Properties of the solar neighbor WISE J072003.20−084651.2

Abstract

Context. The severe crowding towards the Galactic plane suggests that the census of nearby stars in that direction may be incomplete. Recently, Scholz reported a new M9 object at an estimated distance d 7 pc (WISE J072003.20−084651.2; hereafter WISE J0720) at Galactic latitude b = 2.3• . Aims. Our goals are to determine the physical characteristics of WISE J0720, its kinematic properties, and to address the question of whether it is a binary object, as was suggested in the discovery paper. Methods. Optical and infrared spectroscopy from the Southern African Large Telescope and Magellan, respectively, and spectral energy distribution fitting were used to determine the spectral type of WISE J0720. The measured radial velocity, proper motion, and parallax yielded its Galactic velocities. We also investigated if WISE J0720 may show X-ray activity based on archival data. Results. Our spectra are consistent with spectral type L0 ± 1. We find no evidence for binarity, apart from a minor 2σ level difference in the radial velocities taken at two different epochs. The spatial velocity of WISE J0720 does not connect it to any known moving group; instead, it places the object with high probability in the old thin disk or in the thick disk. The spectral energy distribution fit hints at excess in the 12 μm and 22 μm WISE bands which may be due to a redder companion, but the same excess is visible in other late-type objects, and it more likely implies a shortcoming of the models (e.g., problems with the effective wavelengths of the filters for these extremely cool objects, etc.) rather than a disk or redder companion. The optical spectrum shows some Hα emission, indicative of stellar activity. Archival X-ray observations yield no detection. Conclusions. WISE J0720 is a new member of the solar neighborhood, the third nearest L dwarf. Our data do not support the hypothesis of its binary nature.

A non-uniform distribution of the nearest brown dwarfs

“…Single objects or multiple systems nearer than 5.16 pc (half-volume distance) are indicated with a yellow circle. Kirkpatrick et al 2014;Pérez Garrido et al 2014), TYC 3980-1081-1 (∼M3.5; Finch et al 2014), and WISE J0720-0846 AB (M9.5+T5; Scholz 2014;Ivanov et al 2015;Burgasser et al 2015a,b) are three recently discovered stars. Both 2MASS J0533-4257 (M4.5) and 2MASS J1845-1409 AB (M5+M5) have provisional distances (Kirkpatrick et al 2012).…”

WISE J072003.20-084651.2B is a Massive T Dwarf∗ †

“…Scholz (2014) speculated that the star could be a close binary system based on the discrepancy between his parallactic and photometric distance determinations, though the two quantities were formally consistent given the large uncertainties. Ivanov et al (2015) found no convincing evidence for binarity based on the integrated-light spectrum and a refined parallax, though they noted a possible indication of RV variability (at the 2σ level) between measurements separated by three days. Burgasser et al (2015a) suggested that the system harbors a T5-dwarf companion, based on visual identification of peculiarities in the H-band integrated-light spectrum consistent with weak methane absorption.…”

“…As part of a search for such objects using the Two-micron All-sky Survey and Wide-field Infrared Survey Explorer (WISE), Scholz (2014) identified WISEJ072003.20−084651.2 (hereinafter WISEJ0720−0846) as a low-latitude (b=+2°.3) ultracool dwarf with an estimated spectral type of M9±1 and a parallactic distance of 6-8pc based on multicatalog photometry and astrometry. Follow-up by Ivanov et al (2015) derived a spectral type of L0±1 based on optical and near-IR (NIR) spectroscopy and refined the parallax measurement, making it the third closest known L dwarf. They detected weak Hα emission but not LiI absorption, consistent with an old age and a mass above the stellar/substellar boundary.…”

Boosting the public engagement with astronomy through arts

“…Stellar flybys like the one during which Proxima joined the Alpha Cen system are extremely rare on human lifetimes, but common by astronomical standards. Our Solar system underwent one only about 70 thousands of years ago [37,6,22], but the passing star had only ∼15 % of the Solar mass (Proxima is comparable -12 % of the Solar mass). Such events rise the question of the fragility of the planetary systems [47].…”