Long-period, low-amplitude radial velocity variations in the K giant star Herculis: rotation, substellar companion or non-radial pulsations?

Abstract

Precise stellar radial velocity (RV) measurements (σ≈ 20 m s-1 ) spanning more than 2 yr are presented for the K giant star π Herculis. These show variability with a period of 613 ± 57 d and an amplitude of 150 ± 12 m s-1 . Radial pulsations can be excluded as a mechanism for this variability because the observed period is more than an order of magnitude greater than the expected period of the fundamental radial mode. Tenable hypotheses for the RV variability include rotational modulation by surface structure, non-radial pulsations, or a substellar companion of mass at least 27 MJupiter in orbit 3 au from the star. An upper limit of 1400 d to the rotational period for π Herculis is determined using published values of the angular diameter and distance to π Herculis as well as an estimate of the projected rotational velocity. Rotational modulation is thus a viable mechanism for the RV variability. The RV variations were also fitted using two models: (i) a stellar surface covered with cool spots and (ii) non-radial pulsations. Both models could adequately reproduce the RV curve although the spot model predicts photometric variations of Δ V ∼ 0.1 mag. As a result of the long period, non-radial pulsations may be g-mode or r-mode oscillations, which implies that most of the atmospheric motion of the star is in the horizontal direction. If these modes are indeed present then the corresponding photometric amplitude for non-radial pulsations may be quite small. The radial velocity variations were also measured using subsections of the spectral region. Weak spectral lines yielded an RV amplitude of ≈ 140 m s-1 whereas the stronger lines yielded an RV amplitude of ≈ 220 m s-1 with a phase shift of about 60 ° with respect to the weaker lines. This seems to support the pulsation hypothesis as the cause of the RV variability, although an analysis of more lines is needed to confirm this. Also, a period of 90.3 d with an amplitude 50 m s-1 is found in the residual RV measurements after subtracting the 613-d component. The presence of two periods also argues in favour of non-radial pulsations, although at the present time one cannot exclude low-mass companions or surface structure as a cause for at least one of the observed periods. Photometric measurements as well as detailed analysis of the changes in the spectral line shapes using high-resolution data may be required to distinguish between the companion object and non-radial pulsation hypotheses.