Non-conservative mass transfer in stellar evolution and the case of V404 Cyg/GS 2023+338

We consider donor evolution and mass transfer in the microquasar V404 Cyg/GS 2023+338. Based on X-ray observations of its two outbursts, its average mass accretion rate is substantially lower than the theoretical mass-loss rate from its low-mass giant donor. A likely solution to this discrepancy is that a large fraction of the mass flowing from the donor leaves the binary in the form of accretion-associated outflows. We derive an analytical formula describing changes of the donor Roche lobe radius due to its mass change for non-conservative mass transfer, which we parameterize by the fractions of the mass and angular momentum leaving the system. We perform evolutionary calculations for V 404 Cyg. Given our estimated average accretion rate, $>$70 per cent of the mass lost from the donor has to leave the binary. The allowed solution for the actual donor mass loss rate is parameterized by our two outflow parameters. Our results are in agreement with the observed outflows from the outer disc as well as with the variable near-Eddington accretion observed during the outbursts, compatible with outflows from the vicinity of the black hole. In the latter case, the outflowing matter has a negligible angular momentum.