|Abstract ||By virtue of a wide range of critical densities, CO rotational lines are powerful diagnostic tools to probe the physical conditions of molecular gas in diverse environments. |
In the past several years, Herschel spectroscopic observations have been combined with ground-based data
to extensively examine CO Spectral Line Energy Distributions (CO SLEDs) of various Galactic and extragalactic sources, revealing the ubiquitous presence of warm molecular gas (> 100 K).
Several heating sources, e.g., UV photons, X-rays, and cosmic-rays, have been invoked to explain the properties of this warm molecular gas,
and the emerging picture is that non-ionizing sources, e.g., mechanical heating, must play a critical role.
In this talk, I will present a high-resolution (10 pc scales) study on the physical properties and excitation mechanisms of warm molecular gas in 30 Doradus (Lee et al., submitted).
With more than 1000 hot luminous stars, 30 Doradus is the most extreme starburst in the Local Universe,
providing an excellent laboratory for probing the impact of radiative and mechanical feedback into the surrounding ISM.
CO observations obtained with the Herschel SPIRE FTS and ground-based telescopes (from J=1-0 to J=13-12)
were analyzed with the non-LTE radiative transfer code "RADEX", showing that the CO-emitting gas in 30 Doradus is very warm (~400 K) and moderately dense (1e4 cm-3).
To probe the origin of this warm molecular gas, we then compared the CO observations to state-of-the-art PDR and shock models ("Meudon PDR" and "Paris-Durham" shock codes).
We first constrained the properties of PDRs by modeling Herschel observations of [OI] 145 micron, [CII] 158 micron, and [CI] 370 micron fine-structure lines
and found that the constrained PDRs emit very weak CO emission.
X-rays and cosmic-rays were also found to provide a negligible contribution to CO emission, essentially ruling out ionizing sources as the dominant heating source for CO.
On the other hand, mechanical heating by low-velocity C-type shocks (10 km/s) was sufficient to reproduce the observed warm CO across the entire 30 Doradus.
Possible shock drivers are currently under investigation and will be discussed in this talk.