The classification of intrinsic variable stars. I. The red variables of type N.

This is the first in a series of papers based on an examination of the classification system for intrinsic variables and concerns extensive (UB VRI) observations of the 35 N-type variables for which relatively accurate apparent motions are available. The N-type variables are found to be divided into all three population groups, (1) young disk (VD) objects such as the carbon stars in the Large Magellanic Cloud with = -5 mag (W and BL Ori and possibly V Tau), (2) old disk (OD) population stars of mass near 1.25 and 1 = -3 mag, such as the N-type member of NGC 2660 and the brighter component of the common-proper-motion pair HD 75021/2, and (3) the halo population, which consists of CH stars (V Ari and TT CVn) with near -3.5, mag. The ratio of these populations in the present sample is OD/VD/halo = 30/3/2. The old disk objects occur in a distinct break in the distribution along the giant branch, at R - I = +0.9 mag and almost certainly represent the first, and very transient, stage of instability. These stars occur at the luminosity where the ignition of the core helium halts the ascent of the giant branch in the theoretical evolutionary track of 1.25 o stars. The young disk population objects at = -5 mag probably represent the largest (9 o) mass at which N-type stars are formed. No obvious spectroscopic differences appear between the young and old disk population stars, although the halo objects are characterized as CII stars.