Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..837j&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Astronomy and Astrophysics
Astronomy
Scientific paper
Background Simultaneous radio and optical observations of chemically related species in comets promise to supplement each other favorably. High resolution spectra of a submillimeter line provide the distribution of radial velocity. Narrow-band images in the optical region offer the spatial distribution of a species projected into the sky plane perpendicular to the line of sight. Therefore optical and radio observations can in principle be combined into a three-dimensional picture of a comet. A suitable pair of species accessible in the microwave and optical wavelength range is provided by HCN (one of the strongest radio emissions of comets) and CN (strong optical emission). HCN is the most probable parent of CN, but other parents of CN are possible. In this study we use HCN and CN observations of comet it 2P/Encke to address the parental relation of HCN with respect to CN and to investigate the gas outflow from a cometary surface and its dependence on location on the surface (the question of so-called "active vents" or "active areas") and on solar zenith angle. Some known facts about Comet 2P/Encke Comet 2P/Encke is a short period comet. It has the smallest known perihelion distance q = 0.33 AU and a period of 3.28 years. Because of its closeness to the Sun Comet Encke probably is the most evolved comet known. In the optical wavelength range comet Encke does not display a dust tail. Instead a so-called "fan" is observed, a broad feature visible at the solar side of the comet but not directly pointing to the Sun. In the far infrared spectral region Comet Encke displays a huge coma [1] of large dust grains but because of their large size these grains do not contibute significantly to the optical image [2]. In a study based on a large number of historical observations Sekanina [3] has investigated comet Encke's fan-shaped coma. According to this author comet Encke's north rotation pole is located at right ascension 205 and declination 2 (equinox 1950.0). Two vents on the nucleus surface were identified, one at latitude +55 (source I) and another one at latitude -75 (source II). The derivation is based on the incorrect assumption that the fan consists of dust particles. Therefore the location of the active vents may be erroneous but the pole location is reliable. Observations The observations were performed in November 2003. The 4-3 (354.505475 GHz) and 3-2 (265.886436 GHz) transitions of HCN were observed in Comet Encke at the 10m Heinrich-Hertz-Telescope of Steward Observatory and MPI for Radio Astronomy with the heterodyne spectrometer of the MPI for Solar System Science (MPS) during Nov. 18-30 by G. Villanueva and S. Szutowicz. For the optical observations the 2m-telescope of the Institute of Astronomy of the Bulgarian Acad. Sci. was used with the Two-Channel Focal Reducer developed and built at the MPS. 16 images were taken by K. Jockers, T. Bonev, and G. Borisov in the CN 0-0 B 2 + ! X 2 + band at 388 nm. The observations were performed on the inward branch of the cometary orbit. The heliocentric distance varied from 0.98 to 0.77 AU and the solar phase angle from 83 to 123 . In this presentation the data are compared with Monte Carlo particle trajectory models for both species, HCN and CN. Parent and daughter life times: The CN pictures of the comet were averaged in azimuth in a circular area around the comet with approximate radius of 26,000 km to derive the parent life time. In accordance with the radio data the cometary outflow velocity is assumed 1.1 km s-1 and the daughter velocity received at photodissociation of the parent is 0.864 km/s [4]. The Solar2000 space weather solar irradiation model (Kent Tobiska, SpaceWx.com) was applied to the laboratory data [5] to yield CN lifetimes for the time of observations. They are in the range of it 2.2 ·105 s when referred to 1 AU. The model of [6] was applied to the CN observations and the parent life time was determined. When scaled to 1 AU it is in the range of 15,000 s, much shorter than expected from the laboratory data [5] where the lifetime at 1 AU is in the range of 50,000 s, when combined with the appropriate space weather data. EPSC Abstracts, Vol. 3, EPSC2008-A-00492, 2008 European Planetary Science Congress, Author(s) 2008 EPSC Abstract, Vol. 3, EPSC2008-A-xxxx, 2008 European Planetary Science Congress, c Author(s) 2008 Stationary gas coma: For comparison with the observed asymmetric CN gas coma Monte-Carlo particle trajectory models of a spherical homogeneous nucleus were constructed. Models assuming an outgassing flux in proportion to the solar flux received locally on the rotating nucleus do not fit the observations at all. Better agreement between models and observations is achieved if the outgassing flux is assumed in proportion with the longitudinally averaged solar flux, i.e. when the outgassing depends only on cometocentric latitude. Our models assume outgassing from each surface point into a cone of variable width. The maximum opening angle is 180 , i. e. emission into half space. The best agreement between model and observation is achieved for narrow opening angles, i.e. nearly vertical outflow. Non-stationary effects, related to nucleus rotation The stationary Monte Carlo models describe comet Encke's fan surprisingly well. Nevertheless there is evidence for rotation. Close to the nucleus the CN coma is clearly elongated. The position angle of this elongation was measured. It shows a periodic variation with time. The rotation period derived from this variation is in agreement with the period derived by [7] from photometric observations of the comet close to aphelion. A periodic variation of the line center of the observed HCN line profiles is barely noticable and correlates with the phase of the optical observations. There is evidence for the presence of an active longitude. The deviations from the stationary CN gas coma introduced by rotation are, however, very small, and it is very difficult to derive the geometry and cometocentric latitude of the rotationrelated area. References [1] Reach, W. T. et al. (2000), Icarus, 148, 80-94. [2] Jockers, K. et al. (2005), Astron. Astrophys., 441, 773- 782. [3] Sekanina, Z. (1988) Astron. J. 95, 911-924. [4] Fray, N. et al. (2005) Planet. and Space Sci., 53, 1243- 1262. [5] Huebner, W. F. et al. (1992) Atrophys. Space Sci. 195, 1-294. [6] Combi, M.R. and Delsemme, A.H. (1980) Astrophys. J., 237, 633-640 [7] Fernandez, Y. et al. (2005) Icarus, 175, 194-214.
Jockers Klaus
Szutowicz Slawomira
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