Fabry-Perot Imaging of H2 1-0 S(1), 2-1 S(1), and Brackett-gamma Emission in the Orion Nebula
Appeared in Astrophysical Journal, 464, 818.
Authors:
Tomonori USUDA,
Hajime SUGAI, Hironobu KAWABATA, Motoko Y. INOUE, Hirokazu KATAZA, & Masuo TANAKA
Abstract:
We obtained spectral maps of the central 6'x8' (0.9pc x 1.2pc) region of the Orion Nebula.
H2 v=1-0 S(1), 2-1 S(1) (S(1) means J=3-1), and Br-gamma maps were obtained with the wide field Fabry-Perot imager at the Nasmyth focus of the Communications Research Laboratory 1.5m telescope, FINAC.
In the Bright Bar, a typical photodissociation region (PDR), the H2 2-1 S(1)/1-0 S(1) line ratio (= R2-1/1-0 is between 0.2 and 0.6, and has a tendency to be anti-correlated to the intensity of H2 1-0 S(1) (= I1-0 S(1)).
From a comparison of the observed R2-1/1-0 vs. I1-0 S(1) relation with theoretical models, we conclude that the H2 thermal component in this region is due not to shock heating caused by the expansion of the HII region, but to the collisional de-excitation of the lower vibrational levels populated by cascade after UV pumping.
Our results strongly suggest that the Bright Bar region is PDR consisting of a medium density (104-105 cm-3) cloud and a small number of denser (106 cm-3) clumps whose size is smaller than our spatial resolution (8''=0.02pc).
We also find that the ``South region'' and ``East region'' of the Orion Nebula can be described as PDRs similar to the Bright Bar region.
In the Orion KL region, a typical shocked gas of a bipolar outflow source, we find that R2-1/1-0 is independent of I1-0 S(1).
R2-1/1-0 is 0.08 near the central area of KL, the curved bridge-like structure, and the finger-like filaments of the Orion KL region (``finger region'').
On the other hand, R2-1/1-0 0.05 at the edge of the central area.
The difference in the ratio suggests that high-velocity shocks exist at the center of KL, while low-velocity shocks surround the central area.
The Orion S region contains some thermal H2 clumps which may be shock-excited.
The distribution of these shocked-H2 clumps, CO J=2-1 clumps, and FIR sources suggests the existence of at least two sets of bipolar outflows associated with YSOs in this area.
In a global view of the Orion Nebula, the compact distribution of Br-gamma relative to H2 1-0 S(1) indicates the existence of a PDR surrounding the HII region.
Moreover, the intensity ratio of H2 1-0 S(1) from PDRs (R2-1/1-0>0.15; i.e., the PDR including dense molecular clumps) to Br-gamma (=R1-0(PDR)/Br-gamma is 0.17.
By comparing this value for Orion to those for starburst galaxies (cf. R1-0/Br-gamma =0.4-0.9)), we conclude that the typical starburst region has larger H2-emitting areas surrounding HII regions and/or a later average spectral type for exciting stars than the Orion Nebula, if the H2 emission from starburst galaxies is predominately fluorescent.