Stellar winds on the main-sequence

Johnstone, C. P.; Güdel, M.; Brott, I.; Lüftinger, T.

doi:10.1051/0004-6361/201425301

Cited by 202 publications

(222 citation statements)

References 143 publications

(321 reference statements)

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“…These extremes differ by less than a factor of 2, and are smaller than the reported variation in the Sun's corona temperature (e.g. Johnstone et al 2015).…”

Section: Example Comparisons Of LX Vs Age With Datamentioning

confidence: 59%

“…Alternatively, by demanding a specific terminal wind speed, models can also be constructed which then extract the required wind base temperature a posteriori. (Matt & Pudritz 2008;Cranmer & Saar 2011;Johnstone et al 2015). In our framework, the simplest way to incorporate a different coronal and wind temperature at the base would be to assume a prescribed functional form that relates the two.…”

Section: Magnetic Energy As Source Of X-ray Luminosity and Outflowmentioning

confidence: 99%

See 1 more Smart Citation

Minimalist coupled evolution model for stellar X-ray activity, rotation, mass loss, and magnetic field

Blackman

Owen

2016

Mon. Not. R. Astron. Soc.

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Late-type main sequence stars exhibit an x-ray to bolometric flux ratio that depends onRo, the ratio of rotation period to convective turnover time, asRo −ζ with 2 ≤ ζ ≤ 3 forRo > 0.13, but saturates with |ζ| < 0.2 forRo < 0.13. Saturated stars are younger than unsaturated stars and show a broader spread of rotation rates and x-ray activity. The unsaturated stars have magnetic fields and rotation speeds that scale roughly with the square root of their age, though possibly flattening for stars older than the sun. The connection between faster rotators, stronger fields, and higher activity has been established observationally, but a theory for the unified time-evolution of x-ray luminosity, rotation, magnetic field and mass loss that captures the above trends has been lacking. Here we derive a minimalist holistic framework for the time evolution of these quantities built from combining a Parker wind with new ingredients: (1) explicit sourcing of both the thermal energy launching the wind and the x-ray luminosity via dynamo produced magnetic fields; (2) explicit coupling of x-ray activity and mass loss saturation to dynamo saturation (via magnetic helicity build-up and convection eddy shredding); (3) use of coronal equilibrium to determine how magnetic energy is divided into wind and x-ray contributions. For solar-type stars younger than the sun, we infer conduction to be a subdominant power loss compared to x-rays and wind. For older stars, conduction is more important, possibly quenching the wind and reducing angular momentum loss. We focus on the time evolution for stars younger than the sun, highlighting what is possible for further generalizations. Overall, the approach shows promise toward a unified explanation of all of the aforementioned observational trends.

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“…These extremes differ by less than a factor of 2, and are smaller than the reported variation in the Sun's corona temperature (e.g. Johnstone et al 2015).…”

Section: Example Comparisons Of LX Vs Age With Datamentioning

confidence: 59%

Section: Magnetic Energy As Source Of X-ray Luminosity and Outflowmentioning

confidence: 99%

Minimalist coupled evolution model for stellar X-ray activity, rotation, mass loss, and magnetic field

Blackman

Owen

2016

Mon. Not. R. Astron. Soc.

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“…The time-dependent stellar wind models of Holzwarth & Jardine (2007); See et al (2014); Johnstone et al (2015) predict a different (in general, weaker) dependence ofṀ with Fx than that of Eq. (1) and do not predict a sharp decrease inṀ for stars to the right of the WDL.…”

Section: Conclusion and Discussionmentioning

confidence: 85%

Could a change in magnetic field geometry cause the break in the wind-activity relation?

Vidotto¹,

Donati²,

Jardine³

et al. 2015

Monthly Notices of the Royal Astronomical Society: Letters

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Wood et al suggested that mass-loss rate is a function of X-ray flux (Ṁ ∝ F x) for dwarf stars with F x F x,6 ≡ 10 6 erg cm −2 s −1 . However, more active stars do not obey this relation. These authors suggested that the break at F x,6 could be caused by significant changes in magnetic field topology that would inhibit stellar wind generation. Here, we investigate this hypothesis by analysing the stars in Wood et al's sample that had their surface magnetic fields reconstructed through Zeeman-Doppler Imaging (ZDI). Although the solar-like outliers in theṀ -F x relation have higher fractional toroidal magnetic energy, we do not find evidence of a sharp transition in magnetic topology at F x,6 . To confirm this, further wind measurements and ZDI observations at both sides of the break are required. As active stars can jump between states with highly toroidal to highly poloidal fields, we expect significant scatter in magnetic field topology to exist for stars with F x F x,6 . This strengthens the importance of multi-epoch ZDI observations. Finally, we show that there is a correlation between F x and magnetic energy, which implies thatṀ -magnetic energy relation has the same qualitative behaviour as the originalṀ -F x relation. No break is seen in any of the F x -magnetic energy relations.

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“…For all stars other than the Sun, direct measurements of the EUV flux are not possible because of absorption by neutral hydrogen in the ISM. To circumvent this problem, various indirect methods and scaling relations have been developed to estimate the XUV flux on the basis of accessible observables, such as X-ray fluxes (Sanz-Forcada et al 2011;Chadney et al 2015), Lyα fluxes (Linsky et al 2014), and stellar rotational velocities (Wood et al 1994;Johnstone et al 2015;Tu et al 2015).…”

Section: Wasp-18's High-energy Fluxmentioning

confidence: 99%

Suppressed Far-UV Stellar Activity and Low Planetary Mass Loss in the WASP-18 System*

Fossati

Koskinen

Cubillos

et al. 2018

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WASP-18 hosts a massive, very close-in Jupiter-like planet. Despite its young age (<1 Gyr), the star presents an anomalously low stellar activity level: the measured log R ′ HK activity parameter lies slightly below the basal level; there is no significant time-variability in the log R ′ HK value; there is no detection of the star in the X-rays. We present results of far-UV observations of WASP-18 obtained with COS on board of Hubble Space Telescope aimed at explaining this anomaly. From the star's spectral energy distribution, we infer the extinction (E(B − V) ≈ 0.01 mag) and then the interstellar medium (ISM) column density for a number of ions, concluding that ISM absorption is not the origin of the anomaly. We measure the flux of the four stellar emission features detected in the COS spectrum (C II, C III, C IV, Si IV). Comparing the C II/C IV flux ratio measured for WASP-18 with that derived from spectra of nearby stars with known age, we see that the far-UV spectrum of WASP-18 resembles that of old (>5 Gyr), inactive stars, in stark contrast with its young age. We conclude that WASP-18 has an intrinsically low activity level, possibly caused by star-planet tidal interaction, as suggested by previous studies. Re-scaling the solar irradiance reference spectrum to match the flux of the Si IV line, yields an XUV integrated flux at the planet orbit of 10.2 erg s . For such high-mass planets, thermal escape is not energy limited, but driven by Jeans escape.

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Stellar winds on the main-sequence

Cited by 202 publications

References 143 publications

Minimalist coupled evolution model for stellar X-ray activity, rotation, mass loss, and magnetic field

Minimalist coupled evolution model for stellar X-ray activity, rotation, mass loss, and magnetic field

Could a change in magnetic field geometry cause the break in the wind-activity relation?

Suppressed Far-UV Stellar Activity and Low Planetary Mass Loss in the WASP-18 System*

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