Spectroscopy of eclipsing compact hierarchical triples

Abstract

Eclipsing compact hierarchical triples (CHTs) are systems in which a tertiary star orbits an eclipsing binary (EB) in an orbit of fewer than 1000 days. In a CHT, all three stars exist in a space that is less than 5 AU in radius. A low-mass CHT is an interesting case through which we can understand the formation of multiple stars and planets at such small scales. In this study, we combine spectroscopy and photometry to estimate the orbital, stellar, and atmospheric parameters of stars in a sample of CHTs. Using the complete set of parameters, we aim to constrain the metallicity and age of the systems. We used time-series spectroscopy to obtain radial velocities (RVs) and disentangled spectra. Using RV modelling, EB light curve modelling, and spectral analysis, we estimated the metallicities and temperatures. Using isochrone fitting, we constrained the ages of the system. We then combined observations of masses, outer eccentricities ($e_2$), orbital periods, and age estimates of the systems from the literature. We compared the distributions of $e_2$, and the tertiary mass ratio, $q_3 = M_3/(M_1+M_2)$, for three different metallicity ranges and two age ranges. We have estimated the masses, radii, temperatures, metallicities, and ages of 12 stars in four CHTs. The CHT CD-32 6459 shows signs of von Zeipel-Lidov-Kozai oscillations, while CD-62 1257 can evolve to form a triple common envelope. The rest of the CHTs are old and have an M-dwarf tertiary. We find that the $q_3$ distribution for CHTs with sub-solar metallicity has a uniform distribution but the systems with solar and above-solar metallicity peak between 0.5 and 1. When dividing them according to their ages, we find the $q_3$ of old systems to be around 0.5. The eccentricity, $e_2$, favours a value of around 0.3 irrespective of metallicity or age. The distributions of $q_3$ and $e_2$ resemble the distributions of the mass ratio and eccentricity of close field binaries.