Michal Drahus

A recent disruption of the main-belt asteroid P/2010 A2.

Most inner main-belt asteroids are primitive rock and metal bodies in orbit about the Sun between Mars and Jupiter. Disruption, through high-velocity collisions or rotational spin-up, is believed to be the primary mechanism for the production and destruction of small asteroids and a contributor to dust in the Sun’s zodiacal cloud, while analogous collisions around other stars feed dust to their debris disks. Unfortunately, direct evidence about the mechanism or rate of disruption is lacking, owing to the rarity of the events. Here we report observations of P/2010 A2, a previously unknown inner-belt asteroid with a peculiar, comet-like morphology. The data reveal a nucleus of diameter approximately 120 metres with an associated tail of millimetre-sized dust particles. We conclude that it is most probably the remnant of a recent asteroidal disruption in February/March 2009, evolving slowly under the action of solar radiation pressure, in agreement with independent work.

ROTATION STATE OF COMET 103P/HARTLEY 2 FROM RADIO SPECTROSCOPY AT 1 mm*

The nuclei of active comets emit molecules anisotropically from discrete vents. As the nucleus rotates, we expect to observe periodic variability in the molecular emission line profiles, which can be studied through millimeter/ submillimeter spectroscopy. Using this technique we investigated the HCN atmosphere of comet 103P/Hartley 2, the target of NASA’s EPOXI mission, which had an exceptionally favorable apparition in late 2010. We detected short-term evolution of the spectral line profile, which was stimulated by the nucleus rotation, and which provides evidence for rapid deceleration and excitation of the rotation state. The measured rate of change in the rotation period is +1.00 ± 0.15 minutes day−1 and the period itself is 18.32 ± 0.03 hr, both applicable at the epoch of the EPOXI encounter. Surprisingly, the spin-down efficiency is lower by two orders of magnitude than the measurement in comet 9P/Tempel 1 and the best theoretical prediction. This secures rotational stability of the comet’s nucleus during the next few returns, although we anticipate a catastrophic disruption from spin-up as its ultimate fate.

Uncorrelated Volatile Behavior during the 2011 Apparition of Comet C/2009 P1 Garradd

The High Resolution Instrument Infrared Spectrometer (HRI-IR) on board the Deep Impact Flyby spacecraft detected H2O, CO2, and CO in the coma of the dynamically young Oort Cloud comet C/2009 P1 (Garradd) post-perihelion at a heliocentric distance of 2 AU. Production rates were derived for the parent volatiles, Q_(H2O) = 4.6 ± 0.8 × 10^(28), Q_(CO2) = 3.9 ± 0.7 × 10^(27), and Q_(CO) = 2.9 ± 0.8 × 10^(28) molecules s^(–1), and are consistent with the trends seen by other observers and within the error bars of measurements acquired during a similar time period. When compiled with other observations of Garradds dominant volatiles, unexpected behavior was seen in the release of CO. Garradds H_2O outgassing, increasing and peaking pre-perihelion and then steadily decreasing, is more typical than that of CO, which monotonically increased throughout the entire apparition. Due to the temporal asymmetry in volatile release, Garradd exhibited the highest CO to H_2O abundance ratio ever observed for any comet inside the water snow line at ~60% during the HRI-IR observations. Also, the HRI-IR made the only direct measurement of CO_2, giving a typical cometary abundance ratio of CO_2 to H_2O of 8% but, with only one measurement, no sense of how it varied with orbital position.

67P/Churyumov-Gerasimenko at large heliocentric distance

Aims. The Jupiter family comet (JFC) 67P/Churyumov-Gerasimenko (67P/C-G) is the target of ESAs ROSETTA mission. Observational campaigns and theoretical investigations were performed to characterise 67P/C-G in terms of nucleus properties (e.g. size, surface colours, rotational period), gas and dust production, and plasma environment in preparation for the rendezvous with the spacecraft; however, there are still open questions that need to be addressed. Our observations of 67P/C-G are important not only for a good planning of the rendezvous of the ROSETTA spacecraft with 67P/C-G, but also for providing valuable information on the basic physical properties of the nuclei of JFCs at large heliocentric distances. Moreover, this information will help to identify to what extent 67P/C-G is a typical JFC. Methods. We performed broad-band imaging and low-resolution spectroscopy of 67P/C-G in the visible wavelength range during five periods between April 2004 and July 2007 at the ESO Very Large Telescope (VLT) with the FORS2 instrument. At the time of the observations 67P/C-G was at heliocentric distance r > 4.6 AU. The imaging data were used to search for a faint coma, to improve the phase function of the nucleus, to constrain its rotational period, and to analyse the neck-line of dust close to the nucleus. Results. The comet appears point-like and no coma signature was found around the nucleus. The most realistic representative of the phase function of 67P/C-G is the linear approximation: This could be interpreted to mean that the opposition effect is not very pronounced for 67/C-G in the phase angle range between 0.5° and 10°. We determined that the magnitude dependence on the phase angle is very steep, with the linear phase coefficient in the range β = 0.061-0.076 mag/°. The colour indices and reflectance spectrum show that the nucleus of 67P/C-G is slightly redder than the Sun (spectral slope ∼11%/1000 A) and uniform with the rotational phase. A tail-like structure of heavy grains is detected in April 2004, June 2004, and May 2006. Based on its orientation and the variations of its surface brightness we interpreted it as a neck-line phenomenon.

Fast Rotation and Trailing Fragments of the Active Asteroid P/2012 F5 (Gibbs)

While having a comet-like appearance, P/2012 F5 (Gibbs) has an orbit native to the Main Asteroid Belt, and physically is a km-sized asteroid which recently (mid 2011) experienced an impulsive mass ejection event. Here we report new observations of this object obtained with the Keck II telescope on UT 2014 August 26. The data show previously undetected 200 m scale fragments of the main nucleus, and reveal a rapid nucleus spin with a rotation period of 3.24 ± 0.01 hr. The existence of large fragments and the fast nucleus spin are both consistent with rotational instability and partial disruption of the object. To date, many fast rotators have been identified among the minor bodies, which, however, do not eject detectable fragments at the present-day epoch, and also fragmentation events have been observed, but with no rotation period measured. P/2012 F5 is unique in that for the first time we detected fragments and quantified the rotation rate of one and the same object. The rapid spin rate of P/2012 F5 is very close to the spin rates of two other active asteroids in the Main Belt, 133P/Elst-Pizarro and (62412), confirming the existence of a population of fast rotators among these objects. But while P/2012 F5 shows impulsive ejection of dust and fragments, the mass loss from 133P is prolonged and recurrent. We believe that these two types of activity observed in the rapidly rotating active asteroids have a common origin in the rotational instability of the nucleus.

Rotation-stimulated structures in the CN and C3 comae of comet 103P/Hartley 2 close to the EPOXI encounter ,

In late 2010 a Jupiter Family comet 103P/Hartley 2 was a subject of an intensive world-wide investigation. On UT October 20.7 the comet approached the Earth within only 0.12 AU, and on UT November 4.6 it was visited by NASAs EPOXI spacecraft. We joined this international effort and organized an observing campaign. The images of the comet were obtained through narrowband filters using the 2-m telescope of the Rozhen National Astronomical Observatory. They were taken during 4 nights around the moment of the EPOXI encounter. Image processing methods and periodicity analysis techniques were used to reveal transient coma structures and investigate their repeatability and kinematics. We observe shells, arc-, jet- and spiral-like patterns, very similar for the CN and C3 comae. The CN features expanded outwards with the sky-plane projected velocities between 0.1 to 0.3 km/s. A corkscrew structure, observed on November 6, evolved with a much higher velocity of 0.66 km/s. Photometry of the inner coma of CN shows variability with a period of 18.32+/-0.30 h (valid for the middle moment of our run, UT 2010 Nov. 5.0835), which we attribute to the nucleus rotation. This result is fully consistent with independent determinations around the same time by other teams. The pattern of repeatability is, however, not perfect, which is understendable given the suggested excitation of the rotation state, and the variability detected in CN correlates well with the cyclic changes in HCN, but only in the active phases. The revealed coma structures, along with the snapshot of the nucleus orientation obtained by EPOXI, let us estimate the spin axis orientation. We obtained RA=122 deg, Dec=+16 deg (epoch J2000.0), neglecting at this point the rotational excitation.

The HCN molecule as a tracer of the nucleus rotation of comet 73P-C/Schwassmann-Wachmann 3

Context. The causes of cometary break-ups are still uncertain. One suggested mechanism is splitting due to fast rotation of the nucleus. This can be tested by measuring rotation periods of cometary fragments. Aims. The exceptionally close approach of the split comet 73P/Schwassmann-Wachmann 3 to the Earth in May 2006 made it an ideal target to investigate the rotation of its fragments. We used the HCN light curve for this purpose, because it is particularly sensitive to the rotation of the nucleus and at the same time it allows us to study the physics of cometary activity. Methods. Comet 73P/Schwassmann-Wachmann 3 was observed between May 1 and 22, 2006, with the Submillimeter Telescope on Mt. Graham, Arizona, USA. Emission from HCN and CS were clearly detected. In this work we focus exclusively on the observations of the HCN molecule in fragment C, obtained during five nights between May 10 and 22, 2006, which provide the best S /N and the best temporal coverage. Results. The light curve of comet 73P-C/Schwassmann-Wachmann 3 in HCN shows strong non-random variations, most probably stimulated by the nucleus rotation. The variability has an amplitude of about a factor of 2 on a time scale of hours. Among several plausible solutions for periodicity, we found strong indications for a rotation period of between 3.0 and 3.4 h, consistent with the determination from the Hubble Space Telescope. At 1 AU from the Sun the mean-diurnal HCN production rate was 2.7 × 10 25 molec s −1 (with an uncertainty of about 20%) and the coma was expanding with a velocity of 0.8 ± 0. 1k m s −1 . The line position was evolving with a phase angle that is visible in the night-averaged spectra. Conclusions. Evolution of the line position is consistent with the solar-stimulated activity. The mean-diurnal HCN production rate should be considered as very high, and it requires an unusually large fraction of the nucleus area to be active, whereas the coma expansion velocity was typical. The proposed rotation period, being the shortest ever determined for a cometary nucleus, cautiously suggests the disruption of the parent body due to a large centrifugal force, though it cannot be considered as a proof of this scenario. On the other hand, the observed stability of 73P-C against the rotational disruption suggests a bulk tensile strength of at least 14–45 Pa. The rotation period was surprisingly stable, indicating that no more than about 0.2% of the total outgassing was effectively accelerating or decelerating the nucleus spin. This is consistent with the large active fraction of the nucleus.

Preliminary Photometric Results for the 2003 Eclipse of EE Cep

We report multicolour photometric observations of the 2003 eclipse of the long-period (5.6 yr) eclipsing binary EE Cep. Measurements were obtained with ten telescopes at eight observatories in four countries. In most cases, UBV (RI)C broad band filters have been used. The light curve shape shows that the obscuring body is an almost dark disk around a low-luminosity central object. However, variations of the colour indices during the eclipse indicate that the obscuring body emits a considerable amount of radiation in the near infrared.