Tetsuhiro Minamidani

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 the Large Magellanic Cloud in 12CO ( -->J = 1?0) was carried out 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 -->MCO 2 ? 104 M?. 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, and virial mass, of the 164 GMCs that 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 -->Mvir LCO1.1 ? 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 XCO-factor of ~ -->7 ? 1020 cm?2 (K km s?1)?1. The mass spectrum of the clouds is fitted well by a power law of -->Ncloud(> MCO) MCO?0.75 ? 0.06 above the completeness limit of -->5 ? 104 M?. The slope of the mass spectrum becomes steeper if we fit only the massive clouds, e.g., -->Ncloud(> MCO) MCO?1.2 ? 0.2 for -->MCO ? 3 ? 105 M?.

THE SECOND SURVEY OF THE MOLECULAR CLOUDS IN THE LARGE MAGELLANIC CLOUD BY NANTEN. II. STAR FORMATION

We studied star formation activities in the molecular clouds in the Large Magellanic Cloud. We have utilized the second catalog of 272 molecular clouds obtained by NANTEN to compare the cloud distribution with signatures of massive star formation including stellar clusters, and optical and radio H II regions. We find that the molecular clouds are classified into three types according to the activities of massive star formation: Type I shows no signature of massive star formation; Type II is associated with relatively small H II region(s); and Type III with both H II region(s) and young stellar cluster(s). The radio continuum sources were used to confirm that Type I giant molecular clouds (GMCs) do not host optically hidden H II regions. These signatures of massive star formation show a good spatial correlation with the molecular clouds in the sense that they are located within ~100 pc of the molecular clouds. Among possible ideas to explain the GMC types, we favor that the types indicate an evolutionary sequence; i.e., the youngest phase is Type I, followed by Type II, and the last phase is Type III, where the most active star formation takes place leading to cloud dispersal. The number of the three types of GMCs should be proportional to the timescale of each evolutionary stage if a steady state of massive star and cluster formation is a good approximation. By adopting the timescale of the youngest stellar clusters, 10 Myr, we roughly estimate the timescales of Types I, II, and III to be 6 Myr, 13 Myr, and 7 Myr, respectively, corresponding to a lifetime of 20-30 Myr for the GMCs with a mass above the completeness limit, 5 × 104 M ☉.

Temperature and Density Distribution in the Molecular Gas Toward Westerlund 2: Further Evidence for Physical Association

Furukawa et al. reported the existence of a large mass of molecular gas associated with the super star cluster Westerlund 2 and the surrounding H II region RCW49, based on a strong morphological correspondence between NANTEN2 12CO(J = 2-1) emission and Spitzer IRAC images of the H II region. We here present temperature and density distributions in the associated molecular gas at ~3.5 pc resolution, as derived from a large velocity gradient analysis of the 12CO(J = 2-1), 12CO(J = 1-0), and 13CO(J = 2-1) transitions. The kinetic temperature is as high as ~60-150 K within a projected distance of ~5-10 pc from Westerlund 2 and decreases to as low as ~10 K away from the cluster. The high temperature provides robust verification that the molecular gas is indeed physically associated with the H II region, supporting Furukawa et al.s conclusion. The derived temperature is also roughly consistent with theoretical calculations of photodissociation regions (PDRs), while the low spatial resolution of the present study does not warrant a more detailed comparison with PDR models. We suggest that the molecular clouds presented here will serve as an ideal laboratory to test theories on PDRs in future higher resolution studies.

Detection of Molecular Clouds in the Magellanic Bridge: Candidate Star Formation Sites in a Nearby Low-Metallicity System

We present the results of a new, wide-field, and broad-ranging survey for 12CO (1-0) emission regions in the nearest and brightest tidal structure: the Magellanic Bridge. We have detected seven sites of 12CO (1-0) emission using the NANTEN telescope in addition to the one detected previously. The integrated CO brightness for these detections ranges between 30 and 140 mK km s-1, corresponding to an estimated molecular mass of (1-7) × 103 M☉ [assuming the CO-to-H2 conversion factor (X-factor) of ~1.4 × 1021 cm-2 (K km s-1)-1]. The positions of the CO emission regions are generally coincident with sites of bright 100 μm emission, where I100 μm > 2.6 MJy sr-1, and have narrow line widths of <~2 km s-1, indicating gas in a cold and rather quiescent state. The velocity centroids of the CO spectra are generally consistent with those of the H I spectra, and we suggest that CO clouds are formed after the tidal encounter, rather than being extracted from the SMC. This is supported by the small typical lifetime of CO clouds, which is as short as ~107 yr and much less than the estimated 200 Myr age of the Bridge itself.

Submillimeter Observations of Giant Molecular Clouds in the Large Magellanic Cloud: Temperature and Density as Determined from J=3-2 and J=1-0 transitions of CO

We have carried out submillimeter 12CO( -->J = 3?2) observations of six giant molecular clouds (GMCs) in the Large Magellanic Cloud (LMC) with the ASTE 10 m submillimeter telescope at a spatial resolution of 5 pc and very high sensitivity. We have identified 32 molecular clumps in the GMCs and revealed significant details of the warm and dense molecular gas with -->n(H2) ~ 103?105 cm?3 and -->Tkin ~ 60 K. These data are combined with 12CO( -->J = 1?0) and 13CO( -->J = 1?0) results and compared with LVG calculations. The results indicate that clumps that we detected are distributed continuously from cool (~10-30 K) to warm (30-200 K), and warm clumps are distributed from less dense (~103 cm?3) to dense (~103.5-105 cm?3). We found that the ratio of 12CO( -->J = 3?2) to 12CO( -->J = 1?0) emission is sensitive to and is well correlated with the local H? flux. We infer that differences of clump properties represent an evolutionary sequence of GMCs in terms of density increase leading to star formation. Type I and II GMCs (starless GMCs and GMCs with H?II regions only, respectively) are at the young phase of star formation where density does not yet become high enough to show active star formation, and Type III GMCs (GMCs with H?II regions and young star clusters) represent the later phase where the average density is increased and the GMCs are forming massive stars. The high kinetic temperature correlated with H? flux suggests that FUV heating is dominant in the molecular gas of the LMC.

Submillimeter line emission from LMC 30 Doradus: The impact of a starburst on a low-metallicity environment

Context. The 30 Dor region in the Large Magellanic Cloud (LMC) is the most vigorous star-forming region in the Local Group. Star formation in this region is taking place in low-metallicity molecular gas that is exposed to an extreme far-ultraviolet (FUV) radiation field powered by the massive compact star cluster R136. 30 Dor is therefore ideally suited to study the conditions in which stars formed at earlier cosmological times. Aims. Observations of (sub)mm and far-infrared (FIR) spectral lines of the main carbon-carrying species, CO, [Ci ]a nd [Cii], which originate in the surface layers of molecular clouds illuminated by the FUV radiation of young stars, can be used to constrain the physical and chemical state of the star-forming ISM. Methods. We used the NANTEN2 telescope to obtain high-angular resolution observations of the 12 CO J = 4 → 3, J = 7 → 6, and 13 CO J = 4 → 3 rotational lines and [Ci] 3 P 1− 3 P 0 and 3 P 2− 3 P 1 fine-structure submillimeter transitions in 30 Dor-10, the brightest CO and FIR-emitting cloud at the center of the 30 Dor region. We derived the physical and chemical properties of the low-metallicity molecular gas using an excitation/radiative transfer code and found a self-consistent solution of the chemistry and thermal balance of the gas in the framework of a clumpy cloud PDR model. We compared the derived properties with those in the N159W region, which is exposed to a more moderate far-ultraviolet radiation field compared with 30 Dor-10, but has similar metallicity. We also combined our CO detections with previously observed low-J CO transitions to derive the CO spectral-line energy distribution in 30 Dor-10 and N159W. Results. The separate excitation analysis of the submm CO lines and the neutral carbon fine structure lines shows that the mid-J CO and [Ci]-emitting gas in the 30 Dor-10 region has a temperature of about 160 K and a H2 density of about 10 4 cm −3 .W e fi nd that the molecular gas in 30 Dor-10 is warmer and has a lower beam filling factor compared to that of N159W, which might be a result of the effect of a strong FUV radiation field heating and disrupting the low-metallicity molecular gas. We use a clumpy PDR model (including the [Cii] line intensity reported in the literature) to constrain the FUV intensity to about χ0 ≈ 3100 and an average total H density of the clump ensemble of about 10 5 cm −3 in 30 Dor-10.

KINEMATIC STRUCTURE OF MOLECULAR GAS AROUND HIGH-MASS YSO, PAPILLON NEBULA, IN N159 EAST IN THE LARGE MAGELLANIC CLOUD: A NEW PERSPECTIVE WITH ALMA

We present the ALMA Band 3 and Band 6 results of 12CO(2-1), 13CO(2-1), H30α recombination line, free-free emission around 98 GHz, and the dust thermal emission around 230 GHz toward the N159 East Giant Molecular Cloud (N159E) in the Large Magellanic Cloud (LMC). LMC is the nearest active high-mass star-forming face-on galaxy at a distance of 50 kpc and is the best target for studing high-mass star formation. ALMA observations show that N159E is the complex of filamentary clouds with the width and length of ˜1 pc and several parsecs. The total molecular mass is 0.92 × 105 M⊙ from the 13CO(2-1) intensity. N159E harbors the well-known Papillon Nebula, a compact high-excitation H II region. We found that a YSO associated with the Papillon Nebula has the mass of 35 M⊙ and is located at the intersection of three filamentary clouds. It indicates that the formation of the high-mass YSO was induced by the collision of filamentary clouds. Fukui et al. reported a similar kinematic structure toward two YSOs in the N159 West region, which are the other YSOs that have the mass of ≳35 M⊙. This suggests that the collision of filamentary clouds is a primary mechanism of high-mass star formation. We found a small molecular hole around the YSO in Papillon Nebula with a sub-parsec scale. It is filled by free-free and H30α emission. The temperature of the molecular gas around the hole reaches ˜80 K. It indicates that this YSO has just started the distruction of parental molecular cloud.

A Peculiar Jet and Arc of Molecular Gas toward the Rich and Young Stellar Cluster Westerlund 2 and a TeV Gamma Ray Source

Fukui, Yasuo; Furukawa, Naoko; Dame, Thomas M.; Dawson, Joanne R.; Yamamoto, Hiroaki; Rowell, Gavin P.; Aharonian, Felix; Hofmann, Werner; de Ona Wilhelmi, Emma; Minamidani, Tetsuhiro; Kawamura, Akiko; Mizuno, Norikazu; Onishi, Toshikazu; Mizuno, Akira; Nagataki, Shigehiro

Microstructure and kinematics of H2O masers in the massive star-forming region IRAS 06061+2151

We have made multi-epoch very long baseline interferometer (VLBI) observations of H 2 O maser emission in the massive star-forming region IRAS 06061+2151 with the Japanese VLBI network (JVN) from 2005 May to 2007 October. The detected maser features are distributed within a 1 x 1 arcsec 2 area (2000 x 2000 au 2 at the source position) around the ultracompact H ii region seen in radio continuum emission. The bipolar morphology and expanding motion traced through their relative proper motions indicate that they are excited by an energetic bipolar outflow. Our three-dimensional model fitting has shown that the maser kinematical structure in IRAS 06061+2151 can be explained by a biconical outflow with a large opening angle (>50°). The position angle of the flow major axis coincides very well with that of the large-scale jet seen in 2.1 μm hydrogen emission. This maser geometry indicates the existence of dual structures composed of a collimated jet and a less collimated massive molecular flow. We have also detected a large velocity gradient in the southern maser group. This can be explained by a very small (on a scale of several tens of astronomical units) and clumpy (the density contrast by an order of magnitude or more) structure of the parental cloud. Such a structure may be formed by strong instability of the shock front or splitting of the high density core.

Enhancement of CO(3-2)/CO(1-0) ratios and star formation efficiencies in supergiant H II regions

We present evidence that super giant HII regions (GHRs) and other disk regions of the nearby spiral galaxy, M33, occupy distinct locations in the correlation between molecular gas,