C. R. Masson

Properties of jet-driven molecular outflows

We present a critical review of the observed properties of molecular outflows from young stellar objects and show that these properties can be explained by a model in which the molecular flow is driven solely by a collimated jet. Many of the best studied outflows have several characteristics in common, including (1) bipolarity, (2) collimation, (3) partly empty lobes, (4) most material at low velocities, (5) extremely high velocity features, and (6) average momentum directed nearly parallel to the axis of the flow, with small transverse components. From consideration of the conservation of vector momentum, we deduce that energy-driven flows cannot produce the observed distribution of the momentum direction

Molecular line survey of Orion A from 215 to 247 GHz

Molecular line emission from the core of the Orion molecular cloud has been surveyed from 215 to 247 GHz to an average sensitivity of about 0.2 K. A total of 544 resolvable lines were detected, of which 517 are identified and attributed to 25 distinct chemical species. A large fraction of the lines are partially blended with other identified transitions. Because of the large line width in the Orion core, the spectrum is near the confusion limit for the weakest lines identified (≈ 0.2 K). The most abundant complex molecules present are HCOOCH_3, CH_30CH_3, and C_2H_5CN, with beam-averaged column densities of about 3X10^(15) cm^(-2). Together with the simpler species S0_2, CH_30H, and CH_3CN, they account for approximately 70% of the lines in the spectrum. Relatively few unidentified lines are present. There are 27 lines clearly present in the spectrum which are currently unidentified. However, many of these are thought to be high-lying transitions of complex asymmetric rotors such as CH_30H. Present spectroscopic data are inadequate to predict the frequencies of such transitions with sufficient accuracy.

THE ROTATIONAL EMISSION-LINE SPECTRUM OF ORION A BETWEEN 247 AND 263 GHZ

Results are presented from a molecular line survey of the core of the Orion molecular cloud between 247 and 263 GHz. The spectrum contains a total of 243 resolvable lines from 23 different chemical species. When combined with the earlier survey of Orion from 215 to 247 GHz by Sutton et al. (1985), the complete data set includes over 780 emission features from 29 distinct molecules. Of the 23 molecules detected in this survey, only NO, CCH, and HCO^+ were not identified in the lower frequency data. As a result of the supporting laboratory spectroscopy performed to supplement existing millimeter-wave spectral line catalogs, only 33 of the more than 780 lines remain unidentified, of which 16 occur in the upper frequency band. A significant chance remains that a number of these unidentified lines are due to transitions between states of either isotopically substituted or highly excited abundant and complex molecules such as CH_3OH, CH_3OCH_3, and HCOOCH_3, whose rotational spectra are poorly known at present. The very small percentage and weak strength of the unidentified lines implies that the dominant chemical constituents visible at millimeter wavelengths have been identified in the Orion molecular cloud.

Detection of a 45 AU radius source around L1551-IRS 5 : a possible accretion disk

Interferometric and single-dish observations of the continuum emission from L1551-IRS 5 were made at wavelengths from 1.0 mm to 6.2 cm. These observations show that, at millimeter wavelengths, there are two distinct components to the source, an envelope with a radius about 2000 AU, and a compact core with a radius not greater than 64 AU. In both components, the continuum radiation is due predominantly to thermal dust emission. The compact core has a large optical depth, indicating a high column density (about 1000 g/sq cm). By modeling the temperature in the region of the compact core, it is shown that its size must lie in the range 45 + or - 20 AU. The compact core is most plausibly identified with an accretion, or preplanetary, disk around the star, although the present observations do not have sufficient angular resolution to rule out other structures. 63 refs.

Momentum Transfer by Astrophysical Jets

We have used 3-D smoothed particle hydrodynamical simulations to study the basic properties of the outflow that is created by a protostellar jet in a dense molecular cloud. The dynamics of the jet/cloud interaction is strongly affected by the cooling in the shocked gas behind the bow shock at the head of the jet. We show that this cooling is very rapid, with the cooling distance of the gas much less than the jet radius. Thus, although ambient gas is initially driven away from the jet axis by the high thermal pressure odf the post-shock gas, rapid cooling reduces the pressure and the outflow subsequently evolves in a momentum-conserving snowplow fashion. The velocity of the ambient gas is high in the vicinity of the jet head, but decreases rapidly as more material is swept up. Thus, this type of outflow produces extremely high velocity clumps of post shock gas which resemble the features seen in outflows. We have investigated the transfer of momentum from the jet to the ambient medium as a function of the jet parameters. We show that a low Mach number (<6) jet slows down rapidly because it entrains ambient material along its sides. On the other hand, the beam of a high Mach number jet is separated from the ambient gas by a low density cocoon of post-shock gas, and this jet transfers momentum to the ambient medium principally at the bow-shock. In high Mach number jets, as those from young stellar objects, the dominant interaction is therefore at the bow shock at the head of the jet.

The structure of NGC 7027 and a determination of its distance by measurement of proper motions

Radio observations of the PN NGC 7027, obtained at 4.885 and 14.965 GHz (resolution 0.35 arcsec) using the B configuration of the VLA in 12-h runs during August 1982, May 1985, and September 1986, are reported. The data-reduction procedures are outlined, and the results are presented in tables, graphs, and contour maps. Extreme limb brightening is observed and attributed to inverse-square dilution of ionizing radiation from the central star. Comparison of the maps from three epochs reveals steady expansion and implies a distance of 880 + or - 150 pc. 22 references.

Properties of swept-up molecular outflows

Models of momentum-conserving, molecular outflows swept up by radially directed stellar winds from young stellar objects are analyzed, and the new constraint that the models should reproduce the observed spectral line shapes of outflows, which trace the amount of mass as a function of velocity, is introduced. The models can easily reproduce the spatial shape of a typical outflow and the appearance of the highest velocity at the edge of the flow, but they have difficulty in reproducing the observed spectral line shape

Molecular gas in the starburst nucleus of M82

High-resolution interferometric observations of the CO(1-0) emission from the central one arcmin region of the peculiar galaxy M82 are presented. The observations show that the molecular gas can be divided into two components: a high concentration in the central 700 pc x 200 pc and extended regions that may be shell-like and filamentary in structure. The properties of the interstellar medium in the nuclear region of M82 implied by these and other observations are discussed, and the implications of the observations for star formation in M82 and other galaxies are considered. 76 references.

CO J = 3-2 observations of the neutral disk in Sagittarius A West

Emission has been observed from the J = 3-2 rotational transition of CO over a substantial region in Sgr A West. The mapped region encompasses the dense rotating neutral disk, a structure which extends out to a radius of about 8 pc from the Galactic center. The observations show significant differences from previous CO J = 1-0 observations of Serabyn et al. (1986). In particular, the J = 3-2 emission is more centrally concentrated, and there are differences in the relative strengths of emission peaks in the two sets of maps. The derived picture is one of a disk with a significant excitation gradient, since the emission from high-excitation species declines as one moves out from the center. Typical values of the physical conditions present are density n = 20,000-200,000/cu cm and temperature T = 100-200 K. Antenna temperatures indicate optical depths greater than unity. A CO column density along a line of sight through a clump of roughly 2 x 10 to the 19th/sq cm is inferred. 35 refs.

The structures of and distances to BD +30 deg 3639 and NGC 6572

New high-resolution VLA maps of the dense planetary nebulae BD +30 deg 3639 and NGC 6572 are presented. A prolate-ellipsoidal model was used to account for the appearance of these nebulae in quantitative detail, along with a recently developed technique of measuring expansion motions with the VLA to determine the distances to these dense planetary nebulae. Both nebulae have significantly greater distances than previously expected: 2.8 + 47.0 or - 1.2 kpc is the estimated distance to BD +30 deg 3639, and 1.7 + 6.1 or - 0.8 kpc is the one to NGC 6572. The upward revision of the distance, has important implications for the state of the nebulae, reducing their calculated densities and increasing their luminosities. The discrepancy between the new distances for dense nebulae and those calculated from statistical assumptions casts some doubt on the use of statistical methods for such objects. Luminous, optically thick nebulae may have been systematically undercounted, thus affecting studies of mass return to the interstellar medium and of the evolution of central stars. 30 refs.