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 09 11+456 and PG 1219+534, establishing them as members of the EC 14026 stars, a recently discovered class of sdB pulsators. The atmospheric parameters f or 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 a nd log g = 5.82 +/- 0.02 for PG 0911+456. The pulsation periods of the 11 well-studied EC 14026 stars are compared wi th envelope models from the recent work of Charpinet et al. With the except ion of PB8783, the longest period in each star is close to the periods of t he 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 valu e derived in an earlier paper. 'Correcting' the observed periods for the ef fects of differing surface gravity from star to star, comparison is made wi th pulsation models of constant surface gravity and varying effective tempe rature. Reasonable agreement in the width of the instability strip is demon strated, as well as properties of its cool boundary. Overall, the class pro perties of the EC 14026 stars are in impressive agreement with those predic ted 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 su fficient heavy elements accumulate at the appropriate layer in the stellar envelope, the mechanisms (gravitational settling, radiative levitation, wea k stellar winds) that give rise to this must simultaneously be able to prov ide large differences in the abundance of photospheric He.