arXiv subjects

Ludmilla Kolokolova

Publications and source records attributed to Ludmilla Kolokolova.

At least 19 recordsLinked to original sources

arXiv2026

Multi-Wavelength Post-Perihelion Polarimetry of Interstellar Comet 3I/ATLAS

We present new post-perihelion polarimetric observations of the third discovered interstellar object, 3I/ATLAS (C/2025 N1; hereafter 3I), obtained using FORS2 at the Very Large Telescope, ALFOSC at the Nordic Optical Telescope, and FoReRo2 at the 2 m Ritchey-Chrétien-Coudé telescope. The observations span phase angles 1-30$^{\circ}$, providing the most extensive polarimetric phase angle coverage obtained for an interstellar object to date. The post-perihelion measurements reveal that the unusual polarimetric properties of 3I persist across perihelion and match recent independent post-perihelion observations, indicating no significant evolution in the polarimetric properties. The increased phase angle coverage allowed us to further constrain the minimum polarisation to $-2.9\%$ at phase angle $5.5^{\circ}$. Multi-band BVRI observations reveal a wavelength dependence of polarisation, with the negative polarisation branch becoming deeper and shifted toward smaller phase angles at longer wavelengths. The polarimetric colour is predominantly red and increases with phase angle, consistent with behaviour observed in other comets. Imaging and polarimetric maps show a substantially more extended coma after perihelion, while the polarisation distribution itself remains smooth and spatially homogeneous. Numerical modelling suggests that the unusually deep polarisation phase curve originates from moderately porous dust aggregates consisting of weakly absorbing sub-micron monomers, resulting in much brighter dust than that in typical solar system comets.

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arXiv2025

A comprehensive study of comet 67P/Churyumov-Gerasimenko in the 2021/2022 apparition. I. Photometry, spectroscopy, morphology

We present observations of comet 67P/Churyumov-Gerasimenko during its 2021/22 apparition, aiming to investigate its dust and gas environment and compare the results with those obtained in 2015/16 using the same telescope. Quasi-simultaneous photometric, spectroscopic, and polarimetric observations were carried out at the 6-m BTA SAO telescope. The comet was observed on 6 October 2021, 31 days before perihelion, with \textit{g}-SDSS and \textit{r}-SDSS filters, and on 6 February 2022, 96 days after perihelion, using narrowband cometary filters: BC ($\lambda4450/62$~Å), RC ($\lambda6839/96$~Å), and CN ($\lambda3870/58$~Å). These were complemented by images from the 2-m Liverpool Telescope (La Palma). On 6 October 2021, a sunward jet and long dust tail were detected. By 6 February 2022, the dust coma morphology had changed noticeably, revealing a bright sunward neckline structure superimposed on the projected dust tail, along with two jets at position angles of 133$^{\circ}$ and 193$^{\circ}$. Spectra showed strong CN emission, with relatively weak C$_2$, C$_3$ and NH$_2$ emissions. The dust production rate $Afρ$ did not exceed 200~cm (uncorrected for phase angle) in both epochs. An unusual CN coma morphology was observed, with evidence of an additional CN source associated with dust jets. Geometric modeling of the jets' dynamics indicated an active area at latitude $-70^{\circ} \pm 4^{\circ}$ with a jet opening angle of $20^{\circ} \pm 6^{\circ}$ on 6 October 2021, and two active areas at latitudes $-58^{\circ} \pm 5^{\circ}$ and $-53^{\circ} \pm 10^{\circ}$, separated by longitude $150^{\circ} \pm 20^{\circ}$, producing the observed jets on 6 February 2022. The average particle velocity in the jets was about $0.32 \pm 0.04$~km~s$^{-1}$.

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arXiv2025

Extreme Negative Polarisation of New Interstellar Comet 3I/ATLAS

We present the first polarimetric observations of the third discovered interstellar object, 3I/ATLAS (C/2025 N1), obtained pre-perihelion with FORS2/VLT, ALFOSC/NOT, and FoReRo2/RCC, over a phase angle range of 7.7-22.4°. This marks the second ever polarimetric study of an interstellar object, the first distinguishing 2I/Borisov from most Solar System comets by its higher positive polarisation. Our polarimetric measurements as a function of phase angle reveal that 3I is characterised by an deep and narrow negative polarisation branch, reaching a minimum value of -2.7% at phase angle 7°, and an inversion angle of 17° -- a combination unprecedented among asteroids and comets, including 2I/Borisov. At very small phase angles, the extrapolated slope of the polarisation phase curve is consistent with that of certain small trans-Neptunian objects and Centaur Pholus, consistent with independent spectroscopic evidence for a red, possibly water-ice-bearing object. Imaging confirms a diffuse coma present from our earliest observations, though no strong polarimetric features are spatial resolved. These findings may demonstrate that 3I represents a distinct type of comet, expanding the diversity of known interstellar bodies.

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arXiv2024

The pre-perihelion evolution of the activity of comet C/2017 K2 (PANSTARRS) during the water ice-line crossover

Comets, relics from the early solar system, consist of dust and ice. The ice sublimates as comets approach the Sun, ejecting dust from their nuclei seen as activity. Different volatiles sublimate at different Sun-comet distances and eject dust of unique sizes, structures, and compositions. In this study, we present new polarimetric observations of Oort-cloud comet C/2017 K2 (PANSTARRS) in R and I-filter domains before, during, and after its crossover of the water-ice sublimation regime at phase angles of 15.9\arcdeg, 10.5\arcdeg, and 20.0\arcdeg, respectively. Combining multiband optical imaging data covering a wide range of heliocentric distances ($\sim$14$-$2.3 au), we aim to characterize the preperihelion evolution of cometary activity as well as the properties of its coma dust. Two discontinuous brightening events were observed: at $\sim$6 au presumably associated with changes in CO-like supervolatile ice activity, and at $\sim$2.9 au when water ice took over. Particularly, the latter activation is accompanied by changes in coma morphology and color whose trends differ between the inner ($\sim$10$^3$-km) and outer ($\sim$10$^4$-km) parts of the coma. No polarimetric discontinuities on the comet were observed over the inner coma region, all epochs showing phase-angle and wavelength dependencies compatible with those of active comets observed in similar observing geometry. During this period, the underlying dust continuum overwhelmed H$α$ emission at around 656.3 nm, suggesting less water ice on the comet's surface than expected. We discuss K2's coma environment by combining numerical simulations of light scattered by dust and place the observations within the context of the comet's evolution.

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arXiv2024

Imaging Polarimetry of Comet 67P/Churyumov-Gerasimenko: Homogeneous Distribution of Polarisation and its Implications

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° 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.

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arXiv2024

A Uniform Analysis of Debris Disks with the Gemini Planet Imager II: Constraints on Dust Density Distribution Using Empirically-Informed Scattering Phase Functions

Spatially-resolved images of debris disks are necessary to determine disk morphological properties and the scattering phase function (SPF) which quantifies the brightness of scattered light as a function of phase angle. Current high-contrast imaging instruments have successfully resolved several dozens of debris disks around other stars, but few studies have investigated trends in the scattered-light, resolved population of debris disks in a uniform and consistent manner. We have combined Karhunen-Loeve Image Projection (KLIP) with radiative-transfer disk forward modeling in order to obtain the highest quality image reductions and constrain disk morphological properties of eight debris disks imaged by the Gemini Planet Imager at H-band with a consistent and uniformly-applied approach. In describing the scattering properties of our models, we assume a common SPF informed from solar system dust scattering measurements and apply it to all systems. We identify a diverse range of dust density properties among the sample, including critical radius, radial width, and vertical width. We also identify radially narrow and vertically extended disks that may have resulted from substellar companion perturbations, along with a tentative positive trend in disk eccentricity with relative disk width. We also find that using a common SPF can achieve reasonable model fits for disks that are axisymmetric and asymmetric when fitting models to each side of the disk independently, suggesting that scattering behavior from debris disks may be similar to Solar System dust.

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arXiv2023

A Uniform Analysis of Debris Disks with the Gemini Planet Imager I: An Empirical Search for Perturbations from Planetary Companions in Polarized Light Images

The Gemini Planet Imager (GPI) has excelled in imaging debris disks in the near-infrared. The GPI Exoplanet Survey (GPIES) imaged twenty-four debris disks in polarized $H$-band light, while other programs observed half of these disks in polarized $J$- and/or $K1$-bands. Using these data, we present a uniform analysis of the morphology of each disk to find asymmetries suggestive of perturbations, particularly those due to planet-disk interactions. The multi-wavelength surface brightness, the disk color and geometry permit identification of any asymmetries such as warps or disk offsets from the central star. We find that nineteen of the disks in this sample exhibit asymmetries in surface brightness, disk color, disk geometry, or a combination of the three, suggesting that for this sample, perturbations, as seen in scattered light, are common. The relationship between these perturbations and potential planets in the system are discussed. We also explore correlations among stellar temperatures, ages, disk properties, and observed perturbations. We find significant trends between the vertical aspect ratio and the stellar temperature, disk radial extent, and the dust grain size distribution power-law, $q$. We also confirm a trend between the disk color and stellar effective temperature, where the disk becomes increasingly red/neutral with increasing temperature. Such results have important implications on the evolution of debris disk systems around stars of various spectral types.

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arXiv2023

Polarimetry of Didymos-Dimorphos: Unexpected Long-Term Effects of the DART Impact

We have monitored the Didymos-Dimorphos binary system in imaging polarimetric mode before and after the impact from the Double Asteroid Redirection Test (DART) mission. A previous spectropolarimetric study showed that the impact caused a dramatic drop in polarisation. Our longer-term monitoring shows that the polarisation of the post-impact system remains lower than the pre-impact system even months after the impact, suggesting that some fresh ejecta material remains in the system at the time of our observations, either in orbit or settled on the surface. The slope of the post-impact polarimetric curve is shallower than that of the pre-impact system, implying an increase in albedo of the system. This suggests that the ejected material is composed of smaller and possibly brighter particles than those present on the pre-impact surface of the asteroid. Our polarimetric maps show that the dust cloud ejected immediately after the impact polarises light in a spatially uniform manner (and at a lower level than pre-impact). Later maps exhibit a gradient in polarisation between the photocentre (which probes the asteroid surface) and the surrounding cloud and tail. The polarisation occasionally shows some small-scale variations, the source of which is not yet clear. The polarimetric phase curve of Didymos-Dimorphos resembles that of the S-type asteroid class.

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arXiv2023

Characterization of the ejecta from NASA/DART impact on Dimorphos: observations and Monte Carlo models

The NASA/DART (Double Asteroid Redirection Test) spacecraft successfully crashed on Dimorphos, the secondary component of the binary (65803) Didymos system. Following the impact, a large dust cloud was released, and a long-lasting dust tail was developed. We have extensively monitored the dust tail from the ground and from the Hubble Space Telescope (HST). We provide a characterization of the ejecta dust properties, i.e., particle size distribution and ejection speeds, ejection geometric parameters, and mass, by combining both observational data sets, and by using Monte Carlo models of the observed dust tail. The differential size distribution function that best fits the imaging data was a broken power-law, having a power index of --2.5 for particles of r$\le$ 3 mm, and of --3.7 for larger particles. The particles range in sizes from 1 $μ$m up to 5 cm. The ejecta is characterized by two components, depending on velocity and ejection direction. The northern component of the double tail, observed since October 8th 2022, might be associated to a secondary ejection event from impacting debris on Didymos, although it is also possible that this feature results from the binary system dynamics alone. The lower limit to the total dust mass ejected is estimated at $\sim$6$\times$10$^6$ kg, half of this mass being ejected to interplanetary space.

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arXiv2023

Ejecta from the DART-produced active asteroid Dimorphos

Some active asteroids have been proposed to be the result of impact events. Because active asteroids are generally discovered serendipitously only after their tail formation, the process of the impact ejecta evolving into a tail has never been directly observed. NASA's Double Asteroid Redirection Test (DART) mission, apart from having successfully changed the orbital period of Dimorphos, demonstrated the activation process of an asteroid from an impact under precisely known impact conditions. Here we report the observations of the DART impact ejecta with the Hubble Space Telescope (HST) from impact time T+15 minutes to T+18.5 days at spatial resolutions of ~2.1 km per pixel. Our observations reveal a complex evolution of ejecta, which is first dominated by the gravitational interaction between the Didymos binary system and the ejected dust and later by solar radiation pressure. The lowest-speed ejecta dispersed via a sustained tail that displayed a consistent morphology with previously observed asteroid tails thought to be produced by impact. The ejecta evolution following DART's controlled impact experiment thus provides a framework for understanding the fundamental mechanisms acting on asteroids disrupted by natural impact.

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arXiv2022

Dust Evolution in the Coma of Distant, Inbound Comet C/2017 K2 (PANSTARRS)

C/2017 K2 (PANSTARRS) is an Oort cloud comet previously observed to be active at heliocentric distances r>20 au on what is likely its first passage through the inner solar system. We observed the comet on 2021 March 19-20 at r=6.82 au pre-perihelion and 8.35 deg phase angle with the Hubble Space Telescope (HST), and obtained high spatial resolution photometry and polarimetry mapping the properties of dust over the coma prior to the onset of water ice sublimation activity on the nucleus. We found clear radial gradients in the color and polarization of the coma: the F475W-F775W (g'-i') reflectance slope increased from ~4.5% per 100 nm within ~10,000 km of the nucleus up to ~7% per 100 nm by ~50,000 km, while the negative polarization in F775W (i') strengthened from about -2% to -3.5% over the same range. The radial intensity profiles moreover strongly deviate from profiles simulated for stable dust grains. Near-infrared imagery obtained with the Palomar Hale Telescope on 2021 May 18 at r=6.34 au revealed a continued absence of micron-sized grains in the tail, but showed no clear spatial gradient in JHKs colors. The observed patterns collectively appear consistent with the inner coma being optically dominated by sublimating, micron-sized water ice grains, unlike the tail of more stable, millimeter-sized grains. Finally, we evaluated these results alongside other Oort cloud comets, and found in a reanalysis of HST observations of C/2012 S1 (ISON) that the near-nucleus polarimetric halo reported for that comet is likely an observational artifact.

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arXiv2022

Do twin spectral peaks of olivine particles in the thermal infrared diagnose their sizes and porosities?

A well-established constraint on the size of non-porous olivine grains or the porosity of aggregates consisting of small olivine grains from prominent narrow peaks in thermal infrared spectra characteristic of crystalline silicates is reexamined. To thoroughly investigate thermal infrared peaks, we make theoretical argument for the absorption and scattering of light by non-porous, non-spherical olivine particles, which is followed by numerical verification. Our study provides perfectly rational explanations of the physics behind the small-particle effect of emission peaks in the framework of classical electrodynamics and convincing evidence of small-particle's emission peaks in the literature. While resonant absorption excited by surface roughness on the order of submicrometer scales can be identified even for non-porous olivine particles with a radius of $10~{\rm μm}$, it makes only a negligible contribution to thermal infrared spectra of the particles. In contrast, the porosity of non-spherical particles has a significant impact on the strength and wavelength of the peaks, while the resonant absorption excited by an ensemble of small grains takes place at a wavelength different than one expects for surface roughness. We finally reaffirm that twin peaks of olivine in thermal infrared spectra of dust particles in astronomical environments are the intrinsic diagnostic characters of submicrometer-sized small grains and their aggregate particles in fluffy and porous configurations.

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arXiv2021

An Update of the Correlation between Polarimetric and Thermal Properties of Cometary Dust

We present a possible correlation between the properties of scattered and thermal radiation from dust and the principal dust characteristics responsible for this relationship. To this end, we use the NASA/PDS archival polarimetric data on cometary dust in the Red (0.62--0.73 $μ$m) and K (2.00--2.39 $μ$m) domains to leverage the relative excess of the polarisation degree of a comet to the average trend at the given phase angle ($P_{\rm excess}$) as a metric of the dust's scattered light characteristics. The flux excess of silicate emissions to the continuum around 10 $μ$m ($F_{\rm Si}/F_{\rm cont}$) is adopted from previous studies as a metric of the dust's MIR feature. The two metrics show a positive correlation when $P_{\rm excess}$ is measured in the K domain. No significant correlation was identified in the Red domain. The gas-rich comets have systematically weaker $F_{\rm Si}/F_{\rm cont}$ than the dust-rich ones, yet both groups retain the same overall tendency with different slope values. The observed positive correlation between the two metrics indicates that composition is a peripheral factor in characterising the dust's polarimetric and silicate emission properties. The systematic difference in $F_{\rm Si}/F_{\rm cont}$ for gas-rich versus dust-rich comets would rather correspond with the difference in their dust size distribution. Hence, our results suggest that the current MIR spectral models of cometary dust should prioritise the dust size and porosity over the composition. With light scattering being sensitive to different size scales in two wavebands, we expect the K-domain polarimetry to be sensitive to the properties of dust aggregates, such as size and porosity, which might have been influenced by evolutionary processes. On the other hand, the Red-domain polarimetry reflects the characteristics of sub-$μ$m constituents in the aggregate.

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arXiv2021

Comet 2P/Encke in apparition of 2017: II. Polarization and color

We present results of imaging polarimetry of comet 2P/Encke performed on January 23, 2017 at the heliocentric (1.052 au) and geocentric (1.336 au) distances and phase angle 46.8 deg, 46 days before perihelion. Observations were made through the medium-band SED500 (λ5019/246 Å) and broadband r-sdss (λ6200/1200 Å) filters with the multimode focal reducer SCORPIO-2 at the 6-m BTA telescope of the Special Astrophysical Observatory (Russia). Dust in comet 2P/Encke was mainly concentrated in the near-nucleus region of the coma: the maximum dust/gas ratios were 1.5 and 2.9 in the SED500 and the r-sdss filters near the nucleus but dropped sharply to ~0.2 and ~1 at the distance ~2500 km, respectively. Then these ratios began to increase at distances ~12000 km from the nucleus, the ratio was ~0.3 (SED500) and ~1.3 (r-sdds). There were significant variations of polarization over the coma, which correlated with the variations in the dust color and dust/gas ratio. Changes in polarization and color across the 2P/Encke coma indicate changes in physical properties of the dust particles with the distance from the nucleus. Our Sh-matrix computer simulations of light scattering by Gaussian particles allow us to suggest that the observed trends in color and polarization are mainly result from changing particle size.

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arXiv2019

Photometry, spectroscopy, and polarimetry of distant comet C/2014 A4 (SONEAR)

We analyze the dust environment of the distant comet C/2014 A4 (SONEAR), with a perihelion distance near 4.1~au, using comprehensive observations obtained by different methods. We present an analysis of spectroscopy, photometry, and polarimetry of comet C/2014 A4 (SONEAR), which were performed on November 5~--~7, 2015, when its heliocentric distance was 4.2~au and phase angle was 4.7$^\circ$. Long-slit spectra and photometric and linear polarimetric images were obtained using the focal reducer SCORPIO-2 attached to the prime focus of the 6-m telescope BTA (SAO RAS, Russia). We simulated the behavior of color and polarization in the coma presenting the cometary dust as a set of polydisperse polyshapes rough spheroids. No emissions were detected in the 3800~--~7200~$Å$ wavelength range. The continuum showed a reddening effect with the normalized gradient of reflectivity 21.6$\pm$0.2\% per 1000~$Å$ within the 4650~--~6200~$Å$ wavelength region. The fan-like structure in the sunward hemisphere was detected. The radial profiles of surface brightness differ for r-sdss and g-sdss filters, indicating predominance of submicron and micron-sized particles in cometary coma. The dust color (g--r) varies from 0.75$ \pm $0.05$^m$ to 0.45$ \pm $0.06$^m$ along the tail. For aperture radius near 20~000~km, the dust productions in various filters were estimated as $Afρ$~=~680$\pm$18~cm (r-sdss) and 887$ \pm $16~cm (g-sdss). The polarization map showed spatial variations of polarization over the coma from about --3\% near the nucleus to --8\% at cometocentric distance about 150~000~km. Our simulations show that the dust particles were dominated (or covered) by ice and tholin-like organics. Spatial changes in the color and polarization can be explained by particle fragmentation.

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arXiv2019

Debris Disk Composition: A Diagnostic Tool for Planet Formation and Migration

Debris disks are exoplanetary systems containing planets, minor bodies (such as asteroids and comets) and debris dust. Unseen planets are presumed to perturb the minor bodies into crossing orbits, generating small dust grains that are detected via remote sensing. Debris disks have been discovered around main sequence stars of a variety of ages (from 10 Myr to several Gyr) and stellar spectral types (from early A-type to M-type stars). As a result, they serve as excellent laboratories for understanding whether the architecture and the evolution of our Solar System is common or rare. This white paper addresses two outstanding questions in debris disk science: (1) Are debris disk minor bodies similar to asteroids and comets in our Solar System? (2) Do planets separate circumstellar material into distinct reservoirs and/or mix material during planet migration? We anticipate that SOFIA/HIRMES, JWST, and WFIRST/CGI will greatly improve our understanding of debris disk composition, enabling the astronomical community to answer these questions. However, we note that despite their observational power, these facilities will not provide large numbers of detections or detailed characterization of cold ices and silicates in the Trans Neptunian zone. Origins Space Telescope is needed to revolutionize our understanding of the bulk composition and mixing in exoplanetary systems.

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arXiv2018

Models of Rosetta/OSIRIS 67P dust coma phase function

The phase function of the dust coma of comet 67P has been determined from Rosetta/OSIRIS images \citep{Bertini17}. This function show a deep minimum at phase angles near 100$^\circ$, and a strong backscattering enhancement. These two properties cannot be reproduced by regular models of cometary dust, most of them based on wavelength-sized and randomly-oriented aggregate particles. We show, however, that an ensamble of oriented elongated particles of a wide variety of aspect ratios, with radii $r \gtrsim$10 $μ$m, and whose long axes are perpendicular to the direction of the solar radiation, are capable of reproducing the observed phase function. These particles must be absorbing, with an imaginary part of the refractive index of about 0.1 to match the expected geometric albedo, and with porosity in the 60-70\% range.

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arXiv2018

Icy Grains from the Nucleus of Comet C/2013 US10 (Catalina)

We present IRTF/SpeX and NEOWISE observations of the dynamically new comet C/2013 US$_{10}$ (Catalina), hereafter US10, from 5.8 au inbound, to near perihelion at 1.3 au, and back to 5.0 au outbound. We detect water ice in the coma of US10, assess and monitor the physical properties of the ice as insolation varies with heliocentric distance, and investigate the relationship between water ice and CO$_{2}$. This set of measurements is unique in orbital coverage and can be used to infer the physical evolution of the ice and, potentially, the nucleus composition. We report (1) nearly identical near-infrared spectroscopic measurements of the coma at $-$5.8 au, $-$5.0 au, +3.9 au (where $<$0 au indicates pre-perihelion epochs), all presenting evidence of water-ice grains, (2) a dust-dominated coma at 1.3 au and 2.3 au and, (3) an increasing CO$_{2}$/$Afρ$ ratio from $-$4.9 au to 1.8 au. We propose that sublimation of the hyper-volatile CO$_{2}$ is responsible for dragging water-ice grains into the coma throughout the orbit. Once in the coma, the observability of the water-ice grains is controlled by the ice grain sublimation lifetime, which seems to require some small dust contaminant (i.e., non-pure ice grains). At |R$_{h}$|>=3.9 au, the ice grains are long-lived and may be unchanged since leaving the comet nucleus. We find the nucleus of comet US10 is made of, among other components, $\sim$1-micron water-ice grains containing up to 1% refractory materials.

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