IDENTIFICATION OF H₂CCC AS A DIFFUSE INTERSTELLAR BAND CARRIER

Abstract

We present strong evidence that the broad, diffuse interstellar bands (DIBs)\nat 4881 and 5450\\,\\AA are caused by the\n$B\\,^1$B$_1$\\,$\\leftarrow$\\,$X\\,^1$A$_1$ transition of H$_2$CCC (l-C$_3$H$_2$).\nThe large widths of the bands are due to the short lifetime of the $B\\,^1$B$_1$\nelectronic state. The bands are predicted from absorption measurements in a\nneon matrix and observed by cavity ring-down in the gas phase and show exact\nmatches to the profiles and wavelengths of the two broad DIBs. The strength of\nthe 5450\\,\\AA DIB leads to a l-C$_3$H$_2$ column density of\n$\\sim5\\times10^{14}$ cm$^{-2}$ towards HD\\,183143 and\n$\\sim2\\times10^{14}$\\,cm$^{-2}$ to HD\\,206267. Despite similar values of\n$E$($B-V$), the 4881 and 5450\\,\\AA DIBs in HD\\,204827 are less than one third\ntheir strength in HD\\,183143, while the column density of interstellar C$_3$ is\nunusually high for HD\\,204827 but undetectable for HD\\,183143. This can be\nunderstood if C$_3$ has been depleted by hydrogenation to species such as\nl-C$_3$H$_2$ towards HD\\,183143. There are also three rotationally resolved\nsets of triplets of l-C$_3$H$_2$ in the 6150$-$6330\\,\\AA region. Simulations,\nbased on the derived spectroscopic constants and convolved with the expected\ninstrumental and interstellar line broadening, show credible coincidences with\nsharp, weak DIBs for the two observable sets of triplets. The region of the\nthird set is too obscured by the $\\alpha$-band of telluric O$_2$.\n