Weighing The Options: The Unseen Companion in LAMOST J2354 is Likely a Massive White Dwarf

Abstract

LAMOST J235456.73+335625 (J2354) is a binary system hosting a $\sim 0.7{\mathrm{M}}_{\odot }$ K dwarf and a $\sim 1.4{\mathrm{M}}_{\odot }$ 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 ($T_{\mathrm{eff}}$, $\log g$, [Fe/H], $v_{\mathrm{rot}}\sin i$) and abundances for 8 elements for the K dwarf. We refine the mass of the compact object to $M_{\mathrm{co}}\sim 1.3{\mathrm{M}}_{\odot }$ with a minimum mass of $M_{\mathrm{co},\min }=1.23\pm 0.04{\mathrm{M}}_{\odot }$. 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 $\sim 3.3{\mathrm{R}}_{\odot }$. 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.

8 pages, 4 figures, and 3 tables in the main text, an additional 1 page, 1 figure, and 1 table in the appendix. Accepted to the Open Journal of Astrophysics. The spectra are available on Zenodo at https://zenodo.org/records/15477171

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