Hidden massive eclipsing binaries in red supergiant systems: The hierarchical triple system KQ Puppis and other candidates

TL;DR

We investigate hierarchical multiplicity in massive-star systems by focusing on KQ Puppis, a VV Cephei–type RSG binary. Through a multi-technique approach that combines VLTI-GRAVITY Br$\gamma$ astrometry, high-cadence spectroscopy, and TESS photometry, we reveal an inner eclipsing Ba+Bb binary with $P_{Ba+Bb}=17.2596$ d and a wider AB orbit with $P_{AB}\approx 9500$ d, deriving $M_A\approx9\,M_\odot$ and $M_{Ba+Bb}\approx14\,M_\odot$. The Balmer and Br$\gamma$ emission patterns support wind Roche Lobe Overflow feeding the inner disk, modulated by orbital phase and periastron. Our search of additional RSG binaries with TESS suggests that roughly $\sim10\%$ of known Galactic binary RSGs may host eclipsing third components, implying a substantial population of hierarchical triples among massive stars with important implications for their evolution and SN progenitors.

Abstract

The majority of massive stars are part of binary systems that may interact during their evolution. This has important consequences for systems in which one star develops into a Red supergiant (RSG); however, not many RSGs are known binaries. We aim to better constrain the properties of some of the known RSGs in binaries. We first focus on the VV Cephei type RSG KQ Pup (RSG+B-type companion, orbital period of 26 yr), where we have enough data to constrain the system's properties. We use archival photometry and UV spectroscopy, along with newly taken optical spectra and interferometric data. For KQ Pup, as well as for all other Galactic RSGs, we also analyzed the available TESS data. Using TESS photometry, we discovered eclipses with a period of $17.2596 \: \rm d$, associated with the hot B companion, making it a Ba+Bb pair. Meanwhile, the detection of the hydrogen Br$γ$ line with VLTI-GRAVITY enabled us to track the orbital motion of the KQ Pup Ba+Bb pair and thus to determine the astrometric orbit. The dynamical masses agree with independent estimates from asteroseismology and evolutionary models. The results give a mass of $ \sim 9 \: \rm M_{\odot} $ for the RSG and $ \sim 14 \: \rm M_{\odot} $ 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 constrain the minimum mass of KQ Pup Bb as $ \gtrsim 1.2 \: \rm M_{\odot} $. The variability of Balmer lines and the detection of Br$γ$ represent a strong signature of Wind Roche Lobe Overflow, with enhanced signatures of disk-accretion to the Ba+Bb pair during the periastron. Meanwhile, TESS light curves show that about $\sim 10 \%$ of known Galactic binary RSGs may be eclipsing hierarchical triple systems, which suggests that a large fraction of other binary RSGs could also be triples.

Figures (22)

• Figure 1: Photometric light curve of KQ Pup, comprising of several archival sources - APT, Hipparcos, Gaia, and ASAS-SN. Other data are shifted to match Hipparcos and Gaia. We also mark an approximate time of conjunction before periastron and periastron in January 2024.
• Figure 2: Near-IR spectra from VLTI GRAVITY. There are prominent spectral features related to RSGs, such as the CO bands at $\sim 2.29-2.4 \: \rm \mu m$. Based on the interferometric properties, the only feature clearly corresponding to the hot companion (KQ Pup Ba) is the hydrogen Br$\gamma$ feature at $2.167 \: \rm \mu m$. The only other two prominent lines of the hot companion in the region, Hei at $2.059 \: \rm \mu m$ and $2.113 \: \rm \mu m$, are not detected.
• Figure 3: Best-fit of PoWR model atmosphere to median IUE data of KQ Pup, showing full SWP range, for $T_\mathrm{eff} = 19.9$ kK. Main fitted lines of hot stars are shown (purple lines). Narrow lines (green background) are not reproduced by the high-rotating model and likely come from absorption in the RSG wind, see Fig. \ref{['fig:kq_pup_narrow_lines']}. Detailed plots zoomed on specific features are available in Fig. \ref{['fig:kq_pup_spectra_uv_fit']}.
• Figure 4: Left panel: Determined RVs of KQ Pup A and B (based H$\alpha$ and H$\beta$) during the post-periastron epoch covered by STELLA. The vertical lines show the dates of VLTI-GRAVITY observations and TESS sector 88. Right panel: The same, but zoomed to the beginning of 2025. There is evidence for $\sim 17 \rm \: d$ period variations in H$\beta$, while longer trends due to the 26-year orbital period are also present. PLATOSpec spectrum taken in April 2025 is also included.
• Figure 5: PMOIRED fit of hydrogen Br$\gamma$ line for 3 VLTI epochs, red line shows the best fit in each epoch. UV coverage is shown in the left upper corner, and flux in the left bottom corner. In the remaining plots, the 3 main interferometric observables are shown - closure phase (T3PHI), differential phase (DPHI), and absolute visibility ($|V|$).