Esteban Araya

RESOLUTION OF DISTANCE AMBIGUITIES OF INNER GALAXY MASSIVE STAR FORMATION REGIONS. II.

We report simultaneous H110α and H2CO line observations with the NRAO Green Bank Telescope toward 72 H II regions in the Spitzer Space Telescope GLIMPSE survey area (|l| = 10°-65° and |b| ≤ 1°). We used the H110α line to establish the velocity of the H II regions and H2CO absorption lines to distinguish between near and far distances. Accurate distances are crucial for the determination of physical properties of massive star formation regions. We resolved the distance ambiguity of 44 H II regions. We detected multiple H II regions along 18 lines of sight located in the longitude interval 12°-31°, primarily a result of the relatively large telescope beam width. We could not resolve distance ambiguities for lines of sight with multiple H II regions, since we could not determine which H2CO lines were being absorbed against which H II region. We examined the projected location of H II regions whose distance ambiguities have been resolved (in this work and other similar studies) in the Galactic plane and in a longitude-velocity diagram for a recognizable spiral arm pattern. Although the highest density of points in the position-position plot approximately follows the spiral arms proposed by Taylor & Cordes, the dispersion is still about as large as the separation between their proposed arms. The longitude-velocity plot shows an increase in the density of sources at the points where the spiral arm loci proposed by Taylor & Cordes are approaching the locus of tangent point velocities and a lower density between the arm loci. However, it is not possible to trace spiral arms over significant segments of Galactic longitude in the longitude-velocity plot. We conclude that a very large number of H II regions in combination with more sophisticated Galactic rotation models will be required to obtain a more continuous spiral pattern from kinematic studies of H II regions than from fully sampled surveys of H I or CO.

CH3CN Observations toward Southern Massive Star-forming Regions

In an effort to identify very young sites of massive star formation, we have conducted a survey for hot and dense molecular cores toward a sample of 17 southern sources. The sample consists of sources with IRAS color characteristics of ultracompact H II regions for which high-density molecular material had previously been detected. We observed the J = 5-4, 6-5, 8-7, and 12-11 rotational transitions of CH3CN and derived rotation temperatures and column densities using the population-diagram technique. We identify four sources with a high-temperature molecular component (Trot > 90 K) as new candidates for hot molecular cores. We also observed the transitions H35α, CS J = 3-2, and the continuum in the 3, 2 mm bands toward 17 sources, and the 1.3 mm continuum, H41α and 13CO J = 2-1 transitions toward 10 sources. Eight sources show blue and red wings in the CS J = 3-2 line, whereas three sources show wings in the 13CO J = 2-1 spectra, suggestive of molecular outflows. Our continuum and recombination line data show that the 91 GHz continuum emission is dominated by free-free emission from ionized regions, whereas at 147 GHz emission from dust grains contributes significantly.

Sub-arcsecond resolution radio continuum observations of IRAS 20126+4104

Context. The detailed physical processes that lead to the formation of massive stars are still unknown. Observations that probe linear scales as small as

RESOLVING THE CIRCUMSTELLAR DISK AROUND THE MASSIVE PROTOSTAR DRIVING THE HH 80-81 JET

100,

A Search for Formaldehyde 6 cm Emission toward Young Stellar Objects. II. H2CO and H110α Observations

AU are necessary for improving our understanding in this area. Aims. We present high angular-resolution and high-sensitivity multi-frequency radio observations of the deeply embedded massive protostar located in the IRASu200920126+4104u2000core region, with the aim of investigating the nature of the radio continuum emission from the deeply embedded massive protostar. Methods. The observations were performed with the Very Large Array in several continuum bands with wavelengths between 20 and 0.7 cm in the A and/or B configurations. Results. At 3.6 cm we resolve the emission of the IRASu200920126+4104u2000core into 3 components. The emission from the two northern sources is consistent with free-free emission from ionized gas with a density gradient. Most likely the ionization is caused by UVxa0radiation from the cooling region of a shock; i.e. the ionization is caused by the jet driven by the IRASu200920126+4104u2000protostar. The morphology and measured flux densities of the southern source is consistent with emission from an optically thin jet, most likely also due to shock ionization. A simple radiative transfer model shows that the spectral energy distribution of IRASu200920126+4104u2000is consistent with an accretion disk embedded in a spherical halo. We also report the discovery of a highly variable radio source near the IRASu200920126+4104u2000core, which is most likely gyrosynchotron emission from a low-mass pre-main sequence star.

DISCOVERY OF THE FIRST METHANOL (CH3OH) MASER IN THE ANDROMEDA GALAXY (M31)

We present new high angular resolution observations toward the driving source of the HH 80-81 jet (IRAS 18162-2048). Continuum emission was observed with the Very Large Array at 7 mm and 1.3 cm, and with the Submillimeter Array at 860 μm, with angular resolutions of ~01 and ~08, respectively. Submillimeter observations of the sulfur oxide (SO) molecule are reported as well. At 1.3 cm the emission traces the well-known radio jet, while at 7 mm the continuum morphology is quadrupolar and seems to be produced by a combination of free-free and dust emission. An elongated structure perpendicular to the jet remains in the 7 mm image after subtraction of the free-free contribution. This structure is interpreted as a compact accretion disk of ~200 AU radius. Our interpretation is favored by the presence of rotation in our SO observations observed at larger scales. The observations presented here add to the small list of cases where the hundred-AU scale emission from a circumstellar disk around a massive protostar has been resolved.

Discovery of an H2CO 6 Centimeter Maser in IRAS 18566+0408

We report the results of our second survey for Galactic H2CO maser emission toward young stellar objects. Using the GBT and the VLA in the A configuration we observed 58 star-forming regions and discovered the fifth H2CO 6xa0cm maser region in the Galaxy (G23.71-0.20). We have discussed the detection toward G23.71-0.20 in a previous paper. Here we present all the other results from our survey, including detection of H2CO absorption features toward 48 sources, detection of the H110α recombination line toward 29 sources, detection (including tentative detections) of the carbon recombination line C110α toward 14 sources, subarcsecond angular resolution images of 6xa0cm continuum emission toward five sources, and observations of the H2CO masers in IRASxa018566+0408 and NGCxa07538. In the case of NGCxa07538, we detected the two main H2CO maser components, and our observations confirm variability of the blueshifted component recently reported by Hoffman et al. The variability of both maser components in NGCxa07538 could be caused by a shock wave that reached the redshifted component approximately 14 yr before reaching the blueshifted component. If that were the case, we would expect to detect an increase in the flux density rate of change of the blueshifted component sometime after the year 2009. Our data also support the use of H2CO/H110α observations as a tool to resolve the kinematic distance ambiguity of massive star-forming regions in the Galaxy.

Studies of Extragalactic Formaldehyde and Radio Recombination Lines

We present the first detection of a 6.7 GHz Class II methanol (CH3OH) maser in the Andromeda galaxy (M31). The CH3OH maser was found in a VLA survey during the fall of 2009. We have confirmed the methanol maser with the new EVLA, in operation since 2010 March, but were unsuccessful in detecting a water maser at this location. A direct application for this methanol maser is the determination of the proper motion of M31, such as was previously obtained with water masers in M33 and IC10. Unraveling the three-dimensional velocity of M31 would solve for the biggest unknown in the modeling of the dynamics and evolution of the Local Group of galaxies.

Thermal Methanol Observations of the Outflow from the G31.41+0.31 Hot Molecular Core

We report VLA observations toward IRAS 18566+0408 conducted to determine the nature of the H2CO 6 cm emission line recently detected by Araya and coworkers. Our observations clearly show that the H2CO source is due to maser emission, making IRAS 18566+0408 only the fourth Galactic H2CO 6 cm maser source, out of five known H2CO emitters. We also report detection of a weak 2 cm continuum source that is coincident with the H2CO maser. Given the current observational constraints, the maser could be due to the radiative pumping mechanism proposed by Boland & de Jong; however, the coincidence of the new H2CO maser with 22 GHz H2O masers suggests that shocked molecular gas could also play a role in its excitation.

An H2CO 6 cm Maser Pinpointing a Possible Circumstellar Torus in IRAS?18566+0408

ABSTRACTWe present the most sensitive and extensive survey yet performed of extragalactic H 2 CO 6 cm (4.829 GHz)emission/absorption. Sixty-two sources were observed with the C-band system of the Arecibo Telescope to a 1rms noise level of 0.3 mJy. We report a new detection of H 2 CO 6 cm absorption toward NGC 520 and theconfirmation of H 2 CO 6 cm absorption toward several sources. We report confirmation of H 2 CO 6 cm emissiontoward the OH megamasers Arp 220, IC 860, and IRAS 15107+0724. At present these are the only extragalacticH 2 CO 6 cm emitters independently confirmed. A characterization of the properties of formaldehyde absorbersand emitters based on infrared properties of the galaxiesis discussed. We also conducted a simultaneous surveyof the H110 hydrogen recombination line toward a sample of 53 objects. We report the detection of H110toward the giant extragalactic H ii region NGC 604 in M33.Subject headinggs: galaxies: ISM — ISM: molecules — radio lines: galaxies1. INTRODUCTIONMore than 20 molecular species have been detected towardextragalactic regions during the last decades (e.g., Henkel MTakanoetal.1995),formaldehyde(H