Mapping the carbon monoxide coma of comet 29P/Schwassmann-Wachmann 1

Abstract

Context. CO is assumed to be the main driver behind the activity of comet 29P/Schwassmann-Wachmann 1, which resides in a nearcircular orbit at 6 AU from the Sun. Several properties of the outgassing of CO can be deduced from its millimetre-wave emission. Earlier studies have indicated CO production from the nucleus as well as an extended source. Aims. We have sought to further investigate the nature of the CO production in comet 29P/Schwassmann-Wachmann 1, through the use of newly available instrumentation. Methods. We used the HERA receiver array on the 30-m IRAM telescope to map the 230 GHz CO(J = 2−1) line in the comet with an unprecedented sensitivity and spatial coverage, and a high spectral resolution (20 Results. The CO emission line has a characteristic asymmetric profile. Our analysis is based on a coma model, where the outgassing pattern is derived from the shape of this line at the central position of the map. When comparing to the observations, both maps show a line intensity at offset positions that is 2−3 times stronger than the model prediction. Different explanations for this are evaluated, and it is found that for the global coma character, an extremely low gas temperature in the inner coma reproduces the observed radial profile. A cold inner coma depletes the population of the CO J = 2 rotational level in the region closest to the nucleus, making spectra observed at offset positions relatively stronger. From the global appearance of the maps, the coma was found to be largely axisymmetric, and the presence of a strong extended source of CO, as indicated from earlier observations using the SEST telescope, was not seen. When examining the maps in more detail, a possible exception to this was seen in an area ∼30 south of the comet, where an excess in emission is present in both maps. Model fits to the spectra based on the cold inner coma that we find, with an intital kinetic temperature T kin = 4 K, give a measure of Q CO , the CO production rate. Q CO was found to be (3.9 ± 0.2) × 10 28 mol s −1 in June 2003, and (3.7 ± 0.2) × 10 28 mol s −1 in January 2004. These values are a factor ∼1.5 higher than that derived using only the information available from non-mapped data, and this adjustment applies also to previously published production rates.

Carbon Monoxide in the Distantly Active Centaur (60558) 174P/Echeclus at 6 au

“…We also assumed an optically thin gas and rotational and excitation temperatures of 10K and a gas expansion velocity of 0.2 km s −1 . These are consistent with the spectral line profile parameters described in the previous section and with values for CO emission in comets and Centaurs at large heliocentric distances (Biver et al 2002, Gunnarsson et al 2008, Drahus et al 2017). We calculate a production rate of Q(CO) = (7.7 ± 3.3)×10 26 mol s −1 , assuming isotropic outgassing of CO from the nucleus and a photodissociation decay model (Haser 1957).…”

“…Since its first cometary detection in 1994, CO is repeatedly seen in 29P, an unusual object that has a nearly circular orbit, ranging from r = 5.7 to 6.3 au and is continuously outgassing (Senay & Jewitt 1994, Gunnarsson et al 2008). In contrast, CO emission was marginally detected in Chiron at 8.5 au (Womack & Stern 1999), and now in Echeclus at 6.1 au, both at much lower production rates than 29P.…”

“…Most comets are not active at heliocentric distances beyond ∼ 3 au, where the temperature is too low for the water ice to sublimate efficiently (e.g., (Duffard et al 2014b, Ruprecht et al 2015, Sicardy et al 2016, and there are reports of Echeclus (as described earlier in this section) and 29P both having a secondary source of dust emission (and a second small CO outgassing region for 29P), possibly emitting from a chunk or fragment in the coma (Stansberry et al 2004, Gunnarsson et al 2008, Womack et al 2017. In order to test physical and chemical models of Centaurs (and hence by extension models of KBOs and JFCs), it is important to determine whether gaseous activity is common amongst Centaurs.…”

TNOs are Cool: A survey of the trans-Neptunian region

“…The outgassing geometry is also uncertain. For the distant comets 29P/SchwassmannWachmann 1 and C/1995 O1 (Hale-Bopp), CO outgassing was observed to mainly be in the Sun's direction (Gunnarsson et al 2003(Gunnarsson et al , 2008.…”

Herscheland IRAM-30 m observations of comet C/2012 S1 (ISON) at 4.5 AU from the Sun

“…(2013). The upper limit on the water production rate was calculated against two different values of the kinetic temperature, 8 K and 20 K. The 8 K kinetic temperature value is inferred from that derived for comet 29P/Schwassmann-Wachmann 1 (Gunnarsson et al 2008) and the 20 K temperature value from that derived for comet HaleBopp (Biver et al 2002) when both were at the same heliocentric distance as ISON at the time of the observations. Table 1 provides the 3σ upper limit on the line integrated intensity and the corresponding production rate.…”