The coordinated radio and infrared survey for high-mass star formation

Abstract

A catalogue of 239 ultra-compact H II regions (UCHIIs) found in the CORNISH survey at 5 GHz and 1.5 resolution in the region 10 • < l < 65 • , |b| < 1 • is presented. This is the largest complete and well-selected sample of UCHIIs to date and provides the opportunity to explore the global and individual properties of this key state in massive star formation at multiple wavelengths. The nature of the candidates was validated, based on observational properties and calculated spectral indices, and the analysis is presented in this work. The physical sizes, luminosities and other physical properties were computed by utilising literature distances or calculating the distances whenever a value was not available. The near-and mid-infrared extended source fluxes were measured and the extinctions towards the UCHIIs were computed. The new results were combined with available data at longer wavelengths and the spectral energy distributions (SEDs) were reconstructed for 177 UCHIIs. The bolometric luminosities obtained from SED fitting are presented. By comparing the radio flux densities to previous observational epochs, we find about 5% of the sources appear to be time variable. This first high-resolution area survey of the Galactic plane shows that the total number of UCHIIs in the Galaxy is ∼ 750 -a factor of 3-4 fewer than found in previous large area radio surveys. It will form the basis for future tests of models of massive star formation.

GLOSTAR: Radio Source Catalog I. 28° < ℓ < 36° and |b| < 1°

“…The discrepancy between this number and that of the number of CORNISH sources not detected in GLOSTAR (Table 5) suggests that variability alone cannot fully account for the 26 CORNISH source not detected in GLOSTAR. The highest flux ratio is 3 and so the differences are relatively modest compared to those reported by Kalcheva et al (2018) (range between 2.4 and 73.9). All of the sources are either marginally detected or unresolved, have integrated fluxes below 10 mJy, and all but one have negative spectral indices, and so the variability appears to be restricted to weak and unresolved extragalactic sources.…”

“…In the case of variable sources, we should expect to find approximately as many sources with significantly larger fluxes in CORNISH than measured in GLOSTAR and comparing the fluxes we find 7 matches for which this is the case (following Kalcheva et al 2018 we set this criterion to be > 50% more flux). The radio names of these sources and the flux ratio are given in Table 6.…”

“…There are two possible explanations for these missing sources: 1) the flux density is variable and has decreased between the time the CORNISH observations were completed and the GLOSTAR observations were made; and 2) these are spuri- ous CORNISH detections. The variability of radio sources has been investigated by Kalcheva et al (2018) from an analysis of the CORNISH and MAGPIS integrated flux densities (we will discuss the MAGPIS survey in the next subsection). Kalcheva et al (2018) estimate that ∼5% of the CORNISH UC H ii region sample are potentially variable sources (flux increases of over 50 % over a 15 year time period), which is consistent with the number of CORNISH detections that do not have a matching GLOSTAR counterpart (∼ 7%).…”

“…The variability of radio sources has been investigated by Kalcheva et al (2018) from an analysis of the CORNISH and MAGPIS integrated flux densities (we will discuss the MAGPIS survey in the next subsection). Kalcheva et al (2018) estimate that ∼5% of the CORNISH UC H ii region sample are potentially variable sources (flux increases of over 50 % over a 15 year time period), which is consistent with the number of CORNISH detections that do not have a matching GLOSTAR counterpart (∼ 7%). In Table 5 we provide the source name and integrated and peak flux densities of the unmatched CORNISH sources (taken from Purcell et al 2013) along the local rms noise determined from the GLOSTAR maps.…”

“…We note that these radio properties are similar to those reported for T Tauri stars (e.g., Dzib et al 2013), however, given these radio sources appear to be mid-infrared dark this possibility is considered less likely. Kalcheva et al (2018) considered only H ii regions in their analysis, whereas none of the variable candidates we have identified have been classified as being H ii regions, PNe or radio stars. However, Kalcheva et al (2018) only found one variable H ii region in the region being considered here and given the shorter temporal baseline (∼7 yrs) the lack of variable H ii regions and the lower levels of variation are not surprising.…”