Classification of intrinsic variable stars. II. The red variables S and related types

Observations in the (UBVRI) system, mainly obtained with the 404nch (102-cm) reflector at Siding Spring Mountain, are presented for 34 stars of S or related type. Twenty-four of these stars are named variables (first light curves were obtained for two of these, 1 Oru and Renize 120), suspected variation in three stars (HD 58881 = RV 100, RD 75021, and RD 168227 = RV 170) is confirmed, and five new variables (RD 34738, RD 49368, RD 52432, RD 216672, and Henize 244) were found. Membership in the Wolf 630 group (R And and RR 1556), a Puppis group (HR 363) and Hyades group (iti Gru) is used to demonstrate that the S stars occur on the giant sequence of the old disk population. Several other group members are indicated, but the available proper motions are too uncertain for use in calibrating the luminosities. The available space motions of the S stars are characteristic of the old disk population. The present results, combined with those from Paper I and a separate discussion of the R and barium stars, lead to the following conclusions. (1) The energy distribution in the (VRI) passbands of the peculiar red giants match those of normal K and M giants. (2) The photometry indicates a division into the four groups Ba ii, R, N, and S stars, with the MS and SN objects associated with S stars, the NS objects associated with the N, stars and the high-velocity CR stars associated with type R. (3) All of the N stars and (with only one or two exceptions) all of the S stars are variable, whereas the barium stars and (again with one or two exceptions) the R stars are constant. (4) All of the Ba H, R, and S stars and 90 percent of the N stars are old disk-population objects of less than 2 or 3 solar masses. (5) The R, S, and N stars populate the giant sequence whereas the barium stars, like the hydrogenpoor variable R Coronae Borealis, are more luminous. (6) The N stars probably represent the first, and very transient, stage of instability resulting from core helium ignition, but the order, if any, of the transitions through R, Ba ii, S, and M variable stages is unknown.