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

GIOVANNI CARRARO; RUBEN A. VÁZQUEZ; CHRISTIAN MONI BIDIN; JOAO VICTOR SALES SILVA

ASTRONOMICAL JOURNAL

IOP PUBLISHING LTD

Lugar: Londres; Año: 2017

0004-6256

We employ optical photometry and high-resolution spectroscopy to study a fieldtoward 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 isthe spatial distribution of the young population, which consists of dwarfs with spectraltype as early as B6, and distribute in a blue plume feature in the colour-magnitudediagram. For the first time we confirm spectroscopically that most of these stars areearly type stars, and not blue stragglers nor halo/thick disk sub-dwarfs. Moreover, theyare not evenly distributed along the line of sight, but crowd at heliocentric distancesbetween 6.6 and 8.2 kpc. We compare these results with present-day understanding ofthe spiral structure of the Ga;axy and suggest that they traces the outer arm. Thisrange in distances challenges current Galactic models adopting a disk cut-off at 14 kpcfrom the Galactic center. The young dwarfs overlap in space with an older componentwhich identifies the old Galactic thin disk. Both young and old populations are confinedin space since the disk is warped at the latitude and longitude of Tombaugh 1. Themain effects of the warp are that the line of sight intersects the disk and entirely crossesit 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 theformal Galactic plane. We finally analysed a group of giant stars, which turn out tobe located at very different distances, and to possess very different chemical properties,with no obvious relation with the other populations.