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

We present our investigations of the physical and dynamical properties of selected Earth co-orbital asteroids. The photometric optical light curves as well as rotation periods and pole solutions for a sample of four Earth co-orbital asteroids, namely (138175) 2000 EE104 (P=13.9476+/-0.0051 hrs), (418849) 2008 WM64 (P=2.4077+/-0.0001 hrs), 2016 CA138 (P=5.3137+/-0.0016 hrs) and 2017 SL16 (P=0.3188+/-0.0053 hrs), are determined or improved and presented in this work. For this investigation, we combine observations carried out at the Bulgarian National Astronomical Observatory - Rozhen using the FoReRo2 instrument attached to the 2mRCC telescope as well as sparse data from AstDys2 database. Parallel to the rotational properties we did numerical dynamical simulations to investigate the orbital stability of those objects and to find out if there is a relation with their rotational properties. Our results show that the orbit stability is affected by the orbit itself and mainly its eccentricity and inclination, which are responsible for the close encounters with other Solar system planets. We cannot make a definitive conclusion about the relation between orbit stability and the rotational state of the asteroids, so we need further investigations and observations in order to prove or disprove it., Comment: 13 pages, 12 figures and 4 tables

In December 2018, the main-belt asteroid (6478)~Gault was reported to display activity. Gault is an asteroid belonging to the Phocaea dynamical family and was not previously known to be active, nor was any other member of the Phocaea family. In this work we present the results of photometric and spectroscopic observations that commenced soon after the discovery of activity. We obtained observations over two apparitions to monitor its activity, rotation period, composition, and possible non-gravitational orbital evolution. We find that Gault has a rotation period of $P = 2.4929 \pm 0.0003$ hours with a lightcurve amplitude of $0.06$ magnitude. This short rotation period close to the spin barrier limit is consistent with Gault having a density no smaller than $\rho = 1.85$~g/cm$^3$ and its activity being triggered by the YORP spin-up mechanism. Analysis of the Gault phase curve over phase angles ranging from $0.4^{\circ}$ to $23.6^{\circ}$ provides an absolute magnitude of $H = 14.81 \pm 0.04$, $G1=0.25 \pm 0.07$, and $G2= 0.38 \pm 0.04$. Model fits to the phase curve find the surface regolith grain size constrained between 100-500 $\rm{\mu}$m. Using relations between the phase curve and albedo we determine that the geometrical albedo of Gault is $p_{\rm v} = 0.26 \pm 0.05$ corresponding to an equivalent diameter of $D = 2.8^{+0.4}_{-0.2}$ km. Our spectroscopic observations are all consistent with an ordinary chondrite-like composition (S, or Q-type in the Bus-DeMeo taxonomic classification). A search through archival photographic plate surveys found previously unidentified detections of Gault dating back to 1957 and 1958. Only the latter had been digitized, which we measured to nearly double the observation arc of Gault. Finally, we did not find any signal of activity during the 2020 apparition or non-gravitational effects on its orbit., Comment: 15 pages, 11 figures; Accepted in MNRAS

We investigate the mineralogy of L5 Martian Trojan asteroids via reflectance spectroscopy, in particular (101429) 1998 $\mbox{VF}_{31}$, the only L5 Trojan that does not belong to the Eureka family (Christou, 2013). We find that this asteroid most likely belongs to the Bus-Demeo S-complex, in agreement with Rivkin et al. (2007) and obtain good spectral matches with Sq- or S-type asteroids, the lunar surface and of Martian and lunar meteorites. Mixture fitting to spectral endmembers suggests a surface abundance of Mg-rich orthopyroxene and iron metal or, alternatively, a mixture of plagioclase and metal with a small amount of Mg-poor orthopyroxene. The metallic component may be part of the intrinsic mineral makeup of the asteroid or an indication of extreme space weathering. We discuss several origin scenarios for (101429). The asteroid could be related to iron-rich primitive achondrites (Rivkin et al.), may have originated as impact ejecta from Mars - as proposed recently for the Eureka family asteroids (Polishook et al., 2017) - or could be a relic fragment of the Moon's original solid crust. If, on the other hand, (101429) is a relatively recent addition to the Martian Trojan clouds (Christou et al., 2020), its origin is probably traced to high-inclination asteroid families in the Inner Main Belt. For the olivine-dominated Eureka family, we find that the two smaller asteroids are more spectrally similar to one another than to (5261) Eureka. Spectral profiles of all three asteroids are closely similar shortward of $\sim$0.7$\mu$m but diverge at longer wavelengths. For the two smaller asteroids in particular, we find the spectra are virtually identical up to $0.8$$\mu$m. We attribute spectral differences in the near-IR region to differences in either: degree of space weathering, olivine chemical composition and/or regolith grain size., Comment: 22 pages, 13 figures and 7 tables

Twin STEREO spacecraft pre-perihelion photometric and polarimetric observations of the sungrazing Kreutz comet C/2010 E6 (STEREO) in March 2010 at heliocentric distances $3-28~R_{\odot}$ were investigated using a newly-created set of analysis routines. The comet fully disintegrated during its perihelion passage. Prior to that, a broadening and an increase of the intensity peak with decreasing heliocentric distance was accompanied by a drop to zero polarisation at high phase angles (~105-135{\deg}, STEREO-B) and the emergence of negative polarisation at low phase angles (~25-35{\deg}, STEREO-A). Outside the near-comet region, the tail exhibited a steep slope of increasing polarisation with increasing cometocentric distance, with the slope becoming less prominent as the comet approached the Sun. The steep slope may be attributed to sublimation of refractory organic matrix and the processing of dust grains, or to presence of amorphous carbon. The change in slope with proximity to the Sun is likely caused by the gradual sublimation of all refractory material. The polarisation signatures observed at both sets of phase angles closer to the comet photocentre as the comet approached the Sun are best explained by fragmentation of the nucleus, exposing fresh Mg-rich silicate particles, followed by their gradual sublimation. The need for further studies of such comets, both observational and theoretical, is highlighted, as well as the benefit of the analysis routines created for this work., Comment: 13 pages, 9 figures, 1 table. Published by MNRAS in April 2022

We explore the hypothesis that the population of Martian Trojans is the result of a balance between the production of new asteroids ("YORPlets") through the YORP effect and their eventual escape from the Trojan clouds through Yarkovsky-driven orbital evolution. Our principal observables are: (5261) Eureka, its family of 8 asteroids and the family-less Trojans (101429) 1998 VF31 & (121514) 1999 UJ7. We model the population evolution as a birth-death random process and assume it is in a steady state. We then simulate the discovery of Trojans to-date and find that family members of 101429 and 121514, if they exist, are intrinsically more difficult to detect than Eureka's. Their non-discovery can be used as evidence of their non-existence only under the assumption that their brightness relative to the parent asteroid is similar to that in the Eureka family. To find out how efficiently Mars Trojans are lost from the Trojan clouds due to the Yarkovsky effect, we carry out dynamical simulations of test particles originating from these parent bodies. We find that objects originating from Eureka and 121514 begin escaping after ~1 Gyr, but that those from 101429 are already lost by that time, probably due to that asteroid's proximity to an eccentricity-type secular resonance within Mars's co-orbital region. This is the likely cause behind the absence of Trojans in the orbital vicinity of 101429. In contrast, the solitary status of 121514 points to an intrinsic inability of the asteroid to produce YORPlets during the most recent ~20% of the solar system's history, a finding potentially related to 121514's present, low angular momentum rotational state, unless the Eureka family formed rapidly during a single fission event., Comment: 34 pages, 9 figures, to be published in Icarus

We explore the hypothesis that the Eureka family of sub-km asteroids in the L5 region of Mars could have formed in a collision. We estimate the size distribution index from available information on family members; model the orbital dispersion of collisional fragments; and carry out a formal calculation of the collisional lifetime as a function of size. We find that, as initially conjectured by Rivkin et al (2003), the collisional lifetime of objects the size of (5261) Eureka is at least a few Gyr, significantly longer than for similar-sized Main Belt asteroids. In contrast, the observed degree of orbital compactness is inconsistent with all but the least energetic family-forming collisions. Therefore, the family asteroids may be ejecta from a cratering event sometime in the past ~1 Gyr if the orbits are gradually dispersed by gravitational diffusion and the Yarkovsky effect (Cuk et al, 2015). The comparable sizes of the largest family members require either negligible target strength or a particular impact geometry under this scenario (Durda et al, 2007; Benavidez et al, 2012). Alternatively, the family may have formed by a series of YORP-induced fission events (Pravec.et.al, 2010). The shallow size distribution of the family is similar to that of small MBAs (Gladman et al, 2009) interpreted as due to the dominance of this mechanism for Eureka-family-sized asteroids (Jacobson et al, 2014). However, our population index estimate is likely a lower limit due to the small available number of family asteroids and observational incompleteness. Future searches for fainter family members, further observational characterisation of the known Trojans' physical properties as well as orbital and rotational evolution modelling will help distinguish between different formation models., Comment: 3 Tables, 13 Figures, Accepted for publication in Icarus

We have obtained imaging polarimetry of the comet C/2013 R1 (Lovejoy) with 2-Channel-Focal-Reducer Rozhen instrument at 2m Ritchey-Chr\'etien-Coud\'e telescope of the Bulgarian National Astronomical Observatory Rozhen in two dust continuum filters covering wavelength intervals clear from molecular emissions and centred at $4430\AA$ in (blue filter) and at $6840\AA$ in (red filter). In imaging mode we measured the degree of linear polarisation $17.01\pm0.09$% in the blue and $18.81\pm0.02$% in the red, which is in a very good agreement with measurements of other comets at the similar phase angle. We have also obtained polarisation maps in both filters. We found a strong correlation between the spatial distribution of the polarisation and the dust colour. Spectropolarimetry of the nucleus region shows an increase of the polarisation with wavelength, and a depolarisation in the spectral regions with gas emission lines, most noticeable in C$_2$ emission band, which shows a polarisation of $6.0\pm1.1$%., Comment: 6 pages, 8 figures, ACM2014 conference proceedings

We present R-band observations of comet C2004B1 obtained in the period June, 21 - August 20, 2006. The data have been reduced to surface brightness maps, light curves, and mean radial profiles of the coma. In two cases a decrease of the brightness was recorded, which lasted for several days. The brightness decrease was accompanied by morphological changes in the coma., Comment: 6 pages, 6 figures, gzipped tar file. To appear in Proceeding of the 1st conference of BAS (Bulgarian Astronomical Society): "25 years National Astronomical Observatory"