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

¹ Astronomical Observatory of the Jagiellonian University, Orla 171, 30-244 Krakow, Poland
e-mail: waclaw.waniak@uj.edu.pl
² Institute of Astronomy of the Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria
e-mail: gborisov@astro.bas.bg
³ University of California at Los Angeles, 595 Charles E. Young Dr. East, Los Angeles, CA 90024, USA
e-mail: mdrahus@ucla.edu
⁴ Institute of Astronomy of the Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria
e-mail: tbonev@astro.bas.bg

Received: 30 September 2011
Accepted: 1 April 2012

Abstract

Context. In late 2010, a Jupiter family comet 103P/Hartley 2 was the 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 the NASA EPOXI spacecraft.

Aims. We joined this international effort and organized a ground-based observing campaign with three key goals to: (1) measure the parameters of the nucleus rotation in a time series of CN; (2) investigate the compositional structure of the coma by comparing the CN images with nightly snapshots of C₃; and (3) investigate the photochemical relation of CN to HCN, using the HCN data collected nearly simultaneously with our images.

Methods. The images were obtained through narrowband filters using the two-meter telescope of the Rozhen National Astronomical Observatory. They were taken over four nights about the moment of the EPOXI encounter. Image processing methods and periodicity analysis techniques were used to identify transient coma structures and investigate their repeatability and kinematics.

Results. We observe shells, arc-, jet- and spiral-like patterns that are very similar for the CN and C₃ comae. The CN features expanded outwards with the sky-plane projected velocities of between 0.1 to 0.3 km s^-1. A corkscrew structure, observed on November 6, evolved with a much higher velocity of 0.66 km s^-1. The 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, imperfect, 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 identified coma structures, along with the snapshot of the nucleus orientation obtained by EPOXI, enable us to estimate the spin axis orientation. We obtain RA = 122°, Dec = +16° (epoch J2000.0), neglecting at this point the rotational excitation.