The first high-resolution spectra of 1.3 L subdwarfs.
REINERS A. and BASRI G.
Abstract (from CDS):
We present the first high-resolution (R~31,000) spectra of the cool L subdwarf (sdL) 2MASS 0532+8246 and what was originally identified as an early-type sdL, LSR 1610-0040. Our work, in combination with contemporaneous work by Cushing and Vacca, makes it clear that the latter object is more likely a mid-M dwarf with an unusual composition that gives it some subdwarf spectral features. We use the data to derive precise radial velocities for both objects and to estimate space motion; both are consistent with halo kinematics. We measure the projected rotational velocities, revealing a very slow rotation for the old sd?M6 object LSR 1610-0040. The object 2MASS 0532+8246 exhibits a rapid rotation of vsini=65±15 km/s, consistent with the behavior of L dwarfs. This means that the braking time for L dwarfs is extremely long, or that perhaps they never slow down. A detailed comparison of the atomic Rb and Cs lines with spectra of field L dwarfs shows that the spectral type 2MASS 0532+8246 is consistent with being mid- to late-L. The Rb I and K I lines of LSR 1610-0040 are like those of an early-L dwarf, but the Cs I line is like that of a mid-M dwarf. The appearance of the Ca II triplet in absorption in this object is very hard to understand if it is not at least as warm as M6. We explain these effects in a consistent way using a mildly metal-poor mid-M model. M subdwarfs have weak metal oxides and enhanced metal hydrides relative to normal M dwarfs. LSR 1610-0040 exhibits metal hydrides like an M dwarf but metal oxides like a subdwarf. The same explanation that resolves the atomic-line discrepancy explains this as well. Our spectra cover the spectral region around a previously unidentified absorption feature at 9600 Å and the region around 9400 Å in which detection of TiH has been claimed. We identify the absorption around 9600 Å as being due to atomic lines of Ti and a small contribution of FeH, but we cannot confirm a detection of TiH in the spectra of cool sdLs. In 2MASS 0532+8246, both metal oxides and metal hydrides are extremely strong relative to normal L dwarfs. It may be possible to explain the strong oxide features in 2MASS 0532+8246 by invoking effects due to inhibited dust formation. High-resolution spectroscopy has aided in beginning to understand the complex molecular chemistry and spectral formation in metal-deficient and ultracool atmospheres and the properties of early ultra-low-mass objects.