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We have used the Jansky VLA to observe a sample of 5 known aurorally emitting late L and T dwarfs ranging in age from 0.2-3.4 Gyr. We observed each target for seven hours, extending to higher frequencies than previously attempted for objects in this sample. We establish proportionally higher limits on maximum surface magnetic field strengths while simultaneously placing constraints on rotation periods through detections of repeating pulses. Observations at 8{12 GHz yield measurements of 3.7{4.1 kG localized field strengths (corresponding to minimum mean surface fields between 2.7{2.9 kG) on four of our targets, including the archetypal cloud variable T2.5 dwarf SIMP J01365663+0933473 recently proposed to be a possible planetary-mass object in the Carina-Near moving group. We detect a circularly polarized radio pulse at 15{16.5 GHz for the T6.5 dwarf 2MASS 10475385+2124234, corresponding to a localized 5.6 kG field strength and minimum mean surface field of 4.0 kG. For the same object, we also tentatively detect a circularly polarized radio pulse at 16.5{18 GHz corresponding to a localized 6.2 kG field strength and minimum mean surface field of 4.4 kG. We measure rotation periods between 1.44-2.88 hr for all targets, supporting i) the emerging consensus in convective dynamo models that rapid rotation may be important for producing strong dipole fields and/or ii) rapid rotation is a key ingredient for driving the current systems powering auroral radio emission. We do not detect a clear cutoff in the pulsed emission for any targets, which would correspond to a maximum local surface magnetic field strength. However, we do observe evidence of variable structure in the frequency-dependent timeseries of our targets on timescales shorter than a rotation period, suggesting a higher degree of variability in the current systems near the surfaces of brown dwarfs, where emission at the highest frequencies are expected to probe. Finally, we find that old brown dwarfs may generate fields as strong as young brown dwarfs.
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Signs of binary evolution in seven magnetic DA white dwarfs
ABSTRACT We present our findings on the spectral analysis of seven magnetic white dwarfs that were presumed to be double degenerates. We obtained time-resolved spectroscopy at the Gemini Observatory to look for evidence of binarity or fast rotation. We find three of our targets have rotation periods of less than an hour based on the shifting positions of the Zeeman-split H α components: 13, 35, and 39 min, and we find one more target with a approximately an hour long period that is currently unconstrained. We use offset dipole models to determine the inclination, magnetic field strength, and dipole offset of each target. The average surface field strengths of our fast rotators vary by 1–2 MG between different spectra. In all cases, the observed absorption features are too shallow compared to our models. This could be due to extra flux from a companion for our three low-mass targets, but the majority of our sample likely requires an inhomogeneous surface composition. Including an additional magnetic white dwarf with similar properties presented in the literature, we find that five of the eight targets in this sample show field variations on minute/hour time-scales. A crystallization driven dynamo can potentially explain the magnetic fields in three of our targets with masses above 0.7 M⊙, but another mechanism is still needed to explain their rapid rotation. We suggest that rapid rotation or low-masses point to binary evolution as the likely source of magnetism in seven of these eight targets.
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- PAR ID:
- 10426675
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 523
- Issue:
- 4
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 5598-5609
- Size(s):
- p. 5598-5609
- Sponsoring Org:
- National Science Foundation
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