X-SHYNE: X-shooter spectra of young exoplanet analogs

Abstract

We present simultaneous 0.65–2.5 μm medium resolution (3300 ≤ Rλ ≤ 8100) VLT/X-shooter spectra of the relatively young (150–300 Myr) low-mass (19 ± 5MJup) L–T transition object VHS 1256−1257 b, a known spectroscopic analog of HR8799d. The companion is a prime target for the JWST Early Release Science (ERS) and one of the highest-amplitude variable brown dwarfs known to date. We compare the spectrum to the custom grids of cloudless ATMO models, exploring the atmospheric composition with the Bayesian inference tool ForMoSA. We also reanalyze low-resolution HST/WFC3 1.10–1.67 μm spectra at minimum and maximum variability to contextualize the X-shooter data interpretation. The models reproduce the slope and most molecular absorption from 1.10 to 2.48 μm self-consistently, but they fail to provide a radius and a surface gravity consistent with evolutionary model predictions. They do not reproduce the optical spectrum and the depth of the K I doublets in the J band consistently. We derived Teff = 1380±54 K, log(g) = 3.97±0.48 dex, [M/H] = 0.21±0.29, and C/O > 0.63. Our inversion of the HST/WFC3 spectra suggests a relative change of $ 27^{+6}_{-5} $ K of the disk-integrated Teff correlated with the near-infrared brightness. Our data anchor the characterization of that object in the near-infrared and could be used jointly to the ERS mid-infrared data to provide the most detailed characterization of an ultracool dwarf to date.

The JWST Early-release Science Program for Direct Observations of Exoplanetary Systems II: A 1 to 20 μm Spectrum of the Planetary-mass Companion VHS 1256–1257 b

“…The crest-to-crest width of the oscillation waves in the calibrator star between 1.65 and 1.75 is ∼0.01-0.02 μm, and the width of the potential methane feature is 0.02 μm. Previously published near-infrared spectra show no methane absorption at 1.67 μm (Gauza et al 2015;Petrus et al 2023). If this feature remains after further pipeline updates, the opacity We also find a relatively shallow methane feature between 2.8 and 3.8 μm that is consistent with the previously published L-band spectrum in Miles et al (2018).…”

“…VHS 1256 b's effective temperature is cool enough that a small degree of carbon monoxide will be detected at 2.3 μm. The depth and shape of the central feature are similar to previously published K-band spectra in Hoch et al (2022) and Petrus et al (2023). We also detect a densely packed set of carbon monoxide features around 4.6 μm in VHS 1256 b's spectrum.…”

“…The first published near-infrared spectrum of VHS 1256 b in Gauza et al (2015) showed no K doublet features and no detection of the 1.134 μm Na line, possibly due to insufficient resolution. Follow-up work by Petrus et al (2023) obtained medium-resolution (R ∼ 8000) spectra of VHS 1256 b at a resolution higher than the JWST spectrum presented here. The Na line at 1.134 μm and K doublets at 1.173 and 1.248 μm are easier to distinguish from the continuum in the JWST spectrum; however, the published spectrum from Petrus et al (2023) likely has the resolution to capture a broader range of absorption features such as Fe, FeH, and TiO.…”

“…Follow-up work by Petrus et al (2023) obtained medium-resolution (R ∼ 8000) spectra of VHS 1256 b at a resolution higher than the JWST spectrum presented here. The Na line at 1.134 μm and K doublets at 1.173 and 1.248 μm are easier to distinguish from the continuum in the JWST spectrum; however, the published spectrum from Petrus et al (2023) likely has the resolution to capture a broader range of absorption features such as Fe, FeH, and TiO. However, the standard NIRSpec pipeline procedures are still in development.…”

“…VHS 1256 b was discovered by Gauza et al (2015) ∼8″ (∼150 au) away from an M-dwarf binary (Stone et al 2016;Rich et al 2016). Spectra of the host M-star binary and the planetary-mass companion are both consistent with youth (300 Myr); the companion has red colors and a triangular Hband feature that is seen in other low-mass companions, as well as weak alkali lines (Gauza et al 2015;Petrus et al 2023). The photometry (Gauza et al 2015) and distance (22.2 pc; Dupuy et al 2020) of VHS 1256 b place it in the vicinity of other faint red directly imaged planets, such as HR 8799 cde (Marois et al 2008(Marois et al , 2010 and 2MASS 1207 b (Chauvin et al 2004) in a color-magnitude diagram (see Figure 1).…”