| 1. |
- Belitsky, Victor, 1955, et al.
(författare)
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A new 3 mm band receiver for the Onsala 20 m antenna
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 580
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Tidskriftsartikel (refereegranskat)abstract
- A new receiver for the Onsala 20 m antenna with the possibility of being equipped with 3 mm and 4 mm bands has been built and the 3 mm channel has been commissioned during the Spring 2014. For single-dish operation, the receiver uses an innovative on-source/off-source optical switch. In combination with additional optical components and within the same optical layout, the switch provides two calibration loads (for the 3 mm and 4 mm channels), sideband rejection measurement, and tuning possibilities. The optical layout of the receiver employs all cold (4 K) offset elliptical mirrors for both channels, whereas the on-off switch employs flat mirrors only. The 3 mm channel employs a sideband separation (2SB) dual polarization receiver with orthomode transducer (OMT), 4-8 GHz intermediate frequency (IF), x? 2pol x? upper and lower sidebands (USB? +? LSB). The cryostat has four optical windows made of high density polyethylene (HDPE) with anti-reflection corrugations, two for the signal and two for each frequency band cold load. The cryostat uses a two-stage cryocooler produced by Sumitomo HI? RDK? 408D2 with anti-vibration suspension of the cold-head to minimize impact of the vibrations on the receiver stability. The local oscillator (LO) system is based on a Gunn oscillator with aphase lock loop (PLL) and four mechanical tuners for broadband operation, providing independently tunable LO power for each polarization. This paper provides a technical description of the receiver and its technology and could be useful for instrumentation engineers and observers using the Onsala 20 m telescope.
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| 2. |
- Bergman, Per, 1960, et al.
(författare)
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Emission from HCN and CH3OH in comets Onsala 20-m observations and radiative transfer modelling
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 660
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Tidskriftsartikel (refereegranskat)abstract
- Aims. The aim of this work is to characterise HCN and CH3OH emission from recent comets. Methods. We used the Onsala 20-m telescope to search for millimetre transitions of HCN towards a sample of 11 recent and mostly bright comets in the period from December 2016 to November 2019. Also, CH3OH was searched for in two comets. The HCN sample includes the interstellar comet 2I/Borisov. For the short-period comet 46P/Wirtanen, we were able to monitor the variation of HCN emission over a time-span of about one month. We performed radiative transfer modelling for the observed molecular emission by also including time-dependent effects due to the outgassing of molecules. Results. HCN was detected in six comets. Two of these are short-period comets and four are long-period. Six methanol transitions were detected in 46P/Wirtanen, enabling us to determine the gas kinetic temperature. From the observations, we determined the molecular production rates using time-dependent radiative transfer modelling. For five comets, we were able to determine that the HCN mixing ratios lie near 0.1% using contemporary water production rates, Q(H2O), taken from other studies. This HCN mixing ratio was also found to be typical in our monitoring observations of 46P/Wirtanen but here we notice deviations of up to 0.2% on a daily timescale which could indicate short-time changes in outgassing activity. From our radiative transfer modelling of cometary comae, we find that time-dependent effects on the HCN level populations are of the order of 5-15% when Q(H2O) is around 2 x 10(28) mol s(-1). The effects may be stronger for comets with lower Q(H2O). The exact details of the time-dependent effects depend on the amount of neutral and electron collisions, radiative pumping, and molecular parameters such as the spontaneous rate coefficient.
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| 3. |
- Haas, Rüdiger, 1966, et al.
(författare)
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Status of the Onsala Twin Telescopes – One Year After the Inauguration
- 2019
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Ingår i: IVS 2018 General Meeting Proceedings "Global Geodesy and the Role of VGOS – Fundamental to Sustainable Development". ; NASA/CP-2019-219039, s. 17-19
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Konferensbidrag (refereegranskat)abstract
- We briefly describe the status of the Onsala twin telescopes and the experience gained since the official inauguration in May 2017.
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| 4. |
- Lerner, Mikael, 1967, et al.
(författare)
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Bar-induced mass relocation in galactic discs
- 1999
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Ingår i: Astronomy and Astrophysics. ; 344, s. 483-493
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Tidskriftsartikel (refereegranskat)abstract
- The appearance of a bar in a galaxy considerably changes the mass distribution within the galactic disc. We have therefore performed a detailed study of the mass relocation within a stellar Kuzmin disc using two-dimensional numerical simulations with fixed 3D-potentials representing the bulge and the halo. We have started from a fully axisymmetric stellar distribution and followed the galaxy through bar formation and subsequent evolution. Once the bar has formed, the radial surface density profile will split up into three domains. The two innermost domains both have exponential profiles but with very different slopes. The outermost domain remain Kuzmin-like. Except for the very centre, the inner parts of the galaxy are depopulated out to a distance of approximately twice the length of the bar. The region just outside the bar experiences the most severe depopulation; in the presented simulation, the stellar density in this area falls to one-third its original value. All stars orbiting just outside the bar will be strongly perturbed on each orbit. Whether the star will be accelerated into a more eccentric orbit or decelerated into a less eccentric one depends on the position of the pericentre passage relative to the bar position. Circular orbits within this region are unstable. Stars from the region are thus spread out all over the disc. The stellar density in the outer parts of the galaxy thus increases. Depending on their original orbits, the stars will belong to either of two dynamical populations; stars originating in the outer parts will orbit on roughly circular orbits, while stars ejected by the bar from the inner parts of the disc will move on highly eccentric orbits. Stars can even be ejected into such extreme orbits that they escape from the galaxy. Barred galaxies can thus make a minor contribution to the population of intergalactic stars found in galaxy clusters without requiring near-collisions of galaxies.
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| 5. |
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| 6. |
- Lerner, Mikael, 1967
(författare)
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Numerical Studies of Extragalactic Objects. Analysis of mmVLBI Observations and Simulations of Barred Spiral Galaxies
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Numerical studies related to high-frequency VLBI observations of quasars and dynamics of barred spiral galaxies have been performed. Data from a global 100 GHz VLBI experiment has been reduced and a radio image of the quasar 3C 446 with the highest resolution achieved to date has been produced. A new computer program package ( NEW_LSD ) is presented. This software provides a highly automatic platform for model fitting of Gaussian components to small or incomplete sets of VLBI data. Fundamental problems related to multi-parametric model fitting are analysed, as well as the strategies implemented in this software. A comparison of algorithms included in NEW_LSD shows that methods based on ``simulated annealing'' are the most successful ones. Dynamical simulations of barred galaxies have been performed with a two-dimensional, polar-grid, particle-mesh code. The results show that bars usually grow in length until they reach out to their co-rotation radii, which generally are located where the swing amplification is at maximum. There are clear trends in bar behaviour when the properties of the bulge is changed systematically. A more compact bulge gives a shorter bar rotating with a higher pattern speed. Slow-down rates are, however, rather unaffected by changes to the bulge parameters unless the bulge is very massive and compact. The surface density profile of the disc is radically altered by the bar. The stellar density increases in the very centre and in the outer parts of the disc while there is a massive depopulation just outside the bar region. This depopulation is caused by repeated perturbations from the bar, sending stars outwards on highly elliptical orbits. Stars can even be ejected to escape velocities, and barred galaxies can thus contribute to the population of intergalactic stars. An extra bar-like dynamical component, ``the oscillating feature'', may appear between the end points of the bar and its co-rotation radius. Angular momentum is continuously transferred back and forth between the bar and the oscillating feature leading to periodic oscillations in the pattern speeds of the two components. The oscillations diminish as the oscillating feature is gradually replaced by the growing bar.
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| 7. |
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| 9. |
- Löfdahl, Lennart, 1948, et al.
(författare)
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Determination of Derivative Moments using Two Slanted Hot-Wires and a Spectral method
- 1993
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Ingår i: Experiments in Fluids. ; 15, s. 393-400
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Tidskriftsartikel (refereegranskat)abstract
- A method to determine derivative moments using slanted hot-wires and a spectral method has been developed. The general idea is to minimize measuring volume by using only two hot-wires, and a post-processing of the measurement data. It is shown that the spectral method is equivalent to conventional determinations of correlations. The method developed has been tested in the self-preserving region of a plane wake by computing profiles of the Reynolds stresses as well as the derivative moments in the expression for the total energy dissipation. One term in this expression, which with conventional methods requires two triple-wires or four cross-wires, has been determined using the spectral method.
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| 10. |
- Wirström, Eva, 1977, et al.
(författare)
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HCN observations of comets C/2013 R1 (Lovejoy) and C/2014 Q2 (Lovejoy)
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 588, s. Art. no. A72-
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Tidskriftsartikel (refereegranskat)abstract
- HCN J = 1-0 emission from the long-period comet C/2013 R1 (Lovejoy) was observed from the Onsala Space Observatory on multiple occasions during the month before its perihelion passage on December 22, 2013. We report detections for seven different dates, spanning heliocentric distances (Rh) decreasing from 0.94 to 0.82 au. Estimated HCN production rates are generally higher than previously reported for the same time period, but the implied increase in production rate with heliocentric distance, QHCN Rh -3.2, represent well the overall documented increase since it was first observed at Rh = 1.35. The implied mean HCN abundance relative to water in R1 Lovejoy is 0.2%. We also report on a detection of HCN with the new 3 mm receiver system at Onsala Space Observatory in comet C/2014 Q2 (Lovejoy) on January 14, 2015, when its heliocentric distance was 1.3 au. Relative to comet C/2013 R1 (Lovejoy), the HCN production rate of C/2014 Q2 (Lovejoy) was more than 5 times higher at similar heliocentric distances, and the implied HCN abundance relative to water 0.09%.
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