Initial phases of massive star formation in high infrared extinction clouds

Abstract

Aims. The earliest phases of massive star formation are found in cold and dense infrared dark clouds (IRDCs). Since the detection method of IRDCs is very sensitive to the local properties of the background emission, we present here an alternative method to search for high column density in the Galactic plane by using infrared extinction maps. Using this method we find clouds between 1 and 5 kpc, of which many were missed by previous surveys. By studying the physical conditions of a subsample of these clouds, we aim at a better understanding of the initial conditions of massive star formation. Methods. We have made extinction maps of the Galactic plane based on the 3.6−4.5 μm color excess between the two shortest wavelength Spitzer IRAC bands, reaching to visual extinctions of ∼100 mag and column densities of 9 × 10 22 cm −2 . From this we compiled a new sample of cold and compact high extinction clouds. We used the MAMBO array at the IRAM 30 m telescope to study the morphology, masses and densities of the clouds and the dense clumps within them. The latter were followed up by pointed ammonia observations with the 100 m Effelsberg telescope, to determine rotational temperatures and kinematic distances. Results. Extinction maps of the Galactic plane trace large scale structures such as the spiral arms. The extinction method probes lower column densities, N H 2 ∼ 4 × 10 22 cm −2 , than the 1.2 mm continuum, which reaches up to N H 2 ∼ 3 × 10 23 cm −2 but is less sensitive to large scale structures. The 1.2 mm emission maps reveal that the high extinction clouds contain extended cold dust emission, from filamentary structures to still diffuse clouds. Most of the clouds are dark in 24 μm, but several show already signs of star formation via maser emission or bright infrared sources, suggesting that the high extinction clouds contain a variety of evolutionary stages. The observations suggest an evolutionary scheme from dark, cold and diffuse clouds, to clouds with a stronger 1.2 mm peak and to finally clouds with many strong 1.2 mm peaks, which are also warmer, more turbulent and already have some star formation signposts.

Hi-GAL, theHerschelinfrared Galactic Plane Survey: photometric maps and compact source catalogues

“…A similar distribution was also found by Rygl et al (2010) using high-extinction clouds identified with Spitzer colour excess. Therefore it is reasonable to posit that AT-LASGAL typically traces the higher flux fraction of the Hi-GAL sources, although this depends on the intrinsic SEDs of the various objects.…”

TheHerschelview of massive star formation in G035.39–00.33: dense and cold filament of W48 undergoing a mini-starburst

“…4b) but its compact 70 μm emission is three times that of the flux predicted by the grey-body model that fits its cold envelope. This MDC also coincides with a 40 -beam water maser source (Rygl et al 2010) suggesting that it harbours an intermediateor high-mass protostar. The SiO emission at these two positions could easily originate from shocks within protostellar outflows because they are comparable to the weakest ones associated with IR-quiet MDCs in Cygnus X (Motte et al 2007 after distance correction).…”

Deuterium fractionation and the degree of ionisation in massive clumps within infrared dark clouds

“…We note that the dust opacities are likely to be uncertain by a factor of 2 (Ossenkopf & Henning 1994;Motte & André 2001). For comparison, Williams et al (2004), Enoch et al (2006), and Rygl et al (2010) used the same dust model as we did and interpolated the values of κ ν at 850 μm, 1.1 mm, and 1.2 mm to be 0.154, 0.114, and 0.1 m 2 kg −1 , respectively. These are slightly lower than our values, and the difference is likely to be caused by different extrapolation methods (e.g., log-interpolation) and/or different dust emissivity index, β, used by the authors to determine the value of κ ν ∝ ν β .…”