EUVE Science Highlights -- November 9, 2000
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Flares as Coronal Heating Agents in Active Stars: The EUVE View 
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•  o We study the hypothesis that cool-star coronae are heated by 
     explosive events, i.e. flares.

•  o EUVE archival data of 12 late-type stars (early-F to late-M) 
     have been used to construct distributions of coronal flare 
     rates and establish their relations to activity indicators 
     and rotation parameters.

•  o Power laws have been found to describe flare rate distributions 
     in energy, similarly to power laws found for solar flares.

•  o Their power-law indices suggest that the statistical ensemble 
     of all flares can contribute significantly to the coronal 
     heating mechanism in active stars (the "microflare hypothesis").

•  o The flare occurrence rate has been found to be correlated 
     with activity indicators and rotation parameters. The power-
     law index, however, does not show such correlations.

(figure)

• Above, left: Energetic flares occur more often in x-ray bright 
  stars than in x-ray faint stars.

• Above, right: The normalized occurrence rate increases with 
  "activity" and stays constant for saturated (L_x/L_bol ~ 10-3) 
  stars. The flare rate "saturates" as well.

Notes:
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Contributed by Marc Audard, Paul Scherrer Institute (Switzerland). 

This Science Highlight is based on the published paper: Audard, M., Guedel, 
M., Drake, J.J., and Kashyap, V.L., "Extreme-Ultraviolet Flare Activity in 
Late-Type Stars", ApJ, 541, 396 (2000).

Extreme Ultraviolet Explorer Deep Survey observations of cool stars 
(spectral type F to M) have been used to investigate the 
distribution of coronal flare rates in energy and its relation to 
activity indicators and rotation parameters. Cumulative and 
differential flare rate distributions were constructed and fitted 
with different methods. Power laws are found to approximately 
describe the distributions. A trend toward flatter distributions 
for later-type stars is suggested in our sample.

Assuming that the power laws continue below the detection limit, 
we have estimated that the superposition of flares with radiated 
energies of about 10^29-10^31 ergs could explain the observed 
radiative power loss of these coronae, while the detected flares 
are contributing only ~10%.

While the power-law index is not correlated with rotation 
parameters (rotation period, projected rotational velocity, 
Rossby number) and only marginally with the x-ray luminosity, the 
flare occurrence rate is correlated with all of them. The 
occurrence rate of flares with energies larger than 10^32 ergs is 
found to be proportional to the average total stellar x-ray 
luminosity. Thus, energetic flares occur more often in x-ray 
bright stars than in x-ray faint stars. The normalized occurrence 
rate of flares with energies larger than 10^32 ergs increases with 
increasing L_x/L_bol and stays constant for saturated stars. A 
similar saturation is found below a critical Rossby number.

The findings are discussed in terms of simple statistical flare 
models in an attempt to explain the previously observed trend for 
higher average coronal temperatures in more active stars. It is 
concluded that flares can contribute a significant amount of 
energy to coronal heating in active stars.