Search

The coldest metal-poor population made of T and Y dwarfs are archaeological tracers of our Galaxy because they are very old and have kept the pristine material. The optical properties of these objects are important to characterise their atmospheric properties. We aim at characterising further the optical properties of ultracool metal-poor population with deep far-red optical images and parallax determinations. We solve trigonometric parallaxes of five metal-poor T dwarf candidates using 2-year monitoring with Calar-Alto 3.5-m telescope. We obtain $z'$-band photometry for the other 12 metal-poor T dwarf candidates using the 10.4-m GTC, the 8.2-m VLT, and the DES, increasing the sample of T subdwarfs with optical photometry from 12 to 24. We report a 3-$\sigma$ limit for the Accident in five optical bands using the 10.4-m GTC. We confirm four T subdwarfs and the Accident as a Y subdwarf, and propose two more Y subdwarf candidates. We emphasise that the $z_{PS1}-W1$ colour combining with the $W1-W2$ colour could break the metallicity-temperature degeneracy for T and possibly for Y dwarfs. The $z_{PS1}-W1$ colour shifts redward when metallicity decreases for a certain temperature, which is not predicted by state-of-the-art ultracool models. The Accident has the reddest $z_{PS1}-W1$ colour among our sample. The $z_{PS1}-W1$ colour will be useful to search for other examples of this cold and old population in upcoming and existing deep optical and infrared large-area surveys., Comment: 17 pages, 7 figures, 2 appendices, submitted to A&A, comments are welcome

Context. Euclid will carry out a deep survey benefiting the discovery and characterisation of ultracool dwarfs (UCDs), especially in the Euclid Deep Fields (EDFs), which the telescope will scan repeatedly throughout its mission. The photometric and spectroscopic standards in the EDFs are important benchmarks, crucial for the classification and characterisation of new UCD discoveries and for the calibration of the mission itself. Aims. We aim to provide a list of photometric UCD candidates and collect near-infrared reconnaissance spectra for M, L, and T-type UCDs in the EDFs as future Euclid UCD references. Methods. In EDF North, we cross-matched public optical and infrared surveys with certain photometric criteria to select UCDs. In EDF Fornax and EDF South, we used photometrically classified samples from the literature. We also include UCDs identified by Gaia DR2. We selected 7 UCD targets with different spectral types from the lists and obtained low-resolution 0.9-2.5 {\mu}m spectra of them using GTC/EMIR and the VLT/X-shooter. We also selected a young, bright L dwarf near EDF Fornax to test the coherence of these two facilities. We included an extra T dwarf in EDF North with its published J-band spectrum. Results. We retrieved a list of 81 (49, 231) M, 8 (29, 115) L, and 1 (0, 2) T dwarf candidates in EDF North, Fornax, and South, respectively. They are provided to guide future UCD discoveries and characterisations by Euclid. In total, we collected near-infrared spectra for 9 UCDs, including 2 M types, 3 L types, and 4 T types in or close to the 3 EDFs. The Euclidised spectra show consistency in their spectral classification, which demonstrates that slitless Euclid spectroscopy will recover the spectral types with high fidelity for UCDs, both in the EDFs and in the wide survey. We also demonstrate that Euclid will be able to distinguish different age groups of UCDs., Comment: 9 pages, 4 figures, 3 appendices, accepted for publication in A&A on Mar 12 2024. Late-M-type, and L-type UCD lists in EDF North were corrected, reference added

Observations of five Y dwarfs with three optical and near-infrared instruments at the 10.4 m Gran Telescopio Canarias are reported. Deep images of the five targets and a low-resolution far-red optical spectrum for one of the targets were obtained. One of the Y dwarfs, WISE J173835+273258 (Y0), was clearly detected in the optical (z- and i-bands) and another, WISE J182831+265037 (Y2), was detected only in the z-band. We measured the colours of our targets and found that the z-J and i-z colours of the Y dwarfs are bluer than those of mid- and late-T dwarfs. This optical blueing has been predicted by models, but our data indicates that it is sharper and happens at temperatures about 150 K warmer than expected. Likely, the culprit is the K I resonance doublet, which weakens more abruptly in the T- to Y-type transition than expected. We show that the alkali resonance lines (Cs I and K I) are weaker in Y dwarfs than in T dwarfs; the far-red optical spectrum of WISE J173835+273258 is similar to that of late-T dwarfs, but with stronger methane and water features; and we noted the appearance of new absorption features that we propose could be due to hydrogen sulphide. The optical properties of Y dwarfs presented here pose new challenges to understanding grain sedimentation in extremely cool objects. The weakening of the very broad K I resonance doublet due to condensation in dust grains is more abrupt than theoretically anticipated. Consequently, the observed blueing of the z-J and i-z colours of Y dwarfs with respect to T dwarfs is more pronounced than predicted by models and could boost the potential of upcoming deep large-area optical surveys regarding their ability to detect extremely cool objects, Comment: accepted for publication in Astronomy and Astrophysics

The ESA space mission Euclid was launched on July 1st, 2023 and is undergoing its science verification phase. In this invited review we show that Euclid means a before and an after for our understanding of ultra-cool dwarfs and substellar-mass objects and their connections with stars, exoplanets and the Milky Way. Euclid enables the study with unprecedented statistical significance a very large ensemble of ultracool dwarfs, the identification of new types of substellar objects, and the determination of the substellar binary fraction and the Initial Mass Function (IMF) in diverse galactic environments from the nearest stellar nurseries to the ancient relics of Galactic formation., Comment: 8 pages

Context. Metal-poor brown dwarfs are poorly understood because they are extremely faint and rare. Only a few candidates have been identified as T-type subdwarfs in infrared surveys and their optical properties remain unconstrained. Aims. We aim to improve the knowledge of the optical properties of T subdwarf candidates to break the degeneracy between metallicity and temperature and to investigate their atmospheric properties. Methods. Deep $z$-band images of 10 known T subdwarf candidates were collected with the 10.4-m Gran Telescopio Canarias. Low-resolution optical spectra for two of them were obtained with the same telescope. Photometric measurements of the $z$-band flux were performed for all the targets and they were combined with infrared photometry in $J, H, K, W1$ and $W2$-bands from the literature to obtain the colours. The spectra were compared with solar-metallicity T dwarf templates and with laboratory spectra. Results. We found that the targets segregate into three distinct groups in the $W1 - W2$ vs. $z - W1$ colour-colour diagram. Group I objects are mixed with solar-metallicity T dwarfs. Group III objects have $W1 - W2$ colours similar to T dwarfs but very red $z - W1$ colours. Group II objects lie between Group I and III. The two targets for which we obtained spectra are located in Group I and their spectroscopic properties resemble normal T dwarfs but with water features that are deeper and have a shape akin to pure water. Conclusions. We conclude that the $W1 - W2$ vs. $z - W1$ colour-colour diagram is excellent to break the metallicity-temperature degeneracy for objects cooler than L-type. A revision of the spectral classification of T subdwarf might be needed in the future, according to the photometric and spectroscopic properties of WISE1810 and WISE0414 in Group III discussed in this work., Comment: 13 pages, 7 figures, accepted for publication in A&A on 18th Aug 2023

We report the discovery and characterization of TOI-1759~b, a temperate (400 K) sub-Neptune-sized exoplanet orbiting the M~dwarf TOI-1759 (TIC 408636441). TOI-1759 b was observed by TESS to transit on sectors 16, 17 and 24, with only one transit observed per sector, creating an ambiguity on the orbital period of the planet candidate. Ground-based photometric observations, combined with radial-velocity measurements obtained with the CARMENES spectrograph, confirm an actual period of $18.85019 \pm 0.00014$ d. A joint analysis of all available photometry and radial velocities reveal a radius of $3.17 \pm 0.10\,R_\oplus$ and a mass of $10.8 \pm 1.5\,M_\oplus$. Combining this with the stellar properties derived for TOI-1759 ($R_\star = 0.597 \pm 0.015\,R_\odot$; $M_\star = 0.606 \pm 0.020\,M_\odot$; $T_{\textrm{eff}} = 4065 \pm 51$ K), we compute a transmission spectroscopic metric (TSM) value of over 80 for the planet, making it a good target for transmission spectroscopy studies. TOI-1759 b is among the top five temperate, small exoplanets ($T_\textrm{eq} < 500$ K, $R_p < 4 \,R_\oplus$) with the highest TSM discovered to date. Two additional signals with periods of 80 d and $>$ 200 d seem to be present in our radial velocities. While our data suggest both could arise from stellar activity, the later signal's source and periodicity are hard to pinpoint given the $\sim 200$ d baseline of our radial-velocity campaign with CARMENES. Longer baseline radial-velocity campaigns should be performed in order to unveil the true nature of this long period signal., Comment: 22 pages, 10 figures, 7 tables. AJ in press

The theory of substellar evolution predicts that there is a sharp mass boundary between lithium and non-lithium brown dwarfs, not far below the substellar-mass limit. The imprint of thermonuclear burning is carved on the surface lithium abundance of substellar-mass objects during the first few hundred million years of their evolution, leading to a sharp boundary between lithium and non-lithium brown dwarfs, so-called, the lithium test. New optical spectroscopic observations of the binaries DENIS+J063001.4-184014 and DENIS+J225210.7-173013 obtained using the 10.4-m Gran Telescopio de Canarias are reported here. They allow us to re-determine their combined optical spectral types (M9.5 and L6.5, respectively) and to search for the presence of the LiI resonance doublet. The non detection of the LiI feature in the combined spectrum of DENIS\,J063001.4$-$184014AB is converted into estimates for the depletion of lithium in the individual components of this binary system. In DENIS\,J225210.7$-$173013AB we report the detection of a weak LiI feature which we tentatively ascribe as arising from the contribution of the T3.5-type secondary. Combining our results with data for seven other brown dwarf binaries in the literature treated in a self-consistent way, we confirm that there is indeed a sharp transition in mass for lithium depletion in brown dwarfs, as expected from theoretical calculations. We estimate such mass boundary is observationally located at 51.48$^{+0.22}_{-4.00}$ $M_\mathrm{Jup}$, which is lower than the theoretical determinations., Comment: 12 pages, 6 figures, accepted for publication in MNRAS

We present updated radial-velocity (RV) analyses of the AU Mic system. AU Mic is a young (22 Myr) early M dwarf known to host two transiting planets - $P_{b}\sim8.46$ days, $R_{b}=4.38_{-0.18}^{+0.18}\ R_{\oplus}$, $P_{c}\sim18.86$ days, $R_{c}=3.51_{-0.16}^{+0.16}\ R_{\oplus}$. With visible RVs from CARMENES-VIS, CHIRON, HARPS, HIRES, {\sc {\textsc{Minerva}}}-Australis, and TRES, as well as near-infrared (NIR) RVs from CARMENES-NIR, CSHELL, IRD, iSHELL, NIRSPEC, and SPIRou, we provide a $5\sigma$ upper limit to the mass of AU Mic c of $M_{c}\leq20.13\ M_{\oplus}$ and present a refined mass of AU Mic b of $M_{b}=20.12_{-1.57}^{+1.72}\ M_{\oplus}$. Used in our analyses is a new RV modeling toolkit to exploit the wavelength dependence of stellar activity present in our RVs via wavelength-dependent Gaussian processes. By obtaining near-simultaneous visible and near-infrared RVs, we also compute the temporal evolution of RV-``color'' and introduce a regressional method to aid in isolating Keplerian from stellar activity signals when modeling RVs in future works. Using a multi-wavelength Gaussian process model, we demonstrate the ability to recover injected planets at $5\sigma$ significance with semi-amplitudes down to $\approx$ 10\,m\,s$^{-1}$ with a known ephemeris, more than an order of magnitude below the stellar activity amplitude. However, we find that the accuracy of the recovered semi-amplitudes is $\sim$50\% for such signals with our model.

Double-lined spectroscopic binaries (SB2) allow us to determine a lower limit of the masses of their components directly to test stellar models. In this work, our aim is to derive the orbital and physical parameters of GJ1284, a young SB2. We also revise the membership of this system and its two wide co-moving companions, GJ898 and GJ897AB, to a young moving group to assess, along with other youth indicators, their age. Afterwards, we compare the results from these analyses and the photometry of these systems with several pre-main-sequence evolutionary models. We determine the orbit of the GJ1284 system alongside its systemic velocity from high resolution spectra. Additionally, we use TESS photometry to derive the rotational period of the GJ1284 and its two wide companions. GJ1284 is a binary system located at approximately 16 pc with an eccentric orbit ($ e = 0.505 $) of 11.83 d period made up of an M2-M2.5 + M3-M3.5. The revised systemic velocity of $ \gamma = 0.84 \pm 0.14\,\mathrm{km\,s}^{-1} $ suggests that it is a member of the Local Association. The kinematics together with other activity and youth indicators imply an age of 110-800 Myr for this system and its two companions. The isochronal ages derived from the comparison of the photometry with several evolutionary models are younger than the age estimated from the activity indicators for the three co-moving systems. The masses for the components of GJ1284, derived from their luminosity and age using the different models, are not consistent with the masses derived from the photometry, except for the PARSEC models, but are compatible with dynamical masses of double-lined eclipsing binaries with similar ages and spectral types. The effect of magnetic activity in the form of spots can reconcile to some extent the photometric and dynamical masses, but is not considered in most of the evolutionary models., Comment: 18 pages, 13 figures, Accepted for publication in A&A

We highlight the importance of eclipsing double-line binaries in our understanding on star formation and evolution. We review the recent discoveries of low-mass and sub-stellar eclipsing binaries belonging to star-forming regions, open clusters, and globular clusters identified by ground-based surveys and space missions with high-resolution spectroscopic follow-up. These discoveries provide benchmark systems with known distances, metallicities, and ages to calibrate masses and radii predicted by state-of-the-art evolutionary models to a few percent. We report their density and discuss current limitations on the accuracy of the physical parameters. We discuss future opportunities and highlight future guidelines to fill gaps in age and metallicity to improve further our knowledge of low-mass stars and brown dwarfs., Comment: 30 pages, 5 figures, no table. Review paper

We present the results of a study to discover prospective new white dwarf-L dwarf binaries as identified by their near-infrared excesses in the UKIDSS catalogue. We obtained optical spectra to validate the white dwarf nature for 22 of the candidate primary stars, confirming ten as white dwarfs and determining their effective temperatures and gravities. For all ten white dwarfs we determined that the near-infrared excess was indeed indicative of a cool companion. Six of these are suggestive of late M dwarf companions, and three are candidate L dwarf companions, with one straddling the M$-$L boundary. We also present near-infrared spectra of eight additional candidate white dwarf-ultracool dwarf binaries where the white dwarf primary had been previously confirmed. These spectra indicate one candidate at the M$-$L boundary, three potential L dwarf companions, and one suspected M dwarf, which showed photometric variability on a $\sim$6 hour period, suggesting the system may be close. Radial velocity follow up is required to confirm whether these systems are close, or widely separated., Comment: Accepted to MNRAS

MAAT is proposed as a visitor mirror-slicer optical system that will allow the OSIRIS spectrograph on the 10.4-m Gran telescopio CANARIAS (GTC) the capability to perform Integral Field Spectroscopy (IFS) over a seeing-limited FoV 14.20''x10'' with a slice width of 0.303''. MAAT@GTC will enhance the resolution power of OSIRIS by 1.6 times as compared to its 0.6'' wide long-slit. All the eleven OSIRIS grisms and volume-phase holographic gratings will be available to provide broad spectral coverage with moderate resolution (R=600 up to 4100) in the 3600 - 10000 {\AA} wavelength range. MAAT unique observing capabilities will broaden its use to the needs of the GTC community to unveil the nature of most striking phenomena in the universe well beyond time-domain astronomy. The GTC equipped with OSIRIS+MAAT will also play a fundamental role in synergy with other facilities, some of them operating on the northern ORM at La Palma. This White Paper presents the different aspects of MAAT@GTC - including scientific and technical specifications, outstanding science cases, and an outline of the instrument concept., Comment: 49 pages, 36 figures

The mass of a star is the most fundamental parameter for its structure, evolution, and final fate. It is particularly important for any kind of stellar archaeology and characterization of exoplanets. There exists a variety of methods in astronomy to estimate or determine it. In this review we present a significant number of such methods, beginning with the most direct and model-independent approach using detached eclipsing binaries. We then move to more indirect and model-dependent methods, such as the quite commonly used isochrone or stellar track fitting. The arrival of quantitative asteroseismology has opened a completely new approach to determine stellar masses and to complement and improve the accuracy of other methods. We include methods for different evolutionary stages, from the pre-main sequence to evolved (super)giants and final remnants. For all methods uncertainties and restrictions will be discussed. We provide lists of altogether more than 200 benchmark stars with relative mass accuracies between $[0.3,2]\%$ for the covered mass range of $M\in [0.1,16]\,\msun$, $75\%$ of which are stars burning hydrogen in their core and the other $25\%$ covering all other evolved stages. We close with a recommendation how to combine various methods to arrive at a "mass-ladder" for stars., Comment: Invited review article for The Astronomy and Astrophysics Review. 146 pages, 16 figures, 11 tables. Accepted version by the Journal. It includes summary figure of accuracy/precision of methods for mass ranges and summary table for individual methods

We mainly aim to search for the lithium depletion boundary (LDB) among the sub-stellar population of the open star cluster Coma Berenices. We carried out a search for brown dwarf (BD) candidates using colour-magnitude diagrams combining optical and infrared photometry from the latest public releases of the following large-scale surveys: UKIRT/UKIDSS, Pan-STARRS, SDSS, and AllWISE. We checked astrometric consistency with cluster membership using $Gaia$ DR2. A couple dozen new candidate BDs located inside the tidal radius of Coma Ber are reported, but none of these are significantly fainter and cooler than previously known members. A search for Li in three new and five previously known BD candidate cluster members was performed via spectroscopic observations using the OSIRIS instrument at the 10.4-m GTC. No LiI resonance doublet at 6707.8 A 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. The large Li depletion factors found among the four bona fide BD 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 BDs, 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$\pm$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., Comment: 10 pages, 7 figures, accepted for publication in A&A

(Abridged) Our aim is to identify analogues of gas giant planets, as companions at wide separations of very young stars. To identify these objects, we cross correlated the Visible and Infrared Survey Telescope for Astronomy (VISTA) Hemisphere Survey (VHS) and the United Kingdom Infrared Telescope Infrared Deep Sky Survey Galactic Clusters Survey (UKIDSS GCS) catalogues to search for common proper motion companions to 1195 already known members of Upper Scorpius (USco; age $\sim$5-10 Myr, distance $\sim$145 pc). We present the discovery and spectroscopic characterisation of two very wide substellar companions of two early-M stars in Upper Scorpius: USco1621 B and USco1556 B. We obtained optical and near-infrared low-resolution spectroscopy of the candidates to characterise their spectral energy distribution and confirm their membership to the association. We also acquired adaptive optics images of the primaries and secondaries to search for signs of binarity and close companions. We determined a spectral type of M8.5 in the optical for both companions, along with L0 and L0.5 in the near-infrared for USco1621 B and USco1556 B, respectively. The spectra of the two companions show evident markers of youth. The comparison with theoretical evolutionary models gives estimated masses of 0.015$\pm$0.002 and 0.014$\pm$0.002 M$_{\odot}$, with temperatures of 2270$\pm$90 and 2240$\pm$100 K, respectively. The physical separations between the components of both systems are 2880$\pm$20 and 3500$\pm$40 AU for USco1621 and USco1556 systems, respectively. We did not find any additional close companion in the adaptive optics images. The probability that the two secondaries are physically bound to their respective primaries, and not chance alignments of USco members, is 86%, and the probability that none of them are physically related is 1.0%., Comment: 17 pages, 15 figures, accepted for publication in A&A

(Abridged) The Maunakea Spectroscopic Explorer (MSE) is an end-to-end science platform for the design, execution and scientific exploitation of spectroscopic surveys. It will unveil the composition and dynamics of the faint Universe and impact nearly every field of astrophysics across all spatial scales, from individual stars to the largest scale structures in the Universe. Major pillars in the science program for MSE include (i) the ultimate Gaia follow-up facility for understanding the chemistry and dynamics of the distant Milky Way, including the outer disk and faint stellar halo at high spectral resolution (ii) galaxy formation and evolution at cosmic noon, via the type of revolutionary surveys that have occurred in the nearby Universe, but now conducted at the peak of the star formation history of the Universe (iii) derivation of the mass of the neutrino and insights into inflationary physics through a cosmological redshift survey that probes a large volume of the Universe with a high galaxy density. MSE is positioned to become a critical hub in the emerging international network of front-line astronomical facilities, with scientific capabilities that naturally complement and extend the scientific power of Gaia, the Large Synoptic Survey Telescope, the Square Kilometer Array, Euclid, WFIRST, the 30m telescopes and many more., Comment: 9 chapters, 301 pages, 100 figures. This version of the DSC is a comprehensive update of the original version, released in 2016, which can be downloaded at arXiv:1606.00043. A detailed summary of the design of MSE is available in the MSE Book 2018, available at arXiv:1810.08695

The lowest-mass stars, brown dwarfs and giant exoplanets span a minimum in the mass-radius relationship that probes the fundamental physics of extreme states of matter, magnetism, and fusion. This White Paper outlines scientific opportunities and the necessary resources for modeling and measuring the mass-radius relationship in this regime., Comment: 7 pages, submitted to Astro2020 Science White Paper call

The Maunakea Spectroscopic Explorer (MSE) is a planned 11.25-m aperture facility with a 1.5 square degree field of view that will be fully dedicated to multi-object spectroscopy. A rebirth of the 3.6m Canada-France-Hawaii Telescope on Maunakea, MSE will use 4332 fibers operating at three different resolving powers (R ~ 2500, 6000, 40000) across a wavelength range of 0.36-1.8mum, with dynamical fiber positioning that allows fibers to match the exposure times of individual objects. MSE will enable spectroscopic surveys with unprecedented scale and sensitivity by collecting millions of spectra per year down to limiting magnitudes of g ~ 20-24 mag, with a nominal velocity precision of ~100 m/s in high-resolution mode. This white paper describes science cases for stellar astrophysics and exoplanet science using MSE, including the discovery and atmospheric characterization of exoplanets and substellar objects, stellar physics with star clusters, asteroseismology of solar-like oscillators and opacity-driven pulsators, studies of stellar rotation, activity, and multiplicity, as well as the chemical characterization of AGB and extremely metal-poor stars., Comment: 31 pages, 11 figures; To appear as a chapter for the Detailed Science Case of the Maunakea Spectroscopic Explorer

WFIRST will be capable of providing Hubble-quality imaging performance over several thousand square degrees of the sky. The wide-area, high spatial resolution survey data from WFIRST will be unsurpassed for many decades into the future. With the current baseline design, the WFIRST filter complement will extend from the bluest wavelength allowed by the optical design to a reddest filter (F184W) that has a red cutoff at 2.0 microns. In this white paper, we outline some of the science advantages for adding a Ks filter with a 2.15 micron central wavelength in order to extend the wavelength coverage for WFIRST as far to the red as the possible given the thermal performance of the observatory and the sensitivity of the detectors., Comment: 14 Pages, 10 figures, white paper for WFIRST

This chapter reviews our current knowledge of metal-poor ultracool dwarfs with spectral types later than M7. The current census of M, L, and T subdwarfs is explored. The main colour trends of subdwarfs from the optical to the mid-infrared are described and their spectral features presented, which led to a preliminary and tentative spectral classification subject to important changes in the future when more of these metal-poor objects are discovered. Their multiplicity and the determination of their physical parameters (effective temperature, gravity, metallicity, and mass) are discussed. Finally, some suggestions and future guidelines are proposed to foster our knowledge on the oldest and coolest members of our Galaxy., Comment: 12 pages, 3 figures, Review chapter to appear in "Handbook of Exoplanets", eds. Deeg, H.J. & Belmonte, J.A, Springer (2018)

Determination of the lithium depletion boundary (LDB), i.e., the observational limit below which the cores of very low-mass objects do not reach high enough temperature for Li destruction, has been used to obtain ages for several open clusters and stellar associations younger than 200 Myr, which until now has been considered as the practical upper limit on the range of applicability of this method. In this work we show that the LDB method can be extended to significant older ages than previously thought. Intermediate resolution optical spectra of six L-type candidate members in the Hyades cluster obtained using OSIRIS at the 10.4-m Gran Telescopio Canarias are presented. The Li~I 670.8~nm resonance doublet is clearly detected only in the two faintest and coolest of these objects, which are classified as L3.5 to L4 brown dwarf cluster members with luminosities around 10$^{-4}$ solar. Lithium depletion factors are estimated for our targets with the aid of synthetic spectra and they are compared with predictions from evolutionary models. A LDB age of 650$\pm$70 Myr for the Hyades provides a consistent description of our data using a set of state-of-the-art evolutionary models for brown dwarfs calculated by \citet{2015A&A..577..42}., Comment: 10 pages, 4 figures, accepted for publication in The Astrophysical Journal (February 2018)

Our aim is to identify brown dwarf members of the nearby Hyades open star cluster to determine the photometric and spectroscopic properties of brown dwarfs at moderately old ages and extend the knowledge of the substellar mass function of the cluster. We cross-matched the 2MASS and AllWISE public catalogues and measured proper motions to identify low-mass stars and brown dwarf candidates in an area of radius eight degrees around the central region of the Hyades cluster. We identified objects with photometry and proper motions consistent with cluster membership. For the faintest ($J$\,=\,17.2 mag) most promising astrometric and photometric low-mass candidate 2MASS\,J04183483$+$2131275\@, with a membership probability of 94.5\%, we obtained low-resolution (R\,=\,300--1000) and intermediate-resolution (R\,=\,2500) spectroscopy with the 10.4m Gran Telescopio Canarias. From the low-resolution spectra we determined a L5.0$\pm$0.5 spectral type, consistent with the available photometry. In the intermediate dispersion spectrum we detected H$\alpha$ in emission (marginally resolved with a full width half maximum of $\sim$2.8 \AA{}) and determined a $\log ($L$_{\rm{H}\alpha}$/L$_{\rm bol})=-6.0$ dex. From H$\alpha$ we obtained a radial velocity of 38.0$\pm$2.9 km$\cdot$s$^{-1}$, which combined with the proper motion leads to space velocities which are fully consistent with membership in the Hyades cluster.

In a search for common proper motion companions using the VISTA Hemisphere Survey and 2MASS catalogs we have identified a very red (J-Ks=2.47 mag) late-L dwarf companion of a previously unrecognized M dwarf VHS J125601.92-125723.9, located at a projected angular separation of 8.06"+/-0.03". From low-resolution optical and near-IR spectroscopy we classified the primary and the companion as an M7.5+/-0.5 and L7+/-1.5, respectively. The primary shows weaker alkali lines than field dwarfs of similar spectral type, but still consistent with either a high-gravity dwarf or a younger object of hundreds of millions of years. The secondary shows spectral features characteristic for low surface gravity objects at ages below several hundred Myr, like the triangular shape of the H-band continuum and alkali lines weaker than in field dwarfs of the same spectral type. The absence of lithium in the atmosphere of the primary and the likely membership to the Local Association allowed us to constrain the age of the system to the range of 150-300 Myr. We report a measurement of the trigonometric parallax pi=78.8+/-6.4 mas, which translates into a distance of 12.7+/-1.0 pc; the pair thus has a projected physical separation of 102+/-9 AU. We derived the Lbol of the components and compared them with theoretical evolutionary models to estimate the masses and effective temperatures. For the primary, we determined log(Lbol/LSun)=-3.14+/-0.10, and a mass of 73 (+20,-15} MJup at the boundary between stars and brown dwarfs and Teff of 2620+/-140 K. For the companion we obtained log(Lbol/LSun)=-5.05+/-0.22 and a mass of 11.2 (+9.7,-1.8) MJup placing it near the deuterium-burning mass limit. The effective temperature derived from evolutionary models is 880 (+140,-110) K, about 400-700 K cooler than expected for field late-L dwarfs., Comment: 19 pages, 13 figures. Accepted for publication in ApJ

Abridged: We report the discovery of two, new, rare, wide, double-degenerate binaries that each contain a magnetic and a non-magnetic star. The components of SDSSJ092646.88+132134.5 + J092647.00+132138.4 and SDSSJ150746.48+521002.1 + J150746.80+520958.0 have angular separations of only 4.6 arcsec (a~650AU) and 5.1 arcsec (a~750AU), respectively. They also appear to share common proper motions. Follow-up optical spectroscopy reveals each system to consist of a DA and a H-rich high-field magnetic white dwarf (HFMWD). Our measurements of the effective temperatures and the surface gravities of the DA components reveal both to have larger masses than are typical of field white dwarfs. By assuming that these degenerates have evolved essentially as single stars, due to their wide orbital separations, we use them to place limits on the total ages of our stellar systems. These argue that in each case the HFMWD is probably associated with an early type progenitor (M_init > 2M_solar). We find that the cooling time of SDSSJ150746.80+520958.0 (DAH) is somewhat lower than might be expected had it followed the evolutionary path of a typical single star. This mild discord is in the same sense as that observed for two of the small number of other HFMWDs for which progenitor mass estimates have been made, REJ0317-853 and EG59. The mass of the other DAH, SDSSJ092646.88+132134.5, appears to be smaller than expected on the basis of single star evolution. If this object was/is a member of a hierarchical triple system it may have experienced greater mass loss during an earlier phase of its life as a result of it having a close companion. The large uncertainties on our estimates of the parameters of the HFMWDs suggest a larger sample of these objects is required to firmly identify any trends in their inferred cooling times and progenitor masses., Comment: Accepted for MNRAS

The discovery of epsilon Indi Ba, Bb, a binary brown dwarf system very close to the Sun, makes possible a concerted campaign to characterise the physical parameters of two T dwarfs. Recent observations suggest substellar atmospheric and evolutionary models may be inconsistent with observations, but there have been few conclusive tests to date. We therefore aim to characterise these benchmark brown dwarfs to place constraints on such models. We have obtained high angular resolution optical, near-infrared, and thermal-infrared imaging and medium-resolution (up to R~5000) spectroscopy of epsilon Indi Ba, Bb with the ESO VLT and present VRIzJHKL'M' broad-band photometry and 0.63--5.1 micron spectroscopy of the individual components. Furthermore, we use deep AO-imaging to place upper limits on the (model-dependent) mass of any further system members. We derive luminosities of log L/L_sun = -4.699+/-0.017 and -5.232+/-0.020 for epsilon Indi Ba, Bb, respectively, and using the dynamical system mass and COND03 evolutionary models predict a system age of 3.7--4.3 Gyr, in excess of previous estimates and recent predictions from observations of these brown dwarfs. Moreover, the effective temperatures of 1352--1385 K and 976--1011 K predicted from the COND03 evolutionary models, for epsilon Indi Ba and Bb respectively, are in disagreement with those derived from the comparison of our data with the BT-Settl atmospheric models where we find effective temperatures of 1300--1340 K and 880--940 K, for epsilon Indi Ba and Bb respectively, with surface gravities of log g=5.25 and 5.50. Finally, we show that spectroscopically determined effective temperatures and surface gravities for ultra-cool dwarfs can lead to underestimated masses even where precise luminosity constraints are available., Comment: 27 pages, 30 figures, 9 tables, accepted for publication in Astronomy and Astrophysics

The discovery of Epsilon Indi Ba and Bb, a nearby binary brown dwarf system with a main-sequence companion, allows a concerted campaign to characterise the physical parameters of two T dwarfs providing benchmarks against which atmospheric and evolutionary models can be tested. Some recent observations suggest the models at low mass and intermediate age may not reflect reality with, however, few conclusive tests. We are carrying out a comprehensive characterisation of these, the nearest known brown dwarfs, to allow constraints to be placed upon models of cool field dwarfs. We present broadband photometry from the V- to M-band and the individual spectrum of both components from 0.6-5.1 microns at a resolution of up to R=5000. A custom analytic profile fitting routine was implemented to extract the blended spectra and photometry of both components separated by 0.7 arcsec. We confirm the spectral types to be T1 and T6, and notably, we do not detect lithium at 6708A in the more massive object which may be indicative both of the age of the system and the mass of the components., Comment: 4 pages, 2 figures, to appear in proceedings of Cool Stars 15

Ultracool subdwarfs are low luminosity, late-type M and L dwarfs that exhibit spectroscopic indications of subsolar metallicity and halo kinematics. Their recent discovery and ongoing investigation have led to new insights into the role of metallicity in the opacity structure, chemistry (e.g. dust formation) and evolution of low-temperature atmospheres; the long-term evolution of magnetic activity and angular momentum amongst the lowest-mass stars; the form of the halo luminosity and mass functions down to the hydrogen-burning mass limit; and even fundamental issues such as spectral classification and absolute brightness scales. This Splinter Session was devoted to bringing advances in observational and theoretical ultracool subdwarf research to the attention of the low-mass stellar and brown dwarf communities, as well as to share results among ultracool subdwarf enthusiasts., Comment: 8 pages, 5 figures, Splinter Session contribution for Cool Stars 15 (AIP formatting)

We present a deep, wide-field optical survey of the young stellar cluster Alpha Per, in which we have discovered a large population of candidate brown dwarfs. Subsequent infrared photometric follow-up shows that the majority of them are probable or possible members of the cluster, reaching to a minimum mass of 0.035 Msun. We have used this list of members to derive the luminosity and mass functions of the substellar population of the cluster (alpha=0.59+-0.05) and compared its slope to the value measure for the Pleiades. This comparison indicates that the two cluster mass functions are, indeed, very similar., Comment: Astronomy and Astrophysics, accepted

We have identified two late M dwarfs within 10 parsecs of the Sun, by cross-correlating the Luyten NLTT catalogue of stars with proper motions larger than 0.18 arcsec/yr, with objects lacking optical identification in the 2MASS data base. The 2MASS photometry was then combined with improved optical photometry obtained from the SuperCOSMOS Sky Surveys. The two objects (LP775-31 and LP655-48) have extremely red optical-to-infrared colours ((R-K)~7) and very bright infrared magnitudes (K_s<10): follow-up optical spectroscopy with the ESO 3.6-m telescope gave spectral types of M8.0 and M7.5 dwarfs, respectively. Comparison of their near-infrared magnitudes with the absolute magnitudes of known M8 and M7.5 dwarfs with measured trigonometric parallaxes yields spectroscopic distance estimates of 6.4+/-1.4 parsecs and 8.0+/-1.6 parsecs for LP775-31 and LP655-48, respectively. In contrast, Cruz & Reid (2002) recently determined spectral types of M6 for both objects, and commensurately larger distances of 11.3+/-1.3 parsecs and 15.3+/-2.6 parsecs. LP655-48 is also a bright X-ray source (1RXS J044022.8-053020). With only a few late M dwarfs previously known within 10 parsecs, these two objects represent an important addition to the census of the Solar neighbourhood., Comment: Astronomy & Astrophysics (Letters), in press; 5 pages, 1 figure, uses aa.cls version 5.1