A spectroscopically confirmed Gaia-selected sample of 318 new young stars within ∼200 pc

In the Gaia era, the majority of stars in the Solar neighbourhood have parallaxes and proper motions precisely determined while spectroscopic age indicators are still missing for a large fraction of low-mass young stars. In this work, we select 756 overluminous late K and early M young star candidates in the southern sky and observe them over 64 nights with the ANU 2.3-m Telescope at Siding Spring Observatory using the Echelle (R = 24 000) and Wide Field spectrographs (WiFeS, R = 3000-7000). Our selection is kinematically unbiased to minimize the preference against low-mass members of stellar associations that dissipate first and to include potential members of diffuse components. We provide measurements of Hα and calcium H&K emission, as well as of Li i 6708 Å in absorption. This enables identification of stars as young as 10-30 Myr-a typical age range for stellar associations. We report on 346 stars showing detectable lithium absorption, 318 of which are not included in existing catalogues of young stars. We also report 125 additional stars in our sample presenting signs of stellar activity indicating youth but with no detectable lithium. Radial velocities are determined for WiFeS spectra with a precision of 3.2 km s-1 and 1.5 km s-1 for the Echelle sample

Two Intermediate-mass Transiting Brown Dwarfs from the TESS Mission

We report the discovery of two intermediate-mass transiting brown dwarfs (BDs), TOI-569b and TOI-1406b, from NASA's Transiting Exoplanet Survey Satellite mission. TOI-569b has an orbital period of P = 6.55604 +- 0.00016 days, a mass of M b = 64.1 +- 1.9 ${M}_{{\rm{J}}}$, and a radius of R b = 0.75 +- 0.02 ${R}_{{\rm{J}}}$. Its host star, TOI-569, has a mass of M sstarf = 1.21 +- 0.05 $\,{M}_{\odot }$, a radius of R sstarf = 1.47 +- 0.03 $\,{R}_{\odot }$, $[\mathrm{Fe}/{\rm{H}}]=+0.29\pm 0.09$ dex, and an effective temperature of T eff = 5768 +- 110 K. TOI-1406b has an orbital period of P = 10.57415 +- 0.00063 days, a mass of M b = 46.0 +- 2.7 ${M}_{{\rm{J}}}$, and a radius of R b = 0.86 +- 0.03 ${R}_{{\rm{J}}}$. The host star for this BD has a mass of M sstarf = 1.18 +- 0.09 $\,{M}_{\odot }$, a radius of R sstarf = 1.35 +- 0.03 $\,{R}_{\odot }$, $[\mathrm{Fe}/{\rm{H}}]=-0.08\pm 0.09$ dex, and an effective temperature of T eff = 6290 +- 100 K. Both BDs are in circular orbits around their host stars and are older than 3 Gyr based on stellar isochrone models of the stars. TOI-569 is one of two slightly evolved stars known to host a transiting BD (the other being KOI-415). TOI-1406b is one of three known transiting BDs to occupy the mass range of 40-50 ${M}_{{\rm{J}}}$ and one of two to have a circular orbit at a period near 10 days (with the first being KOI-205b). Both BDs have reliable ages from stellar isochrones, in addition to their well-constrained masses and radii, making them particularly valuable as tests for substellar isochrones in the BD mass-radius diagram.Funding for this work was provided by the National Science Foundation Graduate Research Fellowship Program Fellowship (GRFP). This work makes use of observations from the LCOGT network. A.J.M. acknowledges support from the Knut & Alice Wallenberg Foundation (project grant 2014.0017) and the Walter Gyllenberg Foundation of the Royal Physiographical Society in Lund. C.M.P. and M.F. gratefully acknowledge the support of the Swedish National Space Agency (DNR 163/16). A.J. and R.B. acknowledge support by the Ministry for the Economy, Development, and Tourism’s Programa Iniciativa Científica Milenio through grant IC 120009, awarded to the Millennium Institute of Astrophysics (MAS). A.J. acknowledges additional support from FONDECYT project 1171208

The GALAH survey: Temporal chemical enrichment of the galactic disc

We present isochrone ages and initial bulk metallicities ([Fe/H]bulk, by accounting for diffusion) of 163 722 stars from the GALAH Data Release 2, mainly composed of main-sequence turn-off stars and subgiants (7000 K > Teff > 4000 K and log g > 3 dex). The local age-metallicity relationship (AMR) is nearly flat but with significant scatter at all ages; the scatter is even higher when considering the observed surface abundances. After correcting for selection effects, the AMR appears to have intrinsic structures indicative of two star formation events, which we speculate are connected to the thin and thick discs in the solar neighbourhood. We also present abundance ratio trends for 16 elements as a function of age, across different [Fe/H]bulk bins. In general, we find the trends in terms of [X/Fe] versus age from our far larger sample to be compatible with studies based on small (∼100 stars) samples of solar twins, but we now extend them to both sub- and supersolar metallicities. The α-elements show differing behaviour: the hydrostatic α-elements O and Mg show a steady decline with time for all metallicities, while the explosive α-elements Si, Ca, and Ti are nearly constant during the thin-disc epoch (ages ≦ 12 Gyr). The s-process elements Y and Ba show increasing [X/Fe] with time while the r-process element Eu has the opposite trend, thus favouring a primary production from sources with a short time delay such as core-collapse supernovae over long-delay events such as neutron star mergers.Y-ST was supported by the NASA Hubble Fellowship grant HSTHF2-51425.001 awarded by the Space Telescope Science Institute.JK, TZ, and KˇC acknowledge financial support of the Slovenian Research Agency (research core funding no. P1-0188)

The GALAH+ Survey : Third Data Release

© 2021 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This is the accepted manuscript version of an article which has been published in final form at https://doi.org/10.1093/mnras/stab1242The ensemble of chemical element abundance measurements for stars, along with precision distances and orbit properties, provides high-dimensional data to study the evolution of the Milky Way. With this third data release of the Galactic Archaeology with HERMES (GALAH) survey, we publish 678 423 spectra for 588 571 mostly nearby stars (81.2% of stars are within 75 stellar clusters. We derive stellar parameters $T_\text{eff}$, $\log g$, [Fe/H], $v_\text{mic}$, $v_\text{broad}$ & $v_\text{rad}$ using our modified version of the spectrum synthesis code Spectroscopy Made Easy (SME) and 1D MARCS model atmospheres. We break spectroscopic degeneracies in our spectrum analysis with astrometry from $Gaia$ DR2 and photometry from 2MASS. We report abundance ratios [X/Fe] for 30 different elements (11 of which are based on non-LTE computations) covering five nucleosynthetic pathways. We describe validations for accuracy and precision, flagging of peculiar stars/measurements and recommendations for using our results. Our catalogue comprises 65% dwarfs, 34% giants, and 1% other/unclassified stars. Based on unflagged chemical composition and age, we find 62% young low-$\alpha$, 9% young high-$\alpha$, 27% old high-$\alpha$, and 2% stars with $\mathrm{[Fe/H]} \leq -1$. Based on kinematics, 4% are halo stars. Several Value-Added-Catalogues, including stellar ages and dynamics, updated after $Gaia$ eDR3, accompany this release and allow chrono-chemodynamic analyses, as we showcase.Peer reviewe

The GALAH survey: accurate radial velocities and library of observed stellar template spectra

GALAH is a large-scale magnitude-limited southern stellar spectroscopic survey. Its second data release (GALAH DR2) provides values of stellar parameters and abundances of 23 elements for 342 682 stars (Buder et al.). Here we add a description of the public release of radial velocities with a typical accuracy of 0.1 km s-1 for 336 215 of these stars, achievable due to the large wavelength coverage, high resolving power, and good signal-to-noise ratio of the observed spectra, but also because convective motions in stellar atmosphere and gravitational redshift from the star to the observer are taken into account. In the process we derive medians of observed spectra that are nearly noiseless, as they are obtained from between 100 and 1116 observed spectra belonging to the same bin with a width of 50 K in temperature, 0.2 dex in gravity, and 0.1 dex in metallicity. Publicly released 1181 median spectra have a resolving power of 28 000 and trace the well-populated stellar types with metallicities between -0.6 and +0.3. Note that radial velocities from GALAH are an excellent match to the accuracy of velocity components along the sky plane derived by Gaia for the same stars. The level of accuracy achieved here is adequate for studies of dynamics within stellar clusters, associations, and streams in the Galaxy. So it may be relevant for studies of the distribution of dark matter

TOI-216b and TOI-216 c : two warm, large exoplanets in or slightly wide of the 2:1 orbital resonance

K.H. acknowledges support from STFC grant ST/R00824/1.Warm, large exoplanets with 10–100 day orbital periods pose a major challenge to our understanding of how planetary systems form and evolve. Although high eccentricity tidal migration has been invoked to explain their proximity to their host stars, a handful reside in or near orbital resonance with nearby planets, suggesting a gentler history of in situ formation or disk migration. Here we confirm and characterize a pair of warm, large exoplanets discovered by the TESS Mission orbiting K-dwarf TOI-216. Our analysis includes additional transits and transit exclusion windows observed via ground-based follow-up. We find two families of solutions, one corresponding to a sub-Saturn-mass planet accompanied by a Neptune-mass planet and the other to a Jupiter in resonance with a sub-Saturn-mass planet. We prefer the second solution based on the orbital period ratio, the planet radii, the lower free eccentricities, and libration of the 2:1 resonant argument, but cannot rule out the first. The free eccentricities and mutual inclination are compatible with stirring by other, undetected planets in the system, particularly for the second solution. We discuss prospects for better constraints on the planets' properties and orbits through follow-up, including transits observed from the ground.Publisher PDFPeer reviewe

Chronostar: a novel Bayesian method for kinematic age determination - I. Derivation and application to the ss Pictoris moving group

Gaia DR2 provides an unprecedented sample of stars with full 6D phase-space measurements, creating the need for a self-consistent means of discovering and characterizing the phase-space overdensities known as moving groups or associations. Here we present Chronostar, a new Bayesian analysis tool that meets this need. Chronostar uses the Expectation-Maximization algorithm to remove the circular dependency between association membership lists and fits to their phase-space distributions, making it possible to discover unknown associations within a kinematic data set. It uses forward-modelling of orbits through the Galactic potential to overcome the problem of tracing backward stars whose kinematics have significant observational errors, thereby providing reliable ages. In tests using synthetic data sets with realistic measurement errors and complex initial distributions, Chronostar successfully recovers membership assignments and kinematic ages up to approximate to 100 Myr. In tests on real stellar kinematic data in the phase-space vicinity of the ss Pictoris Moving Group, Chronostar successfully rediscovers the association without any human intervention, identifies 10 new likely members, corroborates 48 candidate members, and returns a kinematic age of 17.8 +/- 1.2 Myr. In the process we also rediscover the Tucana-Horologium Moving Group, for which we obtain a kinematic age of 36.3(-1.4)(+1.3) Myr

Single Transit Analysis (DU650) Software Requirements Specification

International audienc

Single Transit Analysis (DU650) Software Requirements Specification: Gaia Report to ESA (GAIA-C6-SP-OPM-YV-001-13)

International audienc

Single transit Analysis (DU650) Software Test Report for cycle 12

International audienc