A high resolution study of isotopic composition and chemical abundances of blue horizontal branch stars in the globular clusters NGC6397 and NGC6752

Large abundance anomalies have been previously detected in Horizontal Branch B-type stars. We present the first high resolution study of isotopic anomalies and chemical abundances in six Horizontal Branch B-type stars in globular clusters NGC6397 and NGC6752 and compare them to those observed in HgMn stars. We obtained high-resolution (up to R~115000) UVES spectra of a representative sample of six B-type stars (T183, T191, T193, B652, B2151, B2206). It is the first time an abundance analysis is performed for all elements for which spectral lines were detected in UVES spectra of Horizontal Branch B-type stars. Our study of these stars revealed no signs of He isotopic anomalies which would produce a ^3He/^4He ratio different from the solar one. The isotopic anomaly of Ca is detected in all six studied stars. The chemical abundance analysis reveals an overabundance over the solar values of P, Ti, Mn, Fe, and Y and an overabundance over the cluster metallicity of Mg, Ca, and Cr. This behaviour is very similar in all six studied stars of both clusters with a few exceptions: The Na abundance is by more than 1.4dex larger than the cluster metallicity in B652, and by more than 0.8dex larger than the cluster metallicity in B2206; the Co abundance is 1.0dex over the solar abundance for T191, while Zr is overabundant over the solar abundance by 0.4dex in B2206. No lines of Hg or other heavy elements were observed in the spectra. Weak emission lines of Ti II, similar to those frequently observed in HgMn stars were discovered in one Horizontal Branch B-type star T191. Further, we detected a radial velocity change of 0.9km s^-1 from one night to the next for T183 and 0.4km s^-1 for B2151.