Gaia - The Galactic Census Project:
The galactic astrophysics mission Gaia, launched by the
European Space Agency in December 2013, will survey the stellar content
of a large part of the Milky Way Galaxy, with the ultimate goal
to trace the origin and evolution of our galaxy and understand its
present structure.
The satellite is a unique and powerful optical
space observatory, combining large-scale photometric and
radial-velocity surveys
with determinations of stellar distances and transverse motions through
microarcsecond astrometry for more than a billion stars brighter than V =
21. Parallax and proper motion
accuracies are such that the space densities and kinematics of common
tracer
stars (e.g. K giants) can be accurately mapped across the whole Galaxy.
For more information on the mission and its current status, please
refer to the
main science portal for Gaia.
Recent and current work on Gaia:
Lund Observatory has a major long-term commitment in the development
and implementation of software for part of 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.
Lund Observatory provides the basic algorithms and some of the
software for AGIS, which are implemented at one of the Gaia
Data Processing Centres, located at the
European Space
Astronomy Centre (ESAC) outside of Madrid. The scientific team in
Lund collaborates intensely with the technical teams at
ESAC to develop and refine the software used to calculate the
astrometric solutions (together with instrument and attitude data)
for about 100 million stars. Methods and algorithms
published before the launch (Lindegren et al.,
A&A 538, A78, 2012)
are continually improved as more data become available and the
attitude and calibration models become more sophisticated after
confrontation with the real data. The first release of Gaia data
(Gaia DR1)
in September 2016 successfully demonstrated the validity of concepts
that had been developed over many years in Lund and elsewhere.
The group also develops algorithms and software for improved error analysis
in the future Gaia Archive. A theoretical foundation for this was developed
in the thesis work of Dr. Berry Holl
(A&A 543, A14, 2012; A&A 543, A15, 2012).
The combination of Gaia data with other astrometric catalougues or
prior information from Galactic models were considered in the PhD project
of Dr. Daniel Michalik. Ideas developed in this work proved to be instrumental for
providing the first accurate astrometric results in Gaia DR1, in particular
the so-called Tycho-Gaia Astrometric Solution (TGAS).
The reference frame and fundamental physics aspects
of Gaia were studied as part of the PhD project of Rajesh K. Bachchan.
The use of Gaia data for exoplanet detection was studied in a project
with Dr. Pieor Ranalli. The exploitation of Gaia data for Galactic studies
is actively pursued by severala researchers in Lund, in particular
Dr. Paul McMillan and PhD student Daniel Mikkola.
The historical involvement of Lund Observatory in the mission:
Gaia was originally proposed in October 1993 by Lindegren, Perryman (ESTEC)
et al. as a concept for an ESA cornerstone mission. Called GAIA,
an acronym for Global Astrometric Interferometer for Astrophysics,
it envisaged using two optical interferometers with 30 cm apertures and 3 m
baselines, capable of measuring some 50 million stars brighter than
V = 15-16 mag to an accuracy of about 20 microarcsec.
(The 1993 proposal can be found here.)
Soon after, it was realized that
that a much better performance could be achieved with a direct imaging
system, using filled apertures and a mosaic of CCDs operated in TDI mode,
rather than the complex fringe
detection method described in the 1993 proposal. These ideas had already
earlier in 1993 been developed by E. Høg, Lindegren and others for the
Roemer mission proposed to ESA
as a medium-size mission. After further studies by
scientists together with ESA and industry, a mature concept was
presented in 2000 to the ESA advisory bodies and eventually approved
as a future mission in the ESA programme. Gaia was then no longer
an interferometer, and in fact incorporated many features from the
Roemer concept, but it retained its proper name Gaia - which is
therefore not an acronym. A summary description of the project as
it was at the time of approval in 2000 can be found
here.
Since then the project has continued to evolve considerably, and the
reader should consult the official
Gaia information sheets
for up-to-date information.
Early mission studies (Lindegren, Söderhjelm, Holmberg, Quist):
Studies performed by the Lund group during the Concept and Technology Study
phase (up to 2000) include predictions
of star counts and the detection of binaries, brown-dwarf and planetary
companions, and the determination of orbits and masses. It has been
shown, for instance, that Gaia will measure the individual stellar masses
for many thousand stars, and that the mission will provide a nearly complete
census of brown-dwarf companions to stars within 100 pc from the Sun.
The impact of Gaia for the determination of astrometric radial velocities
has been examined. The main mission accuracy analysis was performed
in Lund, using semi-analytical tools developed here and subsequently refined
as a consequence of the industrial studies. Numerous technical studies
of optics and detector performance have been carried out, e.g. modelling
the effects of charge transfer inefficiency due to radiation-induced charge
trapping. Aspects of the data analysis for Gaia have been studied,
including data-base architectures for an observatory-type facility.
