The earliest phases of high-mass star formation, as seen in NGC 6334 by Herschel-HOBYS

Abstract

Aims. To constrain models of high-mass star formation, the Herschel-HOBYS key program aims at discovering massive dense cores (MDCs) able to host the high-mass analogs of low-mass prestellar cores, which have been searched for over the past decade. We here focus on NGC 6334, one of the best-studied HOBYS molecular cloud complexes. Methods. We used Herschel/PACS and SPIRE 70−500 µm images of the NGC 6334 complex complemented with (sub)millimeter and mid-infrared data. We built a complete procedure to extract ∼0.1 pc dense cores with the getsources software, which simultaneously measures their far-infrared to millimeter fluxes. We carefully estimated the temperatures and masses of these dense cores from their spectral energy distributions (SEDs). We also identified the densest pc-scale cloud structures of NGC 6334, one 2 pc × 1 pc ridge and two 0.8 pc × 0.8 pc hubs, with volume-averaged densities of ∼10 5 cm −3 . Results. A cross-correlation with high-mass star formation signposts suggests a mass threshold of 75 M for MDCs in NGC 6334. MDCs have temperatures of 9.5−40 K, masses of 75−1000 M , and densities of 1 × 10 5 −7 × 10 7 cm −3 . Their mid-infrared emission is used to separate 6 IR-bright and 10 IR-quiet protostellar MDCs while their 70 µm emission strength, with respect to fitted SEDs, helps identify 16 starless MDC candidates. The ability of the latter to host high-mass prestellar cores is investigated here and remains questionable. An increase in mass and density from the starless to the IR-quiet and IR-bright phases suggests that the protostars and MDCs simultaneously grow in mass. The statistical lifetimes of the high-mass prestellar and protostellar core phases, estimated to be 1−7 × 10 4 yr and at most 3 × 10 5 yr respectively, suggest a dynamical scenario of high-mass star formation. Conclusions. The present study provides good mass estimates for a statistically significant sample, covering the earliest phases of high-mass star formation. High-mass prestellar cores may not exist in NGC 6334, favoring a scenario presented here, which simultaneously forms clouds, ridges, MDCs, and high-mass protostars.

APEX observations of ortho-H2D+ towards dense cores in the Orion B9 filament

“…The process of low-mass star formation is characterised by at least an order of magnitude longer (and colder) starless core phase compared to that of high-mass stars (e.g. Brünken et al 2014;Tigé et al 2017), which supports the development of matured deuterium chemistry in low-mass dense cores.…”

KFPA Examinations of Young STellar Object Natal Environments (KEYSTONE): Hierarchical Ammonia Structures in Galactic Giant Molecular Clouds

“…This sample of molecular cloud complexes presented in Motte et al (2018a) (see also Schneider et al (2011)) gives a complete view of high-mass star formation at distances less than 3 kpc. This sample notably contains the Cygnus X molecular complex (Hennemann et al 2012;Schneider et al 2016), the M16/M17 complex (Hill et al 2012b;Tremblin et al 2013Tremblin et al , 2014, the Monoceres complex (Didelon et al 2015;Rayner et al 2017), Rosette Di Francesco et al 2010;Schneider et al 2010bSchneider et al , 2012, W48 (Nguyen Luong et al 2011;Rygl et al 2014), the W3/KR140 complex (Rivera-Ingraham et al 2013, NGC7538 (Fallscheer et al 2013) plus southern regions not presented here (Hill et al 2012a;Minier et al 2013;Tigé et al 2017). Thus, KEYSTONE provides the kinematic counterpart to the HOBYS survey that is required to understand the relationship between dense gas dynamics and massive stars.…”

Probing fragmentation and velocity sub-structure in the massive NGC 6334 filament with ALMA

“…If β = 1.5 instead of β = 2, the core masses would be a factor of ∼3 lower than the values listed in Table 2. For most sources, we adopted T dust = 20 K which corresponds to the median dust temperature derived from Herschel data along the crest of the filament Tigé et al 2017). For source ID 1, we adopted a higher temperature value (T dust = 50 K) as this object coincides with a bright Spitzer 8 µm source and is most likely an internallyheated, relatively massive protostellar core.…”

“…and G, ρ clump , σ N 2 H + , T k , m, and m N 2 H + are the gravitational constant, the density of each ArTéMiS clump (see Table 4), the velocity dispersion measured in N 2 H + , the gas kinetic temperature, the mean molecular mass, and the mass of the N 2 H + molecule, respectively. Assuming a gas kinetic temperature of 20 K, which corresponds to the median dust temperature derived from Herschel data along the crest of the filament Tigé et al 2017) and using the velocity dispersion measured in N 2 H + for each ArTéMiS clump (σ N 2 H + = δV FWHM / √ 8 ln (2), where δV FWHM is the FWHM linewidth), the effective Jeans length in the clumps of the filament is estimated to range from ∼0.04 pc to ∼0.3 pc (median: 0.08 pc). This is roughly consistent with the typical projected separation between ALMA dense cores (0.04±0.01 pc) assuming random projection effects within the ArTéMiS clumps.…”

“…Our recent APEX/ArTéMiS 350 µm study of the massive star-forming complex NGC 6334 showed that the main filament of the complex has an observed line mass of ∼1000 M /pc, consistent within uncertainties with the estimated virial mass per unit length ∼500 M /pc, and an inner width ∼ 0.15 ± 0.05 pc all along its length , within ∼50% of the typical inner filament width observed with Herschel in nearby clouds . NGC 6334 is a very active star-forming region at a distance d ∼ 1.7 kpc, which contains 150 OB stars and more than 2000 young stellar objects (Persi & Tapia 2008;Russeil et al 2013;Willis et al 2013;Tigé et al 2017), for a total gas mass of ∼2.2×10 5 M . Here, thanks to the high angular resolution and sensitivity of Atacama Large Millimeter Array (ALMA) data at 3 mm, we investigate the density and velocity sub-structure of the massive NGC 6334 filament and compare the results with those obtained for nearby, lower-mass star-forming filaments.…”