12CO and 13CO J=3-2 observations toward N11 in the Large Magellanic Cloud

Abstract

After 30 Doradus, N11 is the second largest and brightest nebula in the LMC. This large nebula has several OB associations with bright nebulae at its surroundings. N11 was previously mapped at the lowest rotational transitions of $^{12}$CO (J=1--0 and 2--1), and in some particular regions pointings of the $^{13}$CO J=1--0 and 2--1 lines were also performed. Using ASTE we mapped the whole extension of the N11 nebula in the $^{12}$CO J=3--2 line, and three sub-regions in the $^{13}$CO J=3--2 line. The regions mapped in the $^{13}$CO J=3--2 were selected based on that they may be exposed to the radiation at different ways: a region lying over the nebula related to the OB association LH10 (N11B), another one that it is associated with the southern part of the nebula related to the OB association LH13 (N11D), and finally a farther area at the southwest without any embedded OB association (N11I). We found that the morphology of the molecular clouds lying in each region shows some signatures that could be explained by the expansion of the nebulae and the action of the radiation. Fragmentation generated in a molecular shell due to the expansion of the N11 nebula is suggested. The integrated line ratios $^{12}$CO/$^{13}$CO show evidences of selective photodissociation of the $^{13}$CO, and probably other mechanisms such as chemical fractionation. The CO contribution to the continuum at 870 $\mu$m was directly derived. The distribution of the integrated line ratios $^{12}$CO J=3--2/2--1 show hints of stellar feedback in N11B and N11D. The ratio between the virial and LTE mass (M$_{\rm vir}$/M$_{\rm LTE}$) is higher than unity in all analyzed molecular clumps, which suggests that the clumps are not gravitationally bounded and may be supported by external pressure. A non-LTE analysis suggests that we are mapping gas with densities about a few 10$^{3}$ cm$^{-3}$.