Near-infrared Spectroscopy of Three Nearby L Dwarfs^*

The number of L dwarfs is scarce within 10 pc from the Sun (about 20), but the census is still incomplete (Kirkpatrick et al. 2012). As part of our efforts to characterize this population of small dwarfs within 10 pc from the Sun, we present new near-infrared spectra of three nearby late-L dwarfs reported by Scholz & Bell (2018), including one possibly within 10 pc and a red L dwarf member of the Carina moving group.

We obtained low-resolution near-infrared spectroscopy of all three L dwarf candidates identified by Scholz & Bell (2018) with the Son of ISAAC (SofI) instrument on the 3.5 m New Technology Telescope (NTT; Moorwood et al. 1998). We observed all targets with a 10 slit and the blue and red gratings, covering 0.95–1.64 and 1.53–2.52 microns with a resolving power of ∼600.

We observed 2M0755430−3259589 on the night of 2018 June 22, 2M07414279−0506464 on 2014 April 27, and 2M19251275+0700362 on 2018 November 3 using AB-BA nodding patterns for the sky subtraction. For 2M0755−3259 and 2M1925+0700 we acquired four individual exposures of 300 s in both gratings, and for 2M0741−0506, four and six individual exposures of 180 s in the blue and red gratings, respectively. We observed a B-type standard immediately after each target at a similar airmass to correct for telluric absorption. Dome flats and arcs were taken during the afternoon calibration plan with the the same set-up as our observations.

We flat fielded each frame and subtracted the corresponding A-B and B-A pairs to remove the sky contribution. We aligned and stacked all A-B and B-A frames to create the final 2D image. We repeated the same process with the telluric stars. We extracted the spectra selecting manually the aperture and divided them by the telluric stars after removing stellar lines. We multiplied by blackbodies of similar temperatures as B stars smoothed to our resolution. The final spectra (black) are displayed in Figure 1 and compared to known L dwarfs (red) for classification.

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By comparison with spectroscopic standards, we classify 2M0741−0506 in the near-infrared as a L5–L6 dwarf (top left panel in Figure 1), in agreement with its infrared colors and the classification of Cushing et al. (2018). This source is marked as duplicate in Gaia DR2 and resolved as a 03 binary in DR1 with similar magnitudes (Cushing et al. 2018).

We classify 2M1925+0700 as a L5–L6 dwarf in the infrared (top right panel in Figure 1), slightly earlier than Faherty et al. (2018) who assigned L7. Gaia DR2 gives a proper motion of (${\mu }_{\alpha }\cos \delta$, μ_δ) = (44.9 ± 1.4, 215.2 ± 1.2) mas yr⁻¹, consistent with Scholz & Bell (2018). The Gaia parallax is 89.26 ± 0.67 mas, translating into 11.20 ± 0.08 pc. Its infrared colors appear typical of mid-L dwarfs.

We classify 2M0755−3259 as a red low-gravity L7 ± 1 by direct comparison with near-infrared spectra of red L dwarfs (bottom panel in Figure 1). Its redness lies between the reddest spectra of Schneider et al. (2017) and VHS1256b (Gauza et al. 2015). The potassium lines in the J band are very weak, and the H-band shows a triangular shape, features characteristic of low gravity young brown dwarfs. From its spectroscopic distance of 16 pc, infrared photometry and bolometric corrections of mid-L dwarfs, we infer a luminosity of log(L/L_⊙) = −4.4 dex, a mass of 0.025 ± 0.005 M_⊙, and a temperature of ∼1300 K. This is the first spectral type determination and youth confirmation for 2M0755−3259, indicating that it might indeed be a member of the Carina Young Moving Group (200 ± Myr; Zuckerman et al. 2006) close to the deuterium-burning limit Scholz & Bell (2018).

N.L., V.J.S.B., and A.P.G. were financially supported by the Spanish Ministry of Economy and Competitiveness (MINECO) under the grants AYA2015-69350-C3-2-P and AYA2015-69350-C3-3-P. M.T.R. aknowledges support from project Basal AFB-170002. B.G. acknowledges support from the CONICYT through FONDECYT Postdoctoral Fellowship grant No. 3170513. This work is based on observations collected at the European Southern Observatory, Chile, under programme numbers 093.C-0224(B) (PI Ruiz), 0101.C-0389(A), and 0102.C-0762(A) (PI Chinchilla).

Facilities: VISTA Hemisphere Survey (VHS) - , ESO NTT SofI. -

Software: astropy (Astropy Collaboration et al. 2013), IRAF (Tody 1986, 1993).