Water production activity of nine long-period comets from SOHO/SWAN observations of hydrogen Lyman-alpha: 2013–2016

Abstract

Nine recently discovered long-period comets were observed by the Solar Wind Anisotropies Of these 9 comets 6 were long-period comets and 3 were possibly dynamically new. Water production rates were calculated from each of the 885 images using our standard time-resolved model that accounts for the whole water photodissociation chain, exothermic velocities and collisional escape of H atoms. For most of these comets there were enough observations over a broad enough range of heliocentric distances to calculate power-law fits to the variation of production rate with heliocentric distances for pre-and post-perihelion portions of the orbits.Comet C/2014 Q1 (PanSTARRS), with a perihelion distance of only ~0.3 AU, showed the most unusual variation of water production rate with heliocentric distance and the resulting active area variation, indicating that when the comet was within 0.7 AU its activity was dominated by the continuous release of icy grains and chunks, greatly increasing the active sublimation area by more than a factor of 10 beyond what it had at larger heliocentric distances. A possible interpretation suggests that a large fraction of the comet's mass was lost during the apparition.

Near-UV and optical spectroscopy of comets using the ISIS spectrograph on theWHT

“…This water production rate is a factor of ∼4 smaller than expected from the reported post-perihelion activity curve. We note that the observations in the Combi et al (2018) paper do not extend to the post-perihelion heliocentric distance where we observed C/2013 US10. Therefore a possible reason for the discrepancy is that the derived power law for Q(OH) in this comet was not valid at r h = 2.21 au.…”

“…Combi et al (2018) have pub-lished water production rates that were determined using SOHO/SWAN observations of hydrogen Lyman-α. We observed this comet at a post-perihelion distance of 2.21 au, and the power law fit in the Combi et al (2018) study yields a water production rate of 3.6×10 28 mol s −1 at this distance. Using the OH (0-0) emission observed in the spectrum of this comet and assuming a 90% branching ratio for H 2 O photodissociation (Crovisier 1989), we derive a water production rate of (8.8±1.7)×10 27 mol s −1 .…”

Volatile Abundances, Extended Coma Sources, and Nucleus Ice Associations in Comet C/2014 Q2 (Lovejoy)

“…Values derived here are in the range of ∼(5-8) × 10 29 molecules s −1 (Table 2), which are in good agreement, accounting for uncertainties and variability, with production rates determined around this time period by multiple techniques (Biver et al 2015(Biver et al , 2016Faggi et al 2016;Paganini et al 2017;Combi et al 2018;Feldman et al 2018). At these large production rates, some H 2 O lines in the M-band setting could be subject to optical depth effects within ∼100 km of the nucleus (Dello Russo et al 2000); however, there is no evidence of this when comparing the multiplicative growth factors and spatial profiles of measured H 2 O lines in the M-band and KL settings.…”

“…Investigations of C/2014 Q2 (Lovejoy) have already revealed its detailed organic chemistry (Biver et al 2015) and volatile isotopic ratios (Biver et al 2016;Shinnaka & Kawakita 2016;Paganini et al 2017). Other longterm observations show asymmetric gas production around perihelion, with significantly higher productivity after perihelion (Venkataramani et al 2016;Combi et al 2018). Here we report IR spectroscopic observations of C/2014 Q2 (Lovejoy) with NIRSPEC at the W. M. Keck Observatory obtained on UT 2015 February 2 and 3 (hereafter all given dates are in UT) a few days after perihelion.…”

BepiColombo Science Investigations During Cruise and Flybys at the Earth, Venus and Mercury

“…For each target, it will be possible to compute the time when it intersects the PHEBUS instrument FOV without a change of the spacecraft attitude. Depending on the brightness of the object and its distance to the spacecraft, observations of the hydrogen coma at Lyman-α (EUV) may be done, as done regularly by SWAN/SOHO (Bertaux et al 1995), to derive the water production rates (e.g., Combi et al 2018). If possible, OH (FUV line at 308 nm) and other weaker emissions could also be observed, as done in the past by SPICAV on Venus Express (Chaufray and Bertaux 2015).…”