2024/07/26 by Tucker, M. A., Wheeler, A. J., Rowan, D. M. +1
#FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE) #Solar and Stellar Astrophysics (astro-ph.SR)
paper · doi:10.48550/arxiv.2407.19004
LAMOST J235456.73+335625 (J2354) is a binary system hosting a ∼ 0.7~\rm M_\odot K dwarf and a ∼ 1.4~\rm M_\odot dark companion, supposedly a neutron star, in a 0.48d orbit. Here we present high- and low-resolution spectroscopy to better constrain the properties of the system. The low-resolution spectrum confirms that the luminous star is a slightly metal-poor K dwarf and strengthens the limits on any optical flux from the dimmer companion. We use the high-resolution spectra to measure atmospheric parameters (T\rm eff, log g, [Fe/H], v\rm rotsin i) and abundances for 8 elements for the K dwarf. We refine the mass of the compact object to M\rm co ∼ 1.3~\rm M_\odot with a minimum mass of M\rm co, min = 1.23±0.04~\rm M_\odot. The expected overabundance of intermediate-mass elements from the incident supernova ejecta is not detected in the K-dwarf atmosphere. This contrasts with known binaries hosting neutron stars where almost all companions show evidence for polluting material. Moving the neutron-star progenitor further from the K-dwarf at the time of explosion to minimize atmospheric pollution requires a finely-tuned kick to produce the current orbital separation of ∼ 3.3~\rm R_\odot. Instead, we find that a massive white dwarf with a cooling age of \gtrsim 3~Gyr satisfies all observational constraints. The system likely experienced two common-envelope phases leading to its current state because the white dwarf progenitor was massive enough to ignite He-shell burning. The system will become a cataclysmic variable in the distant future when the K-dwarf evolves off of the main sequence. These short-period high-q binaries represent an intriguing formation pathway for compact double white dwarf binaries and thermonuclear supernovae. An ultraviolet spectrum is the most promising avenue for directly detecting the white dwarf companion.