On the reality of a boundary in the H-R diagram between late-type stars with and without high temperature outer atmospheres

Simon, T.; Linsky, J. L.; Stencel, R. E.

doi:10.1086/159981

Cited by 61 publications

(23 citation statements)

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“…Its value of V − R = 0.72 mag places it to the left of the coronal dividing line in the Hertzsprung-Russel diagram (hereafter HRD) where stars are supposed to have a chromosphere, a transition region, and a corona. That dividing line was proposed by Linsky & Haisch (1979) near V −R = 0.80 mag and later confirmed by the study of Simon et al (1982) with a larger sample of stars. In the solar neighborhood (d ≤ 30 pc), β Ceti is the single giant star with the highest X-ray luminosity log L x = 30.2 erg/s (Maggio et al 1998;Hünsch et al 1996), which is calculated assuming a distance of d = 29.5 pc based on the Hipparcos parallax (van Leeuwen 2007).…”

Section: Introductionsupporting

confidence: 56%

Magnetic field structure in single late-type giants:βCeti in 2010–2012

Tsvetkova

Petit

Aurière

et al. 2013

A&A

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Aims. We study the behavior of the magnetic field and the line activity indicators of the single late-type giant β Ceti. Using spectropolarimetric data, we aim to reconstruct the magnetic field structure on the star's surface and to present the first magnetic maps for β Ceti. Methods. The data were obtained using two spectropolarimeters -Narval at the Bernard Lyot Télescope, Pic du Midi, France, and ESPaDOnS at CFHT, Hawaii. Thirty-eight circularly-polarized spectra have been collected in the period June 2010-January 2012. The least square deconvolution method was applied for extracting high signal-to-noise ratio line profiles, from which we measured the surface-averaged longitudinal magnetic field B l . Chromospheric activity indicators CaII K, Hα, CaII IR (854.2 nm), and radial velocity were simultaneously measured, and their variability was analyzed along with the behavior of B l . The Zeeman Doppler imaging (ZDI) inversion technique was employed for reconstructing the large-scale magnetic field and two magnetic maps of β Ceti are presented for two periods (June 2010-December 2010 and June 2011-January 2012). Results. The B l stays with a same positive polarity for the whole observational period and shows significant variations in the interval 0.1-8.2 G. The behavior of the line activity indicators is in good agreement with the B l variations. Searching for periodic signals in the Stokes V time series, we found a possible rotation period of 215 days. The two ZDI maps show a mainly axisymmetric and poloidal magnetic topology and a simple surface magnetic field configuration dominated by a dipole. Little evolution is observed between the two maps, in spite of a 1 yr interval between both subsets. We also use state-of-the-art stellar evolution models to constrain the evolutionary status of β Ceti. We derive a mass of 3.5 M and propose that this star is already in the central helium-burning phase. Conclusions. Considering all our results and the evolutionary status of the star, we suggest that dynamo action alone may not be efficient enough to account for the high magnetic activity of β Ceti. As an alternate option, we propose that it is a descendant of an Ap star presently undergoing central helium-burning and still exhibiting a remnant of the Ap star magnetic field.

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Section: Introductionsupporting

confidence: 56%

Magnetic field structure in single late-type giants:βCeti in 2010–2012

Tsvetkova

Petit

Aurière

et al. 2013

A&A

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“…The group of non-coronal stars (giant stars cooler than K2) display emission lines formed at temperatures cooler than 20 000 K, which can originate only in chromospheres. The Linsky-Haisch dividing line separates the groups of coronal and non-coronal stars in the Herzsprung-Russell (HR) diagram and was extensively studied in the 80's (Simon et al 1982;Haisch 1987;Brown & Carpenter 1984;Carpenter et al 1985). Even if this division of giant stars into two groups seemed too simplistic, hybrid giant stars with mixed coronae and significant wind activity have been revealed (Reimers 1982;Hall 2008;and Ayres 2010).…”

Section: Introductionmentioning

confidence: 99%

Chromosphere of K giant stars

Bério

Merle

Thévenin

et al. 2011

A&A

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Context. Interferometers provide accurate diameter measurements of stars by analyzing both the continuum and the lines formed in photospheres and chromospheres. Tests of the geometrical extent of the chromospheres are therefore possible by comparing the estimated radius in the continuum of the photosphere and the estimated radii in chromospheric lines. Aims. We aim to constrain the geometrical extent of the chromosphere of non-binary K giant stars and detect any spatial structures in the chromosphere. Methods. We performed observations with the CHARA interferometer and the VEGA beam combiner at optical wavelengths. We observed seven non-binary K giant stars (β and η Cet, δ Crt, ρ Boo, β Oph, 109 Her, and ι Cep). We measured the ratio of the radii of the photosphere to the chromosphere using the interferometric measurements in the H α and the Ca II infrared triplet line cores. For β Cet, spectro-interferometric observations are compared to a non-local thermal equilibrium (NLTE) semi-empirical model atmosphere including a chromosphere. The NLTE computations provide line intensities and contribution functions that indicate the relative locations where the line cores are formed and can constrain the size of the limb-darkened disk of the stars with chromospheres. We measured the angular diameter of seven K giant stars and deduced their fundamental parameters: effective temperatures, radii, luminosities, and masses. We determined the geometrical extent of the chromosphere for four giant stars (β and η Cet, δ Crt and ρ Boo). Results. The chromosphere extents obtained range between 16% to 47% of the stellar radius. The NLTE computations confirm that the Ca II/849 nm line core is deeper in the chromosphere of β Cet than either of the Ca II/854 nm and Ca II/866 nm line cores. We present a modified version of a semi-empirical model atmosphere derived by fitting the Ca II triplet line cores of this star. In four of our targets, we also detect the signature of a differential signal showing the presence of asymmetries in the chromospheres. Conclusions. It is the first time that geometrical extents and structure in the chromospheres of non-binary K giant stars are determined by interferometry. These observations provide strong constrains on stellar atmosphere models.

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“…A combination of the two processes can exist as well. Arguments for each of the mechanisms can be found in Zirin (1982); Simon et al (1982); Smith (1983);…”

Section: Introductionmentioning

confidence: 99%

Active cool stars and He I 10 830 Å: the coronal connection

Sanz-Forcada¹,

Dupree

2008

A&A

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Context. The mechanism of formation of the He i 10 830 Å triplet in cool stars has been subject of debate for the last 30 years. A relation between the X-ray luminosity and the He i 10 830 Å flux was found in cool stars, but the dominant mechanism of formation in these stars (photoionization by coronal radiation followed by recombination and cascade, or collisional excitation in the chromosphere), has not yet been established.Aims. We use modern instrumentation (NOT/SOFIN) and a direct measurement of the EUV flux, which photoionizes He i, to investigate the formation mechanism of the line for the most active stars which are frequently excluded from analysis.Methods. We have observed with an unprecedented resolution (R ∼ 170 000) the He i 10 830 Å triplet in a set of 15 stars that were also observed with the Extreme Ultraviolet Explorer (EUVE) in order to compare the line strengths with their EUV and X-ray fluxes. Conclusions. Active dwarf stars may have high chromospheric densities thus allowing collisional excitation to dominate photoionization/recombination processes in forming the He i 10 830 Å line. Active giant stars possess lower gravities, and lower chromospheric densities than dwarfs, allowing for photoexcitation processes to become important. Moreover, their extended chromospheres allow for scattering of infrared continuum radiation, producing strong absorption in He i and tracing wind dynamics.

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On the reality of a boundary in the H-R diagram between late-type stars with and without high temperature outer atmospheres

Cited by 61 publications

References 0 publications

Magnetic field structure in single late-type giants:βCeti in 2010–2012

Magnetic field structure in single late-type giants:βCeti in 2010–2012

Chromosphere of K giant stars

Active cool stars and He I 10 830 Å: the coronal connection

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