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Comet 67P/Churyumov-Gerasimenko (67P) become observable for the first time in 2021 since the Rosetta rendezvous in 2014--16. Here, we present pre-perihelion polarimetric measurements of 67P from 2021 performed with the Very Large Telescope (VLT), as well as post-perihelion polarimetric measurements from 2015--16 obtained with the VLT and the William Herschel Telescope (WHT). This new data covers a phase angle range of ~4-50{\deg} and presents polarimetric measurements of unprecedentedly high S/N ratio. Complementing previous measurements, the polarimetric phase curve of 67P resembles that of other Jupiter family comets and high-polarisation, dusty comets. Comparing pre- and post-perihelion data sets, we find only a marginal difference between the polarimetric phase curves. In our imaging maps, we detect various linear structures produced by the dust in the inner coma of the comet. Despite this, we find a homogeneous spread of polarisation around the photocentre throughout the coma and tail, in contrast to previous studies. Finally, we explore the consequences of image misalignments on both polarimetric maps and aperture polarimetric measurements., Comment: 15 pages, 10 figures, 2 tables. Accepted for publication in MNRAS, May 13th 2024

We present a method for automatised detection and analysis of quasi-periodic lineament structures from images at pixel-precision. The method exploits properties of the images' frequency domain found by using the Fourier transform. We developed this method with the goal of detecting lineament structures in an image of the Hathor cliff of comet 67P/Churyumov-Gerasimenko, which are caused by layerings and furrows in the nucleus material. Using our method, we determined the orientation and wavelength-range of these structures. The detected layering edges have similar orientations, spatial separations of 9-20 m, and are ubiquitous throughout the image. We suggest that the layerings are a global feature of the comet nucleus that provide information about formation and evolution of comet 67P. The furrows are non-uniformly distributed throughout the image. Their orientation is broadly parallel to the direction of the local gravity vector at the Hathor cliff, with spacings similar to that of the layering structures. The furrows are interpreted as signatures of local down-slope movement of cliff material. We demonstrate that the developed method is broadly applicable to the detection and analysis of various kinds of quasi-periodic structures like geological layering, folding and faulting, and texture analysis in general. In order to facilitate the application of our method, this paper is accompanied by a demo program written in Matlab.

Comet 41P/Tuttle-Giacobini-Kresak (41P), a Jupiter family comet with three discoveries over about 100 years, is in a short-periodic orbit around the Sun with the perihelion close to the Earth distance. The 2017 apparition of 41P offered a long-lasting visibility of the comet at a close distance to Earth. The four month-long imaging campaign with the 2 m telescope at the Mount Wendelstein Observatory was aimed at characterizing dust activity and nucleus properties of the comet. Using a new analysis method of the inner coma flux, we derived a small mean equivalent radius of about 600 m for the nucleus with an unusual body axes ratio that is higher than two. The nucleus rotation axis was determined from the geometric appearance of coma structures, which were enhanced in the images. A long-lasting coma fan was produced by an extended region at high latitudes on the slowly rotating nucleus, whereas isolated jets originated from narrow, low latitude active regions on the nucleus. The dust activity of 41P, despite being difficult to quantify exactly because of an unknown phase function correction for the comet, indicates a steep radial profile that falls off with an increasing distance from the Sun. Colors and flux profiles provide evidence for dust fragmentation in the inner coma of the comet. A singular outburst event created various dust structures in the coma. The outburst came from an extended region on the nucleus and was due to either a landslide on the nucleus or a sudden material release from a subsurface pocket of volatile ice., Comment: Published on 29 May 2020 in A&A, 25 pages, 12 figures, 9 tables

We analysed layering-related linear features on the surface of comet 67P/Churyumov-Gerasimenko (67P) to determine the internal configuration of the layerings within the nucleus. We used high-resolution images from the OSIRIS Narrow Angle Camera onboard the Rosetta spacecraft, projected onto the SHAP7 shape model of the nucleus, to map 171 layering-related linear features which we believe to represent terrace margins and strata heads. From these curved lineaments, extending laterally to up to 1925 m, we extrapolated the subsurface layering planes and their normals. We furthermore fitted the lineaments with concentric ellipsoidal shells, which we compared to the established shell model based on planar terrace features. Our analysis confirms that the layerings on the comet's two lobes are independent from each other. Our data is not compatible with 67P's lobes representing fragments of a much larger layered body. The geometry we determined for the layerings on both lobes supports a concentrically layered, `onion-shell' inner structure of the nucleus. For the big lobe, our results are in close agreement with the established model of a largely undisturbed, regular, concentric inner structure following a generally ellipsoidal configuration. For the small lobe, the parameters of our ellipsoidal shells differ significantly from the established model, suggesting that the internal structure of the small lobe cannot be unambiguously modelled by regular, concentric ellipsoids and could have suffered deformational or evolutional influences. A more complex model is required to represent the actual geometry of the layerings in the small lobe.

C/2002 CE$_{10}$ is an object in a retrograde elliptical orbit with Tisserand parameter $-0.853$ indicating a likely origin in the Oort Cloud. It appears to be a rather inactive comet since no coma and only a very weak tail was detected during the past perihelion passage. We present multi-color optical photometry, lightcurve and thermal mid-IR observations of the asteroidal comet. \textcolor{blue}{ With the photometric analysis in $BVRI$, the surface color is found to be redder than asteroids, corresponding to cometary nuclei and TNOs/Centaurs. The time-resolved differential photometry supports a rotation period of 8.19$\pm$0.05 h. The effective diameter and the geometric albedo are 17.9$\pm$0.9 km and 0.03$\pm$0.01, respectively, indicating a very dark reflectance of the surface. The dark and redder surface color of C/2002 CE$_{10}$ may be attribute to devolatilized material by surface aging suffered from the irradiation by cosmic rays or from impact by dust particles in the Oort Cloud. Alternatively, C/2002 CE$_{10}$ was formed of very dark refractory material originally like a rocky planetesimal. In both cases, this object lacks ices (on the surface at least). The dynamical and known physical characteristics of C/2002 CE$_{10}$ are best compatible with those of the Damocloids population in the Solar System, that appear to be exhaust cometary nucleus in Halley-type orbits. The study of physical properties of rocky Oort cloud objects may give us a key for the formation of the Oort cloud and the solar system., Comment: 9 pages, 4 figures accepted to Icarus

Comet 67P/Churyumov-Gerasimenko (67P) was imaged with the 2m telescope at Mt. Wendelstein Observatory in the Alps. Coma and tail monitoring was performed during 51 nights between 22 August 2015 and 9 May 2016. The images through r and i Sloan Digital Sky Survey (SDSS) filters show the dust distribution around the comet, while images in the SDSS g filter indicate also the presence of coma gas in early September 2015. The dust color of 67P implies intrinsic reddening of 9 %/100nm. After maximum shortly after perihelion passage the dust activity decreased with a heliocentric exponent of 4.1 to 4.2 from late September 2015 until May 2016. The opposition surge during early 2016 can be explained by a linear light scattering phase function (beta ~ 0.04) or an asteroid-like HG-type phase function (G ~ 0.15). The radial brightness profile indicates a 'quasi-steady-state' dust coma from late September to the end of 2015. Dust fragmentation during about a month after perihelion may be responsible for radial coma profiles with slopes below unity, while dust accumulation due to very slow dust expansion velocity may result in steeper than unity profiles during 2016. Three fan-shape dust structures are characterized in the coma of 67P. A short dust ejection event on 22 -23 August 2015 has produced a dust arc-let and jet feature in the coma. In September 2015 the appearance of cometary dust tail is dominated by young dust produced around perihelion. The older dust dominates the tail appearance as of mid November 2015., Comment: 29 pages, 10 figures, published in MNRAS

We report photometric observations for comet C/2012 S1 (ISON) obtained during the time period immediately after discovery (r=6.28 AU) until it moved into solar conjunction in mid-2013 June using the UH2.2m, and Gemini North 8-m telescopes on Mauna Kea, the Lowell 1.8m in Flagstaff, the Calar Alto 1.2m telescope in Spain, the VYSOS-5 telescopes on Mauna Loa Hawaii and data from the CARA network. Additional pre-discovery data from the Pan STARRS1 survey extends the light curve back to 2011 September 30 (r=9.4 AU). The images showed a similar tail morphology due to small micron sized particles throughout 2013. Observations at sub-mm wavelengths using the JCMT on 15 nights between 2013 March 9 (r=4.52 AU) and June 16 (r=3.35 AU) were used to search for CO and HCN rotation lines. No gas was detected, with upper limits for CO ranging between (3.5-4.5)E27 molec/s. Combined with published water production rate estimates we have generated ice sublimation models consistent with the photometric light curve. The inbound light curve is likely controlled by sublimation of CO2. At these distances water is not a strong contributor to the outgassing. We also infer that there was a long slow outburst of activity beginning in late 2011 peaking in mid-2013 January (r~5 AU) at which point the activity decreased again through 2013 June. We suggest that this outburst was driven by CO injecting large water ice grains into the coma. Observations as the comet came out of solar conjunction seem to confirm our models., Comment: 8 pages, 2 figures, 3 tables

Comet 103P/Hartley~2 was observed on Nov. 1-6, 2010, coinciding with the fly-by of the space probe EPOXI. The goal was to connect the large scale phenomena observed from the ground, with those at small scale observed from the spacecraft. The comet showed strong activity correlated with the rotation of its nucleus, also observed by the spacecraft. We report here the characterization of the solid component produced by this activity, via observations of the emission in two spectral regions where only grain scattering of the solar radiation is present. We show that the grains produced by this activity had a lifetime of the order of 5 hours, compatible with the spacecraft observations of the large icy chunks. Moreover, the grains produced by one of the active regions have a very red color. This suggests an organic component mixed with the ice in the grains., Comment: 11 pages, 7 figures, Icarus in press

The peculiar object P/2010 A2 was discovered by the LINEAR near-Earth asteroid survey in January 2010 and given a cometary designation due to the presence of a trail of material, although there was no central condensation or coma. The appearance of this object, in an asteroidal orbit (small eccentricity and inclination) in the inner main asteroid belt attracted attention as a potential new member of the recently recognized class of 'Main Belt Comets' (MBCs). If confirmed, this new object would greatly expand the range in heliocentric distance over which MBCs are found. Here we present observations taken from the unique viewing geometry provided by ESA's Rosetta spacecraft, far from the Earth, that demonstrate that the trail is due to a single event rather than a period of cometary activity, in agreement with independent results from the Hubble Space Telescope (HST). The trail is made up of relatively large particles of millimetre to centimetre size that remain close to the parent asteroid. The shape of the trail can be explained by an initial impact ejecting large clumps of debris that disintegrated and dispersed almost immediately. We determine that this was an asteroid collision that occurred around February 10, 2009., Comment: Published in Nature on 14/10/2010. 25 pages, includes supplementary material

We report on observations of the dust trail of comet 67P/Churyumov-Gerasimenko (CG) in visible light with the Wide Field Imager at the ESO/MPG 2.2m telescope at 4.7 AU before aphelion, and at 24 micron with the MIPS instrument on board the Spitzer Space Telescope at 5.7 AU both before and after aphelion. The comet did not appear to be active during our observations. Our images probe large dust grains emitted from the comet that have a radiation pressure parameter beta<0.01. We compare our observations with simulated images generated with a dynamical model of the cometary dust and constrain the emission speeds, size distribution, production rate and geometric albedo of the dust. We achieve the best fit to our data with a differential size distribution exponent of -4.1, and emission speeds for a beta=0.01 particle of 25 m/s at perihelion and 2 m/s at 3 AU. The dust production rate in our model is on the order of 1000 kg/s at perihelion and 1 kg/s at 3 AU, and we require a dust geometric albedo between 0.022 and 0.044. The production rates of large (>10 micron) particles required to reproduce the brightness of the trail are sufficient to also account for the coma brightness observed while the comet was inside 3 AU, and we infer that the cross-section in the coma of CG may be dominated by grains of the order of 60-600 micron., Comment: 79 pages, 13 figures, 6 tables. Accepted for publication in Icarus

We report Spitzer Space Telescope observations of comet 67P/Churyumov-Gerasimenko at 5.5 and 4.3 AU from the Sun, post-aphelion. Comet 67P is the primary target of the European Space Agency's Rosetta mission. The Rosetta spacecraft will rendezvous with the nucleus at heliocentric distances similar to our observations. Rotationally resolved observations at 8 and 24 microns (at a heliocentric distance, rh, of 4.8 AU) that sample the size and color-temperature of the nucleus are combined with aphelion R-band light curves observed at the Very Large Telescope (VLT) and yield a mean effective radius of 2.04 +/- 0.11 km, and an R-band geometric albedo of 0.054 +/- 0.006. The amplitudes of the R-band and mid-infrared light curves agree, which suggests that the variability is dominated by the shape of the nucleus. We also detect the dust trail of the comet at 4.8 and 5.5 AU, constrain the grain sizes to be less than or similar to 6 mm, and estimate the impact hazard to Rosetta. We find no evidence for recently ejected dust in our images. If the activity of 67P is consistent from orbit to orbit, then we may expect the Rosetta spacecraft will return images of an inactive or weakly active nucleus as it rendezvous with the comet at rh = 4 AU in 2014., Comment: 19 pages, 2 tables, 10 figures. Accepted for publication in the Astronomical Journal

METIS, the Mid-infrared ELT Imager and Spectrograph (formerly called MIDIR), is a proposed instrument for the European Extremely Large Telescope (E-ELT), currently undergoing a phase-A study. The study is carried out within the framework of the ESO-sponsored E-ELT instrumentation studies. METIS will be designed to cover the E-ELT science needs at wavelengths longward of 3um, where the thermal background requires different operating schemes. In this paper we discuss the main science drivers from which the instrument baseline has been derived. Specific emphasis has been given to observations that require very high spatial and spectral resolution, which can only be achieved with a ground-based ELT. We also discuss the challenging aspects of background suppression techniques, adaptive optics in the mid-IR, and telescope site considerations. The METIS instrument baseline includes imaging and spectroscopy at the atmospheric L, M, and N bands with a possible extension to Q band imaging. Both coronagraphy and polarimetry are also being considered. However, we note that the concept is still not yet fully consolidated. The METIS studies are being performed by an international consortium with institutes from the Netherlands, Germany, France, United Kingdom, and Belgium., Comment: 15 pages, to be published in Proc SPIE 7014: Ground-based & Airborne Instrumentation for Astronomy II

The collision of Deep Impact with comet 9P/Tempel 1 generated a bright cloud of dust which dissipated during several days after the impact. The brightness variations of this cloud and the changes of its position and shape are governed by the physical properties of the dust grains. We use a Monte Carlo model to describe the evolution of the post-impact dust plume. The results of our dynamical simulations are compared to the data obtained with FORS2, the FOcal Reducer and low dispersion Spectrograph for the VLT of the European Southern Observatory (ESO), to derive the particle size distribution and the total amount of material contained in the dust ejecta cloud., Comment: 8 pages, 4 JPEG figures, to appear in the proceedings of "Deep Impact as a World Observatory Event - Synergies in Space, Time and Wavelength", ed. Hans Ulrich K{\"a}ufl and Chris Sterken, Springer

Comet C/2006 W3 (Christensen) remained outside a heliocentric distance (Rh) of 3.1 au throughout its apparition, but it presented an exceptional opportunity to directly sense a suite of molecules released from its nucleus. The Cryogenic Infrared Echelle Spectrograph at ESO-VLT detected infrared emissions from the three hypervolatiles (CO, CH4, and C2H6) that have the lowest sublimation temperatures among species that are commonly studied in comets by remote sensing. Even at Rh 3.25 au, the production rate of each molecule exceeded those measured for the same species in a number of other comets, although these comets were observed much closer to the Sun. Detections of CO at Rh = 3.25, 4.03, and 4.73 au constrained its post-perihelion decrease in production rate, which most likely dominated the outgassing. At 3.25 au, our measured abundances scaled as CO/CH4/C2H6 approx. = 100/4.4/2.1. The C2H6/CH4 ratio falls within the range of previously studied comets at Rh < 2 au, while CO/CH4 is comparatively high and similar to in situ measurements from Rosetta at approx.10 km from the nucleus of 67P/Churyumov-Gerasimenko conducted at a very similar Rh (3.15 au). The independent detections of H2O (Herschel Space Observatory) and CO (this work) imply a coma abundance H2O/CO approx. = 20% in C/2006 W3 near Rh = 5 au. All these measurements are of high value for constraining models of nucleus sublimation (plausibly CO-driven) beyond Rh = 3au, where molecular detections in comets are still especially sparse.

We acquired near-infrared spectra of the Oort cloud comet C/2012 F6 (Lemmon) at three different heliocentric distances (R h) during the comet's 2013 perihelion passage, providing a comprehensive measure of the outgassing behavior of parent volatiles and cosmogonic indicators. Our observations were performed pre-perihelion at R h = 1.2 AU with CRIRES (on 2013 February 2 and 4), and post-perihelion at R h = 0.75 AU with CSHELL (on March 31 and April 1) and R h = 1.74 AU with NIRSPEC (on June 20). We detected 10 volatile species (H2O, OH* prompt emission, C2H6, CH3OH, H2CO, HCN, CO, CH4, NH3, and NH2), and obtained upper limits for two others (C2H2 and HDO). One-dimensional spatial profiles displayed different distributions for some volatiles, confirming either the existence of polar and apolar ices, or of chemically distinct active vents in the nucleus. The ortho-para ratio for water was 3.31 +/- 0.33 (weighted mean of CRIRES and NIRSPEC results), implying a spin temperature >37 K at the 95% confidence limit. Our (3σ) upper limit for HDO corresponds to D/H < 2.45 × 10-3 (i.e., <16 Vienna Standard Mean Ocean Water, VSMOW). At R h = 1.2 AU (CRIRES), the production rate for water was Q(H2O) = 1.9 +/- 0.1 × 1029 s-1 and its rotational temperature was T rot ~ 69 K. At R h = 0.75 AU (CSHELL), we measured Q(H2O) = 4.6 +/- 0.6 × 1029 s-1 and T rot = 80 K on March 31, and 6.6 +/- 0.9 × 1029 s-1 and T rot = 100 K on April 1. At R h = 1.74 AU (NIRSPEC), we obtained Q(H2O) = 1.1 +/- 0.1 × 1029 s-1 and T rot ~ 50 K. The measured volatile abundance ratios classify comet C/2012 F6 as rather depleted in C2H6 and CH3OH, while HCN, CH4, and CO displayed abundances close to their median values found among comets. H2CO was the only volatile showing a relative enhancement. The relative paucity of C2H6 and CH3OH (with respect to H2O) suggests formation within warm regions of the nebula. However, the normal abundance of HCN and hypervolatiles CH4 and CO, and the enhancement of H2CO, may indicate a possible heterogeneous nucleus of comet C/2012 F6 (Lemmon), possibly as a result of radial mixing within the protoplanetary disk

The evolution of the spin rate of comet 9P/Tempel 1 through two perihelion passages (CYs 2000 and 2005) is determined from 1922 Earth-based observations taken over a period of 13y as part of a World-Wide observing campaign and 2888 observations taken over a period of 50d from the Deep Impact spacecraft. We determine the following sidereal spin rates (periods): 209.023 +/- 0.025 degrees /day (41.335 +/- 0.005 h) prior to the 2000 perihelion passage, 210.448 +/- 0.016 degrees/day (41.055 +/- 0.003 h) for the interval between the 2000 and 2005 perihelion passages, 211.856 +/- 0.030 degrees/day (40.783 +/- 0.006 h) from Deep Impact photometry just prior to the 2005 perihelion passage, and 211.625 +/- 0.012 degrees /day (40.827 +/- 0.002 h) in the interval 2006-2010 following the 2005 perihelion passage. The period decreased by 16.8 +/- 0.3 min during the 2000 passage and by 13.7 +/- 0.2 min during the 2005 passage suggesting a secular decrease in the net torque. The change in spin rate is asymmetric with respect to perihelion with the maximum net torque being applied on approach to perihelion. The Deep Impact data alone show that the spin rate was increasing at a rate of 0.024 +/- 0.003 degree/d/d at JD2453530.60510 (i.e., 25.134 d before impact) and provides independent confirmation of the change seen in the Earth-based observations. The rotational phase of the nucleus at times before and after each perihelion and at the Deep Impact encounter is estimated based on the Thomas et al. pole and longitude system. The possibility of a 180 degree error in the rotational phase is assessed and found to be significant. Analytical and physical modeling of the behavior of the spin rate through of each perihelion is presented and used as a basis to predict the rotational state of the nucleus at the time of the nominal (i.e., prior to February 2010) Stardust-NExT encounter on 2011 February 14 20:42. We find that a net torque in the range of 0.3 - 2.5 x 10(exp 7) kg.square m2/square s acts on the nucleus during perihelion passage. The spin rate initially slows down on approach to perihelion and then passes through a minimum. It then accelerates rapidly as it passes through perihelion eventually reaching a maximum post-perihelion. It then decreases to a stable value as the nucleus moves away from the sun. We find that the pole direction is unlikely to precess by more than approximately 1 degree/perihelion passage. The trend of the period with time and the fact that the modeled peak torque that occurs before perihelion is in agreement with published accounts of trends in water production rate and suggests that widespread H2O out-gassing from the surface is largely responsible for the observed spin-up.

The evolution of the spin rate of Comet 9P/Tempel 1 through two perihelion passages (in 2000 and 2005) is determined from 1922 Earth-based observations taken over a period of 13 year as part of a World-Wide observing campaign and from 2888 observations taken over a period of 50 days from the Deep Impact spacecraft. We determine the following sidereal spin rates (periods): 209.023 +/- 0.025deg/dy (41.335 0.005 h) prior to the 2000 perihelion passage, 210.448 +/- 0.016deg/dy (41.055 +/- 0.003 h) for the interval between the 2000 and 2005 perihelion passages, 211.856 +/- 0.030deg/dy (40.783 +/- 0.006 h) from Deep Impact photometry just prior to the 2005 perihelion passage, and 211.625 +/- 0.012deg/dy (40.827 +/- 0.002 h) in the interval 2006-2010 following the 2005 perihelion passage. The period decreased by 16.8 +/- 0.3 min during the 2000 passage and by 13.7 +/- 0.2 min during the 2005 passage suggesting a secular decrease in the net torque. The change in spin rate is asymmetric with respect to perihelion with the maximum net torque being applied on approach to perihelion. The Deep Impact data alone show that the spin rate was increasing at a rate of 0.024 +/- 0.003deg/dy/dy at JD2453530.60510 (i.e., 25.134 dy before impact), which provides independent confirmation of the change seen in the Earth-based observations. The rotational phase of the nucleus at times before and after each perihelion and at the Deep Impact encounter is estimated based on the Thomas et al. (Thomas et al. [2007]. Icarus 187, 4-15) pole and longitude system. The possibility of a 180deg error in the rotational phase is assessed and found to be significant. Analytical and physical modeling of the behavior of the spin rate through of each perihelion is presented and used as a basis to predict the rotational state of the nucleus at the time of the nominal (i.e., prior to February 2010) Stardust-NExT encounter on 2011 February 14 at 20:42. We find that a net torque in the range of 0.3-2.5 x 10(exp 7) kg/sq m/sq s acts on the nucleus during perihelion passage. The spin rate initially slows down on approach to perihelion and then passes through a minimum. It then accelerates rapidly as it passes through perihelion eventually reaching a maximum post-perihelion. It then decreases to a stable value as the nucleus moves away from the Sun. We find that the pole direction is unlikely to precess by more than approx. 1deg per perihelion passage. The trend of the period with time and the fact that the modeled peak torque occurs before perihelion are in agreement with published accounts of trends in water production rate and suggests that widespread H2O out-gassing from the surface is largely responsible for the observed spin-up.