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

Abstract

Infrared-dark clouds (IRDCs) harbor the early phases of cluster and high-mass star formation and are comprised of cold (∼20 K), dense (n > 10 4 cm −3 ) gas. The spectral energy distribution (SED) of IRDCs is dominated by the far-infrared and millimeter wavelength regime, and our initial Herschel study examined IRDCs at the peak of the SED with high angular resolution. Here we present a follow-up study using the SABOCA instrument on APEX which delivers 7.8 angular resolution at 350 µm, matching the resolution we achieved with Herschel/PACS, and allowing us to characterize substructure on ∼0.1 pc scales. Our sample of 11 nearby IRDCs are a mix of filamentary and clumpy morphologies, and the filamentary clouds show significant hierarchical structure, while the clumpy IRDCs exhibit little hierarchical structure. All IRDCs, regardless of morphology, have about 14% of their total mass in small scale core-like structures which roughly follow a trend of constant volume density over all size scales. Out of the 89 protostellar cores we identified in this sample with Herschel, we recover 40 of the brightest and re-fit their SEDs and find their properties agree fairly well with our previous estimates ( T ∼ 19 K). We detect a new population of "cold cores" which have no 70 µm counterpart, but are 100 and 160 µm-bright, with colder temperatures ( T ∼ 16 K). This latter population, along with SABOCA-only detections, are predominantly low-mass objects, but their evolutionary diagnostics are consistent with the earliest starless or prestellar phase of cores in IRDCs.Key words. catalogs -stars: formation -ISM: structure -submillimeter: ISM Background and motivationDespite the importance of high-mass stars to the energy budget of galaxies, their formation remains a major open question in astronomy (Zinnecker & Yorke 2007). A major hindrance to progress is the inherent difficulty in obtaining well-defined initial conditions observationally. Because high-mass stars are rare, they are on average at large distances, thus angular resolution is of paramount importance. With the advent of Spitzer Space Telescope and Herschel Space Observatory (Pilbratt et al. 2010), coupled with extensive ground-based survey efforts, we now can approach this observational task statistically.Ever since their discovery in absorption in mid-infrared Galactic plane surveys, ISO data (Perault et al. 1996) and MSX observations (Egan et al. 1998), infrared-dark clouds (IRDCs) have been subject to intense study because they are the cold (T < 20 K), dense (n > 10 4 cm −3 ) environments believed to be required for the formation of high-mass stars and clusters (cf. Rathborne et al. 2006;Ragan et al. 2009;Battersby et al. 2010). They are located throughout the Galactic plane, concentrated within the spiral arms (Jackson et al. 2008). The formation of IRDCs, their kinematic structure and population of Based on observations carried out with the Atacama Pathfinder Experiment (APEX). APEX is a collaboration between Max Planck Institut für Radioastronomie (MPIf...

Fragmentation and kinematics of dense molecular cores in the filamentary infrared-dark cloud G011.11–0.12

“…Due to the small sample size under discussion here, we merely use T bol as a proxy for relative evolutionary stage and do not speculate further whether the trends seen in the line emission hold universally. Strictly speaking, all cores fall below the fiducial T bol = 70 K threshold proposed by André et al (1993), qualifying all sources as Class 0 cores, but as was demonstrated in Ragan et al (2013) and Stutz et al (2013), T bol faithfully follows the evolution within this phase, such that more evolved cores have higher values of T bol .…”

“…The SEDs of cores were revisited in Ragan et al (2013) with the addition of high-resolution (7. 8) SABOCA data at 350 µm.…”

“…The updated properties derived from SED fits are summarised in Table 1. For the calculations of masses and column densities, we use the cold component temperatures of 13, 19 and 15 K for regions 1, 2 and 3, respectively, from Ragan et al (2013).…”

Physical Processes in the Interstellar Medium

“…In addition, high-mass clumps are not observed to be less dense than low-mass ones. If anything, they tend to be denser, and they are typically highly Jeans-unstable (Battersby et al, 2010;Ragan et al, 2012Ragan et al, , 2013Marsh et al, 2014).…”

The Bolocam Galactic Plane Survey. Xiv. Physical Properties of Massive Starless and Star-Forming Clumps

“…The study of low-mass (<10 M e ) starless cores has received considerable attention owingto their proximity and the systematic mapping of nearby molecular clouds in the GouldBelt (Ward- Thompson et al 1994;Tafalla et al 2004;André et al 2007André et al , 2013. In contrast, the study of intermediateand high-mass starless cores and clumps is still in a nascent state owing in part to their greater distances and few systematic observations to identify them (Tackenberg et al 2012;Wilcock et al 2012;Ragan et al 2013;Traficante et al 2015a). Our understanding of bothhow cluster formation is initiated and theensuing protocluster evolutionultimately depends on identifying and constraining the physical properties of representative samples of high-mass starless clumps.…”