HST -WFPC2 images in the ultraviolet (F255W) and in the visible (U, V, I) have been used to search for faint blue stars - down to an ultraviolet magnitude slightly below the turnoff - in the core of the globular cluster M 3.
The paucity of such stars already noted from ground-based observations in the outer regions of this cluster is also confirmed in the very cluster centre. One remarkable feature in this respect is that the only two blue stars lying on the HB extension are confined within from the cluster centre. We were unable to detect any other faint blue star in the three surrounding WF's chips. This could perhaps suggest that they might be somehow the product of dynamical interactions.
Another low luminosity (namely, #7290), possibly variable has been discovered in the very central region of the cluster. The still unidentified, non-pulsating radio source detected in the very centre of M 3 has been precisely positioned and its error box is not closer than from this peculiar object, too far to be safely identified as its optical counterpart. In turn, the radio source position is close to an intermediate-temperature HB star (#7251) which might be variable and to a bright BSS (#7785). There are not objects with colors representative of that of a QSO in the PC1 field of view.
Our conclusion is that probably none of the selected objects can be safely identified as the optical counterpart of the radio-source, though a few objects located within the error box deserve dedicated follow-up studies.
© European Southern Observatory (ESO) 1998
Online publication: March 10, 1998