Poster Presentation
ALMA imaging of trans-cis conformers of formic acid in Orion KL
Presenter: Ya-Wen Yo (National Taiwan Normal University)
The trans-conformer of formic acid has long been observed in interstellar space. However, the cis-HCOOH, a wobbly conformer of formic acid, was not expected to exist in the interstellar medium due to its very high trans-to-cis conversion energy barrier. Recently cis-HCOOH was detected in the Orion Bar PDR with the IRAM 30m and a FUV photoswitching mechanism for trans-to-cis isomerization was proposed, nevertheless, no cis-HCOOH was detected toward Orion KL (Cuadrado et al. 2016). Thanks to the mighty ALMA with her very high angular resolution and superb sensitivity, we are able to detect an isolated, clean line of cis-HCOOH in Orion KL unambiguously the first time; additionally we also detect four trans-HCOOH lines in Orion KL. The spectral emission of both conformers of formic acid is located in a region near the continuum sources SMA 1, HKKH 8 and HKKH 9, with the cis-HCOOH concentrated mainly on HKKH 9, and the trans-HCOOH, on SMA 1. Weaker, diffuse emission of trans-HCOOH encircling the region between the Compact Ridge and Southern Peak is also apparent which is consistent with previous BIMA and SMA lower angular-resolution results. Our ALMA finding of the existence of cis-HCOOH in the optically thick Orion KL hot molecular core presents an immediate challenge to the FUV photoswitching scenario thus may require a new trans-to-cis conversion mechanism.
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