the outflow. Alternatively, this component may trace infalling and rotating gas in an envelope with a vertical structure.

DOI： 10.1086/421939

arXiv

Takakuwa, S., Ohashi, N., & Hirano, N. . Interaction between the Outflow and the Core in IRAM 04191+1522 . The Astrophysical Journal590 ( 2 ) 932 - 943 2003.6Reviewed

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：The American Astronomical Society

We have carried out mapping observations of the molecular core associated with the young Class 0 protostar, IRAM 04191+1522, in the CH3OH (JK=2K-1K) and C34S (J=2-1) lines, using the 45 m telescope at Nobeyama Radio Observatory. Our observations have revealed that there is a condensation associated with the protostar, elongated in the east-west direction mostly perpendicular to the axis of the associated CO outflow. Its size and mass are estimated to be 0.07×0.04 pc and 2.3 Msolar, respectively, from the CH3OH data. In addition to the elongated envelope, two compact (~0.03 pc) condensations were found in the CH3OH line at the southern edge of the elongated envelope, where the blueshifted CO outflow emerging from the protostar is located. In contrast to the elongated envelope, those compact CH3OH condensations show much larger line width (up to 2.0 km s-1), with centroid velocities blueshifted by ~0.8 km s-1. The compact condensations have momenta (~0.06 Msolar km s-1) comparable to that of the blueshifted molecular outflow. In addition, they are gravitationally unbound, and most probably will dissipate eventually. These results suggest that the compact condensations are probably formed in the course of interaction between the outflow and the ambient gas surrounding the protostar and that such interaction may cause dissipation of a part of the ambient gas. No drastic, localized enhancement of the CH3OH abundance is, however, observed toward the compact condensations, implying that there seems to be no significant shock heating at the compact condensations, in spite of the interaction with the outflow. This may be because the CO outflow velocity (<10 km s-1) is too low to cause effective heating to release CH3OH on dust grains into the gas phase.

DOI： 10.1086/375190

arXiv

Takakuwa, S., Kamazaki, T., Saito, M., & Hirano, N. . H13CO+ and CH3OH Line Observations of Prestellar Dense Cores in the TMC-1C Region. II. Internal Structure . The Astrophysical Journal584 ( 2 ) 818 - 831 2003.2Reviewed

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：American Astronomical Society

We have carried out detailed H13CO+ (J=1-0) and CH3OH (JK=20-10 A+) observations of two dense cores in the TMC-1C region with the 45 m telescope at Nobeyama Radio Observatory and the Nobeyama Millimeter Array (NMA). These two cores, H13CO+ core 2 and CH3OH core 6, were identified previously by the H13CO+ and CH3OH mapping observations of the entire TMC-1C region and show no sign of protostar formation. We found that the internal structure of H13CO+ core 2 is different from that of cores with class 0 protostars; this core consists of three velocity components and shows the radial column density distribution of ~r-0.43+/-0.08, which is flatter than that in the protostellar cores (~r-1.0). The internal structure of CH3OH core 6 is different from that of H13CO+ core 2; the combined images of the 45 m and NMA data have revealed that there are three smaller scale CH3OH ``clumps'' ~2500 AU in size inside this core. The mass of the small-scale clumps, ~0.02 Msolar, is much smaller than the virial mass (~0.3 Msolar), suggesting that these clumps are gravitationally unbound. The velocity dispersion observed in these small-scale clumps is 0.45-0.5 km s-1, which is comparable to that of clumps with 0.02-0.03 pc in radius. A comparison of the velocity dispersion of the small-scale clumps and that of larger scale cores suggests that the velocity dispersion that follows the power law in the radius larger than ~0.02-0.03 pc turns into a constant value of ~0.45-0.5 km s-1 in the smaller size scale. The mass-size relation of the prestellar gas structures suggests that they follow the trend of constant gas density (~105 cm-3) in the radius from 0.006 to ~0.3 pc and that the smaller scale structures tend to be gravitationally unbound. The presence of the small clumps with substellar masses in prestellar cores implies that coalescence processes are likely to play an important role in forming protostars inside the cores. The observational difference of the internal structures among CH3OH core 6, H13CO+ core 2, and protostellar cores might represent the difference of the evolutionary stages.

DOI： 10.1086/345845

Takakuwa, S., Kawaguchi, K., Mikami, H., & Saito, M. . The Ortho-to-Para Ratio and the Chemical Properties of C3 H2 in Dark Cloud Cores . Publications of the Astronomical Society of Japan53 ( 2 ) 251 - 257 2001.4Reviewed

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：日本天文学会

We have observed cyclic C3H2 (JKaKc = 212-101, 202 - 111, 312 - 303) lines at the 3 mm wavelength region toward the starless TMC-1C cloud and the protostellar core of L 1527 with the 45 m telescope at Nobeyama Radio Observatory. In both clouds, the 212-101 and 202 - 111 lines are strongly detected, while the 312 - 303 line is only detected toward the center of the protostellar core L 1527. From our statistical equilibrium analyses of the ortho-C3H2(212-101) and the para-C3H2(202-111)lines, the ortho-to-para ratios of C3H2 are determined to be 2.4 +/- 0.1 and 2.5 +/- 0.5 in TMC-1C and L 1527, respectively. These values are lower than the statistical value of 3, although the error of the ratio in L 1527 is large. Since thermal equilibrium with a dust temperature of 10 K still gives a ratio of 3 in c-C3H2, this result should suggest the lower ortho-to-para ratio of the precursor molecule, c-C3H+3, which is thought to be mainly produced by C3H++H2. Thus, we conclude that the lower ortho-to-para ratio of c-C3H2 is due to a lower ortho-to-para ratio of H2 than the statistical value.

DOI： 10.1093/pasj/53.2.251

Takakuwa, S., Mikami, H., Saito, M., & Hirano, N. . A Comparison of the Spatial Distribution of H13CO+, CH3OH, and C34S Emission and Its Implication in Heiles Cloud 2 . The Astrophysical Journal542 ( 1 ) 367 - 379 2000.10Reviewed

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：American Astronomical Society

We have mapped the Heiles cloud 2 region in the Taurus molecular cloud complex with H13CO+ (J=1-0), CH3OH (JK=20-10 A+), and C34S (J=2-1) lines. Dense gas traced by the mapped lines with critical densities higher than 104 cm-3 is concentrated in four condensations, that is, the TMC 1 and TMC 1C filaments, L1527, and TMC 1A. We have found that the three emission lines have remarkably different spatial distributions. The H13CO+ emission traces well dense cores harboring protostars, while the CH3OH emission is weak toward the protostars and is rather enhanced toward cores without protostars. We found that there are two starless cores with enhanced CH3OH emission at the northwestern ends of the TMC 1 and TMC 1C filaments, toward which the H13CO+ emission is barely seen. On the basis of the analyses using the large velocity gradient (LVG) model, we show that the CH3OH abundance relative to H13CO+ is enhanced by up to 1 order of magnitude in the cores without protostars. The C34S abundance relative to H13CO+ also shows a similar trend to that of CH3OH. Such an abundance variation between H13CO+ and CH3OH and C34S can be explained in the scheme of time-dependent gas-phase chemical evolution, which predicts that CH3OH and C34S are abundant in the early stages of chemical evolution and become deficient in the later stages. A comparison of the spatial-velocity structures in TMC 1 observed with the three molecular lines suggests that this cloud consists of multiple components with different velocities and different chemical compositions along the line of sight.

DOI： 10.1086/309505

Takakuwa, S., Mikami, H., & Saito, M. . H13CO+ and CH3OH Line Observations of Prestellar Dense Cores in the TMC-1C Region . The Astrophysical Journal501 ( 2 ) 723 - 730 1998.7Reviewed

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：American Astronomical Society

We have mapped the entire TMC-1C region in the H13CO+ (J = 1-0) and CH3OH (JK = 20-10A+) lines at a grid spacing of 50" with the 45 m telescope at Nobeyama Radio Observatory. We have also conducted high spatial resolution mapping observations of the TMC-1C region at a grid spacing of 34" in both lines toward a 6' × 6' portion in the south and a 4' × 4' portion in the north. We found that the structure of TMC-1C is filamentary in both molecular lines. The size and position angle of the filament are 0.75 pc × 0.17 pc and 135°, respectively. The filament consists of dense (~105 cm-3) cores which are traced by either H13CO+ or CH3OH lines. We found that the distribution of cores seen in H13CO+ is quite different from the distribution of cores seen in CH3OH. The large velocity gradient analyses indicate that this difference is a result of the relative abundance variation between H13CO+ and CH3OH in the cores by about 1 order of magnitude. We have also carried out multitransitional observations of C3H2 (JK',K'' = 21,2-10,1 and 31,2-30,3) at two positions in the same cloud in order to estimate the molecular hydrogen densities for H13CO+ and CH3OH cores and found that the densities are around 105 cm-3 for both cores. These starless cores (no IRAS source), considered to be prestellar cores, seem to be at chemically different evolutionary stages; the H13CO+ cores are more evolved and closer to protostar formation than CH3OH cores. On the other hand, we found no difference in physical properties, i.e., the size, line width, and mass, between H13CO+ and CH3OH cores; the averages are about 0.07 pc, 0.3 km s-1, and 2 M⊙, respectively.

DOI： 10.1086/305855

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