Estimation of the Parameters of the Spiral Pattern in the Galaxy Based on a Sample of Classical Cepheids

Abstract

We consider a sample of Galactic classical Cepheids with highly accurate estimates of their distances taken from Skowron et al., where they were determined based on the period-luminosity relation. We have refined the geometric characteristics of two spiral arm segments—the Carina-Sagittarius and Outer ones. For this purpose, we have selected 269 Cepheids belonging to the Carina-Sagittarius arm with ages in the range 80-120 Myr. From them we have estimated the pitch angle of the spiral pattern $i={-}11.9^{\circ}± 0.2^{\circ}$ and the position of this arm $a₀=7.32± 0.05$ kpc for the adopted $R₀=8.1± 0.1$ kpc. In the Outer arm we have selected 343 Cepheids with ages in the range 120-300 Myr. From them we have found $i={-}11.5^{\circ}± 0.5^{\circ}$ and $a₀=12.89± 0.06$ kpc. Adhering to the model of a grand-design spiral pattern in the Galaxy with one pitch angle for all arms, we can conclude that this angle is close to ${-}12^{\circ}$.