Abstract
We present a library of near-infrared (1.1-2.45 microns) medium-resolution (R~1500-2000) integral field spectra of 15 young M6-L0 dwarfs, composed of companions with known ages and of isolated objects. We use it to (re)derive the NIR spectral types, luminosities and physical parameters of the targets, and to test (BT-SETTL, DRIFT-PHOENIX) atmospheric models. We derive infrared spectral types L0+-1, L0+-1, M9.5+-0.5, M9.5+-0.5, M9.25+-0.25, M8+0.5-0.75, and M8.5+-0.5 for AB Pic b, Cha J110913-773444, USco CTIO 108B, GSC 08047-00232 B, DH Tau B, CT Cha b, and HR7329B, respectively. BT-SETTL and DRIFT-PHOENIX models yield close Teff and log g estimates for each sources. The models seem to evidence a 600-300+600 K drop of the effective temperature at the M-L transition. Assuming the former temperatures are correct, we derive new mass estimates which confirm that DH Tau B, USco CTIO 108B, AB Pic b, KPNO Tau 4, OTS 44, and Cha1109 lay inside or at the boundary of the planetary mass range. We combine the empirical luminosities of the M9.5-L0 sources to the Teff to derive semi-empirical radii estimates that do not match "hot-start" evolutionary models predictions at 1-3 Myr. We use complementary data to demonstrate that atmospheric models are able to reproduce the combined optical and infrared spectral energy distribution, together with the near-infrared spectra of these sources simultaneously. But the models still fail to represent the dominant features in the optical. This issue casts doubts on the ability of these models to predict correct effective temperatures from near-infrared spectra alone. We advocate the use of photometric and spectroscopic data covering a broad range of wavelengths to study the properties of very low mass young companions to be detected with the planet imagers (Subaru/SCExAO, LBT/LMIRCam, Gemini/GPI, VLT/SPHERE).
Highlights
Since the discovery of the first bound substellar objects GD 165 B (Becklin & Zuckerman 1988) and Gl 229 B (Nakajima et al 1995), the development of large infrared surveys has led to an explosion of discoveries of very low-mass stars and mature brown dwarfs in the field
We have retrieved several narrow absorptions of FeH in the spectra of TWA 22 AB, TWA 5B, KPNO Tau 4, OTS 44, 2MASS J01415823-4633574 (2M0141), USco CTIO 108B, CT Cha b, HR7329 B, AB Pic b, and Gl 417 B that were previously identified in the spectra of field M–L dwarfs (Cushing et al 2003)
We decided to make additional tests to evaluate the robustness of the atmospheric models using complementary material found in the literature, the results presented above, and the three synthetic spectral libraries: 1. We plotted the BT-SETTL10 and BT-SETTL12 spectra over the 1.1–2.5 μm range around Teff = 1600−2000 K and confirmed that the spectral slope vary rapidly with the temperature and the surface gravity between 1600 K and 2000 K
Summary
Since the discovery of the first bound substellar objects GD 165 B (Becklin & Zuckerman 1988) and Gl 229 B (Nakajima et al 1995), the development of large infrared surveys has led to an explosion of discoveries of very low-mass stars and mature brown dwarfs in the field. Many of these “ultra-cool dwarfs” failed to enter the MK spectroscopic classification scheme and required the creation of two new classes “L” (Kirkpatrick et al 1999; Martín et al 1999) and “T”. Models aiming at reproducing the emergent flux of ultracool atmospheres showed that these changes are best explained by a decrease in the effective temperature that leads in turn to the formation of a dusty cloud deck below 2700 K (Tsuji et al 1996; Allard et al 2001; Helling et al 2008a)
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