W. Waniak

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.

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.

A Monte Carlo approach to the analysis of the dust tail of comet P/Halley

Abstract A quantitative analysis of three wide-field photographs of Comet P/Halley obtained between 3 and 6 May 1986 has been carried out. The digitally processed maps were used as input for the Monte Carlo approach to interpreting cometary dust tails. The developed model considers the conditions of the ejection of the dust particles from the collision zone of the cometary coma, the time-averaged dust size distribution, and the dependency between the dust production rate and the heliocentric distance. The lack of any photometric standardization has not allowed an absolute value of the dust production rate to be obtained. The use of panchromatic plates without filters may cause ion plasma interference and therefore a certain biasing of the results. The adjustment procedure gives the model parameters in general agreement with the results of the investigations by Fulle et al. (1989, Astron. Astrophys. 201, 362–372) and by Cremonese and Fulle (1989, Icarus 80, 267–279), and the results of in situ experiments. The maximum value of the dust ejection velocity is 0.46 (±0.23) km sec −1 and is achieved generally after the perihelion passage. The dust ejection velocity dependency on the particle radius is very weak with a power index of −0.25 (±0.10). The dust number production rate varies with heliocentric distance as r −3.8(±0.7) and attains its maximum value for the true anomaly 8°. Power indices of the dust size distribution function are −3.22 (±0.36) and −3.71 (±0.22) for particles of radius less than and greater than 7.3 μm, respectively. These values are in good agreement with the time-averaged size distribution obtained for P/Halley by Fulle et al.

Narrowband photometry of comet Okazaki-Levy-Rudenko 1989r

The results of the photoelectric photometry with the narrowband CN, C3, C2 and Blue Continuum (BC) IHW interference filters are presented. Observations were carried out with a set of diaphragms of different effective radii. On the base of the Haser model the production rates of the radicals have been obtained. The CN and C2 molecules scale lengths (3.4 × 105 km and 8.5 × 104 km respectively for 1.0 AU heliocentric distance) have been also derived. The dust continuum spectrum is negligibly low in comparison with the molecular one, which stay in agreement with the results of other observations of comet Okazaki-Levy-Rudenko.