J/A+A/623/A146 Radial Velocity and BIS measurements of Polaris (Anderson, 2019)
Probing Polaris' puzzling radial velocity signals.
Pulsational (in-)stability, orbital motion, and bisector variations.
Anderson R.I.
<Astron. Astrophys. 623, A146 (2019)>
=2019A&A...623A.146A 2019A&A...623A.146A (SIMBAD/NED BibCode)
ADC_Keywords: Binaries, orbits ; Binaries, spectroscopic ; Line Profiles ;
Radial velocities ; Stars, variable ; Stars, supergiant ;
Stars, double and multiple
Keywords: stars: individual: Polaris - stars: variables: Cepheids -
binaries: spectroscopic - binaries: visual -
stars: oscillations - techniques: radial velocities
Abstract:
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
1.2m 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 ∼30m/s 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 29.3yr 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 3.97d and 40.22d
using Hermes BIS measurements and identify a third signal at a period
of 60.17d. Although the 60.17d signal dominates the BIS periodogram,
we caution that this signal may not be independent of the 40.22d
signal. Finally, we show that the 40.22d 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.
Description:
We present 161 high-precision Radial Velocity (RV) and Bisector
Inverse Span (BIS) measurements, cf. Table A.1 in the article. The
measurements are based on high-resolution (R∼85000) optical echelle
spectra recorded using the Hermes spectrograph mounted to the 1.2m
Flemish Mercator telescope at the Roque de los Muchachos Observatory,
La Palma, Canary Islands, Spain. RVs are measured by cross-correlation
and defined by the center of a Gaussian profile fitted to the
cross-correlation function (CCF). BIS is defined as the velocity
difference between the top and bottom parts of the CCF and provides a
convenient quantification of the time-variable line profile asymmetry.
Objects:
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RA (2000) DE Designation(s)
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02 31 49.09 +89 15 50.8 Polaris = * alf UMi
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 42 161 RV and BIS measurements of Polaris
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See also:
J/ApJ/683/433 : Polaris monitoring (Bruntt+, 2008)
J/MNRAS/452/715 : Optical polarization of the Polaris Flare (Panopoulou+, 2015)
Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 12 F12.6 d BJD Barycentric Julian Date (BJD-2400000)
14- 20 F7.3 km/s RV Barycentric Radial Velocity
22- 26 F5.3 km/s e_RV Radial Velocity precision (short-term) (1)
28- 33 F6.3 km/s BIS Bisector Inverse Span (line asymmetry) (2)
35- 42 I8 --- HermesID Unique Identifier of Hermes spectrum
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Note (1): The short-term RV precision of 15m/s is dominated by ambient pressure
variations. Standard star observations indicate a long-term instrumental
zero-point stability of approximately 50-70m/s.
Note (2): BIS measurements are intrinsically more precise than RVs thanks to the
high S/N of the spectra and stable instrumental line profile. BIS precision
is approximately 6-10 m/s as indicated by groups of consecutive
measurements.
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History:
Richard I. Anderson, randerso(at)eso.org
Acknowledgements:
This work would not have been possible without the help of several
observers, including Lovro Palaversa, Berry Holl, Maria Sueveges,
Michal Pawlak, Andreas Postel, Kateryna Kravchenko, Maroussia Roelens,
Nami Mowlavi, and May Gade Petersen. The competent and friendly
assistance of the Mercator support staff and staff at KU Leuven's
astronomy department (in particular Jesus Perez Padilla, Saskia Prins,
Florian Merges, Hans van Winckel, and Gert Rasking) was much
appreciated. The observations were made with the Mercator Telescope,
operated on the island of La Palma by the Flemish Community, at the
Spanish Observatorio del Roque de los Muchachos of the Instituto de
Astrofisica de Canarias. Hermes is supported by the Fund for
Scientific Research of Flanders (FWO), Belgium, the Research Council
of K.U. Leuven, Belgium, the Fonds National de la Recherche
Scientifique (F.R.S.-FNRS), Belgium, the Royal Observatory of Belgium,
the Observatoire de Geneve, Switzerland, and the Thueringer
Landessternwarte, Tautenburg, Germany.
(End) Richard I. Anderson [ESO, Germany], Patricia Vannier [CDS] 23-Feb-2019