Polaris, the nearest Cepheid in the Galaxy: atmosphere parameters, reddening and chemical composition

Abstract

We present the results of our analysis of spectroscopic observations of α UMi (Polaris) obtained in 1994 (five spectra) and 2001‐2004 (30 spectra). The mean atmospheric parameters we derived are as follows: T eff = 5968 K, � T eff = 50 K; log g = 2.2; � log g = 0.1; V t = 4.35 km s −1 , � V t = 0.1 km s −1 in 1994 and T eff = 6015 K, � T eff = 170 K; log g = 2.2, � log g = 0.3; V t = 4.35 km s −1 , � V t = 0.9 km s −1 in 2001‐2004. A comparison of our Teff with those obtained from (B − V )0 data during the last 60 yr shows both sporadic and regular changes with a period of 25‐30 yr, which is close to the orbital period, 29.9‐30.5 yr. We also determined the colour excess E(B − V ) = 0.034 mag, AV = 0.102 mag, and chemical composition for 30 elements to verify the hypothesis that the star crosses the Cepheid instability strip for the first time. With [Fe/H] =+ 0.07 dex, Polaris shows [C/H] =− 0.17 dex, [N/H] = +0.42 dex and [O/H] =− 0.00 dex, which corresponds to the third (or fifth) crossing. On the basis of the distance, radius and surface gravity values, Polaris is probably a first-overtone pulsator. Polaris is located near the blue edges of the Cepheid instability strip for the fundamental mode and the first overtone, and an abrupt decrease of the pulsational amplitude and its recent increase could be due to its binarity. Ke yw ords: stars: abundances ‐ stars: individual: α UMi.