New Findings for the Blue Plume Stars in the Canis Major Over-Density

TL;DR

This study uses new photometry and spectroscopy to analyze Blue Plume stars in the Canis Major Over-Density, finding they are likely part of the Perseus spiral arm rather than a dwarf galaxy, challenging previous assumptions.

Contribution

It provides improved reddening estimates and stellar parameters, offering new evidence against the dwarf galaxy hypothesis for the CMa over-density.

Findings

BP stars are at ~6 kpc, consistent with the Perseus arm.

Most BP stars are main-sequence B/A types with typical surface gravities.

Results challenge the dwarf galaxy origin hypothesis for CMa.

Abstract

We obtained new UBV photometry and spectroscopy of Blue Plume (BP) stars in fields near the center of the Canis Major Over-Density (CMa). We combined analysis of the color-color diagrams with a new comparison of the hydrogen Balmer-line profile to the reddening-free Q parameter to improve the reddening and extinction estimates for this low-latitude, differentially reddened, area of the sky. Results of our stellar parameter analysis for B/A spectral type stars associated with the BP show that the majority of the stars have main-sequence surface gravities placing them at an average heliocentric distance of <D> = 6.0 +/- 2.7 kpc. This distance is more consistent with membership in the intervening Perseus spiral arm and strongly suggests that the BP stars are not associated with the other stellar populations previously reported to make up the CMa. This result casts serious doubt on the proposed dwarf galaxy origin for the CMa.