GALEX J201337.6+092801: The lowest gravity subdwarf B pulsator

TL;DR

This paper reports the discovery of a unique low-gravity subdwarf B pulsator with unusually long periods, expanding understanding of pulsation modes in such stars and identifying a rare subclass above the extreme horizontal branch.

Contribution

It presents the first detailed spectroscopic and photometric analysis of a low-gravity sdB pulsator, revealing its unique properties and its classification as a rare high-amplitude pulsator.

Findings

Identified a low-gravity sdB star with long-period pulsations.

Found only one similar star with comparable parameters, V338 Ser.

Classified the star as part of a rare subclass above the extreme horizontal branch.

Abstract

We present the recent discovery of a new subdwarf B variable (sdBV), with an exceptionally low surface gravity. Our spectroscopy of J20136+0928 places it at Teff = 32100 +/- 500, log(g) = 5.15 +/- 0.10, and log(He/H) = -2.8 +/- 0.1. With a magnitude of B = 12.0, it is the second brightest V361 Hya star ever found. Photometry from three different observatories reveals a temporal spectrum with eleven clearly detected periods in the range 376 to 566 s, and at least five more close to our detection limit. These periods are unusually long for the V361 Hya class of short-period sdBV pulsators, but not unreasonable for p- and g-modes close to the radial fundamental, given its low surface gravity. Of the ~50 short period sdB pulsators known to date, only a single one has been found to have comparable spectroscopic parameters to J20136+0928. This is the enigmatic high-amplitude pulsator V338 Ser, and we conclude that J20136+0928 is the second example of this rare subclass of sdB pulsators located well above the canonical extreme horizontal branch in the HR diagram.