Galactic structure in the outer disk: the field in the line of sight to
  the intermediate-age open cluster Tombaugh 1
  We employ optical photometry and high-resolution spectroscopy to study a
field toward the open cluster Tombaugh 1, where we identify a complex
population mixture, that we describe in terms of young and old Galactic thin
disk. Of particular interest is the spatial distribution of the young
population, which consists of dwarfs with spectral type as early as B6, and
distribute in a {\it blue plume} feature in the colour-magnitude diagram. For
the first time we confirm spectroscopically that most of these stars are early
type stars, and not blue stragglers nor halo/thick disk sub-dwarfs. Moreover,
they are not evenly distributed along the line of sight, but crowd at
heliocentric distances between 6.6 and 8.2 kpc. We compare these results with
present-day understanding of the spiral structure of the Galaxy and suggest
that they traces the outer arm. This range in distances challenges current
Galactic models adopting a disk cut-off at 14 kpc from the Galactic center. The
young dwarfs overlap in space with an older component which identifies the old
Galactic thin disk. Both young and old populations are confined in space since
the disk is warped at the latitude and longitude of Tombaugh~1. The main
effects of the warp are that the line of sight intersects the disk and entirely
crosses it at the outer arm distance, and that there are no traces of the
closer Perseus arm, which would then be either un-important in this sector, or
located much closer to the formal Galactic plane. We finally analysed a group
of giant stars, which turn out to be located at very different distances, and
to possess very different chemical properties, with no obvious relation with
the other populations.
