2018A&A...618A..86S

Several B-type main-sequence stars show chemical peculiarities. A particularly striking class are the ³He stars, which exhibit a remarkable enrichment of ³He with respect to ⁴He. This isotopic anomaly has also been found in blue horizontal branch (BHB) and subdwarf B (sdB) stars, which are helium-core burning stars of the extreme horizontal branch. Recent surveys uncovered 11 ³He sdBs. The ³He anomaly is not due to thermonuclear processes, but caused by atomic diffusion in the stellar atmosphere. Using a hybrid local/non-local thermodynamic equilibrium (LTE/NLTE) approach for B-type stars, we analyzed high-quality spectra of two known ³He BHBs and nine known ³He sdBs to determine their isotopic helium abundances and ⁴He/³He abundance ratios. We redetermined their atmospheric parameters and analyzed selected HeI lines, including λ4922Å and λ6678Å, which are very sensitive to ⁴He/³He. Most of the ³He sdBs cluster in a narrow temperature strip between 26000K and 30000K and are helium deficient in accordance with previous LTE analyses. BD+48° 2721 is reclassified as a BHB star because of its low temperature (T_eff=20700K). Whereas ⁴He is almost absent (⁴He/³He<0.25) in most of the known ³He stars, other sample stars show abundance ratios up to ⁴He/³He∼2.51. A search for ³He stars among 26 candidate sdBs from the ESO SPY survey led to the discovery of two new ³He sdB stars (HE 0929-0424 and HE 1047-0436). The observed helium line profiles of all BHBs and of three sdBs are not matched by chemically homogeneous atmospheres, but hint at vertical helium stratification. This phenomenon has been seen in other peculiar B-type stars, but is found for the first time for sdBs. We estimate helium to increase from the outer to the inner atmosphere by factors ranging from 1.4 (SB 290) up to 8.0 (BD+48° 2721).