Nagayoshi Ohashi

We present the first interferometric CO(J=3-2) observations (beam size of 3.9"x1.6" or 160pc x 65pc) with the Submillimeter Array (SMA) toward the center of the Seyfert 2 galaxy M51. The image shows a strong concentration at the nucleus and weak emission from the spiral arm to the northwest. The integrated intensity of the central component in CO(J=3-2) is almost twice as high as that in CO(J=1-0), indicating that the molecular gas within an ~80 pc radius of the nucleus is warm (>~100 K) and dense (~10^4 cm^-3). Similar intensity ratios are seen in shocked regions in our Galaxy, suggesting that these gas properties may be related to AGN or starburst activity. The central component shows a linear velocity gradient (~1.4 km/s/pc) perpendicular to the radio continuum jet, similar to that seen in previous observations and interpreted as a circumnuclear molecular disk/torus around the Seyfert 2 nucleus. In addition, we identify a linear velocity gradient (~0.7 km/s/pc) along the jet. Judging from the energetics, the velocity gradient can be explained by supernova explosions or energy and momentum transfer from the jet to the molecular gas via interaction, which is consistent with the high intensity ratio.

ABSTRACT We present combined SubMillimeter Array (SMA) + Atacama Submillimeter Telescope Experiment (ASTE) images of the Class I protobinary L1551 IRS 5 in the CS ($J$ = 7--6) line, the submillimeter images of L1551 IRS 5 with the most complete spatial sampling ever achieved ($0''.9$ -- $36''$). The SMA image of L1551 IRS 5 in the 343 GHz dust-continuum emission is also presented, which shows an elongated feature along the northwest to southeast direction ($\sim$160 AU $\times$ 80 AU), perpendicular to the associated radio jets. The combined SMA+ASTE images show that the high-velocity ($\gtrsim$1.5 km s$^{-1}$) CS emission traces the structure of the dust component and shows a velocity gradient along the major axis, which is reproduced by a geometrically-thin Keplerian-disk model with a central stellar mass of $\sim$0.5 $M_{\odot}$. The low-velocity ($\lesssim$1.3 km s$^{-1}$) CS emission shows an extended ($\sim$1000 AU) feature that exhibits slight south (blueshifted) to north (redshifted) emission offsets, which is modeled with a rotating and infalling envelope with a conserved angular momentum. The rotational motion of the envelope connects smoothly to the inner Keplerian rotation at a radius of $\sim$64 AU. The infalling velocity of the envelope is $\sim$three times lower than the free-fall velocity toward the central stellar mass of 0.5 $M_{\odot}$. These results demonstrate transition from the infalling envelope to the Keplerian disk, consistent with the latest theoretical studies of disk formation. We suggest that sizable ($r\sim$50--200 AU) Keplerian disks are already formed when the protostars are still deeply embedded in the envelopes.

We present high angular resolution (1"; 200 AU) observations from a large program studying low-mass protostars with the Submillimeter Array. Here we focus on the object, NGC 1333-IRAS2: continuum emission at submillimeter wavelengths resolve its circumstellar disk with a size of about 300 AU. High excitation lines of a number of molecular species, i.p., complex organic molecules including CH3OH and

Observational evidence for circumstellar disks surrounding high mass (M > 8 M_sun) stars are currently limited to the deeply embedded, protostellar phase, where it is not clear if the central object is a single protostar, a multiple system or an unresolved star cluster. One of the ways to overcome the ambiguity about the central source is to search for the presence of disk around massive young stars at a less embedded, more evolved phase. We are carrying out an interferometric study of circumstellar disks surrounding optically visible, young high mass ( M = 8 18 M_sun) stars, which exhibit strong evidence for the presence of accreting disks associated with them. Here we present Sub-Millimeter Array (SMA) observations at 230 GHz of the circumstellar environment of a B1.5 star MWC 297, which, at a distance of 250 pc, is one of the closest massive (M=12 M_sun) star associated with a star forming cloud. Investigation of the structure and properties of circumstellar disks in these rel...

ABSTRACT We present SMA interferometric observations of the 12CO J=2-1, 13CO (J=2-1), and C18O (J=2-1) lines and 225 GHz continuum emission and SMT single-dish observations of C18O (J=2-1) toward L43, a protostellar object in transition from Class I to II. The 225 GHz continuum emission shows a weak (~23.6 mJy), compact (<1000 AU) component associated with the central protostar. Our simulated observations show that it can be explained by dust thermal emission arising from an envelope which has a hole or a constant intensity region within a few hundred AU of the protostar. This suggests the disappearance or a lower concentration distribution of the envelope on a small scale. The 12CO and 13CO emission exhibit molecular outflows to the south and north. The C18O emission shows two molecular blobs, which correspond to the reflection nebulosity seen in near-infrared images, while there is no C18O emission associated with the protostar. The near-infrared features are likely due to the scattering at the positions of the blobs. The visible scattering features should result from the optical thinness of the envelope material, which is consistent with the less-concentrated distribution in the continuum emission. From single-dish observations, we found that the mass of the envelope (~1.5 Msun) + protostar (~0.5 Msun) is comparable with the virial mass of M_vir=1.0 Msun within 40 arcsec. This suggests that the envelope is likely gravitationally bound. We suggest that the protostellar envelope of L43 has been disappearing by consumption through accretion, at least in the close vicinity of the protostar.

Millimeter-wave continuum sources in the NGC 7538 region were observed with the NRO 45-m telescope and Nobeyama Millimeter Array (NMA). NRO 45-m telescope observations showed that the compact region which includes IRS1, 2, and 3 has a strong millimeter-wave intensity excess. This region was also studied with NMA at 49 and 98 GHz, and the obtained millimeter-wave spectrum was analyzed. It has been proposed from this analysis that new compact quasi-spherical and homogeneous H II sources may exist in the IRS1 region, rather than the VLA ultra-compact H II regions. It is suggested that the new sources have a small linear size, of the order of 10 exp 15 cm, and a high electron density of about 10 exp 7/cu cm; they are still optically thick, even in the 100 GHz range. This gives an H II evolution time as short as about 100 yr. These small, but intense, H II emission sources in IRS1 may well be identified by cocoon stars.

In Japan, there is a plan for Large Millimeter Array proposed by Nobeyama Radio Observatory. The proposed array may consist of 50 10-m antennas distributed in a 2-3 km area at very high altitude. In this paper, scientific directions, planned system, and site survey for the LMA are reported

Studying young stellar objects with extremely low luminosity (Very Low Luminosity Objects, VeLLOs) provide us great opportunity for investigating the initial conditions of star formation and the formation of brown dwarfs. We study the dynamical and chemical properties of a VeLLO - DCE 065 using Submillimeter Array (SMA). The most promising result from DCE 065 is that both CO and

The direct imaging of circumstellar disks in the optical and near infrared is challenging, due to both the small angular size and the brightness of the central star. The combination of a large telescope, adaptive optics and coronography now makes direct study possible. I will present results from a NIR coronographic study of circumstellar disks and low mass stellar companions with the Subaru telescope.

ABSTRACT We report interferometric observations of two embedded protostar candidates, IRAS 04169]2702 and IRAS 04365]2535 (TMC-1A), in the Taurus molecular cloud. The C18O J \ 1»0 emission from IRAS 04169]2702 reveals a —attened envelope 2200 AU ] 1100 AU in ...

ABSTRACT FN Tau is a rare example of very low-mass T Tauri stars that exhibits a spatially resolved nebulosity in near-infrared scattering light. To directly derive the parameters of a circumstellar disk around FN Tau, observations of dust continuum emission at 340 GHz are carried out with the Submillimeter Array (SMA). A point-like dust continuum emission was detected with a synthesized beam of ~ 0.7" in FWHM. From the analysis of the visibility plot, the radius of the emission is estimated to be <= 0.29", corresponding to 41 AU. This is much smaller than the radius of the nebulosity, 1.85" for its brighter part at 1.6 micron. The 340 GHz continuum emission observed with the SMA and the photometric data at lambda <= 70 micron are explained by a power-law disk model whose outer radius and mass are 41 AU and (0.24 - 5.9) x 10^{-3} M_{sun}, respectively, if the exponent of dust mass opacity (beta) is assumed to be 0-2. The disk model cannot fully reproduce the flux density at 230 GHz obtained with the IRAM 30-meter telescope, suggesting that there is another extended "halo" component that is missed in the SMA observations. By requiring the halo not to be detected with the SMA, the lower limit to the size of the halo is evaluated to be between 174 AU and 574 AU, depending on the assumed beta value. The physical nature of the halo is unclear, but it may be the periphery of a flared circumstellar disk that is not described well in terms of a power-law disk model, or a remnant of a protostellar envelope having flattened structure. Comment: 21 pages with 5 figures; Accepted for publication in The Astrophysical Journal

... Shigehisa Takakuwa, 1, 2 Nagayoshi Ohashi, 3 Tyler L. Bourke, 4 Naomi Hirano, 3 Paul TP Ho, 3, 4 Jes K. Jørgensen, 4 Yi-Jehng Kuan, 3, 5 David J. Wilner, 4 and Sherry CC Yeh 3 Received 2006 April 11; accepted 2007 February 6 ... If we assume RÃ ¼ 4 R (Stahler et al. ...

ABSTRACT We report interferometric observations of the VeLLO L1521F-IRS, in 12CO (2-1) line emission and the 1.3 mm continuum emission, using the Submillimeter Array (SMA). With the 12CO high-resolution observations, we have spatially resolved a compact but poorly collimated molecular outflow associated with L1521F-IRS for the first time. The blueshifted and redshifted lobes are aligned along the east and west side of L1521F-IRS with a lobe size of ~1000 AU. The estimated outflow mass, maximum outflow velocity, and outflow force are (9.0-80)x10^{-4} M_{sol}, 7.2 km s^{-1}, and (7.4-66)x10^{-7} M_{sol} km s^{-1} yr^{-1}, respectively. The estimated outflow parameters such as size, mass, and momentum rate are similar to values derived for other VeLLOs, and are located at the lower end of values compared to previously studied outflows associated with low- to high-mass star forming regions. Low-velocity less collimated (1.5 km s^{-1}/1200 AU) and higher-velocity compact (4.0 km s^{-1}/920 AU) outflow components are suggested by the data. These velocity structures are not consistent with those expected in the jet driven or wind driven outflow models, perhaps suggesting a remnant outflow from the FHSC as well as an undeveloped outflow from the protostar. Detection of an infrared source and compact millimeter continuum emission suggest the presence of the protostar, while its low bolometric luminosity (0.034-0.07 L$_{\odot}$), and small outflow, suggests that L1521F is in the earliest protostellar stage ($<10^{4}$ yr) and contains a substellar mass object. The bolometric (or internal) luminosity of L1521F-IRS suggests that the current mass accretion rate is an order-of-magnitude lower than expected in the standard mass accretion model (~10^{-6} M_{sol} yr^{-1}), which may imply that L1521F-IRS is currently in a low activity phase.