2011A&A...536A.101C

The spiral structure of the Milky Way is nowadays receiving renewed attention thanks to the combined efforts of observational campaigns in different wavelength regimes from the optical to the radio. We start in the paper exploration of several key sectors (line of sights) in the inner Milky Way, where the spiral structure is still poorly known. We searched for density enhancements of young stars that might plausibly be associated with spiral structure. To this aim we collected sufficiently wide-field UBVI photometry to allow us to probe in a statistical sense the distribution in reddening and distance of young stars in the field. Although heavily demanding in terms of observational efforts, the intensive usage of U-band photometry ensures robust determination of reddening, hence distance for stars with earlier than A0 spectral type, which are well-known spiral arm tracers, even though no spectroscopic information are available. The fields we use are large enough to include in most cases well-studied Galactic clusters, which we use as benchmarks to assess the quality and standardization of the data and to validate our method. We focus on the line of sight to the Galactic longitude l=314°, where previous surveys have already detected H_α emitters at different standard-of-rest velocities, hence distances. The difficulty, however, to translate velocity into distance make predictions on the spiral structure quite vague. First of all, we made exhaustive tests to show that our dataset is in the standard system, and calibrated our method using the two open clusters NGC 5617 and Pismis 19, which happen to be in the field and for which we found estimates of the basic parameters in full agreement with the literature. We then applied the method to the general field stars and detected signatures of three different groups of stars, evenly distributed across the field of view, at 1.5^+0.5_–0.2, 2.5^+0.3_–0.5, and 5.1^+1.5_–1.1kpc. These distances are compatible with the location of the nowadays commonly accepted description of the Carina-Sagittarius and Scutum-Crux arms, at heliocentric distance of ∼2 an 5kpc, respectively. As a consequence, we consider these groups to be good candidates for tracing the location of these two inner arms. In line with previous studies, this investigation demonstrates once again how powerful is to use of U-band photometry to characterize ensembles of young stars, and make predictions on the spiral structure of the Milky Way.