On the existence of warm H-rich pulsating white dwarfs. (arXiv:1911.02442v1 [astro-ph.SR])

On the existence of warm H-rich pulsating white dwarfs. (arXiv:1911.02442v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Althaus_L/0/1/0/all/0/1">Leandro G. Althaus</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Corsico_A/0/1/0/all/0/1">Alejandro H. C&#xf3;rsico</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Uzundag_M/0/1/0/all/0/1">Murat Uzundag</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Vuckovic_M/0/1/0/all/0/1">Maja Vu&#x10d;kovi&#x107;</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Baran_A/0/1/0/all/0/1">Andrzej S. Baran</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bell_K/0/1/0/all/0/1">Keaton J. Bell</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Camisassa_M/0/1/0/all/0/1">Mar&#xed;a E. Camisassa</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Calcaferro_L/0/1/0/all/0/1">Leila M. Calcaferro</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Geronimo_F/0/1/0/all/0/1">Francisco C. De Ger&#xf3;nimo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kepler_S/0/1/0/all/0/1">S. O. Kepler</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Silvotti_R/0/1/0/all/0/1">Roberto Silvotti</a>

The possible existence of warm ($T_{rm eff}sim19,000$ K) pulsating DA
white dwarf (WD) stars, hotter than ZZ Ceti stars, was predicted in theoretical
studies more than 30 yr ago. However, to date, no pulsating warm DA WD has been
discovered. We re-examine the pulsational predictions for such WDs on the basis
of new full evolutionary sequences. We analyze all the warm DAs observed by
TESS satellite up to Sector 9 in order to search for the possible pulsational
signal. We compute WD evolutionary sequences with H content in the range $-14.5
lesssim log(M_{rm H}/M_{star}) lesssim -10$, appropriate for the study of
warm DA WDs. We use a new full-implicit treatment of time-dependent element
diffusion. Non-adiabatic pulsations were computed in the effective temperature
range of $30,000-10,000$ K, focusing on $ell= 1$ $g$ modes with periods in
the range $50-1500$ s. We find that extended He/H transition zones inhibit the
excitation of $g$ modes due to partial ionization of He below the H envelope,
and only in the case that the H/He transition is assumed much more abrupt,
models do exhibit pulsational instability. In this case, instabilities are
found only in WD models with H envelopes in the range of $-14.5 lesssim
log(M_{rm H}/M_{star}) lesssim -10$ and at effective temperatures higher
than those typical of ZZ Ceti stars, in agreement with previous studies. None
of the 36 warm DAs observed so far by TESS satellite are found to pulsate. Our
study suggests that the non-detection of pulsating warm DAs, if WDs with very
thin H envelopes do exist, could be attributed to the presence of a smooth and
extended H/He transition zone. This could be considered as an indirect proof
that element diffusion indeed operates in the interior of WDs.

The possible existence of warm ($T_{rm eff}sim19,000$ K) pulsating DA
white dwarf (WD) stars, hotter than ZZ Ceti stars, was predicted in theoretical
studies more than 30 yr ago. However, to date, no pulsating warm DA WD has been
discovered. We re-examine the pulsational predictions for such WDs on the basis
of new full evolutionary sequences. We analyze all the warm DAs observed by
TESS satellite up to Sector 9 in order to search for the possible pulsational
signal. We compute WD evolutionary sequences with H content in the range $-14.5
lesssim log(M_{rm H}/M_{star}) lesssim -10$, appropriate for the study of
warm DA WDs. We use a new full-implicit treatment of time-dependent element
diffusion. Non-adiabatic pulsations were computed in the effective temperature
range of $30,000-10,000$ K, focusing on $ell= 1$ $g$ modes with periods in
the range $50-1500$ s. We find that extended He/H transition zones inhibit the
excitation of $g$ modes due to partial ionization of He below the H envelope,
and only in the case that the H/He transition is assumed much more abrupt,
models do exhibit pulsational instability. In this case, instabilities are
found only in WD models with H envelopes in the range of $-14.5 lesssim
log(M_{rm H}/M_{star}) lesssim -10$ and at effective temperatures higher
than those typical of ZZ Ceti stars, in agreement with previous studies. None
of the 36 warm DAs observed so far by TESS satellite are found to pulsate. Our
study suggests that the non-detection of pulsating warm DAs, if WDs with very
thin H envelopes do exist, could be attributed to the presence of a smooth and
extended H/He transition zone. This could be considered as an indirect proof
that element diffusion indeed operates in the interior of WDs.

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