KQ Pup radial velocities

We focused on the best candidate, the VV Cephei-type RSG binary KQ Pup, which is made up of an RSG, KQ Pup A, and a B-type companion, KQ Pup B and has an orbital period of 26 years. For this system, we have enough data to constrain the system properties. We used archival photometry and UV spectroscopy along with newly acquired optical spectra and interferometric data from VLTI-GRAVITY.

Using TESS light curves, we discovered eclipses with a period of 17.2596d associated with KQ Pup B, making it a binary. We refer to the two components as KQ Pup Ba and Bb. Meanwhile, the detection of the hydrogen Br{gamma} line with VLTI-GRAVITY enabled us to track the orbital motion of the Ba+Bb pair relative to A and thus determine the astrometric orbit of A+B. The dynamical masses agree with independent estimates from asteroseismology and evolutionary models.

The results give a mass of ~10M_{sun}_ for the RSG KQ Pup A and ~14M_{sun}_ for the sum of the hot components Ba+Bb. The observed properties of the system are compatible with a coeval hierarchical triple star, where we constrained the mass of KQ Pup Bb to be >~1.2M_{sun}_. We determined an orbital parallax of {pi}=1.24^+0.05^_-0.04_mas based on the detailed orbital model of the system, which is the first such parallax measurement for an RSG, and the value is consistent with Gaia DR3.

KQ Pup represents a unique demonstration of mass transfer in a wide, eccentric RSG system. The variability of Balmer emission lines and the detection of Br{gamma} are a strong signature of accretion to the Ba+Bb pair near periastron. With the RSG filling its Roche lobe by only ~70% at periastron ({phi}_A+B_~0), the mass transfer is instead driven by accretion from its extended atmosphere via the wind Roche-lobe overflow.

The accretion disk is formed already by {phi}_A+B_~0.95, while TESS light curves also begin to show a signature of occultations by the circumbinary disk, similar to young stellar objects. Through apastron ({phi}_A+B_~0.5), Balmer emissions weaken and indicate a decaying disk. Overall, using TESS, we discovered that several previously assumed RSG binaries host eclipsing inner systems, corresponding to ~10% of all known Galactic RSG binaries.

This suggests that many of the other RSG binaries may also be hierarchical triples.