The Radial Velocity and Spectral Line Bisector Variability of Polaris

Abstract

We present precise stellar radial velocity measurements for the Cepheid-type star Polaris taken in 1992È1993. The peak-to-peak amplitude of the pulsations was 1.555^0.056 km s~1 in epoch 1992.4 and 1.517^0.047 km s~1 in epoch 1993.2. These amplitudes are comparable to the one measured by Dinshaw et al. in 1987.75 and Mg I j5528 lines also show variations with the 40 day period, strongly indicating that this period is indeed real. The presence of line bisector variations excludes a low-mass companion object as a cause of the residual radial velocity variations. Two models are considered for the residual radial velocity and bisector span variations : starspots and nonradial pulsations. Although both can reproduce the amplitude and shape of the residual radial velocity variations, the spot models considered were unable to reproduce the overall shape and amplitude of the bisector span variations. The best-Ðt spot model was provided by a single spot with a temperature 500 K cooler than the photosphere with a macroturbulent velocity of zero in the spot. If surface features do exist on Polaris and are responsible for the residual radial variations for this star, then possibly they have a temperature structure and distribution more complicated than the simple models considered here. The best Ðt to all the observed variations was provided by a nonradial m \ 4 mode, although these were not entirely satisfactory having a phase shift of about 0.25 between the observed and predicted bisector variations. Possible explanations for this shift include temperature e †ects, a source function originating in a dynamic stellar atmosphere, or a di †erent pulsation mode than the one that was considered. Although the exact pulsational mode is yet to be identiÐed, we believe that the residual radial velocity variations are due to a long-period pulsation mode. Supporting evidence comes from the fact that three di †erent residual variations with periods near 40 days have now been seen in Polaris and it is unlikely that these variations are due to rotational modulation by surface features on a star that is di †erentially rotating.

Line-depth-ratio temperatures for the close binary ν Octantis: new evidence supporting the conjectured circumstellar retrograde planet

“…Such an example of a variation of otherwise common stellar surface features (namely sunpots and starspots) are the low-contrast 'starpatches' proposed by Toner & Gray (1988). Similarly, Hatzes & Cochran (2000) investigated the possibility of 'macroturbulent' spots for the behaviour of Polaris (where the spot was distinguished by having a substantially lower macroturbulent velocity than the surrounding surface). However, predictions of such new phenomena have their own risks, since these can in turn be erroneous.…”

Line profile variations in classical Cepheids

“…Hatzes & Cochran (2000) have found a residual component in high-accuracy radial velocities with an amplitude of 400 m s −1 and a period of about 40 days. They argue that this is more likely to be due to a non-radial pulsation, rather than to a low-mass companion or to rotational modulation from spots.…”

Precise radial velocities of giant stars

“…It is recognized that intrinsic stellar activity, such as cool spots, can create RV variations in giant stars that can mimic the presence of companions (e.g., Hatzes & Cochran 2000;Hatzes et al 2004). This poses an additional challenge for validating the interpretation of a periodic RV change as a bona fide planet, when compared to inactive MS stars.…”

“…Nevertheless, hypothetic surface structure phenomena might mimic the presence of an exoplanet. For example, Hatzes & Cochran (2000) investigated the possible existence of a macroturbulent spot to explain the RV variation of Polaris. Given the right conditions, this dark spot might cause a large RV oscillation without a significant photometric variation.…”