Abstract
Photodissociation regions (PDRs), where the (far-)ultraviolet light from hot young stars interact with the gas in surrounding molecular clouds, provide laboratories for understanding the ...nature and role of feedback by star formation on the interstellar medium. While the general nature of PDRs is well understood—at least under simplified conditions—the detailed dynamics and chemistry of these regions, including gas clumping, evolution over time, etc., can be very complex. We present interferometric observations of the 21 cm atomic hydrogen line, combined with C
ii
158
μ
m observations, toward the nearby reflection nebula IC 63. We find a clumpy H
i
structure in the PDR, and a ring morphology for the H
i
emission at the tip of IC 63. We further unveil kinematic substructure, of the order of 1 km s
−1
, in the PDR layers and several legs that will disperse IC 63 in <0.5 Myr. We find that the dynamics in the PDR explain the observed clumpy H
i
distribution and lack of a well-defined H
i
/H
2
transition front. However, it is currently not possible to conclude whether H
i
self-absorption and nonequilibrium chemistry also contribute to this clumpy morphology and missing H
i
/H
2
transition front.
Abstract
We used high-resolution C
ii
158
μ
m mapping of two nebulae IC 59 and IC 63 from SOFIA/upGREAT in conjunction with ancillary data of the gas, dust, and polarization to probe the kinematics, ...structure, and magnetic properties of their photodissociation regions (PDRs). The nebulae are part of the Sh 2-185 H
ii
region that is illuminated by the B0 IVe star
γ
Cas. The velocity structure of each PDR changes with distance from
γ
Cas, which is consistent with driving by the radiation. Based on previous far-ultraviolet (FUV) flux measurements of, and the known distance to,
γ
Cas, along with the predictions of 3D distances to the clouds, we estimated the FUV radiation field strength (
G
0
) at the clouds. Assuming negligible extinction between the star and clouds, we find their 3D distances from
γ
Cas. For IC 63, our results are consistent with earlier estimates of distance from Andersson et al., locating the cloud at ∼2 pc from
γ
Cas at an angle of 58° to the plane of the sky behind the star. For IC 59, we derive a distance of 4.5 pc at an angle of 70° in front of the star. We do not detect any significant correlation between the orientation of the magnetic field and the velocity gradients of C
ii
gas, which indicates a moderate magnetic field strength. The kinetic energy in IC 63 is estimated to be an order of 10 higher than the magnetic energies. This suggests that kinetic pressure in this nebula is dominant.
We report on the design, fabrication, and measured results of a turnstile quad-ridge orthomode transducer (OMT) suitable for millimeter-wavelength receivers used in radio telescopes for astronomy. ...The quad-ridge OMT is directly compatible with a quad-ridge feed horn to implement dual-polarization low-noise receivers that exceed the single-mode bandwidth of a rectangular waveguide. Using a bandwidth of 24-51 GHz, the OMT demonstrated an insertion loss of 0.3-0.4 dB along with cross-polarization leakage and isolation below <inline-formula> <tex-math notation="LaTeX">-</tex-math> </inline-formula>40 dB. An OMT is a key part of a dual polarization receiver front-end, and our long-term goal is to realize low-noise, wideband receivers for radio astronomy utilizing ridge waveguides throughout the entire front-end. Design details are provided for each ridge waveguide feature within the OMT, including a quad-ridge waveguide turnstile junction, <inline-formula> <tex-math notation="LaTeX">E</tex-math> </inline-formula>-plane bend, and <inline-formula> <tex-math notation="LaTeX">E</tex-math> </inline-formula>-plane T-junction. To facilitate future layouts and integration, additional ridge waveguide component designs have been included for a 3-dB coupler, <inline-formula> <tex-math notation="LaTeX">H</tex-math> </inline-formula>-plane bend, and <inline-formula> <tex-math notation="LaTeX">H</tex-math> </inline-formula>-plane T-junction.
The James Clerk Maxwell Telescope (JCMT) Gould Belt Survey (GBS) was one of the first legacy surveys with the JCMT in Hawaii, mapping 47 deg2 of nearby (<500 pc) molecular clouds in dust continuum ...emission at 850 and 450 m, as well as a more limited area in lines of various CO isotopologues. While molecular clouds and the material that forms stars have structures on many size scales, their larger-scale structures are difficult to observe reliably in the submillimeter regime using ground-based facilities. In this paper, we quantify the extent to which three subsequent data reduction methods employed by the JCMT GBS accurately recover emission structures of various size scales, in particular, dense cores, which are the focus of many GBS science goals. With our current best data reduction procedure, we expect to recover 100% of structures with Gaussian sizes of ≤30″ and intensity peaks of at least five times the local noise for isolated peaks of emission. The measured sizes and peak fluxes of these compact structures are reliable (within 15% of the input values), but source recovery and reliability both decrease significantly for larger emission structures and fainter peaks. Additional factors such as source crowding have not been tested in our analysis. The most recent JCMT GBS data release includes pointing corrections, and we demonstrate that these tend to decrease the sizes and increase the peak intensities of compact sources in our data set, mostly at a low level (several percent), but occasionally with notable improvement.
We use Two-Micron All-Sky Survey and Midcourse Space Experiment infrared observations, along with new molecular line (CO) observations, to examine the distribution of young stellar objects (YSOs) in ...the molecular cloud surrounding the halo H ii region KR 140 in order to determine if the ongoing star formation activity in this region is dominated by sequential star formation within the photodissociation region (PDR) surrounding the H ii region. We find that KR 140 has an extensive population of YSOs that have ‘spontaneously’ formed due to processes not related to the expansion of the H ii region. Much of the YSO population in the molecular cloud is concentrated along a dense filamentary molecular structure, traced by C18O, that has not been erased by the formation of the exciting O star. Some of the previously observed submillimetre clumps surrounding the H ii region are shown to be sites of recent intermediate- and low-mass star formation while other massive starless clumps clearly associated with the PDR may be the next sites of sequential star formation.
A large fraction of the mass of the interstellar medium in our Galaxy is in the form of warm (103-104 K) and cool (50-100 K) atomic hydrogen (H i) gas. Cold (10-30 K) regions are thought to be ...dominated by dense clouds of molecular hydrogen. Cold H i is difficult to observe, and therefore our knowledge of its abundance and distribution in the interstellar medium is poor. The few known clouds of cold H i are much smaller in size and mass than typical molecular clouds. Here we report the discovery that the H i supershell GSH139-03-69 is very cold (10 K). It is about 2 kiloparsecs in size and as massive as the largest molecular complexes. The existence of such an immense structure composed of cold atomic hydrogen in the interstellar medium runs counter to the prevailing view that cold gas resides almost exclusively in clouds dominated by molecular hydrogen.
We present observations of intermediate-velocity clouds (IVCs) found at positive velocities in the Canadian Galactic Plane Survey Phase I region (74° < l < 148°, -35 < b < +56). A catalog of 138 ...distinct IVCs and 13 IVC complexes has been compiled. From the global characteristics of our sample we demonstrate that the majority of these IVCs are directly associated with energetic phenomena, such as supernova explosions and massive star-forming regions, within the disk of the Galaxy and thus form a different population than the high-velocity clouds and high-latitude IVCs. We report what we believe to be the first detection of H I 21 cm absorption of a background source by an IVC and attempt to determine its physical properties and distance. We discuss in some detail a few of the IVCs that illustrate the connection between the clouds and energetic phenomena in the Galaxy. We suggest that the velocity shift of these IVCs away from the bulk of the Galactic H I emission provides an opportunity to study the structure of the low-latitude H I emission in a relatively confusion-free environment and present two examples of large (degree-scale) complexes of H I emission particularly suitable for such studies.
We used high-resolution C II 158 \(\mu\)m mapping of two nebulae IC 59 and IC 63 from SOFIA/upGREAT in conjunction with ancillary data on the gas, dust, and polarization to probe the kinematics, ...structure, and magnetic properties of their photo-dissociation regions (PDRs). The nebulae are part of the Sh 2-185 H II region illuminated by the B0 IVe star \(\gamma\) Cas. The velocity structure of each PDR changes with distance from \(\gamma\) Cas, consistent with driving by the radiation. Based on previous FUV flux measurements of, and the known distance to \(\gamma\) Cas along with the predictions of 3D distances to the clouds, we estimated the FUV radiation field strength (G0) at the clouds. Assuming negligible extinction between the star and clouds, we find their 3D distances from \(\gamma\) Cas. For IC 63, our results are consistent with earlier estimates of distance from Andersson et al. (2013), locating the cloud at 2 pc from \(\gamma\) Cas, at an angle of 58 to the plane of the sky, behind the star. For IC 59, we derive a distance of 4.5 pc at an angle of 70 in front of the star. We do not detect any significant correlation between the orientation of the magnetic field (Soam et al. 2017) and the velocity gradients of C II gas, indicating a moderate magnetic field strength. The kinetic energy in IC 63 is estimated to be order of ten higher than the magnetic energies. This suggests that kinetic pressure in this nebula is dominant.
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