A New Generation of Cool White Dwarf Atmosphere Models. III. WD J2356$-$209: Accretion of a Planetesimal with an Unusual Composition. (arXiv:1902.03219v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Blouin_S/0/1/0/all/0/1">S. Blouin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dufour_P/0/1/0/all/0/1">P. Dufour</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Allard_N/0/1/0/all/0/1">N.F. Allard</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Salim_S/0/1/0/all/0/1">S. Salim</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rich_R/0/1/0/all/0/1">R.M. Rich</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Koopmans_L/0/1/0/all/0/1">L.V.E. Koopmans</a>
WD J2356$-$209 is a cool metal-polluted white dwarf whose visible spectrum is
dominated by a strong and broad sodium feature. Although discovered nearly two
decades ago, no detailed and realistic analysis of this star had yet been
realized. In the absence of atmosphere models taking into account the nonideal
high-density effects arising at the photosphere of WD J2356$-$209, the origin
of its unique spectrum had remained nebulous. We use the cool white dwarf
atmosphere code presented in the first paper of this series to finally reveal
the secrets of this peculiar object and details about the planetesimal that
polluted its atmosphere. Thanks to the improved input physics of our models, we
find a solution that is in excellent agreement with the photometric
observations and the visible spectrum. Our solution reveals that the
photosphere of WD J2356$-$209 has a number density ratio of $log,{rm Na/Ca}=
1.0 pm 0.2$, which is the highest ever found in a white dwarf. Since we do not
know how long ago the accretion episode stopped (if it has), we cannot
precisely determine the composition nor the mass of the accreted planetesimal.
Nevertheless, all scenarios considered indicate that its composition is
incompatible with that of chondrite-like material and that its mass was at
least $10^{21},{rm g}$.
WD J2356$-$209 is a cool metal-polluted white dwarf whose visible spectrum is
dominated by a strong and broad sodium feature. Although discovered nearly two
decades ago, no detailed and realistic analysis of this star had yet been
realized. In the absence of atmosphere models taking into account the nonideal
high-density effects arising at the photosphere of WD J2356$-$209, the origin
of its unique spectrum had remained nebulous. We use the cool white dwarf
atmosphere code presented in the first paper of this series to finally reveal
the secrets of this peculiar object and details about the planetesimal that
polluted its atmosphere. Thanks to the improved input physics of our models, we
find a solution that is in excellent agreement with the photometric
observations and the visible spectrum. Our solution reveals that the
photosphere of WD J2356$-$209 has a number density ratio of $log,{rm Na/Ca}=
1.0 pm 0.2$, which is the highest ever found in a white dwarf. Since we do not
know how long ago the accretion episode stopped (if it has), we cannot
precisely determine the composition nor the mass of the accreted planetesimal.
Nevertheless, all scenarios considered indicate that its composition is
incompatible with that of chondrite-like material and that its mass was at
least $10^{21},{rm g}$.
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