Introduction to MIRSI Science
MIRSI is a mid-infrared camera system with both spectroscopic and imaging
capabilities which will penetrate the dusty environments around very
young and very old stars in order to study the process of stellar
evolution. Circumstellar envelopes are found around stars at
different evolutionary phases, and material is continuously exchanged
between these envelopes and their ambient mediums through mass
accretion and mass outflow. The properties of the gas and dust of
which this material is comprised are altered as the material is
cycled. Since new stars will ultimately form from the material
ejected by evolved stars, the processing of gas and dust through
stellar evolution has important consequences for the stellar
populations of galaxies. Our understanding of stellar evolution is
limited in part by our incomplete knowledge of the physical conditions
in circumstellar envelopes and the relation of the envelopes to their
ambient mediums. In order to study these environments, we must
observe at the infrared wavelengths at which these dusty regions emit
most of their luminosity. In addition, since dust absorbs most direct
stellar radiation at shorter wavelengths and re-emits it at longer
infrared wavelengths, the embedded luminosity sources which power
these regions can only be studied indirectly at longer wavelengths.
There are a host of diagnostic spectral features of both dust and gas
which appear in the infrared.
(Hoffmann et al. 1993, Proc. SPIE, 1946, 449)
12-image mosaic at 20.6 microns of the Orion Nebula, taken at IRTF. The white
box represents MIRSI's large field of view. MIRSI will be suited to study galactic
star-forming regions such as these using both imaging and spectroscopy in
the 10 and 20 micron atmospheric windows.
Equipped with wide-field imaging and low resolution spectroscopic modes,
MIRSI is uniquely suited to study:
- Star Formation - MIRSI will be able to probe
the circumstellar dust in collapsing molecular cloud cores to test models
of the star formation process. MIRSI will determine the role of small dust grains
and large molecules, such as polycyclic aromatic hydrocarbons (PAHs), and place
constraints on the extent of the dust envelope. In addition, MIRSI's wide field
of view will enable quick mapping of entire molecular cloud complexes at high sensitivity.
The high resolution will enable spatial separation of individual sources, and thereby allow
comparison with radio and sub-mm, in order to develop accurate spectral energy distributions
for each source. MIRSI can quickly obtain low resolution spectra to identify major
spectral features, such as silicate absorption and ionization lines.
High sensitivity permits the detection of low-level diffuse
emission and faint sources. MIRSI can efficiently map photodissociation regions in HII
regions thereby providing detailed study of the cooling processes at work.
The ability to observe through relatively high amounts of
obscuration permits the use of molecular hydrogen emission at 12.28 microns to trace,
over a wide field, shocks in circumstellar shells of embedded objects.
- Planetary Nebulae - During the last stages of their
evolution, low mass stars lose mass in the form of cool, dusty wind. The
central star becomes hot enough to ionize the wind, forming a planetary nebula.
MIRSI's high sensitivity, large field of view, and imaging and spectroscopic capabilities
will provide constraints on the spatial and chemical properties of planetary nebulae.
- Starburst Galaxies - MIRSI will study hydrocarbon emission and
silicate absorption in starburst galaxies. The combination of spectroscopy and imaging
will detect spectrally-distinct spatial regions and the composition and processes at work
in these galaxies.
- Extrasolar Systems - Solar systems are thought to form from cool
dust disks around young stars. MIRSI's sensitivity at 20 microns wavelength
will allow it to detect these disks and study their spectral properties in detail.
- Solar System Bodies - Mid-IR is a key wavelength region with
which to study solar system objects. Mid-IR imaging and spectra can probe Jupiter's atmospheric
properties using CH4 (7.8 microns), H2 (13.0 microns), and NH3 (10.74 and 8.57 microns, gas
and ice). MIRSI can identify mid-IR spectral properties across planetary surfaces to study,
for example, mineral emission. MIRSI can also be used to study cometary nuclei.
MIRSI instrument description and specifications
Infrared Astronomy Lab at the BU IAR
MIRAC: A Mid-Infrared Array Camera
BU Institute for Astrophysical Research