An HST imaging survey of low-mass stars in the Chamaeleon i star-forming region

We present new Hubble Space Telescope/WFPC2 observations of 20 fields centered around T Tauri stars in the Chamaeleon I star-forming region. Images have been obtained in the F631N ([O I] λ6300), F656N (Hα), and F673N ([S II] λλ6716, 6731) narrow-band filters, plus the Johnson V-band equivalent F547M filter. We detect 31 T Tauri stars falling within our fields. We discuss the optical morphology of 10 sources showing evidence of either binarity, circumstellar material, or mass loss. We supplement our photometry with a compilation of optical, infrared, and submillimeter (sub-mm) data from the literature, together with new sub-mm data for three objects, to build the spectral energy distributions (SEDs) of 19 single sources. Using an SED model fitting tool, we self-consistently estimate a number of stellar and disk parameters, while mass accretion rates are directly derived from our Hα photometry. We find that bolometric luminosities derived from dereddened optical data tend to be underestimated in systems with high α _2-24 IR spectral index, suggesting that disks seen nearly edge-on may occasionally be interpreted as low-luminosity (and therefore more evolved) sources. On the other hand, the same α _2-24 IR spectral index, a tracer of the amount of dust in the warmer layers of the circumstellar disks, and the mass accretion rate appear to decay with the isochronal stellar age, suggesting that the observed age spread (≃ 0.5-5Myr) within the cluster is real. Our sample contains a few outliers that may have dissipated their circumstellar disks on a shorter timescale.