Identification of H${}_2$CCC as a diffuse interstellar band carrier

Abstract

We present strong evidence that the broad, diffuse interstellar bands (DIBs) at 4881 and 5450\,Åare caused by the $B{}^1$B${}_1$\,$\leftarrow$\,$X{}^1$A${}_1$ transition of H${}_2$CCC (l-C${}_3$H${}_2$). The large widths of the bands are due to the short lifetime of the $B{}^1$B${}_1$ electronic state. The bands are predicted from absorption measurements in a neon matrix and observed by cavity ring-down in the gas phase and show exact matches to the profiles and wavelengths of the two broad DIBs. The strength of the 5450\,ÅDIB leads to a l-C${}_3$H${}_2$ column density of $\sim 5\times 10^{14}$ cm${}^{-2}$ towards HD\,183143 and $\sim 2\times 10^{14}$\,cm${}^{-2}$ to HD\,206267. Despite similar values of $E$($B-V$), the 4881 and 5450\,ÅDIBs in HD\,204827 are less than one third their strength in HD\,183143, while the column density of interstellar C${}_3$ is unusually high for HD\,204827 but undetectable for HD\,183143. This can be understood if C${}_3$ has been depleted by hydrogenation to species such as l-C${}_3$H${}_2$ towards HD\,183143. There are also three rotationally resolved sets of triplets of l-C${}_3$H${}_2$ in the 6150$-$6330\,Åregion. Simulations, based on the derived spectroscopic constants and convolved with the expected instrumental and interstellar line broadening, show credible coincidences with sharp, weak DIBs for the two observable sets of triplets. The region of the third set is too obscured by the $\alpha$-band of telluric O${}_2$.

22 pages, 9 figures

References in corpus (2)

• A Catalog of Diffuse Interstellar Bands in the Spectrum of HD 204827
• Radio Search for H2CCC toward HD 183143 as a Candidate for a Diffuse Interstellar Band Carrier

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