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Astron. Astrophys. 320, 525-539 (1997)

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7. Conclusions and summary

In our study of the coronal properties of young stars we show that the X-ray spectra can be well fitted with a continuous emission measure distribution assuming a power-law temperature dependence. We argue that this model might describe the coronal plasma in a more physical way than the frequently used 2T model. Although the fitting results alone do not allow to decide which model better describes the coronal temperature structure, our assessment is supported by recent extreme ultraviolet and X-ray observations with higher spectral resolution. In spectral fitting simulations we show that a fit with a 2T model can significantly underestimate the maximum coronal temperature if there is a continuous temperature distribution in the emitting plasma. We also show that possible abundance variations are not expected to strongly adulterate the plasma temperatures inferred from spectral fits to PSPC spectra, compared to the intrinsic uncertainties of the fitted temperatures.

While we can find no correlation between the coronal temperature and basic stellar parameters such as spectral type, mass, radius or surface gravity, a clear dependence on the X-ray activity is found. Our data show a very good correlation according to [FORMULA], extending over several orders of magnitude. This means that the high level of magnetic activity found for many young stars is determined not simply by a larger filling factor but also by the presence of higher temperature plasma. Since the very young TTS as well as the much older main-sequence stars in our sample follow the same relation, we conclude that the coronal temperatures are not directly related to stellar age. In accordance to other studies we find the X-ray activity to be determined by stellar rotation. This means that the high coronal temperatures of young stars are ultimately caused by their fast rotation.

Assuming that the coronae are composed by equal X-ray emitting loops that are consistently described by the RTV loop model and have lengths not exceeding the pressure scale height, we find that the dominant X-ray emitting structures in the coronae of the young stars must be high-pressure loops that cover only a rather small fraction of the stellar surface. Such loops are quite different from solar active region loops and might be related to the very strong magnetic activity of the young stars. Alternatively, the high pressures might also be taken as an indication of microflaring activity.

Although our results depend on the validity of our, necessarily quite simplifying, model assumptions, they are in good agreement with recent results of high resolution EUV and X-ray spectroscopy. Nevertheless, it is worth checking how strongly the results depend on the use of the CED model. If one would assume a 2T model to be actually more realistic and use [FORMULA] instead of [FORMULA], the basic results would change only slightly: One would also find a relation between [FORMULA] and [FORMULA], however with a decreasing slope for [FORMULA]. In the loop model analysis one would find pressures up to one order of magnitude lower for the objects with the highest temperatures. However, these pressures would still be significantly higher than in typical solar active region loops. Our basic conclusion would thus remain unchanged.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998