Stellar tracers of the Cygnus Arm I. Spectroscopic study of bright photometric candidates

TL;DR

This spectroscopic study identifies distant early-type stars in the second Galactic quadrant, revealing their association with the Outer or Cygnus Arm and suggesting a reevaluation of certain stellar associations.

Contribution

The paper provides new spectroscopic data and distance estimates for stars, supporting the identification of the Cygnus Arm and refining the understanding of stellar associations in that region.

Findings

Many stars are located beyond 4 kpc, tracing the Outer or Cygnus Arm.

The association Cam OB3 is confirmed to be on the Cygnus Arm.

The extent of Cas OB4 and Aur OB2 associations should be reevaluated.

Abstract

We present medium-resolution spectroscopy of a sample of stars in the second Galactic quadrant selected from the literature because their colours suggest that they are moderately-reddened early-type stars at very large distances. From the derived spectral types and observed colours, we calculate distances to all these objects. For a sizable fraction of our sample, we find distances well in excess of what is expected for Perseus Arm objects, even allowing for rather generous errors. In the interval l=150degr-180degr, there is a large number of objects with distances in excess of 4 kpc, which are likely tracing the Outer or Cygnus Arm. In particular, we find that the association Cam OB3 is placed on this Arm. Based on our results, the extent and definition of the associations Cas OB4 and Aur OB2 need to be reevaluated.