The initial conditions of star formation in the ρ Ophiuchi main cloud: wide-field millimeter continuum mapping

We present the results of an extensive 1.3 mm continuum mosaicing study of the ρ Ophiuchi central region obtained at the IRAM 30-m telescope with the MPIfR 19–channel bolometer array. The mosaiced field covers a total area of ∼ 480 arcmin2, corresponding to ∼ 1 pc2 at a distance of 160 pc, and includes the DCO dense cores Oph-A, Oph-B1, Oph-B2, Oph-C, Oph-D, Oph-E, and Oph-F. Our mosaic is sensitive to features down to N H2 ∼ 1022 cm−2 in column density. It is consistent with, but goes significantly deeper than, previous dust continuum studies of the cloud. For the first time, compact circumstellar dusty structures around young stellar objects are detected simultaneously with more extended emission from the dense cores and the ambient cloud. Thus, it becomes possible to directly study the genetic link between dense cores and young stars. The diffuse cloud emission is itself fragmented in at least 58 small-scale, starless clumps harboring no infrared or radio continuum sources in their centers. Most of these starless fragments are probably gravitationally bound and pre-stellar in nature. Nineteen of them exhibit a relatively flat inner intensity profile, indicating they are not as centrally condensed as the envelopes seen around the embedded (Class I and Class 0) protostars of the cloud. Ten other clumps appear to be sharply peaked, however, and may represent candidate ‘isothermal protostars’, i.e., collapsing cloud fragments which have not yet developed a central hydrostatic core. The ∼ 6 000 AU fragmentation sizescale estimated from our ρ Oph 1.3 mm mosaic is consistent with the typical Jeans length in the DCO cores and is at least five times smaller than the diameter of isolated dense cores in the Taurus cloud. In agreement with this short lengthscale for fragmentation, the circumstellar envelopes surrounding ρ Oph Class I and Class 0 protostars are observed to have finite sizes and to be significantly more compact than their Taurus counterparts. The measured frequency distribution of pre-stellar clump masses is relatively shallow below ∼ 0.5 M , being consistent with ∆N/∆m ∝ m−1.5, but steepens to ∆N/∆m ∝ m−2.5 above ∼ 0.5 M . This is reminiscent of the stellar initial mass Send offprint requests to: F. Motte ? e-mail: [email protected] ?? e-mail: [email protected] function (IMF), suggesting the clumps we detect may be the direct progenitors of individual stars. Our observations therefore support theoretical scenarios in which gravitational fragmentation plays a key role in determining the stellar mass scale and the IMF. Finally, the presence of several remarkable alignments of young stars and starless clumps in the 1.3 mm dust continuum mosaic supports the idea that various external agents, such as a slow shock wave originating in the Sco OB2 association, have induced core fragmentation and star formation in at least part of the cloud.