Magnitude-range brightness variations of overactive K giants

K. Oláh, A. Moór, Z. Kővári, T. Granzer, K. G. Strassmeier, L. Kriskovics, K. Vida

Research output: Contribution to journalArticle

6 Citations (Scopus)

Abstract

Context. Decades-long, phase-resolved photometry of overactive spotted cool stars has revealed that their long-term peak-to-peak light variations can be as large as one magnitude. Such brightness variations are too large to be solely explained by rotational modulation and/or a cyclic, or pseudo-cyclic, waxing and waning of surface spots and faculae as we see in the Sun. Aims. We study three representative, overactive spotted K giants (IL? Hya, XX? Tri, and DM? UMa) known to exhibit V-band light variations between 0.m65-1.m05. Our aim is to find the origin of their large brightness variation. Methods. We employ long-term phase-resolved multicolor photometry, mostly from automatic telescopes, covering 42? yr for IL? Hya, 28? yr for XX? Tri, and 34? yr for DM? UMa. For one target, IL? Hya, we present a new Doppler image from NSO data taken in late 1996. Effective temperatures for our targets are determined from all well-sampled observing epochs and are based on a V - IC color-index calibration. Results. The effective temperature change between the extrema of the rotational modulation for IL Hya and XX Tri is in the range 50-200 K. The bolometric flux during maximum of the rotational modulation, i.e., the least spotted states, varied by up to 39% in IL? Hya and up to 54% in XX? Tri over the course of our observations. We emphasize that for IL? Hya it is just about half of the total luminosity variation that can be explained by the photospheric temperature (spots/faculae) changes, while for XX? Tri it is even about one third. The long-term, 0.m6 V-band variation of DM? UMa is more difficult to explain because little or no B - V color index change is observed on the same timescale. Placing the three stars with their light and color variations into H-R diagrams, we find that their overall luminosities are generally too low compared to predictions from current evolutionary tracks. Conclusions. A change in the stellar radius due to strong and variable magnetic fields during activity cycles likely plays a role in explaining the anomalous brightness and luminosity of our three targets. At least for IL? Hya, a radius change of about 9% is suggested from mbol and Teff, and is supported by independent v sin i measurements.

Original languageEnglish
Article numberA94
JournalAstronomy and Astrophysics
Volume572
DOIs
Publication statusPublished - Dec 9 2014

Fingerprint

brightness
temperature
faculae
luminosity
extremely high frequencies
color
modulation
photometry
diagram
magnetic field
calibration
timescale
cool stars
radii
prediction
range (extremes)
coverings
diagrams
time measurement
telescopes

Keywords

  • Stars: activity
  • Stars: individual: DM UMa
  • Stars: individual: IL Hya
  • Stars: individual: XX Tri
  • Stars: late-type
  • Starspots

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Magnitude-range brightness variations of overactive K giants. / Oláh, K.; Moór, A.; Kővári, Z.; Granzer, T.; Strassmeier, K. G.; Kriskovics, L.; Vida, K.

In: Astronomy and Astrophysics, Vol. 572, A94, 09.12.2014.

Research output: Contribution to journalArticle

Oláh, K. ; Moór, A. ; Kővári, Z. ; Granzer, T. ; Strassmeier, K. G. ; Kriskovics, L. ; Vida, K. / Magnitude-range brightness variations of overactive K giants. In: Astronomy and Astrophysics. 2014 ; Vol. 572.
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title = "Magnitude-range brightness variations of overactive K giants",
abstract = "Context. Decades-long, phase-resolved photometry of overactive spotted cool stars has revealed that their long-term peak-to-peak light variations can be as large as one magnitude. Such brightness variations are too large to be solely explained by rotational modulation and/or a cyclic, or pseudo-cyclic, waxing and waning of surface spots and faculae as we see in the Sun. Aims. We study three representative, overactive spotted K giants (IL? Hya, XX? Tri, and DM? UMa) known to exhibit V-band light variations between 0.m65-1.m05. Our aim is to find the origin of their large brightness variation. Methods. We employ long-term phase-resolved multicolor photometry, mostly from automatic telescopes, covering 42? yr for IL? Hya, 28? yr for XX? Tri, and 34? yr for DM? UMa. For one target, IL? Hya, we present a new Doppler image from NSO data taken in late 1996. Effective temperatures for our targets are determined from all well-sampled observing epochs and are based on a V - IC color-index calibration. Results. The effective temperature change between the extrema of the rotational modulation for IL Hya and XX Tri is in the range 50-200 K. The bolometric flux during maximum of the rotational modulation, i.e., the least spotted states, varied by up to 39{\%} in IL? Hya and up to 54{\%} in XX? Tri over the course of our observations. We emphasize that for IL? Hya it is just about half of the total luminosity variation that can be explained by the photospheric temperature (spots/faculae) changes, while for XX? Tri it is even about one third. The long-term, 0.m6 V-band variation of DM? UMa is more difficult to explain because little or no B - V color index change is observed on the same timescale. Placing the three stars with their light and color variations into H-R diagrams, we find that their overall luminosities are generally too low compared to predictions from current evolutionary tracks. Conclusions. A change in the stellar radius due to strong and variable magnetic fields during activity cycles likely plays a role in explaining the anomalous brightness and luminosity of our three targets. At least for IL? Hya, a radius change of about 9{\%} is suggested from mbol and Teff, and is supported by independent v sin i measurements.",
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AU - Moór, A.

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AU - Granzer, T.

AU - Strassmeier, K. G.

AU - Kriskovics, L.

AU - Vida, K.

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N2 - Context. Decades-long, phase-resolved photometry of overactive spotted cool stars has revealed that their long-term peak-to-peak light variations can be as large as one magnitude. Such brightness variations are too large to be solely explained by rotational modulation and/or a cyclic, or pseudo-cyclic, waxing and waning of surface spots and faculae as we see in the Sun. Aims. We study three representative, overactive spotted K giants (IL? Hya, XX? Tri, and DM? UMa) known to exhibit V-band light variations between 0.m65-1.m05. Our aim is to find the origin of their large brightness variation. Methods. We employ long-term phase-resolved multicolor photometry, mostly from automatic telescopes, covering 42? yr for IL? Hya, 28? yr for XX? Tri, and 34? yr for DM? UMa. For one target, IL? Hya, we present a new Doppler image from NSO data taken in late 1996. Effective temperatures for our targets are determined from all well-sampled observing epochs and are based on a V - IC color-index calibration. Results. The effective temperature change between the extrema of the rotational modulation for IL Hya and XX Tri is in the range 50-200 K. The bolometric flux during maximum of the rotational modulation, i.e., the least spotted states, varied by up to 39% in IL? Hya and up to 54% in XX? Tri over the course of our observations. We emphasize that for IL? Hya it is just about half of the total luminosity variation that can be explained by the photospheric temperature (spots/faculae) changes, while for XX? Tri it is even about one third. The long-term, 0.m6 V-band variation of DM? UMa is more difficult to explain because little or no B - V color index change is observed on the same timescale. Placing the three stars with their light and color variations into H-R diagrams, we find that their overall luminosities are generally too low compared to predictions from current evolutionary tracks. Conclusions. A change in the stellar radius due to strong and variable magnetic fields during activity cycles likely plays a role in explaining the anomalous brightness and luminosity of our three targets. At least for IL? Hya, a radius change of about 9% is suggested from mbol and Teff, and is supported by independent v sin i measurements.

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KW - Stars: individual: XX Tri

KW - Stars: late-type

KW - Starspots

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