Surround and Squash: the impact of superbubbles on the interstellar medium in Scorpius–Centaurus OB2

Abstract

Context. Feedback by massive stars shapes the interstellar medium and is thought to influence subsequent star formation. Details of this process are under debate. Aims. We exploited observational constraints on stars, gas and nucleosynthesis ashes for the closest region of recent massive-star formation, Scorpius-Centaurus OB2, and combined them with 3D hydrodynamical simulations, in order to address physics and history for the case of the Scorpius-Centaurus superbubble. Methods. We used published cold gas observations through PLANCK survey data processing, HERSCHEL and APEX, continuum and molecular line observations. We analysed the Galactic All Sky Survey (GASS) to investigate shell structures in atomic hydrogen, and used HIPPARCOS and Gaia data in combination with interstellar absorption against stars to obtain new constraints for the distance to the Hi features. Hot gas is traced in soft X-rays via the ROSAT all sky survey. Nucleosynthesis ejecta from massive stars were traced with new INTEGRAL spectrometer observations via 26 Al radioactivity. We also performed 3D hydrodynamical simulations for the Sco-Cen superbubble. Results. Soft X-rays and a now more significant detection of 26 Al confirm recent (≈ 1 Myr ago) input of mass, energy and nucleosynthesis ejecta, likely by a supernova in the Upper Scorpius (USco) subgroup. We confirm a large supershell around the entire OB association and perform a 3D hydrodynamics simulations with a conservative massive star population that reproduces the morphology of the superbubble. High resolution GASS observations of a nested supershell reveal that it is filamentary possibly related to the Vishniac clumping instability, but molecular gas (Lupus I) is only present where the shell coincides with the connecting line between the subgroups of the OB association, suggesting a connection to the cloud, probably an elongated sheet, out of which the OB association formed. Stars have formed sequentially in the subgroups of the OB association and currently form in Lupus I. To investigate the impact of massive star feedback on extended clouds, we simulate the interaction of a turbulent cloud with the hot, pressurised gas in a superbubble. The hot gas fills the tenuous regions of the cloud and compresses the denser parts. Stars formed in these dense clumps would have distinct spatial and kinematic distributions. Conclusions. The combined results from observations and simulations are consistent with a scenario where dense gas was initially distributed in a band elongated in the direction now occupied by the OB association. Superbubbles powered by massive stars would then repeatedly break out of the elongated parent cloud, surround and squash the denser parts of the gas sheet and thus induce more star formation. The expected spatial and kinematic distribution of stars is consistent with observations of Sco-Cen. The scenario might apply to many similar regions in the Galaxy and also to AGN-related superbubbles.

Comparing simulated 26Al maps to gamma-ray measurements

“…from Sco-Cen or regions along the spiral arm tangents (e.g. del Rio et al 1996;Diehl et al 2010;Krause et al 2018). We find indications that one of the most-nearby Sco-Cen 26 Al-filled superbubbles may have overrun the Solar System already and thus contribute an omnidirectional emission component (cf.…”

“…We find indications that one of the most-nearby Sco-Cen 26 Al-filled superbubbles may have overrun the Solar System already and thus contribute an omnidirectional emission component (cf. Krause et al 2018). This characteristic seen in the observations, and its relative infrequency in simulated sky maps, suggests that the Milky Way may have a more coarse-grained superbubble structure than modelled in the simulation.…”

Stochastic Chemical Evolution of Radioactive Isotopes with a Monte Carlo Approach

“…Radioactive nuclei with half-lives of the order of a few tens to a few hundreds Myr (short-lived radionuclides, SLRs hereafter) are one of the most promising tools to investigate a variety of current astrophysical topics including ongoing stellar nucleosynthesis (Diehl 2018), the properties of different rapid neutron-capture (r) process sites (Hotokezaka et al 2015), the physics of the interstellar medium (ISM) (Krause et al 2018) and of the local bubble (Feige et al 2018), the formation of the Solar System (Lugaro et al 2018), and the thermo-mechanical evolution of protoplanets (Lichtenberg et al 2019). The peculiarity of radioactive nuclei is that they decay into a daughter nucleus with a characteristic timescale determined by their half-life.…”

“…These heterogeneities, coupled with the fact that radioactive nuclei decay, can result in large SLR abundance fluctuations in different parcels of gas within the Galaxy (Meyer & Clayton 2000;Wasserburg et al 2006). 3D hydrodynamical simulations have been used to follow the heterogeneous evolution of one or two selected SLRs, 26 Al (half-life = 0.72 Myr) and 60 Fe (2.62 Myr), in giant molecular clouds (Vasileiadis et al 2013), star forming regions (Krause et al 2018), and the ISM (Fujimoto et al 2018). While these studies have provided the first best attempts at the problem of heterogeneous evolution of SLRs, their results should be treated with caution because the stellar production of 26 Al and 60 Fe from massive stars winds and core-collapse supernovae, one of the main inputs in these simulations, is still extremely uncertain (see, e.g.…”

“…Note that, within a star-forming region, potential contributions from massive stars into the gas from which new stars are born could also contribute to the SLR abundances. In this case the relevant time intervals need to be modified to take into account, for example, the time that elapses between the birth of different stellar populations within the same star-forming region, and/or the time it may take for material from a particular nearby star or supernova to be injected into a collapsing cloud (see, e.g., Boss 2017;Krause et al 2018). In the case of the actinides such as 244 Pu, however, this scenario is not relevant as we do not expect rare r-process sources to be present within a star-forming region.…”