[Paper Review] Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. IX. The giant LMC HII region complex N11

F. P. Israel, Th. de Graauw+8 more|arXiv (Cornell University)|Nov 20, 2002

Astrophysics and Star Formation Studies17 references20 citations

TL;DR

This study maps the N11 HII region complex in the Large Magellanic Cloud using CO(1-0), CO(2-1), and 13CO(1-0) transitions, identifying 29 molecular clouds in a ring structure around OB associations LH 9 and a northeastern ridge. The ring region exhibits extreme photon-dominated region (PDR) characteristics with a high CO-to-H2 conversion factor, indicating significant unobserved diffuse molecular gas not traced by CO, while the ridge shows minimal intercloud emission and lower processing, suggesting quiescent conditions.

ABSTRACT

We present maps and a catalogue containing the J=1-0 12CO parameters of 29 individual molecular clouds in the second-brightest LMC star formation complex, N11. In the southwestern part of N11, molecular clouds occur in a ring or shell surrounding the major OB star association LH9. In the northeastern part, a chain of molecular clouds delineates the rim of one of the so-called supergiant shells in the LMC. The well-defined clouds have dimensions close to those of the survey beam (diameters of 25 pc or less). Some of the clouds were also observed in J=2-1 12CO, and in the lower two transitions of 13CO. Clouds mapped with a twice higher angular resolution in J=2-1 12CO show substructure with dimensions once again comparable to those of the mapping beam. The few clouds for which we could model physical parameters have fairly warm (T(kin) = 60 - 150 K) and moderately dense (n(H2) = 3000 cm-3) gas. The northeastern chain of CO clouds, although lacking in diffuse intercloud emission, is characteristic of the more quiescent regions of the LMC and appears to have been subject to relatively little photo-processing. The clouds forming part of the southwestern shell or ring, however, are almost devoid of diffuse intercloud emission and also exhibit other characteristics of an extreme photon-dominated region (PDR).

Motivation & Objective

• To map the distribution and physical properties of molecular gas in the N11 HII region complex in the Large Magellanic Cloud.
• To investigate the nature of molecular gas in different regions of N11, particularly the ring around OB association LH 9 and the northeastern ridge.
• To determine the CO-to-H2 conversion factor and assess the presence of diffuse intercloud molecular gas.
• To compare the physical conditions and evolutionary state of molecular clouds in the ring (PDR-dominated) versus the ridge (quiescent) regions.
• To evaluate the role of UV radiation from OB associations in shaping molecular cloud structure and gas processing.

Proposed method

• Conducted a CO(1-0) survey of the N11 complex using the SEST telescope with 11 pc beam resolution.
• Obtained additional CO(2-1) and 13CO(1-0) line data for a subset of clouds to improve spatial resolution and assess excitation conditions.
• Used the virial theorem to estimate cloud masses from line widths and sizes, assuming self-gravity dominates.
• Applied CO luminosity to H2 mass conversion factors (X) to infer total molecular hydrogen masses, comparing observed and expected values.
• Analyzed spatial distribution and kinematics of CO emission to distinguish between dense clouds and diffuse intercloud gas.
• Compared CO luminosity sums from individual clouds with total complex emission to estimate missing diffuse CO emission.

Experimental results

Research questions

• RQ1 What is the spatial distribution and morphology of molecular gas in the N11 HII region complex, and how does it correlate with OB associations?
• RQ2 How do the physical conditions (temperature, density, mass) of individual molecular clouds differ between the ring and ridge regions of N11?
• RQ3 To what extent is diffuse molecular gas present between the detected CO clouds, and how does this vary between the ring and ridge?
• RQ4 Why is the CO-to-H2 conversion factor significantly higher in the ring region compared to individual clouds, and what does this imply about gas processing?
• RQ5 What is the total molecular hydrogen mass in N11, and how much remains undetected by CO emission due to PDR effects?

Key findings

• Twenty-nine individual molecular clouds were identified in the N11 complex, with most having diameters ≤25 pc and masses between 0.5×10⁴ and 7.5×10⁴ M⊙, with a mean of 2.5×10⁴ M⊙.
• The northeastern ridge shows a CO-to-H2 conversion factor only 25% higher than the Solar Neighborhood, indicating minimal photo-processing and quiescent conditions.
• The southwestern ring region exhibits a CO-to-H2 conversion factor 5±2 times higher than individual clouds, indicating extreme PDR conditions with significant unobserved diffuse molecular gas.
• The total molecular mass in the ridge region is estimated at ≥2.2×10⁵ M⊙ from CO data, but total H2 mass is ≈5.5×10⁵ M⊙, implying at least 3.3×10⁵ M⊙ of unobserved H2.
• In the ring, the total H2 mass is ≈16.5×10⁵ M⊙, with 3.2×10⁵ M⊙ accounted for by CO-emitting clouds, indicating at least 13.3×10⁵ M⊙ of unobserved H2, or more than twice the atomic hydrogen mass.
• Diffuse intercloud CO emission is minimal overall, but the ring region likely hosts ≥0.5×10⁴ K km s⁻¹ pc² of unobserved CO luminosity, consistent with a PDR environment rich in untraced molecular gas.