tokens: ) shows that these photodissociation products are likely to dominate the gas mass, since molecules including CO are photodestroyed very rapidly after release (in less than 100 years) due to the strong UV radiation environment found around an A0V star. The same radiation field is able to ionise these atomic species, explaining the detection of CII around several debris discs (e.g. Riviere-Marichalar et al. 2014 ). HR 4796 A was also searched for atomic oxygen (OI) and ionised carbon (CII) emission by Herschel ( Meeus et al. 2012 ), which set upper limits of 6.3 and Wm, respectively. Due to the low gas densities expected in this scenario, species with high critical densities such as CO and OI are expected to be out of LTE, meaning that fluxes are dependent on the density of collider species and on the radiation field as well as on the gas temperature and OI mass (Eq. (7) in Matrà et al. 2015 ). This means that the derivation of OI mass upper limits from the observed fluxes is highly degenerate; the Herschel observations therefore cannot tightly constrain the oxygen mass in these low-density discs. On the other hand, taking electrons as the dominant collider species (as shown to be likely in secondary gas discs, Matrà et al. 2015 ), CII has a much lower critical density of order 5–10 electrons cm ( Goldsmith et al. 2012 ). The electron density predicted by this exocometary model in HR 4796 A is much higher (of order at the location of the belt, Kral et al. 2017 ). This means that LTE is a good approximation for CII, yielding a mass upper limit between for the same 20100 K temperature range considered above for CO. However, our secondary gas model predicts an average ionised fraction in the disc that is very low (), meaning that we expect a much higher total carbon mass in HR 4796 A ring of 0.19 . Then, assuming the dominant molecule released by the comets to be CO, yielding a C/O abundance of one, we can estimate the total gas mass in the secondary origin scenario to be 0.44 . However, we note that the Kral et al. (2016) model shows that the bulk of the gas will rapidly expand viscously to form an accretion disc. This means that only 6% of this total gas mass (0.03 ) will be co-located within the AU wide dust ring. In summary, the secondary model predicts a gas mass co-located with the ring of 0.03 , whereas in the primordial scenario we expect a total gas mass of at most 0.11 . On the other hand, SED modelling of the ring’s infrared excess ( Milli et al. 2015 ) yields a total dust mass in the ring of 0.5 distributed among particles between 1 m and 10 mm. Regardless of the gas origin scenario, we therefore derive an upper limit on the gas to dust ratio at the ring’s location of 0.2, which rules out the presence of gas in sufficient amounts to explain the observed ring morphology in the HR 4796 A system. A similar conclusion was recently achieved for the narrow HD 181327 debris ring ( Marino et al. 2016 ), where very low levels of
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