Minho Choi

Extremely high velocity outflows

Extremely high velocity (EHV) wings, with full widths of 72 to 140 km/s, are seen on the CO J=3-2 lines toward W3 IRS 5, GL 490, NGC 2071, W28 A2, GL 2591, S140, and Cepheus A. The results of our survey suggest that EHV wings are common around infrared sources of moderate to high luminosity (500 to 4x10^5 Lsun) in dense regions. Line ratios imply that the EHV gas is usually optically thin and warm. Characteristic velocities range from 20 to 40 km/s, yielding timescales of 1600-4200 yr. Since most sources in this study are producing some ionizing photons, these short timescales suggest that neutral winds coexist with ionizing photons. We examined two possible sources for the EHV CO emission: a neutral stellar wind; and swept-up or entrained molecular gas. Neither can be ruled out. If the high-velocity (HV) gas is swept up by a momentum-conserving stellar wind traced by the extremely high velocity CO emission, most of the C in the winds from luminous objects cannot be in CO. If the EHV and HV forces are equal, the fraction of C in a form other than CO increases with source luminosity and with the production rate of ionizing photons.

B335: A LABORATORY FOR ASTROCHEMISTRY IN A COLLAPSING CLOUD

We present observations of 25 transitions of 17 isotopologues of nine molecules toward B335. With a goal of constraining chemical models of collapsing clouds, we compare our observations, along with data from the literature, to models of chemical abundances. The observed lines are simulated with a Monte Carlo code, which uses various physical models of density and velocity as a function of radius. The dust temperature as a function of radius is calculated self-consistently by a radiative transfer code. The gas temperature is then calculated at each radius, including gas-dust collisions, cosmic rays, photoelectric heating, and molecular cooling. The results provide the input to the Monte Carlo code. We consider both ad hoc step function models for chemical abundances and abundances taken from self-consistent modeling of the evolution of a star-forming core. The step function models can match the observed lines reasonably well, but they require very unlikely combinations of radial variations in chemical abundances. Among the self-consistent chemical models, the observed lines are matched best by models with somewhat enhanced cosmic-ray ionization rates and sulfur abundances. We discuss briefly the steps needed to close the loop on the modeling of dust and gas, including off-center spectra of molecular lines.

The Structure of the Monoceros R2 Molecular Cloud Core

We present the results of molecular line observations of Mon R2. Maps in lines tracing high density show an arclike structure surrounding the H II region. Maps of temperature deduced from H2CO line ratios reveal a hot spot with TK > 100 K coincident with a submillimeter continuum peak. Maps of density deduced from CS line ratios indicate that the density peaks near IRS 1 and declines away from it. The spherically averaged data can be modeled equally well with a power law (with an exponent of 0.8-0.9) or a Gaussian with an FWHM of 1.0 pc. In either case, densities in the central region are around 106 cm-3. The derived density gradient suggests that the thermal pressure is not the only source of cloud support, and the relation between the density gradient and the cloud mass suggested from similar studies of other regions also holds in the Mon R2 cloud core.

KINEMATICS OF THE AMMONIA DISK AROUND THE PROTOSTAR NGC 1333 IRAS 4A2

The NGC 1333 IRAS 4A protobinary was observed in the ammonia (2, 2) and (3, 3) lines with an angular resolution of 0.3 arcsec. The ammonia emission source of IRAS 4A2 is elongated in the direction perpendicular to the bipolar jet and has a size of 0.55 arcsec or 130 AU. This emission structure was interpreted as a circumstellar disk around the IRAS 4A2 protostar, and the rotation kinematics of the disk was investigated by making a position-velocity diagram along the major axis. Assuming a Keplerian rotation, the disk has a rotation velocity of 1.8 km s{sup -1} at a radius of 20 AU, which implies a central object of about 0.08 solar masses. The collapse age of the protostar is about 50,000 yr. The mass, accretion rate, and age are consistent with what are expected from the standard theory of low-mass star formation. If IRAS 4A2 grows at this rate, it may become a star similar to the Sun.

Structure of the Dense Molecular Gas in the NGC 1333 IRAS 4 Region

The NGC 1333 IRAS 4 region was observed in the HCN and HCO+ J = 1 → 0 lines using a single-dish telescope and in the 2.1 mm continuum and the H2CO J = 212 → 111 line using an interferometer. The single-dish maps show that there are at least two velocity components in emission: one at VLSR = 6.7 km s-1 associated with the IRAS 4 core, and the other at ~8 km s-1 associated with a cloud extended from the SVS 13 complex. In addition, there is a foreground cold layer at ~8 km s-1 that causes absorption over most of the mapped area. The cloud structure suggests that the blue-skewed line profile of IRAS 4A/B may not be a sign of protostellar collapse. Examinations of both single-dish and interferometric maps suggest that the dip previously seen in the interferometric spectra toward IRAS 4A/B may be caused mostly by the large-scale foreground layer and partly by missing short-spacing flux. Absorption by an infalling envelope with an unusual velocity profile cannot be ruled out. The HCO+ map revealed other molecular cores, one associated with SK 1, and the other with SK 10/14. They are probable sites of star formation.

Detection of infall in the protostar B335 with ALMA

Observations of the isolated globule B335 with ALMA have yielded absorption features against the continuum that are redshifted from the systemic velocity in both HCN and HCO

A CO LINE AND INFRARED CONTINUUM STUDY OF THE ACTIVE STAR-FORMING COMPLEX W51

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L1448-MM OBSERVATIONS BY THE HERSCHEL KEY PROGRAM, ''DUST, ICE, AND GAS IN TIME'' (DIGIT)

lines. These features provide unambiguous evidence for infall toward a central luminosity source. Previously developed models of inside-out collapse can match the observed line profiles of HCN and HCO

MAGNETIC FIELD STRUCTURE OF THE HH 1-2 REGION: NEAR-INFRARED POLARIMETRY OF POINT-LIKE SOURCES

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Numerical Simulations of a Protostellar Outflow Colliding with a Dense Molecular Cloud

averaged over the central 50 AU. At the new distance of 100 pc, the inferred infall radius is 0.012 pc, the mass infall rate is