Long period variables and Gaia

The period-luminosity (P-L) relation of various types of variable stars is a key tool for determining the distance to an individual object or a whole galaxy. Thus it has a high relevance for stellar and extragalactic astrophysics. The Gaia mission, to be launched in 2011, will provide parallax measurements for a vast number of objects in the Milky Way, which will allow to improve the existing P-L relations considerably. One of the P-L relations that will profit a lot from this improvement is the one for long period variables. The Hipparcos data did no allow to establish a good relation for the Milky Way in that case. From the Magellanic Clouds we now know that the long period variables are found on several parallel P-L relations. A similar picture could not be established for the Milky Way up to now. The proposed project aims to provide the preparatory results necessary to produce a P-L diagram with the Gaia data. More specifically, the project will provide the following main output: (1) a relation for the calculation of a bolometric correction for the Gaia G photometry using Gaia low resolution spectroscopy; (2) a database of LPVs with well determined periods that will be correlated with the Gaia results for producing accurate P-L diagrams for various parts of the Milky Way; (3) high quality example light curves for training and optimizing the period finding algorithm of the Gaia data analysis; (4) improved classification criteria for LPVs according to their pulsation mode; (5) a better understanding of the global parameters influencing the P-L relations with the help of a study on the LPVs in stellar clusters and pulsation models. As part of the findings of this project will directly go into the Gaia data analysis the work has to be started in advance of the launch of Gaia. The project is a contribution to the Gaia Data Processing and Analysis Centre, and the applicant is a member of Gaia Coordination Unit 7 (Variable Stars). However, the outcome of the project will be of use also beyond the Gaia mission. Especially the proposed study of the parameters influencing the shape of the P-L diagram will provide new insight into the late stages of stellar evolution of low and intermediate mass stars.

Long period variables (LPVs) represent stars like our sun in a highly evolved state - shortly before they end their life and become white dwarfs. Their study is interesting from the point of stellar evolution, from their role within stellar populations, and from the aspect of distance determination. The Gaia mission, which is going to measure accurate distance for 1 billion stars down to 20 mag, will provide fundamental input for the investigation of LPVs, in particular the possibility to precisely determine their luminosity and to calibrate the period-luminosity relations of these variables found from observations in stellar systems. This FWF project had the aim to provide preparatory work necessary for the analysis and interpretation of the data on LPVs Gaia will deliver. This included the goal to improve our understanding on what stellar parameters shape the P-L-relations in stellar system (e.g. a stellar cluster or a galaxy). Here we could achieve major progress both by studying Galactic globular clusters and two dwarf galaxies. Together with earlier studies by our team we now reached an understanding of how mass, mass loss, chemistry, metallicity, helium abundance, and possibly also the star formation history of a galaxy (via the mass distribution among the evolved stars) affect the distribution of stars in a P-L-diagram. This was achieved by a combination of new observations and linear pulsation models. The second major goal was to provide the necessary algorithms and codes for the analysis and classification of LPVs within the Gaia Data Analysis Center. Obviously, the extraordinary large amount of data that will be obtained by this satellite requires a well prepared and highly efficient analysis pipeline. Our aim is to use the unique possibility of having a distance measurement for the Gaia LPVs to establish a new, physically more meaningful classification according to a star`s position in the P-L-diagram. All necessary steps for this have been developed during this project. Finally, a closer look on the shape of the light curves of LPVs was in order. Within the FWF project we analyzed the light change of these stars with a yet unprecedented photometric accuracy and time resolution using data from another satellite mission, namely CoRoT. A long standing debate on the occurrence of short time irregularities in the light curves of miras could be solved, the light curves of these stars turned out to be surprisingly smooth. Gaia is an outstanding scientific mission. It will produce a 3-D map of a large part of the Milky Way with implications for many fields of physics and astronomy, and it will also lead to a better impression of the local Universe for everyone. The research work funded here by the FWF contributed a piece to the success of this mission and enhanced our understanding of long period variables, the future of our sun.