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

• Published in: Publications of the Astronomical Society of the Pacific Vol. 123; no. 910; pp. 1383 - 1390
• Main Authors: Dahm, S. E., Lyke, J. E.
• Format: Journal Article
• Language: English
• Published: Chicago, IL University of Chicago Press 01.12.2011
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Spectroscopic binary stars
• ISSN: 0004-6280; 1538-3873
• DOI: 10.1086/663724

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 NIRSPEC and the Keck II adaptive optics system. Positioned∼1.23″ ∼ 1.23 ″ southeast of the primary pair, the companion, HBC 425C, is∼2.4 mag ∼ 2.4     mag fainter at 2.2 μm. Moderate-resolution ( R ∼ 2500 R ∼ 2500 ) J J - and K K -band spectroscopy reveals HBC 425C to have an M5.5 (± 0.5 ± 0.5 ) spectral type. Comparisons with pre–main-sequence evolutionary models imply a mass of∼0.09 M ⊙ ∼ 0.09     M ⊙ and ages of 8–10 Myr, assuming the nominal distance of Taurus-Auriga (∼140 pc ∼ 140     pc ), or∼25 Myr ∼ 25     Myr if placed at∼90 pc ∼ 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 iλ6708 absorption in the spectrum of HBC 425C. Using curves of growth for the Li iλ6708 doublet, we estimate its abundance level to lie betweenlog N(Li) = 1.9 log N ( Li ) = 1.9 and 3.1 dex. The spectrum of HBC 425 exhibits Ca iiH and K lines, He iλλ5876, 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% ∼ 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.