Large Heliocentric Distances Research Articles

Rotation of an erupting filament observed by the STEREO EUVI and COR1 instruments

On August 31, 2007, a prominence eruption was observed by the Solar TErrestrial RElations Observatory (STEREO) in the Extreme-UltraViolet Imager (EUVI) 304 images and later on, as the core of a three-part coronal mass ejection (CME) in images acquired by the inner STEREO coronagraph (COR1). Because they were covered by both STEREO spacecraft from right vantage points, these observations provide an excellent opportunity to perform a three-dimensional (3D) prominence reconstruction and study its evolution. We employed the tie-pointing technique to reconstruct the 3D shape and trajectory of the prominence, which has been followed from an heliocentric distance of ~1.3 up to ~2.4 R ⊙ during the first 1.3 h of eruption. Data show evidence for a progressive clockwise prominence rotation by ~90° occurring not only in the early phase of the eruption sampled by EUVI, but also at larger heliocentric distances as seen by COR1. Interestingly, a counter-clockwise rotation of the filament was observed in Hα images in the week before the eruption; the filament does not show a twisted shape. In the same period, the potential field extrapolated at different times shows a clockwise rotation of closed lines overlying the filament. This suggests that a magnetic helicity storage occurred not in the filament itself, but in the global magnetic field configuration of the surrounding corona. Moreover, close inspection to the high-resolution EUVI images revealed a small scale helical feature along the erupting prominence. The sense of rotation of this helix agrees with the observed prominence rotation, providing evidence for the conversion of twist into writhe. The observed rotation of an erupting prominence, if representative of the flux rope rotation, may have a strong impact on the definition of geo-effectiveness of CMEs for space weather forecasting purposes.

THERMODYNAMICS OF THE SOLAR CORONA AND EVOLUTION OF THE SOLAR MAGNETIC FIELD AS INFERRED FROM THE TOTAL SOLAR ECLIPSE OBSERVATIONS OF 2010 JULY 11

We report on multi-wavelength observations of the corona taken simultaneously in broadband white light, and in seven spectral lines, H-alpha 656.3 nm, Fe IX 435.9 nm, Fe X 637.4 nm, Fe XI 789.2 nm, Fe XIII 1074.7 nm, Fe XIV 530.3 nm and Ni XV 670.2 nm. The observations were made during the total solar eclipse of 11 July 2010 from the atoll of Tatakoto in French Polynesia. Simultaneous imaging with narrow bandpass filters in each of these spectral lines and in their corresponding underlying continua maximized the observing time during less than ideal observing conditions and yielded outstanding quality data. The application of two complementary image processing techniques revealed the finest details of coronal structures at resolution in white light, and 6.5 in each of the spectral lines. This comprehensive wavelength coverage confirmed earlier eclipse findings that the solar corona has a clear two-temperature structure: The open field lines, expanding outwards from the solar surface, are characterized by electron temperatures near 1 X 10(exp 6) K, while the hottest plasma around 2X 10(exp 6) K resides in loop-like structures forming the bulges of streamers. The first images of the corona in the forbidden lines of Fe IX and Ni XV, showed that there was very little coronal plasma at temperatures below 5 X 10(exp 5) K and above 2.5X 10(exp 6) K. The data also enabled temperature differentiations as low as 0:2 X 10(exp 6) K in different density structures. These observations showed how the passage of CMEs through the corona, prior to totality, produced large scale ripples and very sharp streaks, which could be identified with distinct temperatures for the first time. The ripples were most prominent in emission from spectral lines associated with temperatures around 10(exp 6) K. The most prominent streak was associated with a conical-shaped void in the emission from the coolest line of Fe IX and from the hottest line of Ni XV. A prominence, which erupted prior to totality, appeared in the shape of a hook in the cooler lines of Fe X and Fe XI, spanning 0.5 R(solar) in extent starting at a heliocentric distance of 1.3 R(solar), with a complex trail of hot and cool twisted structures connecting it to the solar surface. Simultaneous Fe X 17.4 nm observations from space by Proba2/SWAP provided an ideal opportunity for comparing emission from a coronal forbidden line, namely Fe X 637.4 nm, with a space-based EUV allowed line. Comparison of the Fe X 17.4 nm and 637.4 nm emission provided the first textbook example of the role of radiative excitation in extending the detectability of coronal emission to much larger heliocentric distances than its collisionally excited component. These eclipse observations demonstrate the unique capabilities of coronal forbidden lines for exploring the evolution of the coronal magnetic field in the heliocentric distance range of 1 - 3 R(solar), which is currently inaccessible to any space-borne or ground-based observatory.

Determining CME parameters by fitting heliospheric observations: Numerical investigation of the accuracy of the methods

Transients in the heliosphere, including coronal mass ejections (CMEs) and corotating interaction regions can be imaged to large heliocentric distances by heliospheric imagers (HIs), such as the HIs onboard STEREO and SMEI onboard Coriolis. These observations can be analyzed using different techniques to derive the CME speed and direction. In this paper, we use a three-dimensional (3-D) magneto-hydrodynamic (MHD) numerical simulation to investigate one of these methods, the fitting method of Sheeley et al. (1999) and Rouillard et al. (2008). Because we use a 3-D simulation, we can determine with great accuracy the CME initial speed, its speed at 1 AU and its average transit speed as well as its size and direction of propagation. We are able to compare the results of the fitting method with the values from the simulation for different viewing angles between the CME direction of propagation and the Sun-spacecraft line. We focus on one simulation of a wide (120–140°) CME, whose initial speed is about 800 km s −1. For this case, we find that the best-fit speed is in good agreement with the speed of the CME at 1 AU, and this, independently of the viewing angle. The fitted direction of propagation is not in good agreement with the viewing angle in the simulation, although smaller viewing angles result in smaller fitted directions. This is due to the extremely wide nature of the ejection. A new fitting method, proposed to take into account the CME width, results in better agreement between fitted and actual directions for directions close to the Sun–Earth line. For other directions, it gives results comparable to the fitting method of Sheeley et al. (1999). The CME deceleration has only a small effect on the fitted direction, resulting in fitted values about 1–4° higher than the actual values.

67P/Churyumov-Gerasimenko at large heliocentric distance

Aims. The Jupiter family comet (JFC) 67P/Churyumov-Gerasimenko (67P/C-G) is the target of ESA's ROSETTA mission. Observational campaigns and theoretical investigations were performed to characterise 67P/C-G in terms of nucleus properties (e.g. size, surface colours, rotational period), gas and dust production, and plasma environment in preparation for the rendezvous with the spacecraft; however, there are still open questions that need to be addressed. Our observations of 67P/C-G are important not only for a good planning of the rendezvous of the ROSETTA spacecraft with 67P/C-G, but also for providing valuable information on the basic physical properties of the nuclei of JFCs at large heliocentric distances. Moreover, this information will help to identify to what extent 67P/C-G is a typical JFC. Methods. We performed broad-band imaging and low-resolution spectroscopy of 67P/C-G in the visible wavelength range during five periods between April 2004 and July 2007 at the ESO Very Large Telescope (VLT) with the FORS2 instrument. At the time of the observations 67P/C-G was at heliocentric distance r > 4.6 AU. The imaging data were used to search for a faint coma, to improve the phase function of the nucleus, to constrain its rotational period, and to analyse the neck-line of dust close to the nucleus. Results. The comet appears point-like and no coma signature was found around the nucleus. The most realistic representative of the phase function of 67P/C-G is the linear approximation: This could be interpreted to mean that the opposition effect is not very pronounced for 67/C-G in the phase angle range between 0.5° and 10°. We determined that the magnitude dependence on the phase angle is very steep, with the linear phase coefficient in the range β = 0.061-0.076 mag/°. The colour indices and reflectance spectrum show that the nucleus of 67P/C-G is slightly redder than the Sun (spectral slope ∼11%/1000 A) and uniform with the rotational phase. A tail-like structure of heavy grains is detected in April 2004, June 2004, and May 2006. Based on its orientation and the variations of its surface brightness we interpreted it as a neck-line phenomenon.

Influence of spacecraft outgassing on the exploration of tenuous atmospheres with in situ mass spectrometry

In situ mass spectrometry has been a powerful tool in many space missions to investigate atmospheres and exospheres of different bodies in the solar system. Applying new technologies, the mass spectrometers have become increasingly more sensitive. In this study, we show that spacecraft outgassing, which can never be completely prevented, will be the limiting factor in future missions that investigate very tenuous atmospheres and exospheres of moons, asteroids, or comets at large heliocentric distances. The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument on the European Space Agency Rosetta mission has monitored spacecraft outgassing for 6 years during the cruise phase with unprecedented instrument sensitivity. It is shown that diffusion of gas from materials and from the spacecraft interior plays an important role in maintaining a relatively permanent thin gas cloud around the spacecraft for many years. The density and composition of this gas cloud depends on location on the spacecraft, maneuvers, and payload activity. The main contaminants are water, which is adsorbed on cold surfaces, and organics from the spacecraft structure, electronics, and insulations. Decomposed lubricant material can give a significant contribution to the total background. Fortunately for Rosetta, outgassing of the spacecraft will play a minor role when the comet is close to perihelion; only in the early phase of the mission the outgassing may be larger than the cometary signature.

Observations of the long-lasting activity of the distant Comets 29P Schwassmann–Wachmann 1, C/2003 WT42 (LINEAR) and C/2002 VQ94 (LINEAR)

We investigated three comets, which are active at large heliocentric distances, using observations obtained at the 6-m BTA telescope (SAO RAS, Russia) in the photometric mode of the focal reducer SCORPIO. The three comets, 29P/Schwassmann–Wachmann 1, C/2003 WT42 (LINEAR), and C/2002 VQ94 (LINEAR), were observed after their perihelion passages at heliocentric distances between 5.5 and 7.08 AU. The dust production rates in terms of Afρ was measured for these comets. Using the retrieved values, an average dust production rate was derived under different model assumptions. A tentative calculation of the total mass loss of the comet nucleus within a certain observation period was executed. We calculated the corresponding thickness of the depleted uppermost layer where high-volatile ices completely sublimated. The results obtained in our study strongly support the idea that the observed activity of Comet SW1 requires a permanent demolition of the upper surface layers.

The effect of internal inhomogeneity on the activity of comet nuclei – Application to Comet 67P/Churyumov–Gerasimenko

We present thermal evolution calculations of inhomogeneous asymmetric initial configurations of a spherical model of Comet 67P/Churyumov–Gerasimenko, using a fully 3-dimensional numerical code. The initial composition is amorphous H 2O ice and dust, in a “layered-pile” configuration, where layers differing in ice/dust ratio and thermal properties extend over a fraction of the surface area and about 10 m in depth and may overlap. We analyze the effect of one such layer, as well as the combined effect of many layers, randomly distributed. We find that internal inhomogeneities affect both the surface temperature and the activity pattern of the comet. In particular, they may lead to outbursts at large heliocentric distances and also to activity on the night-side of the nucleus. The rates of ablation and depths of dust mantle and crystalline ice outer layer as functions of longitude and latitude are shown to be affected as well.

Effect of the heliospheric interface on the distribution of interstellar hydrogen atom inside the heliosphere

This paper deals with the modeling of the interstellar hydrogen atoms (H atoms) distribution in the heliosphere. We study influence of the heliospheric interface, that is the region of the interaction between solar wind and local interstellar medium, on the distribution of the hydrogen atoms in vicinity of the Sun. The distribution of H atoms obtained in the frame of the self-consistent kinetic-gasdynamic model of the heliospheric interface is compared with a simplified model which assumes Maxwellian distribution of H atoms at the termination shock and is called often as 'hot' model. This comparison shows that the distribution of H atoms is significantly affected by the heliospheric interface not only at large heliocentric distances, but also in vicinity of the Sun at 1-5 AU. Hence, for analysis of experimental data connected with direct or undirect measurements of the interstellar atoms one necessarily needs to take into account effects of the heliospheric interface. In this paper we propose a new model that is relatively simple but takes into account all major effects of the heliospheric interface. This model can be applied for analysis of backscattered Ly-alpha radiation data obtained on board of different spacecraft.

Probing the relation between the structure of initial proto-planetary disc and the Oort-cloud formation

Aims. The Oort cloud consists of cometary nuclei which were ejected from the once existing proto-planetary disc to large heliocentric distances by the giant planets. The process of the cloud formation depended on the initial structure and mass of the disc. Considering four models of an initial proto-planetary disc, we roughly probe this dependence.Methods. We use the resultant data of our previous simulation of the Oort cloud formation for the first two Gyr. The considered disc models consist of a set of representative test particles. The new models are created subtracting a fraction of the particles from the model considered in our previous work, in a way to obtain the required heliocentric-distance distribution. Specifically, we focus on the situations in which a part of the small bodies in the disc is assumed to be already spent in the previous process of the giant planet formation. We omit the particles from an originally smooth profile in the regions adjacent to the planet orbits. With the reduced data, we construct the comet cloud characteristics we are interested in.Results. We find that it is difficult to construct the proto-planetary disc if (i) the amount of heavy chemical elements in Jupiter and Saturn is as high as currently accepted (≈ 20 and ≈ ; respectively) and (ii) the total mass of the minimum-mass solar nebula is assumed to be lower than ≈ . The behaviour of the Oort cloud formation does not crucially depend on the initial disc model. Some quantitative differences in its structure are obvious: since the cloud is known to be filled mainly by Uranus and Neptune, the efficiency of its formation is higher if the initial amount of particles in the Uranus-Neptune region is relatively higher. The efficiency is also higher in the gapped-disc models because a less amount of particles experience a very close encounter with a planet resulting in their ejection into the interstellar space.

A survey on the distant activity of short period comets

Aims. The aim of this paper is to build up an inventory of observations of short period comets (SPCs) far from the Sun, and perform an analysis of general properties of the family, investigating the link between distant activity and present and historical orbital parameters.Methods. We gathered all the data available in the literature (as per June 2009) on SPCs ground-based and space observations at heliocentric distance r h > 3 AU. We analyzed the occurrence of distant activity with respect to: position on orbital branch, present orbital parameters (in particular the perihelion distance q ), and their recent variations.Results. There is no sharp cut-off at any heliocentric distance, beyond which the cometary activity fades and only bare nuclei are observed. SPCs are more likely to be active post-perihelion than pre-perihelion: among a dataset of 90 comets (for which a heliocentric distance of analysis has been unequivocally established), 59% of the comets observed post-perihelion were detected as active targets, while only 22% of the comets observed pre-perihelion were active. There is a weak trend of comets with increasing perihelion distance to be more likely active at large heliocentric distance: among comets with q > 3 AU, 14 out of 16 comets (88%) have been classified as active. A much less clear trend has been observed for distant cometary activity with semi-major axis a . No apparent appreciable link of a recent (or secular) increase of the perihelion temperature (due to a rapid decrease in the perihelion distance) with the present degree of distant activity has been observed.

A TEM study of four particles extracted from the Stardust track 80

Abstract— Four particles extracted from track 80 at different penetration depths have been studied by analytical transmission electron microscopy (ATEM). Regardless of their positions within the track, the samples present a comparable microstructure made of a silica rich glassy matrix embedding a large number of small Fe‐Ni‐S inclusions and vesicles. This microstructure is typical of strongly thermally modified particles that were heated and melted during the hypervelocity impact into the aerogel. X‐ray intensity maps show that the particles were made of Mg‐rich silicates (typically 200 nm in diameter) cemented by a fine‐grained matrix enriched in iron sulfide. Bulk compositions of the four particles suggest that the captured dust particle was an aggregate of grains with various iron sulfide fraction and that no extending chemical mixing in the bulb occurred during the deceleration. The bulk S/Fe ratios of the four samples are close to CI and far from the chondritic meteorites from the asteroidal belt, suggesting that the studied particles are compatible with chondritic‐porous interplanetary dust particles or with material coming from a large heliocentric distance for escaping the S depletion.

Some active processes in comet icy nuclei: nucleus splitting and anti tail formation

AbstractThe most dramatic display of variable activity of a comet is splitting of the nucleus. For the purpose of revealing the trends of splitting of comet nuclei and of formation of abnormal cometary tails, we have created two catalogues of comets: a catalogue of split nuclei, containing 99 comets, and a catalogue of comets with abnormal tails, including 60 objects. Statistical investigation reveals some general trends of these phenomena. The greatest number of recorded cases of nucleus splitting and abnormal tail (60%) occurs within an interval of heliocentric distance ranging from 0.6 AU to 1.6 AU (maximum at 1.1 AU) and geocentric distance ranging from 0.6 AU to 1.8 AU (maximum at 1.15 AU). Splitting of nuclei and abnormal tails are more often (75%) recorded close to the perihelia of the cometary orbits. Only 16% of splitting comets also exhibit abnormal tails. Some cases of nuclear splitting and large velocity (some km/s) eruptions of dust from a nucleus, as well as cases of abnormal tails developed at large heliocentric distances, may indicate collisions of comet nuclei with other bodies. Our results are of interest for the physics of comets, and for the distribution of meteoroids in solar system.

THE ROTATION PERIOD AND LIGHT-CURVE AMPLITUDE OF KUIPER BELT DWARF PLANET 136472 MAKEMAKE (2005 FY9)

Kuiper Belt dwarf planet 136472 Makemake, formerly known as 2005 FY9, is currently the third-largest known object in the Kuiper Belt, after the dwarf planets Pluto and Eris. It is currently second only to Pluto in apparent brightness, due to Eris' much larger heliocentric distance. Makemake shows very little photometric variability, which has prevented confident determination of its rotation period until now. Using extremely precise time-series photometry, we find that the rotation period of Makemake is 7.7710 ± 0.0030 hr, where the uncertainty is a 90% confidence interval. An alias period is detected at 11.41 hr, but is determined with approximately 95% confidence not to be the true period. Makemake's 7.77 hr rotation period is in the typical range for Kuiper Belt objects, consistent with Makemake's apparent lack of a substantial satellite to alter its rotation through tides. The amplitude of Makemake's photometric light curve is 0.0286 ± 0.0016 mag in V. This amplitude is about 10 times less than Pluto's, which is surprising given the two objects' similar sizes and spectral characteristics. Makemake's photometric variability is instead similar to that of Eris, which is so small that no confident rotation period has yet been determined. It has been suggested that dwarf planets such as Makemake and Eris, both farther from the Sun and colder than Pluto, exhibit lower photometric variability because they are covered with a uniform layer of frost. Such a frost is probably the correct explanation for Eris. However, it may be inconsistent with the spectrum of Makemake, which resembles reddish Pluto more than neutrally colored Eris. Makemake may instead be a more Pluto-like object that we observe at present with a nearly pole-on viewing geometry—a possibility that can be tested with continuing observations over the coming decades.

The distant activity of the Long Period Comets C/2003 O1 (LINEAR) and C/2004 K1 (Catalina)

Aims. We study the distant dust environment of two Long Period Comets: C/2003 O1 (LINEAR) (observed at r h = 7.39 AU) and C/2004 K1 (Catalina) (observed at r h = 3.43 AU). The case of C/2003 O1 is particularly interesting since the comet has a quite large perihelion heliocentric distance (r h = 6.85 AU).Methods. We analysed R -band images taken at the CAHA 2.2 m telescope to characterise the properties of the cometary dust coma. The images of both comets were also used as input into an inverse numerical model, to derive information about the dynamical parameters of coma dust grains until a time ~500 days before the observation.Results. Both the comets appeared to be active, and of similar shape and size despite the great difference in their heliocentric distance of observation. C/2003 O1 exhibited a tail extending to at least 3.7 105 km. It is a very active object, since its Af ρ is 552 ± 36 cm within the inner 5”. C/2004 K1 showed a similar long tail, extending to 2.6 105 km. Its Af ρ is 539 ± 35 cm within the inner 5”. An almost constant dust grain ejection velocity between 0.5 and 0.9 m/s at 1 cm dust size has been derived for C/2003 O1 and an increase from 0.5 to 2 m/s at 1 cm dust size for C/2004 K1, reflecting the different volatile dragging the dust environment (probably CO for C/2003 O1 and water for C/2004 K1). Model results allow some speculations about the comet nucleus size of C/2003 O1 (LINEAR) and its CO content: for a Q CO similar in value to those observed for other distant cometary objects, a comet radius R n from 13 to 17 km can be inferred.

Few Comments on the Relation Between the Initial Proto-planetary Disc Model and the Oort Cloud Formation

A fraction of small bodies from the once existing proto-planetary disc was ejected, by the giant planets, to large heliocentric distances and start to build the comet Oort cloud. Considering four models of initial proto-planetary disc, we attempt to roughly map a dependence between the initial disc’s structure and some properties of the Oort cloud. We find that it is difficult to construct the proto-planetary disc if (i) the amount of heavy chemical elements in Jupiter and Saturn is as high as currently accepted and (ii) the total mass of the minimum-mass solar nebula is assumed to be lower than \(\approx0.05\,\hbox{M}_{\odot}.\) The behaviour of the Oort cloud formation does not crucially depend on the initial disc model. Some differences in its structure are obvious: since the cloud is known to be filled mainly by Uranus and Neptune, the efficiency of its formation is higher when the initial amount of particles in the Uranus-Neptune region is relatively higher. A significantly large number of Jupiter Trojans in our simulation appears, however, only in the case of the initially non-gapped disc, with the particles situated also close to the Jupiter’s orbit.

Activity of comets at large heliocentric distances pre-perihelion

We present observational data for two long-period and three dynamically new comets observed at heliocentric distances between 5.8 to 14.0 AU. All of the comets exhibited activity beyond the distance at which water ice sublimation can be significant. We have conducted experiments on gas-laden amorphous ice samples and show that considerable gas emission occurs when the ice is heated below the temperature of the amorphous–crystalline ice phase transition ( T ∼ 137 K ). We propose that annealing of amorphous water ice is the driver of activity in comets as they first enter the inner Solar System. Experimental data show that large grains can be ejected at low velocity during annealing and that the rate of brightening of the comet should decrease as the heliocentric distance decreases. These results are consistent with both historical observations of distant comet activity and with the data presented here. If observations of the onset of activity in a dynamically new comet are ever made, the distance at which this occurs would be a sensitive indicator of the temperature at which the comet had formed or represents the maximum temperature that it has experienced. New surveys such as Pan STARRS, may be able to detect these comets while they are still inactive.

Saturn Forms by Core Accretion in 3.4 Myr

We present two new in situ core accretion simulations of Saturn with planet formation timescales of 3.37 Myr (model S0) and 3.48 Myr (model S1), consistent with observed protostellar disk lifetimes. In model S0, we assume rapid grain settling reduces opacity due to grains from full interstellar values (Podolak 2003). In model S1, we do not invoke grain settling, instead assigning full interstellar opacities to grains in the envelope. Surprisingly, the two models produce nearly identical formation timescales and core/atmosphere mass ratios. We therefore observe a new manifestation of core accretion theory: at large heliocentric distances, the solid core growth rate (limited by Keplerian orbital velocity) controls the planet formation timescale. We argue that this paradigm should apply to Uranus and Neptune as well.

C/2002 VQ94 (LINEAR) and 29P/Schwassmann–Wachmann 1 — CO + and N +2 rich comets

We investigated comets active at large heliocentric distances using observations obtained at the 6-m BTA telescope (SAO RAS, Russia). Long-slit and photometric modes of the focal reducer SCORPIO were used. Two of the comets, 29P/Schwassmann–Wachmann 1 (SW1) and C/2002 VQ94 (LINEAR) were observed to be emission rich. Detection of CO + and N + 2 emissions in the comae of these comets is evidence that they were formed in the outer regions of the Solar System or in a pre-solar interstellar cloud in a low temperature environment with T ⩽ 25 K . The ratio of N + 2/CO + is equal to 0.011 and 0.027 for SW1 and LINEAR, respectively. Comet LINEAR is the most distant object in the Solar System (7.332 AU) for which CO + and N + 2 are measured. The photometric maximum of the isolated CO + coma in Comet LINEAR is shifted by 1.4 arcsec ( 7.44 × 10 3 km ) relative to the photometric maximum of the dust coma. This shift deviates from the sunward direction by 63 degrees.

Comet 67P/Churyumov-Gerasimenko at a large heliocentric distance

Aims. The Jupiter family comet 67P/Churyumov-Gerasimenko (67P/C-G) is the target comet of ESA's ROSETTA mission. A detailed portrait of this comet has been drawn from observations around perihelion, but what needs to be enhanced is the description of the comet's behaviour at large heliocentric distance. It is not only important for planning the rendezvous of the ROSETTA spacecraft with the comet, but will also provide valuable information on basic physical properties of the nucleus and the possible distant activity of JFCs. Methods. We performed broad-band imaging and longslit spectroscopy of 67P/C-G in the visible wavelength range in June 2004, May 2006, and August 2006, when the comet was at the heliocentric distance of r ≥ 4.9 AU. The observations were performed at the ESO VLT with the FORS2 instrument. Results. The comet appears point-like at all observing epochs, indicating that no significant coma is present around the nucleus. From our 67P/C-G images, we determined a synodic rotational period of the nucleus of 12.7047 ± 0.0011 h and a linear phase function coefficient β = 0.076 ± 0.003 mag/°. We estimated the large-to-small axis ratio a/b ≥ 1.45 ± 0.09 and, assuming an albedo of 0.04, an effective nucleus radius of 2.38 ± 0.04 km, which corresponds to the mean absolute magnitude H = 15.35 ± 0.04 mag. No colour variation with the rotational phase is found, based on broadband colour indices and reflectance spectra of the nucleus. A trail-like structure of heavy grains is clearly detected in June 2004, displaying a surface brightness of ∼28 mag/ 2 in R the filter. For the first time we also determined visible colours of the trail-like structure: (V - R) = 0.95 ± 0.14 mag/ 2 and (R - 1) = 0.39 ± 0.14 mag/ 2 . A trail-like structure is also barely visible in the May 2006 images, but a quantitative study was not possible.

The distant activity of Short Period Comets★- II.

The activity of the Short Period Comets (SPCs) at large heliocentric distance (Rh > 3 au) occurs in a region of the Solar system where the water sublimation rate is low and so the sublimation of other volatiles, for example CO or CO2, could drive the presence of a coma. The detection of distant activity in a SPC can therefore give important hints on its composition. Moreover, a complete characterization of the distant SPCs degree of activity is crucial in order to give correct estimates of the nucleus size and to obtain more reliable size-distribution curves of cometary nuclei. The aim of this paper is to present the last results of a program of CCD imaging of distant SPCs, started in 2004 December and concluded with observing runs at the 3.5-m Telescopio