NLTE spectroscopic analysis of the 3He anomaly in subluminous B-type stars

Several B-type main-sequence stars show chemical peculiarities. A particularly striking class are the He-3 stars, which exhibit a remarkable enrichment of He-3 with respect to He-4. 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 He-3 sdBs. The He-3 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 He-3 BHBs and nine known He-3 sdBs to determine their isotopic helium abundances and He-4/He-3 abundance ratios. We redetermined their atmospheric parameters and analyzed selected He I lines, including lambda 4922 angstrom and lambda 6678 angstrom, which are very sensitive to He-4/He-3. Most of the He-3 sdBs cluster in a narrow temperature strip between 26 000K and 30 000K and are helium deficient in accordance with previous LTE analyses. BD + 48 degrees 2721 is reclassified as a BHB star because of its low temperature (T-eff = 20 700 K). Whereas He-4 is almost absent (He-4/He-3 < 0.25) in most of the known He-3 stars, other sample stars show abundance ratios up to He-4/He-3 similar to 2.51. A search for He-3 stars among 26 candidate sdBs from the ESO SPY survey led to the discovery of two new He-3 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 degrees 2721).