Gas and dust productions of Comet 103P/Hartley 2 from millimetre observations: Interpreting rotation-induced time variations

Abstract

Comet 103P/Hartley 2 made a close approach to the Earth in October 2010. It was the target of an extensive observing campaign including ground-and orbit-based observatories and was visited by the Deep Impact spacecraft in the framework of its mission extension EPOXI. We present observations of HCN and CH 3 OH emission lines conducted with the IRAM Plateau de Bure interferometer on 22-23, 28 October and 4, 5 November 2010 at 1.1, 1.9 and 3.4 mm wavelengths. The thermal emission from the dust coma and nucleus is detected simultaneously. Interferometric images with unprecedented spatial resolution of ∼100 to ∼500 km are obtained. A sine-wave like variation of the thermal continuum is observed in the 23 October data, that we associate with the nucleus thermal light curve. The nucleus contributes up to 30-55 % of the observed continuum emission. The dust thermal emission is used to measure the dust production rate. The inferred large dust-to-gas ratio (in the range 2-6) can be explained by the unusual activity of the comet for its size, which allows decimeter size particles and large boulders to be entrained by the gas due to the small nucleus gravity. The rotational temperature of CH 3 OH is measured with beam radii from ∼150 km to ∼1500 km. We attribute the increase from ∼35 K to ∼46 K with increasing beam size to radiative processes. The HCN production rate displays strong rotation-induced temporal variations, varying from ∼0.3×10 25 s −1 to ∼2.0×10 25 s −1 in the 4-5 November period. The HCN production curve, as well as the CO 2 and H 2 O production curves measured by EPOXI, are interpreted with a geometric model which takes into account the complex rotational state of 103P/Hartley 2 and its shape. The HCN and H 2 O production curves are in phase, showing that these molecules have common sources. The ∼ 1.7 h delay, in average, of the HCN and H 2 O production curves with respect to the CO 2 production curve suggests that HCN and H 2 O are mainly produced by subliming icy grains. The scale length of production of HCN is determined to be on the order of 500-1000 km, implying a mean velocity of 100-200 m s −1 for the icy grains producing HCN. From the time evolution of the insolation of the nucleus, we show that the CO 2 production is modulated by the insolation of the small lobe of the nucleus. The three-cycle pattern of the production curves reported earlier is best explained by an overactivity of the small lobe in the longitude range 0-180• . The good correlation between the insolation of the small lobe and CO 2 production is consistent with CO 2 being produced from small depths below the surface. The time evolution of the velocity offset of the HCN lines, as well as the displacement of the HCN photocenter in the interferometric maps, are overall consistent with this interpretation. Other localized sources of gas on the nucleus surface are also suggested.

Searches for HCl and HF in comets 103P/Hartley 2 and C/2009 P1 (Garradd) with theHerschelSpace Observatory

“…Since the upper levels of the transitions have similar energy and Einstein-A coefficient, we used their combined intensity (7 ± 1.6 mK km s −1 ) to derive a rotational temperature of methanol of T rot = 57 ± 5 K. For comet 103P/Hartley 2, the three lines are well detected and we infer T rot = 60 ± 3 K. Methanol production rates are given in Table 2. For 103P/Hartley 2, the mean production rate, averaging values from the three lines is Q(CH 3 OH) = (3.2 ± 0.4) × 10 26 s −1 , which is consistent with values published by Boissier et al (2014) for 28 October 2010, keeping in mind that the gaseous activity of comet 103P/Hartley 2 was strongly variable Drahus et al 2012). For comet C/2009 P1 (Garradd), we infer a weighted-mean value of Q(CH 3 OH) = (2.6 ± 0.4) × 10 27 s −1 , similar to the value measured a week earlier with the 30-m telescope of the Institut de radioastronomie millimétrique (IRAM; Biver et al 2012).…”

Jet Morphology and Coma Analysis of Comet 103P/Hartley 2

“…Icy grains, which are ejected more slowly from the nucleus than gases and have a shorter scale length in the coma, are the next best candidates for C3 production. The scale lengths of icy chunks reported by Boissier et al (2013) are consistent with the values needed to give reasonably good fits with the Haser model, but there is no empirical measurement of these ices to indicate whether or not they are composed of the materials necessary (e.g., CHON particles) to produce C3 and subsequently C2. Nonetheless, given the limited options for parentage, the extended ices detected by EPOXI serve as the most viable source of the carbon-chain radicals in the Hartley 2 coma.…”

“…Although modeling grains as a grandparent is not completely reliable with the Haser code used in this work, the scale lengths and ejection speeds of grains reported by Boissier et al (2013) are still within reason to permit CHON particles in icy grains as a possible parent or even grandparent source of CH.…”

“…Coma studies of short-period comet Encke showed a similar inconsistency when using HCN as the sole CN parent with the vectorial model (Ihalawela et al, 2011). Boissier et al (2013) hypothesized that the HCN scale length issue may be due to HCN emanating from extended icy grains, but having grains as a source of HCN would then exacerbate the Haser fits since the overall scale length would unavoidably increase (i.e., the scale length needs to be lower for HCN to have a better fit for the data presented here). Since the measured mixing ratios of HCN from the near-infrared studies are lower than the necessary ratios generated by the Haser model in this work, there is not enough of this parent present in the Hartley 2 coma to account for CN production alone, even when considering the high Qwater uncertainty.…”

“…Knight and Schleicher (2011) and Samarasinha et al (2011) used image filters to study the Hartley 2 coma over time and successfully linked the coma morphology with at least one nucleus rotational mode. Boissier et al (2013) and Lara et al (2011) examined the dust and gas in the coma and discovered unique coma structure and distribution relationships between species. Near-infrared spectral measurements at Keck Observatory in Hawaii provided estimates for the abundances of larger volatile species in the coma, including hydrogen cyanide (HCN), formaldehyde (H2CO), ammonia (NH3), ethane (C2H6), acetylene (C2H2), and methanol (CH3OH), along with several other species for which upper limits were established (Dello Kawakita et al, 2011;and Mumma et al, 2011).…”