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Apr 1
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Tim Abbott
Cerro Tololo Inter-American Observatory, CTIO-NOAO
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(host: Edward Robinson) "NOAO, CTIO, and the Blanco 4-m telescope, v2.0"
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Apr 8
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Joan Schmelz
University of Memphis, and the NSF
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(hosts: J. Craig Wheeler and Shardha Jogee) "Unconscious Bias in Hiring, Promotions, and Tenure"
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Apr 29
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Michael Gully-Santiago
University of Texas at Austin
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PhD Defense Talk: "Innovative Technologies for - and Observational Studies of - Star and Planet Formation"
The lowest mass objects in nearby star forming regions now accessible to spectroscopy are in the sub-brown dwarf regime, with estimated masses comparable to the masses of extrasolar planets and bordering the threshold of opacity limited fragmentation. We confirm and characterize, using R=2000 near-infrared spectroscopy, 17 candidate young substellar objects in nearby star forming regions. Using deep optical, near-infrared, and mid-infrared photometry, we have searched an off-core region of the nearby ~1 Myr Ophiuchus star forming cluster for candidate young stars and brown dwarfs. Multi-object I-band spectroscopy of 419 candidates reveals 12 new young stars and brown dwarfs. Ten of these have no evidence for mid-IR emission from 3.6 to 8.0 micron. The other two are indicative of edge-on disks, based on their mid-infrared slopes rising with wavelength. The disk fraction for spectral types M4 and later towards this region of Ophiuchus is 33%, lower than the typical disk fraction of 1-3 Myr clusters.
I also summarize the optical design, fabrication, and performance of silicon diffractive optics for astronomical spectrographs. The optical devices include diffraction-limited, high-throughput silicon grisms for JWST-NIRCam. These grisms served as pathfinders to Silicon immersion gratings, which offer size and cost savings for high-resolution near-infrared spectrographs. I demonstrate the production and optical evaluation of the immersion grating that enabled IGRINS at the McDonald Observatory. This grating provides spectral resolution R=40,000 over the H- and K- near-infrared band atmospheric windows (1.5-2.5 micron). Electron-beam lithography offers much higher precision over contact mask photolithography for the production of Si immersion gratings. Using electron-beam lithography, I designed, fabricated, and tested diffraction-limited Si immersion gratings over a 30 x 100 mm beam. These prototypes are stepping stones to monolithic Si gratings for iSHELL and GMTNIRS. The monolithic design of Si immersion gratings presents a limitation for scaling up the grating size, since existing fabrication equipment cannot handle monolithic silicon pucks. The size limitation can be overcome by direct-bonding Si substrates to optical prisms. I demonstrate a technique to measure interfacial gaps as small as 14 nm between the bonding interface. Gaps this small produce 0.2% transmission loss. These technologies will enable the direct measurement of the atmospheric properties of extrasolar planets in the next decade.
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