Characterizing brown dwarf companions with IRDIS long-slit spectroscopy: HD 1160 B and HD 19467 B

Abstract

The determination of the fundamental properties (mass, separation, age, gravity, and atmospheric properties) of brown dwarf companions allows us to infer crucial informations on their formation and evolution mechanisms. Spectroscopy of substellar companions is available to date only for a limited number of objects (and mostly at very low resolution, R < 50) because of technical limitations, i.e. contrast and angular resolution. We present medium resolution (R = 350), coronagraphic long-slit spectroscopic observations with SPHERE of two substellar companions, HD 1160 B and HD 19467 B. We found that HD 1160 B has a peculiar spectrum that cannot be fitted by spectra in current spectral libraries. A good fit is possible only considering separately the Y+J and the H spectral band. The spectral type is between M5 and M7. We also estimated a Teff of 2800–2900 K and a log g of 3.5–4.0 dex. The low surface gravity seems to favour young age (10–20 Myr) and low mass (∼20 MJup ) for this object. HD 19467 B is instead a fully evolved object with a Teff of ∼1000 K and log g of ∼5.0 dex. Its spectral type is T6 ± 1.

Orbital and spectral characterization of the benchmark T-type brown dwarf HD 19467B

“…It points toward lower effective temperatures of ∼800-890 K using the empirical relation of Filippazzo et al (2015). Our absolute magnitude in the J band of 15.08 ± 0.11 mag recomputed from the apparent magnitude in Crepp et al (2014) agrees with the value of 15.13 ± 0.02 mag reported by Mesa et al (2020).…”

“…6 from the apparent magnitude in Crepp et al (2014). The fainter Hband magnitude found by Mesa et al (2020) implies a redder H-L color by 0.47 mag in the bottom-right panel of Fig. 9.…”

“…For comparison, the model isochrones of Baraffe et al (2003) are indicated. Mesa et al (2020) estimated from a long-slit spectrum an absolute photometry in the H band of 15.84 ± 0.08 mag assuming the distance derived from the Gaia parallax. This is fainter by ∼3.4σ than the photometry of 15.37 ± 0.11 mag derived in Sect.…”

“…−44 deg (the last two parameters are restricted to the interval [0,180) deg because of ambiguities due to the use of imaging data only). Mesa et al (2020) present a long slit spectrum at a resolution of ∼350 over the Y JH bands, which was obtained with the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument. They derived a spectral type of T6 ± 1 and an effective temperature of 1000 ± 100 K by fitting BT-Settl spectra, in agreement with Crepp et al (2015).…”

Medium-resolution spectrum of the exoplanet HIP 65426 b

“…Since 2013, the high-contrast imagers and spectrographs such as GPI (Macintosh et al 2006), SPHERE (Beuzit et al 2019) or SCExAO/CHARIS (Groff et al 2015;Jovanovic et al 2015) have gathered low-resolution (R λ = 30-350) near-infrared (0.95-2.45 µm) spectra of exoplanets down to 100 mas from their host star. The spectra confirmed some of the results inferred earlier from photometric spectral energy distributions (e.g., Marois et al 2008;Bonnefoy et al 2013): the reduced surface gravity affects the vertical mixing and the gravitational settling of condensates, which leads to thicker cloud layers, upper atmosphere submicron hazes, and causes cloud opacities to remain down to T eff = 600 K at early ages (Bonnefoy et al 2016;Rajan et al 2017;Chauvin et al 2017Chauvin et al , 2018Samland et al 2017;Greenbaum et al 2018;Uyama et al 2020;Delorme et al 2017;Mesa et al 2020). These spectra, however had led to contradicting conclusions on the derivation of the atmospheric composition (Samland et al 2017;Rajan et al 2017), possibly owing to the limited resolution of the observations and uncertainties in the atmospheric models.…”