206 research outputs found

    Ray-tracing and physical-optics analysis of the aperture efficiency in a radio telescope

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    The performance of telescope systems working at microwave or visible/IR wavelengths is typically described in terms of different parameters according to the wavelength range. Most commercial ray tracing packages have been specifically designed for use with visible/IR systems and thus, though very flexible and sophisticated, do not provide the appropriate parameters to fully describe microwave antennas, and thus to compare with specifications. In this work we demonstrate that the Strehl ratio is equal to the phase efficiency when the apodization factor is taken into account. The phase efficiency is the most critical contribution to the aperture efficiency of an antenna, and the most difficult parameter to optimize during the telescope design. The equivalence between the Strehl ratio and the phase efficiency gives the designer/user of the telescope the opportunity to use the faster commercial ray-tracing software to optimize the design. We also discuss the results of several tests performed to check the validity of this relationship that we carried out using a ray-tracing software, ZEMAX and a full Physical Optics software, GRASP9.3, applied to three different telescope designs that span a factor of ≃10intermsofD/lambda.ThemaximummeasureddiscrepancybetweenphaseefficiencyandStrehlratiovariesbetween\simeq 10 in terms of D/lambda. The maximum measured discrepancy between phase efficiency and Strehl ratio varies between \simeq 0.4 and 1.9 up to an offset angle of >40 beams, depending on the optical configuration, but it is always less than 0.5 where the Strehl ratio is >0.95.Comment: 34 pages, 7 figure

    Herschel-HIFI observations of H2O, NH3 and N2H+ toward high-mass starless and proto-stellar clumps identified by the Hi-GAL survey

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    Our present understanding of high-mass star formation still remains very schematic. In particular, it is not yet clear how much of the difference between low-mass and high-mass star formation occurs during the earliest star formation phases. The chemical characteristics of massive cold clumps, and the comparison with those of their low-mass counterparts, could provide crucial clues about the exact role that chemistry plays in differentiating the early phases of low-mass and high-mass star formation. Water, in particular, is a unique probe of physical and chemical conditions in star-forming regions. Using the HIFI instrument of Herschel we have observed the ortho-NH3 (1_0-0_0) (572GHz), ortho-H2O (1_10-1_01) (557GHz) and N2H+ (6-5) (559GHz) lines toward a sample of high-mass starless and proto-stellar clumps selected from the "Herschel} Infrared Galactic Plane Survey" (Hi-GAL). We compare our results to previous studies of low-mass and high-mass proto-stellar objects. At least one of the three molecular lines was detected in 4 (out of 35) and 7 (out of 17) objects in the l=59deg and l=30deg galactic regions, respectively. All detected sources are proto-stellar. The water spectra are complex and consist of several kinematic components, identified through a Gaussian decomposition, and in a few sources inverse and regular P-Cygni profiles have been detected. All water line profiles of the l=59deg region are dominated by a broad Gaussian emission feature, indicating that the bulk of the water emission arises in outflows. No such broad emission is detected toward the l=30deg objects. The ammonia line in some cases also shows line wings and an inverse P-Cygni profile, thus confirming that NH3 rotational transitions can be used to probe the dynamics of high-mass star forming regions. Both bolometric and water line luminosity increase with the continuum temperature.Comment: This paper includes 7 main figures and 6 tables, in addition to the figures with the spectra of the individual sources which are presented as on-line material. Accepted for publication on Astronomy and Astrophysic

    Constraints on particle acceleration sites in the Crab Nebula from relativistic MHD simulations

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    The Crab Nebula is one of the most efficient accelerators in the Galaxy and the only galactic source showing direct evidence of PeV particles. In spite of this, the physical process behind such effective acceleration is still a deep mystery. While particle acceleration, at least at the highest energies, is commonly thought to occur at the pulsar wind termination shock, the properties of the upstream flow are thought to be non-uniform along the shock surface, and important constraints on the mechanism at work come from exact knowledge of where along this surface particles are being accelerated. Here we use axisymmetric relativistic MHD simulations to obtain constraints on the acceleration site(s) of particles of different energies in the Crab Nebula. Various scenarios are considered for the injection of particles responsible for synchrotron radiation in the different frequency bands, radio, optical and X-rays. The resulting emission properties are compared with available data on the multi wavelength time variability of the inner nebula. Our main result is that the X-ray emitting particles are accelerated in the equatorial region of the pulsar wind. Possible implications on the nature of the acceleration mechanism are discussed.Comment: 12 pages, 7 figures, 2 table

    Arecibo Observations of Formaldehyde in L1551

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    We report observations of the formaldehyde (H2CO) 6 cm (4.8 GHz) line toward L1551. The observations were conducted with the Arecibo Telescope (beam FWHP ~ 1') to verify the tentative detection of H2CO thermal emission reported by Duncan and collaborators in 1987. The H2CO emission lines were expected to be present with a signal-to-noise ratio of 10 in our spectra. However, we did not detect H2CO emission; i.e., our data rule out their tentative detection. The absence of H2CO emission is also confirmed by the fact that the H2CO line profiles at the two positions of expected emission are well fitted by a single absorption component (accounting for the hyperfine structure of the line) in one of the positions and by a single absorption line plus a red-wing absorption component in the second position. The Orion BN/KL region remains the only H2CO 6 cm thermal emitter known. Our observations also demonstrate that the H2CO 6 cm absorption line traces not only the quiescent molecular cloud but also the kinematics associated with the star formation process in L1551-IRS 5

    Thermal Methanol Observations of the Outflow from the G31.41+0.31 Hot Molecular Core

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    The G31.41+0.31 region hosts one of the most prominent hot molecular cores known. Coincident with the hot molecular core is an outflow whose orientation has been controversial. We report VLA-C observations of thermal methanol (70–61 A+, 44 GHz) toward the position of the G31.41+0.31 hot molecular core. Our goals are to clarify the orientation of the outflow and to study the properties of a molecular outflow from a very young region of massive star formation. We confirm that the outflow is indeed associated with the hot molecular core. Our observations strongly suggest that the outflow is oriented in the northeast-southwest direction. The outflow is massive (15 M☉), with a dynamical time of the order of ~4 × 103 yr, and has a wide-angle bipolar morphology

    A comparison of radome- and astrodome-enclosed large radio telescopes at millimeter wavelengths: The Large Millimeter Telescope

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    We present a systematic comparison of the main figures of merit for an open-air radio telescope and two different types of enclosed antennas: (1) an ordinary radome, with a metal space frame providing the required mechanical rigidity and a dielectric membrane, and (2) an “astrodome,” i.e., a corotating rigid dome with a large window covered by a tensile membrane structure. The analysis is limited to submillimeter and millimeter wavelengths and large (≳30 m) antenna/enclosure systems, where the window tensile structure is very unlikely to be removable and is supported by either a metal space frame or cable networks. As compared with previous studies of this type, here we concentrate on the specific effects that these large metallic support structures have on sensitive astronomical observations. In particular, we critically discuss how the wind-induced random motions of the metal space frame can limit the sensitivity of continuum observations, as a result of fluctuating shadowing and spillover effects combined with various beam-chopping techniques. Using the Large Millimeter Telescope as a benchmark, we provide baselines for future projects where a similar comparison is needed

    A Search for Formaldehyde 6 cm Emission toward Young Stellar Objects. II. H2CO and H110α Observations

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    We report the results of our second survey for Galactic H2CO maser emission toward young stellar objects. Using the GBT and the VLA in the A configuration we observed 58 star-forming regions and discovered the fifth H2CO 6 cm maser region in the Galaxy (G23.71-0.20). We have discussed the detection toward G23.71-0.20 in a previous paper. Here we present all the other results from our survey, including detection of H2CO absorption features toward 48 sources, detection of the H110α recombination line toward 29 sources, detection (including tentative detections) of the carbon recombination line C110α toward 14 sources, subarcsecond angular resolution images of 6 cm continuum emission toward five sources, and observations of the H2CO masers in IRAS 18566+0408 and NGC 7538. In the case of NGC 7538, we detected the two main H2CO maser components, and our observations confirm variability of the blueshifted component recently reported by Hoffman et al. The variability of both maser components in NGC 7538 could be caused by a shock wave that reached the redshifted component approximately 14 yr before reaching the blueshifted component. If that were the case, we would expect to detect an increase in the flux density rate of change of the blueshifted component sometime after the year 2009. Our data also support the use of H2CO/H110α observations as a tool to resolve the kinematic distance ambiguity of massive star-forming regions in the Galaxy

    The BLAST Survey of the Vela Molecular Cloud: Dynamical Properties of the Dense Cores in Vela-D

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    The Vela-D region, according to the nomenclature given by Murphy & May (1991), of the star forming complex known as the Vela Molecular Ridge (VMR), has been recently analyzed in details by Olmi et al. (2009), who studied the physical properties of 141 pre- and proto-stellar cold dust cores, detected by the ``Balloon-borne Large-Aperture Submillimeter Telescope'' (BLAST) during a much larger (55 sq. degree) Galactic Plane survey encompassing the whole VMR. This survey's primary goal was to identify the coldest, dense dust cores possibly associated with the earliest phases of star formation. In this work, the dynamical state of the Vela-D cores is analyzed. Comparison to dynamical masses of a sub-sample of the Vela-D cores estimated from the 13CO survey of Elia et al. (2007), is complicated by the fact that the 13CO linewidths are likely to trace the lower density intercore material, in addition to the dense gas associated with the compact cores observed by BLAST. In fact, the total internal pressure of these cores, if estimated using the 13CO linewidths, appears to be higher than the cloud ambient pressure. If this were the case, then self-gravity and surface pressure would be insufficient to bind these cores and an additional source of external confinement (e.g., magnetic field pressure) would be required. However, if one attempts to scale down the 13CO linewidths, according to the observations of high-density tracers in a small sample of sources, then most proto-stellar cores would result effectively gravitationally bound.Comment: This paper has 12 pages and 6 figures. Accepted for publication by the Astrophysical Journal on July 19, 201
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