Abstract
We report new light curves and phase functions for nine Jupiter-family comets (JFCs). They were observed in the period 2004–2015 with various ground telescopes as part of the Survey of ...Ensemble Physical Properties of Cometary Nuclei as well as during devoted observing campaigns. We add to this a review of the properties of 35 JFCs with previously published rotation properties. The photometric time series were obtained in Bessel R, Harris R and SDSS r
΄ filters and were absolutely calibrated using stars from the Pan-STARRS survey. This specially developed method allowed us to combine data sets taken at different epochs and instruments with absolute-calibration uncertainty down to 0.02 mag. We used the resulting time series to improve the rotation periods for comets 14P/Wolf, 47P/Ashbrook–Jackson, 94P/Russell and 110P/Hartley 3 and to determine the rotation rates of comets 93P/Lovas and 162P/Siding Spring for the first time. In addition to this, we determined the phase functions for seven of the examined comets and derived geometric albedos for eight of them. We confirm the known cut-off in bulk densities at ∼0.6 g cm−3 if JFCs are strengthless. Using a model for prolate ellipsoids with typical density and elongations, we conclude that none of the known JFCs requires tensile strength larger than 10–25 Pa to remain stable against rotational instabilities. We find evidence for an increasing linear phase function coefficient with increasing geometric albedo. The median linear phase function coefficient for JFCs is 0.046 mag deg−1 and the median geometric albedo is 4.2 per cent.
Context. Main belt comets are a recently identified population of minor bodies with stable asteroid-like orbits but cometary appearances. Sublimation of water ice is the most likely mechanism for ...their recurrent activity (i.e. dust tails and dust comae), although there has been no direct detection of gas. These peculiar objects could hold the key to the origin of water on Earth. Aims. In this paper we present a search for the gas responsible for lifting dust from P/2012 T1 (Pan-STARRS), and review previous attempts at such measurements. To date such searches have mainly been indirect, looking for the common cometary gas CN rather than gasses related to water itself. Methods. We use the VLT and X-shooter to search for emission from OH in the UV, a direct dissociation product of water. Results. We do not detect any emission lines, and place an upper limit on water production rate from P/2012 T1 of 8−9 × 1025 molecules s-1. This is similar to limits derived from observations using the Herschel space telescope. Conclusions. We conclude that the best current facilities are incapable of detecting water emission at the exceptionally low levels required to produce the observed activity in main belt comets.
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The physical properties of cometary nuclei observed today relate to their complex history and help to constrain their formation and evolution. In this article, we review some of the main physical ...properties of cometary nuclei and focus in particular on the thermal, mechanical, structural and dielectric properties, emphasising the progress made during the Rosetta mission. Comets have a low density of
480
±
220
kg
m
−
3
and a low permittivity of 1.9–2.0, consistent with a high porosity of 70–80%, are weak with a very low global tensile strength
<
100
Pa, and have a low bulk thermal inertia of
0
–
60
J
K
−
1
m
−
2
s
−
1
/
2
that allowed them to preserve highly volatiles species (e.g. CO, CO
2
, CH
4
, N
2
) into their interior since their formation. As revealed by 67P/Churyumov-Gerasimenko, the above physical properties vary across the nucleus, spatially at its surface but also with depth. The broad picture is that the bulk of the nucleus consists of a weakly bonded, rather homogeneous material that preserved primordial properties under a thin shell of processed material, and possibly covered by a granular material; this cover might in places reach a thickness of several meters. The properties of the top layer (the first meter) are not representative of that of the bulk nucleus. More globally, strong nucleus heterogeneities at a scale of a few meters are ruled out on 67P’s small lobe.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Context. Comet 67P/Churyumov-Gerasimenko was selected in 2003 as the new target of the Rosetta mission. It has since been the subject of a detailed campaign of observations to characterise its ...nucleus and activity. Aims. We present previously unpublished data taken around the start of activity of the comet in 2007/8, before its last perihelion passage. We constrain the time of the start of activity, and combine this with other data taken throughout the comet’s orbit to make predictions for its likely behaviour during 2014/5 while Rosetta is operating. Methods. A considerable difficulty in observing 67P during the past years has been its position against crowded fields towards the Galactic centre for much of the time. The 2007/8 data presented here were particularly difficult, and the comet will once again be badly placed for Earth-based observations in 2014/5. We make use of the difference image analysis technique, which is commonly used in variable star and exoplanet research, to remove background sources and extract images of the comet. In addition, we reprocess a large quantity of archival images of 67P covering its full orbit, to produce a heliocentric lightcurve. By using consistent reduction, measurement and calibration techniques we generate a remarkably clean lightcurve, which can be used to measure a brightness-distance relationship and to predict the future brightness of the comet. Results. We determine that the comet was active around November 2007, at a pre-perihelion distance from the Sun of 4.3 AU. The comet will reach this distance, and probably become active again, in March 2014. We find that the dust brightness can be well described by Afρ ∝ r-3.2 pre-perihelion and ∝ r-3.4 post-perihelion, and that the comet has a higher dust-to-gas ratio than average, with log (Afρ/Q(H2O)) = − 24.94 ± 0.22 cm s molecule-1 at r < 2 AU. A model fit to the photometric data suggests that only a small fraction (1.4%) of the surface is active.
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The refractory-to-ice mass ratio in comets Fulle, Marco; Blum, J; Green, S F ...
Monthly notices of the Royal Astronomical Society,
01/2019, Volume:
482, Issue:
3
Journal Article
Peer reviewed
Open access
We review the complex relationship between the dust-to-gas mass ratio usually estimated in the material lost by comets, and the refractory-to-ice mass ratio inside the nucleus, which constrains the ...origin of comets. Such a relationship is dominated by the mass transfer from the perihelion erosion to fallout over most of the nucleus surface. This makes the refractory-to-ice mass ratio inside the nucleus up to 10 times larger than the dust-to-gas mass ratio in the lost material, because the lost material is missing most of the refractories which were inside the pristine nucleus before the erosion. We review the refractory-to-ice mass ratios available for the comet nuclei visited by space missions, and for the Kuiper Belt Objects with well-defined bulk density, finding the 1-σ lower limit of 3. Therefore, comets and KBOs may have less water than CI-chondrites, as predicted by models of comet formation by the gravitational collapse of cm-sized pebbles driven by streaming instabilities in the protoplanetary disc.
•We show results from a Spitzer mid-IR survey of Jupiter-family (JF) comets.•We present 89 new radii and 57 new beaming parameters for the nuclei.•Mean beaming parameter is 1.03±0.11, so ensemble ...thermal inertia is low.•Our independent cumulative size distribution is similar to earlier work.•There are likely low-perihelion, large JF nuclei still undiscovered.
We present results from SEPPCoN, an on-going Survey of the Ensemble Physical Properties of Cometary Nuclei. In this report we discuss mid-infrared measurements of the thermal emission from 89 nuclei of Jupiter-family comets (JFCs). All data were obtained in 2006 and 2007 using imaging capabilities of the Spitzer Space Telescope. The comets were typically 4–5AU from the Sun when observed and most showed only a point-source with little or no extended emission from dust. For those comets showing dust, we used image processing to photometrically extract the nuclei. For all 89 comets, we present new effective radii, and for 57 comets we present beaming parameters. Thus our survey provides the largest compilation of radiometrically-derived physical properties of nuclei to date. We have six main conclusions: (a) The average beaming parameter of the JFC population is 1.03±0.11, consistent with unity; coupled with the large distance of the nuclei from the Sun, this indicates that most nuclei have Tempel 1-like thermal inertia. Only two of the 57 nuclei had outlying values (in a statistical sense) of infrared beaming. (b) The known JFC population is not complete even at 3km radius, and even for comets that approach to ∼2AU from the Sun and so ought to be more discoverable. Several recently-discovered comets in our survey have small perihelia and large (above ∼2km) radii. (c) With our radii, we derive an independent estimate of the JFC nuclear cumulative size distribution (CSD), and we find that it has a power-law slope of around −1.9, with the exact value depending on the bounds in radius. (d) This power-law is close to that derived by others from visible-wavelength observations that assume a fixed geometric albedo, suggesting that there is no strong dependence of geometric albedo with radius. (e) The observed CSD shows a hint of structure with an excess of comets with radii 3–6km. (f) Our CSD is consistent with the idea that the intrinsic size distribution of the JFC population is not a simple power-law and lacks many sub-kilometer objects.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Context. Near-Earth asteroid (25143) Itokawa was visited by the Hayabusa spacecraft in 2005, resulting in a highly detailed shape and surface topography model. This model has led to several ...predictions for the expected radiative torques on this asteroid, suggesting that its spin rate should be decelerating. Aims. To detect changes in rotation rate that may be due to YORP-induced radiative torques, which in turn may be used to investigate the interior structure of the asteroid. Methods. Through an observational survey spanning 2001 to 2013 we obtained rotational lightcurve data at various times over the last five close Earth-approaches of the asteroid. We applied a polyhedron-shape-modelling technique to assess the spin-state of the asteroid and its long term evolution. We also applied a detailed thermophysical analysis to the shape model determined from the Hayabusa spacecraft. Results. We have successfully measured an acceleration in Itokawa’s spin rate of dω/dt = (3.54 ± 0.38) × 10-8 rad day-2, equivalent to a decrease of its rotation period of ~45 ms year-1. From the thermophysical analysis we find that the centre-of-mass for Itokawa must be shifted by ~21 m along the long-axis of the asteroid to reconcile the observed YORP strength with theory. Conclusions. This can be explained if Itokawa is composed of two separate bodies with very different bulk densities of 1750 ± 110 kg m-3 and 2850 ± 500 kg m-3, and was formed from the merger of two separate bodies, either in the aftermath of a catastrophic disruption of a larger differentiated body, or from the collapse of a binary system. We therefore demonstrate that an observational measurement of radiative torques, when combined with a detailed shape model, can provide insight into the interior structure of an asteroid. Futhermore, this is the first measurement of density inhomogeneity within an asteroidal body, that reveals significant internal structure variation. A specialised spacecraft is normally required for this.
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We present a general framework for matching the point-spread function (PSF), photometric scaling and sky background between two images, a subject which is commonly referred to as difference image ...analysis (DIA). We introduce the new concept of a spatially varying photometric scale factor which will be important for DIA applied to wide-field imaging data in order to adapt to transparency and airmass variations across the field-of-view. Furthermore, we demonstrate how to separately control the degree of spatial variation of each kernel basis function, the photometric scale factor and the differential sky background. We discuss the common choices for kernel basis functions within our framework, and we introduce the mixed-resolution delta basis functions to address the problem of the size of the least-squares problem to be solved when using delta basis functions. We validate and demonstrate our algorithm on simulated and real data. We also describe a number of useful optimizations that may be capitalized on during the construction of the least-squares matrix and which have not been reported previously. We pay special attention to presenting a clear notation for the DIA equations which are set out in a way that will hopefully encourage developers to tackle the implementation of DIA software.
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