The dust temperatures of the pre-stellar cores in the ρ Oph main cloud and in other star-forming regions: consequences for the core mass function

We estimate the dust temperatures of the clumps in the ρ Oph main cloud taking into account the 3D geometry of the region, and external heating from the interstellar radiation field and from HD 147879, a nearby luminous B2V star, which is believed to dominate the radiation field in the region. We find that the regions where pre-stellar cores are observed (i.e. at optical visual extinctions >7 mag) are colder than ∼10 –11 K. These dust temperatures are smaller than those which previous studies of the same region have assumed. We use the new dust temperatures to estimate the masses of the pre-stellar cores in the ρ Oph main cloud from millimetre observations, and we find core masses that are larger than previous estimates by a factor of ∼2 –3. This affects the core mass function (CMF) of the region; we find that the mass at which the core mass spectrum steepens from a slope α∼ 1.5 to a slope α∼ 2.5 has moved from ∼0.5 to ∼1 M⊙. In contrast with the CMF in other star-forming regions (e.g. Orion), there is no indication for a turnover down to the completeness limit (∼0.2 M⊙), but the CMF may flatten at around ∼0.4 M⊙. We generalize our results to the pre-stellar cores in Taurus and in Orion. In Taurus, the ambient radiation field heating the pre-stellar cores is believed to be weaker than that in ρ Oph. Hence, the dust temperatures of the cores in Taurus are expected to be below ∼10 –11 K. In Orion, the radiation field is believed to be 103 times stronger than the standard interstellar radiation field. Based on this assumption, we estimate that the dust temperatures of the pre-stellar cores in Orion are around ∼20 –30 K.