SOFIA Instrument Science Case

The outline of this page follows the outline of the four major science themes in the SOFIA Science Vision.

The Formation of Stars and Planets

What physical, chemical, and dynamical processes result in the formation of stars and planets?

High-Mass Star Formation

The star formation rate in galaxies is traced by massive stars. Massive stars dominate the energy budget of young galaxies. Understanding galaxy evolution will require a better understanding of massive star formation, as we can study the process in details within our own galaxy. Massive starts are also responsible for the creation and distribution of elements in the galaxy and are therefore ultimately responsible for the chemical building blocks necessary for the origin of planets and life. Despite their overall importance, the formation of massive stars is poorly understood.

Probing the Interiors of Massive Star Forming Cores
What are are the chemistry and dynamics of core interiors?

Far-infrared lines constrain models of dense cores; sub-mm data from ground can be fit equally well by many models. High spectral resolving power (R>10^5) of far-infrared rotational lines are needed.
  • high-J transitions of CO, HCN, HNC, and HCO+
  • cooling lines: CO and NII
Technical Questions:
  • How large are these dense cores? Given beamwidth of 20" at 1.4 THz and typical pixel-to-pixel spacing of two beamwidths, will multiple pixels increase efficiency?
    • At the galactic center the beam subtends 0.56 pc.
  • What velocity range (in km/s) and resolution is needed?
  • Any advantage in observing multiple lines simultaneously, i.e. one per sideband?

Low-Mass Star Formation

In the coldest densest clouds, the only molecule not frozen out onto dust grains is H2D+. The ground state transition of para-H2D+ is at 1370.08530 GHz. The ground state transition for ortho-H2D+ is at 373 GHz but it is notoriously hard to see. (There is yet another at 1.1 THz, weaker yet because it arises from a level 200K above ground). From purely excitation considerations, the 1.37 THz line can be order(s) of magnitude brighter than the 373 GHz line. The 1.37 GHz line is not covered by Herschel HIFI. This makes SOFIA the ONLY probe of completely depleted gas and an excellent probe of kinematics of very cold low mass cores.
  • It might be good to do a simple illustration for H2D+
  • One should also investigate the strengths of the simple hydride lines in low mass stellar cores.

Technical Questions:
  • What is the typical size (in arcsec) of these cold dense clouds?

The Interstellar Medium of the Milky Way

The ISM plays a central role in the evolution of galaxies as the birthsite of new stars and the repository of old stellar ejecta. The main cooling lines of the neutral ISM are spread across the far-IR peak. SOFIA's high resolution spectroscopy can resolve the narrow features of ISM dust and kinematics of the ISM gas.

Photodissociation Regions (PDRs)

The intense radiation from massive stars dominates the energetics and chemistry of its surrounding molecular clouds creating HII and photodissociation regions (PDRs).

Main cooling lines can be mapped with high velocity resolution
  • NII (1461 GHz)
  • CO (1265, 1380, 1495 GHz)

Identification of PAHs (polycyclic aromatic hydrocarbons)???

With a very sensitive (300K) SIS receiver, could we detect emission of higher-frequency modes to uniquely identify individual PAHs? This would be a follow-up to FIFI-LS observations at mid-IR with medium spectral resolution.

HIFI tried this, can't be done


Searching for metal hydrides (e.g. FeH, MgH, NiH, CaH, etc.). HIFI perhaps not looking at the right source. Look at more extreme environments, such as supernova explosions. Typical linewidths 20-30 km/sec and extents up to an arcminute.
  • para-NH3 (2-1) multiplet: 1214.85 and 1215.24 GHz
  • HF: 1232 GHz
  • H2O: 1296 GHz (predicted maser)
  • LiH (3-2): 1329 GHz
  • p-H2D+ (1-0): 1370 GHz
  • o-D2H+ (1-0): 1477 GHz
  • SH: 1381 GHz

Galaxies and the Galactic Center

Galactic Center

  • Spatial mapping of high-J CO lines will allow one to identify the presence and extent of any XDRs (X-ray dominated regions) in Galactic Center clouds. (For example, the CO16-15 line is about three times stronger in XDRs than in PDRs). Compared to GREAT, more pixels and lower receiver noise temperature will allow one to map much larger areas in the GC in the same amount of time.

External Galaxies at z=0.3-0.6

SOFIA is uniquely poised to detect CII (158 um, 1.899 THz) from galaxies in the redshift range between 0.3 and 1.1 (this receiver would cover z between 0.3-0.6), from the time of the peak in star formation activity in the Universe to nearly the present day, thereby constraining the strength and spatial extent of the starbursts.

Planetary Science

Not enough spatial resolution to do thermal/wind sounding, but can study chemistry.
  • HCl on Venus at 1251 GHz (5 times stronger than at 625 GHz)
    • ClOx cycle believed to play crucial role in photochemistry of Venusian atmosphere
  • Methane (best line: 1256 GHz) and 15-NH3 on Gas Giants
    • Stratospheric abundance of methane on Uranus and Neptune poorly known
  • High-resolution spectroscopy to identify heavy-C hydrocarbons and nitriles (e.g. HCO and HCN) on Titan
  • Comets
    • mapping of the extended coma (water isotopes, CH, etc.)
      • could use tens of m/s velocity resolution here
  • Even with 20" beam, can do limited coarse wind mapping of Jupiter and Venus


Atmospheric Transparency from SOFIA (12.5 km)

Spectral Lines Covered

  • para-NH3 (2-1) multiplet: 1214.85 and 1215.24 GHz
  • HF: 1232 GHz (<50% transmission at 12.5 km)
  • H2O: 1296 GHz (predicted maser)
  • LiH (3-2): 1329 GHz
  • H2D+ (1-0): 1370 GHz
  • SH: 1381 GHz
  • FeH (9/2-7/2) doublet: 1411.09 and 1411.35 GHz (blocked by atmosphere at 12.5 km)
  • CO (11-10): 1265 GHz
  • CO (12-11): 1380 GHz
  • CO (13-12): 1495 GHz
  • NII: 1461 GHz
  • HCN (14-13): 1240 GHz
  • HCN (15-14): 1328 GHz
  • HCN (16-15): 1417 GHz
  • HNC (14-13): 1286 GHz
  • HNC (15-14): 1359 GHz
  • HNC (16-15): 1449 GHz
  • HCO+ (14-13): 1248 GHz
  • HCO+ (15-14): 1337 GHz
  • HCO+ (16-15): 1426 GHz

Angular Diameter of Solar System Objects (SOFIA beamwidth is 20" at 1.4 THz)

  • Venus: 10-60"
  • Mars: 5-25"
  • Jupiter: 45"
  • Saturn: 18"
  • Uranus: 3.5"
  • Neptune: 2"
  • Io: 1"
  • Titan: 0.8"

GREAT Science Case

Topic revision: r15 - 2011-07-15, EricBryerton
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