ESAC Gaia group
Gaia's key objective is a detailed study of the Milky Way that will reveal our Galaxy's content, dynamics, current state and formation history. By surveying celestial bodies down to the very faint magnitude 20, Gaia takes in a representative fraction of the Milky Way's population, providing data to tackle unanswered questions about our home galaxy. The all-sky survey of about one billion stars also provides unique insight into many other areas of astronomy. Further details on the main science cases can be found at:
Some active research lines within the ESAC Gaia team are listed below, together with contact points.
ESAC plays a key role in the Gaia data processing, in particular the so-called Astrometric Global Iterative Solution (AGIS). This is a central part of the science data analysis for Gaia where the reference frame for the observations is established together with the corresponding instrument calibrations and attitude parameters. Within the framework of the DPAC consortium, extensive work has been devoted to develop new algorithms, run AGIS solutions and analyse the results, in collaboration with science institutes distributed all over Europe.
The software will eventually provide the astrometric solutions (together with instrument and attitude data) for about 100 million stars. Details of the methods and algorithms have been published (Lindegren et al., 2012 A&A 538, A78, 2016 A&A 595A, 4L) including the results of test runs with several million stars. The development effort will continue during and after the mission in order to cope with (as yet unforeseen) complications in the real data.
The Gaia Archive, the main data distribution hub, is developed and located at ESAC (Mora et al. 2017 arXiv:1706.09954). Surveying more than a billion sources, the final data release volume may reach the PB scale. One key objective in modern archives is to carry out as much processing as possible on the server side, according to the “code to the data” paradigm. The Gaia archive is likewise experimenting with these concepts.
The system architecture is based on Virtual Observatory standards and provides extensions for authenticated access, persistent uploads and table sharing. The main contents of Gaia Data Release 1 (DR1) are included in one big catalogue table gaia_source, including astrometric parameters and average photometry for 1.14 billion sources. An emphasis is put in enhancing usability while the data complexity increases with the successive data releases.
Contact point: Alcione Mora
Very bright stars
Very bright stars with magnitudes G<6, i.e. the ~6000 stars observable with the naked eye, are among the best studied astronomical objects. Securing Gaia data for those stars is a unique science opportunity, in particular in what concerns astrometry because no other current or planned observatory can obtain global astrometry at sub-milli-arcsecond level of this stellar sample.
Science cases include but are not limited to:
- Parallaxes and proper motions about 10 more precise than from Hipparcos, e.g. of bright massive stars that are fundamental anchor points for stellar astrophysics.
- Orbit constraints for very bright binary stars (at least 25% of the sample)
- Discover new exoplanets, in particular around very bright A and F stars
- Accurate masses of known exoplanets discovered by radial velocity monitoring
Advanced techniques have been applied to ensure Gaia acquires these key objects. The original Gaia bright limit of G=6 was improved to G=3 by tuning the onboard parameters of the SkyMapper star detection algorithm (Martin-Fleitas et al 2014 Proc. SPIE 9143E 0YM, Sahlmann et al. 2016 Proc. SPIE 9904E 2ES). For the 230 stars brighter than G=3, we are pursuing two solutions to observing them as well. The first consists in forcing the acquisition of full-frame Sky Mapper images and has been in operation since the beginning of Gaia’s nominal mission. The second method uses Virtual Objects whose associated CCD windows are placed at defined locations.
Black holes and globular clusters
Merging of massive stars that are ejected from dense stellar clusters leads to the formation of intermediate mass black holes (IMBH) that can be detected by (a) the presence of hypervelocity stars and (b) ultraluminous X-ray sources (Portegies Zwart & McMillan 2002 ApJ 576, 899P, Maccarone 2014 MNRAS 440, 1626M, Rasio et al. (2004)). There are clear signatures of the presence of globular cluster black holes on the basis of strong, highly variable X-ray emission (Maccarone et al. 2008 IAUS 246 336M).
However, the detection of IMBH via the velocity dispersion of the visible stars has been challenging until now: for a cluster of mass Mc containing an IMBH of mass MBH the influence of the IMBH becomes significant only at a fraction 2.5 MBH / Mc of the half-mass radius, affecting only a small number of stars, thus, a mission like Gaia that can determine proper motions and parallaxes for visible stars down to magnitude 20.7 opens revolutionary new methods for detecting black holes in clusters.
The main objective consists of developing a model to detect Intermediate Mass Black Holes (IMBH) based on the study of the dynamic and astrophysical properties of globular clusters. It will be based in two main sources of information: the proper motions from the Gaia Archive, available for most stars from Data Release 2 (DR2), and X-ray observatory data.
Massive Walkway, Runaway, Hyper-runaway and Hyper-velocity stars
We know that most massive stars from in clusters, yet we are also aware that there are many isolated massive isolated stars in our Milky Way and in other Galaxies of the Local Group. Explanations for their existence include ejection from massive binary systems after the primary explodes as a supernova, dynamical interaction in massive dense cluster and, for stars with extreme velocities (hyper-velocity stars) interaction with a massive black hole.
Some of the most massive of these objects are found in the 30 Doradus (Tarantula Nebula) region of the Large Magellanic Cloud (LMC). Those objects that have radial velocities that are peculiar relative to their local environment are termed runaway stars, and VFTS018 in the Tarantula nebula is one of the most massive examples of such a runaway (Evans et al, 2010, ApJ, 715, L74). Other isolated massive stars have radial velocities that are typical of their environment and a walkaway scenario (i.e. slow runaway) has been proposed (Bestenlehner et al, 2011, A&A, 530, L14) and even isolated star formation (Bressert et al, 2012, A&A, 542, 49). Using data from the Hubble Space Telescope we are attempting to measure proper motions in the field of 30 Doradus to distinguish between these various scenarios Platais et al, 2015, AJ, 150, 89). When Gaia Data Release 2 (DR2) data are available they will further improve out knowledge of these stars' original birthplace.
Already using data from the first Gaia Date Release, and in particular the TGAS catalogue, a candidate hyper-velocity (or hyper-runaway) star has been found in the LMC ( Lennon et al, 2016 arXiv:1611.05504). In addition evidence was found supporting the idea that the Luminous Blue Variable, R71, while a moderate runaway, might also be a product of a previous stellar merger. Gaia DR2 will enable extension of this work to much fainter magnitudes, covering most isolated massive stars in the Magellanic Clouds and Bridge.
Contact point: Danny Lennon