The RMS survey - 6 cm continuum VLA observations towards candidate massive YSOs in the northern hemisphere

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 ~2000 MYSO candidates located throughout the Galaxy by comparing the colours of MSX and 2MASS point sources to those of known MYSOs. The aim of these follow-up observations is to identify other objects with similar colours such as ultra compact (UC) HII regions, evolved stars and planetary nebulae (PNe) and distinguish between genuine MYSOs and nearby low-mass YSOs.

Aims. To identify the populations of UCHII regions and PNe within the sample and examine their Galactic distribution.

Methods. We have conducted high resolution radio continuum observations at 6 cm towards 659 MYSO candidates in the northern hemisphere (10° < l < 250°) using the Very Large Array (VLA). These observations have a spatial resolution of ~1–2´´?and typical image rms noise values of ~0.22 mJy – sensitive enough to detect a HII region powered by B0.5 star at the far side of the Galaxy. In addition to these targeted observations we present archival data towards a further 315 RMS sources extracted from a previous VLA survey of the inner Galaxy.

Results. We present the results of radio continuum observations made towards 974 MYSO candidates, 272 (~27% of the observed sample) of which are found to be associated with radio emission above a 4$\sigma$ detection limit (~1 mJy). Using results from other parts of our multi-wavelength survey we separate these RMS-radio associations into two distinct types of objects, classifying 51 as PNe and a further 208 as either compact or UC HII regions. Including all HII regions and PNe identified either from the literature or from the multi-wavelength data these numbers increase to 391 and 79, respectively. Using this well selected sample of HII regions we estimate their Galactic scale height to be 0.6°. In addition to the RMS-radio associations we are able to set upper limits on the radio emission of $\leq$1 mJy for the 702 non-detections, which is below the level expected if they had already begun to ionise their surroundings.

Conclusions. Using radio continuum and archival data we have identified 79 PNe and 391 HII regions within the northern RMS catalogue. We estimate the total fraction of contamination by PNe in the RMS sample is of order 10%. The sample of HII regions is probably the best representation to date of the Galactic population of HII regions as a whole.