Mid-infrared PAH and H2 emission as a probe of physical conditions in extreme PDRs

Abstract

Mid-infrared (IR) observations of polycyclic aromatic hydrocarbons (PAHs) and molecular hydrogen emission are a potentially powerful tool to derive physical properties of dense environments irradiated by intense UV fields. We present new, spatially resolved, \emph{Spitzer} mid-IR spectroscopy of the high UV-field and dense photodissocation region (PDR) around Monoceros R2, the closest ultracompact \hII region, revealing the spatial structure of ionized gas, PAHs and H${}_2$ emissions. Using a PDR model and PAH emission feature fitting algorithm, we build a comprehensive picture of the physical conditions prevailing in the region. We show that the combination of the measurement of PAH ionization fraction and of the ratio between the H${}_2$ 0-0 S(3) and S(2) line intensities, respectively at 9.7 and 12.3 $\mu$m, allows to derive the fundamental parameters driving the PDR: temperature, density and UV radiation field when they fall in the ranges $T=250-1500$K, $n_H=10^4-10^6$cm${}^{-3}$, $G_0=10^3-10^5$ respectively. These mid-IR spectral tracers thus provide a tool to probe the similar but unresolved UV-illuminated surface of protoplanetary disks or the nuclei of starburst galaxies.

Accepted for publication in ApJ Letters

References in corpus (9)

• Variations of the Mid-IR Aromatic Features Inside and Among Galaxies
• Spectral Mapping Reconstruction of Extended Sources
• Spitzer/IRAC and ISOCAM/CVF insights on the origin of the Near to Mid-IR Galactic diffuse emission
• Formation of simple organic molecules in inner T Tauri disks
• The penetration of FUV radiation into molecular clouds
• Aromatic emission from the ionised mane of the Horsehead nebula
• On the chemistry and distribution of HOC+ in M82: More evidence for extensive PDRs
• Carriers of the mid-IR emission bands in PNe reanalysed. Evidence of a link between circumstellar and interstellar aromatic dust
• What can we learn about protoplanetary disks from analysis of mid-infrared carbonaceous dust emission?

Cited by in corpus (27)

• Evaporating Very Small Grains as tracers of the UV radiation field in Photo-dissociation Regions
• The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR
• Dynamics of cluster-forming hub-filament systems: The case of the high-mass star-forming complex Monoceros R2
• Waves on the surface of the Orion molecular cloud
• Spectral line survey of the ultracompact HII region Mon R2
• Spitzer Mapping of PAHs and H2 in Photodissociation Regions
• PHANGS-JWST First Results: Variations in PAH Fraction as a Function of ISM Phase and Metallicity
• Probing the role of polycyclic aromatic hydrocarbons in the photoelectric heating within photodissociation regions
• Hydrogenation of PAH cations: a first step towards H2 formation
• The Mystery of Unidentified Infrared Emission Bands
• Spatial distribution of small hydrocarbons in the neighborhood of the Ultra Compact HII region Monoceros R2
• Deuteration around the ultracompact HII region Mon R2
• Polycyclic Aromatic Hydrocarbon emission in Spitzer/IRS maps I: Catalog and simple diagnostics
• Herschel / HIFI observations of CO, H2O and NH3 in Mon R2
• Observational studies on the Near-Infrared Unidentified Emission Bands in Galactic HII regions
• Multiple shells around G79.29+0.46 revealed from near-IR to millimeter data
• Kinematics of the ionized-to-neutral interfaces in Monoceros R2
• Isentropic thermal instability in atomic surface layers of photodissociation regions
• The first CO+ image: Probing the HI/H2 layer around the ultracompact HII region Mon R2
• First science results from SOFIA/FORCAST: The mid-infrared view of the compact HII region W3A
• Herschel observations in the ultracompact HII region Mon R2: Water in dense Photon-dominated regions (PDRs)
• The Submillimeter Polarization Spectrum of M17
• Spitzer Observations of Dust Emission from HII Regions in the Large Magellanic Cloud
• JWST Observations of Starbursts: Relations between PAH features and CO clouds in the starburst galaxy M 82
• A Principal Component Analysis of polycyclic aromatic hydrocarbon emission in NGC 7023
• High-resolution images of two wiggling stellar jets, MHO 1502 and MHO 2147, obtained with GSAOI+GeMS
• Diverse stages of star formation in the IRAS 18162-2048 region. Emergence of UV Feedback