HD144941: The most extreme helium-strong star. (arXiv:2110.11267v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Przybilla_N/0/1/0/all/0/1">N. Przybilla</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fossati_L/0/1/0/all/0/1">L. Fossati</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jeffery_C/0/1/0/all/0/1">C.S. Jeffery</a>
Since its discovery about 50 years ago, HD144941 has generally been
classified as a peculiar member of the extreme helium (EHe) supergiant stars, a
very rare class of low-mass hydrogen-deficient stars. We report the detection
of a strong longitudinal magnetic field based on spectropolarimetry with FORS2
on the ESO VLT with surface-averaged longitudinal field strengths as large as
-9kG. This is further constrained by the detection of Zeeman splitting of
spectral lines to a field strength of at least 15kG, explaining the recent
finding of surface spots for this star. The quantitative analysis of the
stellar atmosphere based on a hybrid non-local thermodynamic equilibrium
approach and new optical spectra yields an effective temperature of
22000$pm$500K, a logarithmic surface gravity of 4.20$pm$0.10, and a surface
helium fraction of 0.950$pm$0.002 by number. While the metal abundances are
about a factor of 10 sub-solar in absolute number, the metal-to-hydrogen ratios
are typical of massive early-type stars, indicating that helium fallback in a
weak, fractionated stellar wind in the presence of a magnetic field took place
— the canonical mechanism for the formation of the helium-strong phenomenon.
Both the spectroscopic and the Gaia EDR3 parallax imply HD144941 to be a
luminous massive star. Kinematically, we argue that HD144941 has reached its
high Galactic latitude as a runaway star. We conclude that instead of being a
comparatively high-gravity low-mass EHe star, HD144941 is by far the most
extreme member of the magnetic massive helium-strong stars, with almost all
atmospheric hydrogen substituted by helium.
Since its discovery about 50 years ago, HD144941 has generally been
classified as a peculiar member of the extreme helium (EHe) supergiant stars, a
very rare class of low-mass hydrogen-deficient stars. We report the detection
of a strong longitudinal magnetic field based on spectropolarimetry with FORS2
on the ESO VLT with surface-averaged longitudinal field strengths as large as
-9kG. This is further constrained by the detection of Zeeman splitting of
spectral lines to a field strength of at least 15kG, explaining the recent
finding of surface spots for this star. The quantitative analysis of the
stellar atmosphere based on a hybrid non-local thermodynamic equilibrium
approach and new optical spectra yields an effective temperature of
22000$pm$500K, a logarithmic surface gravity of 4.20$pm$0.10, and a surface
helium fraction of 0.950$pm$0.002 by number. While the metal abundances are
about a factor of 10 sub-solar in absolute number, the metal-to-hydrogen ratios
are typical of massive early-type stars, indicating that helium fallback in a
weak, fractionated stellar wind in the presence of a magnetic field took place
— the canonical mechanism for the formation of the helium-strong phenomenon.
Both the spectroscopic and the Gaia EDR3 parallax imply HD144941 to be a
luminous massive star. Kinematically, we argue that HD144941 has reached its
high Galactic latitude as a runaway star. We conclude that instead of being a
comparatively high-gravity low-mass EHe star, HD144941 is by far the most
extreme member of the magnetic massive helium-strong stars, with almost all
atmospheric hydrogen substituted by helium.
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