SZ Lyncis: An Eccentric wide Binary with a possible neutron star

TL;DR

SZ Lyn is a wide, low-eccentricity binary hosting a δ Scuti primary (SZ Lyn A) and an unseen compact companion (SZ Lyn B). The study combines radial-velocity measurements, light-travel-time effects from pulsations, and detailed asteroseismic modeling with MESA to determine a precise mass for SZ Lyn A ($M_1\approx1.83\,M_\odot$) and to constrain SZ Lyn B's mass to roughly $1.7$–$2.0\,M_\odot$, suggesting a neutron-star interpretation over a luminous star or white dwarf. The orbital solution from RV, LTE, and Hipparcos astrometry yields an eccentricity $e\approx0.186$, period $P\approx1188.5$ d, and a mass function $f(M_2)\approx0.10$–$0.14\,M_\odot$, with an inferred inclination $i\approx39.6^\circ$ enabling dynamical mass estimates. The results demonstrate a powerful multi-technique approach for detecting compact objects in wide binaries and illustrate how asteroseismology can tightly constrain the primary to infer the nature of its unseen companion; Gaia-era astrometry and follow-up observations could decisively confirm SZ Lyn B's NS status.

Abstract

SZ Lyn is a low-eccentricity ($e=0.186$) wide binary system hosting a post-main-sequence $δ$ Scuti variable (SZ Lyn A) and an unseen compact companion (SZ Lyn B), orbiting with a $3.32$-yr period. Combining radial velocity measurements, asteroseismology, and LAMOST spectroscopy, we characterize both components. Asteroseismic modeling yields precise parameters for SZ Lyn A: $M_1 = 1.83^{+0.06}_{-0.01} M_{\odot}$, $R_1 = 2.899^{+0.027}_{-0.000} R_{\odot}$, $L_1 = 16.111^{+0.721}_{-0.354} L_{\odot}$, and core hydrogen abundance $X_{\mathrm{c}} = 0.089^{+0.032}_{-0.005}$. For SZ Lyn B, radial velocity ($M_2 = 1.69_{-0.634}^{+0.933} $M$_{\odot}$) and $O\!-\!C$ analysis ($M_2 = 1.97_{-1.12}^{+1.13} $M$_{\odot}$) yield consistent masses within uncertainties, combined with the astrometric orbital inclination from \textit{Hipparcos} mission. We identify SZ Lyn B as a neutron star candidate, though a massive white dwarf possibility remains. This demonstrates the efficacy of combining radial velocity, light travel time effect, and asteroseismology for compact object detection in wide binaries.

Figures (6)

• Figure 1: The low-resolution spectrum of SZ Lyn observed with LAMOST. Upper panel: The original spectrum, the four Balmer absorbed lines H$_{\alpha}$, H$_{\beta}$, H$_{\gamma}$, and H$_{\delta}$ and the metal lines CaII are marked. Lower panel: The fitting model of A-type star for the spectrum of SZ Lyn.
• Figure 2: The radial velocity curve of SZ lyn.
• Figure 3: The radial $O-C$ curve of SZ lyn.
• Figure 4: Corner plot of the MCMC fitting code for the model of a slash line plus LTTE. Blue vertical lines indicate the median values of the histograms presented for each parameter. Posterior distributions at the 16 per cent and 84 per centquantiles are shown by black vertical dashed lines. The units for $A$, $P$, $\omega$, $\Delta T_{0}$, $\Delta P_{0}$ and d are days, years, degree, HJD and days, respectively. In the plot, $mm_0$ =$\frac{2\pi(2458683.4042-T)}{365.25 \times P}$, and the P and T are the period and the periastron passage of SZ Lyn A.
• Figure 5: This visualization presents the fitting metric $S^{2}_m$ and physical parameters: stellar mass ($M$), radius ($R$), luminosity ($L$), effective temperature ($T\mathrm{eff}$), central hydrogen abundance ($X_c$), and gravitational acceleration ($\log g$) with a red horizontal line marking the threshold $S^{2}_m = 0.10$, where black dots indicate the minimum $S^{2}_m$ for each model while the red dot identifies the minimum $S^{2}_m$ of the best-fitting model.