The Na-O anticorrelation in horizontal branch stars. II. NGC1851

We studied the Na-O anti-correlation from moderately high resolution spectra for 35 stars on the blue HB (BHB), one RR Lyrae, and 55 stars are on the red HB (RHB) of NGC1851. We also derived abundances for He and N in BHB stars, and Ba and upper limits for N in RHB stars. The RHB stars clearly separate into two groups: the vast majority are O-rich and Na-poor, while about 10-15% are Na-rich and moderately O-poor. Most Na-rich RHB stars are also Ba-rich and there is an overall correlation between Na and Ba abundances within the RHB. The group of Ba-rich RHB stars resides on the warmer edge and includes ~10% of the RHB stars. We propose that they are the descendant of the stars on the RGB sequence with very red v-y colour. This sequence is known also to consist of Ba and perhaps CNO-rich stars. However, the upper limit we obtain for N ([N/Fe]<1.55) for one of the Ba-rich stars coupled with the low C-abundances for RGB Ba-rich stars from the literature suggests that the total CNO might not be particularly high ([(C+N+O)/Fe]<=0.15). The other Na-rich RHB stars are also at the warm edge of the RHB and the only RR Lyrae is Na-rich and moderately O-poor. We also find a Na-O anticorrelation among BHB stars, partially overlapping that found among RHB stars, though generally BHB stars are more Na-rich and O-poor. However, there is no clear correlation between temperature and Na and O abundances within the BHB. The average He abundance in BHB stars is Y=0.29+/-0.05. N abundances are quite uniform at [N/Fe]=1.16+/-0.14 among BHB stars, with a small trend with temperature. This value is consistent with normal CNO abundance and excludes that BHB stars are very CNO-rich: this leaves an age spread of ~1.5 Gyr as the only viable explanation for the split of the SGB. [Abridged]