Found 14 talks width keyword low-mass stars

Gaia Data Release 3 (13 June 2022) contains astrophysical parameters for up to 1.5 billion sources derived from the low resolution BP and RP prism spectra, the high resolution RVS spectra, photometry and astrometry.

These include object classifications (star, galaxy, stellar spectral type,...), unresolved galaxies and quasar redshifts (~6 million), outlier objects, interstellar medium characterisation (extinction and DIBs), and spectroscopic and evolutionary parameters (~470 million) for a large variety of stellar types from ultra-cool dwarfs to hot OB stars.

In this talk, I will present an overview of the astrophysical parameter content of Gaia DR3 that was derived using the Astrophysical Parameters Inference System (Apsis) software. I will first give a brief description of the data, models and methods that were employed, and then I will focus on describing what type of parameters you can find in the archive and where to find them among the 30+ new tables. I will then describe the overall performance and present some pre-Gaia DR3 highlights.

Rotation plays an important role in the life of stars and offers a potential diagnostic to infer their ages and that of their planets. This idea is known as gyrochronology, and if properly calibrated, its applications to Galactic, stellar, and exoplanetary astrophysics would be far-reaching. Nevertheless, while potentially fruitful over a wide range of ages and masses, recent results have raised concerns regarding gyrochronology’s applicability. In this talk, I will present the opportunities that the Gaia astrometry has opened to address these issues. First, regarding rotation’s classical calibrators, I will illustrate the impact that removing the non-member contamination has on the rotational sequences of open clusters. Second, I will present a novel method that tests the state-of-the-art gyrochronology relations in under-explore domains using wide binary stars. Finally, I will discuss the prospects for expanding the existing rotational constraints in unprecedented regimes using data from the TESS mission.

The field of Galactic archaeology has been very active in recent years, with a major influx of data from the Gaia satellite and large spectroscopic surveys. The major science questions in the field include Galactic structure and dynamics, the accretion history of the Milky Way, chemical tagging, and age-abundance relations. I will give an overview of GALAH as a large spectroscopic survey, and describe how it is complementary to other ongoing and future survey projects. I will also discuss recent science highlights from the GALAH team and compelling questions for future work.

The new generation of spectrometers designed for extreme precision radial velocities enable correspondingly precise stellar spectroscopy. It is now fruitful to theoretically explore what the information content would be if stellar spectra could be studied with spectral resolutions of a million or more, and to deduce what signatures remain at lower resolutions. Hydrodynamic models of stellar photospheres predict how line profiles shapes, asymmetries, and convective wavelength shifts vary from disk center to limb. Corresponding high-resolution spectroscopy across spatially resolved stellar disks is now practical using differential observations during exoplanet transits, thus enabling the testing of such models. A most demanding task is to understand and to model spectral microvariability toward the radial-velocity detection of also low-mass planets in Earth-like orbits around solar-type stars. Observations of the Sun-as-a-star with extreme precision spectrometers now permit searches for spectral-line modulations on the level of a part in a thousand or less, feasible to test against hydrodynamic models of various solar features.

Planetary systems have been found systematically orbiting main sequence stars and red giants. But the detection of planets per se during the white dwarf phase has been more elusive with only 3 systems.  We have, however, ample indirect evidence  of the existence of planetary debris around these systems in the form of material acreted onto the white dwarf, disks and even planetesimals. In this talk, I will review how we can put the pieces together: how we can reconcile what we see in white dwarfs with what we can infer regarding the evolution of planetary systems from the main sequence phase.

I will summarize the two well proved techniques for high spatial resolution: Lucky Imaging and Adaptive Optics and the work of our group in this field. I will also introduce the state-of-the-art new instrument Adaptive Optics Lucky Imager (AOLI). On AOLI, both techniques merge providing a very versatile answer on the visible range. Some first science on the T-Tauri system LkHa 262/263 in the MBM 12 cloud will be reported together with a review of the next steps to be developed.

 A comprehensive understanding of sub-stellar objects (brown dwarfs and extrasolar giant planets) and their population characteristics (e.g. IMF, formation history) is only possible through the robust interpretation of ultra-cool objects spectroscopy. However, the physics of ultra-cool atmospheres is complicated by a variety of challenging ingredients (dust properties, non-equilibrium chemistry, molecular opacities). Moreover, while hydrogen-burning stars stabilize on the stellar main-sequence, sub-stellar objects continuously cool down (since they lack an internal source of energy) and evolve towards later spectral types. Their atmospheric parameters are a strong function of age. In this talk I will present the spectroscopic analysis of a large sample of L and T dwarfs, complementing the spectroscopic data with astrometry from the PARSEC program, in order to constrain the sub-stellar initial mass function and formation history. I will then describe our new effort to identify and characterize a large sample of benchmark systems, combining Gaia capabilities with large area near-infrared surveys such as UKIDSS, SDSS, and VVV, in order to calibrate effectively the theoretical models.
    

Understanding the atmospheric and evolutive properties of very low mass stars, brown dwarfs, and gas giant exoplanets have been important challenges for modelers around the world since the discovery of the first brown dwarfs in the Pleiades cluster (Rebolo et al. 1995) and in the field (Nakajima et al. 1995). The early studies of brown dwarfs have provided rich insights into atmospheric physics, with discoveries ranging from cloud formation (Tsuji et al. 1996), methane bands (Oppenheimer et al. 1995) and ammonia bands (Delorme et al. 2008), to the formation of wasi-molecular KI-H2 absorption (Allard et al. 2007), and to disequilibrium chemistry (Yelle & Griffith 2001). New classical 1D models yield spectral energy distribution (SED) that match relatively well despite these complexities. These models have for instance explained the spectral transition from M to L, T and now Y brown dwarf spectral types (Allard et al. 2013). However, in presence of surface inhomogeneities revealed recently for a nearby (2 pc) brown dwarf (Crossfield et al. 2014), the SED may well fit even exactly, but the model parameters could be far from exact, e.g. with the effective temperature by several hundred kelvins too cool in the case of dusty brown dwarfs and young gas giant exoplanets! I will review the progress achieved in reproducing the spectral properties of very low mass stars, brown dwarfs and gas giant exoplanets, and review progress in modeling more accurately their atmospheres using Radiation HydroDynamical (RHD) simulations.

Over the past two decades, advances in infrared instrumentation have allowed us to identify a vast and previously unseen population of low-temperature stars, brown dwarfs and free-floating extrasolar planets, collectively called ultracool dwarfs. These sources, with surface temperatures reaching below 0ºC, encompass three new spectral classes and include some of the nearest systems to the Sun. Research in this field is now concentrating on the physical characterization of the ultracool dwarf population and application to Galactic studies. In this talk, I will summarize the recent observational advances in ultracool dwarf research, including the recent discovery of the Y dwarf spectral class. I will then describe our ongoing IRTF/SpeX survey, which has measured the low-resolution, near-infrared spectra of over 1500 late M, L and T dwarfs and uncovered new subpopulations of young (5-30 Myr) brown dwarf, metal-poor halo brown dwarfs and short-period spectral binaries.

Stellar-mass black holes have all been discovered through X-ray emission, which arises from the accretion of gas from their binary companions. Currently known black holes are fed by material stripped from a low-mass star or by the wind of a massive companion. Binary evolution models also predict the existence of black holes accreting from the equatorial envelope of rapidly spinning Be-type stars. However, among the ~80 Be X-ray binaries known in the Galaxy (~150 including the Magellanic Clouds), only pulsating neutron stars have been found as companions, which is known as the missing Be/black-hole X-ray binary problem. In this talk I present the first dynamical evidence for a 3.8-6.9 Msun black hole orbiting the Be star and gamma-ray candidate MWC 656 (=AGL J2241+4454). This discovery has been allowed by the detection of a HeII emission line from an accretion disc encircling the black hole. We find the black hole is X-ray quiescent with Lx<1.6 × 10−7 times the Eddington luminosity. This implies that Be binaries with black-hole companions are difficult to detect by conventional X-ray surveys and may be more abundant than predicted by population synthesis models.