Searching Beyond the Obscuring Dust Between the Cygnus-Aquila Rifts for Cepheid Tracers of the Galaxy's Spiral Arms

Volume 37 number 2 (2009)

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Daniel J. Majaess

David G. Turner

David J. Lane

Saint Mary’s University, Halifax, Nova Scotia, Canada

Abstract

A campaign is described, open to participation by interested AAVSO members, of follow-up observations for newly-discovered Cepheid variables in undersampled and obscured regions of the Galaxy, a primary objective being to use these supergiants to clarify the Galaxy’s spiral nature. Preliminary multiband photometric observations are presented for three Cepheids discovered beyond the obscuring dust between the Cygnus and Aquila Rifts (40° ≤ ℓ ≤ 50°), a region reputedly tied to a segment of the Sagittarius-Carina arm which appears to cease unexpectedly. The data confirm the existence of exceptional extinction along the line of sight at upwards of AV≅6 magnitudes (d ≅ 2 kpc, ℓ ≅ 47°), however, the noted paucity of optical spiral tracers in the region does not arise solely from incompleteness owing to extinction. A hybrid spiral map of the Galaxy comprised of classical Cepheids, young open clusters and H II regions, and molecular clouds presents a consistent picture of the Milky Way and confirms that the three Cepheids do not populate the main portion of the Sagittarius-Carina arm, which does not emanate locally from this region. The Sagittarius-Carina arm, along with other distinct spiral features, is found to deviate from the canonical logarithmic spiral pattern. Revised parameters are also issued for the Cepheid BY Cas, and it is identified on the spiral map as lying in the foreground to most young associations in Cassiopeia. A Fourier analysis of the light curve of BY Cas implies overtone pulsation, and the Cepheid is probably unassociated with the open cluster NGC 663 since the distances, ages, and radial velocities do not match.