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Trojan horses within molecular clouds: How do a few massive stars determine the fate of a whole galaxy?

Out of hundreds of new-born stars in a star-forming galaxy only a handful have a mass exceeding 8 solar masses. Nevertheless, these massive stars are special as they can determine the fate of a whole galaxy. Each massive star is acting like a Trojan horse disrupting its parental molecular cloud from within. Using high-resolution computer simulations, scientists at the Max Planck Institute for Astrophysics have shown how the ionising UV radiation from a single massive star initiates the dispersal of the surrounding molecular cloud. When the star subsequently explodes as a Supernova the gas is further accelerated and eventually expelled from the galaxy, making it unavailable for future star formation. These feedback processes are likely to regulate the efficiency of galaxy-wide star formation in the Universe. The team has been awarded over 40 million CPU hours on SuperMUC, Europe's fastest super-computer, to simulate the whole life cycle of molecular clouds - from assembly and star formation to feedback and dispersal - in a galactic environment. For the first time it will be possible to directly follow the impact of feedback from massive stars from the sites of star formation on milli-parsec scales to kilo-parsec, galactic, scales in unprecedented detail.

Fig. 1: 3D simulation of an HII region (a largely ionised molecular gas cloud) expanding into a molecular cloud with 10,000 solar masses. The colouring indicates the column density along the projected direction. Black dots are new stars, which form in these self-gravitating simulations.

Fig. 2: Temperature profiles, showing the impact of a Supernova explosion in a 10 times more massive molecular cloud (100,000 solar masses). The three panels show from top to bottom: a) the adiabatic case (without radiative cooling); b) the case with radiative cooling; c) the full model, in which the cloud has been affected by ionising radiation even before the Supernova explodes.

Fig. 3: The SILCC project aims to capture the full life cycle of a molecular cloud in one simulation. The panels show the individual steps from cloud assembly to star formation, feedback and driving of large-scale galactic outflows. Each step has been studied in an individual simulation by members of the SILCC team.

Less than 1 % of all new-born stars have a mass of more than 8 solar masses by the time nuclear burning in their centres is initiated and they start to shine. Both, the life and death of a massive star are intriguingly different and much more exciting than those of a low mass star, such as our Sun. During its lifetime, a massive star heavily affects its parental gas cloud -- mostly consisting of cold (10 Kelvin), molecular gas -- by strong UV radiation and a fast stellar wind. Due to the emitted UV radiation, the surrounding molecular cloud is quickly ionised and heated to about 10,000 Kelvin, and a so-called HII region is formed. The hot, ionised bubble expands into the cold, turbulent environment, thereby sweeping up more and more gas and possibly triggering new star formation. The massive star exhausts its fuel fairly rapidly, burning for only a few million years. In death, it explodes as a Supernova Type II and releases an enormous amount of energy, which further accelerates and heats the surrounding gas to up to 100 million Kelvin.

Even though massive stars are rare, they are most important for galaxy formation and evolution. They represent the main source of stellar feedback energy and are able to destroy molecular clouds from within, thus regulating the star formation efficiency in the galaxy. Moreover, their feedback, and in particular their death in form of a Supernova explosion, may even drive large-scale galactic winds and outflows. Gas which is driven out of the galaxy by this process might ultimately be unavailable for new star formation.

Scientists at MPA study the dispersal of molecular clouds by UV radiation (see Fig. 1) and Supernova explosions of massive stars (see Fig. 2) in highly resolved, three-dimensional computer simulations. They show that relatively small molecular clouds with a mass of 10,000 solar masses may easily be disrupted by ionising radiation alone long before the star explodes as a Supernova. However, the disruption of more massive molecular clouds requires more drastic measures. While the initial ionising feedback due to radiation is still an essential ingredient when modelling the disruption of high-mass clouds, only Supernovae are actually able to disrupt clouds with 100,000 to 1 million solar masses. Modelling the stellar feedback involves complex, non-linear cooling processes in the interstellar medium. This means that the Supernova explosion is much more efficient when taking place in a pre-ionised, low-density HII region. By performing simulations of Supernova explosions in clouds with and without previous ionisation, the scientists are able to quantify how much the efficiency improves. In fact, for simulations with previous ionisation and cooling the results are remarkably close to the ideal and well-studied Sedov explosion case, in which the cloud is not at all allowed to cool radiatively. This result is very important for correctly estimating the impact of feedback in the interstellar medium.

Understanding how this feedback propagates over more than six orders of magnitude in spatial scale, from milli-parsec scales, at the sites of massive star formation, to galactic kilo-parsec scales, is a computationally challenging quest. The team is now ready to set the next milestone in performing deeply resolved SImulations of the whole LifeCycle of a molecular Cloud (SILCC-project). They have been awarded more than 40 million CPU hours on SuperMUC, the new 3 petaflop supercomputer, which has just been launched at the Leibniz-Rechenzentrum Garching. Currently, linkPfeilExtern.gifSuperMUC is Europe's fastest super-computer and number four in the known universe. The SILCC project will shed light into the intricate impact of massive stars, from molecular cloud assembly, over star formation and feedback, to gas ejection from the galactic disk (see Fig. 3). These complex three-dimensional simulations will involve a multitude of physical effects that, to date, have not yet been included in a single computation. Investigating all of these processes at the same time will give detailed insight about how feedback from massive stars can regulate the star formation efficiency in the galaxies.

Stefanie Walch, Thorsten Naab

Original publications:

Walch, S.K.; Whitworth, A.P.; Bisbas, T.; W√ľnsch, R., Hubber, D., "Dispersal of molecular clouds by ionising radiation", accepted for publication in MNRAS (2012); linkPfeilExtern.gifarXiv1206.6492

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