Astron. Astrophys. 351, L23-L26 (1999)

Letter to the Editor

The effects of opacity in the transition region of YZ CMi ^*

M. Mathioudakis ¹, J. McKenny ¹, F.P. Keenan ¹, D.R. Williams ¹ and K.J.H. Phillips <sup>2

1</sup> The Queen's University of Belfast, Department of Pure and Applied Physics, Belfast, BT7 1NN, Northern Ireland, UK (M.Mathioudakis@qub.ac.uk)
² Space Science Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon. OX11 0QX, UK

Received 13 September 1999 / Accepted 19 October 1999

Abstract

It has generally been assumed that the emission from the upper atmosphere of late-type stars is optically thin. In the present paper we use the Si IV and C IV resonance lines to investigate this assumption for the active dMe star YZ CMi. The significant deviations of the line ratios from their optically thin values, demonstrate that opacity can be quite important particularly in the case of stellar flares. These deviations are combined with a method of escape probabilities to derive optical depths of approximately unity for the lines under consideration. We demonstrate that, if the electron density in the atmosphere is known, opacity can provide important information on the linear dimensions of the scattering layer. Using this technique, we have estimated path lengths of a few kilometers for one of the flares under consideration.

Key words: atomic data stars: activity stars: atmospheres stars: individual: YZ CMi stars: late-type ultraviolet: stars

* Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.

Send offprint requests to: M. Mathioudakis

© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999

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