Signs of deep mixing in starspot variability

Lämmer

et al. 2016

ApJ

The preliminary results on deep-mixing manifestations in stellar variability are tested using our improved method and extended data set. We measure the timescales τ m of the stochastic change in the spectral power of rotational harmonics with numbers m 3 in the light curves of 1361 main-sequence stars from the Kepler mission archive. We find that the gradient t t --log log log 2 log 1 2 1has a histogram maximum at −2/3, demonstrating agreement with Kolmogorov's theory of turbulence and therefore confirming the manifestation of deep mixing. The squared amplitudes of the first and second rotational harmonics, corrected for integral photometry distortion, also show a quasi-Kolmogorov character with spectral index ≈−5/3. Moreover, the reduction of τ 1 and τ 2 to the timescales τ lam1 and τ lam2 of laminar convection in the deep stellar layers reveals the proximity of both τ lam1 and τ lam2 to the turnover time τ MLT of standard mixing length theory. Considering this result, we use the obtained stellar variability timescales instead of τ MLT in our analysis of the relation between stellar activity and the Rossby number P/τ MLT . Comparison of our diagrams with previous results and theoretical expectations shows that best-fit correspondence is achieved for τ lam1 , which can therefore be used as an analog of τ MLT . This means that the laminar component (giant cells) of stellar turbulent convection indeed plays an important role in the physics of stars. Additionally, we estimate the diffusivity of magnetic elements in stellar photospheres.

Section: Introductionmentioning

confidence: 76%

Section: Modified Methods For Harmonics' Timescale Measurementmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 91%

Section: Research Noveltiesmentioning

confidence: 99%

Section: Extended Stellar Setmentioning

confidence: 99%

See 3 more Smart Citations

Deep Mixing in Stellar Variability: Improved Method, Statistics, and Applications

Lämmer

et al. 2016

ApJ

Timescales of starspot variability in slow rotators

Güdel

et al. 2018

A&A

Self Cite

There is an intriguing proximity between the turnover time τMLT of the standard mixing length theory of the Sun and the timescale τlam of solar activity patterns at the space scale of giant laminar convection assumed in deep layers of the Sun. To verify the reliability of this correspondence, we analyzed the light curves of 637 slowly rotating stars, observed by the Kepler mission, with periods from 16 to 30 days. The proximity τMLT ≈ τlam is confirmed. The performed study also confirms the manifestation of large scale turbulence in the dynamics of surface activity such as that in the Sun. These results open a new way to measure the key astrophysical parameter τMLT and to study deep convection that has been undetected with asteroseismology.

Empirically revealed properties of Rieger-type cycles of stellar activity

2021

A&A

Self Cite

Context. The Rieger cycles were discovered in the Sun as a specific 154-day periodicity of flare occurrence; they strongly influence terrestrial space weather. This phenomenon is far from being understood. Various proposed mechanisms for this periodicity need further verification in stars with stellar parameters different from those of the Sun. Aims. In this work, we study the Rieger-type cycle (RTC) periods PRTC of stellar activity surveyed in the photometric data of the Kepler space telescope. Methods. The processing of 1726 stellar light curves reveals statistics of PRTC values for different main-sequence stars with different effective temperatures Teff and periods of rotation P. This study uses as an index of stellar activity the squared amplitude of the first rotational harmonic A12 of the stellar light curve variability. Results. The obtained information on PRTC of the considered stars confirms the phenomenological analogy between stellar RTCs and the solar Rieger cycles. Two types of RTCs were found: (1) activity cycles with PRTC independent on the stellar rotation, which are typical for the stars with Teff ≲ 5500 K, and (2) activity cycles with PRTC proportional to the stellar rotation period P, which take place on stars with Teff ≳ 6300 K. These two types of RTCs can be driven by the Kelvin and Rossby waves, respectively. The Rossby wave-driven RTCs show a relation with the location of tachocline at shallow depths in the hot stars. This confirms the theoretical predictions of the connection of the RTC with the tachocline. At the same time, the Kelvin wave-driven RTCs do not show this connection. Apparently, both types of wave drivers of RTCs can coexist, resulting in the joint modulation of the magnetic flux tubes emergence by Kelvin and Rossby waves, and the corresponding behavior of PRTC. Conclusions. The signatures of two types of wave drivers discovered for RTCs and their different relations with the tachocline call for a revision and further elaboration of the theory of RTCs.