CO formation in the metal-rich ejecta of SN 1987A

Abstract

We present a model for the formation of CO in the expanding metal-rich ejecta of SN 1987A. Our model is motivated by the presence of 4.6 micron and 2.3 micron emission bands in the spectrum of the supernova, ascribed, respectively, to the fundamental and first vibrational overtone of CO. The observations show that the CO expansion velocities are lower than ~2000 km s^-1^, suggesting that the emission emanates from the inner, metal-rich, regions of the ejecta. In addition, we ascribe a 2.26 micron excess feature in the high-resolution IR spectra to the v = 2 - 0 overtone of CO^+^, and offer a possible identification of a 2.95 micron feature with the v = 2 - 0 transition of electronically excited CO^*^ (a ^3<SUP>pi</SUP>). The concurrence of the CO^+^ and CO^*^(a ^3<SUP>phi</SUP>) with the ground state of CO supports a scenario in which the formation, excitation and ionization of the CO molecule take place in a warm environment. If reaction rates used in this paper are even approximately correct, then the regions in which CO formation is taking place must be ionized to a substantially lower degree than regions which dominate the line emission of heavier metals. The infrared observations of SN 1987A represent the first evidence for the formation of CO in a warm (T &gt; 1500 K), hydrogen-deficient environment, under conditions unique in the interstellar medium. Furthermore, our identification of CO^*^ (a ^3<SUP>phi</SUP>) is the first assignment of this species in an astronomical object outside the solar system.