S. Kurtz

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.

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. I.

Fifty-four ultracompact (UC) H ii regions in the GLIMPSE survey region (jbj < 1 � and 30 � < l < 70 � ) were observed in H2CO and H110� using the 305 m Arecibo telescope. By analyzing H2CO absorption against the UC H ii region continuum emission, we resolve the distance ambiguity toward 44 sources. This determination is critical to measure global physical properties of UC H ii regions (e.g., luminosity, size, mass) and properties of the Galaxy (e.g., spiral structure, abundance gradients). We find that the distribution of UC H ii regions in this survey is consistent with a ‘‘ local spur,’’ the Perseus, Sagittarius, and Scutum arms as delineated by Taylor & Cordes. However, departures from model velocities produce distance uncertainties only slightly smaller than the proposed arm separations. Subject headings: Galaxy: disk — radio lines: general — stars: formation On-line material: machine-readable tables

The Coordinated Radio and Infrared Survey for High-mass Star Formation. II. Source Catalog

The CORNISH project is the highest resolution radio continuum survey of the Galactic plane to date. It is the 5 GHz radio continuum part of a series of multi-wavelength surveys that focus on the northern GLIMPSE region (10° < l < 65°), observed by the Spitzer satellite in the mid-infrared. Observations with the Very Large Array in B and BnA configurations have yielded a 1.″5 resolution Stokes I map with a root mean square noise level better than 0.4 mJy beam -1 . Here we describe the data-processing methods and data characteristics, and present a new, uniform catalog of compact radio emission. This includes an implementation of automatic deconvolution that provides much more reliable imaging than standard CLEANing. A rigorous investigation of the noise characteristics and reliability of source detection has been carried out. We show that the survey is optimized to detect emission on size scales up to 14″ and for unresolved sources the catalog is more than 90% complete at a flux density of 3.9 mJy. We have detected 3062 sources above a 7σ detection limit and present their ensemble properties. The catalog is highly reliable away from regions containing poorly sampled extended emission, which comprise less than 2% of the survey area. Imaging problems have been mitigated by down-weighting the shortest spacings and potential artifacts flagged via a rigorous manual inspection with reference to the Spitzer infrared data. We present images of the most common source types found: H II regions, planetary nebulae, and radio galaxies. The CORNISH data and catalog are available online at http://cornish.leeds.ac.uk.

Triggered star formation on the borders of the Galactic Hii region RCW 82

Context. To investigate the process of star formation triggered by th e expansion of an H region, we present a multi-wavelength analysis of the Galactic H region RCW 120 and its surroundings. The collect and collaps e model predicts that the layer of gas and dust accumulated between the ionization and shock fronts during the expansio n of the H region collapses and forms dense fragments, giving rise to p otential sites of massive-star formation. Aims. The aim of our study is to look for such massive fragments and m assive young stars on the borders of RCW 120. Methods. We mapped the RCW 120 region in the cold dust continuum emissi on at 1.2 mm to search for these fragments. We supplemented this study with the available near(2MASS) and mid-IR (GLIM PSE) data to locate the IR sources observed towards this regi on and to analyse their properties. We then compared the observational resul ts with the predictions of Hosokawa & Inutsuka’s model (2005 , 2006). Results. At 1.2 mm we detected eight fragments towards this region, fiv e located on its borders. The largest fragment has a mass of ab out 370 M⊙. Class I and Class II young stellar objects are detected all o ver the region, with some observed far from the ionization fr ont. This result emphasises the possible importance of distant interactions between the radiation, escaping from the ioni zed region, and the surrounding

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.

The Coordinated Radio and Infrared Survey for High-Mass Star Formation (The CORNISH Survey). I. Survey Design

We describe the motivation, design and implementation of the CORNISH survey, an arcsecond resolution radio continuum survey of the inner Galactic plane at 5GHz using the Karl G. Jansky Very Large Array (VLA). It is a blind survey co-ordinated with the northern Spitzer GLIMPSE I region covering 10 o < l <65 o and |b| <1 o at similar resolution. We discuss in detail the strategy that we employed to control the shape of the synthesised beam across this survey that covers a wide range of fairly low declinations. Two snapshots separated by 4 hours in hour angle kept the beam elongation to less that 1.5 over 75% of the survey area and less than 2 over 98% of the survey. The prime scientific motivation is to provide an unbiased survey for ultra-compact H II regions to study this key phase in massive star formation. A sensitivity around 2mJy will allow the automatic distinction between radio loud and quiet mid-IR sources found in the Spitzer surveys. This survey has many legacy applications beyond star formation including evolved stars, active stars and binaries, and extragalactic sources. The CORNISH survey for compact ionized sources complements other Galactic plane surveys that target diffuse and non-thermal sources as well as atomic and molecular phases to build up a complete picture of the ISM in the Galaxy.

ATLASGAL – properties of compact H ii regions and their natal clumps

We present a complete sample of molecular clumps containing compact and ultracompact (UC) Hii regions betweenl = 10 ◦ and 60 ◦ and|b|< 1 ◦ , identified by combining the the ATLASGAL sub-mm and CORNISH radio continuum surveys wit h visual examination of archival infrared data. Our sample is complete to optically thin, compact and UC Hii regions driven by a zero age main sequence star of spectral type B0 or earlier embedded within a 1,000 M⊙ clump. In total we identify 213 compact and UC Hii regions, associated with 170 clumps. Unambiguous kinematic distances are derived for these clumps and used to estimate their masses and physical sizes, as well as the Lyman continuum fluxes and sizes of their embedded Hii regions. We find a clear lower envelope for the surface densit y of molecular clumps hosting massive star formation of 0.05 g cm −2 , which is consistent with a similar sample of clumps associated with 6.7 GHz masers. The mass of the most massive embedded stars is closely correlated with the mass of their natal clump. Young B stars appear to be significantly more luminous in the ultraviolet than predicted by current stellar atmosphere models. The properties of clumps associated with compact and UC Hii regions are very similar to those associated with 6.7 GHz methanol masers and we speculate that there is little evolution in the structure of the molecular clumps between these two phases. Finally, we identify a significant peak in the surface density of compact and UC Hii regions associated with the W49A star-forming complex, noting that this complex is truly one of the most massive and intense regions of star formation in the Galaxy.

ARECIBO OBSERVATIONS OF FORMALDEHYDE AND RADIO RECOMBINATION LINES TOWARD ULTRACOMPACT H II REGIONS

We report observations of the H110α radio recombination line and H2CO (110-111) toward 21 ultracompact H II regions with the Arecibo 305 m radio telescope. We detect the H110α line in 20 sources, and for each of these we also detect a H2CO absorption feature at nearly the same velocity, demonstrating the association between molecular and ionized gas. We determine kinematic distances and resolve the distance ambiguity for all observed H II regions, as well as for 19 intervening molecular clouds. A plot of the Galactic distribution of these objects traces part of the spiral structure in the first Galactic quadrant. We compare flux densities and velocities as measured with the Arecibo Telescope with interferometric measurements of our sample of ultracompact H II regions. In general, the single-dish fluxes exceed the interferometric values, consistent with an extended component of radio continuum emission.

A 1000 AU ROTATING DISK AROUND THE MASSIVE YOUNG STELLAR OBJECT G192.16(3.82

We present evidence for a circumstellar disk around the luminous young stellar object G192.16-3.82. Observations of 2.6 mm, 7 mm, and 3.6 cm continuum emission and 13CO, C18O, and H2O maser line emission support the interpretation that a 1000 AU rotating disk exists around the central B2 star. The disk major axis is roughly perpendicular to the outflow, and the maser velocities are consistent with Keplerian rotation. The mass of the disk plus circumstellar envelope traced by millimeter continuum emission is ~15 M☉. The source is embedded in a 500 M☉ molecular cloud from which a massive outflow emerges. Updated outflow parameters yield a total flow mass of 95 M☉ and a mass flow rate of 5.6 × 10-4 M☉ yr-1. The outflow opening angle, defined by a 13CO shell, is approximately 90°. A weak, one-sided, ionized jet emanates from the young stellar object system. The estimated momentum rate of the jet (~10-3 M☉ km s-1 yr-1) is about a factor of 4 too low to power the flow. These results are consistent with the idea that the G192.16 outflow mechanism can be best characterized by a combination of a weak jet and a strong, wide-angle wind. Approximately 05 to the north of G192.16-3.82 is a band of 3.6 cm continuum emission and redshifted H2O maser components which we interpret to be due to a newly discovered jet from the embedded object G192.16-3.82 N. The jet does not affect the dynamics of the massive outflow significantly which indicates that it is probably due to a low-mass young stellar object.