We analyse C 18 O (J = 3−2) data from a sample of 99 infrared-bright massive young stellar objects (MYSOs) and compact Hii regions that were identified as potential molecular-outflow sources in the Red MSX source (RMS) survey. We extract a distance limited (D < 6 kpc) sample shown to be representative of star formation covering the transition between the source types. At the spatial resolution probed, Larsonlike relationships are found for these cores, though the alternative explanation, that Larson's relations arise where surface-density-limited samples are considered, is also consistent with our data. There are no significant differences found between source properties for the MYSOs and Hii regions, suggesting that the core properties are established prior to the formation of massive stars, which subsequently have little impact at the later evolutionary stages investigated. There is a strong correlation between dust-continuum and C 18 O-gas masses, supporting the interpretation that both trace the same material in these IR-bright sources. A clear linear relationship is seen between the independently established core masses and luminosities. The position of MYSOs and compact Hii regions in the mass-luminosity plane is consistent with the luminosity expected a cluster of protostars when using a ∼40 percent star-formation efficiency and indicates that they are at a similar evolutionary stage, near the end of the accretion phase.

The absorption mean free path L of star-producing radiation was determined by means of the local production of neutrons in condensed materials; for that purpose a neutron monitor was installed in an aircraft, and two flights were performed at the geomagnetic latitudes 23.3°S (Buenos Aires, Reptiblica Argentina) and 15°S (Asunci6n, Reptiblica del Paraguay). The results show an increase in L of about 20% for an altitude increase of 6000 m in Buenos Aires, and of 40% in Asuncion, for the same altitude increase; the total L value in Asuncion was found to be 175 g cm -2 , while in Buenos Aires it was 163 g cm -2 , that is, approximately 8% lower.

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