On the origin of white dwarfs with carbon-dominated atmospheres: the case of H1504+65

TL;DR

This study investigates the evolutionary origins of the helium-deficient, carbon-rich atmosphere of H1504+65, evaluating whether mass-loss episodes can account for its unique composition and challenging previous evolutionary links.

Contribution

It demonstrates that stationary winds are insufficient to explain the helium deficiency, suggesting non-stationary mass-loss episodes are necessary for the star's evolution.

Findings

Stationary winds during post-born-again evolution cannot fully remove helium-rich envelopes.

Mass-loss during Sakurai's stage might be needed to explain H1504+65's composition.

The proposed evolutionary link from born-again stars to H1504+65 to carbon-rich white dwarfs is unlikely without non-stationary mass loss.

Abstract

We explore different evolutionary scenarios to explain the helium deficiency observed in H1504+65, the most massive known PG1159 star. We concentrate mainly on the possibility that this star could be the result of mass loss shortly after the born-again and during the subsequent evolution through the [WCL] stage. This possibility is sustained by recent observational evidence of extensive mass-loss events in Sakurai's object and is in line with the recent finding that such mass losses give rise to PG1159 models with thin helium-rich envelopes and large rates of period change, as demanded by the pulsating star PG1159-035. We compute the post born again evolution of massive sequences by taking into account different mass-loss rate histories. Our results show that stationary winds during the post-born-again evolution fail to remove completely the helium-rich envelope so as to explain the helium deficiency observed in H1504+65. Stationary winds during the Sakurai and [WCL] stages only remove at most half of the envelope surviving the violent hydrogen burning during the born-again phase. In view of our results, the recently suggested evolutionary connection born-again stars --> H1504+65 --> white dwarfs with carbon-rich atmospheres is difficult to sustain unless the whole helium-rich envelope could be ejected by non-stationary mass-loss episodes during the Sakurai stage.