Early Results from VLT-SPHERE: Long-Slit Spectroscopy of 2MASS 0122-2439B, a Young Companion Near the Deuterium Burning Limit

TL;DR

This study presents high-resolution long-slit spectroscopy of a young, planetary-mass companion near the deuterium-burning limit, revealing its atmospheric properties and spectral classification, bridging the gap between hot planets and cooler substellar objects.

Contribution

First high-resolution long-slit spectroscopy of 2MASS J0122-2439 B, providing detailed atmospheric characterization and spectral classification of a young, low-mass companion.

Findings

Spectral type L3-L4, low gravity, young age confirmed.

Effective temperature around 1600 K, surface gravity log(g)=4.5.

Spectral features indicate intermediate gravity and youth.

Abstract

We present 0.95-1.80 $\mu$m spectroscopy of the $\sim$12-27 $M_{\rm Jup}$ companion orbiting the faint ($R$$\sim$13.6), young ($\sim$120 Myr) M-dwarf 2MASS J01225093--2439505 ("2M0122--2439 B") at 1.5 arcsecond separation (50 AU). Our coronagraphic long-slit spectroscopy was obtained with the new high contrast imaging platform VLT-SPHERE during Science Verification. The unique long-slit capability of SPHERE enables spectral resolution an order of magnitude higher than other extreme AO exoplanet imaging instruments. With a low mass, cool temperature, and very red colors, 2M0122-2439 B occupies a particularly important region of the substellar color-magnitude diagram by bridging the warm directly imaged hot planets with late-M/early-L spectral types (e.g. $\beta$ Pic b and ROXs 42Bb) and the cooler, dusty objects near the L/T transition (e.g. HR 8799bcde and 2MASS 1207b). We fit BT-Settl atmospheric models to our $R$$\approx$350 spectrum and find $T_{\rm eff}$=1600$\pm$100 K and $\log(g)$=4.5$\pm$0.5 dex. Visual analysis of our 2M0122-2439 B spectrum suggests a spectral type L3-L4, and we resolve shallow $J$-band alkali lines, confirming its low gravity and youth. Specifically, we use the Allers & Liu (2013) spectral indices to quantitatively measure the strength of the FeH, VO, KI, spectral features, as well as the overall $H$-band shape. Using these indices, along with the visual spectral type analysis, we classify 2M0122-2439 B as an intermediate gravity (INT-G) object with spectral type L3.7$\pm$1.0.