Bibcode
Zapatero Osorio, M. R.; Béjar, V. J. S.; Peña Ramírez, K.
Referencia bibliográfica
The Astrophysical Journal, Volume 842, Issue 1, article id. 65, 23 pp. (2017).
Fecha de publicación:
6
2017
Revista
Número de citas
26
Número de citas referidas
22
Descripción
We have obtained low-resolution optical (0.7–0.98 μm) and
near-infrared (1.11–1.34 μm and 0.8–2.5 μm) spectra of
12 isolated planetary-mass candidates (J = 18.2–19.9 mag) of the 3
Myr σ Orionis star cluster with the aim of determining the
spectroscopic properties of very young, substellar dwarfs and assembling
a complete cluster mass function. We have classified our targets by
visual comparison with high- and low-gravity standards and by measuring
newly defined spectroscopic indices. We derived L0–L4.5 and
M9–L2.5 using high- and low-gravity standards, respectively. Our
targets reveal clear signposts of youth, thus corroborating their
cluster membership and planetary masses (6–13 M Jup).
These observations complete the σ Orionis mass function by
spectroscopically confirming the planetary-mass domain to a confidence
level of ∼75%. The comparison of our spectra with BT-Settl solar
metallicity model atmospheres yields a temperature scale of
2350–1800 K and a low surface gravity of log g ≈ 4.0 [cm
s‑2], as would be expected for young planetary-mass
objects. We discuss the properties of the cluster’s least-massive
population as a function of spectral type. We have also obtained the
first optical spectrum of S Ori 70, a T dwarf in the direction of
σ Orionis. Our data provide reference optical and near-infrared
spectra of very young L dwarfs and a mass function that may be used as
templates for future studies of low-mass substellar objects and
exoplanets. The extrapolation of the σ Orionis mass function to
the solar neighborhood may indicate that isolated planetary-mass objects
with temperatures of ∼200–300 K and masses in the interval
6–13 M Jup may be as numerous as very low-mass stars.
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