A review is presented of the past 165 years of observation of the 4-day Cepheid Polaris, including the exciting results of the last 50 years, an interval that has produced three orbital solutions for the spectroscopic binary subsystem, recently resolved by HST, parameters for the optical companion, precise measurement of the star's trigonometric parallax and angular diameter, evidence for a rapid increase in its pulsation period, and observations of the dramatic decline and recent partial recovery of its light amplitude. There has been considerable discussion about the exact nature of the star, with potential resolutions summarized here. It is also noted that many of the star's characteristics are shared by a small number of other Cepheids that display rapid period increases identical to those predicted for stars in the first crossing of the instability strip, small light amplitudes, and intrinsic colors typical of variables lying near the center of the strip, where Cepheids of largest amplitude reside. While all members of the group appear to display the canonical traits of first crossers of the instability strip, Polaris has one unique peculiarity: a brief hiatus in its monotonic period increment between 1963 and 1966 during which the pulsation period underwent a dramatic decrease. Has the average brightness of the Cepheid also increased over the last millennium?
We present the results of an analysis of 21 spectra of alpha UMi (Polaris) obtained in September - December 2015. Frequency analysis shows an increase of the pulsation period up to 8.6 min in comparison to the 2007 observational set. The radial velocity amplitude comes to 4.16 km s^-1, and it approximately twice the one found in 2007. The average Teff = 6017 K, and it is close to the value determined for the 2001-2004 set. Therefore Polaris moves to the red edge of the Cepheid instability strip (CIS)
A recently presented HST/FGS parallax measurement of the Polaris system has been interpreted as evidence for the Cepheid Polaris Aa to be pulsating in the second overtone. An age discrepancy between components A and B has been noted and discussed in terms of a stellar merger. Here I show that the new parallax of Polaris is consistent with a simpler interpretation of Polaris as a $7 \rm{M}_\odot,$ first-overtone, classical Cepheid near the hot boundary of the first instability strip crossing. This picture is anchored to rates of period change, the period-luminosity relation, the location in col…
As part of a program to determine dynamical masses of Cepheids, we have imaged the nearest and brightest Cepheid, Polaris, with the Hubble Space Telescope Wide Field Planetary Camera 2 and Wide Field Camera 3. Observations were obtained at three epochs between 2007 and 2014. In these images, as in HST frames obtained in 2005 and 2006, which we discussed in a 2008 paper, we resolve the close companion Polaris Ab from the Cepheid Polaris Aa. Because of the small separation and large magnitude difference between Polaris Aa and Ab, we used PSF deconvolution techniques to carry out astrometry of th…
Classical Cepheids are essential objects in the study of stellar evolution and cosmology; however, we know little about their magnetic properties. We report the detection of Stokes $V$ features interpreted as Zeeman signatures in four classical Cepheids using high-resolution spectropolarimetric observations obtained with ESPaDOnS at CFHT. Eight observations of $η$ Aql were acquired in 2017 covering its 7.2 d pulsation period, and single observations of Polaris, $ζ$ Gem, $δ$ Cep and RT Aur were obtained in 2020 as part of our ongoing systematic survey. We use mean circular polarization Stokes $…
Polaris, the nearest and brightest Cepheid, is a potential anchor point for the Leavitt period-luminosity relation. However, its distance is a matter of contention, with recent advocacy for a parallax of ~10 mas, in contrast with the Hipparcos measurement of 7.54+/-0.11 mas. We report an independent trigonometric parallax determination, using the Fine Guidance Sensors (FGS) on the Hubble Space Telescope. Polaris itself is too bright for FGS, so we measured its 8th-magnitude companion Polaris B, relative to a network of background reference stars. We converted the FGS relative parallax to absol…
Polaris is the nearest Cepheid to us and as such holds a special place in our understanding of Cepheids in general and the Leavitt Law. In the past couple of decades, we have learned many new things about the star as a Cepheid and as the primary component of a multiple star system. As such, we are more precisely measuring the mass, radius and evolution of Polaris. However, as we learn more, it is becoming clear that we understand less. There is evidence that Polaris is much less massive than stellar evolution models suggest and that Polaris is a much younger star than its main sequence compani…
X-ray observations have been made of a sample of 20 classical Cepheids, including two new observations (Polaris and {\it l} Car) reported here. The occurrence of X-ray flux around the pulsation cycle is discussed. Three Cepheids are detected ($δ$ Cep, $β$ Dor, and Polaris). X-rays have also been detected from the low--mass F, G, and K companions of 4 Cepheids (V473 Lyr, R Cru, V659 Cen, and W Sgr), and one hot companion (S Mus). Upper limits on the X-ray flux of the remaining Cepheids provide an estimate that 28\% have low mass companions. This fraction of low--mass companions in intermediate …
The VELOCE modulation zoo III. Detecting additional pulsation modes in optical spectra of classical Cepheids using semi-partial distance correlation periodograms
We investigate temporally changing variability amplitudes and the multi-periodicity of the type-I Cepheid Polaris using 161 high-precision radial velocity (RV) and bisector inverse span (BIS) measurements based on optical spectra recorded using Hermes at the 1.2 m Flemish Mercator telescope on La Palma, Canary Islands, Spain. Using an empirical template fitting method, we show that Polaris' RV amplitude has been stable to within $\sim 30 \rm{m s}^{-1}$ between September 2011 and November 2018. We apply the template fitting method to publicly accessible, homogeneous RV data sets from the litera…
In a step toward understanding the origin of the Galactic Halo, we have reexamined Type II Cepheids (T2C) in the field with new input from the second data release (DR2) of Gaia. For 45 T2C with periods from 1 to 20 days, parallaxes, proper motions, and [Fe/H] values are available for 25 stars. Only 5 show [Fe/H] < -1.5, while the remaining stars show thick disk kinematics and [Fe/H] > -0.90. We have compared the T2C stars of the field with their cousins in the globular clusters of the Halo and found that the globular clusters with T2C stars show metallicities and kinematics of a pure Halo popu…
Classical Cepheids (DCEPs) are the most important primary indicators for the extragalactic distance scale, but they are also important objects per se, allowing us to put constraints on the physics of intermediate-mass stars and the pulsation theories. We have investigated the peculiar DCEP HD 344787, which is known to exhibit the fastest positive period change among DCEPs along with a quenching amplitude of the light variation. We have used high-resolution spectra obtained with HARPS-N@TNG for HD 344787 and the more famous Polaris DCEP, to infer their detailed chemical abundances. Results from…
The investigation of the non-linearity of the Leavitt law is a topic that began more than seven decades ago, when some of the studies in this field found that the Leavitt law has a break at about ten days. The goal of this work is to investigate a possible statistical cause of this non-linearity. By applying linear regressions to OGLE-II and OGLE-IV data, we find that, in order to obtain the Leavitt law by using linear regression, robust techniques to deal with influential points and/or outliers are needed instead of the ordinary least-squares regression traditionally used. In particular, by u…
Classical Cepheids in open clusters are key ingredients for stellar population studies and the characterization of variable stars, as they are tracers of young and massive populations and of recent star formation episodes. Cluster Cepheids are of particular importance since they can be age dated by using the cluster's stellar population to obtain the Cepheid period-luminosity-age relation. In this contribution, we present the preliminary results of an all-sky search for classical Cepheids in Galactic open clusters by taking advantage of the unprecedented astrometric precision of the second dat…
The VELOCE Modulation Zoo I. Spectroscopic detection of non-radial modes in the first-overtone Cepheids BG Crucis, QZ Normae, V0391 Normae, and V0411 Lacertae
As the RR Lyrae stars evolve on the Hertzsprung-Russell diagram they are believed to become short period Type II Cepheids, known as BL Her type (with a pulsation period from $1$ to $3-8$ days). Assuming that their mass is around $0.5 - 0.6 {\rm M}_{\odot}$, and that they are low metallicity objects, they were thought to belong to the halo of the Milky Way. We investigated seven Galactic short period Type II Cepheids (BL Her, SW Tau, V553 Cen, DQ And, BD Cas, V383 Cyg, and KT Com) in order to establish their membership within the Galactic structure using the kinematic approach. $Gaia$ should pr…
