Accretion in the ρ-Ophiuchi pre-main sequence stars

Aims. The aim of this paper is to provide a measurement of the mass accretion rate in a large, complete sample of objects in the core of the star forming region rho Oph. Methods. The sample includes most of the objects (104 out of 111) with evidence of a circumstellar disk from mid-infrared photometry; it covers a stellar mass range from about 0.03 to 3 M-circle dot and it is complete to a limiting mass of similar to 0.05 M-circle dot. We used J and K-band spectra to derive the mass accretion rate of each object from the intensity of the hydrogen recombination lines, Pa beta or Br gamma. For comparison, we also obtained similar spectra of 35 diskless objects. Results. The results show that emission in these lines is only seen in stars with disks, and can be used as an indicator of accretion. However, the converse does not hold, as about 50% of our disk objects do not have detectable line emission. The measured accretion rates show a strong correlation with the mass of the central object (M-acc proportional to M-*(1.8 +/- 0.2)) and a large spread, of two orders of magnitude at least, for any interval of M-*. A comparison with existing data for Taurus shows that the objects in the two regions have similar behaviour, at least for objects more massive than similar to 0.1 M-circle dot. The implications of these results are briefly discussed.