Title:Preliminary detection to a stellar overdensity associated to the Perseus arm

Abstract:
The cartography and dynamics of the spiral structure of the Milky Way is still, nowadays, highly 
controversial. Different spiral tracers lead to different conclusions about basic parameters such as the 
strength and number of arms and, more important, we are far from understanding the mechanisms acting to 
create this perturbation in the galactic disk.
Aiming to shed light on these issues we present a photometric and spectroscopic survey in the anticenter 
direction, that is ongoing to detect the overdensity and kinematic perturbation of the Perseus arm. We show 
how the preliminary radial distribution of young stars, with distances obtained from the Strömgren 
photometry in the first 3sqd survey completed up to now (INT/WFC, La Palma), suggests an overdensity 
around 1.5-2kpc, probably associated with the Perseus arm. 
We also present some simulations trying to evaluate how the future Gaia data will help in tracing the spiral 
arms of the Galaxy. Using actual expectations on the Gaia mission accuracy on astrometric and 
spectroscopic data, we show the uncertaities for the fitted parameters, and the improvements needed for 
the used model.