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We present Atacama Large Millimeter/submillimeter Array (ALMA) observationsof the submillimeter dust continuum and H2D+ 1_{10}-1_{11} emission toward twoevolved, potentially protostellar cores within the Ophiuchus molecular cloud,Oph A SM1 and SM1N. The data reveal small-scale condensations within bothcores, with mass upper limits of M <~ 0.02M_Sun (~ 20 M_Jup). The SM1condensation is consistent with a nearly-symmetric Gaussian source with a widthof only 37 AU. The SM1N condensation is elongated, and extends 500 AU along itsmajor axis. No evidence for substructure is seen in either source. A Jeansanalysis indicates these sources are unlikely to fragment, suggesting that bothwill form single stars. H2D+ is only detected toward SM1N, offset from thecontinuum peak by ~150-200 AU. This offset may be due to either heating from anundetected, young, low luminosity protostellar source or first hydrostaticcore, or HD (and consequently H2D+) depletion in the cold centre of thecondensation. We propose that SM1 is protostellar, and that the condensationdetected by ALMA is a warm (T ~ 30-50 K) accretion disk. The less concentratedemission of the SM1N condensation suggests that it is still starless, but wecannot rule out the presence of a low-luminosity source, perhaps surrounded bya pseudodisk. These data reveal observationally the earliest stages of theformation of circumstellar accretion regions, and agree with theoreticalpredictions that disk formation can occur very early in the star formationprocess, coeval with or just after the formation of a first hydrostatic core orprotostar.

Revealing H2D+ depletion and compact structure in starless and protostellar cores with ALMA