A comparative study of high-mass cluster forming clumps

Abstract

Aims. We have searched for star formation activity (mainly infall and outflow signatures) in a sample of high-mass molecular clumps (M > 100 M ) in different evolutionary stages and with a wide range of surface densities, with the aim of looking for evolutionary trends and testing observationally recent theoretical models which predict the need for a minimum surface density to form high-mass stars. Methods. Our sample has been selected from single-dish 1.2 mm continuum surveys and is composed of 48 massive molecular clumps, of which 29 are IR-loud and 19 are IR-dark. Each of these has been mapped in the HCO + (1-0), HCN(1-0) and C 18 O(2-1) transitions with the IRAM-30 m telescope on Pico Veleta (Spain). We derive basic parameters (mass, momentum, kinetic energy) for the clumps and their associated outflows and examine the HCO + (1-0) line profiles for evidence of infall or expansion. Results. Molecular outflows have been detected in 75% of our targets from the presence of high-velocity wings in the HCO + (1-0) spectra. These are equally frequent and massive (between ∼1 and ∼100 M ) in IR-dark and IR-loud clumps, implying similar levels of star formation activity in both kinds of objects. A surface density threshold at Σ = 0.3 g cm −2 has been found above which the outflow detection rate increases significantly and the outflows are on average more massive. The infall detection rate in our sample is low, but significantly higher in the IR-dark sub-sample. Our clump mass estimates using the mm dust emission and C 18 O(2-1) are sensitive to the temperature, but assuming a value of 15 K for the IR-dark sub-sample, we find evidence that C 18 O is depleted by a factor ∼4.5. The HCO + (1-0) to HCN(1-0) integrated intensity ratios measured reveal a greater dispersion about the mean value in the IR-dark sub-sample than in the IR-loud by a factor of about 5. We find that a considerable number of IR-dark sources are self-absorbed in HCN(1-0) suggesting that radiative transport effects in the ground state transitions have an important influence on the integrated intensity ratio. Conclusions. Our results indicate that, in terms of outflow frequency and energetics, both IR-dark and IR-loud molecular clumps present equivalent signatures of star formation activity, and that the formation of high-mass stars requires sufficiently high clump surface densities. The higher infall detection rate measured for the IR-dark subsample suggests that these objects could be associated with the onset of star formation.

Evolution and excitation conditions of outflows in high-mass star-forming regions

“…2). We have identified the lines detected with signal-to-noise ratio > ∼ 5, which correspond to typical main beam temperatures ∼0.1 K. Most of the lines detected toward all the sources of the sample correspond to simple molecules such as N 2 H + , C 2 H, NH 2 D, or H 13 CN, which are typically found tracing the dense gas in YSOs (e.g., Tafalla et al 2004;Padovani et al 2011;Busquet et al 2011;Fontani et al 2012), and molecules such as SiO and HCO + typically used to trace the outflow emission (e.g., Tafalla et al 2010;López-Sepulcre et al 2010;Codella et al 2013). For twelve of the fourteen sources, we also detected CH 3 CN emission, which is a tracer typically found in association with hot cores (e.g., Olmi et al 1993Olmi et al , 1996Sánchez-Monge et al 2010.…”

“…according to its emission or not at mid-IR wavelengths according to López-Sepulcre et al (2010). (d) Parameters obtained from the singletemperature, modified black body fit to the spectral energy distribution (see Sect.…”

“…Some outflow parameters, such as the energy and mechanical luminosity, can be greatly affected if inclination >60 • (see Table 6 of López-Sepulcre et al 2010, for the correction factor that must be applied for different inclinations).…”

Magnetically regulated fragmentation of a massive, dense, and turbulent clump

“…1, Beltrán et al 2006), and found to be not blended with nearby millimeter clumps, which allows a clear identification of the fragments. Its high mass and column density make it a potential site for the formation of massive stars and rich clusters, according to observational findings (Kauffmann & Pillai 2010;Lopez-Sepulcre et al 2010). The clump was classified as an infrared dark cloud because it was undetected in the images of the Midcourse Space Experiment (MSX) infrared satellite, although more sensitive images of the Spitzer satellite revealed infrared emission at a wavelength of 24 µm (panel A in Fig.…”

Formation of a protocluster: A virialized structure from gravoturbulent collapse

“…Observationally, the infall motion is detected with a double-peaked, blue-skewed line profile, while complicated by density, molecular abundance, and excitation temperature. Nevertheless, clear global infall at clump scale is difficult to detect (López-Sepulcre et al 2010;Reiter et al 2011) since local star formation also creates infall and outflow signatures. Infall rates of star-forming clumps are observed to be small, about 10% of the free-fall velocity (Rolffs et al 2011;Tan et al 2014;Wyrowski et al 2016), supporting a quasi-static picture.…”