The earliest phases of high-mass star formation: a 3 square degree millimeter continuum mapping of Cygnus X

Abstract

Aims. Our current knowledge of high-mass star formation is mainly based on follow-up studies of bright sources found by IRAS, and is thus biased against its earliest phases, inconspicuous at infrared wavelengths. We therefore started searching, in an unbiased way and in the closest high-mass star-forming complexes, for the high-mass analogs of low-mass pre-stellar cores and class 0 protostars. Methods. We have made an extensive 1.2 mm continuum mosaicing study of the Cygnus X molecular cloud complex using the MAMBO cameras at the IRAM 30 m telescope. The ∼ 3 • 2 imaged areas cover all the high-column density (A V ≥ 15 mag) clouds of this nearby (∼ 1.7 kpc) cloud complex actively forming OB stars. We then compared our millimeter maps with mid-infrared images, and have made SiO(2-1) follow-up observations of the best candidate progenitors of high-mass stars. Results. Our complete study of Cygnus X with ∼ 0.09 pc resolution provides, for the first time, an unbiased census of massive young stellar objects. We discover 129 massive dense cores (FWHM size ∼ 0.1 pc, M 1.2 mm = 4 − 950 M ⊙ , volume-averaged density ∼ 10 5 cm −3 ), among which ∼ 42 are probable precursors of high-mass stars. A large fraction of the Cygnus X dense cores (2/3 of the sample) remain undetected by the MSX satellite, regardless of the mass range considered. Among the most massive (> 40 M ⊙ ) cores, infrared-quiet objects are driving powerful outflows traced by SiO emission. Our study qualifies 17 cores as good candidates for hosting massive infrared-quiet protostars, while up to 25 cores potentially host high-luminosity infrared protostars. We fail to discover in the high-mass analogs of pre-stellar dense cores (∼ 0.1 pc, > 10 4 cm −3 ) in Cygnus X, but find several massive starless clumps (∼ 0.8 pc, 7 × 10 3 cm −3 ) that might be gravitationally bound. Conclusions. Since our sample is derived from a single molecular complex and covers every embedded phase of high-mass star formation, it gives the first statistical estimates of their lifetime. In contrast to what is found for low-mass class 0 and class I phases, the infrared-quiet protostellar phase of high-mass stars may last as long as their better-known high-luminosity infrared phase. The statistical lifetimes of high-mass protostars and pre-stellar cores (∼ 3 × 10 4 yr and < 10 3 yr) in Cygnus X are one and two order(s) of magnitude smaller, respectively, than what is found in nearby, low-mass star-forming regions. We therefore propose that high-mass pre-stellar and protostellar cores are in a highly dynamic state, as expected in a molecular cloud where turbulent processes dominate.

APEX/SABOCA observations of small-scale structure of infrared-dark clouds

“…While Spitzer may have missed the deeply embedded protostars that Herschel now captures, indirect signposts, such as molecular outflows traced by SiO (e.g. Motte et al 2007) and excess 4.5 µm emission (e.g. Cyganowski et al 2008;Chambers et al 2009), help to provide a more complete census of star formation.…”

“…Motte et al 2007;Hatchell & Fuller 2008). The HMPO lifetime in Cygnus X is 3.2 × 10 4 yr (Motte et al 2007). Adopting this timescale for the massive cores, then the upper limit to τ SL is 2.1 × 10 4 yr.…”

“…Because we lack spectral information at shorter wavelengths (in part due to high extinction and poor sensitivity in distant clouds such as IRDCs) which in nearby clouds provide empirical divisions between Class 0/I/II protostellar stages in nearby clouds, we do not further refine the protostellar classification scheme here. However, based on our evolutionary diagnostics, we assume that the PACS cores are in the high-mass protostellar object (HMPO) stage (Sridharan et al 2002;Beuther et al 2002;Motte et al 2007), previous to the formation of an HII region.…”

“…One elusive quantity needed to constrain theoretical models is the lifetime of the earliest "starless" phase (τ SL ) for high-mass star forming cores/clumps (Motte et al 2007;Chambers et al 2009;Miettinen 2012;Tackenberg et al 2012). For such a calculation, we first assume that objects that are mid-infrared (MIR) dark (λ < 100 µm) are starless, and cores bright at 70 µm (our PACS cores) are actively forming protostars, which has been shown as a good diagnostic for embedded protostars (Dunham et al 2008; R12;Stutz et al 2013).…”

“…Because we lack spectral information at shorter wavelengths (in part due to high extinction and poor sensitivity in distant clouds such as IRDCs) which in nearby clouds provide empirical divisions between Class 0/I/II protostellar stages in nearby clouds, we do not further refine the protostellar classification scheme here. However, based on our evolutionary diagnostics, we assume that the PACS cores are in the high-mass protostellar object (HMPO) stage (Sridharan et al 2002;Beuther et al 2002;Motte et al 2007), previous to the formation of an HII region.In the simplest case, if we assume that all the candidate starless objects will ultimately form protostars, and the rate of this progession is independent of the core mass, then the ratio of the number of starless to protostellar cores is the same as the ratio of the lifetimes (see Enoch et al 2008). If we assume that our protostars are predominantly in the "Class 0"-like phase as our evolutionary diagnostics indicate (see Figs.…”