Acetylene bands observed in carbon stars - A laboratory study and an illustrative example of its application to IRC plus 10216

Abstract

Acetylene (C<SUB>2</SUB> H<SUB>2</SUB>) has been identified as a constituent of the atmospheres of cool carbon stars based on Fourier transform stellar spectra in the near-infrared. To interpret these astrophysical data on a quantitative basis, a systematic laboratory study of the absorption spectrum of acetylene occurring in this spectral region has been made at a resolution of ∼0.03 cm<SUP>-1</SUP>, both at room temperature and at an elevated temperature of 160° C. Interpretation of the acetylene lines in the 3 μm region has led to the identification of 27 vibration-rotation bands which have been assigned to transitions between 41 different vibrational states of <SUP>12</SUP>C<SUB>2</SUB>H<SUB>2</SUB> and <SUP>12</SUP>C<SUP>13 </SUP>CH<SUB>2</SUB> and absolute strengths were measured for 101 selected unblended spectral lines distributed among some of the stronger bands including the 0010<SUP>0</SUP>0<SUP>0</SUP>0<SUP>0</SUP> band of <SUP>12</SUP>C<SUP>13</SUP>CH<SUB>2</SUB>. In addition, the integrated strengths of the Q-branches of the 1000<SUP>0</SUP>1<SUP>1</SUP>-0000<SUP>0</SUP>0<SUP>0</SUP> band at 4091 cm<SUP>-1</SUP> and the 0100<SUP>0</SUP>1<SUP>1</SUP>-0000<SUP>0</SUP>0<SUP>0</SUP> band at 2701 cm<SUP>-1</SUP> of <SUP>12</SUP>C<SUB>2</SUB>H<SUB>2</SUB> were measured, and by comparing with the stellar data for IRC+ 10216, an estimate has been made for the acetylene column density in the circumstellar shell of this carbon star.