The Photodissociation of Formaldehyde in Comets

Abstract

Observations of comets in the 905-1180 Å spectral band made with the Far Ultraviolet Spectroscopic Explorer in 2001 and 2004 show unusual features in the fluorescent emissions of CO and H 2 . These include emission from a non-thermal high-J rotational population of CO and solar Lyα induced fluorescence from excited vibrational levels of H 2 , both of which are attributed to the photodissociation of formaldehyde. In this paper we model the large number of observed H 2 lines and demonstrate the dependence of the pumping on the heliocentric velocity of the comet and the solar line profiles. We also derive the rotational and vibrational populations of H 2 and show that they are consistent with the results of laboratory studies of the photodissociation of H 2 CO. In addition to the principal series of H I and O I, the residual spectrum is found to consist mainly of the Rydberg series of C I multiplets from which we derive the mean carbon column abundance in the coma. Fluorescent emissions from N I and N 2 are also searched for.

Far-ultraviolet Spectroscopy of Recent Comets with the Cosmic Origins Spectrograph on the Hubble Space Telescope

“…Thus, we can confirm H 2 CO as the source of the Lyα pumped lines in the spectrum of comet Lovejoy, even though we cannot unambiguously detect the non-thermal wings, separated by 1.5Å from the band origin, on the CO Fourth Positive bands. From the FUSE data of Feldman et al (2009) andFeldman (2015), and using the mean of the H 2 populations given in Table 3, we expect the non-thermal CO population, peaking at J = 40, to be 3.1 ± 0.7% that of the total CO population, which is consistent with the spectrum shown in Fig. 4.…”

“…The high signal-to-noise ratio of this spectrum allows for the detection of lines of the Lyman band system (B 1 Σ + u − X 1 Σ + g ) of H 2 , fluorescently pumped from the ground vibrational level by solar Lyβ (Feldman et al 2002) (in green) and from excited vibrational levels by solar Lyα (in blue). The latter result from the photodissociation of H 2 CO (Feldman 2015) and were first identified in spectra of Jupiter following the impact of comet Shoemaker-Levy 9 in 1994 (Wolven et al 1997). These are discussed in detail below in Section 4.…”

“…The red line represents a synthetic CO Fourth Positive fluorescence spectrum. The green lines are solar Lyβ pumped H 2 fluorescence while the blue lines are a synthetic spectrum of Lyα pumped H 2 vibrationally excited in the photodissociation of H 2 CO(Feldman 2015).…”

Strong CO+ and Emission in Comet C/2016 R2 (Pan-STARRS)*

“…Indeed, it is much higher than other observations of N + 2 /CO + , as listed in Table III of Cochran et al Korsun et al (2014) measured the N + 2 /CO + in comet C/2002 VQ94 (LINEAR), a comet active at > 8 AU, as 0.06. Feldman (2015) placed a 3-σ upper limit on N 2 /CO of 0.027 for comet C/2001 Q4 (NEAT) using FUSE observations. Ivanova et al (2016) measured N + 2 /CO + as 0.013 for comet 29P/Schwassmann-Wachmann 1 at 5.25 au, though the N + 2 feature is not well defined in these low-resolution spectra.…”

The 1600 Å Emission Bump in Protoplanetary Disks: A Spectral Signature of H2O Dissociation∗

“…Initial modeling work argued that the majority of this population was produced by the dissociation of H 2 O via Lyα photons (Liu et al 2007), analogous to the process we describe for protoplanetary disks. More recent analyses by Feldman et al (2009) and Feldman (2015) have demonstrated that both the H 2 and the rovibrationally excited CO fluorescence spectra are better described as the fragments of formaldehyde (H 2 CO) dissociation. We do not favor the formaldehyde hypothesis for the T Tauri star case because while the population resulting from H 2 CO dissociation can produce highly excited vibrational levels (Zhang et al 2005), the rotational populations peak at J = 5 or 7 (Chambreau et al 2006), whereas J > 15 is required to produce the observed 1600 Å Bump.…”