We present the detection of neutral atomic carbon CI ( ^ { 3 } P _ { 1 } - ^ { 3 } P _ { 0 } ) line emission towards omi Cet .
This is the first time that CI is detected in the envelope around an oxygen-rich M-type asymptotic giant branch ( AGB ) star .
We also confirm the previously tentative CI detection around V Hya , a carbon-rich AGB star .
As one of the main photodissociation products of parent species in the circumstellar envelope ( CSE ) around evolved stars , CI can be used to trace sources of ultraviolet ( UV ) radiation in CSEs .
The observed flux density towards omi Cet can be reproduced by a shell with a peak atomic fractional abundance of 2.4 \times 10 ^ { -5 } predicted based on a simple chemical model where CO is dissociated by the interstellar radiation field .
However , the CI emission is shifted by \sim 4 km s ^ { -1 } from the stellar velocity .
Based on this velocity shift , we suggest that the detected CI emission towards omi Cet potentially arises from a compact region near its hot binary companion .
The velocity shift could , therefore , be the result of the orbital velocity of the binary companion around omi Cet .
In this case , the CI column density is estimated to be 1.1 \times 10 ^ { 19 } cm ^ { -2 } .
This would imply that strong UV radiation from the companion and/or accretion of matter between two stars is most likely the origin of the CI enhancement .
However , this hypothesis can be confirmed by high-angular resolution observations .