Advances in manufacturing and metrology of silicon immersion gratings and spectroscopy of young, low-mass stars with IGRINS

Silicon immersion gratings will allow the Giant Magellan Telescope Near-IR Spectrograph (GMTNIRS) to achieve continuous coverage over the entire J, H, K, L and M photometric bands with resolution R~65,000, at J, H and K and R~80,000 at L and M. I describe the manufacturing process and metrology techniques for silicon immersion gratings. I overview updates to the production process and new grating metrology techniques that I have developed in order to successfully manufacture silicon immersion gratings for GMTNIRS. Most of the changes to our manufacturing process that I have contributed to, were required because of our need to produce gratings on silicon substrates larger than those we have used in the past. Gratings for J, H and K will be blazed at R3, while the L and M gratings will be blazed at R4 to achieve the desired resolution. The higher blaze angle of the L and M gratings requires that we use 150mm diameter substrates rather than the 100mm substrates that our standard process was built for. In order to accommodate the larger substrates my colleagues and I constructed a custom UV exposure system for contact printing of grating lines, and constructed fixtures for coating and etching of the larger substrates. Additionally, we implemented new process checks and metrology techniques to improve our overall grating yield for 100mm and 150mm gratings. These updates to our process have resulted in the successful production of a complete set of gratings for GMTNIRS. In addition to my work in the area of manufacturing silicon immersion gratings, I have completed two observation based projects using our existing instrument, the Immersion Grating Near-Infrared Spectrometer (IGRINS), which employs a silicon immersion grating as its primary dispersing element. The first of these projects uses IGRINS to provide updated membership information for young low-mass stars in relation to the TW Hya young moving group. Nearby young moving groups provide unique samples of similar age stars for testing the evolution of physical properties. Incomplete and/or incorrect group membership classifications reduce the usefulness of the group, which we assume to be coeval. With near-infrared spectra of two candidate members of the TW Hya Association, 2MASS J12354615-4115531 (TWA 46) and 2MASS J12371238-4021480 (TWA 47), we test their membership by adding radial velocity measurements to the literature. We find that 2MASS J12354615-4115531 is a close spectroscopic binary system with a center-of-mass radial velocity of -6.5±3.9 km s⁻¹. This radial velocity and a Gaia parallax produces a TWA membership probability of 41.9% using the Banyan ∑ tool for 2MASS J12354615-4115531. The spectrum of 2MASS J12371238-4021480 shows that it appears to be a single star with a radial velocity consistent with the TW Hya Association and a membership probability of 99.5%. The reduced probability of TWA 46 as a true member of TWA highlights the importance of high-resolution, near-infrared spectra in validating low-mass moving group members. The second astronomical observation based project in this document concerns veiling in spectra of young stars in the Taurus-Auriga star forming region. I present measurements of the H and K band veiling for 142 young stellar objects in the Taurus-Auriga star forming region using high-resolution spectra from IGRINS. In addition to providing measurements for r [subscript H] and r [subscript K] we produce low-resolution spectra of the excess emission across the H and K bands. We fit single-temperature blackbodies to the excess spectra of 42 members of our sample and measure near-IR excess temperatures ranging from 1200-2400K. We compare the luminosity of the excess continuum emission in Class II and Class III young stellar objects and find that a large number of Class III sources display a significant amount of excess flux in the near-infrared. We conclude that the mid-infrared SED slope and therefore young stellar object classification is a poor predictor of the amount of remaining inner-disk (<1AU) material. I find that 6 members of our sample contain a prominent feature in their H-band excess spectra of unknown origin that is definitively inconsistent with a single or multi-temperature blackbody.