L-band Spectroscopy with Magellan-AO/Clio2: First Results on Young Low-Mass Companions

TL;DR

This paper presents initial results from L-band spectroscopy of young low-mass companions using Magellan-AO/Clio2, revealing insights into their atmospheres and cloud structures, and constraining deviations from field objects below 1500 K.

Contribution

First implementation of L-band spectroscopy with Magellan-AO/Clio2 for directly imaged companions, providing new atmospheric data and constraints on cloud and chemistry differences.

Findings

Spectra consistent with previous near-infrared data for late M to early L types.

Deviations from field sequence occur below 1500 K.

Discrepancies likely due to cloud structure and CO/CH4 chemistry differences.

Abstract

L-band spectroscopy is a powerful probe of cool low-gravity atmospheres: The P, Q, and R branch fundamental transitions of methane near 3.3 $\mu$m provide a sensitive probe of carbon chemistry; cloud thickness modifies the spectral slope across the band; and H$_{3}^{+}$ opacity can be used to detect aurorae. Many directly imaged gas-giant companions to nearby young stars exhibit L-band fluxes distinct from the field population of brown dwarfs at the same effective temperature. Here we describe commissioning the L-band spectroscopic mode of Clio2, the 1-5 $\mu$m instrument behind the Magellan adaptive-optics system. We use this system to measure L-band spectra of directly imaged companions. Our spectra are generally consistent with the parameters derived from previous near-infrared spectra for these late M to early L type objects. Therefore, deviations from the field sequence are constrained to occur below 1500 K. This range includes the L-T transition for field objects and suggests that observed discrepancies are due to differences in cloud structure and CO/CH$_{4}$ chemistry.