Quadruple system HD 135160 in a unique 2+2 configuration

TL;DR

HD 135160 is unveiled as a rare, bright 2+2 quadruple system consisting of an eccentric heartbeat ellipsoidal binary Aa-Ab ($P_A=8.23459$ d) and a shallowly eclipsing binary Ba-Bb ($P_B=5.8528657$ d), with three visible early-type components Aa, Ab, and Ba detected in high-resolution spectroscopy. A four-year CHIRON campaign combined with TESS photometry enables a joint orbital analysis using RVs, spectral disentangling (KOREL), and light-curve modelling (PHOEBE 2; PYTERPOL), revealing a long mutual orbit between the binaries with a period between $1600$ and $2200$ days and a complex, partially constrained architecture. The A-binary exhibits a pronounced heartbeat-like brightening near periastron, while the B-binary shows shallow, shallow eclipses; the RVs and disentangling indicate Aa, Ab, and Ba are hot, massive stars with Teff of $31700$, $22000$, and $16070$ K, respectively, and Bb is cooler (~6000 K). The work highlights the rarity and dynamical richness of massive quadruple systems and points to future interferometric and spectroscopic monitoring to precisely determine masses, inclinations, and the long outer orbit, with implications for models of massive star formation and evolution.

Abstract

Analysing a large body of observational data, we found that HD 135160 is a quadruple 2+2 system, composed of a massive ellipsoidal binary ('heartbeat' star) with components Aa and Ab in an eccentric 8.234 d orbit and an eclipsing binary (with components Ba and Bb), with a 5.853 d period and partial eclipses that have already been reported from the space photometry secured by the Transiting Exoplanet Survey Satellite (TESS). Our systematic echelle spectroscopy, secured since September 2021, led to the discovery that the optical spectra are dominated by spectral lines of three early-type stars, two moving around each other on a 8.234 d orbit of a high eccentricity, which causes periodic brightenings near the periastron passage, and the third one (component Ba) being the brighter component of the 5.853 d binary. Both pairs are physically bounded and revolve around each other with a period somewhere between 1600 and 2200 days (4.4 to 6 years). The object exhibits small cyclic light variations of a variable amplitude and characteristic time scale of 0.071 d (14.14 c d-1), seen throughout the whole orbit. The nature of these tiny changes deserves further investigation. It also seems that the earlier classification of the object as a Be star is unfounded.

Figures (11)

• Figure 1: Amplitude periodogram of RVs measured on Heii 4686 Å line. The highest peak at frequency of 0.1214 c$\,$d$^{-1}$ corresponds to period of 8234.
• Figure 2: Top: RV curve of the 8234 primary measured in reSPEFO on Heii 4686 Å line. Bottom: RV curves of the 8234 primary (blue) and secondary (red) based on the reSPEFO RVs of the clean Hei 5016 Å line. A preliminary ephemeris $T_{\rm RVmin.}$ = RJD $60003.740 + 8\fd2347\times E$ was used in both plots. Errors are taken as a mean of two independent measurements and are too small to be seen.
• Figure 3: Parts of the CHIRON spectra with H$\alpha$ and Hei 6678 Å ordered according to their phase for ephemeris $T_\textrm{0, perpass}=\textrm{RJD } 59995.6424 + 8.23459\times E$. Only some of the spectra are plotted, corresponding to around a third of the orbital period, so that change of the position of lines of 3 components (Aa, Ab, and Ba) is visible. Distances between spectra do not correspond to the distance in phase. Difference in $S/N$ can be seen between 30 s, 300 s, and 500 s exposures.
• Figure 4: Locally derived systemic ($\gamma$) velocities of binary A, based on the orbital solution with FOTEL from RVs measured on Heii 4686 Å line.
• Figure 5: Scheme of the HD 135160 system based on this study.