How Type Ia Supernovae Explosions Affect The Formation Of Second Generation Stars? (Cosmology)

By using 3D hydrodynamic simulations, Lacchin and colleages investigated how Type Ia Supernovae (SNe) explosions affect the star formation history and the chemical properties of second generation (SG) stars in globular clusters. They showed that gas from Type Ia SNe might have played a key role in the formation of second generation stars. Their study recently appeared in Arxiv.

Second Generation stars are assumed to form once first generation (FG) AGB stars start releasing their ejecta; during this phase, external gas is accreted by the system and SNe Ia begin exploding, carving hot and tenuous bubbles.

Recent work of Elena Lacchin et al. is inspired by D’Ercole et al. and Calura et al. Focusing on AGB scenario, D’Ercole et al. showed that the gas ejected by AGBs collects in a cooling flow towards the cluster core and later condenses forming SG stars, which are therefore more centrally concentrated than the FG ones. While, Calura et al. concluded that the most He-rich SG stars were concentrated in more central regions than the less enriched ones. However, in this study the feedback of Type Ia SNe was not included. Now, Elena Lacchin and colleagues intend to verify whether stellar feedback from Type Ia SNe of the FG could have the same effect in denser systems like Galactic Globular clusters (GCs).

“We for the first time using series of 3D hydrodynamic simulations explored the effects of the feedback of SN Ia explosions on the duration of the SG star formation phase and on the chemical properties of the SG stars.”

Given the large uncertainties still present on the timing of Type Ia SN explosions, they have considered two delay time distributions: firstly, they have assumed that SNe Ia start exploding and releasing gas at the beginning of the SG formation epoch together with the most massive AGBs; then, they have performed a simulation, in the low-density scenario, where they have delayed the beginning of the SN Ia by 25 Myr. Finally, they ran a simulation in which cooling is switched off locally, to assess the possible effects of the cooling overestimation in high density regions, a typical artificial phenomenon in numerical simulations.

© Lacchin et al.

In the low-density scenario with short delay time, they found that the explosions start at the beginning of the SG star formation, halting it in its earliest phases. The external gas hardly penetrates the system, therefore most SG stars present extreme helium abundances (Y > 0.33).

The low-density model with delayed SN explosions has a more extended SG star formation epoch and includes SG stars with modest helium enrichment.

Figure 1. Upper panel: evolution of the SG stellar mass for all their simulations. Lower panel: evolution of the SFR for all their models. The blue lines represent the standard low-density models while the red lines the high-density ones. The purple line in the upper panel represents the m result of the low-density model at high resolution (LD_HR). The orange lines represent the high-density model with delayed cooling while the light-blue lines represent the low-density model with delayed Type Ia SNe. The green dashed lines represent the low density model where the infall is switched off once the first SN bubble have reached the negative 𝑥 boundary. © Lacchin et al.

On the contrary, the high-density model is weakly affected by SN explosions, with a final SG mass similar to the one obtained without SNe Ia. Most of the stars form from a mix of AGB ejecta and pristine gas and have a modest helium enrichment. They showed that gas from SNe Ia may produce an iron spread of ∼ 0.14 dex, consistent with the spread found in about 20% of Galactic GCs, suggesting that SNe Ia might have played a key role in the formation of this sub-sample of GCs.

“Our simulations have shown that the SG formation can continue during the Type Ia SNe epoch and the ejecta of these supernovae can contribute to the mix of gas out of which SG stars form. The formation of SG stars from mixed gas of SN Ia ejecta and AGB gas revealed by our study indicates a possible avenue for the formation of Galactic GC with multiple population characterized by a spread in iron; these clusters represent about 20 per cent of the Galactic globular cluster system and have been shown to host stellar populations enriched also in s-process elements which could be produced by low-mass AGBs.”

— they added.

They will further extend the initial set of simulations presented in this paper to study a broader range of initial conditions and explore the possible dynamics and formation history of these complex clusters.

Reference: E. Lacchin, F. Calura, E. Vesperini, “On the role of Type Ia supernovae in the second generation star formation in globular clusters”, Arxiv, pp. 1-19, 2021.

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