The different origins of magnetic fields and activity in the Hertzsprung gap stars, OU Andromedae and 31 Comae

Abstract

Context. When crossing the Hertzsprung gap, intermediate-mass stars develop a convective envelope. Fast rotators on the main sequence, or Ap star descendants, are expected to become magnetic active subgiants during this evolutionary phase. Aims. We compare the surface magnetic fields and activity indicators of two active, fast rotating red giants with similar masses and spectral class but different rotation rates -OU And (P rot = 24.2 d) and 31 Com (P rot = 6.8 d) -to address the question of the origin of their magnetism and high activity. Methods. Observations were carried out with the Narval spectropolarimeter in 2008 and 2013. We used the least-squares deconvolution (LSD) technique to extract Stokes V and I profiles with high signal-to-noise ratio to detect Zeeman signatures of the magnetic field of the stars. We then provide Zeeman-Doppler imaging (ZDI), activity indicators monitoring, and a precise estimation of stellar parameters. We use state-of-the-art stellar evolutionary models, including rotation, to infer the evolutionary status of our giants, as well as their initial rotation velocity on the main sequence, and we interpret our observational results in the light of the theoretical Rossby numbers. Results. The detected magnetic field of OU Andromedae (OU And) is a strong one. Its longitudinal component B l reaches 40 G and presents an about sinusoidal variation with reversal of the polarity. The magnetic topology of OU And is dominated by largescale elements and is mainly poloidal with an important dipole component, as well as a significant toroidal component. The detected magnetic field of 31 Comae (31 Com) is weaker, with a magnetic map showing a more complex field geometry, and poloidal and toroidal components of equal contributions. The evolutionary models show that the progenitors of OU And and 31 Com must have been rotating at velocities that correspond to 30 and 53%, respectively, of their critical rotation velocity on the zero age main sequence. Both OU And and 31 Com have very similar masses (2.7 and 2.85 M , respectively), and they both lie in the Hertzsprung gap. Conclusions. OU And appears to be the probable descendant of a magnetic Ap star, and 31 Com the descendant of a relatively fast rotator on the main sequence. Because of the relatively fast rotation in the Hertzsprung gap and the onset of the development of a convective envelope, OU And also has a dynamo in operation.

The magnetic strip(s) in the advanced phases of stellar evolution

“…At the effective temperature of one of the lowest-mass stars observed by Aurière et al (2015, HD 203387; T eff = 5012 K, log L/L = 1.87) the theoretical period of the 2 M model varies between 10 and 60 days (which implies a factor of 6 variation on the theoretical Rossby between the two models) depending on the initial rotation velocity, while the observed period for this star is 68 days. While the overall agreement is relatively good, comparison with the magnetic and rotational properties of individual stars requires tailor-made models with different initial rotation rates for all individual sample stars (e.g., Aurière et al 2009Aurière et al , 2011Aurière et al , 2012Konstantinova-Antova et al 2010Tsvetkova et al 2013;Borisova et al 2016;Tsvetkova et al 2017), which is out of the scope of the present paper.…”

“…stars). Recently, magnetic fields have been unambiguously detected via Zeeman signatures in a large sample of single G-K giants observed with the spectropolatimeters TBL/Narval and CFHT/ESPaDOnS (Konstantinova-Antova et al 2013;Aurière et al 2015;Borisova et al 2016;Tsvetkova et al 2017). Interestingly, the cool intermediate-mass evolved stars with surface magnetic fields are found to cluster in certain regions of the HRD that correspond to precise moments of their evolution where convective envelopes make up a significant fraction of the stellar mass.…”

Active red giants: Close binaries versus single rapid rotators

“…Moreover, the chromospheric index S index and the Zeeman measurements are not sensitive to the stellar inclination contrarily to S ph , which makes trends clearer to appear. Aurière et al (2015) also identify a few outliers that could be descendent of magnetic Ap stars (e.g., Aurière et al 2011;Borisova et al 2016;Tsvetkova et al 2019). These outliers are characterized by an enhanced chromospheric activity S index with respect to their peers with similar rotation periods.…”

Magnetic field structure in single late-type giants: The weakG-band giant 37 Comae from 2008 to 2011

“…interpreted as another hint in favour of a fossil origin of their surface fields, as discussed by Aurière et al (2011), Tsvetkova et al (2013), and Borisova et al (2016). One additional constraint is the measurement of the differential rotation parameters.…”

“…These giants are V390 Aur (at the first dredge-up phase, KonstantinovaAntova et al 2012), EK Eri (at the first dredge-up phase, Aurière et al 2011), β Ceti (at the core He-burning phase, Tsvetkova et al 2013), and OU And and 31 Com (both at the Hertzsprung gap, Borisova et al 2016). The magnetic activity of V390 Aur is very likely due to the operation of a dynamo and its ZDI map reveals a complex surface magnetic topology.…”