% GaiaSAM introduction
% UH

   %Background on Gaia, DPAC and astrophysical parametrization of Gaia sources
   %(see "Background" section on GaiaSAM web page)

% Astrophysical context
A major task of modern Astrophysics is to determine the origin, structure, and
evolutionary history of our Galaxy. Progress in this task depends crucially on
our knowledge of the physics of stars as building blocks of the Galaxy, in
particular their atmospheres. Low-mass stars play a very important role in
this context, because most of them exist for longer than the present age of
the Universe. They also display, to a high degree, their original chemical
composition at their surfaces.

% Gaia mission
The ESA cornerstone mission Gaia will provide astronomers with a detailed
astrometric and spectrophotometric census of the Galaxy and the local
Universe. For one billion stars brighter than an apparent visual magnitude of
V$\approx$20, astrometry with an accuracy of down to 20 microarcseconds and
optical and near-IR spectrophotometry will be obtained. This will allow a
detailed mapping of the content of our Galaxy, both in space and in terms of
astrophysical parameters: surface temperature ($T_{\rm eff}$), surface gravity (log\,$g$)
and metal content ([Fe/H]). For brighter objects, the astrometry and
spectrophotometry will be supplemented by near-IR spectroscopic observations,
which will allow, besides radial velocities, some individual elemental
abundances to be measured.

% Gaia science goals
The scientific goals of Gaia range from understanding the origin and history of our Galaxy (e.g. its star formation history, its chemical and dynamical evolution, and the distribution of dust) to understanding the formation and evolution of stars (e.g. the dynamics of star forming regions, a complete and detailed local census, and a census of multiple stars). Additional scientific products include the detection of extra-solar planetary systems, as well as comprehensive surveys of Solar System and extragalactic objects.

% DPAC
The responsibility for processing and analysing Gaia data lies with
the Gaia Data Processing and Analysis Consortium (DPAC). This consortium
of over 300 European scientists and software engineers is developing the software system which will
convert the raw data recorded by the Gaia satellite into scientifically
meaningful data, for immediate use by the Astronomical community.

% CU8
Within DPAC, the different data processing tasks are carried out by
Coordination Units (CUs). CU8, "Astrophysical Parameters", is developing (1)
classification algorithms which will determine, for each Gaia source, a
probablity of its being a single star, galaxy, quasar, etc.,
and (2) parametrisation algorithms which will determine astrophysical
parameters ($T_{\rm eff}$, log\,$g$, metallicity, $\alpha$ element abundance, interstellar extinction) for each single star (or resolved binary).  
This is the area where the work of the Gaia-SAM collaboration will contribute.

% SAM
We are aiming to improve synthetic spectra by improving the underlying physical models, to the extent needed to allow Gaia to achieve its science goals based on astrophysical parameters. We focus on the effects of 3D hydrodynamical modelling of convection, non-LTE effects, and improving input data (e.g. atomic data).
We concentrate our efforts on F/G/K-type stars, since the effects metioned above are most important in those stars. 
Furthermore, F/G/K-type stars will be the most numerous among the objects observed by Gaia -- 86\% of all stars with $V<20$ according to estimations by \cite{2005ESASP.576...83R}.
Model validation will be based on a limited number of benchmark stars, with the best known physical properties, and dedicated high-quality observations. In this way we want to make sure that the astrophysical parameters in the Gaia catalogue are based on theoretical grounds beyond the current state of the art of stellar atmosphere modelling. Although Gaia data processing is the main driver for this work, the final goal may not be achieved until after publication of the Gaia catalogue, since our ambition is to eventually include non-LTE effects directly in the 3D stellar atmosphere modelling (for about 10 elements, the most important electron donors).

% include something on theoretical vs. semi-empirical calibration of photometry here?
% (e.g. �nehag et al. 2009)

% document outline
In the remainder of this section we will briefly review the status of current model grids, following \cite{2007IAUS..241...47G}.
In Section~\ref{sect:aims} we motivate this work in more detail and expand upon the specific aims.
In Section~\ref{sect:status} we review the current status, the methods we are using and what has been achieved up to now.
In Section~\ref{sect:plans} we present our long-term plans, while Section~\ref{sect:schedule} focuses on the short-term schedule, followed by some concluding remarks.

   %Status of current model grids (see Gustafsson et al. 2007, IAU 241)
   % 1) physical consistency of current models (basic physical assumptions)
   % 2) comparison of models from various computer programs (recent spectrum grids)
   % 3) comparison of model spectra with corresponding observed stellar spectra

   %Summing up, we see that the classical models are seriously questioned when the basic assumptions are scrutinized. I.e., their spectra may be systematically off as compared with corresponding stars, and this may have important effects on the interpretation of galaxy spectra. However, due to the very incomplete coverage of the HR diagram by these more advanced models, it is still not possible to replace the classical grids. In fact, the more realistic models are still so scant that more precise statements cannot yet be made on what errors are caused by using them.

% Currently published 3D models:
% * Sun - Asplund et al., Ludwig et al.
% * Supergiant stars (Freytag; Plez et al.) - Teff = 3482 K, log g = -0.39
% * AGB star (Freytag & Höfner 2008):  Teff = 2800 K, log g = -0.6 (cgs)
% * Metal-poor red giant stars - Collet et al.
%   Teff = 4700-5100 K, log g = 2.2 (cgs), met from solar to -3
% * Metal-poor subgiants - Collet et al., HD 140283
%   Teff = 5690 K, logg = 3.67 (cgs), and [Fe/H]= -2.50
% * Extreme halo stars (Collet, Asplund et al.)
% * alf Cen (Bigot et al.)
% * Metal-poor stars (Ludwig et al., CFIST)
%   Teff = 4000 ... 6500 K, log g = 1.5 ... 5.0, met = -2.0
% * K dwarfs (Ramirez et al.)
%   Teff=4830 K, log g=4.50, [Fe/H]=0
% * M dwarfs (Ludwig et al.) - Teff = 2800 K, log g = 5.0

% CFIST report 2007: "... 28 new 3D model atmospheres have been computed. More
% models are being computed, the goal is that by the end of the project a
% small grid of models, and tools for their exploitation will be made
% available to the community. These models are now being actively exploited,
% like in the case of CS 22876-32 or in the systematic study of 3D effects in
% the Sun and other stars."

