The physical and chemical structure of Sagittarius B2

Abstract

Context. Sagittarius B2 (north) is a chemically rich, high-mass star-forming region located within the giant molecular cloud complex Sgr B2 in the central molecular zone of our Galaxy. Dust continuum emission at 242 GHz, obtained at high angular resolution with the Atacama Large Millimeter Array (ALMA), reveals that it has a filamentary structure on scales of 0.1 pc. Aims. We aim to characterize the filamentary structure of Sgr B2(N) and its kinematic properties using multiple molecular dense gas tracers. Methods. We have used an unbiased, spectral line-survey that covers the frequency range from 211 to 275 GHz and obtained with ALMA (angular resolution of 0.′′4, or 3300 au) to study the small-scale structure of the dense gas in Sgr B2(N). In order to derive the kinematic properties of the gas in a chemically line-rich source like Sgr B2(N), we have developed a python-based tool that stacks all the detected line transitions of any molecular species. This allows us to increase the signal-to-noise ratio (S/N) of our observations and average out line blending effects, which are common in line-rich regions. Results. A filamentary network is visible in Sgr B2(N) in the emission maps of the molecular species CH3OCHO, CH3OCH3, CH3OH and H2CS. In total, eight filaments are found that converge to the central hub (with a mass of 2000 M⊙, assuming a temperature of 250 K) and extending for about 0.1 pc (up to 0.5 pc). The spatial structure, together with the presence of the massive central region, suggest that these filaments may be associated with accretion processes, transporting material from the outer regions to the central dense hub. We derive velocity gradients along the filaments of about 20–100 km s−1 pc−1, which are 10–100 times larger than those typically found on larger scales (~1 pc) in other star-forming regions. The mass accretion rates of individual filaments are ≾0.05 M⊙ yr−1, which result in a total accretion rate of 0.16 M⊙ yr−1. Some filaments harbor dense cores that are likely forming stars and stellar clusters. We determine an empirical relation between the luminosity and stellar mass of the clusters. The stellar content of these dense cores is on the order of 50% of the total mass. The timescales required for the dense cores to collapse and form stars, exhausting their gas content, are compared with the timescale of their accretion process onto the central hub. We conclude that the cores may merge in the center when already forming stellar clusters but still containing a significant amount of gas, resulting in a “damp” merger. Conclusions. The high density and mass of the central region, combined with the presence of converging filaments with high mass, high accretion rates and embedded dense cores already forming stars, suggest that Sgr B2(N) may have the potential to evolve into a super stellar cluster.

Exploring molecular complexity with ALMA (EMoCA): complex isocyanides in Sgr B2(N)

“…This sort of structure has been evidenced by the aforementioned recent detection of multiple new hot cores in Sgr B2(N) (Bonfand et al 2017), as well as recent studies of Orion KL, to name one other hot-core source (Wright & Plambeck 2017). In addition to this, the structure of the cloud is more complicated than we assume here, with evidence of filaments that converge toward the main hot core, as suggested recently by Schwörer et al (2019).…”

Sulphur and carbon isotopes towards Galactic centre clouds

“…The GC region harbours one of the most intense and luminous sites of massive star formation in the Galaxy, Sgr B2 (Molinari et al 2014;Ginsburg et al 2018). It provides an extreme environment in terms of pressure, turbulent Mach number, and gas temperature (Ginsburg et al 2016) over a much more extended region than encountered in star-forming regions throughout the Galactic disc (Morris & Serabyn 1996;Ginsburg et al 2016;Schwörer et al 2019;Dale et al 2019). These conditions are comparable to those in starburst galaxies (Belloche et al 2013;Schwörer et al 2019).…”

“…It provides an extreme environment in terms of pressure, turbulent Mach number, and gas temperature (Ginsburg et al 2016) over a much more extended region than encountered in star-forming regions throughout the Galactic disc (Morris & Serabyn 1996;Ginsburg et al 2016;Schwörer et al 2019;Dale et al 2019). These conditions are comparable to those in starburst galaxies (Belloche et al 2013;Schwörer et al 2019). We therefore expect unique results in this GC study from sulphur ratios, which are a tool for tracing stellar processing (see Sect.…”

“…This massive star-forming region harbours five hot cores, namely Sgr B2(N1-N5), with kinetic temperatures ranging from ∼130 to 150 K for N3-N5, between 150 and 200 K for N2, and between 160 and 200 K for N1, assuming LTE conditions in all these cases (Belloche et al 2016(Belloche et al , 2019Bonfand et al 2017). In addition, there are 20 ∼1.3 mm continuum sources associated with dense clouds in Sgr B2(N) that exhibit a rich chemistry (Sánchez-Monge et al 2017;Schwörer et al 2019). Recently, Ginsburg et al (2018) detected 271 compact continuum sources at ∼3 mm in the extended Sgr B2 cloud, thought to be high-mass protostellar cores, representing the largest cluster of high-mass young stellar objects reported to date in the Galaxy.…”

“…Some measurements using the double-isotope ratio method (see Sect. 4.1.1), which take the 12 C/ 13 C ratio into account, may induce a bias since CS may show a non-negligible fractionation into 13 C. There is no specific study of the 13 C fractionation of CS, but the reactivity of C + and C, in particular with CO and CN (but also with CS), can induce an enrichment or depletion in 13 C of carbonaceous species including CS (Smith & Adams 1980;Roueff et al 2015). In that case, the good agreement between the sulphur fractionation measurements using C 32 S/C 34 S and the values obtained using the double-isotope method also suggests a low 13 C fractionation of CS.…”