What can be observed in the mid-infrared: lines and emissions

What can be observed in the infrared?

Line emission

Dust emission lines

One of the principal scientific drivers for observations in the thermal infrared is the presence of a series of scientifically important dust-emission lines, mainly concentrated in the 10-micron window, although interesting features also exist in the 20 and 30 micron windows. Some of the most interesting are:

Wavelength (microns)	Line
6.0	H₂O ice
6.2	PAHs
7.7	PAHs
8.6	PAHs
7 - 11	Silicates
11	Graphite
11.3	PAHs

Both PAHs and silicates are found in a wide variety of astronomical objects, ranging from AGNs, through proto-stellar and proto-planetary disks, to solar system objects, such as comets. Lack of appropriate instrumentation has limited their study so far to the brightest objects in the sky. We would expect CanariCam to make a huge contribution in the study of these objects on the Gran Telescopio Canarias, due to the enormous gain in light grasp and sensitivity that the Gran Telescopio Canarias will offer over previously existing telescopes.

Characterisation of silicate dust from CanariCam observations

Two types of silicate emission are seen in different astrophysical situations:

A broad, smooth 10-micron feature with little structure, always observed in protostellar regions in molecular clouds. This line is attributed to newly formed amorphous, or glassy silicates. The main source is thought to be outflows from O-rich M-stars.

A feature even broader than the smooth silicate emission, with a narrow peak at 11.2 microns. The peak is attributed to crystalline silicates. These may condense out of protostellar material, or be formed by the metamorphosis of amorphous silicates if these are subjected to high temperatures over a long period of time. This process, known as annealing, imbues the grains with a memory of the thermal history of the cloud. Spectroscopy at different points within a protoplanetary disk can provide valuable information on the thermal evolution of the material.

Recent thermal infrared studies of comets, such as Hale-Bopp, have revealed that several minerals can be distinguished, with the recent identification of forsterite (Mg₂SiO₄) being one of the highlights. In the Herbig Ae/Be star HD 100546, the 10-micron spectrum is remarkably similar to that of Comet Hale-Bopp (C/1995 O1). When the range is extended to the full 8-30 microns permitted by the atmosphere, several minerals may be identified: crystalline forsterite, giving narrow emission bands at around 11 microns; amorphous olivines, which give the broad features at 10 and 18 microns; and a broad feature at 23 microns, which is interpreted as due to FeO. Hence spectroscopy in both the 10 and the 20-micron window are required to characterise the mineralogy.

Emission from PAHs

Narrow-band imaging of PAH features is of considerable interest. PAHs are a significant constituent of the interstellar medium and emit as the result of the absorption of a single UV photon. The relative strengths of the different PAH features is a tracer of their molecular size. As heating of the dust is not involved PAH features are emitted independent of the local temperature or the distance from the illuminating source. PAHs thus serve as dust tracers even in cold, regions which do not produce significant thermal emission. PAHs are often thought to be very small grains and are mixed-in with other dust. Their unique emitting characteristics allows them to emit even when "normal" dust grains are at too low a temperature to emit. An excellent example of this capacity to serve as a tracer is seen in the case of the Herbig Ae/Be star WL16. Deutsch et al (1995, Ap. & Spa. Sci., 224, 89) have shown how an extended disk may be seen when the star is imaged with a 8.6 microns PAH filter, but not when a 10.3m m continuum filter is used.

Atomic and molecular transitions

There are many important lines in the mid-infrared. In the following table we list some of the main atomic transitions that occur in the infrared and are accessible from the ground:

Wavelength (microns)	Line	Transition	Photon Energy
7.9	Ar V		60 eV
8.8	K IV		83 eV
8.99	Ar III	^{3 P₁- 3P₂}	28 eV
9.0	Na IV		72 eV
9.0	Mg V		109 eV
10.51	S IV	^{2 P}°_3/2 -^2P °_1/2	35 eV
12.28	H₂	n =0-0 S(2)
12.37	H I	n=7 -6 (Hu-a )
12.81	Ne II	^{2 P}°_1/2 -^2P °_3/2	22 eV
13.1	Ar V		60 eV
13.5	Mg VII		187 eV
17.03	H₂	n =0-0 S(1)
18.71	S III	^{3 P₂- 3P₁}	23 eV
24.28	Ne V	^{3 P₁- 3P₀}	97 eV
25.87	O IV	^{2 P_3/2- 2P_1/2}

These lines are also important probes of heavily obscured regions and can be used to study star formation regions, or the central regions of AGNs, obtaining physical parameters that cannot be obtained from visible spectroscopy. A new field of research is the study of interstellar molecules in the mid-infrared. Many molecules have been detected with radio techniques, but little effort has be directed to the possibility of studying molecules in the mid-infrared where many of them emit. The combination of highly sensitive instruments and very large telescopes promises to open a whole new field of astrochemistry.

Continuum emission
The 3-25 micron range is often referred to as the Thermal Infrared because warm objects (including the Earth's atmosphere) radiate principally at these wavelengths. The peak of black body emission for many classes of warm object (including extra-solar planets and dust in galaxies) falls in this range. One particular interest of CanariCam is that its broad wavelength coverage allows objects at a wide range of different temperatures, ranging from brown dwarfs to cool dust to be studied. Many of these objects are too cool to give significant emission in the near infrared (<2.5 microns) and thus cannot be adequately studied with instruments limited to the 1-2.5 microns range.

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