Disentangling the excitation conditions of the dense gas in M17 SW⋆
Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69,
2 I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
3 Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, 9747 AV Groningen, The Netherlands
Received: 30 April 2015
Accepted: 24 August 2015
Context. Stars are formed in dense molecular clouds. These dense clouds experience radiative feedback from UV photons, and X-ray from stars, embedded prestellar cores, young stellar objects (YSOs), and ultra compact H II regions. This radiative feedback affects the chemistry and thermodynamics of the gas.
Aims. We aim to probe the chemical and energetic conditions created by radiative feedback through observations of multiple CO, HCN, and HCO+ transitions. We measure the spatial distribution and excitation of the dense gas (n(H2) > 104 cm-3) in the core region of M17 SW and aim to investigate the influence of UV radiation fields.
Methods. We used the dual band receiver GREAT on board the SOFIA airborne telescope to obtain a 57 × 37 map of the J = 16 → 15, J = 12 → 11, and J = 11 → 10 transitions of 12CO in M17 SW. We compare these maps with corresponding APEX and IRAM 30 m telescope data for low- and mid-J CO, HCN, and HCO+ emission lines, including maps of the HCN J = 8 → 7 and HCO+J = 9 → 8 transitions. The excitation conditions of 12CO, HCO+, and HCN are estimated with a two-phase non-LTE radiative transfer model of the line spectral energy distributions (LSEDs) at four selected positions. The energy balance at these positions is also studied.
Results. We obtained extensive LSEDs for the CO, HCN, and HCO+ molecules toward M17 SW. These LSEDs can be fit simultaneously using the same density and temperature in the two-phase models and to the spectra of all three molecules over a ~12 square arc minute size region of M17 SW. Temperatures of up to 240 K are found toward the position of the peak emission of the 12CO J = 16 → 15 line. High densities of 106 cm-3 were found at the position of the peak HCN J = 8 → 7 emission.
Conclusions. We found HCO+/HCN line ratios larger than unity, which can be explained by a lower excitation temperature of the higher-J HCN lines. The LSED shape, particularly the high-J tail of the CO lines observed with SOFIA/GREAT, is distinctive for the underlying excitation conditions. The cloudlets associated with the cold component of the models are magnetically subcritical and supervirial at most of the selected positions. The warm cloudlets instead are all supercritical and also supervirial. The critical magnetic field criterion implies that the cold cloudlets at two positions are partially controlled by processes that create and dissipate internal motions. Supersonic but sub-Alfvénic velocities in the cold component at most selected positions indicate that internal motions are likely magnetohydrodynamic (MHD) waves. Magnetic pressure dominates thermal pressure in both gas components at all selected positions, assuming random orientation of the magnetic field. The magnetic pressure of a constant magnetic field throughout all the gas phases can support the total internal pressure of the cold components, but cannot support the internal pressure of the warm components. If the magnetic field scales as B ∝ n2/3, then the evolution of the cold cloudlets at two selected positions, and the warm cloudlets at all selected positions, is determined by ambipolar diffusion.
Key words: ISM: individual objects: M17 SW / ISM: molecules / photon-dominated region (PDR) / submillimeter: ISM / ISM: clouds
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© ESO, 2015