Orientation Parameters of the Cepheid System in the Galaxy

Abstract

Based on the distribution of long-period Cepheids, we have redetermined the orientation parameters of their principal plane in the Galaxy. Based on 299 Cepheids with heliocentric distances $r<20$ kpc and pulsation periods $P\geq5^d$, we have found the directions of the three principal axes of the position ellipsoid: $L_1=281.0\pm0.1^\circ,$ $B_1=-1.9\pm0.1^\circ,$ $L_2= 11.0\pm0.7^\circ,$ $B_2=0.2\pm0.1^\circ$ and $L_3=275.9\pm0.7^\circ,$ $B_3=88.1\pm0.1^\circ$. Thus, the line of nodes $l_\Omega=L_3+90^\circ=5.9^\circ$ is very close to the direction to the Galactic center; the Cepheid symmetry plane is inclined to the Galactic plane approximately by $-2^\circ$ in the direction of the first axis ($L_1$). The direction of the line of nodes found from old Cepheids ($P<5^d$) differs significantly and is $l_\Omega=298^\circ$. The elevation of the Sun above the Galactic plane has been estimated from 365 closer stars ($r<4$ kpc) without any constraint on the pulsation period to be $h_\odot=23\pm5$ pc.