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We employ optical photometry and high-resolution spectroscopy to study afield toward the open cluster Tombaugh 1, where we identify a complexpopulation mixture, that we describe in terms of young and old Galactic thindisk. Of particular interest is the spatial distribution of the youngpopulation, which consists of dwarfs with spectral type as early as B6, anddistribute in a {\it blue plume} feature in the colour-magnitude diagram. Forthe first time we confirm spectroscopically that most of these stars are earlytype stars, and not blue stragglers nor halo/thick disk sub-dwarfs. Moreover,they are not evenly distributed along the line of sight, but crowd atheliocentric distances between 6.6 and 8.2 kpc. We compare these results withpresent-day understanding of the spiral structure of the Galaxy and suggestthat they traces the outer arm. This range in distances challenges currentGalactic models adopting a disk cut-off at 14 kpc from the Galactic center. Theyoung dwarfs overlap in space with an older component which identifies the oldGalactic thin disk. Both young and old populations are confined in space sincethe disk is warped at the latitude and longitude of Tombaugh~1. The maineffects of the warp are that the line of sight intersects the disk and entirelycrosses it at the outer arm distance, and that there are no traces of thecloser Perseus arm, which would then be either un-important in this sector, orlocated much closer to the formal Galactic plane. We finally analysed a groupof giant stars, which turn out to be located at very different distances, andto possess very different chemical properties, with no obvious relation withthe other populations.

Galactic structure in the outer disk: the field in the line of sight to the intermediate-age open cluster Tombaugh 1