UBV stellar photometry of bright stars in GC M5 – I. UV colour–magnitude and colour–colour diagrams and some peculiarities in the HB stellar distribution

We present stellar photometry in the UBV passbands for the globular cluster M5 ≡ NGC 5904. The observations, taken from short‐exposure photographic plates and CCD frames, were obtained in the Ritchey‐Cretien (RC) focus of the 2‐m telescope of the National Astronomy Observatory ‘Rozhen’. All stars in an annulus with radius 1 r 5.5 arcmin were measured. We show that the ultraviolet (UV) colour–magnitude diagrams (CMDs) describe different evolutionary stages in a better manner than the ‘classical’ V , B−V diagram. We use HB stars, with known spectroscopic T eff to check the validity of the colour zero‐point. A review of all known UV‐bright star candidates in M5 is made and some of their parameters are catalogued. Six new stars of this kind are suspected on the basis of their position on the CMD. New assessment of the cluster reddening and metallicity is done using the U−B , B−V diagram. We find that [Fe/H]=−1.38 , which confirms the Zinn & West value, contrasting with recent spectroscopic estimates. In an effort to clarify the question of the gap in the blue horizontal branch (BHB) stellar distribution and to investigate some other peculiarities, we use the relatively long‐base colour index U−V . A comparison of the observed V , (U−V) 0 distribution of horizontal branch (HB) stars with a canonical zero‐age horizontal branch (ZAHB) model reveals that the hottest stars rise above the model line. This is similar to the ‘ u ‐jump’ found in the Strömgren photometry. 18 BHB stars with (B−V) 0 ∈[−0.02/0.18] are used to estimate their ultraviolet deficiency. It is shown that low‐gravity (log g 2) Kurucz's atmospheric models fit the observed distribution of these stars along the two‐colour diagram well.