Mid-Infrared Observations of the Orion Bar

MIRSI instrument team:  PI L. Deutsch, Co-I J. Hora, J. Adams, M. Kassis

MIRSI website:  http://www.cfa.harvard.edu/mirsi

In high mass star forming regions like the Orion Nebula, a photodissociation region (PDR) represents the neutral interface between the optically visible HII region and a cold, dark molecular cloud. PDRs exhibit significant infrared emission because the ionizing radiation heats classical dust grains and excites Polycyclic Aromatic Hydrocarbons (PAHs). Both dust grains and PAHs re-emit the absorbed energy at infrared wavelengths

We have used our new MIRSI (Mid-InfraRed Spectrometer and Imager; see below, also Deutsch et.al. 2002, SPIE, 4841-13) system on the IRTF to observe the PDR in the Orion Bar.   By comparing our observations to theoretical models (Bakes et.al. 2001, Ap.J. 560, 261), we can constrain properties of the environment and composition in the Orion Bar PDR such as ultraviolet photon flux, gas and electron density, and gas temperature.  These properties will vary with depth into the PDR.

Figure 1 is a 4 x 5 MIRSI mosaic of the Orion Nebula at 20.9 µm exhibiting the most prominent features of the region: the Bar, Trapezium, and the BN/KL region.  The display in the mosaic is set to show the faint diffuse emission near Trapezium and the Bar.  Figure 2 shows a blow-up of the BN/KL region as imaged by MIRSI.  (BN/KL is the very bright region in the upper right of Figure 1.)

Figure 1.                                                                                   Figure 2.

Because of the Bar's favorable edge-on geometry, MIRSI grism spectra trace the relative strengths of  ionized gas and PAH features as a function of depth into the Bar (Figure 3). With the MIRSI array, we obtain spatial information along one dimension (y-axis) and spectral information along the other dimension (x-axis).  For this spectrum, the slit was oriented along the Dec direction and was centered at the position 100 in RA and -150 in Dec (see Figure 1).  The prominent spectral features are labeled at the bottom of the spectrum. The sharp transition between emission from ionized gas ([Ar III] at 8.99 µm) and PAHs (at 8.6 and 11.2 µm demonstrates that PAH emission peaks near the edge of the HII region.

Figure 3.

The MIRSI Instrument

MIRSI is a mid-infrared spectrometer and imager ( Deutsch et al 2002; ) utilizing a Raytheon 320x240 Si:As high-background IBC array.  MIRSI offers a large field of view (85 arcsec x 64 arcsec on the IRTF), diffraction-limited spatial resolution, complete spectral coverage over the 8-14 µm  and 17-26 µm atmospheric windows for both imaging and spectroscopy, and high sensitivity. The pixel scale at the IRTF is 0.27 arcsec.  In imaging mode, MIRSI offers broad N- and Q-band filters and discrete filters at 4.9, 7.8, 8.7, 9.8, 11.7, 12.28, 12.3, 18.4, 20.9, and 24.5 µm, as well as a CVF operating from 7.9 to 14.5 µm.  In spectroscopic mode, MIRSI offers a grism operating from 8-14 µm at a resolution of 200 and a grism covering 17-26 µm with resolution of 100.  Slit widths of 0.6 and 1.2 arcsec are available. 

MIRSI is available for collaborative programs at the IRTF.