T. L. Wilson

VLBI observations of the 6.7 GHz methanol masers toward W3(OH)

We have conducted very long baseline interferometric (VLBI) observations of the 6.668 GHz maser transition of interstellar methanol toward the ultracompact H II region W3(OH). We have determined absolute maser positions with an accuracy of 0″.05 and produced maps that show that the methanol masers have a distribution similar to the hydroxyl masers in this source. The intrinsic sizes of individual maser spots are ≃0″.0014 (FWHM), or ≃3 AU, and are not significantly affected by interstellar scattering

Water Masers in Orion

Measurements of the 616 → 523 line of H2O at 1.3 cm in the Orion region of star formation are reported. With a spatial resolution of ≈ 01, H2O maser emission was detected in two regions: Orion BN/KL and Orion S. The well-known masers in the BN/KL region are distributed in a 30 by 30 area. The shell masers, within the BN/KL region, are distributed in a 2 by 05 strip centered on radio source I and are offset from IRc2. The average shell maser spectrum is doubly peaked, resembling the spectrum of the v = 1 SiO masers. The shell masers have deconvolved sizes of 24-38 AU, slightly smaller than the synthesized beam. Newly detected H2O masers in the Orion S region are distributed in a 15 by 20 area. One cluster of masers in the Orion S region is found in a thin 06 strip. The velocity range of the masers in this cluster is nearly 65 km s-1. We suggest that this cluster of masers is associated with the energetic source of the Orion S molecular outflow. A search in the Orion S region for associated maser emission in the ground-state OH main lines and the 92 → 101A+ and 62 → 61E lines of CH3OH gave only upper limits, as did a search for centimeter-wavelength continuum. Near-infrared images of the Orion region are presented in the J, H, and K bands. Three objects with very red near-infrared colors were detected in the Orion S region, near the H2O masers and the previously detected millimeter-wavelength dust continuum peak. One of these objects exhibits near-infrared colors consistent with a B2 zero-age main-sequence star. Although this object may heat the northern part of the Orion S dust cloud, its luminosity and separation from the dust maximum make it unlikely that it alone heats the entire Orion S region. More likely, the primary heat source of the Orion S region is deeply embedded in dust and completely extincted in the near-infrared.

8. 7 GHz hyperfine line of /sup 3/He/sup +/ in Galactic H II regions

A search for the 8.7 GHz hyperfine line of /sup 3/He/sup +/ in galactic H II regions has yielded detections in three regions has yielded detections in three regions and significant upper limits for three others. We derive /sup 3/He/H abundance ratios of 4 x 10/sup -5/ in W43, 1 x 10/sup -4/ in W51, and 5 x 10/sup -4/ in W3. The upper limits found are 6 x 10/sup -5/ in Orion A, and 2 x 10/sup -5/ in W49 and M17S. These abundances have been corrected (by up to a factor of 2) for source structure using simple core-halo models. Our detections have line-to-continuum ratios in the range of 10/sup -3/ to 10/sup -4/. For lines this weak, systematic errors can be much larger than random noise, so we have simultaneously observed many weak recombination lines in order to estimate the magnitude of the systematic errors. We believe the /sup 3/He/H abundance for W43 of 4 x 10/sup -5/ is our most accurate measurement because it has the smallest structure corrections and the smallest systematic errors.

ISO observations of the Galactic center interstellar medium. Neutral gas and dust

The 500 central pc of the Galaxy (hereafter GC) exhibit a widespread gas component with a kinetic temperature of 100-200 K. The bulk of this gas is not associated to the well-known thermal radio continuum or far infrared sources like Sgr A or Sgr B. How this gas is heated has been a longstanding problem. With the aim of studying the thermal balance of the neutral gas and dust in the GC, we have observed 18 molecular clouds located at projected distances far from thermal continuum sources with the Infrared Space Observatory (ISO). In this paper we present observations of several fine structure lines ((Ouf769) 63 and 146 µm, (Cuf769uf769) 158 µm, (Si uf769uf769 )3 5µm, (S uf769 )2 5µ ma nd (Feuf769uf769 )2 6µm), which are the main coolants of the gas with kinetic temperatures of several hundred K. We also present the full continuum spectra of the dust between 40 and 190 µm. All the clouds exhibit a cold dust component with a temperature of ∼15 K. A warmer dust component is also required to fit the spectra. The temperature of this dust component changes between 27 and 42 K from source to source. We have compared the gas and the dust emission with the predictions from J-type and C-type shocks and photodissociation region (PDRs) models. We conclude that the dust and the fine structure lines observations are best explained by a PDR with a density of 10 3 cm −3 and an incident far-ultraviolet field 10 3 times higher than the local interstellar radiation field. The fine structure line emission arises in PDRs in the interface between a diffuse ionized gas component and the dense molecular clouds. The (Cuf769uf769) 158 µ ma nd (Siuf769uf769) 35 µm lines also have an important contribution from the ionized gas component. PDRs can naturally explain the discrepancy between the gas and the dust temperatures. However, these PDRs can only account for 10-30% of the total H2 column density with a temperature of ∼150 K. We discuss other possible heating mechanisms for the rest the warm molecular gas, such as non-stationary PDRs, X-ray Dominated Regions (XDRs) or the dissipation of supersonic turbulence. The central 600 pc of the Galaxy (hereafter the Galactic center, GC) contain up to 10% of the neutral gas of the Milky Way. In spite of the high gas surface density in the GC (several or- ders of magnitude higher than in the disk of the Galaxy) the star formation rate is 10 times lower than that in the disk and the star formation efficiency is only similar to that in the disk. This is probably due to the particular physical conditions of the � Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) and with the participation of ISAS and NASA. �� Figures 1-4 are only available in electronic form at

The 3He Abundance in Planetary Nebulae

We report measurements of the abundance of 3He for a sample of six Galactic planetary nebulae: IC 289, NGC 3242, NGC 6543, NGC 6720, NGC 7009, and NGC 7662. Based on observations of the 8.665 GHz hyperfine transition of 3He+, we derive 3He/H abundances ranging from ~0.1 to 1.0 × 10-3 by number. These abundances are more than an order of magnitude larger than those found in any H II region, the local interstellar medium, or the proto-solar system. If planetary nebulae are surrounded by large, low-density, ionized halos, modeling suggests that these abundances will decrease by a factor of about 2. Our source sample is highly biased in that we selected objects on the basis of several criteria that maximized the likelihood of 3He detections. The abundances are nonetheless consistent with the idea that 3He is produced in significant quantities by stars of 1-2 M☉. We conclude that there is some stellar production of 3He.

Coupling the dynamics and the molecular chemistry in the galactic center

The physical conditions of the Galactic center (GC) clouds moving with non-circular velocities are not well-known. We have studied the physical conditions of these clouds with the aim of better understanding the origin of the outstanding physical conditions of the GC molecular gas and the possible effect of the large scale dynamics on these physical conditions.Using published CO(1-0) data, we have selected a set of clouds belonging to all the kinematical components seen in the longitude-velocity diagram of the GC. We have done a survey of dense gas in all the components using the J=2-1 lines of CS and SiO as tracers of high density gas and shock chemistry. We have detected CS and SiO emission in all the kinematical components. The gas density and the SiO abundance of the clouds in non-circular orbits are similar those in the nuclear ring (GCR). Therefore, in all the kinematical components there are dense clouds that can withstand the tidal shear. However, there is no evidence of star formation outside the GCR. The high relative velocity and shear expected in the dust-lanes along the bar major axis could inhibit the star formation process, as observed in other galaxies. The high SiO abundances derived in the non-circular velocity clouds are likely due to the large-scale shocks that created the dust lanes

Hubble space telescope nicmos imaging of W3 IRS 5 : A trapezium in the making?

We present Hubble Space Telescope NICMOS imaging of W3 IRS 5, a binary high-mass protostar. In addition to the two protostars, NICMOS images taken in the F222M and F160W filters show three new 2.22 μm sources with very red colors; these sources fall within a region 5600 AU in diameter and are coincident with an ~100 M☉, dense molecular clump. Two additional point sources are found within 04 (800 AU) of one of the high-mass protostars; these may be stellar companions or unresolved emission knots from an outflow. We propose that these sources constitute a nascent Trapezium system in the center of the W3 IRS 5 cluster containing as many as five proto-OB stars. This would be the first identification of a Trapezium still deeply embedded in its natal gas.

A highly collimated outflow in the core of OMC-1

The paper reports the discovery of a 120 arcsec long jet of CO in OMC-1. The feature, which is redshifted up to 15 km s-1 from the ambient CO gas, was presumably ejected from CS3/FIR4 60 arcsec southwest of the Trapezium and 110 arcsec south of IRc2. It might be related to a local bipolar source recently found there in SiO. The jet has very narrow width (not greater than 8 arcsec) and shows no sign of angular divergence; its axis is a straight line. The matter is accelerated all along, with the largest accelerations observed on the central axis. Lower velocity material envelops the high-speed core. Halfway along the jet, there is a sharp break at which the higher velocities terminate abruptly. Beyond the break, acceleration continues without change of direction or strength; the flows final disappearance might be caused by accelerative dilution. The jet is probably denser than the ambient cloud; its temperature (about 45 K) approaches ambient values. Blueshifted emission of narrow lateral extent is seen over the full length of the redshifted jet. 15 refs.

The detection of the (J, K) = (18, 18) line of NH3

The first astronomical detection of the metastable (

CO Isotopes in Planetary Nebulae

J,K