Wave modes in magnetic elements in a stratified atmosphere

Wilson, P.R.

doi:10.1086/156573

Cited by 9 publications

(6 citation statements)

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“…The Ca II H & K lines have regularly been monitored in F-M type stars for many years (e.g. Wilson 1978;Baliunas et al 1995) since their emission cores show strong variability connected with stellar magnetic activity. The S index measured from the H & K lines (Baliunas et al 1995) is known to be a general indicator of activity as it is related to the area and the strength of magnetic activity on a star (Schrijver et al 1989).…”

Section: Ca II 8662 å Activity and Correlation With Hα Variabilitymentioning

confidence: 99%

Precision radial velocities of 15 M5–M9 dwarfs

Barnes¹,

Jenkins²,

Jones³

et al. 2014

Monthly Notices of the Royal Astronomical Society

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We present radial velocity measurements of a sample of M5V -M9V stars from our Red-Optical Planet Survey, ROPS, operating at 0.652 -1.025 µm. Radial velocities for 15 stars, with r.m.s. precision down to 2.5 ms −1 over a week long time scale are achieved using Thorium-Argon reference spectra. We are sensitive to planets with m p sin i 1.5 M ⊕ (3 M ⊕ at 2-σ) in the classical habitable zone and our observations currently rule out planets with m p sin i > 0.5 M J at 0.03 AU for all our targets. A total of 9 of the 15 targets exhibit r.m.s. < 16 ms −1 , which enables us to rule out the presence of planets with m p sin i > 10 M ⊕ in 0.03 AU orbits.Since the mean rotation velocity is of order 8 kms −1 for an M6V star and 15 kms −1 by M9V, we avoid observing only slow rotators that would introduce a bias towards low axial inclination (i ≪ 90 • ) systems, which are unfavourable for planet detection. Our targets with the highest v sin i values exhibit radial velocities significantly above the photon-noise limited precision, even after accounting for v sin i. We have therefore monitored stellar activity via chromospheric emission from the Hα and Ca II infrared triplet lines. A clear trend of log 10 (L Hα / L bol ) with radial velocity r.m.s. is seen, implying that significant starspot activity is responsible for the observed radial velocity precision floor. The implication that most late M dwarfs are significantly spotted, and hence exhibit time varying line distortions, indicates that observations to detect orbiting planets need strategies to reliably mitigate against the effects of activity induced radial velocity variations.

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Section: Ca II 8662 å Activity and Correlation With Hα Variabilitymentioning

confidence: 99%

Precision radial velocities of 15 M5–M9 dwarfs

Barnes¹,

Jenkins²,

Jones³

et al. 2014

Monthly Notices of the Royal Astronomical Society

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“…Long-term magnetic cycles are known to exist on many late-type single stars. The Mount Wilson survey is the longest running project which has successfully measured cyclic periods using Ca II H & K lines (Wilson 1978;Baliunas et al 1995). More recently, long-term seasonal photometric periods for several solar-type stars (Messina & Guinan 2002) have also revealed changes in period from one epoch to the next.…”

Section: Differential Rotationmentioning

confidence: 99%

Differential rotation and star-spot evolution on HK Aqr in 2001 and 2002

Barnes

James

Cameron

2004

Monthly Notices of the Royal Astronomical Society

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We present images of the rapidly rotating (P-rot = 0.431 d) M1.5Ve dwarf HK Aqr, from data obtained during 2001 and 2002. Star-spots are found distributed at a range of latitudes during both observing seasons, but unlike other solar-type rapid rotators of higher mass, HK Aqr does not show a strong polar cap at either epoch. In addition to the surface brightness images, we make use of the four-night time-base over which the 2002 observations were made, in order to derive an estimate of the latitudinal dependent rotation on HK Aqr. We find that the equator-lap-pole time lies in the range -1449 to +448 d. Although very small, we are therefore unable to determine whether the degree of differential rotation is in the solar or anti-solar sense

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“…The correlation between this scatter and the mean rotation period indicates that ∝ 0.24±0.06 (where is the angular velocity and is the corresponding scatter in rad d −1 ), and thus that is almost independent of rotation (H95). The Mount Wilson Survey of chromospheric Ca II H and K emission-line fluxes has shown that many solar-like stars exhibit multiperiodic chromospheric emission flux variations (Wilson 1978;Baliunas et al 1995). The short-period modulation (of order days to weeks) gives the rotation period of the dominant active latitude at C 2004 RAS any epoch.…”

Section: Introductionmentioning

confidence: 99%

“…The Mount Wilson Survey of chromospheric Ca II H & K emission-line fluxes has shown that many solar-like stars exhibit multi-periodic chromospheric emission flux variations (Wilson 1978;Baliunas et al 1995). The short-period modulation (of order days to weeks) gives the rotation period of the dominant active latitude at any epoch.…”

Section: Introductionmentioning

confidence: 99%

The dependence of differential rotation on temperature and rotation

Barnes

Cameron

Donati

et al. 2005

Monthly Notices of the Royal Astronomical Society: Letters

275

352

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The definitive version is available at www.blackwell-synergy.com '. Copyright Blackwell PublishingWe use Doppler imaging techniques to determine the dependence of starspot rotation rates on latitude in an homogeneous sample of young, rapidly-rotating solar analogues. A solar-like differential rotation law is used, where the rotation depends on sin2(??), where ?? is the stellar latitude. By including this term in the image reconstruction process, using starspots as tracers, we are able to determine the magnitude of the shear over more than one rotation cycle. We also consider results from matched filter starspot tracking techniques, where individual starspot rotation rates are determined. In additionwe have re-analysed published results and present a new measurement for the K3 dwarf, Speedy Mic. A total of 10 stars of spectral type G2 - M2 are considered. We find a trend towards decreasing surface differential rotation with decreasing effective temperature. The implied approach to solid body rotation with increasing relative convection zone depth implies that the dynamo mechanism operating in low-mass stars may be substantially different from that in the Sun

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Wave modes in magnetic elements in a stratified atmosphere

Cited by 9 publications

References 0 publications

Precision radial velocities of 15 M5–M9 dwarfs

Precision radial velocities of 15 M5–M9 dwarfs

Differential rotation and star-spot evolution on HK Aqr in 2001 and 2002

The dependence of differential rotation on temperature and rotation

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