The Thickening of the Thin Disk in the Third Galactic Quadrant

CARRARO, GIOVANNI; VÁZQUEZ, RUBÉN A.; COSTA, EDGARDO; AHUMADA, JAVIER A.; GIORGI, EDGAR

ASTRONOMICAL JOURNAL

IOP PUBLISHING LTD

Lugar: Londres; Año: 2015 vol. 149 p. 1 - 17

0004-6256

In the third Galactic quadrant (180 l 270) of the Milky Way, theGalactic thin disk exhibits a signicant warp |shown both by gas and youngstars| bending down a few kpc below the formal Galactic plane (b = 0). Thiswarp shows its maximum at l 240, in the direction of the Canis Major constellation.In a series of papers we have traced the detailed structure of this regionusing open star clusters, putting particular emphasis on the spiral structure ofthe outer disk. We noticed a conspicuous accumulation of young star clusterswithin 2-3 kpc from the Sun and close to b = 0o, that we interpreted as the continuationof the Local (Orion) arm towards the outer disk. While most clusters(and young stars in their background) follow closely the warp of the disk, ourdecade-old survey of the spiral structure of this region led us to identify threeclusters, Haner 18(1 and 2) and Haner 19, which remain very close to b = 0and lie at distances (4.5, 8.0, and 6.4 kpc) where most of the material is alreadysignicantly warped.Here we report on a search for clusters that share the same properties asHaner 18 and 19, and investigate the possible reasons for such an unexpected occurrence.We present UBVRI photometry of 5 young clusters, namely NGC 2345,NGC 2374, Trumpler 9, Haner 20, and Haner 21, which also lie close to theformal Galactic plane. With the exception of Haner 20, in the background ofthese clusters we detected young stars that appear close to b = 0, and are locatedat distances up to 8 kpc from the Sun, thus deviating signicantly fromthe warp. These populations dene a structure that distributes over almost theentire third Galactic quadrant. We discuss this structure in the context of apossible thin disk aring, in full similarity with the Galactic thick disk.