Polaris the Cepheid returns: 4.5 years of monitoring from ground and space

Abstract

We present the analysis of 4.5 years of nearly continuous observations of the classical Cepheid Polaris, which comprise the most precise data available for this star. We have made spectroscopic measurements from ground and photometric measurements from the WIRE star tracker and the SMEI instrument on the Coriolis satellite. Measurements of the amplitude of the dominant oscillation (P = 4 days), that go back more than a century, show a decrease from 120 mmag to 30 mmag (V magnitude) around the turn of the millennium. It has been speculated that the reason for the decrease in amplitude is the evolution of Polaris towards the edge of the instability strip. However, our new data reveal an increase in the amplitude by about 30% from 2003-2006. It now appears that the amplitude change is cyclic rather than monotonic, and most likely the result of a pulsation phenomenon. In addition, previous radial velocity campaigns have claimed the detection of long-period variation in Polaris (P > 40 days). Our radial velocity data are more precise than previous datasets, and we find no evidence for additional variation for periods in the range 3-50 days with an upper limit of 100 m/s. However, in the WIRE data we find evidence of variation on time-scales of 2-6 days, which we interpret as being due to granulation.

Accepted by ApJ. 8 pages. Quality of figures 1+3+7 degraded

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• Observations of Cepheids with the MOST satellite: Contrast between Pulsation Modes
• TESS observations of Cepheid stars: first light results
• The Kepler Cepheid V1154 Cyg revisited: light curve modulation and detection of granulation
• Evolutionary status of Polaris
• Probing Polaris' Puzzling Radial Velocity Signals - Pulsational (In-)Stability, Orbital Motion, and Bisector Variations
• HD344787: a true Polaris analogue?
• The VELOCE modulation zoo III. Detecting additional pulsation modes in optical spectra of classical Cepheids using semi-partial distance correlation periodograms
• Filling the gap: the missing eclipses of $\gamma$ Persei from 2005 and from 2006