Does the Babcock-Leighton dynamo operate in rapidly rotating solar-type stars? Exploration using a 3D dynamo model at different rotation rates

TL;DR

The study addresses whether the Babcock–Leighton dynamo can operate in rapidly rotating solar-type stars where starspots emerge at high latitudes. It employs a 3D kinematic dynamo model (STABLE) driven by rotation-rate–dependent large-scale flows and tests four BMR deposition scenarios (radial rise, parallel-rise, tilt-enhanced, and delayed/large spots) across rotation periods from 1 to 30 days. Key findings show that cyclic BL dynamos persist in most cases, with dipolar parity for radial-rise deposition and quadrupolar parity when high-latitude emergence suppresses cross-equatorial cancellation; Case IV can yield irregular cycles, especially at the fastest rotation. The results advance understanding of magnetic activity in young, fast-rotating stars and provide predictions for how cycle strength, parity, and period depend on rotation and spot emergence geometry.

Abstract

The Babcock-Leighton dynamo, which relies on the generation of a poloidal field through the decay and dispersal of tilted bipolar magnetic regions (BMRs), is a promising paradigm for explaining the features of the solar magnetic cycle. In rapidly rotating stars, BMRs are expected to emerge at high latitudes, which are less efficient in generating the poloidal field due to poor cross-equatorial cancellation. The operation of the Babcock-Leighton dynamo in rapidly rotating stars is therefore questionable. We, for the first time, using a 3D kinematic dynamo model, STABLE, explore this question. By taking large-scale flows from mean-field hydrodynamics models for stars rotating at different speeds, We conduct a series of dynamo simulations in rapidly rotating stars, exploring the following four cases of spot deposition, each based on a different assumption about toroidal flux tube rise: (i) radial rise, (ii) parallel rise to the rotation axis, (iii) parallel rise combined with an increase in Joy's law slope with the stellar rotation rate, and (iv) increasing time delay and spot size. We find cyclic magnetic fields in all cases except case IV of the 1-day rotating star, for which the magnetic field is irregular. For the parallel-rise cases, the magnetic field becomes quadrupolar, and for all other cases, it is dipolar. Our work demonstrates that the Babcock-Leighton dynamo may operate even in rapidly rotating stars with starspots appearing at higher latitudes.

Figures (10)

• Figure 1: (a) Time-latitude distribution of the surface radial magnetic field $B_{\rm r}$ [in kG], and (b) toroidal field along with starspot distribution (black dots) for a star of 1 day rotation period for Case I. (c) Monthly number of spots as function of time.
• Figure 2: Time-latitude distribution of the (a) surface radial magnetic field $B_{\rm r}$ [in kG], and (b) toroidal field along with starspot distribution (black dots) for a star of 1 day rotation period for Case II.
• Figure 3: Same as Figure \ref{['fig:rot1r']}, but for Case III.
• Figure 4: The variation of the mean peak field for a rotation period (in years) of 1 day with the $\zeta$, the factor that shows the dependency of the rotation period on the tilt angle. Here, $B_P$ and $B_T$ represent the poloidal and toroidal magnetic field components.
• Figure 5: Distribution of the time delays (lags between the successive BMR emergences) obtained from Cases III and IV for 1 day rotating star.