Estimating magnetic filling factors from Zeeman–Doppler magnetograms

See, Victor and Matt, Sean P. and Folsom, Colin P. and Boro Saikia, Sudeshna and Donati, Jean-Francois and Fares, Rim and Finley, Adam J. and Hebrard, Elodie M. and Jardine, Moira M. and Jeffers, Sandra J. and Lehmann, Lisa T. and Marsden, Stephen C. ORCID: and Mengel, Matthew W. ORCID: and Morin, Julian and Petit, Pascal and Vidotto, Aline A. and Waite, Ian A. (2019) Estimating magnetic filling factors from Zeeman–Doppler magnetograms. The Astrophysical Journal, 876 (2). pp. 118-127. ISSN 1538-4357

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Low-mass stars are known to have magnetic fields that are believed to be of dynamo origin. Two complementary techniques are principally used to characterize them. Zeeman─Doppler imaging (ZDI) can determine the geometry of the large-scale magnetic field while Zeeman broadening can assess the total unsigned flux including that associated with small-scale structures such as spots. In this work, we study a sample of stars that have been previously mapped with ZDI. We show that the average unsigned magnetic flux follows an activity-rotation relation separating into saturated and unsaturated regimes. We also compare the average photospheric magnetic flux recovered by ZDI, < {B}V> , with that recovered by Zeeman broadening studies, < {B}I> . In line with previous studies, < {B}V> ranges from a few % to ∼20% of < {B}I> . We show that a power-law relationship between < {B}V> and < {B}I> exists and that ZDI recovers a larger fraction of the magnetic flux in more active stars. Using this relation, we improve on previous attempts to estimate filling factors, i.e., the fraction of the stellar surface covered with magnetic field, for stars mapped only with ZDI. Our estimated filling factors follow the well-known activity-rotation relation, which is in agreement with filling factors obtained directly from Zeeman broadening studies. We discuss the possible implications of these results for flux tube expansion above the stellar surface and stellar wind models.

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Item Type: Article (Commonwealth Reporting Category C)
Refereed: Yes
Item Status: Live Archive
Additional Information: Published version deposited in accordance with the copyright policy of the publisher.
Faculty/School / Institute/Centre: Current - Institute for Advanced Engineering and Space Sciences - Centre for Astrophysics (1 Aug 2018 -)
Faculty/School / Institute/Centre: Current - Institute for Advanced Engineering and Space Sciences - Centre for Astrophysics (1 Aug 2018 -)
Date Deposited: 12 Sep 2019 05:38
Last Modified: 25 Sep 2019 06:19
Uncontrolled Keywords: stars; low-mass; magnetic field; rotation; astrophysics; solar and stellar astrophysics
Fields of Research (2008): 02 Physical Sciences > 0201 Astronomical and Space Sciences > 020110 Stellar Astronomy and Planetary Systems
Fields of Research (2020): 51 PHYSICAL SCIENCES > 5101 Astronomical sciences > 510109 Stellar astronomy and planetary systems
Socio-Economic Objectives (2008): E Expanding Knowledge > 97 Expanding Knowledge > 970102 Expanding Knowledge in the Physical Sciences
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