The Perseus spiral arm in the Anticenter direction

The thesis developed by Maria Monguió was just presented this November, 11/2013

The main purpose of her work is to map the radial variation of the stellar density for the young stellar population in the Galactic anticenter direction in order to understand the structure and location of the Perseus spiral arm. A Strömgren photometric survey covering 16 square degrees in the anticenter direction was carried out using the Wide Field Camera at the Isaac Newton Telescope. As a result, a main catalog of 35974 stars with all Strömgren indexes has been obtained, together with a extended one with 96980 stars with partial data. These large samples permit us to analyze the stellar surface density variation associated to the Perseus arm and to study the properties of the stellar component and the interstellar extinction in the anticenter direction. To compute the physical parameters for these stars two different approaches have been used, 1) the available pre-Hipparcos empirical calibrations based on cluster data and trigonometric parallaxes, and 2) a new model based method using atmospheric models and evolutionary tracks, optimized for stars up to Teff >7000K. Results for both of them have been compared with Hipparcos data looking for possible biases and trends. The obtained physical parameters allow us to select the intermediate young stars useful for our studies (B5-A3). These stars are young enough to still have a small intrinsic velocity dispersion (making them respond stronger to a perturbation), but they are also old enough to have approached a dynamic equilibrium with the spiral perturbation. Through their stellar distances, and after defining distance complete samples between 1.2 and 3 kpc, we can trace the density distribution in the anticenter direction, finding a clear overdensity around 1.7 kpc with an amplitude of 10% that can be associated to the Perseus arm. Exponential fittings also allowed us to constrain the radial scale length of the young population of the Galaxy, as well as to estimate the stellar density at the solar vicinity. In addition, all these data allow the creation of a 3D extinction map, that carefully analyzed shows the presence of a dust layer clearly in front of the location of the stellar overdensity of the arm, suggesting that the corotation radius of the spiral pattern is further away of the position of the Perseus arm.