The Low-Mass Companion to the Lithium-Depleted, Spectroscopic Binary HBC 425 (St 34)

TL;DR

This study uses high-resolution imaging and spectroscopy to analyze a low-mass companion to the binary HBC 425, revealing its spectral type, mass, age, and lithium content, which informs the system's evolutionary status.

Contribution

First detailed characterization of the low-mass companion HBC 425C, including spectral type, mass, age, and lithium abundance, in a lithium-depleted binary system.

Findings

HBC 425C has an M5.5 spectral type and ~0.09 solar masses.

HBC 425C retains lithium, indicating a younger age or different evolutionary status.

HBC 425 shows signs of accretion and lithium depletion, complicating age estimates.

Abstract

We present high angular resolution, near-infrared imaging and spectroscopy of a low-mass companion to the lithium-depleted, double-line spectroscopic binary HBC 425 (St 34) obtained using the Near Infrared Spectrograph (NIRSPEC) and the Keck II adaptive optics system. Positioned 1.23" southeast of the primary pair, the companion, HBC 425C, is ~2.4 magnitudes fainter at 2.2 microns. Moderate-resolution (R~2500) J- and K-band spectroscopy reveal HBC 425C to have an M5.5 (+/-0.5) spectral type. Comparisons with pre-main sequence evolutionary models imply a mass of ~0.09 M(Sun) and ages of 8-10 Myr, assuming the nominal distance of Taurus-Auriga (~140 pc), or ~25 Myr if placed at ~90 pc. We also present high dispersion, optical spectra of HBC 425 and HBC 425C obtained using the High Resolution Echelle Spectrometer (HIRES) on Keck I. We detect strong Li I 6708 absorption in the spectrum of HBC 425C. Using curves of growth for the Li I 6708 doublet, we estimate its abundance level to lie between log N(Li)=1.9 and 3.1 dex. The spectrum of HBC 425 exhibits Ca II H & K, He I 5876, 6678, and strong Balmer line emission, consistent with accretion. We place more restrictive upper limits on the surface abundance of lithium and find that HBC 425 retains less than ~0.1% of its primordial abundance. The presence of lithium in the photosphere of HBC 425C does not resolve the discrepancy between isochronal and lithium depletion ages for the primary pair. However, if lithium were depleted relative to interstellar abundance levels, even minimally, considerable support would be gained for the more advanced age of this hierarchical triple system.