Search for the sub-stellar lithium depletion boundary in the open star cluster Coma Berenices

Abstract

Aims. We mainly aim to search for the lithium depletion boundary (LDB) among the sub-stellar population of the open star cluster Coma Berenices. Methods. Since the number of brown dwarf candidates in Coma Ber available in the literature is scarce, we carried out a search for additional candidates photometrically using colour–magnitude diagrams combining optical and infrared photometry from the latest public releases of the following large-scale surveys: the United Kingdom InfraRed Telescope Infrared Deep Sky Survey (UKIRT/UKIDSS), the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), the Sloan Digital Sky Survey (SDSS), and AllWISE. We checked astrometric consistency with cluster membership using Gaia DR2. A search for Li in three new and five previously known brown dwarf candidate cluster members was performed via spectroscopic observations using the OSIRIS instrument at the 10.4 m Gran Telescopio de Canarias (GTC). Results. A couple dozen new photometric candidate brown dwarfs located inside the tidal radius of Coma Ber are reported, but none of these are significantly fainter and cooler than previously known members. No LiI resonance doublet at 6707.8 Å was detected in any of eight Coma Ber targets in the magnitude range J = 15–19 and G = 20–23 observed with the GTC. Spectral types and radial velocities were derived from the GTC spectra. These values confirm the cluster membership of four L2–L2.5 dwarfs, two of which are new in the literature. Conclusions. The large Li depletion factors found among the four bona fide sub-stellar members in Coma Ber implies that the LDB must be located at spectral type later than L2.5 in this cluster. Using the latest evolutionary models for brown dwarfs, a lower limit of 550 Myr on the cluster age is set. This constraint has been combined with other dating methods to obtain an updated age estimate of 780 ± 230 Myr for the Coma Ber open cluster. Identification of significantly cooler sub-stellar cluster members in Coma Ber awaits the advent of the Euclid wide survey, which should reach a depth of about J = 23; this superb sensitivity will make it possible to determine the precise location of the sub-stellar LDB in this cluster and to carry out a complete census of its sub-stellar population.

Light Elements in the Universe

“…We have selected a number of key topics, while we remind to the recent literature for several issues that we will not cover. The latter include the multiple populations of globular clusters (e.g., Gratton et al, 2004Renzini 2008;D'Orazi et al, 2015;Milone et al, 2017;D'Antona et al, 2019;Milone et al, 2020a, Milone et al, 2020b; the abundances of C, N, and O from emission-line spectra of H II regions and planetary nebulae (e.g., Toribio San Cipriano et al, 2016;Toribio San Cipriano et al, 2017;, Esteban et al, 2019Stanghellini and Haywood, 2018); post-main sequence Li evolution (e.g., Charbonnel et al, 2020;Deepak and Reddy, 2020;; and references therein), Li-rich giant stars and their nature (e.g., Casey et al, 2016, Casey et al, 2019Gao et al, 2019;Gonçalves et al, 2020;Jorissen et al, 2020;Martell et al, 2020;Sanna et al, 2020;Wheeler et al, 2020;Yan et al, 2020); boron abundances in massive stars (e.g., Proffitt et al, 2016, and references therein); light elements in the context of exoplanet hosting stars (e.g., Delgado Mena et al, 2012;Delgado Mena et al, 2014); the lithium depletion boundary (see, e.g., Stauffer, 2000;Lodieu, 2020;Martín et al, 2020; an references therein); 6 Li/ 7 Li isotopic ratio and its implications for Li nucleosynthesis (e.g., González Hernández et al, 2019, and references therein).…”

Astrometry and Photometry for ≈1000 L, T, and Y Dwarfs from the UKIRT Hemisphere Survey

“…The Coma Ber Cluster (CBER) is the second closest open cluster to the Sun after the Hyades, and has recent age estimates of 700-800 Myr (Tang et al 2018;Martín et al 2020;Sapozhnikov & Kovaleva 2021;Singh et al 2021). Two CBER candidates were found in this search.…”

New constraints on the minimum mass for thermonuclear lithium burning in brown dwarfs

“…Following the same procedure as in our previous work in the nearby open clusters the Hyades (Martín et al 2018) and Coma Berenices (Martín et al 2020), where we used similar instrumental setup, spectral types were derived by comparison with field late-M and L dwarfs from the Sloan database (Bochanski et al 2010;Schmidt et al 2010). The GTC/OSIRIS mean spectrum of the Sahlmann 2 AB system was found to be intermediate between the M9 and the L0 standards, and hence it is classified as M9.5±0.5 type, i.e., one spectral subclass later than the classification of M8.5 reported by Phan-Bao et al (2008).…”