Period and chemical evolution of SC stars

Abstract

The SC and CS stars are thermal-pulsing AGB stars with C/O ratio close to unity. Within this small group, the Mira variable BH Cru recently evolved from spectral type SC (showing ZrO bands) to CS (showing weak C2). Wavelet analysis shows that the spectral evolution was accompanied by a dramatic period increase, from 420 to 540 days, indicating an expanding radius. The pulsation amplitude also increased. Old photographic plates are used to establish that the period before 1940 was around 490 days. Chemical models indicate that the spectral changes were caused by a decrease in stellar temperature, related to the increasing radius. There is no evidence for a change in C/O ratio. The evolution in BH Cru is unlikely to be related to an on-going thermal pulse. Periods of the other SC and CS stars, including nine new periods, are determined. A second SC star, LX Cyg, also shows evidence for a large increase in period, and one further star shows a period inconsistent with a previous determination. Mira periods may be intrinsically unstable for C/O ~ 1; possibly because of a feedback between the molecular opacities, pulsation amplitude, and period. LRS spectra of 6 SC stars suggest a feature at wavelength > 15 micron, which resembles one recently attributed to the iron-sulfide troilite. Chemical models predict a large abundance of FeS in SC stars, in agreement with the proposed association.Comment: 14 pages, 20 figures. MNRAS, 2004, accepted for publication. Janet Mattei, one of the authors, died on 22 March, 2004. This paper is dedicated to her memor

The evolutionary state of Miras with changing pulsation periods

“…LX Cyg (Zijlstra et al 2004) is a particularly interesting example, which is classified as SC spectral type, just as BH Cru was before its spectrum started to change. Assuming that the sprocess enrichment in LX Cyg is intrinsic, this star would similarly be placed on the TP-AGB.…”

“…However, our sample stars do not necessarily closely follow this relation, because of the change in the pulsation period. For example, Zijlstra et al (2004) argue that the near-IR data of BH Cru is consistent with evolution at constant luminosity, and that any increase in luminosity did not exceed 10 percent. If this were true, then the star would now be fainter than other stars of a similar pulsation period that do follow the Mira period-luminosity relation.…”

“…It was initially classified as being of SC spectral type (with C/O essentially unity; Catchpole & Feast 1971;Keenan & Boeshaar 1980), but its type changed to CS and it experienced a marked period increase (Lloyd Evans 1985). Zijlstra et al (2004) interpret this spectral change as being due to a decrease in the effective temperature by some 200 K, rather than a change in C/O ratio (i.e. by a recent dredge-up of C).…”

“…our sample stars is fairly narrow. The C-star BH Cru does not exhibit the temperature-sensitive TiO bands, so we adopted the temperature of 3000 K reported by Zijlstra et al (2004). For the spectral synthesis, we used model no.…”

“…Ya'ari & Tuchman (1996) and identify a feedback mechanism between the pulsation and the stellar entropy structure, which can lead to mode switching. On the other hand, Zijlstra et al (2004) propose that a possibly chaotic feedback between molecular opacities, pulsation amplitude, and period can cause an unstable period. This is supposed to be most important for stars with a C/O ratio very close to unity (SC and CS spectral types), as small changes in the temperature in the atmosphere can cause large changes in the molecular abundances, and hence the opacities.…”