Compact Radio Continuum Sources Research Articles

Thermal Infrared Imaging of Ultracompact HiiRegions in W49A

Several compact radio continuum sources in W49A show detectable 8-20 μm emission in MIRAC2 images obtained at the IRTF. In general, the infrared morphologies of these sources closely resemble the radio continuum emission. Spectral energy distributions indicate an infrared continuum excess above the level expected from free-free emission, consistent with thermal emission from dust grains heated to a few hundred K. The bright radio continuum sources concentrated at the western end of the ring of ultracompact H II regions are not detected in the mid-infrared, while those at other positions in the ring are detected. This could be due to a localized region of high extinction along the line of sight. In addition, there are a few new infrared sources with no radio continuum counterparts. Finally, several infrared sources show strong 12.8 μm [Ne II] emission, yielding neon abundances that are typically a few percent of the cosmic abundance of neon but are high considering the expected Ne++/Ne+ ratios for the range of spectral types of the ionizing sources. We conclude that the [Ne II] emission must come from shells around the ultracompact H II regions, where the neon is able to survive as Ne+ rather than Ne++ because the radiation field has been softened by absorption of hard UV photons within the H II regions.

350 Micron Images of Massive Star Formation Regions

We present 350 μm continuum images of 24 massive star formation regions obtained with the Caltech Submillimeter Observatory equipped with the SHARC focal-plane array. At this wavelength the spatial resolution is 11''. The 350 μm emission is compared with thermal radio continuum emission and OH, H2O, and CH3OH masers. Emission at 350 μm is believed to be thermal emission from dust heated by embedded or nearby stars. Compact radio continuum sources are usually present in the mapped 350 μm fields, and more than 60% of the 350 μm peaks coincide with radio continuum peaks. This association lends strong support to the notion that the dust is heated primarily by hot stars. Masers are also a common property of massive star formation regions. Usually OH, H2O, and/or CH3OH masers are found near, but generally not coincident with, the 350 μm peaks. Less than 25% of the 350 μm peaks have no reported masers located close by. In most of the observed regions, the 350 μm dust maps show one or more components surrounded by fainter extended emission. In total, we identify 28 separate 350 μm components. Ten of the 28 components do not have radio continuum counterparts. These are luminous sources and should produce detectable H II regions. It is postulated that these sources may be undergoing such rapid accretion that the infalling matter quenches the H II region very close to the protostar, thereby making the H II region undetectable in free-free emission. Objects in this evolutionary state may well represent the long-sought precursors to ultracompact H II regions and should have the properties of accreting massive protostars (e.g., accretion disks, bipolar molecular outflows, hot shocked gas, infall with spin-up toward the protostar). We suggest that the 10 sources in this category in our sample merit further observational study for these properties. Two-temperature graybody models constrained by the observed infrared spectral energy distributions were calculated to estimate the total mass, luminosity, average dust temperature, and hydrogen column and number densities for each source. The graybody models do not account for possible low-level emission from the coolest dust (<25 K) and therefore may underestimate the total mass. There is, however, no evidence for emission in excess of the models at 1.3 mm, and we conclude that the mass contribution of dust colder than ~25 K cannot be large. Thus, the graybody models shown provide useful average global properties required to understand massive star formation environments.

A Search for the CO–H2Dimer in the Galaxy

We have used the 30 m IRAM millimeter radio telescope to search for two low-lying rotational transitions of the CO-H2 dimer in the emission spectra of three Galactic sources and in the absorption spectrum of a low-latitude extragalactic continuum source. We first summarize the structure of this dimer as it can be inferred currently from laboratory infrared spectroscopy. The transitions we have observed and their initial estimated frequencies are the 111-↔000+ line at ~109 GHz and the 110+↔101- line at ~91 GHz. The search targets were the TMC-1 and L134 dark clouds, the L1157 bipolar outflow, and the compact radio continuum source 2013+370. The spectral search covered the frequency ranges from 90.58 to 91.10 GHz and from 108.95 to 109.46 GHz. Several known emission lines were found, including HC3N (12-11), HC3N (10-9), OCS, SO, and HNC, but no other features were seen in the spectra. The rms noise was typically 5 mK per 1 MHz channel (≈ 3 km s-1). Improved laboratory values for the frequencies were obtained after the 30 m observations were made and are also given here for future reference. The frequency of the 111-↔000+ line may now fall outside of our present search range, but the 110+↔101- frequency is likely to be within it. We conclude that the fractional abundance of the CO-H2 dimer with respect to the CO monomer is typically ≤1 × 10-4, if the excitation temperature is at least a few degrees above the cosmic background. This is the most stringent upper limit yet placed on the presence of a dimer in the interstellar medium.

The Position of Sagittarius A*: Accurate Alignment of the Radio and Infrared Reference Frames at the Galactic Center

We present a novel approach to the long-standing problem of locating the position of the compact nonthermal radio source Sgr A* on infrared images of the Galactic center region. Using the Very Large Array, we have detected SiO and H2O maser emission toward several sources within the central parsec of our Galaxy. These masers arise from the innermost parts of circumstellar envelopes of giant and supergiant stars that are members of the nuclear star cluster and appear as compact infrared sources in a diffraction-limited 2.2 μm infrared image. One of the SiO masers is associated with the M-type supergiant IRS 7, the most prominent 2.2 μm point source in the Galactic center region. The radio data allow measurements of the maser positions relative to the compact nonthermal radio continuum source Sgr A* with milliarcsecond accuracy. Because stellar SiO masers near the Galactic center trace their host stars to within a few milliarcseconds, these relative positions can be used to calibrate the plate scale and rotation of the infrared image. Our method allows registration of the radio relative to the infrared reference frame with an estimated accuracy of 003. Using the improved position accuracy we put a stringent upper limit on the 2.2 μm flux density of Sgr A* that is significantly lower than values predicted by recent theoretical model calculations.

Discovery of a Subparsec Jet 4000 Radii Away from the Central Schwarzschild Engine of NGC 4258

We report the VLBI detection of compact continuum emission associated with the maser disk in NGC 4258. The strongest emission is located about 0.5 mas (0.015 pc) north of the dynamical center of the disk. It has an average flux density of about 3 mJy and varies by ~100% on timescales of weeks. We postulate that we have detected nonthermal synchrotron emission in the base of the northern jet, which is seen on parsec-to-kiloparsec scales in radio to X-ray emission. We also report a detection of emission from the southern jet which may be attenuated by thermal absorption in a layer of ionized gas above the molecular disk. The average flux density of the maser emission in the systemic velocity range is correlated with the flux density of the northern continuum emission. Together with the geometry of the disk, these data suggest that the masers amplify the southern continuum emission and that the southern and northern jet emission are correlated in strength. We discuss the results in the context of jet emission models and properties of other compact radio continuum sources.

Detection of 130 “Diffuse” Galactic HiiRegions

The paper reports the results of a search for diffuse Galactic H II regions using radio recombination lines. Unlike most previous surveys, which have been directed toward fairly bright, compact radio continuum sources, we observed unprepossessing objects that might be optically obscured analogs to the Rosette Nebula. The sources observed here are typically a factor of 2 fainter in the continuum than those that have been detected before. One hundred thirty nebulae have been measured in radio recombination lines for the first time; most require at least one main-sequence O star to provide their ionization. The diffuse H II regions appear to have essentially the same distribution in the Galaxy as compact H II regions. There is some indication that the diffuse nebulae have a low abundance of ionized helium, possibly due to their low excitation. Several newly discovered nebulae are associated with peculiar kinematic features in the Galactic nucleus. The surface density of radio H II regions in the inner Galaxy follows the surface density of CO fairly closely from 4 ≤ R ≤ 8 kpc. In this area, there is ~1 radio H II region for each 106 M☉ of molecular hydrogen. However, H II regions are a factor ~2 more confined to the Galactic plane than the molecular gas from which they form.

Radio Continuum Detection of the Exciting Sources of the DG Tauri B and L1551NE Outflows

The exciting sources of molecular outflows are characteristically associated with centimeter radio continuum emission, most probably originating in a partially ionized, collimated outflow. DG Tau B and L1551NE are two low-luminosity pre-main-sequence objects that have recently been found to be associated with molecular outflows. We have used 3.5 cm Very Large Array observations with angular resolution of 02 to detect compact radio continuum sources at both the positions of DG Tau B and L1551NE. The DG Tau B radio source has deconvolved dimensions (038 ± 002) × (022 ± 002), and its major axis is aligned along a position angle of 298° ± 5°, coincident within a few degrees with the position angle of the axis of the optical and molecular outflows (~294°). This result suggests that we are observing the base of the collimated jet that powers the region, on scales of tens of astronomical units. A comparison of the widths of the radio and optical jets in this source supports this interpretation.

Studies of embedded far infrared sources in the vicinity of H2O masers – I. observations

We have undertaken a search for submillimetre (submm) embedded cores associated with known water maser sources. For 44 water maser sources, we found submm continuum emission from 40 (91 per cent) using the James Clerk Maxwell Telescope (JCMT). Some of the sources were then observed further. Maps were made in CO and C18O (⁠|$J=2\rightarrow1$|⁠) and CS (⁠|$J=5\rightarrow4$|⁠), using the JCMT, so that the molecular environment of the sources could be examined. In addition the Very Large Array (VLA) was used to find accurate positions for the water maser emission and hence any association with compact radio continuum sources. A preliminary examination of the data shows that most of the water masers are on average closer than 0.1 pc to the corresponding submm source. A more detailed analysis will follow in a later paper.

What is powering the Orion Kleinmann-low infrared nebula

view Abstract Citations (193) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS What is Powering the Orion Kleinmann-Low Infrared Nebula? Menten, Karl M. ; Reid, Mark J. Abstract We used the Very Large Array (VLA) to observe the Orion BN/KL region simultaneously at frequencies of 8.4 and 43.1 GHz with 0.25 sec resolution. At 8.4 GHz we detect a rich cluster of compact radio continuum sources, some of which are coincident with infrared sources while others do not have known infrared counterparts. At 43.1 GHz we observed continuum emission together with the v = 1, J = 1-0 SiO maser line, allowing precise registration of radio continuum relative to SiO emission. We find that the radio continuum emission from one of the radio sources, I, coincides with the centroid of the SiO maser distribution. Our SiO maser maps show intricate velocity structure indicative of ordered motions that must have a rotating and expanding (or contracting) component. Since source I powers an SiO maser, it must have a high luminosity, most likely exceeding 104 solar luminosity. Precise astrometry shows that it does not coincide with the mid-infrared source IRc2. Given that the radio emission from source I is not affected by dust extinction, it locates the central (proto)star powering the IRc2 complex. This suggests that the location and morphology of the infrared emission from 'IRc2' is determined by inhomogeneities of the dust envelope surrounding the central star. Furthermore, this brings into question all previous luminosity estimates of IRc2 and opens the possibility that other sources contribute significantly to the energetics of the Orion KL region. In particular, we suggest that another infrared source, n, which is coincident with a peculiar double radio source, may be responsible for at least some of the energetic phenomena observed in the region. Publication: The Astrophysical Journal Pub Date: June 1995 DOI: 10.1086/187913 Bibcode: 1995ApJ...445L.157M Keywords: Continuous Spectra; Emission Spectra; Interstellar Masers; Interstellar Matter; Line Spectra; Orion Nebula; Radio Astronomy; Radio Spectra; Silicon Oxides; Star Formation; Centimeter Waves; Extremely High Frequencies; Infrared Radiation; Millimeter Waves; Stellar Luminosity; Superhigh Frequencies; Very Large Array (Vla); Astronomy; STARS: FORMATION; ISM: INDIVIDUAL NAME: BECKLIN-NEUGEBAUER; ISM: INDIVIDUAL NAME: ORION KLEINMANN-LOW; ISM: INDIVIDUAL ALPHANUMERIC: IRC2; MASERS full text sources ADS | data products SIMBAD (19)

The Partially Ionized Gas in the w3 Complex - C90-ALPHA Observations

view Abstract Citations (16) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The partially ionized gas in the W 3 complex: C90 alpha observations. Jaffe, D. T. ; Wilson, T. L. Abstract We have mapped the distribution of the C9Oα line toward W3 with 90" resolution. The emission is centered at α=602h21m50s.6±δ=61°52'31"±9 (1950), that is, southwest of W3A (IRS 2) and toward W3 B (IRS 3). If the source and the beam are Gaussian shaped, the deconvolved source size is (121" ± 17", 88" ± 15") in (α, δ). The carbon emission covers a region which includes all of the compact radio continuum sources at the center of the W3 complex. The C II emission region shows a velocity gradient: from -39 km s-1 in the east to - 42 km s-1 in the west. It is not possible to derive uniquely the electron temperature and density of the partially ionized gas from our data or from our data in conjunction with other C II results. The carbon region may consist of shells surrounding the compact H II regions. Publication: The Astrophysical Journal Pub Date: May 1981 DOI: 10.1086/158904 Bibcode: 1981ApJ...246..113J full text sources ADS | data products SIMBAD (4)