6,125 research outputs found
Zone plate lens antennas for millimeter and submillimeter wavelengths
Zone plate lenses are a type of focusing element which function essentially as differential phase shifters, having a relatively few, coarsely quantized phase delays across the incident beam of radiation. The major advantages are ease of fabrication and much reduced thickness, compared to conventional refractive focusing elements. These considerations are both of particular importance for the submillimeter range, in which manufacturing tolerances for curved optical elements can pose significant problems, and where the absorption of readily available dielectric materials is quite large. In this presentation we briefly review the theory of zone plate lens operations, present a relatively simple method for calculating the aperture efficiency of zone plate lenses used as antennas, and show some theoretical and measured results in the 100 GHz range
Electron Excitation of High Dipole Moment Molecules Reexamined
Emission from high-dipole moment molecules such as HCN allows determination
of the density in molecular clouds, and is often considered to trace the
"dense" gas available for star formation. We assess the importance of electron
excitation in various environments. The ratio of the rate coefficients for
electrons and H molecules, 10 for HCN, yields the requirements
for electron excitation to be of practical importance if $n({\rm H}_2) \leq\
10^{5.5} ~ \rm cm^{-3}X({\rm e}^-) \geq\ 10^{-5}n_{\rm{}c}({\rm H_2})X^*({\rm{}e}^-)X({\rm e}^-)^+X^*({\rm e}^-)_2^-^+^+$ ratios observed in the nuclear regions of luminous galaxies may
be in part a result of electron excitation of high dipole moment tracers. The
importance of electron excitation will depend on detailed models of the
chemistry, which may well be non-steady state and non-static.Comment: published in Ap
CII in the Interstellar Medium: Excitation by H2 Revisited
C is a critical constituent of many regions of the interstellar medium,
as it can be a major reservoir of carbon and, under a wide range of conditions,
the dominant gas coolant. Emission from its 158m fine structure line is
used to trace the structure of photon dominated regions in the Milky Way and is
often employed as a measure of the star formation rate in external galaxies.
Under most conditions, the emission from the single [CII] line is proportional
to the collisional excitation rate coefficient. We here used improved
calculations of the deexcitation rate of [CII] by collisions with H to
calculate more accurate expressions for interstellar C fine structure
emission, its critical density, and its cooling rate. The collision rates in
the new quantum calculation are 25% larger than those previously
available, and narrow the difference between rates for excitation by atomic and
molecular hydrogen. This results in [CII] excitation being quasi-independent of
the molecular fraction and thus dependent only on the total hydrogen particle
density. A convenient expression for the cooling rate at temperatures between
20 K and 400 K, assuming an LTE H ortho to para ration is . The present work
should allow more accurate and convenient analysis of the [\CII] line emission
and its cooling
Herschel Measurements of Molecular Oxygen in Orion
We report observations of three rotational transitions of molecular oxygen (O_2) in emission from the H_2 Peak 1 position of vibrationally excited molecular hydrogen in Orion. We observed the 487 GHz, 774 GHz, and 1121 GHz lines using the Heterodyne Instrument for the Far Infrared on the Herschel Space Observatory, having velocities of 11 km s^(–1) to 12 km s^(–1) and widths of 3 km s^(–1). The beam-averaged column density is N(O_2) = 6.5 × 10^(16) cm^(–2), and assuming that the source has an equal beam-filling factor for all transitions (beam widths 44, 28, and 19"), the relative line intensities imply a kinetic temperature between 65 K and 120 K. The fractional abundance of O_2 relative to H_2 is (0.3-7.3) × 10^(–6). The unusual velocity suggests an association with a ~5" diameter source, denoted Peak A, the Western Clump, or MF4. The mass of this source is ~10 M_⊙ and the dust temperature is ≥150 K. Our preferred explanation of the enhanced O_2 abundance is that dust grains in this region are sufficiently warm (T ≥ 100 K) to desorb water ice and thus keep a significant fraction of elemental oxygen in the gas phase, with a significant fraction as O_2. For this small source, the line ratios require a temperature ≥180 K. The inferred O_2 column density ≃5 × 10^(18) cm^(–2) can be produced in Peak A, having N(H_2) ≃4 × 10^(24) cm^(–2). An alternative mechanism is a low-velocity (10-15 km s^(–1)) C-shock, which can produce N(O_2) up to 10^(17) cm^(–2)
Low Virial Parameters in Molecular Clouds: Implications for High Mass Star Formation and Magnetic Fields
Whether or not molecular clouds and embedded cloud fragments are stable
against collapse is of utmost importance for the study of the star formation
process. Only "supercritical" cloud fragments are able to collapse and form
stars. The virial parameter, alpha=M_vir/M, which compares the virial to the
actual mass, provides one way to gauge stability against collapse.
Supercritical cloud fragments are characterized by alpha<2, as indicated by a
comprehensive stability analysis considering perturbations in pressure and
density gradients. Past research has suggested that virial parameters alpha>2
prevail in clouds. This would suggest that collapse towards star formation is a
gradual and relatively slow process, and that magnetic fields are not needed to
explain the observed cloud structure. Here, we review a range of very recent
observational studies that derive virial parameters <<2 and compile a catalogue
of 1325 virial parameter estimates. Low values of alpha are in particular
observed for regions of high mass star formation (HMSF). These observations may
argue for a more rapid and violent evolution during collapse. This would enable
"competitive accretion" in HMSF, constrain some models of "monolithic
collapse", and might explain the absence of high--mass starless cores.
Alternatively, the data could point at the presence of significant magnetic
fields ~1 mG at high gas densities. We examine to what extent the derived
observational properties might be biased by observational or theoretical
uncertainties. For a wide range of reasonable parameters, our conclusions
appear to be robust with respect to such biases.Comment: accepted to Ap
Dust emission in the Sagittarius B2 molecular cloud core
A model is presented for the dust emission from the Sagittarius B2 molecular cloud core which reproduces the observed spectrum between 30 and 1300 micron, as well as the distribution of the emission at 1300 micron. The model is based on the assumption that Sgr B2(N) continuum source is located behind the dust cloud associated with Sgr B2(M) continuum source. The fact that Sgr B2(N) is stronger at 1300 micron can be attributed to a local column density maximum at the position of this source. Absence of a 53 micron emission peak at the position of Sgr B2(N) suggests that the luminosity of the north source is lower than that of the middle source
The Magnetic Field of L1544: I. Near-Infrared Polarimetry and the Non-Uniform Envelope
The magnetic field (B-field) of the starless dark cloud L1544 has been
studied using near-infrared (NIR) background starlight polarimetry (BSP) and
archival data in order to characterize the properties of the plane-of-sky
B-field. NIR linear polarization measurements of over 1,700 stars were obtained
in the H-band and 201 of these were also measured in the K-band. The NIR BSP
properties are correlated with reddening, as traced using the RJCE (H-M)
method, and with thermal dust emission from the L1544 cloud and envelope seen
in Herschel maps. The NIR polarization position angles change at the location
of the cloud and exhibit their lowest dispersion of position angles there,
offering strong evidence that NIR polarization traces the plane-of-sky B-field
of L1544. In this paper, the uniformity of the plane-of-sky B-field in the
envelope region of L1544 is quantitatively assessed. This allowed evaluating
the approach of assuming uniform field geometry when measuring relative
mass-to-flux ratios in the cloud envelope and core based on averaging of the
envelope radio Zeeman observations, as in Crutcher et al. (2009). In L1544, the
NIR BSP shows the envelope B-field to be significantly non-uniform and likely
not suitable for averaging Zeeman properties without treating intrinsic
variations. Deeper analyses of the NIR BSP and related data sets, including
estimates of the B-field strength and testing how it varies with position and
gas density, are the subjects of later papers in this series.Comment: 16 pages, 9 figures; accepted for publication in The Astrophysical
Journa
Resolving distance ambiguities towards 6.7 GHz methanol masers
Distances to most star forming regions are determined using kinematics,
through the assumption that the observed radial velocity arises from the motion
of the source with respect to the Sun resulting from the differential rotation
of Galaxy. The primary challenge associated with the application of this
technique in the inner Galaxy is the kinematic distance ambiguity. In this
work, we aim to resolve the kinematic distance ambiguity towards a sample of
6.7 GHz methanol masers, which are signposts of the early stages of massive
star formation. We measured 21 cm HI absorption spectra using the Very Large
Array in C and CnB configurations. A comparison of the maximum velocity of HI
absorption with the source velocity and tangent point velocity was used to
resolve the kinematic distance ambiguity. We resolved the distance ambiguity
towards 41 sources. Distance determinations that are in conflict with previous
measurements are discussed. The NE2001 spiral arm model is broadly consistent
with the locations of the star forming complexes. We find that the use of
vertical scale height arguments to resolve the distance ambiguity can lead to
erroneous classifications for a significant fraction of sources.Comment: Accepted by Astronomy & Astrophysic
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