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The Plane

The bulge and mid-plane areas to be surveyed by VVV
In order to understand the Milky Way populations globally (and to account for the disk contribution along the line of sight), it is necessary to survey the inner Galactic plane. In addition to the Bulge Survey, we therefore propose to survey an adjacent region of the mid-plane in order to provide a Legacy Database and 3-D Atlas of a large Population I region. We have selected the inner Galaxy region at −65 < L < −10, |B| < 2 (see figure), where star formation activity is high and there will be complementary optical, mid-IR and far-IR data from VPHAS+ and the SPITZER GLIMPSE and MIPSGAL surveys. The addition of this region will also permit us to settle the controversial question of inner Galactic structure, discriminating between models with a long bar and a ring and triaxial Bulge models with no long bar.

The large survey area will allow several outstanding astrophysical problems to be addressed. For example, the environmental dependence of star formation and in particular the IMF at low masses is presently unknown. This issue will be addressed statistically by observing hundreds of star formation (SF) regions and cross-correlating luminosity functions (LFs) with cluster density, the presence of high-mass stars and galactocentric distance. Other important parameters such as velocity dispersion and metallicity would be determined by follow-up observations. In addition, the LF of the clusters themselves will be measured, both for star forming clusters and more evolved open clusters.

These issues cannot be addressed with optical surveys, owing to the high extinction in the plane. The SPITZER data will be invaluable for detecting the most obscured high-mass protostars within SF regions. However, a near-IR survey will be more sensitive to all but the reddest objects, and the superior spatial resolution of this waveband will be essential to resolve distant clusters and the crowded field population.

The Plane Survey will also be a multi-epoch survey, with the aims described in the goals below. Extinction maps for clusters will be provided by an initial single epoch of ZY JHKs imaging. Distances will then be derived using a variety of methods, e.g. main sequence fitting for mature clusters and analysis of foreground star counts vs. extinction for SF regions. For the field population, the use of several reddening-independent indices such as (J − H) − (Y − J) will be used to probe changes in the stellar population along every line of sight, statistically measuring the ratios of dwarfs to giants and hot stars to cool stars. The multi-colour data will also be valuable for robust detection of rare objects and characterisation of every type of stellar population.

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