Astronomy departmental seminar - Mingjie Jian (SU)

The behavior of the chromospheric lines of helium at 10830Å in late-type stars
The stellar helium abundance is an essential parameter which is expected to provide important clues on the chemical evolution of our Milky Way. It is believed to be enhanced in the second (and later) generation(s) in globular clusters and one of the double red clump, though the latter is still under intense debate. However, it is hard to determine the helium abundance of a late-type star spectroscopically. Previous studies show that the chromospheric helium line at 10830 Å (He 10830) has the potential to be used as a helium abundance indicator, but observations of this line have been insufficient. We collected a large sample of medium- and high-resolution spectra covering the wavelength range of He 10830 for over 700 stars. Systemic trends between line strength and other parameters are found, and the photoionization-recombination formation mechanism dominates in the dwarfs and warm giants. Thus He 10830 is difficult to use as a helium abundance tracer for these stars. The cool giants, on the other hand, are expected to have weak or no corona, thus still have the possibility to trace helium abundance using He 10830.