Early-time light curves of Type Ib/c supernovae from the SDSS-II Supernova Survey

Context. Type Ib/c supernovae (SNe Ib/c) have been investigated in several single-object studies; however, there is still a paucity of works concerning larger, homogeneous samples of these hydrogen-poor transients, in particular regarding the premaximum phase of their light curves. Aims. In this paper we present and analyze the early-time optical light curves (LCs, ugriz) of 20 SNe Ib/c from the Sloan Digital Sky Survey (SDSS) SN survey II, aiming to study their observational and physical properties, as well as to derive their progenitor parameters. Methods. High-cadence, multiband LCs are fitted with a functional model and the best-fit parameters are compared among the SN types. Bolometric LCs (BLCs) are constructed for the entire sample. We also computed the black-body (BB) temperature (T-BB) and photospheric radius (R-ph) evolution for each SN via BB fits on the spectral energy distributions. In addition, the bolometric properties are compared to both hydrodynamical and analytical model expectations. Results. Complementing our sample with literature data, we find that SNe Ic and Ic-BL (broad-line) have shorter rise times than those of SNe Ib and IIb. The decline rate parameter, Delta m(15), is similar among the different subtypes. SNe Ic appear brighter and bluer than SNe Ib, but this difference vanishes if we consider host galaxy extinction corrections based on colors. Templates for SN Ib/c LCs are presented. Our SNe have typical T-BB of similar to 10000 K at the peak and R-ph of similar to 10(15) cm. Analysis of the BLCs of SNe Ib and Ic gives typical ejecta masses M-ej approximate to 3.6-5.7 M-circle dot, energies E-K approximate to 1.5-1.7x10(51) erg, and M(Ni-56) approximate to 0.3 M-circle dot. Higher values for E-K and M(Ni-56) are estimated for SNe Ic-BL (M-ej approximate to 5.4 M-circle dot, E-K 10.7x10(51) erg, M(Ni-56) approximate to 1.1 M-circle dot). For the majority of SNe Ic and Ic-BL, we can put strong limits (<2-4 days) on the duration of the expected early-time plateau. Less stringent limits can be placed on the duration of the plateau for the sample of SNe Ib. In the single case of SN Ib 20061c, a >5.9 days plateau seems to be detected. The rising part of the BLCs is reproduced by power laws with index <2. For two events (SN 2005hm and SN 2007qx), we find signatures of a possible shock break-out cooling tail. Conclusions. Based on the limits for the plateau length and on the slow rise of the BLCs, we find that in most of our SNe Ic and Ic-BL the Ni-56 is mixed out to the outer layers, suggesting that SN Ic progenitors are de facto helium poor. The derived progenitor parameters (Ni-56, E-K, M-ej) are consistent with previous works.