The corona of HD 223460 (HR 9024)

Gondoin, P.

doi:10.1051/0004-6361:20030978

Cited by 12 publications

(14 citation statements)

References 68 publications

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“…OU And is a well known active red giant, with small rotation period for a giant (24.2 d, Strassmeier et al 1999) and very active in X-rays (Gondoin et al 2003, Ayres et al 2007). In the literature is given a T eff value inferred for OU And by Wright et al (2003) from its spectral type.…”

Section: B123 Ou and Hd 223460mentioning

confidence: 99%

The magnetic fields at the surface of active single G-K giants

Aurière

Konstantinova-Antova

Charbonnel

et al. 2015

A&A

117

159

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Aims. We investigate the magnetic field at the surface of 48 red giants selected as promising for detection of Stokes V Zeeman signatures in their spectral lines. In our sample, 24 stars are identified from the literature as presenting moderate to strong signs of magnetic activity. An additional 7 stars are identified as those in which thermohaline mixing appears not to have occured, which could be due to hosting a strong magnetic field. Finally, we observed 17 additional very bright stars which enable a sensitive search to be performed with the spectropolarimetric technique. Methods. We use the spectropolarimeters Narval and ESPaDOnS to detect circular polarization within the photospheric absorption lines of our targets. We treat the spectropolarimetric data using the least-squares deconvolution method to create high signal-to-noise ratio mean Stokes V profiles. We also measure the classical S -index activity indicator for the Ca ii H&K lines, and the stellar radial velocity. To infer the evolutionary status of our giants and to interpret our results, we use state-of-the-art stellar evolutionary models with predictions of convective turnover times. Results. We unambiguously detect magnetic fields via Zeeman signatures in 29 of the 48 red giants in our sample. Zeeman signatures are found in all but one of the 24 red giants exhibiting signs of activity, as well as 6 out of 17 bright giant stars. However no detections were obtained in the 7 thermohaline deviant giants. The majority of the magnetically detected giants are either in the first dredge up phase or at the beginning of core He burning, i.e. phases when the convective turnover time is at a maximum: this corresponds to a "magnetic strip" for red giants in the Hertzsprung-Russell diagram. A close study of the 16 giants with known rotational periods shows that the measured magnetic field strength is tightly correlated with the rotational properties, namely to the rotational period and to the Rossby number Ro. Our results show that the magnetic fields of these giants are produced by a dynamo, possibly of α-ω origin since Ro is in general smaller than unity. Four stars for which the magnetic field is measured to be outstandingly strong with respect to that expected from the rotational period/magnetic field relation or their evolutionary status are interpreted as being probable descendants of magnetic Ap stars. In addition to the weak-field giant Pollux, 4 bright giants (Aldebaran, Alphard, Arcturus, η Psc) are detected with magnetic field strength at the sub-Gauss level. Besides Arcturus, these stars were not considered to be active giants before this study and are very similar in other respects to ordinary giants, with S -index indicating consistency with basal chromospheric flux.Key words. stars: magnetic field -stars: late-type -stars: evolution -stars: rotation Tables 6-8

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Section: B123 Ou and Hd 223460mentioning

confidence: 99%

The magnetic fields at the surface of active single G-K giants

Aurière

Konstantinova-Antova

Charbonnel

et al. 2015

A&A

117

159

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“…Spectral type, bolometric luminosity, effective temperature, radius, mass and X-ray luminosity in the 0.3-10 keV band of the sample stars. References to X-ray data (last column) are as follows: (1) Maggio et al (1990); (2) Hünsch et al (1998); (3) Gondoin (2003a); (4) Gondoin (2002); (5) Gondoin (2005); (6) Gondoin (2004); (7) Gondoin (2003b). substantially.…”

Section: Sample Selectionmentioning

confidence: 99%

The relation between X-ray activity and rotation in intermediate-mass G giants

Gondoin

2005

A&A

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I study the relation between X-ray activity and rotation among intermediate-mass single G giants. The results show evidence that the quiescent coronal activity of these stars, as measured by their X-ray surface flux, increases linearly with the angular rotation velocity and with the inverse of the Rossby number. Even the most rapidly rotating G giants do not reach the canonical log(L X /L bol ) ≈ -3 saturation level. The effect of rapid rotation on these stars could result mainly in an increased coverage of their surface with magnetic close loop structures. The empirical activity-rotation relationship accounts for the occurrence of a maximum of magnetic activity in the atmosphere of intermediate-mass stars as they evolve off the main-sequence near the bottom of the red giant branch. Remarkably, the relation between X-ray to bolometric luminosity ratio and the Rossby number or rotation period for G giants differs from the power law dependence with an index of about -2 that is observed for main-sequence stars. Possible implications for the dynamo generation of magnetic fields on giants are discussed.

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“…OU And is chromospherically active with X-ray luminosity variations. Series of highly ionised iron lines, several Lyman lines of hydrogenlike ions and triplet lines of Helium-like ions in the X-ray region (Gondoin 2003;Ayres et al 2007) are observed in its spectra.…”

Section: Introductionmentioning

confidence: 99%

The different origins of magnetic fields and activity in the Hertzsprung gap stars, OU Andromedae and 31 Comae

Borisova

Aurière

Petit

et al. 2016

A&A

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Context. When crossing the Hertzsprung gap, intermediate-mass stars develop a convective envelope. Fast rotators on the main sequence, or Ap star descendants, are expected to become magnetic active subgiants during this evolutionary phase. Aims. We compare the surface magnetic fields and activity indicators of two active, fast rotating red giants with similar masses and spectral class but different rotation rates -OU And (P rot = 24.2 d) and 31 Com (P rot = 6.8 d) -to address the question of the origin of their magnetism and high activity. Methods. Observations were carried out with the Narval spectropolarimeter in 2008 and 2013. We used the least-squares deconvolution (LSD) technique to extract Stokes V and I profiles with high signal-to-noise ratio to detect Zeeman signatures of the magnetic field of the stars. We then provide Zeeman-Doppler imaging (ZDI), activity indicators monitoring, and a precise estimation of stellar parameters. We use state-of-the-art stellar evolutionary models, including rotation, to infer the evolutionary status of our giants, as well as their initial rotation velocity on the main sequence, and we interpret our observational results in the light of the theoretical Rossby numbers. Results. The detected magnetic field of OU Andromedae (OU And) is a strong one. Its longitudinal component B l reaches 40 G and presents an about sinusoidal variation with reversal of the polarity. The magnetic topology of OU And is dominated by largescale elements and is mainly poloidal with an important dipole component, as well as a significant toroidal component. The detected magnetic field of 31 Comae (31 Com) is weaker, with a magnetic map showing a more complex field geometry, and poloidal and toroidal components of equal contributions. The evolutionary models show that the progenitors of OU And and 31 Com must have been rotating at velocities that correspond to 30 and 53%, respectively, of their critical rotation velocity on the zero age main sequence. Both OU And and 31 Com have very similar masses (2.7 and 2.85 M , respectively), and they both lie in the Hertzsprung gap. Conclusions. OU And appears to be the probable descendant of a magnetic Ap star, and 31 Com the descendant of a relatively fast rotator on the main sequence. Because of the relatively fast rotation in the Hertzsprung gap and the onset of the development of a convective envelope, OU And also has a dynamo in operation.

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The corona of HD 223460 (HR 9024)

Cited by 12 publications

References 68 publications

The magnetic fields at the surface of active single G-K giants

The magnetic fields at the surface of active single G-K giants

The relation between X-ray activity and rotation in intermediate-mass G giants

The different origins of magnetic fields and activity in the Hertzsprung gap stars, OU Andromedae and 31 Comae

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