Galactic Star-forming Regions Research Articles

OH (1720 MHz) Maser Search toward the Large Magellanic Cloud

We have carried out a sensitive search for OH (1720 MHz) masers in the Large Magellanic Cloud (LMC) toward five regions using the Australia Telescope Compact Array. Our source list includes the 30 Doradus region and four supernova remnants (SNRs): N44, N49, N120, and N132D. These data have a typical resolution of ∼8'' and rms noise levels of 5–10 mJy beam-1. We have detected OH (1720 MHz) masers in the northeast part of 30 Doradus and toward the SNR N49. The OH (1720 MHz) maser emission in 30 Doradus is coincident with a cluster of young stars known as "knot 1" and is almost certainly of the star formation variety. Our spectral resolution (0.68 km s-1) is insufficient to detect the Zeeman effect from the strongest (∼320 mJy beam-1) of the 30 Doradus OH (1720 MHz) masers, leading to an upper limit to the field strength of 6 mG. The weak OH (1720 MHz) maser emission (35 mJy beam-1) detected toward the LMC SNR N49 is located just west of a previously identified CO cloud and is indicative of an interaction between the SNR and the molecular cloud. Although the statistics are low, the detection rate seems consistent with that seen for Galactic star-forming region and SNR type OH (1720 MHz) masers—both of which are low.

Observations of cyanoacetylene sources

Thirty-four Galactic star-forming regions have been observed in the J=4−3 line of cyanoacetylene (HC3N) at 36.4 GHz. Emission was detected toward 17 sources. The HC3N column density was determined in the detected sources, and in eight of them (NGC 2264, L379, W51E1/E2, DR 21 West, DR 21 (OH), DR 21, S140, and Cep A), the relative cyanoacetylene abundance was estimated. The HC3N abundance in these objects is about 1–70×10−10.

Parameters of warm molecular clouds from methyl acetylene observations

The results of a survey of 63 Galactic star-forming regions in the 6K–5K and 5K–4K methyl acetylene lines at 102.5 and 85.5 GHz are presented. Fourty-three sources were detected at 102.5 GHz, and twenty-five at 85.5 GHz. Emission was detected toward molecular clouds with kinetic temperatures of 20–60 K (so-called “warm clouds”). The CH3CCH abundances in these clouds are about several ×10−9. Five sources (NGC 2264, G30.8-0.1, G34.26+0.15, DR 21(OH), S140) were mapped using the maximum-entropy method. The sizes of the mapped clouds fall in the range 0.1–1.7 pc, and the clouds have virial masses of 90–6200 M⊙ and densities between 6×104 and 6×105 cm−3. The CH3CCH sources coincide spatially with the CO and CS sources. Chemical-evolution simulations show that the typical methyl acetylene abundances in the observed clouds correspond to ages of ≈6×104 years.

Thermal emission of methanol and other molecules at millimeter wavelengths

The paper reports the results of a survey of Galactic star-forming regions in the methanol lines 8−1–70E at 229.8 GHz, 3−2–4−1E at 230.0 GHz, 00–1−1E at 108.9 GHz, and a series of J1–J0E lines near 165 GHz. In addition to the methanol lines, lines of methyl cyanide (CH3CN), cyanoacetylene (HC3N), methyl formate (HCOOCH3), and sulphur dioxide (SO2) were detected. Analysis of the data indicates that the methanol emission arises in warm (30–50 K) gas.

Cold Sources in Star Forming Regions

Infrared Astronomy Group of the Tata Institute of Fundamental Research (TIFR) has an ongoing programme of high resolution mapping of star forming regions in two trans-IRAS far infrared bands using a 1 metre balloon-borne telescope. The telescope has been flown many times from TIFR Balloon Facility, Hyderabad, India. The photometer consists of two bolometer arrays cooled to 0.3 K by a closed cycle liquid 3 He refrigerator. The effective wavelengths of the two bands are ~140 and 200 μ m and the angular resolution of the MEM deconvolved maps is ~1'.2. These bands are suitable for the study of cold sources (temperature ≤ 30 K). Using this system many Galactic star forming regions have been mapped. Using simultaneous maps in the two bands, maps of dust temperature and optical depth have also been obtained. Our maps are combined with other available data and radiation transfer calculations have been made. In this paper we will describe the telescope and give a sample of the results obtained for some of the star forming regions in the Galaxy.

Full-polarization maps of OH main-line masers in the W51 and G351.78-0.54 star-forming regions

Full-polarization spectral-line VLBA observations were made of the ground state, main-line, 2π3/2J = 3/2 OH masers in two Galactic star-forming regions: the “e1”/ “e2” region of W51 and G351.78-0.54. Two especially interesting results are presented. (1) Two of the 27 Zeeman pairs in W51 were found to be associated with unusually strong magnetic fields (≈ 20 mG), more than twice as strong as for any previously reported OH maser. (2) G351.78-0.54 was found to exhibit a high degree of linear polarization (up to 61%), which constrains the three dimensional structure of its magnetic field.

Maser and thermal methanol emission in the millimeter wave range: new masers at 1.3 mm and 2.8 mm

Results of a survey of Galactic star-forming regions in the lines of methanol 8−1 − 70E at 229.8 GHz, 3−2 − 4−1E at 230.0 GHz, 00 − 1−1E at 108.9 GHz, and a series of methanol lines J1 − J0E near 165 GHz are presented. Two masers, DR 21 (OH) and DR 21 West, and two maser candidates, L 379 IRS3 and NGC 6334I(N), as well as 16 thermal sources are found at 229.8 GHz. This is the first detection of methanol masers at a wavelength as short as 1 mm. At 108.9 GHz, masers were found towards G345.01 + 1.79 and probably, towards M 8E. Thermal emission is found towards 28 objects. Only thermal emission was found at 165 and 230.0 GHz (20 and 7 sources, respectively). The masers at 229.8 GHz belong to class I, whereas those at 108.9 GHz belong to class II, according to the classification by Menten (1991). The masers in DR 21 (OH) and DR 21 West can be roughly fitted by models with the gas kinetic temperature of the order of 50 K. The detection of the 108.9 GHz masers towards G345.01 + 1.79 and M 8E may indicate on a specific geometry of these objects. The combination of the existence of the class II J0 - J−1E masers towards W 3(OH), G345.01 + 1.79, W 48, and Cep A and our non-detection of the 3−2 − 4−1E and J1 - J0E lines is an evidence that the class II masers in these objects are pumped by the radiation of hot dust rather than by that of UC HII-regions.

A New Mid-Infrared Diagnostic between AGN and Starbursts

We present a new set of diagnostics which allow us to trace and classify in a statistical manner the mid-IR emission produced by active galactic nuclei (AGN) and star-forming regions. We construct a diagram based on the strength of the unidentified infrared band (UIB)at 6.2 μm, and the intensity of the continuum at short (6 μm) and long wavelengths (15 μm). We interpret the integrated mid-IR emission in late-type galaxies as resulting from three individual contributions coming from HII regions, diffuse/photodissociation regions (PDRs), and AGN. Based on this assumption, our diagnostic diagram provides a quantitative estimate of the AGN and starburst contribution to an observed mid-IR spectrum. We show that UIB emission is very faint or absent in regions harbouring intense and hard radiation fields as in the case of AGN or ‘pure’ HII starburst regions where UIB carriers can be destroyed by photodissociation. However, contrary to starburst spectra, typical AGN spectra present a strong hot continuum below 9 μm originating from hot dust heated by the AGN radiation field. An extrapolation of this diagnostic towards other mid-IR observations should improve our knowledge of the AGN/starburst connection.

Determination of molecular gas properties using methyl cyanide lines

A survey has been made of 27 Galactic star-forming regions in the (CH3CN) 6K–5K, 5K–4K, and 8K–7K lines of methyl cyanide (CH3CN) at 110, 92, and 147 GHz. Twenty-five sources were detected at 110 GHz, nineteen at 92 GHz, and three at 147 GHz. The strongest CH3CN emission arises in hot cores in regions of massive star formation. The abundance of CH3CN in these objects exceeds 10−9 as a consequence of grain mantle evaporation. Weaker CH3CN lines were found in a number of sources. These can arise in either warm (30–50 K), dense (>104 cm−3) clouds or in hot regions with cooler gas.

An Analysis of Water Line Profiles in Star Formation Regions Observed by the [ITAL]Submillimeter Wave Astronomy Satellite[/ITAL

We present spectral line profiles for the 557 GHz 110 → 101 ground-state rotational transition of ortho-H216O for 18 Galactic star formation regions observed by the Submillimeter Wave Astronomy Satellite. Water is unambiguously detected in every source. The line profiles exhibit a wide variety of shapes, including single-peaked spectra and self-reversed profiles. We interpret these profiles using a Monte Carlo code to model the radiative transport. The observed variations in the line profiles can be explained by variations in the relative strengths of the bulk flow and small-scale turbulent motions within the clouds. Bulk flow (infall, outflow) must be present in some cloud cores, and in certain cases this bulk flow dominates the turbulent motions.

Nonequilibrium Excitation of Methanol in Galactic Molecular Clouds: Multitransitional Observations at 2 Millimeters

We observed 14 methanol transitions near λ = 2 mm in Galactic star-forming regions. Broad, quasi-thermal J0-J-1E methanol lines near 157 GHz were detected toward 73 sources. Together with the 6-1-50E and 5-2-6-1E lines at 133 GHz and the 71-70E line at 166 GHz, they were used to study the methanol excitation. In the majority of the observed objects, the Class I 6-1-50E transition is inverted, and the Class II 5-2-6-1E and 60-6-1E transitions are overcooled. This is exactly as predicted by models of low-gain Class I masers. The absence of the inversion of Class II transitions 5-2-6-1E and 60-6-1E means that quasi-thermal methanol emission in all objects arises in areas without a strong radiation field, which is required for the inversion.

Far‐Infrared Observations of the Galactic Star‐forming Regions Associated with IRAS 00338+6312 and RAFGL 5111

Two Galactic star-forming regions, one in a very early phase of evolution and another evolved one, associated with the IRAS sources 00338)6312 and 03595)5110 (RAFGL 5111), respectively, have been studied in detail. These sources have been mapped simultaneously in two far-infrared bands (j eff 143 and 185 km), with angular resolution, using the Tata Institute of Fundamental Research (TIFR) D1@ 100 cm balloon-borne telescope. The HIRES-processed IRAS maps at 12, 25, 60, and 100 km have been used for comparison. Whereas IRAS 00338)6312 is resolved only in the TIFR bands, RAFGL 5111 is very well resolved in both the TIFR bands as well as in at least three IRAS bands. The neighboring fainter source IRAS 04004)5114 has also been resolved in the TIFR bands. Taking advantage of the identical beams in the two TIFR bands at 143 and 185 km, dust color temperature, T (143/185), and optical depth, maps have been generated for RAFGL 5111. These maps show interesting structural q 150 , details. Radiative-transfer modeling in spherical geometry has been carried out for individual sources to extract information about the cloud size, the type of the embedded source, the radial density distribu- tion, the optical depth, the gas-to-dust ratio, and the dust grain composition. The best-—t models are in good agreement with the observed spectral energy distribution (SED), radio continuum data, and so on. Another scheme of radiative transfer through the interstellar dust-gas cloud including the heavier ele- ments has been used to predict ionic nebular line emission, which is in reasonable agreement with the measurements for RAFGL 5111. An important conclusion from the present study is that, for all three sources (IRAS 00338)6312, 03595)5110, and 04004)5114, a faint source in the neighborhood of RAFGL 5111), the best —t to the observed SED is obtained for a uniform density (n(r) D r0) cloud. Subject headings: H II regionsinfrared: ISM: continuum ¨ ISM: individual (IRAS 00338)6312, IRAS 03595)5110)

A Morphological Study of Infrared Line Emission in Compact Star-Forming Regions

Near-infrared broadband and narrowband images are presented for 11 Galactic star-forming regions. Data were taken using the infrared camera on the Mount Palomar 60 inch (1.5 m) telescope with the broadband filters J, H, and K and with narrowband filters centered on H2 and Brγ. The morphologies of these regions are compared with ultraviolet fluorescence and shock front models of line emission. We find that the majority of these star-forming regions can be described by ultraviolet fluorescence.

Megamaser Disks in Active Galactic Nuclei

Recent spectroscopic and VLBI-imaging observations of bright extragalactic water maser sources have revealed that the megamaser emission often originates in thin circumnuclear disks near the centers of active galactic nuclei (AGNs). Using general radiative and kinematic considerations and taking account of the observed flux variability, we argue that the maser emission regions are clumpy, a conclusion that is independent of the detailed mechanism (X-ray heating, shocks, etc.) driving the collisionally pumped masers. We examine scenarios in which the clumps represent discrete gas condensations (i.e., clouds) and do not merely correspond to velocity irregularities in the disk. We show that even two clouds that overlap within the velocity coherence length along the line of sight could account (through self-amplification) for the entire maser flux of a high-velocity ``satellite'' feature in sources like NGC 4258 and NGC 1068, and we suggest that cloud self-amplification likely contributes also to the flux of the background-amplifying ``systemic'' features in these objects. Analogous interpretations have previously been proposed for water maser sources in Galactic star-forming regions. We argue that this picture provides a natural explanation of the time-variability characteristics of extragalactic megamaser sources and of their apparent association with Seyfert 2-like galaxies. We also show that the requisite cloud space densities and internal densities are consistent with the typical values of nuclear (broad emission-line region-type) clouds.

Two Micron Narrowband Imaging of the Sagittarius D HiiRegion

We present 2 μm narrowband images of the core H II region in the Galactic star-forming region Sagittarius D. The emission-line images are centered on 2.17 μm (Brγ) and 2.06 μm (He I). The H II region appears at the edge of a well-defined dark cloud, and the morphology suggests a blister geometry as pointed out in earlier radio continuum work. There is a deficit of stars in general in front of the associated dark cloud indicating the H II region is located in between the Galactic center and the Sun. The lesser spatial extent of the He I line emission relative to Brγ places the effective temperature of the ionizing radiation field below 40,000 K. The He I 2.06 μm to Brγ ratio and Brγ/far-infrared dust emission put Teff at about 36,500-40,000 K as derived from ionization models.

A comprehensive search for extragalactic 6.7-GHz methanol masers

We have used the Australia Telescope Compact Array (ATCA) to search for 6.7-GHz methanol maser emission towards 87 galaxies. We chose the target sources using several criteria, including far-IR luminosities and the presence of known OH megamasers. In addition, we searched for methanol masers in the nearby starburst galaxy NGC 253, making a full spectral-line synthesis image. No emission was detected in any galaxies, with detection limits ranging from 25 mJy to 75 mJy. This is surprising, given the close association of OH and methanol masers in Galactic star-formation regions, and significantly constrains models of OH megamaser emission. This absence of maser emission may be a result of low methanol abundances in molecular clouds in starburst galaxies.

A comprehensive search for extragalactic 6.7-GHz methanol masers

We have used the Australia Telescope Compact Array (ATCA) to search for 6.7-GHz methanol maser emission towards 87 galaxies. We chose the target sources using several criteria, including far-IR luminosities and the presence of known OH megamasers. In addition, we searched for methanol masers in the nearby starburst galaxy NGC 253, making a full spectral-line synthesis image. No emission was detected in any galaxies, with detection limits ranging from 25 mJy to 75 mJy. This is surprising, given the close association of OH and methanol masers in Galactic star-formation regions, and significantly constrains models of OH megamaser emission. This absence of maser emission may be a result of low methanol abundances in molecular clouds in starburst galaxies.

Multifrequency, High‐Resolution Radio Recombination Line Observations of the Massive Star‐forming Region W49A

We have carried out a multifrequency, multiconfiguration VLA observational program of the massive Galactic star-forming region W49A (D = 11.4 kpc). Radio continuum observations were carried out at 0.7, 1.3, and 3.6 cm. In the highest sensitivity 3.6 cm images, we have detected 45 distinct continuum sources (located within a ~4' [13 pc] diameter region) exhibiting a wide variety of sizes and morphological types. Several sources have rising continuum spectra (α > 0.6) up to the shortest wavelengths observed (7 mm). We observe a larger fraction of sources with shell morphologies (11%) than were observed in the Galactic plane survey of Wood & Churchwell. A correlation between the rms electron density and the radius of the ultracompact H II regions is observed for the spherical, unresolved, and cometary sources in W49A. We have observed W49A in the H92α, He92α, H66α, and H52α recombination lines. We detect hydrogen line emission from 39 of the 45 continuum sources observed at 3.6 cm (H92α). We derive LTE electron temperatures from the H92α, H66α, and H52α lines to have mean values of T -->*e=6700±300 K, 10,400 ± 600 K, and 9100 ± 600 K, respectively. The mean singly ionized helium abundance in the 13 sources where He92α emission is detected is Y+ = 11 ± 1%. Several of the most compact continuum sources in W49A have broad radio recombination lines (ΔVH52α > 45 km s-1), including W49A/B and the western portion of W49A/G (which contains a large number of high-velocity H2O masers). The high-frequency radio recombination line velocities are utilized to discriminate between various models that propose to explain the origin of the central ring of H II regions.

STAR FORMING REGIONS AND GALACTIC STRUCTURE

The main kinematic characteristics of our Galaxy are analyzed using a catalog of about 2700 Galactic star-forming regions. The terminal velocities of the star-forming regions confirm that the rotation curve for the inner Galaxy is flat from |l| ∼ 23°. Bends in the rotation curve are probably due to motions in spiral arms. The Galaxy rotates with a circular velocity of 200 ± 10 km s-1. Evidence for an expanding ring-like structure at 3 kpc is presented. An l - v diagram for the catalog sources in the first and fourth Galactic quadrants is constructed. Density wave theory is used to show that a two-spiral model with a pitch angle of -9° and large-amplitude velocity perturbations in the region of 3 kpc can satisfactory describe the main features of the l - v diagram to Galactocentric radii of 6-7 kpc.

IRASObservations of the Outer Galaxy. I. Discrete Sources and Large‐Scale (Diffuse) Emission

The infrared emission in two fields of the outer Galaxy is analyzed using the IRAS image data. We present a census of discrete (but extended) IRAS sources, identified from the 60 μm images. This sample can be used to characterize the properties of typical star-forming regions.We distinguish class A sources (Galactic star-forming regions) from other Galactic objects on the basis of their flux density distributions. However, our class A sources have different infrared colors than other star formation groups (young stellar objects [YSOs] and ultracompact H II regions), as measured by the IRAS Point Source Catalogue. The class A colors, in particular the high 12 μm/25 μm ratio, are similar to the color sequence found by Boulanger et al. for the O-star-excited California Nebula. They are also similar to the colors measured, on IRAS image products, for larger classical H II regions embedded in large molecular clouds. The similar infrared colors suggest that most class A sources are larger, lower density regions that sample a variety of local radiation field strengths.The infrared colors of class A sources are also found to match the colors of normal spiral galaxies. This suggests that a close correspondence exists between the physical conditions in outer Galaxy star-forming regions and normal spiral galaxies.The discrete sources have been carefully measured on IRAS Coadd images, and detailed comparisons are made with the results from other IRAS data products, especially with the Point Source Catalog (PSC). Great care has been taken to investigate the uncertainties associated with these measurements, and these results may be useful for other studies that make use of IRAS image products.Information from these measurements is combined with larger scale IRAS images to investigate the relative contributions of different components of the outer Galaxy to the total mid- and far-infrared emission. At 60 and 100 km, the diffuse emission (i.e., infrared cirrus) dominates the emission from the discrete sources, which include all possible sites of star formation.