Variability in binary and single red giants

Abstract

Variable red giants populate several sequences in the period-luminosity plane. Most sequences represent radially pulsating variables, but two sequences show more enigmatic behaviour. Sequence D is populated by stars known as Long Secondary Period Variables (LSPVs). Two forms of variation are evident in their light curves, the first of which is an overtone pulsation lying on one of the shorter-period pulsation sequences. The cause of the Long Secondary Period (LSP) has not yet been successfully identified. Many models have been proposed for the elusive LSP cause, including radial and nonradial pulsation, binarity, and orbiting dust clouds. Sequence E consists of stars with small light amplitudes, regular periods, and the symmetric variations characteristic of close binaries. They are speculated to be ellipsoidal variables on the basis of their light curves, although radial velocity curves are necessary to confirm this. Here we study these two groups of red giant variables, the inscrutable LSPVs and the sequence E binaries.

First, we present a sample of LSPVs analysed using new radial velocity curves from VLT spectra, alongside MACHO and OGLE light curves. We find the LSP radial velocity variation is too large for radial pulsation, as it implies a fractional radius change of > 30% over the LSP. These large radius variations are not accompanied by correspondingly large amplitudes in Teff or light, making radial pulsation an unlikely LSP cause. However, the sample's velocity amplitudes show a striking similarity, and small values for binary motion, implying small and similar companion stars, ~ 0.09M0{u2299}. Companions of this mass are thought to be extremely rare. Additionally, the angles of periastron indicate surprisingly similar orbital orientations, unexpected in a sample of binaries. Therefore binarity also seems to be ruled out as an explanation for the LSPs. The mid-IR colours of LSPVs show they have a significant mid-IR excess compared to stars without LSPs, at both 8 and 24 {u03BC}m. This indicates the LSP is associated with enhanced circumstellar dust. We also find that the near-IR J -K colour of LSPVs is unaffected by the 24 {u03BC}m excess. This suggests the circumstellar dust is in a disk or clumpy configuration. Although dusty disks could be associated with binarity, the problems with the binary explanation, described above, remain.

We present two samples of sequence E binaries. Using the first, we show that their combined MACHO light curves and VLT-derived velocity curves constitute unambiguous evidence that sequence E stars are ellipsoidal variables. By comparing the phased light and velocity curves of LSPVs and sequence E variables, we show that LSPs are not caused by ellipsoidal variability, and that these two variable types are fundamentally different. Sequence E stars show no evidence of a mid-IR excess that would indicate excess circumstellar dust, unlike the LSPVs. Our second sequence E sample is comprised of stars with light curve shapes believed to denote eccentric orbits.

Our second sequence E sample is comprised of stars with light curve shapes believed to denote eccentric orbits. We confirm their eccentric nature and model the OGLE light curves and observed radial velocity curves using the Wilson-Devinney code, obtaining complete system parameters. Most of our eccentric sequence E star light curves have unequal maxima as well as minima, a phenomenon not observed in non-eccentric sequence E variables. We find evidence that the shape of the ellipsoidal red giant changes throughout the orbit, due to high eccentricity and the varying influence of the companion. The complete solutions obtained for these systems will allow future modelling to determine how eccentricity is maintained in these and other evolved binaries. Show more