A VOLUME-LIMITED SAMPLE OF 63 M7–M9.5 DWARFS. II. ACTIVITY, MAGNETISM, AND THE FADE OF THE ROTATION-DOMINATED DYNAMO
By A. Reiners, G. Basri. 2009
147 citations recorded by the source corpus.
In a volume-limited sample of 63 ultracool dwarfs of spectral type M7–M9.5, we have obtained high-resolution spectroscopy with UVES at the Very Large Telescope and HIRES at Keck Observatory. In this second paper, we present projected rotation velocities, average magnetic field strengths, and chromospheric emission from the Hα line. We confirm earlier results that the mean level of normalized Hα luminosity decreases with lower temperature, and we find that the scatter among Hα luminosities is larger at lower temperature. We measure average magnetic fields between 0 and 4 kG with no indication for a dependence on temperature between M7 and M9.5. For a given temperature, Hα luminosity is related to magnetic field strength, consistent with results in earlier stars. A few very slowly rotating stars show very weak magnetic fields and Hα emission, and all stars rotating faster than our detection limit show magnetic fields of at least a few hundred Gauss. In contrast to earlier-type stars, we observe magnetic fields weaker than 1 kG in stars rotating faster than ∼3 km s−1, but we find no correlation between rotation and magnetic flux generation among them. We interpret this as a fundamental change in the dynamo mechanism; in ultracool dwarfs, magnetic field generation is predominantly achieved by a turbulent dynamo, while other mechanisms can operate more efficiently at earlier spectral types.
Subfields: Astronomy and Astrophysics
View Paper: https://www.semanticscholar.org/paper/cc420b81785b44905dc0eabca30acbd9253ce542
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Citations in the corpus
- CARMENES input catalogue of M dwarfs. I. Low-resolution spectroscopy with CAFOS (2015, background): “However, Reiners & Basri (2010) observed flaring activity in a J07523+162 spectrum (pEW(Hα) = – 44.4 Å) and Gagné et al. (2014) and Klutsch et al. (2014) were not able to determine membership in any known stellar kinematic group.”
- THE EVOLUTION OF CATACLYSMIC VARIABLES AS REVEALED BY THEIR DONOR STARS (2011, background): “Broadly speaking, there is evidence for systematic changes in both magnetic field strength and topology with spectral type (e.g., Reiners & Basri 2008, 2009, 2010; Donati et al. 2008; Morin et al. 2010).”
- The Formation and Early Evolution of Low-Mass Stars and Brown Dwarfs (2012, result): “Similar objects have been found during other spectroscopic surveys of late-type field dwarfs (Allers et al. 2010; Nakajima, Tsuji & Yanagisawa 2004; Reiners & Basri 2010).”
- A CATALOG OF ROTATION AND ACTIVITY IN EARLY-M STARS (2012, methodology): “A saturation-type rotation-activity relation in M dwarfs is observed for late-M spectral types (later than M5; Mohanty & Basri 2003; Reiners & Basri 2010), and less well studied also in early-M type stars (Delfosse et al. 1998; Reiners & Basri 2007).”
- Observations of Cool-Star Magnetic Fields (2012, background): “0 1.4 Reiners and Basri (2010) 2M 1438082+640836 M9.”
- AN ALL-SKY CATALOG OF BRIGHT M DWARFS (2011, background): “…their enhanced rotation, dynamo activity, and chromospheric emission (Kiraga & Stepien 2007), although the trend for late M stars for which the mechanism of dynamo operation is thought to be different is less clear (West et al. 2009; West & Basri 2009; Browning et al. 2010; Reiners & Basri 2010).”
- Frequent Flaring in the TRAPPIST-1 System—Unsuited for Life? (2017, background): “This latter value is consistent with the measured v sin i = 6 ± 2 km s−1 (Reiners & Basri 2010), that yields Prot = 0.9866d (0.74–1.48 days with the given error) using R = 0.117R (Gillon et al. 2016) and assuming i = 90◦.”