A Laboratory log(gf) Measurement of the Ti II 15873.84 {\AA} H-band Line   in Support of SDSS-III APOGEE
  The SDSS-III APOGEE collaboration has identified a single useable line in the H-band spectra of APOGEE target stars arising from a singly ionized species. This line of Ti II (${\lambda}_{air}$ = 15873.84 {\AA}) is therefore of great importance for use in stellar surface gravity, or log(g), determinations via the Saha equation. While a theoretical estimate of the line strength exists, to date no laboratory measurement of the line strength has been reported. Herein we report an absolute laboratory transition probability measurement for this important Ti II line. A relative line strength measurement is made of the Ti II H-band line of interest and a reference line with a previously reported absolute transition probability. This ratio is measured using multiple spectra of a high-current water-cooled HC lamp recorded with a calibrated FT-IR spectrometer. 
Michael P. Wood, James E. Lawler, Matthew D. Shetrone
astro-ph.SR astro-ph.IM physics.atom-ph