Nikolai Kiselev - Academia.edu (original) (raw)

Papers by Nikolai Kiselev

Kosmìčna nauka ì tehnologìâ, 2012

Long-periodic comet C/2011 KP36 (Spacewatch), defined by Bauer et al. 2013 (Aph. J. 773:22) as a ... more Long-periodic comet C/2011 KP36 (Spacewatch), defined by Bauer et al. 2013 (Aph. J. 773:22) as a Scattered Disc Object, has a period of around 238 years and perihelion distance q=4.88 au. Its semimajor axis (a=38.41 au) is larger than aphelion distance of Neptune (a N =30.07 au). Comprehensive observations of the comet were carried out at the 6-m BTA telescope of the Special Astrophysical Observatory (Russia) with the multimode focal reducer SCORPIO-2.

Icarus, 1998

B2 Hyakutake were carried out at the 70-cm telescope of Khar-70-cm reflector with a Cassegrain co... more B2 Hyakutake were carried out at the 70-cm telescope of Khar-70-cm reflector with a Cassegrain configuration (f/16) at kov State University Observatory from March 25 to April 7, 1996. The goal of the observations was to determine the polar-the Chuguev Observation Station of Kharkov State Uniization maximum of the cometary continuum. We have found versity from March 25 to April 7, 1996. This station is that the maximal degrees of polarization in the blue (4845 Å) located about 70-km to the southeast of Kharkov at 156 m and red (6840 Å) continuum domains are 24.0% and 26.1%, altitude. The observations were made with a set of narrowrespectively, and lie at a phase angle close to 94؇, slightly band comet filters. The ultraviolet, blue, and red contindependent on wavelength. According to the gas-to-dust ratio uum filters centered at 3650 (UC), 4845 (BC), and 6840 Å and color, Comet Hyakutake belongs to the group of dusty (RC) and the molecular band filters for C 3 at 4060 and C 2 comets such as West 1976 VI, P/Churyumov-Gerasimenko at 5140 Å were used. The diaphragm was 88 arcsec. The 1982 VIII, P/Kopff 1983 XIII, P/Hartley-IRAS 1984 III, comet was placed visually near the center of the diaphragm.

In November 2003 Comet 2P/Encke was observed simultaneously with the 10-m Heinrich-Hertz Submilli... more In November 2003 Comet 2P/Encke was observed simultaneously with the 10-m Heinrich-Hertz Submillimeter Telescope on Mount Graham, Arizona, USA, and the 2-m optical telescope on Mount Rozhen, Bulgaria. Simultaneous radio observations of the 4-3 and 3-2 rotational transitions of HCN and the 0-0 transition of the CN violet band system provide a three-dimensional view on the comet. The observations are consistent with outgassing from the source region I with location and pole position of Comet Encke taken from [14]. The outflow speed is 1.2 km. There is some evidence for another possible parent for CN besides HCN. The visual dust coma of Comet Encke is nearly spherical with a diameter of about 1000 km and a slight extension into Comet Encke's fan. The polarization of the observed NH 2 transition at 662 nm is 7% at a phase angle of 94.5 • , close to the value for two-atomic molecules. At this phase angle and a wavelength of 642 nm the polarization of Comet Encke's dust is greater than 30%, i.e., exceeds the value for so-called dusty comets.

Solar System Research, 2020

The results of polarimetric observations of the asteroid (599) Luisa carried out on the same type... more The results of polarimetric observations of the asteroid (599) Luisa carried out on the same type of aperture photoelectric polarimeters of the 2.6-m reflector of the Crimean Astrophysical Observatory and the Zeiss 2000 telescope of the Terskol Peak Observatory in November 2018 and February 2019 are presented. Based on these and available literature data, the parameters of the composite phase dependence of the polarization degree are obtained: P min =-1.68%, α min = 14.7°, α inv = 29.2°, which indicates that the asteroid belongs to the class of "Barbarians". The absolute value of the negative polarization degree of the asteroid in the range of the BVRI photometric bands is shown to increase with wavelength, which contradicts the data of Bagnulo et al. (2015) but is consistent with the spectral polarization behavior of other silicate S-, K-, and Ltype asteroids, including well-known Barbarians. Possible causes of differences in the spectral behavior of the polarization degree of this asteroid are discussed. The properties of the asteroid (599) Luisa are unique in that it combines spectral properties of silicate L-type asteroids and polarization properties (P min and the slope h) of low-albedo C and Ch asteroids.

The Planetary Science Journal

This paper is dedicated to a long-standing problem of the shape of the negative branch of polariz... more This paper is dedicated to a long-standing problem of the shape of the negative branch of polarization (NBP) for Jupiter's moon Europa, determination of which is crucial for the characterization of the icy regolith on this satellite and similar objects, as well as for further progress in understanding light scattering by particulate surfaces. To establish the shape of Europa's NBP, in 2018–2021 we accomplished high-precision disk-integrated polarimetry of Europa in the UBVR I bands using the identical two-channel photoelectric polarimeters mounted on the 2.6 m Shajn reflector of the Crimean Astrophysical Observatory and the 2 m telescope of the Peak Terskol Observatory. We found that the polarization dependence on the phase angle in each filter is an asymmetric curve with a sharp polarization minimum P min ≈ − 0.3 % at phase angle α min ≤ 0 .° 4 , after which the polarization degree gradually increases to positive values, passing the inversion angle at α inv ≈ 6° − 7°. Withi...

Acta Astrophysica Taurica, 2020

We present the method and results of defining coefficients while passing on from the instrument b... more We present the method and results of defining coefficients while passing on from the instrument brightness values in u, b, v, r, i bands to U, B, V, R, I bands of the Johnson - Cousins system at the telescope MTM-500.

Icarus, 2020

We present results of imaging polarimetry of comet 2P/Encke performed on January 23, 2017 at the ... more 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 � , 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 to leave F em /F cont 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 ~2.500 km, respectively. Then these ratios began to increase at distances ~12,000 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. The maximum degree of polarization, ~8% in the r-sdss filter, was observed in the dust shell which was shifted by ~1.000 km towards the Sun. Polarization sharply dropped to ~4% at the distance ~3.000 km and then gradually increased with wave-like fluctuations with the distance from the nucleus, reaching ~8% at the distance ~12,000 km. A similar change in polarization was observed in the SED500 filter. After correction for gas contamination, using the dust/gas ratios from spectroscopy made on the same night, the values of polarization appeared to be ~4% in the near-nucleus region (~1.000 km), and reached 11-12% at the distance ~12,000 km in both filters. We also found an effect of nucleus polarization on the polarization of the dust coma in comet Encke in the r-sdss filter. The maximum value of the nucleus contamination was ~0.7%. 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.

Astronomy & Astrophysics, 2019

Context. The study of distant comets, which are active at large heliocentric distances, is import... more Context. The study of distant comets, which are active at large heliocentric distances, is important for a better understanding of their physical properties and mechanisms of long-lasting activity. Aims. We analyzed 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. 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°. 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. Results. No emission features were detected in the 3800–7200 Å wav...

Monthly Notices of the Royal Astronomical Society, 2017

We present post-perihelion photometric and polarimetric observations of comet 67P/Churyumov-Geras... more We present post-perihelion photometric and polarimetric observations of comet 67P/Churyumov-Gerasimenko performed at the 6-m telescope of the SAO RAS in the gsdss (465/65 nm), r-sdss (620/60 nm) and R filters. Observations in November and December 2015 and April 2016 covered the range of heliocentric distance 1.62-2.72 au and phase angle 33.2 •-10.4 •. The comet was very active. Two persistent jets and long dust tail were observed during the whole observing period; one more jet was detected only in December. The radial profiles of surface brightness, colour and polarization significantly differed for the coma, jets and tail, and changed with increasing heliocentric distance. The dust production Afρ decreased from 162 cm at r = 1.62 au to 51 cm at r = 2.72 au. The dust colour (g-r) gradually changed from 0.8 mag in the innermost coma to about 0.4 mag in the outer coma. The spectral slope was 8.2 ± 1.7 per cent/100 nm in the 465 to 620 nm wavelength domain. In November and December, the polarization in the near-nucleus area was about 8 per cent, dropped sharply to 2 per cent at the distance above 5000 km and then gradually increased with distance from the nucleus, reaching ∼8 per cent at 40 000 km. In April, at a phase angle 10.4 • , the polarization varied between-0.6 per cent in the near-nucleus area and-4 per cent in the outer coma. Circular polarization was not detected in the comet. The spatial variations of brightness, colour and polarization in different structural features suggest some evolution of particle properties, most likely decreasing the size of dust particles.

Astronomy & Astrophysics, 2019

Context. The study of distant comets, which are active at large heliocentric distances, is import... more Context. The study of distant comets, which are active at large heliocentric distances, is important for a better understanding of their physical properties and mechanisms of long-lasting activity. Aims. We analyzed 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. 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°. 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. Results. No emission features were detected in the 3800–7200 Å wav...

Monthly Notices of the Royal Astronomical Society, 2016

Circular polarization in cometary continuum bands has been observed for more than 30 years. Recen... more Circular polarization in cometary continuum bands has been observed for more than 30 years. Recently, imaging data on circular polarization have been obtained for more than 10 comets using the 6-m Big Telescope Alt-azimuthal (BTA) telescope (Russia). However, despite the accumulation of significant amounts of data, the mechanisms that form circular polarization in the cometary environment are still a mystery. Regular mechanisms, such as multiple scattering in an optically thick medium or the domination of particles or materials of a specific mirror asymmetry (including homochirality), could not explain the observations. Particle alignment was considered the most feasible mechanism; however, the cause of the particle alignment was not clear. The most reasonable mechanism, alignment in a magnetic field, was considered doubtful, as comets do not have their own magnetic field and, according to the in situ results for comet Halley, the solar magnetic field could not penetrate deeply into the coma. However, new theoretical studies of cometary coma interaction with the solar magnetic field and, especially, Rosetta observations of the diamagnetic cavity of comet 67P/Churyumov-Gerasimenko showed that the solar wind magnetic field can penetrate as close to the nucleus as several dozen kilometres. This allows us to suggest alignment in the solar magnetic field as a reason for the observed circular polarization. Based on the data obtained for comet 67P, we estimate the time necessary for the alignment of cometary particles in the solar magnetic field. The estimates obtained are consistent with the observations of cometary circular polarization.

Monthly Notices of the Royal Astronomical Society, 2017

We present post-perihelion photometric and polarimetric observations of comet 67P/Churyumov-Geras... more We present post-perihelion photometric and polarimetric observations of comet 67P/Churyumov-Gerasimenko performed at the 6-m telescope of the SAO RAS in the gsdss (465/65 nm), r-sdss (620/60 nm) and R filters. Observations in November and December 2015 and April 2016 covered the range of heliocentric distance 1.62-2.72 au and phase angle 33.2 •-10.4 •. The comet was very active. Two persistent jets and long dust tail were observed during the whole observing period; one more jet was detected only in December. The radial profiles of surface brightness, colour and polarization significantly differed for the coma, jets and tail, and changed with increasing heliocentric distance. The dust production Afρ decreased from 162 cm at r = 1.62 au to 51 cm at r = 2.72 au. The dust colour (g-r) gradually changed from 0.8 mag in the innermost coma to about 0.4 mag in the outer coma. The spectral slope was 8.2 ± 1.7 per cent/100 nm in the 465 to 620 nm wavelength domain. In November and December, the polarization in the near-nucleus area was about 8 per cent, dropped sharply to 2 per cent at the distance above 5000 km and then gradually increased with distance from the nucleus, reaching ∼8 per cent at 40 000 km. In April, at a phase angle 10.4 • , the polarization varied between-0.6 per cent in the near-nucleus area and-4 per cent in the outer coma. Circular polarization was not detected in the comet. The spatial variations of brightness, colour and polarization in different structural features suggest some evolution of particle properties, most likely decreasing the size of dust particles.

Monthly Notices of the Royal Astronomical Society, 2016

Circular polarization in cometary continuum bands has been observed for more than 30 years. Recen... more Circular polarization in cometary continuum bands has been observed for more than 30 years. Recently, imaging data on circular polarization have been obtained for more than 10 comets using the 6-m Big Telescope Alt-azimuthal (BTA) telescope (Russia). However, despite the accumulation of significant amounts of data, the mechanisms that form circular polarization in the cometary environment are still a mystery. Regular mechanisms, such as multiple scattering in an optically thick medium or the domination of particles or materials of a specific mirror asymmetry (including homochirality), could not explain the observations. Particle alignment was considered the most feasible mechanism; however, the cause of the particle alignment was not clear. The most reasonable mechanism, alignment in a magnetic field, was considered doubtful, as comets do not have their own magnetic field and, according to the in situ results for comet Halley, the solar magnetic field could not penetrate deeply into the coma. However, new theoretical studies of cometary coma interaction with the solar magnetic field and, especially, Rosetta observations of the diamagnetic cavity of comet 67P/Churyumov-Gerasimenko showed that the solar wind magnetic field can penetrate as close to the nucleus as several dozen kilometres. This allows us to suggest alignment in the solar magnetic field as a reason for the observed circular polarization. Based on the data obtained for comet 67P, we estimate the time necessary for the alignment of cometary particles in the solar magnetic field. The estimates obtained are consistent with the observations of cometary circular polarization.

Kinematics and Physics of Celestial Bodies, 2015

Results of polarimetric observations of Saturn's satellite Rhea obtained in the range of phase an... more Results of polarimetric observations of Saturn's satellite Rhea obtained in the range of phase angles from 0.76° to 5.96° are presented. The observations were carried out in the WR spectral band (550-750 nm) at the 2.6 m telescope equipped with a single channel photometer-polarimeter at the Crimean Astrophysical Observatory in the period from March 23, 2012, to May 2, 2014. From the results of the observations, the phase dependence of linear polarization of Rhea was built. The parameters of this dependence are compared to those of the high albedo satellites of Jupiter, Japetus, and medium albedo satellites of Uranus. The obtained results are discussed in terms of the currently available models of the light scattering by regolith surfaces.

Kosmìčna nauka ì tehnologìâ, 2012

Long-periodic comet C/2011 KP36 (Spacewatch), defined by Bauer et al. 2013 (Aph. J. 773:22) as a ... more Long-periodic comet C/2011 KP36 (Spacewatch), defined by Bauer et al. 2013 (Aph. J. 773:22) as a Scattered Disc Object, has a period of around 238 years and perihelion distance q=4.88 au. Its semimajor axis (a=38.41 au) is larger than aphelion distance of Neptune (a N =30.07 au). Comprehensive observations of the comet were carried out at the 6-m BTA telescope of the Special Astrophysical Observatory (Russia) with the multimode focal reducer SCORPIO-2.

Icarus, 1998

B2 Hyakutake were carried out at the 70-cm telescope of Khar-70-cm reflector with a Cassegrain co... more B2 Hyakutake were carried out at the 70-cm telescope of Khar-70-cm reflector with a Cassegrain configuration (f/16) at kov State University Observatory from March 25 to April 7, 1996. The goal of the observations was to determine the polar-the Chuguev Observation Station of Kharkov State Uniization maximum of the cometary continuum. We have found versity from March 25 to April 7, 1996. This station is that the maximal degrees of polarization in the blue (4845 Å) located about 70-km to the southeast of Kharkov at 156 m and red (6840 Å) continuum domains are 24.0% and 26.1%, altitude. The observations were made with a set of narrowrespectively, and lie at a phase angle close to 94؇, slightly band comet filters. The ultraviolet, blue, and red contindependent on wavelength. According to the gas-to-dust ratio uum filters centered at 3650 (UC), 4845 (BC), and 6840 Å and color, Comet Hyakutake belongs to the group of dusty (RC) and the molecular band filters for C 3 at 4060 and C 2 comets such as West 1976 VI, P/Churyumov-Gerasimenko at 5140 Å were used. The diaphragm was 88 arcsec. The 1982 VIII, P/Kopff 1983 XIII, P/Hartley-IRAS 1984 III, comet was placed visually near the center of the diaphragm.

In November 2003 Comet 2P/Encke was observed simultaneously with the 10-m Heinrich-Hertz Submilli... more In November 2003 Comet 2P/Encke was observed simultaneously with the 10-m Heinrich-Hertz Submillimeter Telescope on Mount Graham, Arizona, USA, and the 2-m optical telescope on Mount Rozhen, Bulgaria. Simultaneous radio observations of the 4-3 and 3-2 rotational transitions of HCN and the 0-0 transition of the CN violet band system provide a three-dimensional view on the comet. The observations are consistent with outgassing from the source region I with location and pole position of Comet Encke taken from [14]. The outflow speed is 1.2 km. There is some evidence for another possible parent for CN besides HCN. The visual dust coma of Comet Encke is nearly spherical with a diameter of about 1000 km and a slight extension into Comet Encke's fan. The polarization of the observed NH 2 transition at 662 nm is 7% at a phase angle of 94.5 • , close to the value for two-atomic molecules. At this phase angle and a wavelength of 642 nm the polarization of Comet Encke's dust is greater than 30%, i.e., exceeds the value for so-called dusty comets.

Solar System Research, 2020

The results of polarimetric observations of the asteroid (599) Luisa carried out on the same type... more The results of polarimetric observations of the asteroid (599) Luisa carried out on the same type of aperture photoelectric polarimeters of the 2.6-m reflector of the Crimean Astrophysical Observatory and the Zeiss 2000 telescope of the Terskol Peak Observatory in November 2018 and February 2019 are presented. Based on these and available literature data, the parameters of the composite phase dependence of the polarization degree are obtained: P min =-1.68%, α min = 14.7°, α inv = 29.2°, which indicates that the asteroid belongs to the class of "Barbarians". The absolute value of the negative polarization degree of the asteroid in the range of the BVRI photometric bands is shown to increase with wavelength, which contradicts the data of Bagnulo et al. (2015) but is consistent with the spectral polarization behavior of other silicate S-, K-, and Ltype asteroids, including well-known Barbarians. Possible causes of differences in the spectral behavior of the polarization degree of this asteroid are discussed. The properties of the asteroid (599) Luisa are unique in that it combines spectral properties of silicate L-type asteroids and polarization properties (P min and the slope h) of low-albedo C and Ch asteroids.

The Planetary Science Journal

This paper is dedicated to a long-standing problem of the shape of the negative branch of polariz... more This paper is dedicated to a long-standing problem of the shape of the negative branch of polarization (NBP) for Jupiter's moon Europa, determination of which is crucial for the characterization of the icy regolith on this satellite and similar objects, as well as for further progress in understanding light scattering by particulate surfaces. To establish the shape of Europa's NBP, in 2018–2021 we accomplished high-precision disk-integrated polarimetry of Europa in the UBVR I bands using the identical two-channel photoelectric polarimeters mounted on the 2.6 m Shajn reflector of the Crimean Astrophysical Observatory and the 2 m telescope of the Peak Terskol Observatory. We found that the polarization dependence on the phase angle in each filter is an asymmetric curve with a sharp polarization minimum P min ≈ − 0.3 % at phase angle α min ≤ 0 .° 4 , after which the polarization degree gradually increases to positive values, passing the inversion angle at α inv ≈ 6° − 7°. Withi...

Acta Astrophysica Taurica, 2020

We present the method and results of defining coefficients while passing on from the instrument b... more We present the method and results of defining coefficients while passing on from the instrument brightness values in u, b, v, r, i bands to U, B, V, R, I bands of the Johnson - Cousins system at the telescope MTM-500.

Icarus, 2020

We present results of imaging polarimetry of comet 2P/Encke performed on January 23, 2017 at the ... more 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 � , 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 to leave F em /F cont 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 ~2.500 km, respectively. Then these ratios began to increase at distances ~12,000 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. The maximum degree of polarization, ~8% in the r-sdss filter, was observed in the dust shell which was shifted by ~1.000 km towards the Sun. Polarization sharply dropped to ~4% at the distance ~3.000 km and then gradually increased with wave-like fluctuations with the distance from the nucleus, reaching ~8% at the distance ~12,000 km. A similar change in polarization was observed in the SED500 filter. After correction for gas contamination, using the dust/gas ratios from spectroscopy made on the same night, the values of polarization appeared to be ~4% in the near-nucleus region (~1.000 km), and reached 11-12% at the distance ~12,000 km in both filters. We also found an effect of nucleus polarization on the polarization of the dust coma in comet Encke in the r-sdss filter. The maximum value of the nucleus contamination was ~0.7%. 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.

Astronomy & Astrophysics, 2019

Context. The study of distant comets, which are active at large heliocentric distances, is import... more Context. The study of distant comets, which are active at large heliocentric distances, is important for a better understanding of their physical properties and mechanisms of long-lasting activity. Aims. We analyzed 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. 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°. 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. Results. No emission features were detected in the 3800–7200 Å wav...

Monthly Notices of the Royal Astronomical Society, 2017

We present post-perihelion photometric and polarimetric observations of comet 67P/Churyumov-Geras... more We present post-perihelion photometric and polarimetric observations of comet 67P/Churyumov-Gerasimenko performed at the 6-m telescope of the SAO RAS in the gsdss (465/65 nm), r-sdss (620/60 nm) and R filters. Observations in November and December 2015 and April 2016 covered the range of heliocentric distance 1.62-2.72 au and phase angle 33.2 •-10.4 •. The comet was very active. Two persistent jets and long dust tail were observed during the whole observing period; one more jet was detected only in December. The radial profiles of surface brightness, colour and polarization significantly differed for the coma, jets and tail, and changed with increasing heliocentric distance. The dust production Afρ decreased from 162 cm at r = 1.62 au to 51 cm at r = 2.72 au. The dust colour (g-r) gradually changed from 0.8 mag in the innermost coma to about 0.4 mag in the outer coma. The spectral slope was 8.2 ± 1.7 per cent/100 nm in the 465 to 620 nm wavelength domain. In November and December, the polarization in the near-nucleus area was about 8 per cent, dropped sharply to 2 per cent at the distance above 5000 km and then gradually increased with distance from the nucleus, reaching ∼8 per cent at 40 000 km. In April, at a phase angle 10.4 • , the polarization varied between-0.6 per cent in the near-nucleus area and-4 per cent in the outer coma. Circular polarization was not detected in the comet. The spatial variations of brightness, colour and polarization in different structural features suggest some evolution of particle properties, most likely decreasing the size of dust particles.

Astronomy & Astrophysics, 2019

Context. The study of distant comets, which are active at large heliocentric distances, is import... more Context. The study of distant comets, which are active at large heliocentric distances, is important for a better understanding of their physical properties and mechanisms of long-lasting activity. Aims. We analyzed 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. 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°. 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. Results. No emission features were detected in the 3800–7200 Å wav...

Monthly Notices of the Royal Astronomical Society, 2016

Circular polarization in cometary continuum bands has been observed for more than 30 years. Recen... more Circular polarization in cometary continuum bands has been observed for more than 30 years. Recently, imaging data on circular polarization have been obtained for more than 10 comets using the 6-m Big Telescope Alt-azimuthal (BTA) telescope (Russia). However, despite the accumulation of significant amounts of data, the mechanisms that form circular polarization in the cometary environment are still a mystery. Regular mechanisms, such as multiple scattering in an optically thick medium or the domination of particles or materials of a specific mirror asymmetry (including homochirality), could not explain the observations. Particle alignment was considered the most feasible mechanism; however, the cause of the particle alignment was not clear. The most reasonable mechanism, alignment in a magnetic field, was considered doubtful, as comets do not have their own magnetic field and, according to the in situ results for comet Halley, the solar magnetic field could not penetrate deeply into the coma. However, new theoretical studies of cometary coma interaction with the solar magnetic field and, especially, Rosetta observations of the diamagnetic cavity of comet 67P/Churyumov-Gerasimenko showed that the solar wind magnetic field can penetrate as close to the nucleus as several dozen kilometres. This allows us to suggest alignment in the solar magnetic field as a reason for the observed circular polarization. Based on the data obtained for comet 67P, we estimate the time necessary for the alignment of cometary particles in the solar magnetic field. The estimates obtained are consistent with the observations of cometary circular polarization.

Monthly Notices of the Royal Astronomical Society, 2017

We present post-perihelion photometric and polarimetric observations of comet 67P/Churyumov-Geras... more We present post-perihelion photometric and polarimetric observations of comet 67P/Churyumov-Gerasimenko performed at the 6-m telescope of the SAO RAS in the gsdss (465/65 nm), r-sdss (620/60 nm) and R filters. Observations in November and December 2015 and April 2016 covered the range of heliocentric distance 1.62-2.72 au and phase angle 33.2 •-10.4 •. The comet was very active. Two persistent jets and long dust tail were observed during the whole observing period; one more jet was detected only in December. The radial profiles of surface brightness, colour and polarization significantly differed for the coma, jets and tail, and changed with increasing heliocentric distance. The dust production Afρ decreased from 162 cm at r = 1.62 au to 51 cm at r = 2.72 au. The dust colour (g-r) gradually changed from 0.8 mag in the innermost coma to about 0.4 mag in the outer coma. The spectral slope was 8.2 ± 1.7 per cent/100 nm in the 465 to 620 nm wavelength domain. In November and December, the polarization in the near-nucleus area was about 8 per cent, dropped sharply to 2 per cent at the distance above 5000 km and then gradually increased with distance from the nucleus, reaching ∼8 per cent at 40 000 km. In April, at a phase angle 10.4 • , the polarization varied between-0.6 per cent in the near-nucleus area and-4 per cent in the outer coma. Circular polarization was not detected in the comet. The spatial variations of brightness, colour and polarization in different structural features suggest some evolution of particle properties, most likely decreasing the size of dust particles.

Monthly Notices of the Royal Astronomical Society, 2016

Circular polarization in cometary continuum bands has been observed for more than 30 years. Recen... more Circular polarization in cometary continuum bands has been observed for more than 30 years. Recently, imaging data on circular polarization have been obtained for more than 10 comets using the 6-m Big Telescope Alt-azimuthal (BTA) telescope (Russia). However, despite the accumulation of significant amounts of data, the mechanisms that form circular polarization in the cometary environment are still a mystery. Regular mechanisms, such as multiple scattering in an optically thick medium or the domination of particles or materials of a specific mirror asymmetry (including homochirality), could not explain the observations. Particle alignment was considered the most feasible mechanism; however, the cause of the particle alignment was not clear. The most reasonable mechanism, alignment in a magnetic field, was considered doubtful, as comets do not have their own magnetic field and, according to the in situ results for comet Halley, the solar magnetic field could not penetrate deeply into the coma. However, new theoretical studies of cometary coma interaction with the solar magnetic field and, especially, Rosetta observations of the diamagnetic cavity of comet 67P/Churyumov-Gerasimenko showed that the solar wind magnetic field can penetrate as close to the nucleus as several dozen kilometres. This allows us to suggest alignment in the solar magnetic field as a reason for the observed circular polarization. Based on the data obtained for comet 67P, we estimate the time necessary for the alignment of cometary particles in the solar magnetic field. The estimates obtained are consistent with the observations of cometary circular polarization.

Kinematics and Physics of Celestial Bodies, 2015

Results of polarimetric observations of Saturn's satellite Rhea obtained in the range of phase an... more Results of polarimetric observations of Saturn's satellite Rhea obtained in the range of phase angles from 0.76° to 5.96° are presented. The observations were carried out in the WR spectral band (550-750 nm) at the 2.6 m telescope equipped with a single channel photometer-polarimeter at the Crimean Astrophysical Observatory in the period from March 23, 2012, to May 2, 2014. From the results of the observations, the phase dependence of linear polarization of Rhea was built. The parameters of this dependence are compared to those of the high albedo satellites of Jupiter, Japetus, and medium albedo satellites of Uranus. The obtained results are discussed in terms of the currently available models of the light scattering by regolith surfaces.