Probing Polaris’ Puzzling Radial Velocity Signals – Pulsational (In-)Stability, Orbital Motion, and Bisector Variations. (arXiv:1902.08031v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Anderson_R/0/1/0/all/0/1">Richard I. Anderson</a>
We investigate temporally changing variability amplitudes and the
multi-periodicity of the type-I Cepheid Polaris using 162 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
literature and provide an updated solution of Polaris’ eccentric 29.3 yr orbit.
While the inferred pulsation-induced RV amplitudes differ among individual data
sets, we find no evidence for time-variable RV amplitudes in any of the
separately considered, homogeneous data sets. Additionally, we find that
increasing photometric amplitudes determined using SMEI photometry are likely
spurious detections due to as yet ill-understood systematic effects of
instrumental origin. Given this confusing situation, further analysis of
high-quality homogeneous data sets with well-understood systematics is required
to confidently establish whether Polaris’ variability amplitude is subject to
change over time. We confirm periodic bisector variability periods of 3.97 d
and 40.22 d using Hermes BIS measurements and identify a third signal at a
period of 60.17 d. Although the 60.17 d signal dominates the BIS periodogram,
we caution that this signal may not be independent of the 40.22 d signal.
Finally, we show that the 40.22 d signal cannot be explained by stellar
rotation. Further long-term, high-quality spectroscopic monitoring is required
to unravel the complete set of Polaris’ periodic signals, which has the
potential to provide unprecedented insights into the evolution of Cepheid
variables.
We investigate temporally changing variability amplitudes and the
multi-periodicity of the type-I Cepheid Polaris using 162 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
literature and provide an updated solution of Polaris’ eccentric 29.3 yr orbit.
While the inferred pulsation-induced RV amplitudes differ among individual data
sets, we find no evidence for time-variable RV amplitudes in any of the
separately considered, homogeneous data sets. Additionally, we find that
increasing photometric amplitudes determined using SMEI photometry are likely
spurious detections due to as yet ill-understood systematic effects of
instrumental origin. Given this confusing situation, further analysis of
high-quality homogeneous data sets with well-understood systematics is required
to confidently establish whether Polaris’ variability amplitude is subject to
change over time. We confirm periodic bisector variability periods of 3.97 d
and 40.22 d using Hermes BIS measurements and identify a third signal at a
period of 60.17 d. Although the 60.17 d signal dominates the BIS periodogram,
we caution that this signal may not be independent of the 40.22 d signal.
Finally, we show that the 40.22 d signal cannot be explained by stellar
rotation. Further long-term, high-quality spectroscopic monitoring is required
to unravel the complete set of Polaris’ periodic signals, which has the
potential to provide unprecedented insights into the evolution of Cepheid
variables.
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