A Laboratory log(gf) Measurement of the Ti II 15873.84 {\AA} H-band Line in Support of SDSS-III APOGEE

TL;DR

This paper reports the first laboratory measurement of the transition probability for the Ti II 15873.84 Å line in the H-band, crucial for stellar gravity analysis in APOGEE spectra.

Contribution

It provides an absolute laboratory transition probability for the Ti II line, enhancing the accuracy of stellar surface gravity measurements.

Findings

First laboratory measurement of Ti II 15873.84 Å line strength.

Improved data for stellar surface gravity determination.

Enhanced calibration for APOGEE H-band spectra.

Abstract

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.