Kengo Tachihara

A complete search for dense cloud cores in Taurus

We present the results of an H13CO+ J = 1-0 survey for dense molecular condensations in Taurus. The observations were carried out with the 45 m telescope at Nobeyama on the basis of an extensive C18O survey made with the 4 m telescope at Nagoya University. The present survey is based purely on molecular line observations and thus provides the most extensive and uniform sample of high-density condensations. We detected 55 H13CO+ condensations, of which 44 are starless. These starless condensations are compact (R 0.1 pc) and of high density (105 cm-3) and thus are highly probable candidates for protostellar condensations just before star formation. The striking feature of these starless compact condensations is the steep slope of the mass spectrum (dN/dM ~ M-2.5 for 3.5 M? < M < 20.1 M?). Under the assumption of uniform star formation efficiency, the spectrum resembles the stellar initial mass function (IMF) for a mass of a few solar masses. This suggests that the stellar IMF is the result of the fragmentation process from relatively lower density cores. The statistical analysis of the starless condensations yields the lifetime of the condensations of ~4 ? 105 yr, which is several times longer than free-fall timescale. This short timescale compared with magnetic flux loss time may indicate that the timescale is determined by the dissipation timescale of turbulence.

Unveiling the gas-and-dust disk structure in HD 163296 using ALMA observations

Aims. The aim of this work is to study the structure of the protoplan etary disk surrounding the Herbig Ae star HD 163296. Methods. We have used high-resolution and high-sensitivity ALMA observations of the CO(3‐2) emission line and the continuum at 850µm, as well as the 3- dimensional radiative transfer code MCFOST to model the data presented in this work. Results. The CO(3‐2) emission unveils for the first time at sub-millim eter frequencies the vertical structure details of a gaseou s disk in Keplerian rotation, showing the back- and the front-side of a flared disk. Continuum emission at 850 µm reveals a compact dust disk with a 240 AU outer radius and a surface brightness profil e that shows a very steep decline at radius larger than 125 AU. The gaseous disk is more than two times larger than the dust disk, with a similar critical radius but with a shallower radial pr ofile. Radiative transfer models of the continuum data confirms the need for a s harp outer edge to the dust disk. The models for the CO(3‐2) channel map require the disk to be slightly more geometrically thick than previous models suggested, and that the temperature at which CO gas becomes depleted (frozen-out) from the outer regions of the disk midplane is T < 20 K, in agreement with previous studies.

C18O Observations of the Dense Cloud Cores and Star Formation in Ophiuchus

In order to reveal the distribution of the dense gas of ≥104 cm3, C18O (J = 1-0) observations have been made toward the molecular clouds in the Ophiuchus region of ~6.4 deg2 with the two 4 m telescopes of Nagoya University. Forty dense cores have been identified, providing the first complete sample of such dense cores in Ophiuchus. The C18O dense cores are distributed not only in the active star-forming region, ρ Oph cloud core, but also in the North region where star formation is less active. The typical core mass, MLTE, radius, R, and average number density, n(H2), of the cores are 90 M☉, 0.24 pc, and 1.7 × 104 cm-3 in the ρ Oph region, respectively, and 14 M☉, 0.19 pc, and 7.6 × 103 cm-3 in the North region, respectively. Nine of the 40 cores are associated with young stellar objects, and most of the C18O cores are starless. An analysis of the physical parameters of the C18O cores show that star-forming cores tend to have larger N(H2) than the rest by a factor of ~3, although there is no significant trend in the other physical parameters between star forming and starless cores. We have compared the present C18O data with the 13CO data (Nozawa et al.) and with the associated YSOs, in order to understand better the condensing process from molecular gas with density of ~103 cm-3 to protostars. It is found that 55% of the 13CO cores are associated with C18O cores and that the C18O cores are typically less massive, smaller and denser by ~34%, ~32%, and a factor of ~3, respectively, than the 13CO cores. It is also found that the C18O cores have steeper density profiles than the 13CO cores; when we fit the density profile by a power law as ρ ∝ r-β, the values of β for C18O and 13CO are estimated as ~1.5 and 1.2, respectively. This suggests that the C18O cores are gravitationally more relaxed than the 13CO cores. In order to investigate the energetics of the cores, the virial mass, MVIR, has been calculated for each core. It is found that most of the 13CO cores have MVIR larger than MLTE. On the other hand, 22 of the 40 C18O cores have MVIR smaller than MLTE, suggesting that the C18O cores are more deeply gravitationally bound than the 13CO cores. Further, we have found a correlation between the ratio MVIR/MLTE and star formation activity: (1) For 13CO cores, the fraction of the 13CO cores associated with the C18O cores tends to increase with decreasing MVIR/MLTE, and (2) for the C18O cores, the fraction of the C18O cores associated with stars tends to increase with decreasing of MVIR/MLTE. We interpret this to indicate that the gradual dissipation of the internal turbulence leads to formation of denser cores and subsequent star formation. Through the evolution from the 13CO cores to the C18O cores, they should lose the turbulence energy of ~1044 ergs. The supersonic gas motion with the magnetic fields produces shocks, and the radiation from the small shocked region may significantly contribute to the cooling. We suggest that the cores have continuous collisions between turbulent eddies to produce the C-shocks. Also, the Alfvenic energy loss may be viable as the dissipation mechanism.

CLOUD–CLOUD COLLISION AS A TRIGGER OF THE HIGH-MASS STAR FORMATION: A MOLECULAR LINE STUDY IN RCW 120

RCW120 is a Galactic HII region having a beautiful ring shape bright in infrared. Our new CO J=1-0 and J=3-2 observations performed with the NANTEN2, Mopra, and ASTE telescopes have revealed that two molecular clouds with a velocity separation of 20km/s are both physically associated with RCW120. The cloud at -8km/s apparently traces the infrared ring, while the other cloud at -28km/s is distributed just outside the opening of the infrared ring, interacting with the HII region as supported by high kinetic temperature of the molecular gas and by the complementary distribution with the ionized gas. A spherically expanding shell driven by the HII region is usually discussed as the origin of the observed ring structure in RCW120. Our observations, however, indicate no evidence of the expanding motion in the velocity space, being inconsistent with the expanding shell model. We here postulate an alternative that, by applying the model introduced by Habe & Ohta (1992), the exciting O star in RCW120 was formed by a collision between the present two clouds at a colliding velocity ~30km/s. In the model, the observed infrared ring can be interpreted as the cavity created in the larger cloud by the collision, whose inner surface is illuminated by the strong UV radiation after the birth of the O star. We discuss that the present cloud-cloud collision scenario explains the observed signatures of RCW120, i.e., its ring morphology, coexistence of the two clouds and their large velocity separation, and absence of the expanding motion.

The First ALMA view of IRAS 16293-2422: Direct detection of infall onto source B and high-resolution kinematics of source A

Aims: In this paper, we focus on the kinematical properties of a proto-binaryto study the infall and rotation of gas towards its two protostellarcomponents. Methods: We present ALMA Science Verification observations withhigh-spectral resolution of IRAS 16293-2422 at 220.2 GHz. The wealth ofmolecular lines in this source and the very high spectral resolution offered byALMA allow us to study the gas kinematics with unprecedented detail. Results:We present the first detection of an inverse P-Cygni profile towards source Bin the three brightest lines. The line profiles are fitted with a simpletwo-layer model to derive an infall rate of 4.5x10^-5 Msun/yr. This infalldetection would rule-out the previously suggested possibility of source B beinga T Tauri star. A position velocity diagram for source A shows evidence forrotation with an axis close to the line-of-sight.

OPTICALLY THICK H I DOMINANT IN THE LOCAL INTERSTELLAR MEDIUM: AN ALTERNATIVE INTERPRETATION TO “DARK GAS”*

Dark gas in the interstellar medium (ISM) is believed to not be detectable either in CO or H I radio emission, but it is detectable by other means including γ rays, dust emission, and extinction traced outside the Galactic plane at |b| > 5°. In these analyses, the 21 cm H I emission is usually assumed to be completely optically thin. We have reanalyzed the H I emission from the whole sky at |b| > 15° by considering temperature stratification in the ISM inferred from the Planck/IRAS analysis of the dust properties. The results indicate that the H I emission is saturated with an optical depth ranging from 0.5 to 3 for 85% of the local H I gas. This optically thick H I is characterized by spin temperature in the range 10 K-60 K, significantly lower than previously postulated in the literature, whereas such low temperature is consistent with emission/absorption measurements of the cool H I toward radio continuum sources. The distribution and the column density of the H I are consistent with those of the dark gas suggested by γ rays, and it is possible that the dark gas in the Galaxy is dominated by optically thick cold H I gas. This result implies that the average density of H I is 2-2.5 times higher than that derived on the optically thin assumption in the local ISM.

Clustering of Pre-Main-Sequence Stars in the Orion, Ophiuchus, Chamaeleon, Vela, and Lupus Star-forming Regions

We study clustering of pre-main-sequence stars in the Orion, Ophiuchus, Chamaeleon, Vela, and Lupus star-forming regions. We calculate the average surface density of companions, ?(?), as a function of angular distance, ?, from each star. We employ the method developed by Larson in a 1995 study for the calculation. In most of the regions studied, the function can be fitted by two power laws (? ??) with a break as found by Larson for the Taurus star-forming region. The power index, ?, is smaller at small separations than at large separations. The power index at large separations shows significant variation from region to region (-0.8 < ? < -0.1), while the power index at small separations does not (? ~ -2). The power index at large separations relates to the distribution of the nearest-neighbor distance. When the latter can be fitted by the Poisson distribution, the power index is close to 0. When the latter is broader than the Poisson distribution, the power index is negatively large. This correlation can be interpreted as the result of the variation in the surface density within the region. At large separations, the power-law fit may indicate star formation history in the region and not the spatial structure like the self-similar hierarchical, or fractal, one. Because of the velocity dispersion, stars move from their birthplaces, and the surface density of coeval stars decreases with their age. When a star-forming region contains several groups of stars with different ages, a power law may fit the average surface density of companions for it. The break of the power law is located around 0.01-0.1 pc. There is a clear correlation between the break position and the mean nearest-neighbor distance. The break position may reflect dispersal of newly formed stars.

The TWO MOLECULAR CLOUDS IN RCW 38: EVIDENCE FOR THE FORMATION OF THE YOUNGEST SUPER STAR CLUSTER IN THE MILKY WAY TRIGGERED BY CLOUD–CLOUD COLLISION

We present distributions of two molecular clouds having velocities of 2 km s

Transit timing variation and activity in the WASP-10 planetary system★

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H I, CO, and Planck/IRAS dust properties in the high latitude cloud complex, MBM 53, 54, 55 and HLCG 92 – 35. Possible evidence for an optically thick H I envelope around the CO clouds

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