The Period Change of the Cepheid Polaris Suggests Enhanced Mass Loss

Ignace

2015

A&A

Self Cite

The β Cephei stars represent an important class of massive star pulsators that probe the evolution of B-type stars and the transition from main sequence to hydrogen-shell burning evolution. By understanding β Cep stars, we gain insights into the detailed physics of massive star evolution, including rotational mixing, convective core overshooting, magnetic fields, and stellar winds, all of which play important roles. Similarly, modeling their pulsation provides additional information into their interior structures. Furthermore, measurements of the rate of change of pulsation period offer a direct measure of β Cephei stellar evolution. In this work, we compute state-of-the-art stellar evolution models assuming different amounts of initial rotation and convective core overshoot and measure the theoretical rates of period change, that we compare to rates previously measured for a sample of β Cephei stars. The results of this comparison are mixed. For three stars, the rates are too low to infer any information from stellar evolution models, whereas for three other stars the rates are too high. We infer stellar parameters, such as mass and age, for two β Cephei stars: ξ 1 CMa and δ Cet, which agree well with independent measurements. We explore ideas for why models may not predict the higher rates of period change. In particular, period drifts in β Cep stars can artificially lead to overestimated rates of secular period change.

Section: Discussionsupporting

confidence: 64%

Period change and stellar evolution ofβCephei stars

Ignace

2015

A&A

Self Cite

“…In terms of the Pp relation, these differences will not lead to a systematic shift of the Pp relation but will lead to a greater scatter of the p-factors used to fit a relation. Recent observations show that Cepheids undergo mass loss (Marengo et al 2010a,b;Barmby et al 2011;Matthews et al 2011;Neilson et al 2012). This mass loss is believed to lead to a far-infrared excess and may be consistent with the K-band excess observed by Kervella et al (2006);Mérand et al (2006Mérand et al ( , 2007.…”

Section: Why Do Theory and Observations Disagree?mentioning

confidence: 77%

Cepheid limb darkening, angular diameter corrections, and projection factor from static spherical model stellar atmospheres

Nardetto

Ngeow

et al. 2012

A&A

Context. One challenge for measuring the Hubble constant using classical Cepheids is the calibration of the Leavitt law or periodluminosity relationship. The Baade-Wesselink method for distance determination to Cepheids relies on the ratio of the measured radial velocity and pulsation velocity, the so-called projection factor and the ability to measure the stellar angular diameters. Aims. We use spherically-symmetric model stellar atmospheres to explore the dependence of the p-factor and angular diameter corrections as a function of pulsation period. Methods. Intensity profiles are computed from a grid of plane-parallel and spherically-symmetric model stellar atmospheres using the SAtlas code. Projection factors and angular diameter corrections are determined from these intensity profiles and compared to previous results. Results. Our predicted geometric period-projection factor relation including previously published state-of-the-art hydrodynamical predictions is not with recent observational constraints. We suggest a number of potential resolutions to this discrepancy. The model atmosphere geometry also affects predictions for angular diameter corrections used to interpret interferometric observations, suggesting corrections used in the past underestimated Cepheid angular diameters by 3-5%. Conclusions. While spherically-symmetric hydrostatic model atmospheres cannot resolve differences between projection factors from theory and observations, they do help constrain underlying physics that must be included, including chromospheres and mass loss. The models also predict more physically-based limb-darkening corrections for interferometric observations.

“…Therefore, it can be concluded that Polaris must be more distant and is consistent with the previously measured Hipparcos parallax. Similarly, the pulsation mode for Polaris has been previously derived based on the measured distance, hence radius, and combined with a period-radius relation (Neilson et al 2012a;Turner et al 2013). At a distance of 118 pc, the predicted mean radius is R = 41.1 R .…”

Section: Constraining the Distancementioning

confidence: 92%

“…Hence, Polaris is evolving along the third crossing of the instability strip on the Cepheid blue loop and its observed CNO abundances are consistent with dredge up during the previous red giant stage. Neilson et al (2012a) further suggested that to account for discrepancy between measured and predicted rates of period change, Polaris must be losing mass in an enhanced stellar wind withṀ = 10 −7 −10 −6 M yr −1 . Our understanding of Polaris is biased by the assumed distance, therefore measuring a precise and consistent distance is crucial.…”

Section: Introductionmentioning

confidence: 99%

Revisiting the fundamental properties of the Cepheid Polaris using detailed stellar evolution models

2014

A&A

Polaris the Cepheid has been observed for centuries, presenting surprises and changing our view of Cepheids and stellar astrophysics, in general. Specifically, understanding Polaris helps anchor the Cepheid Leavitt law, but the distance must be measured precisely. The recent debate regarding the distance to Polaris has raised questions about its role in calibrating the Leavitt law and even its evolutionary status. In this work, I present new stellar evolution models of Cepheids to compare with previously measured CNO abundances, period change and angular diameter. Based on the comparison, I show that Polaris cannot be evolving along the first crossing of the Cepheid instability strip and cannot have evolved from a rapidly-rotating main sequence star. As such, Polaris must also be at least 118 pc away and pulsates in the first overtone, disagreeing with the recent results of Turner et al. (2013, ApJ, 762, L8).