Is the Effective Temperature of ROB 162 Really 51,000 K?

The UV-bright star ROB 162 in the globular cluster NGC 6397 is a post-AGB star with a mass between 0.55 and 0.6 M⊙. Model fits to its optical spectrum yield an effective temperature Teff = 51,000 ± 2,000 K. A recent analysis of the UV-bright star Y453 in M4 (NGC 6121) found that, while its optical spectrum yields a similar temperature, Teff ~ 56,000 K, its FUV spectrum reveals the star to be much hotter, with Teff ~ 72,000 K. Could the temperature of ROB 162 be similarly underestimated? To find out, we analyze archival FUSE and STIS spectra of the star. By fitting the absorption features of multiple ionization stages of nitrogen and oxygen, we derive an effective temperature of ~ 48,000 K, consistent with the optically-derived value. Why do the optical and FUV spectra yield consistent values for ROB 162, but not for Y453? In a word, metallicity. The metallicy of M4 ([Fe/H] = -1.16) is nearly an order of magnitude greater than that of NGC 6397 ([Fe/H] = -2.02). Furthermore, Y453 is overabundant (relative to M4) in the iron-peak elements Ti, Cr, and Ni. These metal enhancements alter the structure of the photosphere, and thus the shape of the Balmer lines, leading us to underestimate the effective temperature when fitting those lines with H-He stellar-atmosphere models. The lower metallicity of ROB 162 makes it less susceptible to these effects.