The RMS survey - (CO)-C-13 observations of candidate massive YSOs in the southern Galactic plane

Abstract

Context. The Red MSX Source ( RMS) survey is an ongoing multi-wavelength observational programme designed to return a large, well-selected sample of massive young stellar objects (MYSOs). We have identified similar to 2000 MYSOs candidates located within our Galaxy by comparing the colours of MSX and 2MASS point sources to those of known MYSOs. The aim of our follow- up observations is to identify other contaminating objects such as ultra compact (UC) HII regions, evolved stars and planetary nebulae (PNe) and distinguish between genuine MYSOs and nearby low- mass YSOs. Aims. A critical part of our follow- up programme is to conduct (CO)-C-13 molecular line observations in order to determine kinematic distances to all of our MYSO candidates. These distances will be used in combination with far- IR and (sub) millimetre fluxes to determine bolometric luminosities which will allow us to identify and remove nearby low- mass YSOs. In addition these molecular line observations will help in identifying evolved stars which are weak CO emitters. Methods. We have used the 22 m Mopra telescope, the 15 m JCMT and the 20 m Onsala telescope to conduct molecular line observations towards 854 MYSOs candidates located in the 3rd and 4th quadrants. These observations have been made at the J = 1 - 0 ( Mopra and Onsala) and J = 2 - 1 (JCMT) rotational transition frequency of (CO)-C-13 molecules and have a spatial resolution of similar to 20`- 40`, a sensitivity of T-A* A similar or equal to 0.1 K and a velocity resolution of similar to 0.2 km s(-1). Results. We detect (CO)-C-13 emission towards a total of 752 of the 854 RMS sources observed (similar to 88%). In total 2132 emission components are detected above 3s level (typically T*(A) = 0.3 K). Multiple emission profiles are observed towards the majority of these sources - 461 sources (similar to 60%) - with an average of similar to 4 molecular clouds detected along the line of sight. These multiple emission features make it di. cult t