Halpha emission-line stars in molecular clouds III. Canis Major
  A deep objective-prism survey for H-alpha emission stars towards the Canis
Major star-forming clouds was performed. A total of 398 Halpha emitters were
detected, 353 of which are new detections. There is a strong concentration of
these H-alpha emitters towards the molecular clouds surrounding the CMa~OB1
association, and it is likely that these stars are young stellar objects
recently born in the clouds. An additional population of H-alpha emitters is
scattered all across the region, and probably includes unrelated foreground dMe
stars and background Be stars. About 90% of the H-alpha emitters are detected
by WISE, of which 75% was detected with usable photometry. When plotted in a
WISE colour-colour diagram it appears that the majority are Class II YSOs.
Coordinates and finding charts are provided for all the new stars, and
coordinates for all the detections. We searched the Gaia-DR2 catalogue and from
334 Halpha emission stars with useful parallaxes, we selected a subset of 98
stars that have parallax errors of less than 20% and nominal distances in the
interval 1050 to 1350 pc that surrounds a strong peak at 1185 pc in the
distance distribution. Similarly, Gaia distances were obtained for 51 OB-stars
located towards Canis Major and selected with the same parallax errors as the
H-alpha stars. We find a median distance for the OB stars of 1182 pc, in
excellent correspondence with the distance from the H-alpha stars. Two known
runaway stars are confirmed as members of the association. Finally, two new
Herbig-Haro objects are identified.
