Observational constraints on the ERE interpretation ⋆

Empirical relationships on the properties of the Extended Red Emission (ERE) are presented. They are based on published observational data and on new results obtained on reflection nebulae illuminated by cold stars. The plot of the width versus the central wavelength of the ERE band is in agreement with laboratory properties of the materials commonly proposed as the ERE carriers. But this is not the case for the plot of the ERE band width versus the effective temperature of the nebula illuminating star. The Extended Red Emission (ERE) is a continuous emission band observed in the red part (6000–8000Å) of the spectrum of various astrophysical objects : reflection neb-The ERE band is found to vary significantly both in position and width from an object to an other. The diversity of objects where the ERE is detected and the diversity of the observed band characteristics lead us to search for empirical relationships that should help identification of the ERE carriers. For all the available observations, we have plotted in Fig. 1 the width as a function of the central wavelength of the ERE bands. For reflection nebulae, we have used the values of Witt & Boroson (1990). For all the other objects, Send offprint requests to: J.-P. Sivan ⋆ Partly based on observations made at Observatoire de Haute Provence du CNRS and at European Southern Observatory (ESO), La Silla (Chile) we have made measurements according to the same definitions as Witt & Boroson : the central wavelength splits the band luminosity into two equal parts and the width is measured as the difference between the wavelengths of the first and third quartiles. Note that for planetary nebulae the values derived from the published spectra of Furton & Witt (1992) are underestimated. Fig. 1. This diagram plots the position of the maximum (˚ A) versus the width (˚ A) of the ERE band for a number of objects of various types : H ii regions