The Y dwarf population with HST : unlocking the secrets of our coolest neighbours – I. Overview and first astrometric results

In this paper, we present our project that aims at determining accurate distances and proper motions for the Y brown dwarf population using the Hubble Space Telescope. We validate the program with our first results, using a single new epoch of observations of the Y0pec dwarf WISE J163940.83−684738.6. These new data allowed us to refine its proper motion and improve the accuracy of its parallax by a factor of three compared to previous determinations, now constrained to ϖ = 211.11 ± 0.56 mas. This newly derived absolute parallax corresponds to a distance of 4.737 ± 0.013 pc, an exquisite and unprecedented precision for faint ultracool Y dwarfs

DE0823−-49 is a juvenile binary brown dwarf at 20.7 pc

Astrometric monitoring of the nearby early-L dwarf DE0823$-$49 has revealed a low-mass companion in a 248-day orbit that was announced in an earlier work. Here, we present new astrometric and spectroscopic observations that allow us to characterise the system in detail. The optical spectrum shows LiI-absorption indicative of a young age and/or substellar mass for the primary component. The near-infrared spectrum is best reproduced by a binary system of brown dwarfs with spectral types of L1.5 $+$ L5.5 and effective temperatures of $2150\pm100$ K and $1670\pm140$ K. To conform with the photocentric orbit size measured with astrometry and the current understanding of substellar evolution, the system must have an age in the 80--500 Myr range. Evolutionary models predict component masses in the ranges of $M_1\simeq0.028-0.063\,M_\odot$ and $M_2\simeq0.018-0.045\,M_\odot$ with a mass ratio of $q\simeq0.64-0.74$. Multi-epoch radial velocity measurements unambiguously establish the three-dimensional orbit of the system and allow us to investigate its kinematic properties. DE0823$-$49 emerges as a rare example of a nearby brown dwarf binary with orbit, component properties, and age that are characterised well. It is a juvenile resident of the solar neighbourhood, but does not appear to belong to a known young association or moving group.Comment: 9 pages, 11 figures. Accepted for publication in A&

orvara::An Efficient Code to Fit Orbits using Radial Velocity, Absolute, and/or Relative Astrometry

We present an open-source Python package, Orbits from Radial Velocity, Absolute, and/or Relative Astrometry (orvara), to fit Keplerian orbits to any combination of radial velocity, relative astrometry, and absolute astrometry data from the Hipparcos-Gaia Catalog of Accelerations. By combining these three data types, one can measure precise masses and sometimes orbital parameters even when the observations cover a small fraction of an orbit. orvara achieves its computational performance with an eccentric anomaly solver five to ten times faster than commonly used approaches, low-level memory management to avoid python overheads, and by analytically marginalizing out parallax, barycenter proper motion, and the instrument-specific radial velocity zero points. Through its integration with the Hipparcos and Gaia intermediate astrometry package htof, orvara can properly account for the epoch astrometry measurements of Hipparcos and the measurement times and scan angles of individual Gaia epochs. We configure orvara with modifiable .ini configuration files tailored to any specific stellar or planetary system. We demonstrate orvara with a case study application to a recently discovered white dwarf/main sequence (WD/MS) system, HD 159062. By adding absolute astrometry to literature RV and relative astrometry data, our comprehensive MCMC analysis improves the precision of HD 159062B's mass by more than an order of magnitude to $0.6083^{+0.0083}_{-0.0073}\,M_\odot$. We also derive a low eccentricity and large semimajor axis, establishing HD 159062AB as a system that did not experience Roche lobe overflow.Comment: 24 pages, 5 figures, 5 tables. AJ accepted with minor changes. orvara is available at https://github.com/t-brandt/orvar

Development of an Anger camera in Lanthanum Bromide for gamma-ray space astronomy in the MeV range

International audienceLanthanum bromide is a very promising scintillator material for the next generation of g-ray telescopes. We present in this paper first g-ray imaging results obtained by coupling a LaBr3 crystal with a position-sensitive 8×8 multianode photomultiplier tube to form a simple Anger camera module. The readout of the 64 signals is carried out with the most recent evolution of the MultiAnode ReadOut Chip (MAROC) which was initially designed for the luminometer of the ATLAS detector. Measured charge distributions are compared with detailed GEANT4 simulations that include the tracking of the optical photons produced in the scintillation crystal. The depth of interaction (d.o.i.) of 662-keV g-rays inside the crystal is derived from the charge distributions using an artificial neural network. We obtain for an irradiation at detector centre a mean standard deviation of the d.o.i. of 1.69 mm. Such a position-sensitive g-ray detector can form an innovative building block for a future space calorimete

Individual dynamical masses of DENIS J063001.4−184014AB reveal a likely young brown dwarf triple

The binary nature of the M8.5 dwarf DENIS J063001.4$-$184014AB (DE0630$-$18) was discovered with astrometric monitoring from the ground, which determined the unresolved photocentric orbit and the trigonometric parallax of the system. Here we present radial-velocity monitoring and resolved observations in the near-infrared with Keck aperture masking that allow us to measure the system's relative separation and brightness. By combining all available information, we determine the individual dynamical masses of the binary components to be $M_1 = 0.052^{+0.009}_{-0.008} M_\mathrm{Sun}$ and $M_2 = 0.052^{+0.005}_{-0.004} M_\mathrm{Sun}$, both firmly in the substellar regime. These masses are surprising given the object's M8.5 optical spectral type and equivalent absolute magnitude, and the significant difference in brightness between the components ($\Delta{K}$ = 1.74$\pm$0.06 mag). Our results suggest that DE0630$-$18 is a relatively young system ($\sim$200 Myr) with a secondary component that is itself a potentially unresolved binary.Comment: 10 pages, 8 figures, accepted for publication in MNRA

Kinematics, rotation, and multiplicity of ultracool dwarfs with high-resolution near-infrared spectroscopy

Stars and planetary system

An eclipsing substellar binary in a young triple system discovered by SPECULOOS

Mass, radius, and age are three of the most fundamental parameters for celestial objects, enabling studies of the evolution and internal physics of stars, brown dwarfs, and planets. Brown dwarfs are hydrogen-rich objects that are unable to sustain core fusion reactions but are supported from collapse by electron degeneracy pressure. As they age, brown dwarfs cool, reducing their radius and luminosity. Young exoplanets follow a similar behaviour. Brown dwarf evolutionary models are relied upon to infer the masses, radii and ages of these objects. Similar models are used to infer the mass and radius of directly imaged exoplanets. Unfortunately, only sparse empirical mass, radius and age measurements are currently available, and the models remain mostly unvalidated. Double-line eclipsing binaries provide the most direct route for the absolute determination of the masses and radii of stars. Here, we report the SPECULOOS discovery of 2M1510A, a nearby, eclipsing, double-line brown dwarf binary, with a widely-separated tertiary brown dwarf companion. We also find that the system is a member of the $45\pm5$ Myr-old moving group, Argus. The system's age matches those of currently known directly-imaged exoplanets. 2M1510A provides an opportunity to benchmark evolutionary models of brown dwarfs and young planets. We find that widely-used evolutionary models do reproduce the mass, radius and age of the binary components remarkably well, but overestimate the luminosity by up to 0.65 magnitudes, which could result in underestimated photometric masses for directly-imaged exoplanets and young field brown dwarfs by 20 to 35%