He white dwarfs with large H contamination: Convective mixing or accretion?. (arXiv:1812.09405v1 [astro-ph.SR])

He white dwarfs with large H contamination: Convective mixing or accretion?. (arXiv:1812.09405v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Wachlin_F/0/1/0/all/0/1">F. C. Wachlin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Vauclair_G/0/1/0/all/0/1">G. Vauclair</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Vauclair_S/0/1/0/all/0/1">S. Vauclair</a>

White dwarfs are compact objects with atmospheres containing mainly light
elements, hydrogen or helium. Because of their surface high gravitational
field, heavy elements diffuse downwards in a very short timescale compared to
the evolutionary timescale, leaving the lightest ones on the top of the
envelope. This results in the main classification of white dwarfs as hydrogen
rich or helium rich. But many helium rich white dwarfs show also the presence
of hydrogen traces in their atmosphere, whose origin is still unsettled. Here
we study, by means of full evolutionary calculations, the case for a
representative model of the “He-H-Z” white dwarfs, a sub-group of helium rich
white dwarfs showing both heavy elements and a large amount of hydrogen in
their atmosphere. We find it impossible to explain its hydrogen atmospheric
content by the convective mixing of a primordial hydrogen present in the star.
We conclude that the most likely explanation is the accretion of hydrogen rich
material, presumably water-bearing, coming from a debris disk.

White dwarfs are compact objects with atmospheres containing mainly light
elements, hydrogen or helium. Because of their surface high gravitational
field, heavy elements diffuse downwards in a very short timescale compared to
the evolutionary timescale, leaving the lightest ones on the top of the
envelope. This results in the main classification of white dwarfs as hydrogen
rich or helium rich. But many helium rich white dwarfs show also the presence
of hydrogen traces in their atmosphere, whose origin is still unsettled. Here
we study, by means of full evolutionary calculations, the case for a
representative model of the “He-H-Z” white dwarfs, a sub-group of helium rich
white dwarfs showing both heavy elements and a large amount of hydrogen in
their atmosphere. We find it impossible to explain its hydrogen atmospheric
content by the convective mixing of a primordial hydrogen present in the star.
We conclude that the most likely explanation is the accretion of hydrogen rich
material, presumably water-bearing, coming from a debris disk.

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