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Expanding bubbles in SILCC-Zoom simulations

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  • Figure 1: Synthetic emission maps of [CII] of four simulated molecular clouds showing expanding bubbles due to stellar feedback. Stars are represented with different colors and sizes according to their age and temperature. The emission maps are here the result of radiative transfer calculations performed with the original C+ abundance obtained from the simulations, which yields to a relative large line intensity in the inner part of the expanding bubbles.
  • Figure 2: Same as Figure 1, but in this case the C+ is the result of the post-processing tool that we implemented. The bubbles are now considerably darker in [CII], which is in better agreement with real observations, and structures like pillars are visible.

Stefano Ebagezio

Stellar wind and stellar radiation produced by recently formed stars are capable of deeply modify the characteristics of the region surrounding the star-forming region. In particular, two relevant effects are the dispersion of the dense and cold regions which surround the young stars and the ionisation of the gas present in these regions. This yields to the formations of bubbles, which expand with time, containing mostly ionised, hot gas in the inner part, and cold, atomic or molecular gas in the outer part.

We analyse several simulated molecuar clouds within the SILCC-Zoom Project (Seifried et al. 2017), which include stellar formation and stellar radiation, as well as a chemical network simulating the evolution of CO, C+, and hydrogen. We produce synthetic [CII] observations of such clouds by using the RADMC-3D radiative transfer code.

We observe the expanding bubbles in our simulated clouds, too. Figure 1 shows a sample of synthetic [CII] maps of such clouds, and these bubbles, characterized by a roughly circular shape around the star-forming regions and very [CII]-bright rims are clearly visible.

These synthetic observations present, however, qualitative differences with respect to real observations of expanding bubbles in [CII]. The most relevant is that such bubbles are expected to be quite dark in [CII] in the inner part, which means that they should be devoid of C+ there. This is not the case for our bubbles, because the chemical network included in our simulations does not include C+ and the other highly-ionised states of carbon. This is actually important as the stellar radiation is capable of further ionising C+ and, therefore, of producing the [CII]-dark areas withing the bubbles.

In order to reproduce this effect, we build a series of 1D PDR models using the CLOUDY software and then post-process our simulations, updating the carbon abundances according to such models. Then, we do the radiative transfer again to obtain the new synthetic emission maps. The result is shown in Figure 2, where we show the same bubbles after the post-process.

After this post-process, the [CII] luminosity inside the bubbles is considerably lower than before and the rims of the bubbles are the brightest [CII] regions. Furthermore, other typical elements of such regions, like pillars, are now visible. We conclude that applying this post-processing tool is of great importance for obtaining realistic [CII] synthetic maps of regions with stellar feedback and for comparing them with real observations.