A review of SOHO/UVCS observations of sungrazing comets

In the last 10 years more than 1000 sungrazing comets have been discovered by the LASCO coronagraphs aboard SOHO the spacecraft; from this huge amount of data it has been possible to study the common origin of these comets and to explain some of the main peculiarities observed in their lightcurves. Moreover, the UV Coronagraph Spectrometer (UVCS) aboard SOHO allowed EUV spectroscopy of sungrazers in the final stage of their trajectory (i.e. between 1.4 and 10 solar radii), but a few sungrazers have been observed with this instrument. In this paper we review the main results from the UVCS observation of sungrazers C/1996 Y1, C/2000 C6 and C/2001 C2, discussing also the first possible detection of two fragments and the determination of the pyroxene dust grain number density in the latter one. Preliminary results on the UVCS data interpretation of a sungrazer observed in 2002 (C/2002 S2) are also presented here.     

A review of radio observations of planetary nebulae

The observational data for planetary nebulae in the radio region of the spectrum is reviewed and summarized. The methods of determining interstellar extinction, electron temperatures and turnover points in the spectrum from these observations are given, as are the tests for the recombination theory and a discussion of the validity of several models of these objects.     

Radio observations of Jupiter-family comets

Radio observations from decimetric to submillimetric wavelengths are now a basic tool for the investigation of comets. Spectroscopic observations allow us: (i) to monitor the gas production rate of the comets, by directly observing the water molecule, or by observing secondary products (e.g., the OH radical) or minor species (e.g., HCN); (ii) to investigate the chemical composition of comets; (iii) to probe the physical conditions of cometary atmospheres: kinetic temperature and expansion velocity. Continuum observations probe large-size dust particles and (for the largest objects) cometary nuclei.Comets are classified from their orbital characteristics into two separate classes: (i) nearly isotropic, mainly long-period comets and (ii) ecliptic, short-period comets, the so-called Jupiter-family comets (JFCs). These two classes apparently come from two different reservoirs, respectively, the Oort cloud and the trans-Neptunian scattered disc. Due to their different history and—possibly—their different origin, they may have different chemical and physical properties that are worth being investigated.The present article reviews the contribution of radio observations to our knowledge of the JFCs. The difficulty of such a study is the commonly low gas and dust productions of these comets. Long-period, nearly isotropic comets from the Oort cloud are better known from Earth-based observations. On the other hand, JFCs are more easily accessed by space missions. However, unique opportunities to observe JFCs are offered when these objects come by chance close to the Earth (like 73P/Schwassmann-Wachmann 3 in 2006), or when they exhibit unexpected outbursts (as did 17P/Holmes in 2007).About a dozen JFCs were successfully observed by radio techniques up to now. Four to ten molecules were detected in five of them. No obvious evidence for different properties between JFCs and other families of comets is found, as far as radio observations are concerned.     

IUE observations of faint comets

Ultraviolet spectra of seven comets taken with the same instrument are presented. Comets P/Encke (1980), P/Tuttle (1980 h), P/Stephan-Oterma (1980 g), and Meier (1980 q) were observed during November-December 1980 with the International Ultraviolet Explorer (IUE) satellite observatory, while comets P/Borrelly (1980 i) and Panther (1980 u) were observed with IUE on 6 March 1981. The spectra of these comets are compared with those of comet Bradfield (1979 X), studied extensively earlier in 1980 with IUE, as well as with each other. In order to simplify the interpretation of the data and to minimize the dependence upon a specific model, the spectra are compared at approximately the same value of heliocentric distance whenever possible. Effects due to heliocentric velocity, geocentric distance, and optical depth are also discussed. All of the cometary spectra are remarkably similar, which suggests that these comets may have a common composition and origin.     

Polarimetry of comets: A review

Some attempts of polarimetric sounding of Comet Halley will be undertaken from the flyby probes. In order to facilitate the final planning and the future interpretation of these experiments we have done a thorough analysis of practically all available polarimetric observations. An emphasis is made on interpretation attempts and their discussion. The results of the phase dependence of polarization investigations are presented covering a wide range of phase angles . The chief peculiarities of this dependence are: maximum polarization at = 90, diminishing through zero at 20, negative values up to several per cent and a final growth to zero at zero . A division into gaseous and dusty comets on polarimetric basis is revealed. The wavelength dependence of polarization is discussed. The numerous results of detailed polarimetry are compared to the negative results of attempts to detect the elliptical polarization. New observational problems arising from the evidence given by the negative polarization at small phase angles and by the opposition effect recently discovered are discussed.     

The origin of the CN radical in comets: A review from observations and models

The origin of CN radicals in comets is not completely understood so far. We present a study of CN and HCN production rates and CN Haser scale lengths showing that: (1) at heliocentric distances larger than 3 AU, CN radicals could be entirely produced by HCN photolysis; (2) closer to the Sun, for a fraction of comets CN production rates are higher than HCN ones whereas (3) in the others, CN distribution cannot be explained by the HCN photolysis although CN and HCN production rates seem to be similar. Thus, when the comets are closer than 3 AU to the Sun, an additional process to the HCN photolysis seems to be required to explain the CN density in some comets.The photolysis of HC₃N or C₂N₂ could explain the CN origin. But the HC₃N production rate is probably too low to reproduce CN density profile, even if uncertainties on its photolysis leave the place for all possible conclusions. The presence of C₂N₂ in comets is a reliable hypothesis to explain the CN origin; thus, its detection is a challenging issue. Since C₂N₂ is very difficult to detect from ground-based observations, only in situ measurements or space observations could determine the contribution of this compound in the CN origin.Another hypothesis is a direct production of CN radicals by the photo- or thermal degradation of complex refractory organic compounds present on cometary grains. This process could explain the spatial profile of CN inside jets and the discrepancy noted in the isotopic ratio ¹⁴N/¹⁵N between CN and HCN. Laboratory studies of the thermal and UV-induced degradation of solid nitrogenated compounds are required to model and validate this hypothesis.     

A baseband recorder for radio pulsar observations

Digital signal recorders are becoming widely used in several subfields of centimetre-wavelength radio astronomy. We review the benefits and design considerations of such systems and describe the Princeton Mark IV instrument, an implementation designed for coherent-dedispersion pulsar observations. Features of this instrument include corrections for the distortions caused by coarse quantization of the incoming signal, as well as algorithms that effectively excise both narrow-band and broad-band radio frequency interference. Observations at 430 MHz, using the Mark IV system in parallel with a system using a 250-kHz filter bank and incoherent dedispersion, demonstrated timing precision improvement by a factor of 3 or better for typical millisecond pulsars.     

Decametric radio emission from comets: An attempt at detection

Radio continuum emission from comets may arise as thermal emission from ice or dust particles and free electrons, or as nonthermal emission due to plasma effects in the tail. Nonthermal emission is expected to become intense at the longer, possibly hectometer or even kilometer wavelengths. An attempt at the detection of a comet at the longest possible wavelength observable from the ground was made during the passage of P/Crommelin in March 1984 using the Teepe Tee array of the Clark Lake Radio Observatory. Although radio emission was not detected during our observations, we are able to place an upper limit on the thermal emission, and consequently on the electron density in the tail which we estimate was less than 5.6 × 10² − 8.9 × 10³ electrons cm⁻³.     

Probing the interior of asteroids and comets using radio reflection tomography

Abstract— Asteroids and comets are of great scientific interest: their interior structure and composition, which are poorly known, provide information about conditions and processes that occurred during the early stages of solar system development. They are also of interest for social and economic reasons. Their proximity to Earth and abundance in the solar system make them potential sources of raw materials as well as a threat, as evidenced by past catastrophic impacts. Information on their composition and structure is therefore important to assess both the potential benefit of these objects and mitigate the potential risk they pose. This paper describes the use of radio reflection tomography for studying the interiors of asteroids and comets. We discuss technical issues regarding benefits and challenges of implementing a radio reflection tomography instrument and present potential solutions. This paper addresses a range of topics including (1) data collection scenarios, (2) data processing and inversion, and (3) instrument implementation. A “strawman” instrument capable of imaging the full interior of an asteroid or a comet with dimensions of a few kilometers is presented. Such an instrument can play a significant role in studying the near‐Earth objects, both for scientific and socio‐economic purposes.     

A study of filament transition sheath from radio observations

We have observed an H dark filament at 8, 15, and 22 GHz and derived the radio spectrum of the filament. We suggest that the filament has to be optically thick at radio frequencies and that the observed spectrum is due to the presence of a transition sheath surrounding the filament. We examine a model for the transition sheath in which the energy radiated away is balanced by the conduction of heat from the corona, and show that the radio observations indicate that little or no thermal energy is conducted into the main body of the filament. We compare the model with ultraviolet observations of filaments and discuss how the discrepancies can be removed.On leave of absence from Tata Institute of Fundamental Research, Bombay, India.     

A review of penetrometers for subsurface access on comets and asteroids

Abstract— The characterization of comet and asteroid interiors will eventually require in situ exploration with drills, penetrators/penetrometers, hypervelocity impactors, excavators or other devices. Because they offer desirable scientific capabilities and relative mechanical simplicity, penetrators and penetrometers, which use only axial force to push beneath the surface, are a good choice for near‐term missions. Penetrometers are instruments, generally deployed from a larger vehicle, that measure subsurface mechanical properties and may also contain additional scientific instruments. There are three basic types: “fast” penetrometers are released from above and plunge into the surface. Static and dynamic (collectively referred to as “slow”) penetrometers use, respectively, a constant slow penetration speed and periodic hammering impulses. The low gravity environment of asteroids and comets presents a key challenge to instrument deployment and also greatly affects the mechanical properties of surface materials, and in turn penetrometer performance. The Rosetta mission, currently en route to comet 67P/Churyumov‐Gerasimenko, will be the next mission to try both fast and slow, dynamic penetrometry, when it arrives in 2014. We present some new concepts of static penetrometers for small body exploration that are adapted to the low gravity environment. The low gravity environment also presents challenges for the testing of penetrometers on Earth and a number of previous solutions are described and new methods suggested. In the next generation of missions to study comets and asteroids, penetrometers could provide important data on their mechanical, seismic, thermal, electromagnetic, and chemical characteristics, as well as sample collection.     

CCD observations of distant comets from Palomar and Steward Observatories

We are conducting a ground-based observational study of distant cometary nuclei with the aim of increasing the current database of physical parameters of individual objects, and to estimate the overall distributions of size, rotation period, axial ratio, and color indices. Additionally, we are obtaining CCD spectroscopy and photometry of established and potential targets of current and future spacecraft missions. The results presented here are derived from CCD imaging obtained using the 2.3-m Bok telescope of Steward Observatory (Arizona), obtained in May 2001, and the 5-m Hale telescope at Palomar Observatory (California), obtained in May 2000 and March 2001. Comets observed include 4P/Faye, 6P/d'Arrest, 22P/Kopff, 36P/Whipple, 50P/Arend, 78P/Gehrels 2, 92P/Sanguin, 107P/Wilson-Harrington, and 128P/Shoemaker-Holt 1-A. Of the nine comets observed, only Comets 4P/Faye and 50P/Arend displayed visible coma activity. We have performed either single R filter or multi-filter (BVRI) measurements on these comets, from which we obtain radius and broadband color estimates as well as Afρ values for the active comets. For selected objects we have performed time-series R filter imaging from which we have derived the rotation period and lower limits on the nuclear axial ratio and density. The radius results obtained are included in the cometary nucleus size distribution estimate by Weissman and Lowry (2003).     

Suppression of atmospheric radio emission fluctuations in radio astronomical observations

The fluctuations of radio emission of the atmosphere (for the present only cloudy) are a substantial limiting factor for observations of cosmic radio sources on centimetre and millimetre wavelengths. For suppressing these fluctuations the dual-beam method (differential receiving from two slightly separated direction) is widely used. At the same time works for perfecting this method are going on as well as in searching other possibilities with the principal aim of doing away limitations on sizes of sources under observation. There are considered here these methods with a competitive evaluation of their possibilities for suppressing fluctuation of the atmosphere radio emissions.     

MERLIN radio observations of CfA Seyferts

We present new 1.6-GHz (18-cm) MERLIN maps of 15 Seyfert galaxies, with angular resolutions typically 0.1 to 0.3 arcsec. These and previous observations are used to investigate the properties of 19 of the 24 CfA Seyfert galaxies brighter than 2 mJy at 8.4 GHz. This is the first time a significant fraction of the CfA sample has been mapped at this frequency with subarcsecond resolution, and our observations provide the highest resolution radio maps available for several sources. We use our observations to measure the two-point spectral indices of compact radio components, and we investigate the correlation between infrared and radio emission shown by Seyfert galaxies.
Our results can be summarized as follows. Resolved structures as small as 20 pc are found in three previously unresolved radio sources, and only four sources show single, unresolved radio components. The mean 1.6 to 8.4 GHz spectral index of 31 radio components is         , and approximately 25 per cent of the components have a spectral index flatter than     . The spectral index distributions of type 1 and type 2 Seyferts are statistically indistinguishable. The cores of multiple-component sources tend to have flatter radio spectra than secondary components. The low-resolution infrared ( IRAS ) emission from Seyfert galaxies is usually dominated by kiloparsec-scale, extranuclear emission regions.     

Ionospheric refraction in radio source observations at long radio wavelengths

Ionospheric refraction effects encountered in radio source observations in the 30 to 75 MHz range with the Clark Lake TPT telescope are discussed. It is found that simple calibration procedures are sufficient to provide positions of unknown sources with an accuracy of approximately one arcmin. Observations made near sunrise, or during disturbed ionospheric conditions must be discarded. If no corrections are applied, RMS errors of a few arcmin are to be expected. On leave at Radiosterrenwacht Dwingeloo, The Netherlands     

A comparison of type III solar radio burst theories using satellite radio observations and particle measurements

The required electron density to excite a type III solar burst can be predicted from different theories, using the low frequency radio observations of the RAE-1 satellite. Electron flux measurements by satellite in the vicinity of 1 AU then give an independent means of comparing these predicted exciter electron densities to the measured density. On this basis, one theory predicts the electron density in closest agreement with the measured values.NAS/NRC Postdoctoral Resident Research Associate.     

A review of East Asian reports of aurorae and comets circa AD 775

Given that a strong ¹⁴C variation in AD 775 has recently been suggested to be due to the largest solar flare ever recorded in history, it is relevant to investigate whether celestial events observed around that time may have been aurorae, possibly even very strong aurorae, or otherwise related to the ¹⁴C variation (e.g. a suggested comet impact with Earth's atmosphere). We critically review several celestial observations from AD 757 to the end of the 770s, most of which were previously considered to be true, and in some cases, strong aurorae; we discuss in detail the East Asian records and their wording. We conclude that probably none among the events after AD 770 was actually an aurora, including the event in AD 776 Jan, which was misdated for AD 774 or 775; the observed white qi phenomenon that happened above the moon in the south‐east was most probably a halo effect near the full Moon – too late in any case to be related to the ¹⁴C variation in AD 774/5. There is another report of a similar (or identical) white qi phenomenon above the moon, reported just before a comet observation and dated to AD 776 Jan; the reported comet observed by the Chinese was misdated to AD 776, but actually sighted in AD 767. Our critical review of East Asian reports of aurorae circa AD 775 shows some very likely true Chinese auroral displays observed and reported for AD 762; there were also several events prior to AD 771 that may have been aurorae but are questionable. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)