Infrared spectroscopy of the longest-period Galactic Mira variables from Palomar Gattini-IR: Identification of a candidate thermally pulsing Super-Asymptotic Giant Branch star

Abstract

The population of extremely long-period Mira variables in the Milky Way remains poorly characterized. Their rarity and heavy circumstellar obscuration have precluded systematic studies in optical surveys, yet they are expected to trace the most massive stars evolving through the Asymptotic Giant Branch (AGB). We present IRTF/SpeX spectroscopy of the longest-period ($P\gtrsim1000$ d) Galactic Mira variables identified with the Palomar Gattini-IR survey. Twelve of thirteen targets exhibit characteristics consistent with massive O-rich AGB stars. In contrast, one peculiar source IRAS 19552+3142 exhibits i) a large-amplitude ($ΔJ\gtrsim2$ mag) pulsations with $P\approx1060$ d, ii) strong C$_2$ and CO molecular bands indicating C/O$\gtrsim1$, and iii) SiC emission at $\approx11.3\,μ$m in archival mid-infrared spectra. Its spectroscopically confirmed chemistry makes it the longest period C-rich Galactic Mira variable known; together with the rarity of similar objects, this source is difficult to reconcile with standard massive AGB evolution where hot-bottom burning maintains an O-rich surface composition. Instead, its properties may be explained by super-AGB evolution where dredge-out episodes produce a C-rich surface and circumstellar material. Its high luminosity ($M_{[4.5]}\lesssim-10$ mag), red color ($[3.6] - [4.5]\gtrsim2.0$ mag), and high mass-loss rate ($\gtrsim 10^{-4} \mathrm{M_\odot yr^{-1}}$) resemble extreme AGB star progenitors of intermediate-luminosity red transients in nearby galaxies and a known chemically peculiar super-AGB star candidate in the Small Magellanic Cloud. IRAS 19552+3142 may thus provide a rare look at a Galactic super-AGB star in the thermally pulsing phase. Our results motivate improved theoretical predictions and long-baseline infrared searches for these short-lived, dust-enshrouded endpoints of stellar evolution.