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Spectroscopic follow-up of L- and T-type proper-motion member candidates in the Pleiades.

Author/s: M. R. Zapatero Osorio, V.J.S. Béjar, E.L. Martín, M.C. Gálvez-Ortiz, R. Rebolo, G. Bihain, Th. Henning, S. Boudreault, B. Goldman, R. Mundt,K.A. Caballero, P.A. Miles-Páez

Reference: DOI: 10.1051/0004-6361/201424634 | Link

Figure Caption: LIRIS (red dots) and NIRSPEC (black line) spectra of Pleiades proper motion candidates. The NIRSPEC spectrum of Calar 21 is normalized to the <em>K</em>-band LIRIS spectrum. The left panel illustrates the comparison of the Pleiades data with field, high-gravity dwarfs, and the right panel highlights the comparison with young (low-gravity) sources. The reference spectra (blue and gray lines) correspond to 2MASS J22244381<strong>−</strong>0158521 (L4.5, LIRIS data), DENIS J122815.2<strong>−</strong>154733 (L6), and SDSS J042348.57<strong>−</strong>041403.5 (T0) in the left panel, and to G196-3B (L3), 2MASS J00452143<strong>+</strong>1634446 (L2, LIRIS data), 2MASS J03552327<strong>+</strong>1133437 (L5, LIRIS data), and 2MASS J12073347<strong>−</strong>3932540b (L7) in the right panel. To improve the quality of the Pleiades data, the following binning factors (in pixels) were applied: 25 (<em>ZJ</em>) and 29 (<em>HK</em>) for Calar 16, 35 (<em>ZJ</em>) for Calar 18 and 19, 44 (<em>ZJ</em>) and 51 (<em>HK</em>) for Calar 20, 41 (<em>HK</em>) for Calar 21, and 55 (<em>HK</em>) for Calar 22. The vertical error bars of the red dots stand for the errors of the mean fluxes, and the horizontal bars account for the wavelength coverages of the binning intervals. For clarity, regions of strong telluric absorption were intentionally removed from the LIRIS and NIRSPEC spectra, and the data normalized to 1.0 at around 1.3 or 1.68 <strong> μ</strong>m and shifted by a constant.
Figure Caption: LIRIS (red dots) and NIRSPEC (black line) spectra of Pleiades proper motion candidates. The NIRSPEC spectrum of Calar 21 is normalized to the K-band LIRIS spectrum. The left panel illustrates the comparison of the Pleiades data with field, high-gravity dwarfs, and the right panel highlights the comparison with young (low-gravity) sources. The reference spectra (blue and gray lines) correspond to 2MASS J222443810158521 (L4.5, LIRIS data), DENIS J122815.2154733 (L6), and SDSS J042348.57041403.5 (T0) in the left panel, and to G196-3B (L3), 2MASS J00452143+1634446 (L2, LIRIS data), 2MASS J03552327+1133437 (L5, LIRIS data), and 2MASS J120733473932540b (L7) in the right panel. To improve the quality of the Pleiades data, the following binning factors (in pixels) were applied: 25 (ZJ) and 29 (HK) for Calar 16, 35 (ZJ) for Calar 18 and 19, 44 (ZJ) and 51 (HK) for Calar 20, 41 (HK) for Calar 21, and 55 (HK) for Calar 22. The vertical error bars of the red dots stand for the errors of the mean fluxes, and the horizontal bars account for the wavelength coverages of the binning intervals. For clarity, regions of strong telluric absorption were intentionally removed from the LIRIS and NIRSPEC spectra, and the data normalized to 1.0 at around 1.3 or 1.68 μm and shifted by a constant.

We report on the near-infrared low-resolution spectroscopy and red optical (Z-band) photometry of seven proper-motion, very low-mass substellar member candidates of the Pleiades cluster with magnitudes in the interval J=17.5-20.8 and K=16.1-18.5 mag. Spectra were acquired for six objects with the LIRIS and NIRSPEC instruments mounted on the 4.2-m WHT and the 10-m Keck II telescopes. Z-band images of two of the faintest candidates were collected with ACAM/WHT. The new data confirm the low temperatures of all seven Pleiades candidates. From the imaging observations, we find extremely red Z-J and Z-K colors that suggest that the faintest target, Calar Pleiades 25, has a Galactic rather than extragalactic nature. We tentatively classify the spectroscopic targets from early-L to ~T0 and suggest that the L/T transition, which accounts for the onset of methane absorption at 2.1 micron, may take place at J,K ~ 20.3, 17.8 mag in the Pleiades (absolute values of M_J ~ 14.7 and M_K ~ 12.2 mag). We find evidence of likely low-gravity atmospheres based on the presence of triangular-shape H-band fluxes and the high flux ratio K/H (compatible with red H-K colors) of Calar Pleiades 20, 21, and 22, which is a feature also seen in field low-gravity dwarfs. Weak KI absorption lines at around 1.25 micron are probably seen in two targets. These observations add support to the cluster membership of all seven objects in the Pleiades. The trend delineated by the spectroscopic sequence of Pleiades late-M and L dwarfs resembles that of the field. With masses estimated at 0.012-0.015 Msol (solar metallicity and 120 Myr), Calar Pleiades 20 (L6 +/- 1), 21 (L7 +/- 1), and 22 (L/T) may become the coolest and least massive Pleiades members that are corroborated with photometry, astrometry, and spectroscopy. Calar Pleiades 25 (<0.012 Msol) is a firm free-floating planetary-mass candidate in the Pleiades.

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