We extend and generalize to stellar X-ray flares the analysis of the decay phase of flares inside solar coronal loops of constant cross-section, and investigate related diagnostics from X-ray observations. The work is based on hydrodynamic simulations of compact stellar flares for different values of stellar surface gravity, loop length and peak temperature, and characteristics of loop heating during the decay phase. We discuss the validity of the thermodynamic scaling law of Serio et al. (1991) in the wider context of stellar flares, and find that a generalized version of this law holds if the loop half-length L is smaller than the peak pressure scale height h. In the light of the diagnostics which can be derived from the density-temperature (n-T) diagram, already tested for solar flares, we consider how the ratio ζ of the density decay time to the temperature decay time depends on gravity. We find that ζ, as computed at the top of the loop, has a maximum value ζ ∼ 3 for very small L/h and decreases as L/h increases. Alternatively we consider the average flare density and temperature as they would be derived from observations with the ROSAT PSPC or other moderate energy resolution detectors, we obtain a flatter curve, with a maximum value around 2. We discuss also how energy deposition during the flare decay phase influences ζ, and propose a combined diagnostic approach on stellar flares based on the joint use of the n-T diagrams and of the fitting of the light curve with hydrodynamic models. As a first practical result we can safely exclude the presence of sustained heating during the decay of an X-ray flare observed on Prox Cen by the EINSTEIN/IPC instrument.
Astronomy and Astrophysics
- Pub Date:
- May 1993
- stars: atmospheres;
- stars: coronae stars: flare X-rays: stars