The First Direct Measurements of Surface Magnetic Fields on Very Low Mass Stars

By A. Reiners, G. Basri. 2006

222 citations recorded by the source corpus.

We present the first direct magnetic field measurements on M dwarfs cooler than spectral class M4.5. Utilizing a new method based on the FeH band near 1 μm, we categorize the integrated surface magnetic flux as low (well under 1 kG), intermediate (between 1 and about 2.5 kG), or strong (greater than about 3 kG) for a set of more than 20 stars ranging from M2 down to M9. Along with the field, we also measure the rotational broadening (v sin i) and Hα emission strength. Our goal is to advance the understanding of how dynamo field production varies with stellar parameters for very low mass stars, how the field and emission activity are related, and whether there is a connection between the rotation and magnetic flux. We find that fields are produced throughout the M dwarfs. In the mid-M stars, there is a clear connection between slow rotation and weak fields. In the late-M stars, rotation is always measurable, and the strongest fields are associated with with the most rapid rotators. Interestingly, these very cool rapid rotators appear to have the largest magnetic flux in the whole sample (greater than in the classical dMe stars). Hα emission is found to be a good proxy for magnetic fields, although the relation between the fractional emission and the magnetic flux varies with effective temperature. The drop-off in this fractional emission near the bottom of the main sequence is not accompanied by a drop-off in magnetic flux. It is clear that the methodology we have developed can be further applied to discover more about the behavior of magnetic dynamos and activity in cool and fully convective objects.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/2b5ece63ef8596d514c80104814a0ff8f1d16036

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

1. A. Reiners
2. D. Charbonneau
3. M. Browning
4. K. Luhman
5. G. Basri
6. A. Brun
7. P. Hopkins
8. J. Donati

Citations in the corpus

• Chromospheric Activity, Rotation, and Rotational Braking in M and L Dwarfs (2008, methodology): “In the right panel, we show them together with the data from Delfosse et al. (1998), Reiners & Basri (2007), and Mohanty & Basri (2003).”
• M dwarfs: effective temperatures, radii and metallicities (2008, background): “Modeling the internal structure, atmospheric properties and magnetic activity of M dwarfs (e.g. Burrows et al. 1993, Allard et al. 1997, Baraffe et al. 1998, Hauschildt, Allard & Baron 1999, Allred et al. 2006, Reiners & Basri 2007) also presents ongoing theoretical challenges.”
• PERSISTENT MAGNETIC WREATHS IN A RAPIDLY ROTATING SUN (2010, background): “Expressed in this fashion, a common rotation-activity correlation appears to span spectral types ranging from late F to late M (e.g., Mohanty & Basri 2003; Pizzolato et al. 2003; Reiners & Basri 2007).”
• Evolution of low-mass star and brown dwarf eclipsing binaries (2007, background): “This yields typical magnetic length scales l <∼R⋆/50, and thus Beq ≈ a few 10 4 G for LMS average conditions, in good agreement with surface field determinations of a few kG (Donati et al. 2006; Reiners & Basri 2007).”
• Toroidal versus poloidal magnetic fields in Sun-like stars: a rotation threshold (2008, background): “Although being successful on a few objects quite different from the Sun by their mass (Valenti, Marcy & Basri 1995; Reiners & Basri 2007) or rotation rate (Rüedi et al. 1997; Donati et al. 2003; Petit et al. 2005), measurements of Zeeman signatures have always failed at detecting the magnetic…”
• COLORS AND KINEMATICS OF L DWARFS FROM THE SLOAN DIGITAL SKY SURVEY (2010, background): “LHS 2924, a well known active M9 dwarf (e. g., Fleming et al. 1993; Reiners & Basri 2007), was classified as an L0 dwarf during the spectral typing of our sample, which is within our ±1 subtype uncertainty.”
• A CATALOG OF ROTATION AND ACTIVITY IN EARLY-M STARS (2012, methodology): “During the course of our project, Jenkins et al. (2009) and Browning et al. (2010) added more stars to this list, and a few other rotational velocities were presented in Reiners (2007) and Reiners & Basri (2007).”
• Large-scale magnetic topologies of late M dwarfs⋆ (2008, methodology): “Given the previously reported stellar parameters v sin i = 3.0 kms−1 (Reiners & Basri 2007), a rotation period of 2.7 d (Spiesman & Hawley 1986) and R⋆ 6 0.40 R⊙ (see Tab.”
• TESTING A PREDICTIVE THEORETICAL MODEL FOR THE MASS LOSS RATES OF COOL STARS (2011, background): “0.35M⊙) are likely to be fully convective, and thus their dynamos are likely to be driven by fundamentally different processes than exist in more massive stars (Mullan & MacDonald 2001; Reiners & Basri 2007; Irwin et al. 2011).”
• Simulations of Dynamo Action in Fully Convective Stars (2007, background): “At still lower masses, Reiners & Basri (2007) have found that magnetic activity is detectable even in some stars that are not detectably rotating; such stars may conceivably have low enough convective velocities that even rotation rates below the observational detection limit could imply a…”
• The Karl G. Jansky Very Large Array Sky Survey (VLASS). Science Case and Survey Design (2019, background): “…strengths now known to occur around some brown dwarfs were first detected through their effect on cm-wavelength radio emission (Berger et al. 2001; Hallinan et al. 2006) before the signatures were seen through Zeeman splitting of absorption lines at near infrared wavelengths (Reiners & Basri 2007).”
• Confirmation of the Electron Cyclotron Maser Instability as the Dominant Source of Radio Emission from Very Low Mass Stars and Brown Dwarfs (2008, background): “It was recently confirmed that ultracool dwarfs do indeed possess such high strength magnetic fields by Reiners & Basri (2007), who studied the magnetically sensitive Wing-Ford FeH band in a wide spectral range of M dwarfs, thus extending the direct measurement of magnetic field strengths out to M9.”
• A VOLUME-LIMITED SAMPLE OF 63 M7–M9.5 DWARFS. II. ACTIVITY, MAGNETISM, AND THE FADE OF THE ROTATION-DOMINATED DYNAMO (2009, methodology): “Rotation and Average Magnetic Field The analysis of our spectra follows the strategy laid out in Reiners & Basri (2006, 2007) and Reiners et al. (2009a).”
• Large-scale magnetic topologies of late M dwarfs★: Magnetic topologies of late M dwarfs (2010, methodology): “We use v sin i = 17 kms−1 (Reiners & Basri 2007) which allows us to fit the observed polarised and unpolarised profiles, as opposed to the previously reported value (v sin i = 9.2 kms−1, Delfosse et al. 1998).”
• A PHOTOMETRIC VARIABILITY SURVEY OF FIELD K AND M DWARF STARS WITH HATNet (2009, background): “Nonetheless, several studies have found evidence that the rotation–activity connection may continue even to very late M dwarfs (Delfosse et al. 1998; Mohanty & Basri 2003; Reiners & Basri 2007).”