Dust Loss Rate Research Articles

Performance optimization and mechanism analysis of applied Enteromorpha-based composite dust suppressant.

In order to solve the problem of environmental pollution caused by the escape of coal dust in open-pit coal mines, a composite dust suppressant was prepared from Enteromorpha, and the preparation factors (water-soluble polymer, temperature, solid content and surfactant) were optimized. The mechanism of dust suppression and the possibility of large-scale field application were discussed. The research results on the related properties of dust suppressants showed that the performance of Enteromorpha-based dust suppressants prepared by this method was excellent compared with similar studies. Among them, polyacrylamide (PAM) Enteromorpha-based dust suppressant had the best performance, with viscosity of 25.1mPas and surface tension of 27.05mN/m. Moreover, PAM Enteromorpha-based dust suppressant had the best effect, with the mass loss of 2.94% under the wind speed of 10m/s and the coal dust loss rate of 4.6% after rain erosion, and it had strong water retention performance. Through the discussion of dust suppression mechanism, it was found that the mechanical entangled network structure with hydrogen bonds as nodes was formed after the graft copolymerization of PAM and Enteromorpha. It had high permeability and good adhesion. After quickly wetting coal dust, it formed a dense package for coal dust. The field experiment also showed that the use of Enteromorpha-based dust suppressant can effectively inhibit the escape of coal dust. From the point of view of economy and efficiency, Enteromorpha can save 30% of the material cost and the dust suppression efficiency can reach 89-94%. Therefore, the Enteromorpha-based dust suppressant may stably suppress coal dust on the basis of reducing the cost.

A possible selection criterion of the target of Comet Interceptor mission

The ESA mission Comet Interceptor will target an Oort Cloud or interstellar comet during its first approach to the heliocentric distance rh≈1 au, and has been designed to handle the dust loss rate of 1P/Halley as an upper limit of activity level. In the framework of the adopted model of pebble-made nuclei, any extrapolation at rh≈1 au of the dust loss rate observed beyond Jupiter is impossible, because the dust loss rate at rh>3.8 au depends on the nucleus surface area, Ap, composed of water-poor pebbles, whereas at rh<3.8 au the dust loss rate depends on the nucleus surface area, Ar, composed of water-rich pebbles. Pebble-made nuclei are characterized by all possible values Ap/Ar⩾12. The water-driven activity model of pebble-made nuclei has been applied to comets 1P/Halley and C/1995O1 Hale-Bopp. We find that all comets with a nucleus radius smaller than 3.2-0.9+2.3 km will be characterized by a dust loss rate lower than 1P/Halley at rh≈1 au, i.e. will be safe for Comet Interceptor, whichever their ratio Ap/Ar⩾12 will be. We also find that the lower limit of the average refractory-to-water–ice mass ratio of all comets is 3.4.

Activity of the Jupiter co-orbital comet P/2019 LD2(ATLAS) observed with OSIRIS at the 10.4 m GTC

Context.The existence of comets with heliocentric orbital periods close to that of Jupiter (i.e., co-orbitals) has been known for some time. Comet 295P/LINEAR (2002 AR2) is a well-known quasi-satellite of Jupiter. However, their orbits are not long-term stable, and they may eventually experience flybys with Jupiter at very close range, close enough to trigger tidal disruptions like the one suffered by comet Shoemaker-Levy 9 in 1992.Aims.Our aim was to study the observed activity and the dynamical evolution of the Jupiter transient co-orbital comet P/2019 LD2(ATLAS) and its dynamical evolution.Methods.We present results of an observational study of P/2019 LD2carried out with the 10.4 m Gran Telescopio Canarias (GTC) that includes image analyses using a Monte Carlo dust tail fitting code to characterize its level of cometary activity, and spectroscopic studies to search for gas emission. We also presentN-body simulations to explore its past, present, and future orbital evolution.Results.Images of P/2019 LD2obtained on May 16, 2020, show a conspicuous coma and tail, but the spectrum obtained on May 17, 2020, does not exhibit any evidence of CN, C2, or C3emission. The comet brightness in a 2.6′′ aperture diameter isr′ = 19.34 ± 0.02 mag, with colors (g′−r′) = 0.78 ± 0.03, (r′−i′) = 0.31 ± 0.03, and (i′−z′) = 0.26 ± 0.03. The temporal dependence of the dust loss rate of P/2019 LD2can be parameterized by a Gaussian function having a full width at half maximum of 350 days, with a maximum dust mass loss rate of 60 kg s−1reached on August 15, 2019. The total dust loss rate from the beginning of activity until the GTC observation date (May 16, 2020) is estimated at 1.9 × 109kg. Comet P/2019 LD2is now an ephemeral co-orbital of Jupiter, following what looks like a short arc of a quasi-satellite cycle that started in 2017 and will end in 2028. On January 23, 2063, it will experience a very close encounter with Jupiter at perhaps 0.016 au; its probability of escaping the solar system during the next 0.5 Myr is estimated to be 0.53 ± 0.03.Conclusions.Photometry and tail model results show that P/2019 LD2is a kilometer-sized object, in the size range of the Jupiter-family comets, with a typical comet-like activity most likely linked to sublimation of crystalline water ice and clathrates. Its origin is still an open question. Our numerical studies give a probability of this comet having been captured from interstellar space during the last 0.5 Myr of 0.49 ± 0.02 (average and standard deviation), 0.67 ± 0.06 during the last 1 Myr, 0.83 ± 0.06 over 3 Myr, and 0.91 ± 0.09 during the last 5 Myr.

Dust Environment Model of the Interstellar Comet 2I/Borisov

Abstract 2I/Borisov is the first interstellar comet discovered on 2019 August 30, and it soon showed a coma and a dust tail. This study reports the results of images obtained at the Telescopio Nazionale Galileo telescope, on La Palma—Canary Islands, in 2019 November and December. The images have been obtained with the R filter in order to apply our dust tail model. The model has been applied to the comet 67P/Churyumov–Gerasimenko and compared to the Rosetta dust measurements showing a very good agreement. It has been applied to the comet 2I/Borisov, using almost the same parameters, obtaining a dust environment similar to that of 67P/Churyumov–Gerasimenko, suggesting that the activity may be very similar. The dust tail analysis provided a dust-loss rate Q d ≈ 35 kg s−1 in 2019 November and Q d ≈ 30 kg s−1 in 2019 December.

CO-driven activity constrains the origin of comets

Context.An open question in the study of comets is the so-called cohesion bottleneck, that is, how dust particles detach from the nucleus.Aims.We test whether the CO pressure buildup inside the pebbles of which cometary nuclei consist can overcome this cohesion bottleneck.Methods.A recently developed pebble-diffusion model was applied here to comet C/2017K2 PANSTARRS, assuming a CO-driven activity.Results.(i) The CO-gas pressure inside the pebbles erodes the nucleus into the observed dust, which is composed of refractories, H2O ice and CO2ice. (ii) The CO-driven activity onset occurs up to heliocentric distances of 85 au, depending on the spin orientation of the comet nucleus. (iii) The activity onset observed at ≈26 au suggests a low obliquity of the nucleus spin axis with activity in a polar summer. (iv) At 14 au, the smallest size of the ejected dust is ≈0.1 mm, consistent with observations. (v) The observed dust-loss rate of ≈200 kg s−1implies a fallout ≥30%, a nucleus surface active area ≥10 km2, a CO-gas loss rate ≥10 kg s−1, and a dust-to-gas ratio ≤20. (vi) The CO-driven activity never stops if the average refractory-to-all-ices mass ratio in the nucleus is ≤4.5 for a nucleus all-ices-to-CO mass ratio ≈4, as observed in comets Hale–Bopp and Hyakutake. These results make comet C/2017K2 similar to the Rosetta target comet 67P/Churyumov–Gerasimenko. (vii) The erosion lifetime of cometary planetesimals is a factor 103shorter than the timescale of catastrophic collisions. This means that the comets we observe today cannot be products of catastrophic collisions.

The optical characteristics of the dust of sungrazing comet C/2012 S1 (ISON) observed at large heliocentric distances

We present an analysis of the photometric and spectroscopic data of the comet C/2012 S1 (ISON) observed at the heliocentric distances of 6.21–4.81 AU. The photometric observations were made with the 60-cm Zeiss-600 telescope (ICAMER, peak Terskol, Russia) and the spectroscopic observations were performed using the SCORPIO-2 focal reducer mounted in the prime focus of the 6-m BTA telescope (SAO RAS, Russia). We analyse the B, V and R-band images to describe the dusty cometary coma and to investigate its brightness, colours and dust production rate. The spectra cover the wavelength range of 3600–7070 Å. No emissions which are expected in this wavelength region were detected above the 3σ level. The continuum shows a reddening effect with the normalized gradient of reflectivity along dispersion of 9.3 ± 1.1% per 1000 Å. A dust-loss rate was derived using the obtained values and under the different model assumptions. Our simulations clearly indicate that to retrieve dust production from the observational Afρ parameter is an ambiguous task. The result of such a procedure is strongly dependent on dynamical (e.g. effective density and cross-section) as well as optical (e.g. scattering coefficient and phase function) characteristics of dust grains. A variation of the mentioned parameters can lead to dramatic changes in the evaluation of mass production. We demonstrate that the dynamic and optical properties are interconnected via the microscopic properties of dust grains (effective size and porosity).

Physical and dynamical properties of the anomalous comet 249P/LINEAR

Images and low-resolution spectra of the near-Earth Jupiter family comet (JFC) 249P/LINEAR in the visible range obtained with the instrument OSIRIS in the 10.4 m Gran Telescopio Canarias (GTC) (La Palma, Spain) on January 3, 4, 6 and February 6, 2016 are presented, together with a series of images obtained with the 0.4m telescope of the Great Shefford Observatory obtained on Oct. 22 and 27, and Nov. 1 and 24, 2006. The reflectance spectrum of 249P is similar to that of a B-type asteroid. The comet has an absolute (visual) nuclear magnitude HV=17.0±0.4, which corresponds to a radius of about 1-1.3 km for a geometric albedo ∼0.04−0.07. From the analysis of GTC images using a Monte Carlo dust tail code we find that the time of maximum dust ejection rate was around 1.6 days before perihelion. The analysis of the dust tails during the 2006 and 2016 perihelion approaches reveals that, during both epochs, the comet repeated the same dust ejection pattern, with a similar short-lived activity period of about 20 days (FWHM) around perihelion and a dust loss rate peaking at 145 ± 50 kg/s. The total dust mass ejected during its last perihelion passage was (2.5 ± 0.9) × 108 kg, almost all this mass being emitted before the first observation of January 3, 2016. The activity onset, duration, and total ejected mass were very similar during the 2006 perihelion passage. This amount of dust mass is very low as compared with that from other active JFCs. The past orbital evolution of 249P and 100 clones were also followed over a time scale of ∼ 5 × 104 yr. The object and more than 60% of the clones remained bound to the near-Earth region for the whole computed period, keeping its perihelion distance within the range q≃0.4−1.1 au. The combination of photometric and spectroscopic observations and dynamical studies show that the near-Earth comet 249P/LINEAR has several peculiar features that clearly differentiate it from typical JFCs. We may be in front of a new class of near-Earth JFC whose source region is not the distant trans-neptunian population, but much closer in the asteroid belt. Therefore, 249P/LINEAR may be a near-Earth counterpart of the so-called main-belt comets or active asteroids.

EVOLUTION OF THE DUST SIZE DISTRIBUTION OF COMET 67P/CHURYUMOV–GERASIMENKO FROM 2.2 au TO PERIHELION

ABSTRACT The Rosetta probe, orbiting Jupiter-family comet 67P/Churyumov–Gerasimenko, has been detecting individual dust particles of mass larger than 10−10 kg by means of the GIADA dust collector and the OSIRIS Wide Angle Camera and Narrow Angle Camera since 2014 August and will continue until 2016 September. Detections of single dust particles allow us to estimate the anisotropic dust flux from 67P, infer the dust loss rate and size distribution at the surface of the sunlit nucleus, and see whether the dust size distribution of 67P evolves in time. The velocity of the Rosetta orbiter, relative to 67P, is much lower than the dust velocity measured by GIADA, thus dust counts when GIADA is nadir-pointing will directly provide the dust flux. In OSIRIS observations, the dust flux is derived from the measurement of the dust space density close to the spacecraft. Under the assumption of radial expansion of the dust, observations in the nadir direction provide the distance of the particles by measuring their trail length, with a parallax baseline determined by the motion of the spacecraft. The dust size distribution at sizes &gt;1 mm observed by OSIRIS is consistent with a differential power index of −4, which was derived from models of 67P’s trail. At sizes &lt;1 mm, the size distribution observed by GIADA shows a strong time evolution, with a differential power index drifting from −2 beyond 2 au to −3.7 at perihelion, in agreement with the evolution derived from coma and tail models based on ground-based data. The refractory-to-water mass ratio of the nucleus is close to six during the entire inbound orbit and at perihelion.

ON THE DUST ENVIRONMENT OF MAIN-BELT COMET 313 P/Gibbs

We present observations carried out using the 10.4 m Gran Telescopio Canarias and an interpretative model of the dust environment of activated asteroid 313P/Gibbs. We discuss three different models relating to different values of the dust parameters, i.e, dust loss rate, maximum and minimum sizes of particles, power index of the size distribution, and emission pattern. The best model corresponds with an isotropic emission of particles which started on August 1st. The size of grains were in the range of $0.1-2000$ $\mu$m, with velocities for 100 $\mu$m particles between $0.4-1.9$ m$~$s$^{-1}$, with a dust production rate in the range of $0.2-0.8$ kg$~$s$^{-1}$. The dust tails' brightness and morphology are best interpreted in terms of a model of sustained and low dust emission driven by water-ice sublimation, spanning since 2014 August 1st, and triggered by a short impulsive event. This event produced an emission of small particles of about 0.1 $\mu$m with velocities of $\sim$4 m$~$s$^{-1}$. From our model we deduce that the activity of this Main-Belt Comet continued for, at least, four months, since activation.

Dust environment and dynamical history of a sample of short-period comets

Aims. This paper is a continuation of the first paper in this series, where we presented an extended study of the dust environment of a sample of short-period comets and their dynamical history. On this occasion, we focus on comets 81P/Wild 2 and 103P/Hartley 2, which are of special interest as targets of the spacecraft missions Stardust and EPOXI.Methods. As in the previous study, we used two sets of observational data: a set of images, acquired at Sierra Nevada and Lulin observatories, and the Afρ data as a function of the heliocentric distance provided by the amateur astronomical association Cometas-Obs . The dust environment of comets (dust loss rate, ejection velocities, and size distribution of the particles) was derived from our Monte Carlo dust tail code. To determine their dynamical history we used the numerical integrator Mercury 6.2 to ascertain the time spent by these objects in the Jupiter family Comet region.Results. From the dust analysis, we conclude that both 81P/Wild 2 and 103P/Hartley 2 are dusty comets, with an annual dust production rate of 2.8 × 109 kg yr-1 and (0.4−1.5) × 109 kg yr-1 , respectively. From the dynamical analysis, we determined their time spent in the Jupiter family Comet region as ~40 yr in the case of 81P/Wild 2 and ~1000 yr for comet 103P/Hartley 2. These results imply that 81P/Wild 2 is the youngest and the most active comet of the eleven short-period comets studied so far, which tends to favor the correlation between the time spent in JFCs region and the comet activity previously discussed.

Dust environment and dynamical history of a sample of short-period comets

Aims. In this work, we present an extended study of the dust environment of a sample of short period comets and their dynamical history. With this aim, we characterized the dust tails when the comets are active, and we made a statistical study to determine their dynamical evolution. The targets selected were 22P/Kopff, 30P/Reinmuth 1, 78P/Gehrels 2, 115P/Maury, 118P/Shoemaker-Levy 4, 123P/West-Hartley, 157P/Tritton, 185/Petriew, and P/2011 W2 (Rinner). Methods. We use two different observational data: a set of images taken at the Observatorio de Sierra Nevada and the Afrho curves provided by the amateur astronomical association Cometas-Obs. To model these observations, we use our Monte Carlo dust tail code. From this analysis, we derive the dust parameters, which best describe the dust environment: dust loss rates, ejection velocities, and size distribution of particles. On the other hand, we use a numerical integrator to study the dynamical history of the comets, which allows us to determine with a 90% of confidence level the time spent by these objects in the region of Jupiter Family Comets. Results. From the Monte Carlo dust tail code, we derived three categories attending to the amount of dust emitted: Weakly active (115P, 157P, and Rinner), moderately active (30P, 123P, and 185P), and highly active (22P, 78P, and 118P). The dynamical studies showed that the comets of this sample are young in the Jupiter Family region, where the youngest ones are 22P (100 yr), 78P (500 yr), and 118P (600 yr). The study points to a certain correlation between comet activity and time spent in the Jupiter Family region, although this trend is not always fulfilled. The largest particle sizes are not tightly constrained, so that the total dust mass derived should be regarded as lower limits.}

Blending the distinctions among groups of minor bodies: a portrait of the Centaur-comet “transition” object P/2010 C1 (Scotti)

Aims. We present a portrait of the active Centaur P/2010 C1 (Scotti), observed with a well developed comet-like activity (central diffuse coma condensation and an extended sharp tail-like structure) at the heliocentric distance of rh = 5.5 AU. Methods. We analyse multicolour (B, V, R ,a ndI) images taken at the TNG telescope to characterise the dust coma of the active Centaur and to investigate its photometry, colours, and dust production, and to gain hints to its nucleus size. Results. P/2010 C1 (Scotti) has a small nucleus (<4.8 km in radius), with visual colour V − R = 0.49 ± 0.03 typical of the nuclei of Jupiter-family comets, but with a dust coma much redder (B − I = 2.37 ± 0.13) than both the Sun and the typical comet mucleus. It has an integrated R magnitude of mR = 21.41 ± 0.02 and R-Afρ = 34 ± 2 cm (all measured in the reference aperture of radius φ = 2.0 �� ), which is much lower than has been measured for other active Centaurs at similar heliocentric distances. Its dust production rate, Qd = 0.1–15 kg/s, depicts P/2010 C1 (Scotti) as a rather weak dust emitter, with a dust-loss rate comparable to that of the weakest Jupiter-family comets at shorter heliocentric distance and to the weakest active Centaur characterised to now.

The slow flow model of dust efflux in local star-forming galaxies

We develop a dust efflux model of radiation pressure acting on dust grains which successfully reproduces the relation between stellar mass, dust opacity and star formation rate observed in local star-forming galaxies. The dust content of local star-forming galaxies is set by the competition between the physical processes of dust production and dust loss in our model. The dust loss rate is proportional to the dust opacity and star formation rate. Observations of the relation between stellar mass and star formation rate at several epochs imply that the majority of local star-forming galaxies are best characterized as having continuous star formation histories. Dust loss is a consequence of sustained interaction of dust with the radiation field generated by continuous star formation. Dust efflux driven by radiation pressure rather than dust destruction offers a more consistent physical interpretation of the dust loss mechanism. By comparing our model results with the observed relation between stellar mass, dust extinction and star formation rate in local star-forming galaxies we are able to constrain the timescale and magnitude of dust loss. The timescale of dust loss is long and therefore dust is effluxed in a "Slow Flow". Dust loss is modest in low mass galaxies but massive galaxies may lose up to 70~80% of their dust over their lifetime. Our Slow Flow model shows that mass loss driven by dust opacity and star formation may be an important physical process for understanding normal star-forming galaxy evolution.

Dust driven mass loss from carbon stars as a function of stellar parameters

Context. It is well established that the winds of carbon-rich AGB stars (carbon stars) can be driven by radiation pressure on grains of amorphous carbon and collisional transfer of momentum to the gas. This has been demonstrated convincingly by different numerical wind models that include time-dependent dust formation. To simplify the treatment of dust opacities, radiative cross s ections are usually computed using the assumption that the dust grains are small compared to wavelengths around the stellar flux maxi mum. Considering the typical grain sizes that result from these models, however, the applicability of this small-particle l imit (SPL) seems questionable. Aims. We explore grain size effects on wind properties of carbon stars, using a generalized description of radiative cross sections valid for particles of arbitrary sizes. The purpose of the study is to investigate under which circumstances the SPL may give acceptable results, and to quantify the possible errors that may occur when the SPL does not hold. Methods. The time-dependent description of grain growth in our detailed radiation-hydrodynamical models gives information about dust particle radii in every layer at every instant of time. T heses grain radii are used for computing opacities and determining the radiative acceleration of the dust-gas mixture. From the la rge number of models presented in the first paper of this serie s (based on SPL dust opacities; Mattsson et al. 2010) we selected two samples, i.e., a group of models with strong, well-developed outflows that are probably representative of the majority of wind-forming models, and another group, close to thresholds in stellar parameter space for dust-driven winds, which are referred to as critical cas es. Results. We show that in the critical cases the effect of the generalized description of dust opacities can be s ignificant, resulting in more intense mass loss and higher wind velocities compared to models using SPL opacities. For well-developed winds, however, grain size effects on mass-loss rates and wind velocities are found to be small. Both groups of models tend towards lower degrees of dust condensation compared to corresponding SPL models, owing to a self-regulating feedback between grain growth and radiative acceleration. Consequently, the ”dust-loss rates” are low er in the models with the generalized treatment of grain opacities. Conclusions. We conclude that our previous results on mass-loss rates obtained with SPL opacities are reliable within a wide region of stellar parameter space, except for critical cases c lose to thresholds of dust-driven outflows where SPL models w ill tend to underestimate the mass loss rates and wind velocities.

COMET 22P/KOPFF: DUST ENVIRONMENT AND GRAIN EJECTION ANISOTROPY FROM VISIBLE AND INFRARED OBSERVATIONS

We present optical observations and Monte Carlo models of the dust coma, tail, and trail structures of comet 22P/Kopff during the 2002 and 2009 apparitions. Dust loss rates, ejection velocities, and power-law size distribution functions are derived as functions of the heliocentric distance using pre- and post-perihelion imaging observations during both apparitions. The 2009 post-perihelion images can be accurately fitted by an isotropic ejection model. On the other hand, strong dust ejection anisotropies are required to fit the near-coma regions at large heliocentric distances (both inbound at $r_h$=2.5 AU and outbound at $r_h$=2.6 AU) for the 2002 apparition. These asymmetries are compatible with a scenario where dust ejection is mostly seasonally-driven, coming mainly from regions near subsolar latitudes at far heliocentric distances inbound and outbound. At intermediate to near-perihelion heliocentric distances, the outgassing would affect much more extended latitude regions, the emission becoming almost isotropic near perihelion. We derived a maximum dust production rate of 260 kg s$^{-1}$ at perihelion, and an averaged production rate over one orbit of 40 kg s$^{-1}$. An enhanced emission rate, accompanied also by a large ejection velocity, is predicted at $r_h>$2.5 pre-perihelion. The model has also been extended to the thermal infrared in order to be applied to available trail observations with IRAS and ISO spacecrafts of this comet. The modeled trail intensities are in good agreement with those observations, which is remarkable taking into account that those data are sensitive to dust ejection patterns corresponding to several orbits before the 2002 and 2009 apparitions.

Dust-driven mass loss from carbon stars as a function of stellar parameters

[Abridged] In this paper we explore grain size effects on wind properties of carbon stars, using a generalized description of radiative cross sections valid for particles of arbitrary sizes. The purpose of the study is to investigate under which circumstances the small particle limit (SPL) may give acceptable results, and to quantify the possible errors that may occur when it does not hold. The time-dependent description of grain growth in our detailed radiation-hydrodynamical models gives information about dust particle radii in every layer at every instant of time. These grain radii are used for computing opacities and determining the radiative acceleration of the dust-gas mixture. It is shown that in the critical cases the effect of the generalized description of dust opacities can be significant, resulting in more intense mass loss and higher wind velocities compared to models using SPL opacities. For well-developed winds, however, grain size effects on mass loss rates and wind velocities are found to be small. Both groups of models tend towards lower degrees of dust condensation compared to corresponding SPL models, due to a self-regulating feedback between grain growth and radiative acceleration. Consequently, the "dust-loss rates" are lower in the models with the generalized treatment of grain opacities. We conclude that our previous results on mass loss rates obtained with SPL opacities are reliable within a wide region of stellar parameter space, except for critical cases close to thresholds of dust-driven outflows where SPL models will tend to underestimate the mass loss rates and wind velocities.

On the relationship between gas and dust in 15 comets: an application to Comet 103P/Hartley 2 target of the NASAEPOXImission of opportunity

AbstractAfter the success of Deep Impact mission to hit the nucleus of Comet 9P/Tempel 1 with an impactor, the concerns are turned now to the possible reutilization of this dormant flyby spacecraft in the study of another comet, for only about 10% of the cost of the original mission. Comet 103P/Hartley 2 on UT 2010 October 11 is the most attractive target in terms of available fuel at rendezvous and arrival time at the comet. In addition, the comet has a low inclination so that major orbital plane changes in the spacecraft trajectory are unnecessary. In an effort to provide information concerning the planning of this new NASAEPOXIspace mission of opportunity, we use in this work, visual magnitudes measurements available from International Comet Quarterly (ICQ) to obtain, applying the Semi-Empirical Method of Visual Magnitudes - SEMVM (de Almeida, Singh, &amp; Huebner 1997), the water production rates (in molecules/s) related to its perihelion passage of 1997. When associated to the water vaporization theory of Delsemme (1982), these rates allowed the acquisition of the minimum dimension for the effective nuclear radius of the comet. The water production rates were then converted into gas production rates (in g/s) so that, with the help of the strong correlation between gas and dust found for 12 periodic comets and 3 non-period comets (Trevisan Sanzovo 2006), we obtained the dust loss rates (in g/s), its behavior with the heliocentric distance and the dust-to-gas ratios in this physically attractive rendezvous target-comet to Deep Impact spacecraft at a closest approach of 700 km.

Constraints on comet 46P/Wirtanen dust parameters provided by in-situ and ground-based observations

A dust environment working group was encouraged by ESA to provide coma dust environment models useful to plan the ROSETTA operations around the nucleus of short period comet 46P/Wirtanen. Among the many parameters describing the dust released from the nucleus surface, special care was devoted to the dust size distribution. Its present uncertainty makes all environment models sensitive, mainly, to which actual size distribution is adopted. In fact, it must be stressed that no other cometary dust parameter can be derived, such as dust loss rate or dust to gas ratio, if the size distribution remains undetermined. This paper will focus, therefore, on the available information on cometary dust size distributions, starting from the in situ measurement cornerstone provided by the GIOTTO-DIDSY results. Available ground-based observations are then reviewed, in order to disentangle the real sensitivity of them to this quantity; the size distribution is always embedded together with other dust parameters, and its influence on the published results is often forgotten.

The Preperihelion Dust Environment of C/1995 O1 Hale-Bopp from 13 to 4 AU

Two UK Schmidt plates of comet Hale-Bopp dust tail taken in 1996 May are analyzed by means of the inverse dust tail model. The dust tail fits are the only available tools providing estimates of the ejection velocity, the dust-loss rate, and the size distribution of the dust grains ejected during years preceding the comet discovery. These quantities describe the comet dust environment driven by CO sublimation between 1993 and 1996, when the comet approached the Sun from 13 to 4 AU. The outputs of the model are consistent with the available coma photometry, quantified by the Afρ quantity. The dust mass loss rate increases from 500 to 8000 kg s-1, these values being inversely proportional to the dust albedo, assumed here to be 10%. Therefore, the mass ratio between icy grains and CO results is at least 5. Higher values of the dust-to-gas ratio are probable, because the model infers the dust-loss rate over a limited size range, up to 1 mm sized grains, and because the power-law index of the differential size distribution ranges between -3.5 and -4.0, so that most of the dust mass was ejected in the largest boulders that Hale-Bopp was able to eject. The dust ejection velocity close to the observations, between 7 and 4 AU, was close to 100 m s-1 for grains 10 μm in size, much higher than that predicted by R. F. Probstein's theory, thus confirming previous results of Neck-Line photometry. This result is an indicator of CO superheating with respect to a free sublimating CO ice, in agreement with the high observed CO velocity. The fundamental result of the paper is that such a high dust velocity remained constant between 13 and 4 AU, thus providing a strong constraint to all models of the CO-driven activity of the comet during its approach to the Sun: CO superheating must have been active since 13 AU from the Sun. It might be provided by the abundant dust itself, or by seasonal effects heating the subsurface layers, as was suggested for comet 29P/Schwassmann-Wachmann 1. Another similarity between the two comets is provided by the power-law index of the time-averaged size distributions: -3.6 ± 0.1 for C/1995O1 and -3.3 ± 0.3 for 29P/SW1. However, other characteristics of the dust environments are very different, so that, in general, it is impossible to distinguish a CO-driven comet from a typical water-driven one.

Circumstellar shells and mass loss rates: Clues to the evolution of S stars

It is the purpose of this paper to rediscuss the circumstellar properties of S stars and to put these properties in perspective with our current understanding of the evolutionary status of S stars, in particular the intrinsic/extrinsic dichotomy. This dichotomy states that only Tc-rich (intrinsic”) S stars are genuine thermally-pulsing asymptotic giant branch stars, possibly involved in the M–S–C evolutionary sequence. Tc-poor S stars are referred to as extrinsic” S stars, because they are the cooler analogs of barium stars, and like them, owe their chemical peculiarities to mass transfer across their binary system. Accordingly, an extensive data set probing the circumstellar environment of S stars (IRAS flux densities, maser emission, CO rotational lines) has been collected and critically evaluated. This data set combines new observations (9 stars have been observed in the CO line and 3 in the CO line, with four new detections) with existing material (all CO and maser observations of S stars published in the literature). The IRAS flux densities of S stars have been re-evaluated by co-adding the individual scans, in order to better handle the intrinsic variability of these stars in the IRAS bands, and possible contamination by Galactic cirrus. In the () color-color diagram, S stars segregate into five distinct regions according to their Tc content and ZrO/TiO, C/O and IR spectral indices. Stars with photospheric colors (populating Region A”) may be identified with extrinsic S stars. For the other regions characterized by different excess levels in the 12, 25 and 60 μ m bands, several diagnostics (like the IRAS spectral class, maser emission, and shape of CO rotational lines) have been collected to infer the physical properties of the dust shell. A simple radiative-transfer code has also been used to infer the chemical nature (carbonaceous or silicate) of the dust grains from the observed IR colors. S stars with large excesses and moderate [25] - [60] excesses (populating Regions B and C) exhibit the signatures of oxygen-rich shells (9.7 μ m silicate emission and SiO maser emission). The situation is less clear for S stars with small and moderate indices (populating Regions D and E). Their IR colors are consistent with carbonaceous grains (as is their featureless IRAS spectrum, and absence of silicate or SiO maser emission), but these features may equally well be explained by a detached shell. For many of these stars with a large 60 μ m excess, the shell is indeed resolved by the IRAS beam at 60 μ m. The prototypical SC star FU Mon is among these. Since SC stars are believed to be in a very short-lived evolutionary phase where C/O = 1 within 1%, FU Mon may be a good candidate for the interrupted mass-loss” scenario advocated by Willems & de Jong (1988). The CO line profile of FU Mon is also peculiar in being quite narrow ( km s), suggesting that the mass loss has just resumed in this star. Mass loss rates or upper limits have been derived for all S stars observed in the CO rotational lines, and range from y-1 for extrinsic S stars to y-1 (the Mira S star W Aql). These mass-loss rates correlate well with the color index, which probes the dust loss rate, provided that y-1 . Small mass-loss rates are found for extrinsic S stars, consistent with their not being so evolved (RGB or Early-AGB) as the Tc-rich S stars. This result does not support the claim often made in relation with symbiotic stars that binarity strongly enhances the mass-loss rate.