Crystalline Si nanoparticles as carriers of the blue luminescence in the red rectangle nebula

The discovery of a band of blue luminescence in the Red Rectangle proto-planetary nebula has been recently reported by Vijh et al. These authors used the hydrogen Balmer lines and the line-depth technique to extract the intensity of the blue luminescence. This luminescence was attributed to fluorescence from small neutral polycyclic aromatic hydrocarbon molecules in the interstellar medium, consisting of three to four aromatic rings such as anthracene and pyrene. We present in this Letter evidence of another potential carrier responsible for the blue luminescence, namely, ultrasmall silicon nanoparticles of 1 nm in diameter.