AI II as an electron density diagnostic for the upper chromosphere/lower transition region in late-type stars.

Abstract

The ratio of the Al II resonance line 3s2 1S-3sp 1P1 at 1670 A to the inter-system line 3s2 1S-3s3p 3P1 at 2669 A is a useful electron density diagnostic for N(e) greater than 10 exp 10/cu cm. However, it is also sensitive to the adopted electron temperature, and, in fact, should be a useful electron temperature diagnostic when Ne is not greater than 10 exp 9/cu cm. In particular, we present observational data for the RS CVn star II Peg and show that this ratio implies an electron pressure substantially less than that implied from other diagnostic line ratios but in good agreement with the pressure derived from the C III 1176/C III 1908 ratio. We suggest additional observational and theoretical work that may be required.