Spectral library of age-benchmark low-mass stars and brown dwarfs

Abstract

In recent years, some extremely red brown dwarfs have been discovered. They were believed to have a low surface gravity, but many of their spectral characteristics are similar to those of high-surface-gravity brown dwarfs, showing that the spectral characteristics of young brown dwarfs are poorly understood. We aim to test surface-gravity indicators in late-M and early-L brown dwarf spectra using data obtained with the X-shooter spectrograph at the Very Large Telescope. We select a benchmark sample of brown dwarf members of Chamaeleon I (∼2 Myr), Upper Scorpius (5−10 Myr), the Pleiades (132 ± 27 Myr) and Praesepe (590−790 Myr) with well-constrained ages and similar metallicities. We provide a consistent spectral classification of the sample in the optical and in the near-infrared. We measure the equivalent widths of their alkali lines, finding that they have a moderate correlation with age, especially for objects with spectral types M8 and later. We use spectral indices defined in the literature to estimate surface gravity, finding that their gravity assignment is accurate for 75 per cent of our sample. We investigate the correlation between red colour and age, finding that after ∼10 Myr, the colour does not change significantly for our sample with spectral types M6.0–L3.0. In this case, the red colours might be associated with circumstellar discs, ring structures, extinction, or viewing angle. Finally, we calculate the bolometric luminosity, and J and K bolometric corrections for our sample. We find that six objects are overluminous compared with other members of the same association. These objects are flagged as binary candidates by the Gaia survey.

Demographics of young stars and their protoplanetary disks: lessons learned on disk evolution and its connection to planet formation

“…The analyses by Manara et al (2013bManara et al ( , 2017a have shown how this chromospheric emission scales with stellar temperature in PMS stars, allowing one to discriminate between emission lines dominated by chromospheric or accretion-related emission. In recent years, larger libraries of empirical templates covering broad wavelength ranges (λλ ∼3000-25000 Å) have been collected and are publicly available for low-mass PMS stars and brown dwarfs (Manara et al 2013b(Manara et al , 2017aManjavacas et al 2016Manjavacas et al , 2020. Additional empirical templates have been collected with HST (e.g., Ingleby et al 2014).…”

“…Thanks to its sensitivity and to its location in the Southern Hemisphere, this instrument is being used to survey a number of star-forming regions, including Lupus (Alcalá et al 2014), Chamaeleon I (Manara et al 2016, 2017b, η-Chamaeleon (Rugel et al 2018), TWA (Venuti et al 2019b), and Upper Scorpius (Manara et al 2020). Surveys carried out with this instrument for brown dwarfs (Manara et al 2015;Manjavacas et al 2020) are still incomplete, whereas surveys of the Herbig Ae/Be star populations (Fairlamb et al 2015(Fairlamb et al , 2017 include all the known targets prior to Gaia (Vioque et al 2020) visible from the VLT. Photospheric templates of spectra of non-accreting pre-main sequence stars obtained with this instrument are available for a wide range of spectral types from G-to L-type (Manara et al 2013b(Manara et al , 2017aManjavacas et al 2016Manjavacas et al , 2020.…”

“…Surveys carried out with this instrument for brown dwarfs (Manara et al 2015;Manjavacas et al 2020) are still incomplete, whereas surveys of the Herbig Ae/Be star populations (Fairlamb et al 2015(Fairlamb et al , 2017 include all the known targets prior to Gaia (Vioque et al 2020) visible from the VLT. Photospheric templates of spectra of non-accreting pre-main sequence stars obtained with this instrument are available for a wide range of spectral types from G-to L-type (Manara et al 2013b(Manara et al , 2017aManjavacas et al 2016Manjavacas et al , 2020. For earlier type stars the synthetic spectra by Castelli and Kurucz (2003) are typically used.…”

Synergies of THESEUS with the large facilities of the 2030s and guest observer opportunities

“…IRT will provide high S/N spectra of brown dwarfs with R 400. This resolution is higher than that of the G141 grism on H u b b l e WFC3, allowing “snap style” targets to be obtained and comparative studies, such as those of [ 90 ], to be performed at higher resolution than is currently possible. Rotation studies of brown dwarfs have also been performed with H u b b l e , creating spectral and spatial maps of their surfaces (e.g.…”

Synergies of THESEUS with the large facilities of the 2030s and guest observer opportunities

“…IRT will provide high S/N spectra of brown dwarfs with R∼400. This resolution is higher than that of the G141 grism on Hubble WFC3, allowing "snap style" targets to be obtained and comparative studies such as those of [91] to be performed at higher resolution than is currently possible.…”