Abstracts of the FRIday Programme


Invited Talk: How Gaia reveals the diversity of brown dwarfs

by Jacqueline Faherty

Brown Dwarfs are objects with masses that straddle “planet” and star classifications. They are defined by an inability to sustain stable Hydrogen burning. Their spectral energy distributions morph with time as they cool from objects as hot as stars to those as cool as Jupiter.

On the high mass end, brown dwarfs overlap in observable properties with the coolest stars like TRAPPIST-1 which hosts seven terrestrial worlds. On the low mass end, they overlap with the observable properties of directly imaged exoplanets like 51 Eri b and Beta Pictoris b. On the coolest end brown dwarfs like J085510.83-071442.5 -- a ~ 250 K object at just 2 pc from the Sun -- are directly comparable to Jupiter. In order to ascertain brown dwarf fundamental parameters such as mass, temperature, radius, gravity and characteristics like atmosphere composition, precise parallaxes are required.

From the onset of the Gaia mission it was unclear what the full extent of influence that Gaia data would have on our understanding of the lowest mass objects formed through the star formation process. However since the release of the Gaia DR2 catalog, it is clear that Gaia data is impactful on discoveries of new nearby cold brown dwarfs, co-moving low binding energy systems, and evaluating the diversity in the physical properties for the entire population of substellar mass objects.

In this talk I will review the major scientific achievements in low mass into brown dwarf and directly imaged exoplanet science which has emerged from Gaia DR2 studies.


Catalog of Revised Astrophysical Parameters from Gaia DR2

by Morgan Fouesneau, Rene Andrae, Coryn Bailer-Jones, et al.

Gaia DR2 offers us a first glimpse at the most detailed view of the dynamics of field star populations in our Galaxy. However; knowledge of the stellar properties of these populations is critical to understanding fully the Milky-Way’s formation and evolution. We present an all-sky uniformly derived catalog of stellar astrophysical parameters for hundreds of millions of stars implementing our data processing of the next Gaia data release.

As the DR3 BP/RP spectra are not yet available; we combined measurements of Gaia integrated photometry with GALEX; 2MASS; and WISE photometry; where available as an alternative to cover a large wavelength range. We obtain the posterior probability distribution of stellar properties such as age; mass; temperature; bolometric luminosity; distance; and dust extinction by fitting these sets of SEDs with evolutionary models; taking advantage of the GDR2 parallaxes to uniquely link atmospheric parameters to absolute bolometric luminosities.

This catalog bridges the gap between the content of GDR2 and the future GDR3 which will provide stellar parameters using Gaia data alone. This represents a key first step to anchor all current and future spectroscopic surveys to a common ground and provide us with the most comprehensive view of our Galaxy.


Single-star masses from astrometric microlensing using Gaia

by Jonas Klüter, Ulrich Bastian , Joachim Wambsganss, et al.

The mass is one of the most substantial quantities of a star. It determines the temperature; surface gravity and the evolution. Currently; relations concerning stellar masses are mainly based on binary stars; where a direct measurement is possible. However; it is known that single stars evolve differently. Hence it is important to derive the masses of single stars directly. Apart from strongly model-dependent asteroseismology; astrometric microlensing the only other usable tool.

Using precise astrometric measurements of the lensed position of a background source in combination with accurate predictions of the positions of the lensing star and the unlensed source (using Gaia) it is possible to determine the mass of the lens with an accuracy of a few percent. Further; the precise astrometric data from Gaia can be used to predict when such events will happen. This is one of the major advantages of astrometric microlensing as compared to photometric microlensing.

In this talk; I will present the analysis of the recently published Gaia DR2 for the prediction of a number of astrometric microlensing events in the near future. Especially I will present an event which happened in 2018; and show the great opportunities of Gaia for the task of directly determining stellar masses.


The discovery of New Magnetic Transitions in Late-Type Dwarfs from Gaia DR2

by Alessandro Lanzafame, Elisa Distefano, Sydney Barnes, et al.

The second Gaia data release contains the identification of 147,535 low-mass rotational modulation variable candidates on (or close to) the main sequence; together with their rotation period and modulation amplitude. The richness; the period and amplitude range; and the photometric precision of this sample make it possible to unveil; for the first time; signatures of different surface inhomogeneity regimes in the amplitude-period density diagram. The modulation amplitude distribution shows a clear bimodality; with an evident gap at periods P<2 d. The low amplitude branch; in turn; shows a period bimodality with a main clustering at periods of approximately 5 - 10 d and a secondary clustering of ultra-fast rotators at period approximately less than 0.5 d.

The amplitude-period multimodality is correlated with the position in the period-absolute magnitude (or period-color) diagram; with the low- and high-amplitude stars occupying different preferential locations. Such a multimodality represents a further evidence of the existence of different regimes of surface inhomogeneities in young and middle-age low-mass stars and we lay out possible scenarios for their evolution; which manifestly include rapid transitions from one regime to another. In particular; the data indicate that stars spinning up close to break-up velocity undergo a very rapid change in their surface inhomogeneities configuration; which is revealed by Gaia DR2 for the first time. The multimodality can be exploited to identify field stars of age approximately 100 - 600 Myr belonging to the slow-rotator low-amplitude sequence; for which age can be estimated from the rotation period via gyrochronology relationships.


Finding long-period companions in Tycho-2/Hipparcos/Gaia astrometry

by Daniel Michalik

Gaia DR2 provides positions; parallaxes; and instantaneous proper motions for over one billion stars. The underlying model assumes all catalogue sources to exhibit single star behaviour; i.e. to follow a linear uniform proper motion trend. For 2 million stars we can obtain an additional set of long temporal baseline proper motions; by combining Gaia DR2 with Tycho-2/Hipparcos positions from 1991.25. This requires careful calibrations between the two missions to account for; e.g.; frame rotation.

A comparison of the combined Tycho-/Hipparcos/Gaia long-term trends with the instantaneous Gaia DR2 proper motions allows the detection of accelerations; i.e. finding candidate stars hosting stellar or sub-stellar companions below the Gaia detection limits.


Gaia DR2 and Bessel legacy

by François Mignard

In his epoch-making parallax measurement of 61 Cygni in 1838; F.W. Bessel had to make assumptions about the parallaxes and proper motions of the two comparison stars he referred his local sights. None of these stars was in the Hipparcos catalogue and up to DR2 no check of these assumptions could be done with certainty.

Thanks to the Gaia-DR2 it is possible to identify these stars and; with the best orbit of 61 Cygni; reconstruct Bessel's observations to confirm the soundness of the assumptions and estimate the single observation accuracy of his observations with the Heliometer. This is compared to the quoted accuracy and to the large scatter seen in the numerous other measurements of the parallax of 61 Cygni published until ~1900.


New ultracool dwarfs in Gaia DR2

by Céline Reylé

Detailed population studies of ultracool; Very Low Mass (VLM) dwarfs are persistently plagued by incompleteness. While hundreds of VLM dwarfs have been uncovered in red and infrared sky surveys; this population remains incomplete; even in the solar neighborhood; due to their faint magnitudes. The Gaia mission has provided the means to uncover nearby VLM dwarfs through astrometric; rather than purely photometric; selection. This holds the promise of a truly volume-complete sample. I will present a study of the content of Gaia DR2 at the low-mass end.

I will first investigate the known sample of late M; L; T and Y dwarfs retrieved in Gaia DR2; and compare it with the up-to-date BT-Settl atmosphere models. I will next show that thousands of new candidates are revealed by Gaia DR2. They are evenly distributed across the sky; filling in “missing” populations in the Southern hemisphere and Galactic plane. Over 200 candidates have measured distances of less than 30 parsec; showing that even the nearby census is not complete.