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 ¼ 4 days), which go back more than a century, show a decrease from A V ¼ 120 to 30 mmag around the turn of the millennium. It has been speculated that the reason for the decrease in amplitude is the evolution of Polaris toward the edge of the instability strip. However, our new data reveal an increase in the amplitude by $30% from 2003 to 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 data sets, and we find no evidence for additional variation for periods in the range 3Y50 days with an upper limit of 100 m s À1 . However, in the WIRE data we find evidence of variation on timescales of 2Y6 days, which we interpret as being due to granulation.

On the Period and Amplitude Changes in Polaris and Other Short-Period Cepheids

“…The superimposed irregular behavior of Polaris resembles, in some ways, the Blazhko effect in RR Lyrae stars, as Evans et al (2004) and Bruntt et al (2008) have noted. Recently, a new theory of the Blazhko effect has been proposed (Stothers 2006) that may also explain Polaris.…”

“…However, their data reveal no trace of more than one mode being present, and the surprising change from falling to rising amplitude has occurred relatively smoothly. Similar objections have been raised by Bruntt et al (2008), who regarded the remarkable change of amplitude as being cyclic and possibly due to the beating of two very close periods. Although Arellano Ferro (1983) had already made essentially the same suggestion, he rejected it because the beat period would have to be of the order of a century long, which is unrealistic.…”

“…What makes Polaris an unusual Cepheid is its rapid decline of amplitude (in both light and radial velocity) between 1972 and 1985, and then its increase of amplitude since 1994 (Arellano Ferro 1983;Fernie et al 1993;Kamper & Fernie 1998;Turner et al 2005;Bruntt et al 2008). Meanwhile its period has been lengthening since at least 1844, although between 1963 and 1966 a glitch occurred when the period suddenly decreased and the amplitude started to behave erratically (Turner et al 2005).…”

Cepheids with the eyes of photometric space telescopes

“…These observations revealed that Polaris is not switching off and its pulsation amplitude started to increase after reaching a minimum around 2000. Weak excess variation in the low-frequency regime was tentatively connected to granulation patterns on the surface of the star ( [5]). …”

“…Interestingly, the first actual photometric mission was born from another mishap: the small optical star tracker camera on the otherwise unusable WIRE infrared space telescope was utilized to monitor bright stars. The well-known Cepheid Polaris (α UMi) was observed with it multiple times in 2004-5, complementing the more extended, but significantly lower-quality data set of the SMEI all-sky camera aboard the Coriolis satellite ( [5]). These observations revealed that Polaris is not switching off and its pulsation amplitude started to increase after reaching a minimum around 2000.…”