What we can learn from constant
stars, and what means constant?
Ernst Paunzen
Department of Theoretical Physics and Astrophysics
Masaryk University
Brno
Czech Republic
Why do we need non-variable stars?
 Flux standards
 RV standards
 Calibration of
stellar formation
and evolutionary
tracks
 Photometric
calibration of
effective
temperature and
metallicity
Gaia Collaboration, 2019 A&A, 623, A110
What does non-variability mean?
 A star is constant (not variable) for a
given
1. Frequency range
2. Time basis of the observations
3. Amplitude – Noise level
4. Wavelength region – filter
5. Applied “pipeline software”
6. Applied time series analysis method
Motivation
 From Gaia better
estimates of the
luminosities and
hopefully also for
the temperatures
 Zoom in and
define boxes
Motivation
 What determines if a star with “identical”
effective temperature and luminosity
(same age) is variable or not?
 Some guesses
1. Rotation
2. Metallicity
3. Binarity
4. Stellar Magnetic Field
5. Circumstellar Material
6. Inclination
Gaia’s view
Gaia Collaboration, 2019 A&A, 623, A110
The study of amplitudes
 An amplitude-period-metallicity relation
exists for Cepheids and RR-Lyrae stars
Szabados & Klagyivik, 2012, A&A, 537, A81
P > 10.47d
P < 10.47d
1OT
The study of amplitudes
 Amplitude versus Flare Activity and Spots
Yang et al., 2017, ApJ, 849, 36
Jackson&Jeffries,2013,MNRAS,431,1883
Available light curves
 The current available amount of light
curves is huge
 Different cadences, filter/wavelength
ranges, qualities, magnitude ranges, …
 Also a lot of ground based data available
An example – CoRoT
Which one is the known variable star?
An example - TESS
An example - TESS
An example – TESS and Kepler K2
Light curve of the same object – constant or variable?
A short recipe
 Learn about the “instrumental frequencies”
 Study known variable stars in your data set
 Be aware of the time basis
 Be aware of the frequencies removed by
your method/algorithm
 Divide the investigated frequency range
 Be aware of irregular variability
 Use more than one time series analysis
method, i.e. Fourier and String based
techniques
An example – TESS and Kepler K2
Our pipeline software
Thank you for your attention