MEDIUM — Molecular Exploration of the Diffuse Interstellar mediUM

Project Abstract (AGBT21B_316)

MEDIUM is a GBT Large Project to conduct a comprehensive, unbiased molecular absorption survey of diffuse clouds occulting the distant, centimeter-bright Galactic H II region W49N, with the overarching goals of greatly enlarging the molecular inventory of the diffuse interstellar medium and understanding its molecular complexity. Recent observations have yielded detection of C4H, the largest carbon chain radical and acetaldehyde (CH3CHO), the most complex polar molecule observed in diffuse clouds, suggesting that detection of molecules of considerable size and complexity is feasible there. The survey will be so sensitive that it will permit detection of molecular absorption features with optical depths <<0.01 corresponding to column densities that are nearly two orders of magnitude lower than measured to date. The GBT is the only instrument in the world capable of reaching the required level of sensitivity at the spectral resolution required to detect sharp absorption features and characterize the velocity distribution of molecular absorption lines toward W49N. These goals will be achieved through a coordinated campaign of observations, laboratory studies, and complementary chemical modeling, with a strong emphasis on providing the community with science-ready data products in the form of a fully-reduced spectral line survey.

Team Members

Harshal Gupta (PI) (NSF)

Michael McCarthy (CFA)

Kin Long Kelvin Lee (MIT)

Ci Xue (UVA)

Mark Siebert (UVA)

Anthony Remijan (NRAO)

David Frayer (GBO)

Romane Le Gal (CFA)

Maryvonne Gerin (Obs de Paris)

Brett McGuire (MIT)

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