Paolo G. Calisse

Results of SPARO 2003: Mapping Magnetic Fields in Giant Molecular Clouds

We present results from the Austral Winter 2003 observing campaign of SPARO, a 450 μm polarimeter used with a 2 m telescope at the South Pole. We mapped large-scale magnetic fields in four GMCs in the Galactic disk: NGC 6334, the Carina Nebula, G333.6-0.2, and G331.5-0.1. We find a statistically significant correlation of the inferred field directions with the orientation of the Galactic plane. Specifically, three of the four GMCs (NGC 6334 is the exception) have mean field directions that are within 15° of the plane. The simplest interpretation is that the field direction tends to be preserved during the process of GMC formation. We have also carried out an analysis of published optical polarimetry data. For the closest of the SPARO GMCs, NGC 6334, we can compare the field direction in the cloud as measured by SPARO with the field direction in a larger region surrounding the cloud, as determined from optical polarimetry. For purposes of comparison, we also use optical polarimetry to determine field directions for 9-10 other regions of similar size. We find that the region surrounding NGC 6334 is an outlier in the distribution of field directions determined from optical polarimetry, just as the NGC 6334 cloud is an outlier in the distribution of cloud field directions determined by SPARO. In both cases the field direction corresponding to NGC 6334 is rotated away from the direction of the plane by a large angle. This finding is consistent with our suggestion that field direction tends to be preserved during GMC formation. Finally, by comparing the disorder in our magnetic field maps with the disorder seen in magnetic field maps derived from MHD turbulence simulations, we conclude that the magnetic energy density in our clouds is comparable to the turbulent energy density.

Submillimeter Site Testing at Dome C, Antarctica

We have developed a 350 μm radiometer to perform automated site testing in remote regions of Antarctica. In summer 2000–2001 the instrument operated at Concordia, a new station under construction at Dome C on the Antarctic Plateau. We present the results, and compare them with the atmospheric opacity measured at the South Pole in the same five-week period. During these five weeks, observing conditions at Dome C were, on average, substantially better than those at the South Pole.

Site testing for submillimetre astronomy at Dome C, Antarctica

Aims. Over the past few years a major effort has been put into the exploration of potential sites for the deployment of submillimetre astronomical facilities. Amongst the most important sites are Dome C and Dome A on the Antarctic Plateau, and the Chajnantor area in Chile. In this context, we report on measurements of the sky opacity at 200 μm over a period of three years at the French-Italian station, Concordia, at Dome C, Antarctica. We also present some solutions to the challenges of operating in the harsh polar environment. Methods. The 200-μm atmospheric opacity was measured with a tipper. The forward atmospheric model MOLIERE (Microwave Observation LIne Estimation and REtrieval) was used to calculate the atmospheric transmission and to evaluate the precipitable water vapour content (PWV) from the observed sky opacity. These results have been compared with satellite measurements from the Infrared Atmospheric Sounding Interferometer (IASI) on Metop-A, with balloon humidity sondes and with results obtained by a ground-based microwave radiometer (HAMSTRAD). In addition, a series of experiments has been designed to study frost formation on surfaces, and the temporal and spatial evolution of thermal gradients in the low atmosphere. Results. Dome C offers exceptional conditions in terms of absolute atmospheric transmission and stability for submillimetre astronomy. Over the austral winter the PWV exhibits long periods during which it is stable and at a very low level (0.1 to 0.3 mm). Higher values (0.2 to 0.8 mm) of PWV are observed during the short summer period. Based on observations over three years, a transmission of around 50% at 350 μm is achieved for 75% of the time. The 200-μm window opens with a typical transmission of 10% to 15% for 25% of the time. Conclusions. Dome C is one of the best accessible sites on Earth for submillimetre astronomy. Observations at 350 or 450 μm are possible all year round, and the 200-μm window opens long enough and with a sufficient transparency to be useful. Although the polar environment severely constrains hardware design, a permanent observatory with appropriate technical capabilities is feasible. Because of the very good astronomical conditions, high angular resolution and time series (multi-year) observations at Dome C with a medium size single dish telescope would enable unique studies to be conducted, some of which are not otherwise feasible even from space.

ABLE: Development of an Airborne Lidar

Abstract The acronym ABLE (Airborne Lidar Experiment) identifies a project to develop and fly an optical radar on a stratospheric platform for studies related to atmospheric radiation and composition. The prototype, ABLE 1, has been successfully flown on board the M55 Geophysica aircraft in the Arctic campaign of December 1996–January 1997 to observe stratospheric clouds and aerosol. The lidar, which runs automatically, has been installed in the unpressurized bay of the aircraft where the temperature approaches the low values of external air. The lidar transmitter is based on a Nd:YAG laser, with second and third harmonic outputs. The receiver consists of a 0.3-m Cassegrain telescope and several detection channels to look at different wavelengths and polarizations. A fluid circulation unit connected to the aircraft provides heating control. The instrument can point to the zenith or to the nadir. In the past campaign only λ = 532 nm was utilized: observations were carried out at two polarizations, pointing...

Toward a large telescope facility for submm/FIR astronomy at Dome C

Submillimetre astronomy is the prime technique to unveil the birth and early evolution of stars and galaxies in the local and distant Universe. Preliminary meteorological studies and atmospheric transmission models tend to demonstrate that Dome C might offer atmosphere conditions that open the 200-μm atmospheric windows, and could potentially be a site for a large ground-based telescope facility. However, Antarctic climate conditions might also severely impact and deform any telescope mirror and hardware. We present prerequisite conditions and their associate experiments for defining a large telescope facility for submillimetre astronomy at Dome C: (1) Whether the submm/THz atmospheric windows open from 200 μm during a large and stable fraction of time; (2) The knowledge of thermal gradient and (3) icing formation and their impact on a telescope mirror and hardware. This paper will present preliminary results on current experiments that measure icing, thermal gradient and sky opacity at Dome C. We finally discuss a possible roadmap toward the deployment of a large telescope facility at Dome C.

Observing cosmic microwave background polarization through ice

Ice crystal clouds in the upper troposphere can generate polarisation signals at the μK level. This signal can seriously affect very sensitive ground based searches for Eand B-mode of Cosmic Microwave Background polarisation. In this paper we estimate this effect within the ClOVER experiment observing bands (97, 150 and 220 GHz) for the selected observing site (Llano de Chajnantor, Atacama desert, Chile). The results show that the polarisation signal from the clouds can be of the order of or even bigger than the CMB expected polarisation. Climatological data suggest that this signal is fairly constant over the whole year in Antarctica. On the other hand the stronger seasonal variability in Atacama allows for a 50% of clean observations during the dry season.

Observation of polar stratospheric clouds with the ABLE LIDAR during the APE-POLECAT flight of January 9, 1997

Observations of Polar Stratospheric Clouds (PSCs) were carried out with an airborne lidar on the stratospheric M55 Geophysica aircraft during a .ight from Rovaniemi, Finland, on 9 January, 1997. The clouds were observed at the zenith, downwind from the Norwegian Alps: three PSCs, of somewhat di=erent characteristics, were detected at heights between 23 and 28 km. In two of the clouds, di=erent types of particles seem to coexist: echoes attributable to types I and II PSCs are found in di=erent portions of the clouds. The formation of the PSCs is related to an orographic lee-wave, whose development was forecast by a mesoscale dynamical model used to plan the .ight path. The largest observed PSC displays a complex structure, that appears to be in.uenced by waves of di=erent wavelengths. In particular, lidar and in situ data suggest the presence of a wave having a relatively short length (about 18 km) that overlaps on the main lee-wave. The short wavelength oscillation is thought to play a major role in the cloud development, determining the rapid formation and evaporation of particles and therefore the non-stationary character of the PSC. ? 2003 Elsevier Science Ltd. All rights reserved.

The CℓOVER experiment

CℓOVER is a multi-frequency experiment optimised to measure the Cosmic Microwave Background (CMB) polarization, in particular the B-mode component. CℓOVER comprises two instruments observing respectively at 97 GHz and 150/225 GHz. The focal plane of both instruments consists of an array of corrugated feed-horns coupled to TES detectors cooled at 100 mK. The primary science goal of CℓOVER is to be sensitive to gravitational waves down to r ~ 0.03 (at 3σ)in two years of operations.ClOVER is a multi-frequency experiment optimised to measure the Cosmic Microwave Background (CMB) polarization, in particular the B-mode component. ClOVER comprises two instruments observing respectively at 97 GHz and 150/225 GHz. The focal plane of both instruments consists of an array of corrugated feed-horns coupled to TES detectors cooled at 100 mK. The primary science goal of ClOVER is to be sensitive to gravitational waves down to r similar to 0.03 (at 3 sigma) in two years of operations.

The Effect of a Radome on Submillimeter Site-Testing Measurements

We evaluate the effect that radome transparency has on atmospheric opacity measurements performed by the skydip technique. We show that, except at rather high opacities, it is not sufficient to ignore losses in the radome (or ‘window’) during the data analysis and then subtract them from the derived atmospheric opacity. Perhaps surprisingly, unless radome transparency is correctly modelled, the atmosphere will appear to have a minimum opacity that is many times greater that the radome losses. Our conclusion is that some previous site studies may have significantly underestimated the quality of the best submilli-metre sites, and that the difference between these sites and poorer sites may be much greater than currently believed. We also show that part of the residual 857-GHz opacity at the best sites, currently ascribed to ‘dry-air opacity’, can in fact be just an artefact caused by not properly modelling the radome during the data analysis.

AFOS: probing the UV-visible potential of the Antarctic plateau

The Antarctic Fiber-Optic Spectrometer (AFOS) is a 30cm Newtonian optical telescope that injects light through six 30m long optical fibers onto a 240-850nm spectrograph with a 1024 x 256 pixel CCD camera. The telescope is mounted on a dual telescope altitude-azimuth mount and has been designed to measure the transperency of the atmosphere above the South Pole for astronomy in the UV and visible wavelength regions. The instrument has observed a series of bright O and B stars during the austral winters of 2002 and 2003 to probe the UV cutoff wavelength, the auroral intensity and water vapour content in the atmosphere above the plateau. AFOS is the first completely automated optical telescope on the Antarctic Plateau. This paper reports on the results of the past two austral winters of remote observing with the telescope as well as the technical and software modifications required to improve the quality and automation of the observations. The atmospheric absorption bands in the 660-900nm regions of the spectra have been fitted with MODTRAN atmospheric models and used to calculate the precipitable water vapour above the South Pole. These data are then compared to those collected concurrently by radiosonde and by a 350 μm submillimeter tipper at South Pole.