Low Frequency Constraints on Brown Dwarf Radio Emission

Rachel Osten
LOFAR TKP Meeting, Amsterdam, June 2011

Transcript

1. Low Frequency Constraints on Brown Dwarf Radio Emission Rachel Osten LOFAR TKP meeting June 28, 2011 based on a paper by T. Jaeger, J. Lazio, R. Osten, N. Kassim, R. Mutel, to be submitted to ApJ
2. solar metric behavior is rich and varied courtesy Stephen White
3. type II burst -- intrinsic frequency drift (MHD shock travelling outward through the solar atmosphere)
5. Expectations • It was not expected that ultracool dwarfs would display any signatures of magnetic fields -- their atmospheres are “too dense & neutral to sustain magnetic stresses”, their interiors are fully convective, and thus the solar dynamo model is not applicable • Nevertheless, Hα emission, X-ray emission, radio emission is observed from these objects • & strong magnetic fields detected on their surfaces
6. LSR J1835; Hallinan et al. (2008) Xband LP412-31 (Stelzer et al. 2006); note the X-ray flare and optical flare (ΔV=6!) Reid et al. 1999; BRI0021-0214 (also a sporadic radio emitter) X-rays Hα radio (with a twist. . . er spin)
7. Expectations • Still unclear at this point what the dominant radio emitting mechanism is for UCDs -- the periodic bursts are likely ECM emission, but is the underlying emission the same? Gyrosynchrotron has been suggested, in analogy with higher mass flaring stars • The broad-band nature of the radio bursts has been interpreted as masing at different heights in the atmosphere (B0~3 kG), then R/Rs=1--1.5 for fundamental, second harmonic emission at 4.8 & 8.4 GHz assuming dipole configuration • Observations at lower frequencies probe lower field strengths, hence larger spatial scales
8. UCD sp. type mass (Msun) age (GY) distance (pc) Prot (hr) TVLM513-46546 M9 0.09 >1 10.6 1.96 2MASS0036 L3.5 0.06-0.074 >0.8 8.8 3.08 Properties of UCD Under Discussion both objects have exhibited periodically varying radio bursts at cm wavelengths, with period=Prot (confirmed from OIR observations) 2M0036 is the coolest UCD detected at radio wavelengths to date
9. Doyle et al (2010) April & June 2007 cm-wave observation of TVLM513 10s 60s
10. Bursts appear to be phase-stable • Doyle et al. (2010) measured the stability of pulses from TVLM513 ~40 days apart in April & June 2007, finding periodic pulses with P=1.96733±0.00002h • Our P-band measurements happened 25 days later (June 26, 2007). • The lack of evolution seen in the 3.6 cm light curve during this time period can be used to investigate conditions present during the 325 MHz observations
11. What did we see? Nada • No detections at either source location, 2.5 sigma upper limits of 795 μJy and 942 μJy for TVLM513 and 2MASS0036, resp. • no variability on timescales ranging from 10 s to 10.5 hours -- 5 [TVLM513] and 3.4 [2MASS0036] rotation periods
12. TVLM513-46546 2MASS0036 flat/declining spectra below 4 GHz
13. radio flux density at position of TVLM513, folded over the 1.96 hour rotation period, and Lomb-Scargle periodogram radio flux density at position of 2M0036, folded over the 3.08 hour rotation period, and Lomb-Scargle periodogram
14. What does it mean? • plasma environment around an UCD, assuming a dipole field, and exponentially decreasing electron density B0=3kG, ne,0-=109 cm-3 • ECM requires νc/νp>1 • constraints at 1.4 GHz argue against a single extended masing source • other possibilites: change in viewing orientation, source of energetic electrons does not extend far from surface
15. Conclusions and food for LOFAR thought • upper limits consistent with peak of radio bursts at higher frequencies; didn’t get down to quiescent flux density levels at cm λs • low frequency emission may be more sporadic, bursty than cm-wave emission (disconnect with apparently stable emitting structures at high ν) • need multi-frequency observations to put the low frequency emission in context