Scientific/Technical Abstract:

GREAT, our first-generation SOFIA dual-channel instrument will offer opportunities for observations in (up to) three different frequency windows:

• the lower band, roughly 1.4-1.9 THz, will cover the important atomic fine-structure lines of ionized nitrogen and carbon [KOSMA]

• the mid-frequency detector is centered on the cosmologically relevant 1-0 transition of deuterated molecular hydrogen (HD) at 2.6 THz and the rotational ground-state transition of OH( ²_3/2) [MPIfR]

• A high-frequency channel that targets at, e.g., the 63 µm transition of atomic oxygen [DLR]

The receivers will employ sensitive superconducting mixer elements, SIS tunnel junctions or hot electron bolometers. The intermediate frequency band of a few GHz width will be linked to an array of acousto-optical spectrometers (AOS), provided by KOSMA [ PI: R.Schieder, schieder@ph1.uni-koeln.de].

GREAT is designed to investigate a wide range of astronomical questions, which ask for highest spectral resolution. A few examples:

• The 158µm fine-structure transition of ionized carbon (CII) is the most important cooling line of the cold interstellar medium and therefore critical for its energy balance. KAO observations have demonstrated that the integrated emission provides a sensitive tracer of the star forming activity of a galaxy.On smaller scales, comparison with complementary observations of neutral atomic carbon [CI] and of carbon monoxide [CO] will constrain the physical conditions in the photon-dominated surface layers (PDRs) of molecular clouds.

• The 112µm rotational ground-state transition of the deuterated hydrogen molecule, HD, will allow the derivation of the abundance profile of deuterium across the galactic disk and nearby galaxies, thereby providing unique information on the chemical evolution and star formation history of these systems. The ultimate goal is to better confine the cosmological deuterium abundance, which according to models of Big Bang nucleosynthesis, critically constrains the baryon density of the Universe.

• A series of rotational transitions of excited carbon monoxide CO will be accessible (J >13) for high-resolution excitation studies of, e.g., the inner shells of circumstellar envelopes and the physics of PDRs and shock layers.

GREAT Performance Summary – Instrument sensitivity and resolution summary is provided to permit estimating feasibility of scientific investigations.

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All sensitivity and resolution data are preliminary, and based on anticipated performance of the observatory and the instrument.  Actual performance of the SOFIA telescope and instrument combination will be established after flight operations begin.  Telescope performance is expected to be upgraded during the first two years, and instrument performance may be upgraded, or additional modes or capabilities may be added.

Performance estimates given here are based on data supplied by the instrument team. Further details and updates may be obtained from the instrument team via the contact information given above.

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