High precision radial-velocity measurements of late-type evolved stars

Abstract

Late-type evolved stars include the variable red giants, like Mira and semi-regular variables, and the pulsating supergiant RV Tauri stars. Despite this, K-type giants in the past were generally believed to be constant in photometry and radial velocity, but in the last decade it has been discovered that this is not necessarily the case. This was the motivation for spectroscopic observations of 44 late-type evolved stars to be carried out over three years, in the hope of helping to determine how common K giants variable in radial velocity are and whether the variations are related to those seen for other late-type evolved stars.The observations were obtained at Mount John University Observatory and used with digital crosscorrelation to achieve relative radial velocities of 50 m/s precision. At this precision all but eight of the observed stars were found to be variable in radial velocity. Thirteen stars also had broad-band photometry taken, because any one of orbital motion, rotation with starspots, pulsation and the motion of large convective cells could cause apparent radial-velocity variations. Knowledge of the relationship between light and radial-velocity variations can help distinguish between these different mechanisms.The timescales for these different mechanisms were also estimated to help determine which was responsible for the variations observed. Generally the observed K-giant radial-velocity timescales were of the order of a few hundred days, which indicates they are due to one of binary motion, rotational modulation or non-acoustic non-radial pulsation. However, forM giants and supergiants the situation was very different with both short-term and long-term radial-velocity timescales being found in most stars. In most cases the short-term timescale is due to acoustic pulsation, while the long-term timescale could be due to any one of binary motion, surface features (including the motion of large convective cells) or non-acoustic non-radial pulsation.ii

Spectroscopic binaries among Hipparcos M giants

“…This 141 d period is clearly related to pulsation, since that periodicity is found in both the light and radial-velocity curves (Cummings et al 1999;Lebzelter et al 2005). The light curve has a large amplitude (about 2.5 mag in the visual), which cannot therefore be attributed to ellipsoidal variations.…”

The Tennessee State University Automatic Spectroscopic Telescope: Data Processing and Velocity Variation of Cool Giants

“…Walker et al (1989), for instance, discussed the velocity variations of five K giants and supergiants, and Smith et al (1989) followed velocity variations of three M supergiants. Velocity surveys of K and M stars by Larson et al (1999) and Cummings et al (1999), with external errors of ∼0.01 and 0.05 km s Ϫ1 , respectively, show that the normal K giants are all likely to be variable at the 0.03 km s Ϫ1 level. This evidence, along with our own observations, contradicts a recent optimistic prediction (Bizyaev & Smith 2007) that 12%-20% of K giants should have stable radial velocities.…”

The light curve of the semiregular variable L2 Puppis -- II. Evidence for solar-like excitation of the oscillations

“…Adopting L = 1500 L⊙, T eff = 3400 K and M = 1 M⊙ (Jura et al 2002), the original and revised scaling relations predict velocity semi-amplitudes for L2 Pup of about 400 m/s and 3 km s −1 , respectively. From the observational side, there are two published radial velocity measurements for L2 Pup: Cummings et al (1999) measured a semi-amplitude of about 2.5 km s −1 , while Lebzelter et al (2005) reported six data points in a full range of 12 km s −1 over almost three pulsation cycles, implying a semiamplitude of 6 km s −1 . Given the large uncertainties of the extrapolated relations, the observed amplitudes do not contradict predictions.…”