J/A+A/646/A6 sigma Gem BV photometry (Korhonen+, 2021)
Observing the changing surface structures of the active K giant sigma Geminorum
with SONG.
Korhonen H., Roettenbacher R.M., Gu S., Grundahl F., Andersen M.F.,
Henry G.W., Jessen-Hansen J., Antoci V., Palle P.L.
<Astron. Astrophys. 646, A6 (2021)>
=2021A&A...646A...6K 2021A&A...646A...6K (SIMBAD/NED BibCode)
ADC_Keywords: Stars, late-type ; Photometry, CCD ; Spectroscopy
Keywords: stars: activity - stars: late-type - stars: rotation -
starspots - stars: individual: sigma Geminorum
Abstract:
We aim to study the spot evolution and differential rotation in the
magnetically active cool K-type giant star sigma Gem from broadband
photometry and continuous spectroscopic observations that span 150
nights.
We use high-resolution, high signal-to-noise ratio spectra obtained
with the Hertzsprung SONG telescope to reconstruct surface
(photospheric) temperature maps with Doppler imaging techniques. The
303 observations span 150 nights and allow for a detailed analysis of
the spot evolution and surface differential rotation. The Doppler
imaging results are compared to simultaneous broadband photometry from
the Tennessee State University T3 0.4m Automated Photometric
Telescope. The activity from the stellar chromosphere, which is higher
in the stellar atmosphere, is also studied using SONG observations of
Balmer Halpha line profiles and correlated with the photospheric
activity.
The temperature maps obtained during eight consecutive stellar
rotations show mainly high-latitude or polar spots, with the main spot
concentrations above latitude 45°. The spots concentrate around
phase 0.25 near the beginning of our observations and around phase
0.75 towards the end. The photometric observations confirm a small
jump in spot phases that occurred in February 2016. The
cross-correlation of the temperature maps reveals rather strong
solar-like differential rotation, giving a relative surface
differential rotation coefficient of alpha=0.10±0.02. There is a
weak correlation between the locations of starspots and enhanced
emission in the chromosphere at some epochs.
Description:
Photometric and spectroscopic observing logs of sigma Geminorum.
The photometric observations were obtained between 27 October 2015 and
5 May 2016 with the Tennessee State University T3 0.4m Automated
Photometric Telescope at Fairborn Observatory in Arizona. The
observations consist of Johnson B and V differential magnitudes, which
are defined as the variable star (sigma Gem ) minus the comparison
star (HD 60318). The typical standard deviation of the observations is
0.00935mag in B and 0.00840mag in V.
The spectroscopic observations were obtained with the Hertzsprung SONG
telescope between 4 November 2015 and 1 April 2016. We used the
spectrograph with slit #5, giving a resolving power of 77000 and
covering wavelengths from 4400Å to 6900Å in 51 orders, with some
small gaps redwards of 5300Å. Each individual spectrum of sigma Gem
had an exposure time of 180 seconds. All the spectra are freely
available in the SONG Data Archive, SODA: https://soda.phys.au.dk/
Objects:
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RA (2000) DE Designation(s)
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07 43 18.73 +28 53 00.6 sigma Gem = HR 2973
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 41 125 Photometric observations of sigma Gem
table2.dat 52 275 Observing log of the spectroscopic observations
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See also:
J/A+AS/123/209 : Radial velocity curve of Sigma Gem (Duemmler+ 1997)
J/A+A/391/641 : Spectroscopic study of sigma Gem from IUE (Ekmekci+, 2002)
J/A+A/562/A107 : σ Gem V-band differential light curve (Kajatkari+, 2014)
J/ApJ/807/23 : RS CVn primaries companions. I. sig Gem (Roettenbacher+, 2015)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 10 A10 "DD-MM-YYYY" Date Observing date (DD-MM-YYYY)
12- 19 F8.4 d MJD Modified Julian Date (MJD-57000)
21- 25 F5.3 --- Phase Orbital phase
27 A1 --- Filter [BV] Johnson Filter
29- 34 F6.3 mag mag-orig Original differential magnitude
36- 41 F6.3 mag mag-ell Differential magnitude with ellipticity
removed
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 10 A10 "YYYY-MM-DD" Obs.date Observing date (YYYY-MM-DD)
12- 19 A8 "h:m:s" Obs.time Observing time (HH:MM:SS)
21- 30 F10.6 d MJD Modified Julian Date (MJD-57000)
32- 36 F5.3 --- Phase Orbital Phase
38 I1 --- Set [1/7] Observing set number (1)
40- 42 I3 --- SNR Signal-to-noise ratio of the
spectrum (2)
44- 48 F5.3 --- Phase-comb Phase of the combined observations (3)
50- 52 I3 --- SNR-comb Signal-to-noise ratio of the combined
observation (2) (3)
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Note (1): The observations were divided into seven sets covering 0.84-1.62
stellar rotations
Note (2): The SNR is measured per pixel in a continuum region around 6550{AA).
Note (3): In sets 4 and 5 there are several rotational phases during which many
spectra were obtained closely in time, and some of those individual spectra
have SNR of 75 or less. Therefore, for these two subsets, the data obtained
immediately after each other were combined into one higher SNR spectrum.
When no combined spectrum is available the value for Phase_comb is 0.000 and
for SNR_comb 000.
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Acknowledgements:
Heidi Korhonen, heidi.korhonen(at)eso.org
(End) Heidi Korhonen [ESO, Chile], Patricia Vannier [CDS] 19-Jan-2021