The dramatic change of the fossil magnetic field of HD 190073: evidence of the birth of the convective core in a Herbig star ?

TL;DR

This study monitors the magnetic field of Herbig star HD 190073 over several years, revealing a sudden change likely caused by the interaction between a fossil magnetic field and a newly formed convective core.

Contribution

It provides the first observational evidence of a fossil magnetic field interacting with a dynamo field during the early evolution of a Herbig star.

Findings

Stable fossil magnetic field observed from 2004 to 2009.

Sudden change in magnetic field configuration detected between 2009 and 2011.

Proposed interaction between fossil field and dynamo field in a developing convective core.

Abstract

In the context of the ESPaDOnS and Narval spectropolarimetric surveys of Herbig Ae/Be stars, we discovered and then monitored the magnetic field of HD 190073 over more than four years, from 2004 to 2009. Our observations all displayed similar Zeeman signatures in the Stokes V spectra, indicating that HD 190073 hosted an aligned dipole, stable over many years, consistent with a fossil origin. We obtained new observations of the star in 2011 and 2012 and detected clear variations of the Zeeman signature on timescales of days to weeks, indicating that the configuration of its field has changed between 2009 and 2011. Such a sudden change of external structure of a fossil field has never previously been observed in any intermediate or high-mass star. HD 190073 is an almost entirely radiative pre-main sequence star, probably hosting a growing convective core. We propose that this dramatic change is the result of the interaction between the fossil field and the ignition of a dynamo field generated in the newly-born convective core.