Radio Observations of a Large Sample of Late M, L, and T Dwarfs: The Distribution of Magnetic Field Strengths

By E. Berger. 2006

161 citations recorded by the source corpus.

I present radio observations of 90 dwarf stars and brown dwarfs of spectral type M5-T8. Three sources exhibit radio activity, in addition to the six objects previously detected in quiescence and outburst, leading to an overall detection rate of ~10% for objects later than M7. The inferred magnetic field strengths are ~102 G in quiescence and nearly 1 kG during flares, while the majority of the nondetected objects have B ≲ 50 G. Depending on the configuration and size of the magnetic loops, the surface fields may approach 1 kG even in quiescence, at most a factor of a few smaller than in early M dwarfs. With the larger sample of sources I find continued evidence for (1) a sharp transition around spectral type of M7 from a ratio of radio to X-ray luminosity of log(LR/LX) ~ -15.5 to ≳-12, (2) increased radio activity (LR/Lbol) with later spectral type, in contrast to Hα and X-ray observations, and (3) an overall drop in the fraction of active sources from ~30% for M dwarfs to ~5% for L dwarfs, consistent with Hα and X-ray observations. Taken together, these trends suggest that some late M and L dwarfs are capable of generating 0.1-1 kG magnetic fields, but the overall drop in the fraction of such objects likely reflects changes in the structure of the chromospheres and coronae, possibly due to increasingly neutral atmospheres and/or a transition to a turbulent dynamo. These possibilities can best be tested through simultaneous observations, which can trace the effect of magnetic dissipation in a direct, rather than a statistical, manner. Still, a more extended radio survey currently holds the best promise for measuring the magnetic field properties of a large number of dwarf stars.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/34e6c680da24c2300979720849fe5f0cb29cb63f

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In Astronomy and Astrophysics

1. A. Reiners
2. D. Charbonneau
3. A. Burgasser
4. G. Basri
5. J. Donati
6. J. Irwin
7. E. Newton
8. Judd D. Bowman

Citations in the corpus

• Large-scale magnetic topologies of late M dwarfs★: Magnetic topologies of late M dwarfs (2010, background): “However the strong correlation between X-ray and radio luminosities established by Guedel & Benz (1993) for stars of spectral types ranging form F to mid M, is no longer valid for very low mass dwarfs which exhibit very strong radio emission whereas X-ray emissions dramatically drop (Berger 2006).”
• Science with ASKAP (2008, background): “These flares can be used to measure the magnetic field strengths of the stars, and indeed Berger [19] argues that radio observations are the best way to obtain these values.”
• Chromospheric Activity, Rotation, and Rotational Braking in M and L Dwarfs (2008, background): “5, Berger 2006).”
• The First Direct Measurements of Surface Magnetic Fields on Very Low Mass Stars (2006, background): “Berger 2006), but direct measurement of magnetic fields in VLMS is still missing.”
• Confirmation of the Electron Cyclotron Maser Instability as the Dominant Source of Radio Emission from Very Low Mass Stars and Brown Dwarfs (2008, background): “…ultracool dwarf, TVLM 513, and also due to its close proximity (< 6 pc) (Reid et al. 2003), the intensity of its radio emission in a previous short duration survey observation of ∼ 2 hours (0.525±0.015 mJy) (Berger 2006) and its favorable declination for a lengthy single observation with the VLA.”
• THE Hα EMISSION OF NEARBY M DWARFS AND ITS RELATION TO STELLAR ROTATION (2016, background): “…et al. 1981; Vilhu 1984; Pizzolato et al. 2003; Wright et al. 2011), chromospheric (Hα and Ca II) emission (e.g. Wilson 1966; Noyes et al. 1984; Soderblom et al. 1993), and radio emission from accelerated electrons (e.g. Stewart et al. 1988; Slee & Stewart 1989; Berger 2006; McLean et al. 2012).”
• Science with the Murchison Widefield Array (2012, background): “Yet radio variability from these sources is remarkably diverse, from strong (30 mJy peak flux), narrowband, fully polarized, bursting emission with frequency drifts (Burgasser & Putman 2005); to periodic bursts suggestive of pulsar-like beaming (Hallinan et al. 2007, and Fig.”
• Periodic Bursts of Coherent Radio Emission from an Ultracool Dwarf (2007, background): “Surprisingly, despite their low bolometric luminosity, a number of ultracool dwarfs have been detected as relatively bright sources at radio frequencies (Berger et al. 2001; Berger 2002; Burgasser & Putman 2005; Berger 2006; Phan-Bao et al. 2007).”
• Magnetic Fields of Nondegenerate Stars (2009, background): “…confirmed by the detection of highly-polarised rotationally-modulated radio emission from late M and early L dwarfs attributable tointense large-scale magnetic fields (e.g., Berger et al. 2005; Berger 2006; Hallinan et al. 2006)through electron cyclotron maser instability (Hallinan et al. 2008).”