Stability of helium accretion discs in ultracompact binaries

International audienceContext: Stellar companions of accreting neutron stars in ultra compact X-ray binaries (UCXBs) are hydrogen-deficient. Their helium or C/O accretion discs are strongly X-ray irradiated. Both the chemical composition and irradiation determine the disc stability with respect to thermal and viscous perturbations. At shorter periods, UCXBs are persistent, whereas longer-period systems are mostly transient. Aims: To understand this behaviour one has to derive the stability criteria for X-ray irradiated hydrogen-poor accretion discs. Methods: We use a modified and updated version of the Dubus et al. code describing time-dependent irradiated accretion discs around compact objects. Results: We obtained the relevant stability criteria and compared the results to observed properties of UCXBs Conclusions: Although the general trend in the stability behaviour of UCXBs is consistent with the prediction of the disc instability model, in a few cases the inconsistency of theoretical predictions with the system observed properties is weak enough to be attributed to observational and/or theoretical uncertainties. Two systems might require the presence of some amount of hydrogen in the donor star