STELLA meets RAVE: a search for Maunder Minimum candidate stars

• Applicant: Dr. Silva P. Järvinen
• Subject Area: Astrophysics and Astronomy
• Term: from 2014 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 264056313

• Final Report Year: 2017

Final Report Abstract

It is not known what causes the 100-year pattern of waxing and waning solar cycles. A flux-transport dynamo-based prediction attempts to predict the cycle strength and morphology. A major problem is that the validation of a successful cycle forecast would take decades if only the Sun were used as a benchmark. Therefore, finding minimum chromospheric emission-line fluxes for a large sample of stars throughout the H-R diagram is essential. Such quiet-star fluxes could be used as a test parameter for flux-transport dynamos embedded in stellar-structure models with different effective temperatures, gravities, and metallicities, and to discover new solar analogues. The presence of emission in the cores of the Ca ii H&K and infra-red triplet (IRT) resonance lines is a simple diagnostic of magnetic activity in the chromospheres of late-type stars. Spatially resolved Ca ii heliograms and magnetograms have amply demonstrated the relation between H&K-emission strength and the surface magnetic field on our Sun. Furthermore, the fact that we observe generally stronger Ca ii emission in more rapidly rotating stars is widely known as the rotation-activity relation which is heuristically explained by the Ω-effect of the classical αΩ dynamo. Anomalies like grand activity minima are important constraints for such dynamo theories. Within this project, we have selected the late-type dwarfs from the RAdial Velocity Experiment (RAVE) survey database, converted the measured 1-Å core equivalent widths into the fluxes, and obtained follow up high-resolution spectra of selected targets as well as spectra of Morgan-Keenan (MK) standard stars using SES (STELLA échelle spectrograph) at STELLA (STELLar Activity) telescope. From the RAVE and STELLA fluxes we have obtained flux-flux relations between different IRT lines as well as IRT – Hα and IRT – Ca ii H&K lines. All these relations can be described with simple linear fits. Furthermore, we can place measured fluxes in relation to astrophysical parameters, such as temperature or metallicity.

Publications

• HARPS spectropolarimetry of three sharp-lined Herbig Ae stars: New insights, 2015, A&A, 584, 15
  Järvinen, S.P., Carroll, T.A., Hubrig, S., et al.
  
• Chemical spots on the surface of the strongly magnetic Herbig Ae star HD 101412. 2016, AN, 337, 329
  Järvinen, S.P., Hubrig, S., Schöller, M., et al.