A preliminary investigation of the diffuse interstellar line at 8621 Å

We have obtained high-resolution spectra at the Dominion Astrophysical Observatory, the Calar Alto Observatory, and the European Southern Observatory of hot stars in the near-infrared region to study the diffuse interstellar band (DIB) at 8621 angstrom. Field stars as well as selected members of the Perseus spiral arm, the rho Ophiuchi complex, and the Cygnus OB2 association were observed and the equivalent widths of the band measured, as well as interstellar K I absorption at 7699 angstrom. In total we measure the equivalent width of the 8621 band in 64 stars. In a series of figures we show the correlations of the DIB with reddening, polarization, K I, and strengths of other DIBs at 5780, 5797, and 6613 angstrom. The quality of the correlations are discussed by means of the Spearman rank correlation test. Good correlations are found with reddening and, among the other DIBs, 8621 correlates best with 5780. By examining the three special regions mentioned above we confirm the weakness of the 8621 DIB in the hot stars of the rho Oph star-forming region. In the Perseus spiral arm region we find that the DIBs are formed largely in foreground clouds but not in the gas that appears to have been blown away from the young massive stars of the Perseus arm. In Cyg OB2 we find that the equivalent widths of the 8621 line lie above the linear correlation of equivalent width with and their strength does not vary as strongly with E( B - V) reddening. The heavily reddened Cyg OB2 star 12 is the most extreme example of the relative insensitivity of the 8621 equivalent width to reddening in the Cygnus OB2 region.