Two massive star-forming regions at early evolutionary stages

We report sensitive Australia Telescope Compact Array radio continuum observations toward IRAS 15596-5301 and 16272-4837, two luminous objects (L>2x10(4) L-circle dot) thought to represent massive star-forming regions in early stages of evolution ( due to previously undetected radio emission at the 1 sigma level of 2 mJy beam(-1)). Also reported are 1.2 mm continuum and a series of molecular line observations made with the Swedish ESO Submillimeter Telescope. The radio continuum observations toward IRAS 15596-5301 reveal the presence of three distinct compact sources, with angular sizes of 2.7-8.8 (FWHM), all located within a region of 30 in diameter. Assuming that these are regions of ionized gas, we find that they have diameters of 0.06-0.2 pc and electron densities of 8x10(2)-2x10(3) cm(-3) and that they are excited by early B-type stars. The 1.2 mm observations show that the dust emission arises from a region of 42 x 25 ( FWHM) with a total flux of 5.8 Jy, implying a mass of 1.4x10(3) M-circle dot. The line observations indicate that IRAS 15596-5301 is associated with a molecular cloud with a FWHM angular size of 37(similar to0.4 pc radius at the distance of 4.6 kpc), a molecular hydrogen density of similar to4x10(5) cm(-3), and a rotational temperature of similar to27 K. We suggest that the massive dense core associated with IRAS 15596-5301 contains a cluster of B stars that are exciting compact H II regions that are in pressure equilibrium with the dense molecular surroundings. No radio continuum emission was detected from IRAS 16272-4837 up to a 3 sigma limit of 0.2 mJy. However, the 1.2 mm observations show strong dust emission arising from a region of 41 x 25 (FWHM) with a total flux of 13.8 Jy, implying a mass of 2.0x10(3) M-circle dot. The line observations indicate the presence of an elongated molecular cloud with FWHM major and minor axes of 61 and 42(0.50x0.35 pc in radius at the distance of 3.4 kpc), a molecular hydrogen density of similar to2x10(5) cm(-3), and a rotational temperature of similar to27 K. The high luminosity (2.4x10(4) L-circle dot) and lack of radio emission from this massive core suggest that it hosts an embedded young massive protostar that is still undergoing an intense accretion phase. This scenario is supported by the observed characteristics of the line profiles and the presence of a bipolar outflow detected from observations of the SiO emission. We suggest that IRAS 16272-4837 is a bona de massive star-forming region in a very early evolutionary stage, being the precursor of an ultracompact H II region.