Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://star.arm.ac.uk/~jgd/outgoing/Proposals/vla_4_8GHz.ps Äàòà èçìåíåíèÿ: Sun Sep 12 15:09:12 2004 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 10:02:46 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: arp 220

VLA OBSERVING APPLICATION
DEADLINES: 1st of Feb., June., Oct. for next configuration following review
INSTRUCTIONS: Each numbered item must have an entry or N/A
E­MAIL TO: propsoc@nrao.edu (di#erent for some Rapid Response Science)
OR MAIL TO: Director NRAO, 520 Edgemont Rd., Charlottesville, VA 22903­2475
A
rcvd:
(1) Date Prepared: September 9th 2004
(2) Title of Proposal: Confirming Dynamic Radio Spectra from Brown Dwarf Coronae?
Students Only
(3) AUTHORS INSTITUTION E­mail G/U For Ph.D.
(Add * for new location) Thesis? Year
A. Golden National University of Ire­
land, Galway
agolden@it.nuigalway.ie
J.G. Doyle * Armagh Observatory jgd@arm.ac.uk
T. Antonova * Armagh Observatory tan@arm.ac.uk G Yes 1st
(4) Related VLA previous proposal number(s):
(5) Contact author (6) Telephone: 353 91 524411 x3549
for scheduling: A. Golden E­mail: agolden@it.nuigalway.ie
address: Computational Astrophysics Labo­
ratory, IT Building
Fax: 353 91 750501
National University of Ireland, Gal­
way
Ireland
(7) Scientific Category: # solar system # galactic # extragalactic # other:
Rapid Response Science:
# Known Transient # Exploratory # Target of Opportunity
(8) Configurations (one per column)
(A+Pt, A, B, C, D, BnA, CnB, DnC, Any) Any Any
(9) Wavelength(s) 3.6cm, 6cm 3.6cm, 6cm
(400, 90, 20, 6, 3.5, 2, 1.3, 0.7 cm)
(10) Time requested
(hours) 10 10
(11) Type of observation:
# continuum # spectroscopy # multichannel continuum # polarimetry # solar
(check all that apply) # pulsar # high­time resolution # Pie Town link # other:
(12) Suitable for dynamic scheduling? # Suitable # Unsuitable
(13) ABSTRACT (do not write outside this space)
NRAO use only
(08/03)
To date, radio emission has been observed from 7 brown dwarfs, at luminosities that violate our understanding of
traditional coronal emission. Identifying the actual emission process and its location requires new observational data
­ determining its brightness temperature via VLBI imaging, and extending our existing timeseries archive. Recent
ACTA observations yielded a significant flare from DENIS 1048­3956 at # 4 GHz, followed 10 minutes later by
emission at 8 GHz. This contradicts the prevailing view that emission at 4 GHz is predominately self­absorbed. Are
these emissions at 4 & 8 GHz correlated? We request 20 hours to characterise the quiescent & flare characteristics
of two well placed brown dwarfs 2MASSW J0036159+182110 and TVLM513 simultaneously at 4 and 8 GHz with
a view to confirming the reality of this time­lag emission as a function of frequency, which will provide a powerful
constraint to evolving models of brown dwarf coronal emission.
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(14) Observer present for observations? # Yes
# No Data analysis at?
# Home # AOC or CV (2 weeks notice)
(15) Help required: # None # Consultation # Friend (extensive help)
(16) Spectroscopy only line 1 line 2 line 3 line 4
Transition (HI, OH, etc.)
Rest Frequency (MHz)
Velocity (km/s)
Observing frequency (MHz)
Correlator mode
IF bandwidth(s) (MHz)
Hanning smoothing (y/n)
Number of channels per IF
Frequency Resolution (kHz/channel)
Rms noise (mJy/bm, nat. weight., 1 hr)
Rms noise (K, nat. weight., 1 hr)
(17) Number of sources: 2
(If more than 10 please attach list. If more than 30 give only selection criteria and LST range(s).)
Coordinates Band­ Total Required Required Time
1950 # 2000
# Conf. # Corr. width Flux LAS rms dynamic request
(18) NAME RA Dec. (cm) mode per IF (Jy) # (mJy/bm) range (hr)
hh mm ± xx.x # (MHz)
2MASSW
J0036159+182110
00 36 15.9, +18 21 20 Any 3.6 50 2.0e­
5
0.02 10
2MASSW
J0036159+182110
00 36 15.9, +18 21 20 Any 6 50 2.0e­
5
0.03 10
TVLM13­46546 15 10 8.3, +22, 50 02 Any 3.6 50 2.0e­
05
0.02 10
TVLM13­46546 15 10 8.3, +22, 50 02 Any 6 50 2.0e­
05
0.03 10
# For spectral line, this should be the total flux at the peak of the line
Notes to the table (if any):
(19) Restrictions to elevation (other than hardware limits) or HA range (give reason):
(20) Preferred range of dates for scheduling (give reason):
(21) Dates which are not acceptable:
(22) Special hardware, software, or operating requirements:
(23) Please attach a self­contained Scientific Justification not in excess of 1000 words. (Preprints or reprints will be ignored.)
Please include the full addresses (postal and e­mail) for first­time users or for those that have moved (if not contact author).
When your proposal is scheduled, the contents of the cover sheets become public information (Any supporting pages are for
refereeing only).
v4.2 8/03
2

Scientific Case
To date in the literature 7 brown dwarfs have been reported to display quiescent and flaring behaviour
at radio wavelengths. Of the five, two (LP944­20 & BRI0021) have been observed in to flare in X­rays.
The radio and X­ray emission may be correlated, as is the case for a wide range of coronally active stars in
both quiescent and flaring states (Guedel & Benz 1993; Benz & Guedel 1994). There are firm grounds to
believe there is a causal link between the radio­emitting electrons and the X­ray emitting plasma in stellar
coronae (Doyle & Butler 1985), with the latter producing a high speed stellar wind that carries away angular
momentum from the star, depending on the strength and shape of the coronal magnetic (B) field. Such
active stellar coronae implicitly imply rapidly rotating stars and the presence of B fields of a few kG.
The observed radio emission however exceeds the emission predicted by the Guedel--Benz model by many
orders of magnitude. Attempts to interpret the observed emission have assumed incoherent gyrosynchrotron
or synchrotron processes in the corona. Analyses of four active brown dwarfs suggest B fields of # 10--10 2 G,
with electron densities of # 10 12 cm -3 (Berger, 2002). However, their collective 'advanced' age, but rapid
rotation, undermines conventional 'dynamo based' coronal models, due to the fact that they should have
lost considerable angular momentum due to magnetic braking. A plausible alternative, coherent processes
such as plasma radiation or an electron cyclotron maser (both associated with localised coronal loops) may
require B fields of # 1500 G, with similar particle densities.
Perhaps the best way to understand the radio emission process would be the resolving of the dimensions
of the quiescent and/or flaring regions via VLBI. The main di#erence between coherent and incoherent
emission in the corona would be the associated brightness temperature, T b # 10 8-9 K for the former and
# 10 11 K for the latter. One can use the observed T b 's and degrees of circular polarization to di#erentiate
between coherent and incoherent emission processes, provided one can resolve the emission. This is a
function of the dimensions of the emission, the observing frequency and largest baseline. Following on from
the discovery observations of LP944­20 by Berger et al. (2001), the assumption has been that the peak
of the flare continuum emission is # 8 GHz, with emission at lower frequencies e#ectively quenched via
self­absorption. No dwarf has been observed at 15 GHz and reported thus in the literature ­ clearly the
detection of any of the known radio emitting dwarfs at this frequency would have important implications for
VLBI work.
Using the ACTA, Putmann and Burgasser (2003) have recently reported the detection of strong flare
and significant quiescent emission from the brown dwarf DENIS 1048­3956, along with 2 others detected as
quiescent sources (not articulated at this time). Observations were performed simultaneously at the 4.8 and
8.6 GHz bands. The quiescent emission from the three sources correspond to brightness temperatures of 10 8
­ 10 9 K and spectral indices (­1 < # < 0) suggests geosynchrotron emission from relativistic electrons in 10
­ 100 G magnetic fields.
DENIS 1048­3956, however, was reported to emit two significant flares over its 615 minute observation ­
and this is the particularly interesting point ­ initially at 4.8 GHz with a peak of # 20 mJy, and subsequently
at 8.6 GHz with a peak of # 40 mJy. These authors state that the high brightness temperatures and short
timescales involved (# 10 12 K and # 2 minutes), high linear polarisation point to a coherent emission process
­ which supports the contention that has followed the original LP944­20 detection. This time lag in the
emission is the first such reported emission for a brown dwarf corona, and could potentially provide new
insights into the emission phenomena.
The original LP944­20 observations were performed at 2 frequency bands, but Berger et al. (2001) did
not report any such lag in emission ­ on the contrary, the apparant lack of emission at 4 GHz when clearly
obvious at 8 GHz was used to support the contention that the emission was self­absorbed at 4 GHz. The
ACTA observations, whilst indicating evidence for some form of absorption at 4 GHz, clearly show emission
at this band is significant and detectable.
The subsequent observations of Berger (2002) used the self­absorbed argument to make single­band
observations at 8 GHz, which lead to the discovery of three more flaring brown dwarfs: BRI0021­02,
2MASS0036+18 and TVLM513. Critically, there were no simultaneous 4 GHz observations, so one can­
not assert that there is significant emission at this lower band, as is clearly the case based on observations of
DENIS 1048­3956. We are aware of a recent report of variable emission at 4 and 8 GHz from one of these,
TVLM513 (poster presented by R. Olsen at the Cool Stars Meeting, Hamburg, 2004, online review of meet­
ing at http://jilawww.colorado.edu/ jlinsky/cs13suma4.ps). The fact that Olsen argues for geosynchrotron
3

emission suggests they monitored quiescent variability ­ not actual flare events. We await publication of the
results of this work ­ but it does suggest that there is significant emission at 4 GHz.
We propose to perform continuum split­array observations of the two 'brightest' brown dwarfs detected
in the Berger (2002) survey: 2MASS0036+18 and TVLM513 at 4.6 GHz and 8.6 GHz simultaneously for 10
hours each. From the literature, one expects a flare event on average every 2.5 hours, so over the 10 hours
requested for each dwarf, we might hope to get data for as many as 8 flares. This is clearly dependent on
the activity of each dwarf, e.g. Berger et al. (2001) noted a change by a factor of 3 in the quiescent emission
from 2MASS0036 in his two separate pointings. This is why we have requested time for two, rather than
one, dwarf. Both are well placed in the northern sky for the Socorro site.
This would provide the first simultaneous multi­frequency observations of both brown dwarfs, the goals
of which would be the following
. to consolidate the time­series 'data archive' of known radio­emitting brown dwarfs
. to characterise the flare and quiescent emission characteristics of both objects
. to confirm the possibility of correlated flare emission as a function of energy over time
Benz, A.O. & Guedel, M., 1994, A&A 285, 621
Berger, E., et al. 2001, Nat 410, 338
Berger, E., 2002, ApJ 572, 503
Doyle, J.G. & Butler, J.G., Nat 313, 378
Guedel, M. & Benz, A.O., 1993, ApJ 405, L63
Rutledge, R.E., et al. 2000, ApJ 538, L141
4