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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:
(2) Title of Proposal: Understanding Flares on Brown Dwarfs
Students Only
(3) AUTHORS INSTITUTION E­mail G/U For Ph.D.
(Add * for new location) Thesis? Year
J.G. Doyle * Armagh Observatory jgd@arm.ac.uk
A. Golden National University of Ire­
land, Galway
agolden@it.nuigalway.ie
T. Antonova * Armagh Observatory tan@arm.ac.uk G Yes 1st
(4) Related VLA previous proposal number(s):
(5) Contact author (6) Telephone: 44 28 37522928
for scheduling: J.G. Doyle E­mail: jgd@arm.ac.uk
address: Armagh Observatory Fax: 44 28 37527174
Armagh BT61 9DG
N. 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) CnB
(9) Wavelength(s) 2
(400, 90, 20, 6, 3.5, 2, 1.3, 0.7 cm)
(10) Time requested
(hours) 60
(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)
Brown dwarfs are regarded as the 'missing link' in our understanding of stellar and planetary evolution. We propose
to try and resolve the mystery of active coronal emission from these objects by implementing a programme of
complementary observational and theoretical work. These planned activities include the acquisition of high resolution
radio images, long term monitoring of radio flares from several brown dwarfs, and the construction of models of such
flare emission processes. The latter will entail the idea of flux tubes transporting magnetic stresses from the interior
to the upper layers, critically incorporating the role of dust. For this part of the programme, we request 60 hr
observation of 2MASSW J0036159+182110 at 15 GHz in order to accurately flux its integrated emission at this
band, and to look for temporal variations over this time­frame. This source is well characterized at 8 GHz, but not
at 15 GHz.
1

(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: 1
(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, +18.3 CnB 2 50 7.5E­
5
0.02 60
# 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

At present over 270 dwarf stars have been detected with temperatures cooler than the well known M
dwarf class, yielding two new spectral classes, L and T. Many of these are better known collectively as
`brown dwarfs', and are in many respects enigmatic objects in terms of stellar evolution, particularly as they
span the gap between cool M stars and giant Jupiter­like planets. Arguably they represent the missing link
between stars and planets, and as such, have motivated a considerable amount of on­going research into
understanding them in the context of planetary & stellar evolution, and also in their own right.
Some profoundly interesting physics occurs in the `transition' from an M dwarf to a L or T dwarf.
Coronal `flare' emission in early/mid M dwarfs (in radio & X­rays) appears to be correlated in terms of
luminosity, following a power­law first noted by Gudel & Benz (1993) derived for a range of coronal active
spectral classes. This followed on from earlier work by Doyle & Butler (1985) who showed that there was
a very close correlation between the rate of flaring activity on a range of late­type stellar objects and the
quiescent X­ray emission, and introduced the idea that coronal heating results from flare­like events. Such
active stellar coronae implicitly imply rapidly rotating stars and the presence of magnetic fields of a few kG.
What is completely surprising is that whilst a number of brown dwarfs exhibit quiescent and flare emission
at both wavelengths, the radio emission observed from these flare events is far in excess of that predicted
from this model (Berger et al. 2001). Further VLA observations of late M/early L dwarfs indicated flaring
and quiescent radio emission (similar to that seen first for LP944--20) has been presented for three more
rapidly rotating brown dwarfs by Berger (2002). Thus, radio and X­ray activity (Rutledge et al. 2000) is a
reality beyond the main sequence -- but how?
Recent observations show that chromospheric H # activity in late­M/early L dwarfs is much lower than
in the earlier M types despite the fact that these objects are very rapid rotators. One possibility is that the
drop­o# in activity is the result of high electrical resistivities in the atmosphere due to the presence of dust.
As a result, the available free magnetic energy for the support of a chromosphere becomes smaller with later
spectral type. This then raises the question, how are radio and X­ray flares then produced?
The observed radio emission is either a widespread coronal e#ect or is localized in much smaller regions?
One such idea to transport magnetic energy from the interior to the upper layers are flux tubes. Due to
the cool upper atmosphere in BD's, di#erent dissipation mechanisms need to be overcome, e.g. collision
with neutrals and recombination. Central of the analysis is the brightness temperatures, which is critically
dependent on the size of the emission region. It we could resolve the size of the region, we would be able
to constrain T b and as a consequence put limits on the emission process and the likely coronal nature of the
emission (whether quiescent or flaring).
The aspect we wish to address in this proposal is variability, which is a clear characteristic of brown
dwarf radio emission. Monitoring both quiescent and flaring behaviour over long timescales across a range of
frquency bands would allow us to build a good statistical base of coronal activity, to look for periodicities, to
determine the burst statistics, and to constrain coherent/incoherent emission models. This follows successful
observation campaigns of M dwarfs such as AD Leonis (Adabda­Simon et al. 1997, Pestalozzi et al. 2000).
We propose to perform continuum 15 GHz observations of 2MASSW J0036159+182110. To estimate
it's 15 GHz fluz, we use Berger et al. (2001) power law exponent for the 4 to 8 GHz flare emission from
LP 944--20. Assuming an extrapolation to 15 GHz is valid with this spectral index suggests an average flare
flux of 2--3 mJy/ Similar scaling of the quiescent flux suggests # 1 mJy. Using the VLA exposure calculator,
for 43 MHz BW, 26 antenna, a 60 hr integration would ensure rms noise of 0.02 mJy/beam at 15 GHz.
this correspond to # 4# detection of 2MASSW J0036+18. It would also allow us to analyze the dataset for
flare behaviour over this timescale, leading to better constraints for the flare modelling. Probabilistically,
we have a reasonable expectation of flare event incidents at 8 GHz ­ do we see a similar correlation at 15
GHz? Can we definitely confirm Berger's original assertion that brown dwarf emission 'peaks' at # 8 GHz,
being apparently self­absorbed at # 4 GHz (Berger et al. 2001). Recent observations of flare activity from
DENIS 1048­3956 at 8 AND 4 GHz (Putmann & Burgasser, 2003) contradict this latter position. Bearing
in mind the still considerable uncertainty associated with our understanding of this bizarre radio emission
phenomenon, and the paucity of existing data, we believe the case for such deep observations at 15 GHz is
compelling. We note that the definitive detection of this dwarf at 15 GHz would imply the possibility of
subsequent VLBI observations at a higher resolution.
Abada­Simon et al. 1997, A&A, 321, 841
3

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
Pestalozzi, M.R. et al., 2000, A&A, 353, 569
Putmann, M.E. & Burgasser, A.J., 2003, AAS, 203, 4307
Rutledge, R.E., et al. 2000, ApJ 538, L141
4