TESS Pulsation Timing Reveals the JWST Calibration Star HD 158485 as a Double-pulsator Binary

Abstract

HD~158485 is an A-type $\delta$~Scuti star in the CALSPEC network and the JWST absolute-flux calibration program. Gaia DR3 assigns it a binary classification probability close to unity but provides no orbital solution. Using TESS observations from Sectors 14--26, 40--41, 47--60, and 73--86, we measure the pulsation phases of six $\delta$~Scuti modes. The frequencies form two groups with opposite light-travel-time delays, revealing two pulsating components. A direct light-curve forward model yields an orbital period of $P_{\mathrm{orb}}=382.80_{-0.76}^{+0.80}$~d, eccentricity $e=0.242_{-0.046}^{+0.045}$, projected light-time amplitudes of $195.8_{-8.9}^{+9.2}$ and $239.7_{-6.8}^{+6.8}$~s, and a dynamical mass ratio of $q=M_B/M_A=0.818_{-0.045}^{+0.046}$. A blind prediction based only on the first two long TESS campaigns reproduces the third campaign without updating the orbital parameters, with a Pearson correlation coefficient of $r=0.983$. Its held-out rms is 83.8~s, compared with 289.9~s when the period is fixed at 365.25~d. A 5.15-$\sigma$ Hipparcos--Gaia EDR3 proper-motion anomaly independently indicates unresolved astrometric motion. The unresolved companion can also affect spectrophotometric predictions: a conditional PB2-constrained two-atmosphere model differs from the corresponding single-star atmosphere prediction by 1.6\% over 1--5~$\mu$m and 2.5\% over 5--28~$\mu$m. HD~158485 therefore shows that a calibration-relevant binary can lack a catalogued orbital solution while two independent pulsation clocks reveal and measure its orbit.

arXiv commentAccepted for publication in The Astrophysical Journal Letters