Differential rotation of the present and the pre-main-sequence Sun

Küker, M.; Stix, M.

doi:10.1051/0004-6361:20010009

Cited by 61 publications

(88 citation statements)

References 9 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…It would give a Rossby number of 0.13 and, according to the criterium of Durney & Latour (1978), be by far sufficient to run an effective dynamo. We use the differential-rotation code of Küker & Stix (2001) to recompute τ c as a function of depth based on its description through the pressure scale height, H p , and the classical mixing length, α. The characteristic convective length is defined as = α H p , with α = 2.…”

Section: Discussionmentioning

confidence: 99%

Rotation and magnetic activity of the Hertzsprung-gap giant 31 Comae

Strassmeier¹,

Granzer²,

Kopf³

et al. 2010

A&A

Self Cite

View full text Add to dashboard Cite

Context. The single rapidly-rotating G0 giant 31 Comae has been a puzzle because of the absence of photometric variability despite its strong chromospheric and coronal emissions. As a Hertzsprung-gap giant, it is expected to be at the stage of rearranging its moment of inertia, hence likely also its dynamo action, which could possibly be linked with its missing photospheric activity. Aims. Our aim is to detect photospheric activity, obtain the rotation period, and use it for a first Doppler image of the star's surface. Its morphology could be related to the evolutionary status. Methods. We carried out high-precision, white-light photometry with the MOST satellite, ground-based Strömgren photometry with automated telescopes, and high-resolution optical echelle spectroscopy with the new STELLA robotic facility. Results. The MOST data reveal, for the first time, light variations with a full amplitude of 5 mmag and an average photometric period of 6.80 ± 0.06 days. Radial-velocity variations with a full amplitude of 270 m s −1 and a period of 6.76 ± 0.02 days were detected from our STELLA spectra, which we also interpret as due to stellar rotation. The two-year constancy of the average radial velocity of +0.10 ± 0.33 km s −1 confirms the star's single status, as well as the membership in the cluster Melotte 111. A spectrum synthesis gives T eff = 5660 ± 42 K, log g = 3.51 ± 0.09, and [Fe/H] = −0.15 ± 0.03, which together with the revised Hipparcos distance, suggests a mass of 2.6 ± 0.1 M and an age of ≈540 Myr. The surface lithium abundance is measured to be nearly primordial. A detection of a strong He i absorption line indicates nonradiative heating processes in the atmosphere. Our Doppler images show a large, asymmetric polar spot, cooler than T eff by ≈1600 K, and several small low-to-mid latitude features that are warmer by ≈300-400 K and are possibly of chromospheric origin. We computed the convective turnover time for 31 Com as a function of depth and found on average τ C ≈ 5 days. Conclusions. 31 Com appears to be just at the onset of rapid magnetic braking and Li dilution because its age almost exactly coincides with the predicted onset of envelope convection. That we recover a big polar starspot despite the Rossby number being larger than unity, and thus no efficient (envelope) dynamo is expected, leads us to conclude that 31 Com still harbors a fossil predominantly poloidal magnetic field. However, the increasing convective envelope may have just started an interface dynamo that now is the source of the warm surface features and the corresponding UV and X-ray emission.

show abstract

Section: Discussionmentioning

confidence: 99%

Rotation and magnetic activity of the Hertzsprung-gap giant 31 Comae

Strassmeier¹,

Granzer²,

Kopf³

et al. 2010

A&A

Self Cite

View full text Add to dashboard Cite

show abstract

“…We first simulate the solar rotation law and compare the results to those from earlier models by Kitchatinov & Rüdiger (1999) and Küker & Stix (2001) by solving the same set of equations with different numerical methods. KR99 used a simple Kramers' law for the opacity, while KS01 used the opacity from Ahrens et al (1992).…”

Section: The Sunmentioning

confidence: 99%

The differential rotation of G dwarfs

Küker

Rüdiger

Kitchatinov

2011

A&A

Self Cite

108

View full text Add to dashboard Cite

A series of stellar models of spectral type G is computed to study the rotation laws resulting from mean-field equations. The rotation laws of the slowly rotating Sun, the rapidly rotating MOST stars Eri and κ 1 Cet, and the rapid rotators R58 and LQ Lup can be easily reproduced. We also find that differences in the depth of the convection zone cause large differences in the surface rotation law and that the extreme surface shear of HD 171488 can only be explained with an artificially shallow convection layer. We verify the thermal wind equilibrium in rapidly rotating G dwarfs and find that the polar subrotation (dΩ/dz < 0) is due to the baroclinic effect and the equatorial superrotation (dΩ/dr > 0) is caused by the Reynolds stresses. In the bulk of the convection zones where the meridional flow is slow and smooth, the thermal wind equilibrium holds between the centrifugal and the pressure forces. It does not hold, however, in the bounding shear layers including the equatorial region where the Reynolds stresses dominate.

show abstract

“…Pizzolato et al 2003). We determine τ c with the differential-rotation code of Küker & Stix (2001) as a function of depth based on a description of the pressure scale height, H p , and the classical mixing length, α. The characteristic convective length is then defined as = α H p , with α = 2.…”

Section: Rossby Number and Dynamo Activitymentioning

confidence: 99%

Binary-induced magnetic activity?

Strassmeier

Carroll

Weber

et al. 2011

A&A

View full text Add to dashboard Cite

Context. Multi-wavelength time-series observations with high cadence and long duration are needed to resolve and understand the many variations of magnetically active late-type stars, which is an approach often used to observe the Sun. Aims. We present a first and detailed study of the bright and active K0IV-III star HD 123351. Methods. We acquired a total of 955 high-resolution STELLA echelle spectra during the years 2006-2010 and a total of 2260 photometric VI C data points during 1998-2010. These data are complemented by some spectra from CFHT and KPNO. Results. The star is found to be a single-lined spectroscopic binary with a period of 147.8919 ± 0.0003 days and a large eccentricity of e = 0.8086 ± 0.0001. The rms of the orbital solution is just 47 m s −1 , making it the most precise orbit ever obtained for an active binary system. The rotation period is constrained from long-term photometry to be 58.32 ± 0.01 days. It shows that HD 123351 is a very asynchronous rotator, rotating five times slower than the expected pseudo-synchronous value. Two spotted regions persisted throughout the 12 years of our observations. We interpret them as active longitudes on a differentially rotating surface with a ΔP/P of 0.076. Four years of Hα, Ca ii H&K and He i D3 monitoring identifies the same main periodicity as the photometry but dynamic spectra also indicate that there is an intermittent dependence on the orbital period, in particular for Ca ii H&K in 2008. Line-profile inversions of a pair of Zeeman sensitive/insensitive iron lines yield an average surface magnetic-flux density of 542 ± 72 G. The time series for 2008 is modulated by the stellar rotation as well as the orbital motion, such that the magnetic flux is generally weaker during times of periastron and that the chromospheric emissions vary in anti-phase with the magnetic flux. We also identify a broad and asymmetric lithium line profile and measure an abundance of log n(Li) = 1.70 ± 0.05. The star's position in the H-R diagram indicates a mass of 1.2 ± 0.1 M and an age of 6-7 Gyr. Conclusions. We interpret the anti-phase relation of the magnetic flux with the chromospheric emissions as evidence that there are two magnetic fields present at the same time, a localized surface magnetic field associated with spots and a global field that is oriented towards the (low-mass) secondary component. We suggest that the inter-binary field is responsible for the magnetic-flux dilution at periastron. It is also likely to be responsible for the unexpected slow and asynchronous rotation of the primary star.

show abstract

Differential rotation of the present and the pre-main-sequence Sun

Cited by 61 publications

References 9 publications

Rotation and magnetic activity of the Hertzsprung-gap giant 31 Comae

Rotation and magnetic activity of the Hertzsprung-gap giant 31 Comae

The differential rotation of G dwarfs

Binary-induced magnetic activity?

Contact Info

Product

Resources

About