Infrared Dark Clouds: Precursors to Star Clusters

By J. Rathborne, J. Jackson, R. Simon, Qizhou Zhang. 2004

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Infrared dark clouds (IRDCs) are dense molecular clouds seen as extinction features against the bright mid-infrared Galactic background. Millimeter continuum maps toward 38 IRDCs reveal extended cold dust emission to be associated with each of the IRDCs. IRDCs range in morphology from filamentary to compact and have masses of 120 to 16,000 M☉, with a median mass of ~940 M☉. Each IRDC contains at least one compact (≤0.5 pc) dust core and most show multiple cores. We find 140 cold millimeter cores unassociated with MSX 8 μm emission. The core masses range from 10 to 2100 M☉, with a median mass of ~120 M☉. The slope of the IRDC core mass spectrum (α ~ 2.1 ± 0.4) is similar to that of the stellar IMF. Assuming that each core will form a single star, the majority of the cores will form OB stars. IRDC cores have similar sizes, masses, and densities as hot cores associated with individual, young high-mass stars, but they are much colder. We therefore suggest that IRDC represent an earlier evolutionary phase in high-mass star formation. In addition, because IRDCs contain many compact cores and have the same sizes and masses as molecular clumps associated with young clusters, we suggest that IRDCs are the cold precursors to star clusters. Indeed, an estimate of the star formation rate within molecular clumps with similar properties to IRDCs (~2 M☉ yr-1) is comparable to the global star formation rate in the Galaxy, supporting the idea that all stars may form in such clumps.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/b5e422b2bd988fa44a8ab0e33dffdf6a2132745d

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The relation between the most-massive star and its parental star cluster mass (2009, background): “Low-mass young clusters are found to be generally small (<∼ 1 pc, Testi et al. 1998; Gutermuth et al. 2005; Testor et al. 2005; Rathborne et al. 2006; Scheepmaker et al. 2007) so it is unclear if and how such a correlation between the most-massive star and the cluster radius extends to lower masses.”
ATLASGAL - properties of a complete sample of Galactic clumps (2017, background): “If a clump is associated with an infrared dark cloud (IRDC; e.g., Rathborne et al. 2006), it suggests the source is in the foreground with respect to the relatively bright infrared emission that fills the inner part of the Galaxy.”
The earliest phases of high-mass star formation: a 3 square degree millimeter continuum mapping of Cygnus X (2007, methodology): “As a consequence of their small sizes, the Cygnus X dense cores have masses (4−950M⊙, see Table 1) which are up to one order of magnitude smaller than those obtained by Beuther et al. (2002) and Rathborne et al. (2006) (see Table 4 and Fig.”
ATLASGAL - The APEX telescope large area survey of the galaxy at 870 μm (2009, background): “Alternatively, dense condensations in Infrared Dark Clouds (IRDCs, see Simon et al. 2006, for a compilation) are promising hunting grounds for the initial stages of high-mass star formation (e.g., Rathborne et al. 2006; see also the reviewsby Menten et al. 2005; Wyrowski 2008).”
CHEMISTRY IN INFRARED DARK CLOUD CLUMPS: A MOLECULAR LINE SURVEY AT 3 mm (2011, methodology): “Due to the characteristics of IRDCs and the large variety of evolutionary stages that they harbor, it has been suggested that they are the natal sites of all massive stars and stellar clusters (Rathborne et al. 2006, 2010).”
THE “NESSIE” NEBULA: CLUSTER FORMATION IN A FILAMENTARY INFRARED DARK CLOUD (2009, background): “A few of these embedded young stellar objects will evolve into high-mass stars (e.g., Beuther et al. 2005; Rathborne et al. 2005, 2006, 2007; Simon et al. 2006; Pillai et al. 2006; Beuther & Steinacker 2007).”
FRAGMENTATION AT THE EARLIEST PHASE OF MASSIVE STAR FORMATION (2009, methodology): “Figure 2 presents the 1.2 mm continuum emission from the IRAM 30 m telescope (Rathborne et al. 2006), the NH3 (1, 1) emission from the VLA (Wang et al. 2008), and the SMA 1.3 mm dust continuum emission.”
THE BOLOCAM GALACTIC PLANE SURVEY: SURVEY DESCRIPTION AND DATA REDUCTION (2010, methodology): “In the past decade, compelling evidence has emerged which suggests that IRDCs are the pre-cursors to massive stars, and therefore, stellar clusters (see, e.g., Rathborne et al. 2006, 2008).”
KINEMATIC DISTANCES TO MOLECULAR CLOUDS IDENTIFIED IN THE GALACTIC RING SURVEY (2009, background): “Previous studies of dense molecular clouds in our own Galaxy have also shown that stars form in molecular clouds (e.g., Beuther et al. 2005; Beuther & Steinacker 2007; Pillai et al. 2006; Rathborne et al. 2006, 2007, 2008).”
The ATLASGAL survey: a catalog of dust condensations in the Galactic plane (2013, methodology): “These clouds have been cataloged using MSX (Simon et al. 2006; Rathborne et al. 2006) and the more sensitive Spitzer Space Telescope (Peretto & Fuller 2009; Butler & Tan 2009; Peretto & Fuller 2010; Rygl et al. 2010).”
Fragmentation and mass segregation in the massive dense cores of Cygnus X (2009, background): “…2002, Faúndez et al. 2004), and more recently inside the dark filaments seen in the infrared surveys of the Galactic plane, the so-called IRDCs (e.g. Simon et al. 2006, and references therein), have found a number of candidates to host such precursors (Rathborne et al. 2006, Pillai et al. 2006).”
Characterizing precursors to stellar clusters with Herschel (2011, background): “While there exist many small IRDCs (e.g., Peretto & Fuller 2010; Kauffmann & Pillai 2010), the most massive ones (M ∼ 103−4 M⊙, nH > 10 5 cm−3, T < 25 K, Rathborne et al. 2006; Egan et al. 1998; Carey et al. 1998) are consistent with expectations for a proto-cluster.”
STAR FORMATION ACTIVITY OF CORES WITHIN INFRARED DARK CLOUDS (2009, methodology): “Nevertheless, in this paper, we will call the objects identified in the IRAM 30 m observations (Rathborne et al. 2006) cores to differentiate them from the clump-like IRDCs which contain them.”
Ammonia from cold high-mass clumps discovered in the inner Galactic disk by the ATLASGAL survey (2012, background): “This, together with the cold temperatures and fragmented substructure of IRDCs, suggests that they are protoclusters (Rathborne et al. 2006, 2005).”
Cold Dark Clouds: The Initial Conditions for Star Formation (2007, background): “(Sridharan et al. 2005; Ragan et al. 2006; Rathborne et al. 2006).”