The presence of extreme horizontal branch ( EHB ) and blue hook stars in some Galactic globular clusters ( GGCs ) constitutes one of the remaining mysteries of stellar evolution .
While several evolutionary scenarios have been proposed to explain the characteristics of this peculiar population of evolved stars , their observational verification has been limited by the availability of spectroscopic data for a statistically significant sample of such objects in any single GGC .
We recently launched the SHOTGLAS project with the aim of providing a comprehensive picture of this intriguing stellar population in terms of spectroscopic properties for all readily accessible GGCs hosting an EHB .

In this first paper , we focus on $ ω $ Cen , a peculiar , massive GGC that hosts multiple stellar populations .
We use non-LTE model atmospheres to derive atmospheric parameters ( T _ { eff } , log g and N ( He ) / N ( H ) ) and spectroscopic masses for 152 EHB stars in the cluster .
This constitutes the largest spectroscopic sample of EHB stars ever analyzed in a GGC and represents \approx 20 % of the EHB population of $ ω $ Cen .
We also search for close binaries among these stars based on radial velocity variations .

Our results show that the EHB population of $ ω $ Cen is divided into three spectroscopic groups that are very distinct in the T _ { eff } - helium abundance plane .
The majority of our sample consists of sdOB stars that have roughly solar or super-solar atmospheric helium abundances .
It is these objects that constitute the blue hook at V > 18.5 mag in the $ ω $ Cen color-magnitude diagram .
Interestingly , the helium-enriched sdOBs do not have a significant counterpart population in the Galactic field , indicating that their formation is dependent on the particular environment found in $ ω $ Cen and other select GGCs .

Another major difference between the EHB stars in $ ω $ Cen and the field is the fraction of close binaries .
From our radial velocity survey we identify two binary candidates , however no orbital solutions could be determined .
We estimate an EHB close binary fraction of \approx 5 % in $ ω $ Cen .
This low fraction is in line with findings for other GGCs , but in sharp contrast to the situation in the field , where around 50 % of the sdB stars reside in close binaries .

Finally , the mass distribution derived is very similar for all three spectroscopic groups , however the average mass ( 0.38 M _ { \odot } ) is lower than that expected from stellar evolution theory .
While this mass conundrum has previously been noted for EHB stars in $ ω $ Cen , it so far appears to be unique to that cluster .