Not all that glitters is massive: Impostors in the search for massive hot subdwarfs

Hot subdwarfs are core- or helium-shell-burning stars with little or no hydrogen envelope that are formed through binary-star interactions. They are a treasure chest of astrophysical phenomena, such as strong magnetic fields, diffusion processes, and extreme enrichments of trans-iron elements. They are also found in type Ia supernova progenitor systems and are believed to be a major source of the ultraviolet excess observed in elliptical galaxies. Furthermore, hot subdwarfs are powerful laboratories for studying close binary evolution and serve as useful background sources for investigations of the interstellar medium. 

Most hot subdwarfs are thought to originate from low-mass stars that have been stripped near the tip of the red giant branch. However, more massive hot subdwarfs can form when intermediate-mass stars lose their envelopes shortly after core hydrogen exhaustion. While evolutionary models predict a substantial population of these objects, only a few are currently known. They are particularly interesting because of their connections to type Ib/c supernovae, double neutron-star progenitors, and their potential contribution in the reionisation of He II in the early Universe.

In this talk, I will present results from our spectroscopic campaign targeting very blue and luminous candidates selected from the Gaia catalogue of hot subluminous stars. To our surprise, some of the candidates turned out to be impostors, low-mass hot subdwarfs whose photometric properties mimic those of more massive objects. Follow-up ultraviolet observations with STIS on board the Hubble Space Telescope revealed their true nature and demonstrated that such objects should be considered in future observing campaigns. 

Talk in presence and via Zoom:

https://uni-hamburg.zoom.us/j/66006535328?pwd=aGkrSjJIYmZjK0VpYlpGL0ZrdHg2UT09