Monitoring and forward modeling OH sky emission lines in high resolution spectra
Abstract
We present the first results for a new project to monitor the hydroxyl radical OH sky emission lines arising from the Earth's mesosphere with high resolution infrared and optical spectra from the Habitable-zone Planet Finder (Mahadevan et al. 2012, Proc. SPIE, 8446, 84461S), NEID (Schwab et al. 2016, Proc SPIE, 9908, 99087H), IGRINS (Park et al. 2014, Proc. SPIE, 9147, 91471D), and APOGEE (Wilson et al. 2010, Proc. SPIE, 7735, 77351C) which provide an untapped and ever growing number of high quality sky spectra. Chemical reactions form OH in excited rovibrational levels with non-LTE level populations. The OH radiatively decays to the ground rovibrational levels giving rise to a rich spectrum of emission lines that make up the majority of the sky emission in the infrared and present an unwanted source of contamination (Rousselot et al. 2000, A&A, 354, 1134; Meinel 1950, AJ, 111, 555). The OH lines are highly variable making them difficult to fit and remove from spectra, but recent progress has been made on this problem (e.g., Noll et al. 2014, A&A, 567, A25; Davies et al. 2007, MNRAS, 375, 1099). Their variability also presents an opportunity to probe the conditions of the Earth's upper atmosphere where the OH forms (e.g., Singh & Pallamraju 2017, AnGeo, 35, 227; Noll et al. 2015, ACPD, 14, 32979). Deep sky spectra such as those by Cosby & Slanger (2007, CaJPh, 85, 77) and Olivia et al. (2015, A&A, 581, A47) reveal the detailed non-LTE behavior of the OH rovibrational level populations. We forward model the OH emission line intensities based on these known trends in observed level populations and the theoretical transition probabilities from Brook et al. (2016, JQSRT, 168, 142). Our modeling varies the vibrational temperatures, rotational temperatures, and fraction of cold vs. hot OH over multiple iterations to find the best fit to the observed OH spectrum. By carefully measuring each spectrometer's dispersion profile, we can fit and subtract the OH lines in a noise free manner. We also monitor the variability of the OH emission over short and long time scales and compare our model fits to atmospheric conditions during observations.
- Publication:
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American Astronomical Society Meeting Abstracts #233
- Pub Date:
- January 2019
- Bibcode:
- 2019AAS...23324514K