NGC 6334A Research Articles

MAGNETIC FIELD STRENGTHS IN PHOTODISSOCIATION REGIONS

ABSTRACT We measure carbon radio recombination line (RRL) emission at 5.3 GHz ?> toward four H ii regions with the Green Bank Telescope to determine the magnetic field strength in the photodissociation region (PDR) that surrounds the ionized gas. Roshi suggests that the non-thermal line widths of carbon RRLs from PDRs are predominantly due to magneto-hydrodynamic waves, thus allowing the magnetic field strength to be derived. We model the PDR with a simple geometry and perform the non-LTE radiative transfer of the carbon RRL emission to solve for the PDR physical properties. Using the PDR mass density from these models and the carbon RRL non-thermal line width we estimate total magnetic field strengths of B ∼ 100 – 300 ?> &mgr; G ?> in W3 and NGC 6334A. Our results for W49 and NGC 6334D are less well constrained with total magnetic field strengths between B ∼ 200 – 1000 ?> &mgr; G ?> . H i and OH Zeeman measurements of the line of sight magnetic field strength ( B los ?> ), taken from the literature, are between a factor of ∼ 0.5 – 1 ?> of the lower bound of our carbon RRL magnetic field strength estimates. Since ∣ B los ∣ ≤ B ?> , our results are consistent with the magnetic origin of the non-thermal component of carbon RRL widths.

The magnetic field of the H ii region NGC 6334A from Faraday rotation

We have studied the polarization characteristics and Faraday rotation of the extragalactic radio source J17204$-$3554, that appears projected on the north lobe of the galactic H II region NGC 6334A. From observations made with the Very Large Array at 6.0 and 3.6 cm in three different epochs (1994, 1997, and 2006), we estimate a rotation measure of +5100$\pm$900 rad m$^{-2}$ for the extragalactic source. This large rotation measure implies a line-of-sight average magnetic field of $B_\parallel \simeq +36\pm6$ $\mu$G, the largest obtained by this method for an H II region. NGC 6334A is significantly denser than other H II regions studied and this larger magnetic field is expected on the grounds of magnetic flux conservation. The ratio of thermal to magnetic pressure is $\sim$5, in the range of values determined for more diffuse H II regions.

Optically Thick [Oi] and [Cii] Emission toward NGC 6334A

This work focuses on [O I] and [C II] emission towards NGC 6334 A, an embedded H+ region/PDR only observable at infrared or longer wavelengths. A geometry where nearly all the emission escapes out the side of the cloud facing the stars, such as Orion, is not applicable to this region. Instead, we find the geometry to be one where the H+ region and associated PDR is embedded in the molecular cloud. Constant density PDR calculations are presented which predict line intensities as a function of AV (or N(H)), hydrogen density (nH), and incident UV radiation field (G0). We find that a single component model with AV ~650 mag, nH = 5x10^5 cm-3, and G0 = 7x10^4 reproduces the observed [O I] and [C II] intensities, and that the low [O I] 63 to 146 micron ratio is due to line optical depth effects in the [O I] lines, produced by a large column density of atomic/molecular gas. We find that the effects of a density-law would increase our derived AV, while the effects of an asymmetric geometry would decrease AV, with the two effects largely canceling. We conclude that optically selected H+ regions adjacent to PDRs, such as Orion, likely have a different viewing angle or geometry than similar regions detected through IR observations. Overall, the theoretical calculations presented in this work have utility for any PDR embedded in a molecular cloud.

Detection of the Winds from the Exciting Sources of Shell H [CSC]ii[/CSC] Regions in NGC 6334

We present results of high-sensitivity, high-resolution, multifrequency VLA observations toward the star-forming complex NGC 6334. We find that the H II region NGC 6334E, previously described as spherical, has a shell-like morphology. An additional shell-like radio source, G351.02+0.65, is mapped at 330 MHz. Four radio sources in the NGC 6334 complex present shell-like morphology; their diameters vary from 0.12 to 3.5 pc. Compact radio sources are detected at the center of the shells of NGC 6334E and NGC 6334A. These compact sources are believed to be associated with the exciting stars and are probably tracing ionized stellar winds. This is the first time for any H II region that both the shell and the central object are detected simultaneously in the radio. Two compact radio sources are detected toward the NGC 6334I(N) molecular core. One of them lies within 03 of the position of a bright Class II methanol maser, suggesting that the radio source is associated with a young embedded massive star. This is the first detection of a radio continuum source in the NGC 6334I(N) region.

VLA High-Sensitivity [TSUP]4[/TSUP]H[CLC]e[/CLC] Imaging of Galactic H [CSC]ii[/CSC] Regions

We have carried out 8.3 GHz hydrogen and helium radio recombination line and continuum observations of three Galactic H II regions with the NRAO 140 Foot Telescope and the Very Large Array. These observations measure the spatial variations in the singly ionized helium-to-hydrogen abundance ratio, 4He+/H+ (y+), and probe for any localized enhancements in y+. The three observed regions (W43, NGC 6334A, and K3-50) are morphologically complex, and high-resolution, wide field observations are crucial to measure the helium abundances on scales that would be associated with individual massive stars or clusters of stars. The nearly constant y+ value across the W43 source (W43 = 7.7% ± 1%) indicates that any ionization correction over the spatial scales investigated is small. Such ionization corrections are important for calculating total abundances of the light element 3He, which is measured in the Galaxy via a singly ionized hyperfine transition. Localized enhancements in the y+ value have been detected in both NGC 6334A (y+ = 20% ± 4%) and K3-50A (y+ = 24% ± 5%). The 4He abundance results are discussed along with other measurements of the physical properties of these H II regions, such as the kinematics and excitation.

Properties of the Photodissociated Gas in NGC 6334

To investigate the properties of photodissociated gas in strong radiation fields, the massive star formation region NGC 6334 has been mapped with the Far-Infrared Imaging Fabry-Perot Interferometer (FIFI) instrument in the far-infrared fine-structure transitions [C II] 158 μm, [O I] 146 μm, and [O I] 63 μm. Bright [C II] 158 μm emission is found throughout the cloud, whereas the [O I] 146 μm emission is associated only with the star-forming ridge. Comparison of the relative intensities of the lines with single-component photodissociation region models suggests densities of nH ~ 104 cm-3. The models imply unphysically large radiation fields for three sources, particularly for NGC 6334A, which is probably caused by self-absorption in the [O I] 63 μm line. An alternative method for estimating physical conditions, based on the correlation between [C II] 158 μm and CO line intensities, is explored. This method implies hydrogen column densities of NH 1022-23 cm-2, which agree well with those from molecular excitation models. The relative distribution of the [C II] 158 μm and [O I] 146 μm emission supports clumpy photodissociation region models that suggest the [C II] 158 μm emission arises from interclump gas and thus should be more extended than the [O I] 146 μm emission that arises from the denser clumps. The spatial coincidence of [C II] 158 μm emission peaks with minima in the molecular gas emission indicates that at least some of the molecular holes contain photodissociated gas. The relative pressures of the ionized, photodissociated, and molecular gas are compared: the photodissociated and molecular gas are in approximate pressure equilibrium, but the ionized gas is overpressurized by at least an order of magnitude at the continuum sources.

Ground-state OH observations towards NGC 6334

We have made observations of the four hyperfine transitions of the 2Π3/2, ground state of OH at 1612, 1665, 1667 and 1720 MHz and the related 1.6-GHz continuum emission towards NGC 6334 using the Australia Telescope Compact Array. The observations covered all the major radio continuum concentrations aligned along the axis of NGC 6334 (V, A to F). We have detected seven OH masers plus a possible faint eighth maser; two of these masers are located towards NGC 6334-A. Absorption at 1665 and 1667 MHz was detected towards almost all the continuum distribution. All transitions show non-LTE behaviour. The 1667-/1665-MHz intensity ratios range from 1.0 to 1.2, significantly less than their LTE value of 1.8. The results of the OH ‘Sum Rule’ suggest that this discrepancy cannot be explained solely by high optical depths. The 1612- and 1720-MHz line profiles show conjugate behaviour whereby one line is in absorption and the other in emission. In addition, the profiles commonly showed a flip from absorption to emission and vice versa, which is interpreted as a density gradient. The OH line-to-continuum distribution, optical depth and velocity trends are consistent with a bar-like shape for the molecular gas which wraps around the continuum emission.

Oi] 63 Micron Absorption in NGC 6334

The [O I] 63 μm transition has been imaged around five far-infrared (FIR) and radio continuum sources in the southern massive star formation region NGC 6334. The [O I] 63 μm line is found in absorption toward the FIR continuum source NGC 6334V. This is only the second case in which the [O I] 63 μm line has been seen in absorption against a continuum source. From the depth of the absorption line, the minimum column density of oxygen is calculated to be N(O0) 5 × 1018 cm-2. This amount of oxygen is consistent with [O I] 63 μm absorption due to atomic gas in the foreground molecular cloud. The [O I] 63 μm line is found in emission toward the other four sources observed: NGC 6334, sources A, C, D, and E. Single-component photodissociation region (PDR) models suggest densities of n ~ 104 cm-3 for these sources, based on previously observed [O I] 145 μm and [C II] 158 μm intensities. However, unphysically large far-ultraviolet (FUV) fields are implied for three of the sources, particularly for NGC 6334A. Neither one- nor two-component photodissociation region models can explain the anomalously low [O I] 63 μm intensity toward NGC 6334A nor the absorption toward NGC 6334V. We suggest that self-absorption of the [O I] 63 μm line, such as has been suggested toward DR 21, is suppressing the observed [O I] 63 μm intensity. This underestimate leads to an overestimate of the derived FUV field strengths throughout NGC 6334. The discovery of several more star-forming sites in which the [O I] 63 μm is in absorption or is self-absorbed implies that this line is not always a reliable PDR diagnostic because the PDR models do not treat the radiative transfer through the molecular cloud.

Anisotropic Radio Scattering of NGC 6334B

The extragalactic radio source NGC 6334B is the most strongly scattered object known, with an angular size of 3'' at 20 cm wavelength. Its angular size scales as the square of the observing wavelength, and its phase structure function has a power-law dependence on projected baseline length. These properties suggest that NGC 6334B is a point source, probably a quasar, scatter broadened by an intervening region of turbulent plasma. The line of sight to the source intercepts the northern lobe of the bipolar H II region NGC 6334A, which is probably the source of the scattering. We discuss observations of NGC 6334B with the VLA at 1.4, 2, 6, 20, and 90 cm, and with the VLBA at 1.4, 3.6, and 6 cm. The VLA observations indicate that the scattering disk of NGC 6334B is anisotropic, with an axial ratio of ~1.2 at 20 and 6 cm, and ~1.5 at 2 cm. This anisotropy is probably due to the effects of a magnetic field in the scattering medium. The position angle of the scattering disk implies that the mean projected magnetic field direction is parallel to the outflow from NGC 6334A. The position angle of the scattering disk rotates slightly between 6 and 2 cm; we use this rotation to place an upper limit on the outer scale of turbulence, rout 1016 cm. We present a new algorithm to detect the presence of weak (<3 σ) fringes in interferometric data. Detections of low-level excess visibility on long VLBA baselines at 3.6 cm place an upper limit on the inner dissipative scale of turbulence rin of 1011-12 cm, while the baseline scaling of fringe visibility amplitude and the wavelength scaling of scattering-disk size place a lower limit on rin of 5 × 106 cm. Nearby high-velocity (-80 km s-1) H2O maser features show little sign of broadening, implying that they are in front of, or less than 100 pc behind, the scattering region.

A 2000M☉Rotating Molecular Disk around NGC 6334A

We present millimeter and centimeter wave spectroscopic observations of the H II region NGC 6334A. We have mapped the source in several transitions of CO, CS, and NH3. The molecular emission shows a distinct flattened structure in the east-west direction. This structure is probably a thick molecular disk or torus (2.2 × 0.9 pc) responsible for the bipolarity of the near-infrared (NIR) and radio continuum emission which extends in two lobes to the north and south of the shell-like H II region. The molecular disk is rotating from west to east (ω ≈ 2.4 km s-1 pc-1) about an axis approximately parallel to the radio and NIR emission lobes. By assuming virial equilibrium, we find that the molecular disk contains ~2000 M☉. Single-component gas excitation model calculations show that the molecular gas in the disk is warmer and denser (Tk ≈ 60 K, n ≈ 3000 cm-3) than the gas to the north and south (Tk ≈ 50 K, n ≈ 400 cm-3). High resolution (~5'') NH3 (3, 3) images of NGC 6334A reveal several small (~0.1 pc) clumps, one of which lies southwest of the radio continuum shell, and is spatially coincident with a near-infrared source, IRS 20. A second NH3 clump is coincident with an H2O maser and the center of a molecular outflow. The dense gas tracers, CS J = 5 → 4 and 7 → 6, peak near IRS 20 and the H2O maser, not at NGC 6334A. IRS 20 has a substantial far-infrared (FIR) luminosity LFIR ~ 105 L☉, which indicates the presence of an O 7.5 star but has no detected radio continuum (F6 cm < 0.02 Jy). The combination of dense gas, a large FIR luminosity and a lack of radio continuum can best be explained if IRS 20 is a protostar. A third clump of NH3 emission lies to the west of IRS 20 but is not associated with any other molecular or continuum features. The star formation activity in the region has moved west of NGC 6334A to IRS 20 and the H2O maser position. We suggest that NGC 6334A, IRS 20, and the H2O maser spot are part of a protocluster of stars which is condensing from the massive molecular disk. The similarity between the structure around NGC 6334A and other large (r ~ 1 pc), massive (M ~ 103 M☉), rotating disks (K3-50A and G10.6-0.4) suggests that this may be a common mechanism by which open clusters form.

Rotation and Outflow in Compact H II Regions: VLA Observations of the Molecular and Ionized Gas in NGC 6334A and NGC 6334F

Rotation and Outflow in Compact H II Regions: VLA Observations of the Molecular and Ionized Gas in NGC 6334A and NGC 6334F

The large scattering disk of NGC 6334B

view Abstract Citations (65) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Large Scattering Disk of NGC 6334B Moran, James M. ; Rodriguez, Luis F. ; Greene, Brian ; Backer, Donald C. Abstract At centimeter wavelengths, the angular size of NGC 6334B increases as the square of the wavelength suggesting that plasma scattering broadens the image. This source, which has a size of 3" at 20 cm wavelength, has the largest known scattering disk. We present spectral line observations of neutral hydrogen and continuum observations at 20, 6, and 2 cm wavelength made with the VLA that improve our knowledge of this unusual source and its scattering agent. We obtained the following results. (1) The measurements of H I in absorption indicate that NGC 6334B is much more distant than the NGC 6334 H II region-molecular cloud complex. NGC 6334B is probably an extragalactic source whose line of sight passes through the star-forming region. (2) The continuum flux density measurements confirm the time variability of the source, which suggests that the intrinsic angular size of the source is much smaller than its observed angular size. (3) The fringe visibility as a function of baseline length at 6 cm wavelength is accurate1y modeled by a Gaussian function, as expected for scattering by a turbulent plasma with a Gaussian spectrum or with a power-law spectrum and spectral index of 4 or an arbitrary index with an inner length scale greater than 35 km. (4) The northern lobe of the bipolar H II region NGC 6334A, which is centered about 2' south of NGC 6334B, covers the line of sight to NGC 6334B and may be responsible for some or all of the scattering. Publication: The Astrophysical Journal Pub Date: January 1990 DOI: 10.1086/168222 Bibcode: 1990ApJ...348..147M Keywords: Disk Galaxies; Extragalactic Radio Sources; H Ii Regions; Molecular Clouds; Plasma Turbulence; Very Large Array (Vla); Line Of Sight; Line Spectra; Scattering; Star Formation; Astrophysics; NEBULAE: H II REGIONS; NEBULAE: INDIVIDUAL NGC NUMBER: NGC 6334; RADIO SOURCES: 21 CM RADIATION; RADIO SOURCES: VARIABLE full text sources ADS | data products SIMBAD (7) NED (1)

The bipolar H II region NGC 6334A

view Abstract Citations (34) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Bipolar H II Region NGC 6334A Rodriguez, Luis F. ; Canto, Jorge ; Moran, James M. Abstract Interferometric data are used here to make a new, self-calibrated map of the 6 cm continuum emission from the H II region NGC 6344A. The new map has a dynamic range 10 times better than the original one and reveals that the source has an extended bipolar structure. The data show that the lobes of this structure are being ionized by the same star or star cluster that ionizes the core of NGC 6344A. The infrared emission from the bipolar lobes is most probably produced in situ by the ionized gas and not by reflection. The bipolar morphology is probably due to the confinement of the H II region by a flattened structure of gas and dust. The expansion of the core of the H II region may have triggered additional star formation in the confining structure. Publication: The Astrophysical Journal Pub Date: October 1988 DOI: 10.1086/166788 Bibcode: 1988ApJ...333..801R Keywords: H Ii Regions; Molecular Clouds; Radio Interferometers; Star Formation; Stellar Coronas; Stellar Mass; Very Large Array (Vla); Bipolarity; Interstellar Masers; Morphology; Near Infrared Radiation; Water Masers; Astronomy; INTERFEROMETRY; INTERSTELLAR: MOLECULES; NEBULAE: H II REGIONS; NEBULAE: INDIVIDUAL NGC NUMBER: NGC 6334; NEBULAE: INTERNAL MOTIONS full text sources ADS | data products SIMBAD (3)

Ammonia Mapping of the Southern Molecular Cloud NGC 6334

AbstractThe Parkes 64-m antenna has been used to map ammonia emission in the (J,K = 1,1) and (J,K = 2,2) transitions toward molecular cloud complex NGC 6334. This complex contains a number of OH and H2O masers, HII regions and IR sources, and is a rich source of molecular line emission.Distributions of observed and derived quantities are presented with a linear resolution of ∼ 1 pc. Velocity anomalies are present near the two OH masers NGC 6334A and B, and the source of ammonia emission located near the northernmost H2O maser is one of the most intense sources of NH3 emission in the Galaxy.