Radio observations explain certain kinds of stellar activity and reliable estimates of magnetic field strengths are derived for the radio emitting regions. Radio observation of flare stars shows rapidly varying bursts of radiation with frequencies of up to 5 GHz. The inferred brightness temperatures of these bursts (10 to the 10th power to 10 to the 14th power) imply that the radiation is coherent. Two mechanisms are proposed to generate such radiation: (1) plasma radiation, and (2) electron cyclotron masers. The high brightness temperatures of the bursts up to 10 to the 20th power K) makes the latter the most plausible. For bursts in the 5 GHz range, the magnetic field strength would be approximately 900 Gauss

Dulk, G. A.

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ACTIVITY AND MAGNETIC FIELDS ON STARS FROM RADIO OBSERVATIONS
George A. Dulk
DESPA, Observatoire de Paris, 92190 Meudon, France
and
Department of Astrophysical, Planetary and Atmospheric
Sciences, University of Colorado, Boulder, CO 80309, USA
t ' During the past few years, owing partly to the completion of the
VLA, interest in radio observations of stars has had a resurgence. New
Instruments have providee an increase in sensitivity by an order of
magnitude, accompanied by a considerably reduced risk of mistaking radio
1?	 interference for stellar radiation. Gibson1983(	 ) gives a summary of
observations of the class of stars most extensively studied to date,
i.e., flare stars of spectral type dMe. Here I will describe some of the
newest results on several classes of stars and comment on their
Interpretation in terms of stellar activity and stellar magnetic fields.
I will confine my attention to outbursts of radio radiation, omitting
from consideration the quiescent or steady emission that comes from some
stars such as RS CVn and AM Herculis type binaries, late type stars with
coronae such as x1 Orionis; this aspect is discussed by Brown and
Crane (1978), Gary and Linsky (1981), Topka and Marsh (1982) Chanmugam
and Dulk (1982) and Gibson (1983).
Figure 1, from Gary, Linsky and Dulk (1982), shows an example of an
outburst of radiation that has most of the characteristics of the five
examples I will show on viewgraphs. The top frame, in which each data
point represents a five minute integration, shows an approximately
steady component of emission from UV Ceti which could be due to a hot,
magnetically confined corona or to sustained, low level flaring
activity. A drastic, short-lived increase in radiation is seen in the
top panel in only one or two data points and in only one polarization.
The burst is expanded in the lower panel, in which 10 s integrations
were used, i.e., the shortest then available at the VLA. There are
factor-of.-two variations in flux from one 10 s interval to another, and
the radiation is X80% circularly polarized. Careful measurements of
source position revealed that the outburst arose not from UV Ceti itself
but from its distant binary companion, also a dMe star.
r .,.,y
(NASA-CB-175966) AC4IVIUY AND MAGNETIC	 N85-29861
FIELDS ON STABS 2EOM EADIO CESEEVATIONS
d	 (Colorado Univ.) 4 p HC A02,fM2 A01 CSCL 02B
Unclas
G3/90 24063
ORIGINAL PAGE M
OF POOR QUALITY
12
L726-6 AB
4.9 GHz
RH
1	 1	 i	 1
0400	 0600	 0800	 1000
Time (UT)
I6^- b'
	
L726-BA
	
I	 49 GHz
E 12	 V `^ ^^
K 8
	
Y
7
lJ.
LH
4-
0648	 0652	 0656	 0700	 0704
Time (UT)
Fig. 1. Radio emission at 4.9 GHz from W Ceti and its companion L726-
8A. Top: Fluxes in RE and LH circular polarization taken from VLA maps
with 5 min integration times. Bottom: Flare emission on an expanded time
scale, taken from maps with 10 s integration times. ( From Gary, Linsky
and Dulk 1982).
A burst with many of the same characteristics was observed at 1.4
GHz from the flare star AD Leo by Lang at al. ( 1983), but with a much
better time resolution. In that case there were two components to the
burst: ( 1) an erratically varying rise and fall, lasting about 30 min,
with little structure less than a few seconds duration, and with a
polarization	 degree 40.15; ( 2)	 a	 rapidly
-
varying	 portion, >90%
polarized, lasting about 3 min, and having time structure down to the
instrumental limit of 0.2 s. The first component had great similarities
to solar microwave bursts and was probably due to gyrosynchrotron
radiation by electrons of energy >100 keV spiraling in a field of B >
100 Gauss; we consider the interpretation of the second component later.
Bursts from the Sun with high polarization and fast time structure
have been reported by Drtige ( 1977) and Slottje (1978) at frequencies up
to 2.8 GHz. Termed "microwave spike I* bbrsis" they typically vary on Lime
scales > 10 ms, but occasionally exhibit spikes with rise times as
_.
	 v	
tlii`y. Stu ='itp" .^,
	
_d	 ,..
T 6
E
'4
0
1
u^
4
r
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.	 ^	 1
k.
`, I
f`r
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short as the 1 ms instrumental limit. From these fast rise times source
sizes < 300 km are inferred.
Highly polarized bursts, with many of the same characteristics and
with time scales down to instrumental limits of -1 min have been
reported for RS CVu stars (e.g. Brown and Crane, 1978).
f^ Finally, a rather similar outburst was reported at 5 GHz from the
magnetic cataclysmic variable star AM Herculis (Dulk, Bastian and
Chanmugam, 1983). The outburst probably arose from close to the red
dwarf secondary, whose or it lies within the magnetosphere of the
primary, a white dwarf. Because AM Her is -10 times more distant than
typical flare stars and the observed flux density was comparable, the
emitted radiation must have been exceptionally intense.
Turning now to the interpretation of the rapidly varying bursts of
radiation,	 it	 is	 important	 to	 note	 that	 the	 inferred	 brightness
temperatures	 are	 10 10
	K	 to	 10 14
	K	 which,	 with	 the	 high	 degree	 of
circular	 polarization,	 imply	 that	 the	 radiation	 is	 coherent.	 Two
mechanisms
	 have	 been	 proposed	 to	 generate	 such
	
radiation:	 (1)	 plasma
<<,- radiation	 and	 (2)	 electron-cyclotron	 masers.	 While	 plasma	 radiation
(e.g.	 Melrose,	 1980)	 accounts	 for	 many	 kinds	 of	 solar	 bursts	 at	 low
frequencies, 4 1 GRz,	 and
	
may	 produce	 stellar
	
bursts	 at	 similar
frequencies,	 it encounters severe difficulties in accounting for bursts
at higher frequencies. This is due to the fact that the overlying plasma
is likely to reabsorb the radiation very effectively, 	 either because of
free-free absorption if the temperature T is
	
<106 K , or gyroresonance
absorption if T	 >106 K. Hence the electron -cyclatron maser mechanism is
-' the most plausible so far proposed.
This	 maser	 (e.g.	 Melrose	 and	 Dulk,	 1982)	 operates	 at	 the
fundamental of the electron -cyclotron frequency
f = fB = 2 . 8 x 106B
(where	 f	 and	 fB	are	 in	 Hz	 and	 B	 in	 Gauss)	 or	 perhaps	 its	 second
harmonic f = 2f 	 .	 The	 maser	 converts	 electron	 energy	 directly	 into
(amplified)	 radio	 radiation	 which	 is	 highly	 polarized,	 and	 brightness
temperatures
	 to	 1020 K	 or	 higher	 can	 be	 attained	 under
	
reasonablea
C
conditions.	 Radiation	 at	 the	 fundamental f = f 	 is	 probably
gyroresonantly	 reabsorbed	 when	 travelling	 outward	 through
	
the	 stellar
plasma,	 so	 radiation	 at	 f = 2fB	is	 the	 more	 likely	 (note,	 however,
that	 certain	 bursts	 from	 Jupiter,	 Saturn	 and	 Earth	 are	 convincingly
1
'	 J
attributed to electron-cyclotron masers operating at the fundamental).
Assuming that this mechanism is the ralevant one, then the obser-
vation of bursts at frequency f implies that a field of strength B
f/5.6xl06 is present in the source region, e.g. 900 Gauss if f - 5 GHz.
Of course, higher and lower field strengths may also exist in the source
region, with masing at corresponding frequencies, but usually only one
frequency is observed at a time. 1	 .,
In summary, radio observations are now able to elucidate certain
kinds of stellar activity and reliable estimates of magnetic field
strengths are being derived for the radio emitting regions.
This work was sponsored in part by NASA under grants NSG-7287 and
NAGW-91 to the University of Colorado.
References
Brown, R.L., Crane, P.C.: 1978, Astron. J. 83, 1504.
Chanmugam, G., Dulk, G . A.: 1982, Astrophys. J. Letters 255, L107.
Droge, F.: 1977, Astron. Astrophys. 57, 285.
Dulk, G.A., Bastian, T.S., Chanmugam, G.: 1983, Astrophys. J. 273, 249.
Gary, D.E., Linsky, J.L.: 1981, Astrophys. J. 250, 284.
Gary, D.E., Linsky, J.L., Dulk, G.A.: 1982, Astrophys. J. Letters
263, L79.
Gibson, D.M.: 1983, in Activity in Red Dwarf Stars, eds. P. Byrne and
t.^
M. Rodono, ( Dordrecht: Reidel), p. 273.
Lang, K.R., Bookbinder, J., Golub, L., Davis, M.: 1983, Astrophys. J.
Letters 272, L15.
Melrose, D.S.: 1980, Space Sr,i. Rev. 26, 3.
Melrose, D.S., Dulk, G.A.: 1,82, Astrophys. J. 259, 844.
Slottje, C.: 1978, Nature 275, 520.
Topka, K., Marsh, R.A.: 1982, Astrophys. J. 254, 641.
i
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Activity and magnetic fields on stars from radio observations

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