<figure><img src="image_1.png" /></figure>

HAL
open science

# A change in the relationship between chromospheric activity and the large-scale magnetic field for G stars on the main sequence

Emma L. Brown, Sandra V. Jeffers, Stephen C. Marsden, Julien Morin, Sudeshna Boro Saikia, Pascal Petit, Moira M. Jardine, Victor See, Aline A. Vidotto, Matthew W. Mengel, et al.

► To cite this version:

Emma L. Brown, Sandra V. Jeffers, Stephen C. Marsden, Julien Morin, Sudeshna Boro Saikia, et al. A change in the relationship between chromospheric activity and the large-scale magnetic field for G stars on the main sequence. Brun, Allan Sacha; Bouvier, Jérôme; Petit, Pascal. The 21st Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun (CS21), Jul 2022, Toulouse, France., 2022, 10.5281/zenodo.7344161. [hal-04169434](https://hal.science/hal-04169434)

HAL Id: hal-04169434

https://hal.science/hal-04169434

Submitted on 24 Jul 2023

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# A change in the relationship between chromospheric activity and the large-scale magnetic field for G stars on the main sequence

<figure><img src="image_3.png" /></figure>

USQ

E. L. Brown¹, S. V. Jeffers², S. C. Marsden¹, J. Morin³, S. Boro Saikia⁴, P. Petit⁵, M. M. Jardine⁶, V. See⁷,⁸, A. A. Vidotto⁹,¹⁰, M. W. Mengel¹, M. N. Dahlkemper¹¹,¹² and the BCool Collaboration

¹ University of Southern Queensland; ² Max Planck Institut for Solar System Research, Göttingen; ³ Université de Montpellier, CNRS; ⁴ University of Vienna;

⁵ Université de Toulouse, CNRS; ⁶ University of St Andrews; ⁷ European Space Agency; ⁸ University of Exeter; ⁹ Trinity College Dublin; ¹⁰ Leiden University;

¹¹ University of Göttingen; ¹² CERN, Switzerland

<figure><img src="image_4.png" /></figure>

emma.brown@usq.edu.au

<figure><img src="image_5.png" /></figure>

Emma Brown

<figure><img src="image_6.png" /></figure>

The BCool Collaboration

## We present a database of chromospheric activity and magnetic field strengths for 954 F-M dwarf stars

The internal magnetic fields of cool stars are known to power activity in stellar chromospheres, but there is still much to learn about the nature of this relationship and its dependence on stellar properties.

We draw on data from PolarBase, a rich legacy database of observations from the spectropolarimeters ESPaDOnS (CFHT) and NARVAL (TBL), to survey the chromospheric activity and large-scale magnetic field strengths for 954 F-M dwarf stars.

Our active sample complements previous chromospheric activity surveys that usually focus on inactive, planet search targets.

<figure><img src="image_7.png" /></figure>

## Method

We measure mean chromospheric activity, mean longitudinal field strength and their peak-to-peak variability amplitudes over time-scales of up to 15 yrs, and combine our data with archival activity data and published magnetic field geometries derived using Zeeman Doppler Imaging.

Chromospheric activity is measured as $log R'_{HK}$ from the emissions in the cores of the Call H and K lines [1,2,3,4].

Longitudinal magnetic field strength, $B_l$, is measured from high SNR, least-squares deconvolved spectral line profiles [5].

<figure><img src="image_8.png" /></figure>

## Key Findings

**G stars** with a high number of observations (red to dark blue markers) show an initial decrease in chromospheric activity, $logR'_{HK}$, with decreasing magnetic field strength, $log(|B_l|)$, and chromospheric activity variability, $log\Delta R'_{HK}$. This is followed by an **abrupt drop in the mean chromospheric activity** from $logR'_{HK}$ ~-4.4 to -4.8, with **little change in the mean magnetic field strength** or the amplitude of **chromospheric activity variability**. The relationships between activity, activity variability and magnetic field strength then appear to return to a gradually decreasing trend.

<figure><img src="image_9.png" /></figure>

*Note the inverted horizontal axes*

The drop in chromospheric activity on the main sequence is **not the same** as the drop in activity between young and main sequence stars, which occurs in the both the $logR'_{HK}$ and $log(|B_l|)$.

The **relationship change** for main sequence G stars occurs at a **rotation period** of ~15 d, and at a similar activity level as the **transition from mixed (poloidal and toroidal) to dominantly poloidal** magnetic field geometries.

**F, K and M stars** may show a comparatively smooth relation between chromospheric activity, magnetic field strength, and their variability amplitudes, but further data are required to confirm this.

## The takeaway

There is an **abrupt drop in chromospheric activity** for G stars on the main sequence but **no corresponding drop in magnetic field strength, nor activity variability**. The drop is therefore likely to be related to a change in the properties of the magnetic dynamo, rather than a rapid evolutionary phase.

**Possibilities:** a change in the area ratio of spots to plages?

<figure><img src="image_10.png" /></figure>

Read the full paper

**References:** [1] Vaughan et al., 1978, PASP, 90, 267; [2] Noyes et al., 1984, ApJ, 279, 763; [3] Marsden et al., 2014, MNRAS, 444, 3517; [4] Suarez Mascareno et al., 2015, MNRAS, 452, 2745; [5] Donati et al., 1997, MNRAS, 291, 658