P. Hofner

A Catalog of CH3OH 70-61 A+ Maser Sources in Massive Star-forming Regions

We present a Very Large Array survey of 44 massive star-forming regions in the 44 GHz 70-61 A+ methanol transition; 37 fields showed maser emission. Thirty-one sources were also observed in the 23 GHz 92-101 A+ methanol line; two fields showed maser emission. Although the 44 GHz line is a class I maser, we find a large number of these masers in relatively close association with H II regions and water masers. Several sources show strong evidence for a correlation between 44 GHz masers and shocked molecular gas, supporting the interpretation that molecular outflows may give rise to class I maser emission. We provide maser positions with arcsecond accuracy that not only locate the masers with respect to other star formation phenomena, but also provide, for the stronger masers, phase referencing sources that can be used to calibrate future 7 mm (44 GHz) observations of these regions.

A study of the Keplerian accretion disk and precessing outflow in the massive protostar IRAS 20126+4104

We report on interferometric observations at 3.2 and 1.3 mm of the massive young stellar object IRAS 20126+4104 obtained in the C 34 Sa nd CH 3OH lines and in the continuum emission. The C 34 S data confirm the existence of a Keplerian disk, as already suggested by various authors. However, the mass of the central object is ∼7 M� , significantly less than previous estimates. We believe that such a discrepancy is due to the fact that the rotation curve is affected not only by the star but also by the mass in the innermost regions of the disk itself: this leads to an overestimate of the stellar mass when low-density tracers are used to study the velocity field over regions larger than a few seconds of arc (i.e. a few 0.01 pc). On the basis of the line profiles we speculate that accretion onto the star might be still occurring through the disk. This seems consistent with current models of high-mass star formation which predict an accretion luminosity equal to that of IRAS 20126+4104 for a 7 Mprotostar. The CH3OH lines trace both the disk and the bipolar outflow previously detected in other molecules such as HCO + ,S iO, and H 2. New H2 images obtained at 2.2 µm confirm that the outflow axis is undergoing precession. We elaborate a simple model that suitably fits the data thus allowing derivation of a few basic parameters of the precession.

Broad Radio Recombination Lines from Hypercompact H II Regions

ABSTRACTThe H92 recombination line was observed toward six massive star formation regions (MSFRs), and theH76 line was observed toward one MSFR. All seven MSFRs were suspected of harboring hypercompact (HC)H ii regions. The goal was to detect broad-line sources and to investigate their properties. The sources wereselected according to their small sizes, high brightness temperatures, and rising continuum spectra (typicalspectral index þ 1, S / ) at centimeter wavelengths. Two of the HC Hii candidates, G25.5+0.2 and NGC7538 (IRS 1), were previously known to have extremely broad lines (line widths of 160 and 180 km s 1 ,respectively). Sixteen separate, compact, radio continuum components were detected, fourteen of which weredetected in either the H92 or H76 line. Eight sources have line widths (FWHMs) greater than 40 km s 1 ;typical ultracompact (UC) H ii region line widths are 25–30 km s 1 . These broad lines may be produced by acombination of thermal, turbulent, and electron impact broadening, and large-scale motions (rotation, expansion,jets, shocks, inflow, disk, etc.). On the basis of one line and a relatively low spatial resolution, we are unable todetermine the relative contributions from each mechanism. All the MSFRs in the current sample are composed oftwo or more continuum components. The large projected separations between the continuum components withina given MSFR indicate that they are unlikely to be gravitationally bound massive protostars. Possible origins ofthe observed intermediate-sloped power-law spectral energy distributions (SEDs) are discussed. It is suggestedthat hierarchal clumping in HC H ii regions may produce the observed power-law SEDs.Subject headings: H ii regions — radio lines: ISM — stars: formation1. INTRODUCTIONThe formation and earliest evolution of massive stars is oneof astrophysics’ least understood problems. The observationalrecord is far too incomplete to piece together a coherent theoryof massive star formation and subsequent evolution. Theconventional model assumes that ultracompact (UC) H iiregions represent the earliest manifestation of massive starsafter forming via rapid accretion of ambient gas onto a pro-tostellar hydrostatic core. Norberg & Maeder (2000) andBehrend & Maeder (2001) proposed a ‘‘growing accretionrate’’ scenario, in which the accretion rate increases as themass of the protostar increases. However, the hypothesis offormation via accretion for massive stars (M > 8 10 M )hasbeen questioned by Bonnell et al. (1998), who have proposedan alternative formation mechanism based on mergers oflower mass protostars in dense young clusters. The accretionhypothesis predicts infall of molecular gas to form an equa-torial, Keplerian, accretion disk accompanied by bipolar out-flows along the protostar’s spin axis. Massive bipolar outflowshave been observed toward numerous massive star formationregions (MSFRs; Shepherd & Churchwell 1996; Ridge 2000;Beuther et al. 2002, and references therein). However, thereexist only a few massive protostars with candidate accretiondisks (see Garay & Lizano 1999; Churchwell 2002; Shepherdet al. 2002, and references therein). The small number ofsources is not surprising, because the observations are difficultwith current telescopes, and the phase of rapid accretion islikelytobeshortlived(<10

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.

QUASI-PERIODIC FORMALDEHYDE MASER FLARES IN THE MASSIVE PROTOSTELLAR OBJECT IRAS 18566+0408

We report results of an extensive observational campaign of the 6 cm formaldehyde maser in the young massive stellar object IRAS 18566+0408 (G37.55+0.20) conducted from 2002 to 2009. Using the Arecibo Telescope, the Very Large Array, and the Green Bank Telescope, we discovered quasi-periodic formaldehyde flares (P ~ 237 days). Based on Arecibo observations, we also discovered correlated variability between formaldehyde (H2CO) and methanol (CH3OH) masers. The H2CO and CH3OH masers are not spatially coincident, as demonstrated by different line velocities and high angular resolution MERLIN observations. The flares could be caused by variations in the infrared radiation field, possibly modulated by periodic accretion onto a young binary system.

Search for Class I methanol masers in low-mass star formation regions

A survey of young bipolar outflows in regions of low-to-intermediate-mass star formation has been carried out in two Class I methanol maser transitions: 7(0) - 6(1)A+ at 44 GHz and 4(-1) - 3(0)E at 36 GHz. We detected narrow features towards NGC 1333I2A, NGC 1333I4A, HH25MMS and L1157 at 44 GHz, and towards NGC 2023 at 36 GHz. Flux densities of the lines detected at 44 GHz are no higher than 11 Jy and the relevant source luminosities are about 1022 erg s-1, which is much lower than those of strong masers in high-mass star formation regions. No emission was found towards 39 outflows. All masers detected at 44 GHz are located in clouds with methanol column densities of the order of or larger than a few x1014 cm-2. The upper limits for the non-detections are typically of the order of 3-5 Jy. Observations in 2004, 2006 and 2008 did not reveal any significant variability of the 44 GHz masers in NGC 1333I4A, HH25MMS and L1157.

THE ULTRACOMPACT H II REGION G5.97(1.17: AN EVAPORATING CIRCUMSTELLAR DISK IN M8

We present new high spatial resolution optical, infrared, and radio observations of G5.97-1.17, an ultracompact H II region 27 distant from the O7 star Herschel 36 in the center of M8, the Lagoon Nebula. Infrared imaging was obtained using the adaptive optics system ADONIS on the ESO 3.6 m telescope and a thermal-IR camera on the Hale 5 m; optical images taken with the Hubble Space Telescope Wide Field Planetary Camera 2 were extracted from the ST-ECF data archive; and radio continuum images were made using the VLA. The narrowband optical images show G5.97-1.17 to be a bow shock–shaped structure with its apex pointed toward Her 36, with the Hα flux distributed over 06 and consistent with the appearance at 2 cm. The optical continuum and near-infrared images show a star displaced from the peak of the bow shock by 0125 in the direction away from Her 36. This star exhibits a thermal-IR excess, indicating hot circumstellar dust, the presence of which can be reconciled with the optical visibility of the star if the dust is in a disklike (rather than spherical) distribution. Therefore, contrary to previous hypotheses that G5.97-1.17 is an ultracompact H II region intrinsically ionized by an embedded B0 star, our observations strongly suggest that it is actually a young star surrounded by a circumstellar disk that is being photoevaporated by Her 36, similar to the so-called proplyds seen near θ1 Ori C in the Orion Nebula. At a distance of 1.8 kpc, this is the most distant known proplyd. These new observations confirm our previous claims based on earlier adaptive optics and deconvolved WF/PC images.

The structure of hot molecular cores over 1000 AU

Context. Hot molecular cores (HMCs) are believed to be the cradles of stars of mass above ∼6 M� . It is hence important to determine their structure and kinematics and thus study phenomena directly related to the star-formation process, such as outflow, infall, and rotation. Establishing the presence of embedded early-type (proto)stars is also crucial for understanding the nature of HMCs. Aims. To achieve the highest available angular resolution to date, we performed observations of the molecular gas in two well-known HMCs (G10.47+0.03 and G31.41+0.31) with an angular resolution of ∼0. �� 1. Continuum observations were also made at different wavelengths to detect Hii regions associated with early-type stars embedded in the cores. Methods. We used the Very Large Array in its most extended configuration to image the NH3(4,4) inversion transition. Continuum measurements were made at 7 mm, 1.3 cm, and 3.6 cm using the A-array configuration. Results. We detected two new continuum sources in G31.41+0.31, which are possibly thermal jets, and confirmed the presence of one ultracompact and two hypercompact Hii regions in G10.47+0.03. Evidence that the gas is infalling towards the embedded (proto)stars is provided for both G10.47+0.03 and G31.41+0.31, while in G10.47+0.03 part of the ammonia gas also appears to be expanding in two collimated bipolar outflows. From the temperature profile in the cores, we establish an approximate bolometric luminosity for both sources in the range 1 ×10 5 −7 ×10 5 L� . Finally, a clear velocity gradient across the core is detected in G31.41+0.31. The nature of this gradient is discussed and two alternative explanations are proposed: outflow and rotation. Conclusions. We propose a scenario where G10.47+0.03 is in a more advanced evolutionary stage than G31.41+0.31. In this scenario, thermal jets develop until the accretion rate is sufficiently high to trap or even quench any Hii region. When the jets have pierced the core and the stellar mass has grown sufficiently, hypercompact Hii regions appear and the destruction of the HMC begins.

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

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.