The EC 14026 stars - XII. PG 1219+534, PG 0911+456: successes and new challenges for the Fe driving mechanism

We report the discovery of short-period oscillations in the sdB stars PG 0911+456 and PG 1219+534, establishing them as members of the EC 14026 stars, a recently discovered class of sdB pulsators. The atmospheric parameters for both stars are also determined. The results are: periods of (148.8, 143.7, 133.5, 128.1s), T-eff = 32800 +/- 300 K and log g = 5.76 +/- 0.04 for PG 1219+534; and periods of (165.7, 161.5, 155.8s); T-eff = 31900 +/- 200 K and log g = 5.82 +/- 0.02 for PG 0911+456. The pulsation periods of the 11 well-studied EC 14026 stars are compared with envelope models from the recent work of Charpinet et al. With the exception of PB8783, the longest period in each star is close to the periods of the lowest-order modes in the models. Arguments are given that this is also true for PB8783, involving an increase in its surface gravity over the value derived in an earlier paper. 'Correcting' the observed periods for the effects of differing surface gravity from star to star, comparison is made with pulsation models of constant surface gravity and varying effective temperature. Reasonable agreement in the width of the instability strip is demonstrated, as well as properties of its cool boundary. Overall, the class properties of the EC 14026 stars are in impressive agreement with those predicted from models based on the Fe driving mechanism. PG 1219+534 has an atmosphere much richer in He than most sdB stars and all EC 14026 pulsators seen so far. As the Fe driving mechanism operates if sufficient heavy elements accumulate at the appropriate layer in the stellar envelope, the mechanisms (gravitational settling, radiative levitation, weak stellar winds) that give rise to this must simultaneously be able to provide large differences in the abundance of photospheric He.