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

LAMOST J235456.73+335625 (J2354) is a binary system hosting a K dwarf and a 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 (, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>log</mml:mo> <mml:mi>g</mml:mi> </mml:mrow> </mml:math> , [Fe/H], ) and abundances for 8 elements for the K dwarf. We refine the mass of the compact object to with a minimum mass of . 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 . Instead, we find that a massive white dwarf with a cooling age of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>≳</mml:mo> <mml:mn>3</mml:mn> <mml:mspace width="0.222em"/> </mml:mrow> </mml:math> 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- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>q</mml:mi> </mml:math> 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.