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

Abstract

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 He I lines, including λ4922 Å and λ6678 Å, which are very sensitive to 4He/3He. Most of the 3He sdBs cluster in a narrow temperature strip between 26000 K and 30000 K and are helium deficient in accordance with previous LTE analyses. BD+48° 2721 is reclassified as a BHB star because of its low temperature (Teff = 20700 K). 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).