New Limits to CO Outgassing in Centaurs

Abstract

Centaurs are small Solar system objects orbiting between Jupiter and Neptune. They are widely believed to be escapees from the trans-Neptunian region on their way to become Jupiter-family comets. Indeed, some Centaurs exhibit the characteristic cometary appearance. The sublimation of carbon monoxide has been proposed as a driver of activity in distant comets, but no strong detection of gaseous CO in a Centaur other than 29P/SchwassmannWachmann 1 has been reported to date. Here we report the results of a deep search for CO outgassing in three Centaurs: (315898), (342842), and (382004). Our survey was carried out using the Caltech Submillimeter Observatory on nine nights in late 2011. The targeted rotational line J(2-1) of CO is undetected in all three objects in spite of high instrumental sensitivity. We find the model-dependent 3σ upper limits to the CO production rate of 2.13 × 10 27 molecules s −1 for (315898), 1.32 × 10 27 molecules s −1 for (342842), and 1.17 × 10 27 molecules s −1 for (382004), which are among the most sensitive obtained to date. These upper limits are consistently analysed in the context of published CO data of 14 Centaurs and one well-observed long-period comet, C/1995 O1 (Hale-Bopp), and support an earlier suggestion that the surfaces of most Centaurs are not dominated by exposed CO ice.

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).…”

“…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. 3 A recent study set Q(CO) upper limits for 16 Centaurs (including Echeclus at aphelion) and analyzed them with a high-albedo energy balance model (Drahus et al 2017). They ruled out substantial amounts of CO ice being present on their surfaces, but did not constrain models much further.…”

“…Thus, we calculated the specific production rate, Q(CO)/D 2 , of the Echeclus detection and upper limit, and plotted the results in Figure 3, along with values of Q(CO)/D 2 for Hale-Bopp, following the method we introduced in (Womack et al 2017). We also calculated upper limits to specific production rates for several other Centaurs using measurements from Drahus et al (2017), which are also included in the figure.…”

“…When we include the upper limits derived from Drahus et al (2017), even more Centaurs appear deficient, or depleted, in CO and may, in fact, be inactive. As Figure 3 shows, other Centaurs, such as Chariklo and 342842, are each CO-depleted by ∼ 10x compared to Hale-Bopp at the same heliocentric distance.…”

“…It is difficult, however, to draw general conclusions about Centaurs based on such limited data. Previous attempts to detect CO in Echeclus with the JCMT 15-m telescope in 2002 May (r = 14.8 au) yielded an CO production rate upper limit of Q(CO) < 3.6×10 27 mol s −1 near aphelion (Jewitt et al 2008, Drahus et al 2017). This limit was interpreted as being incompatible with there being much solid CO ice on the surface, but does not constrain models much further.…”