Volume-limited radio survey of ultracool dwarfs

A. Antonova, G. Hallinan, J. G. Doyle, S. Yu, A. Kuznetsov, Y. Metodieva, A. Golden, K. L. Cruz

Research output: Contribution to journalArticle

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Abstract

Aims. We aim to increase the sample of ultracool dwarfs studied in the radio domain to allow a more statistically significant understanding of the physical conditions associated with these magnetically active objects. Methods. We conducted a volume-limited survey at 4.9 GHz of 32 nearby ultracool dwarfs with spectral types covering the range M7-T8. A statistical analysis was performed on the combined data from the present survey and previous radio observations of ultracool dwarfs. Results. Whilst no radio emission was detected from any of the targets, significant upper limits were placed on the radio luminosities that are below the luminosities of previously detected ultracool dwarfs. Combining our results with those from the literature gives a detection rate for dwarfs in the spectral range M7-L3.5 of ∼9%. In comparison, only one dwarf later than L3.5 is detected in 53 observations. We report the observed detection rate as a function of spectral type and the number distribution of the dwarfs as a function of spectral type and rotation velocity. Conclusions. The radio observations to date point to a drop in the detection rate toward the ultracool dwarfs. However, the emission levels of detected ultracool dwarfs are comparable to those of earlier type active M dwarfs, which may imply that a mildly relativistic electron beam or a strong magnetic field can exist in ultracool dwarfs. Fast rotation may be a sufficient condition to produce magnetic fields strengths of several hundred Gauss to several kilo Gauss, as suggested by the data for the active ultracool dwarfs with known rotation rates. A possible reason for the non-detection of radio emission from some dwarfs is that maybe the centrifugal acceleration mechanism in these dwarfs is weak (due to a low rotation rate) and thus cannot provide the necessary density and/or energy of accelerated electrons. An alternative explanation could be long-term variability, as is the case for several ultracool dwarfs whose radio emission varies considerably over long periods with emission levels dropping below the detection limit in some instances.

Original languageEnglish (US)
Article numberA131
JournalAstronomy and Astrophysics
Volume549
DOIs
StatePublished - 2013

Fingerprint

radio
radio emission
radio observation
luminosity
relativistic electron beams
magnetic fields
magnetic field
statistical analysis
electron
field strength
coverings
rate
detection
electrons
energy

Keywords

  • Brown dwarfs
  • Masers
  • Radiation mechanisms: general
  • Radio continuum: stars
  • Stars: low-mass

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Antonova, A., Hallinan, G., Doyle, J. G., Yu, S., Kuznetsov, A., Metodieva, Y., ... Cruz, K. L. (2013). Volume-limited radio survey of ultracool dwarfs. Astronomy and Astrophysics, 549, [A131]. https://doi.org/10.1051/0004-6361/201118583

Volume-limited radio survey of ultracool dwarfs. / Antonova, A.; Hallinan, G.; Doyle, J. G.; Yu, S.; Kuznetsov, A.; Metodieva, Y.; Golden, A.; Cruz, K. L.

In: Astronomy and Astrophysics, Vol. 549, A131, 2013.

Research output: Contribution to journalArticle

Antonova, A, Hallinan, G, Doyle, JG, Yu, S, Kuznetsov, A, Metodieva, Y, Golden, A & Cruz, KL 2013, 'Volume-limited radio survey of ultracool dwarfs', Astronomy and Astrophysics, vol. 549, A131. https://doi.org/10.1051/0004-6361/201118583
Antonova A, Hallinan G, Doyle JG, Yu S, Kuznetsov A, Metodieva Y et al. Volume-limited radio survey of ultracool dwarfs. Astronomy and Astrophysics. 2013;549. A131. https://doi.org/10.1051/0004-6361/201118583
Antonova, A. ; Hallinan, G. ; Doyle, J. G. ; Yu, S. ; Kuznetsov, A. ; Metodieva, Y. ; Golden, A. ; Cruz, K. L. / Volume-limited radio survey of ultracool dwarfs. In: Astronomy and Astrophysics. 2013 ; Vol. 549.
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AU - Antonova, A.

AU - Hallinan, G.

AU - Doyle, J. G.

AU - Yu, S.

AU - Kuznetsov, A.

AU - Metodieva, Y.

AU - Golden, A.

AU - Cruz, K. L.

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AB - Aims. We aim to increase the sample of ultracool dwarfs studied in the radio domain to allow a more statistically significant understanding of the physical conditions associated with these magnetically active objects. Methods. We conducted a volume-limited survey at 4.9 GHz of 32 nearby ultracool dwarfs with spectral types covering the range M7-T8. A statistical analysis was performed on the combined data from the present survey and previous radio observations of ultracool dwarfs. Results. Whilst no radio emission was detected from any of the targets, significant upper limits were placed on the radio luminosities that are below the luminosities of previously detected ultracool dwarfs. Combining our results with those from the literature gives a detection rate for dwarfs in the spectral range M7-L3.5 of ∼9%. In comparison, only one dwarf later than L3.5 is detected in 53 observations. We report the observed detection rate as a function of spectral type and the number distribution of the dwarfs as a function of spectral type and rotation velocity. Conclusions. The radio observations to date point to a drop in the detection rate toward the ultracool dwarfs. However, the emission levels of detected ultracool dwarfs are comparable to those of earlier type active M dwarfs, which may imply that a mildly relativistic electron beam or a strong magnetic field can exist in ultracool dwarfs. Fast rotation may be a sufficient condition to produce magnetic fields strengths of several hundred Gauss to several kilo Gauss, as suggested by the data for the active ultracool dwarfs with known rotation rates. A possible reason for the non-detection of radio emission from some dwarfs is that maybe the centrifugal acceleration mechanism in these dwarfs is weak (due to a low rotation rate) and thus cannot provide the necessary density and/or energy of accelerated electrons. An alternative explanation could be long-term variability, as is the case for several ultracool dwarfs whose radio emission varies considerably over long periods with emission levels dropping below the detection limit in some instances.

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KW - Masers

KW - Radiation mechanisms: general

KW - Radio continuum: stars

KW - Stars: low-mass

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