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Register a new userResolved spectral variations of the centimetre-wavelength continuum from the rho Oph W photo-dissociation-region
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Cm-wavelength radio continuum emission in excess of free-free, synchrotron and Rayleigh-Jeans dust emission (excess microwave emission, EME), and often called `anomalous microwave emission, is bright in molecular cloud regions exposed to UV radiation, i.e. in photo-dissociation regions (PDRs). The EME correlates with IR dust emission on degree angular scales. Resolved observations of well-studied PDRs are needed to compare the spectral variations of the cm-continuum with tracers of physical conditions and of the dust grain population. The EME is particularly bright in the regions of the rho Ophiuchi molecular cloud (rho Oph) that surround the earliest type star in the complex, HD 147889, where the peak signal stems from the filament known as the rho Oph-W PDR. Here we report on ATCA observations of rho Oph-W that resolve the width of the filament. We recover extended emission using a variant of non-parametric image synthesis performed in the sky plane. The multi-frequency 17 GHz to 39 GHz mosaics reveal spectral variations in the cm-wavelength continuum. At ~30 arcsec resolutions, the 17-20 GHz intensities follow tightly the mid-IR, Icm propto I(8 um), despite the breakdown of this correlation on larger scales. However, while the 33-39 GHz filament is parallel to IRAC 8 mum, it is offset by 15-20 arcsec towards the UV source. Such morphological differences in frequency reflect spectral variations, which we quantify spectroscopically as a sharp and steepening high-frequency cutoff, interpreted in terms of the spinning dust emission mechanism as a minimum grain size a_cutoff ~ 6 +- 1A that increases deeper into the PDR.
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The $rho$ Oph molecular cloud is one of the best examples of spinning dust emission, first detected by the Cosmic Background Imager (CBI). Here we present 4.5 arcmin observations with CBI 2 that confirm 31 GHz emission from $rho$ Oph W, the PDR exposed to B-type star HD 147889, and highlight the absence of signal from S1, the brightest IR nebula in the complex. In order to quantify an association with dust-related emission mechanisms, we calculated correlations at different angular resolutions between the 31 GHz map and proxies for the column density of IR emitters, dust radiance and optical depth templates. We found that the 31 GHz emission correlates best with the PAH column density tracers, while the correlation with the dust radiance improves when considering emission that is more extended (from the shorter baselines), suggesting that the angular resolution of the observations affects the correlation results. A proxy for the spinning dust emissivity reveals large variations within the complex, with a dynamic range of 25 at 3$sigma$ and a variation by a factor of at least 23, at 3$sigma$, between the peak in $rho$ Oph W and the location of S1, which means that environmental factors are responsible for boosting spinning dust emissivities locally.
We investigate to what degree local physical and chemical conditions are related to the evolutionary status of various objects in star-forming media. rho Oph A displays the entire sequence of low-mass star formation in a small volume of space. Using spectrophotometric line maps of H2, H2O, NH3, N2H+, O2, OI, CO, and CS, we examine the distribution of the atomic and molecular gas in this dense molecular core. The physical parameters of these species are derived, as are their relative abundances in rho Oph A. Using radiative transfer models, we examine the infall status of the cold dense cores from their resolved line profiles of the ground state lines of H2O and NH3, where for the latter no contamination from the VLA 1623 outflow is observed and line overlap of the hyperfine components is explicitly taken into account. The stratified structure of this photon dominated region (PDR), seen edge-on, is clearly displayed. Polycyclic aromatic hydrocarbons (PAHs) and OI are seen throughout the region around the exciting star S1. At the interface to the molecular core 0.05 pc away, atomic hydrogen is rapidly converted into H2, whereas OI protrudes further into the molecular core. This provides oxygen atoms for the gas-phase formation of O2 in the core SM1, where X(O2)~ 5.e-8. There, the ratio of the O2 to H2O abundance [X(H2O)~ 5.e-9] is significantly higher than unity. Away from the core, O2 experiences a dramatic decrease due to increasing H2O formation. Outside the molecular core, on the far side as seen from S1, the intense radiation from the 0.5 pc distant early B-type star HD147889 destroys the molecules. Towards the dark core SM1, the observed abundance ratio X(O2)/X(H2O)>1, which suggests that this object is extremely young, which would explain why O2 is such an elusive molecule outside the solar system.
We present 850 $mu$m imaging polarimetry data of the $rho$ Oph-A core taken with the Submillimeter Common-User Bolometer Array-2 (SCUBA-2) and its polarimeter (POL-2), as part of our ongoing survey project, BISTRO (B-fields In STar forming RegiOns). The polarization vectors are used to identify the orientation of the magnetic field projected on the plane of the sky at a resolution of 0.01 pc. We identify 10 subregions with distinct polarization fractions and angles in the 0.2 pc $rho$ Oph A core; some of them can be part of a coherent magnetic field structure in the $rho$ Oph region. The results are consistent with previous observations of the brightest regions of $rho$ Oph-A, where the degrees of polarization are at a level of a few percents, but our data reveal for the first time the magnetic field structures in the fainter regions surrounding the core where the degree of polarization is much higher ($> 5 %$). A comparison with previous near-infrared polarimetric data shows that there are several magnetic field components which are consistent at near-infrared and submillimeter wavelengths. Using the Davis-Chandrasekhar-Fermi method, we also derive magnetic field strengths in several sub-core regions, which range from approximately 0.2 to 5 mG. We also find a correlation between the magnetic field orientations projected on the sky with the core centroid velocity components.
Models of pure gas-phase chemistry in well-shielded regions of molecular clouds predict relatively high levels of molecular oxygen, O2, and water, H2O. Contrary to expectation, the space missions SWAS and Odin found only very small amounts of water vapour and essentially no O2 in the dense star-forming interstellar medium. Only toward rho Oph A did Odin detect a weak line of O2 at 119 GHz in a beam size of 10 arcmin. A larger telescope aperture such as that of the Herschel Space Observatory is required to resolve the O2 emission and to pinpoint its origin. We use the Heterodyne Instrument for the Far Infrared aboard Herschel to obtain high resolution O2 spectra toward selected positions in rho Oph A. These data are analysed using standard techniques for O2 excitation and compared to recent PDR-like chemical cloud models. The 487.2GHz line was clearly detected toward all three observed positions in rho Oph A. In addition, an oversampled map of the 773.8GHz transition revealed the detection of the line in only half of the observed area. Based on their ratios, the temperature of the O2 emitting gas appears to vary quite substantially, with warm gas (> 50 K) adjacent to a much colder region, where temperatures are below 30 K. The exploited models predict O2 column densities to be sensitive to the prevailing dust temperatures, but rather insensitive to the temperatures of the gas. In agreement with these model, the observationally determined O2 column densities seem not to depend strongly on the derived gas temperatures, but fall into the range N(O2) = (3 to >6)e15/cm^2. Beam averaged O2 abundances are about 5e-8 relative to H2. Combining the HIFI data with earlier Odin observations yields a source size at 119 GHz of about 4 - 5 arcmin, encompassing the entire rho Oph A core.
We present images of C110$alpha$ and H110$alpha$ radio recombination line (RRL) emission at 4.8 GHz and images of H166$alpha$, C166$alpha$ and X166$alpha$ RRL emission at 1.4 GHz, observed toward the starforming region NGC 2024. The 1.4 GHz image with angular resolution $sim$ 70arcsec is obtained using VLA data. The 4.8 GHz image with angular resolution $sim$ 17arcsec is obtained by combining VLA and GBT data. The similarity of the LSR velocity (10.3 kms) of the C110$alpha$ line to that of lines observed from molecular material located at the far side of the HII region suggests that the photo dissociation region (PDR) responsible for C110$alpha$ line emission is at the far side. The LSR velocity of C166$alpha$ is 8.8 kms. This velocity is comparable with the velocity of molecular absorption lines observed from the foreground gas, suggesting that the PDR is at the near side of the HII region. Non-LTE models for carbon line forming regions are presented. Typical properties of the foreground PDR are $T_{PDR} sim 100$ K, $n_e^{PDR} sim 5$ cmthree, $n_H sim 1.7 times 10^4$ cmthree, path length $l sim 0.06$ pc and those of the far side PDR are $T_{PDR} sim$ 200 K, $n_e^{PDR} sim$ 50 cmthree, $n_H sim 1.7 times 10^5$ cmthree, $l sim$ 0.03 pc. Our modeling indicates that the far side PDR is located within the HII region. We estimate magnetic field strength in the foreground PDR to be 60 $mu$G and that in the far side PDR to be 220 $mu$G. Our field estimates compare well with the values obtained from OH Zeeman observations toward NGC 2024.
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