Siddhartha Gupta

Star clusters are one of the most energetic sources in a galaxy. They emit wind and radiation which are so energetic that they can expel gas from their vicinity. Observers have found such gas expulsion, however, the driving mechanism was not clear.

We have studied this problem by considering the time evolution of a star cluster. We use a realistic Initial Mass Function (IMF) and include stellar winds (source of mechanical energy) and radiation in our analysis. After ~3 Myr when one-by-one massive stars die through supernova explosion, they also provide mechanical energy. However, the radiation power decreases with time. Therefore, the contribution from wind and radiation depend on the age of the star cluster. Using 1D numerical simulations, we show that radiation pressure can push the gas before 1 Myr. After 1 Myr, the expansion is controlled by mechanical power and radiation heating. Movie 1, shown here, displays the time evolution of various profiles due to interactions of stellar wind and radiation with the ambient medium.

Ref:
Gupta et al. (2016) MNRAS, 462, 4532; [NASA/ADS]; MNRAS; arXiv

Winds and supernovae drive shocks and produce all necessary conditions to accelerate high energy particles (e.g. Cosmic rays (CRs)). Despite the observations over a century, there are many aspects of CRs that are still unknown, e.g. from where do they come from? are they accelerated in supernova shocks or in stellar winds? To understand this problem, we have developed an analytical model and performed 1D and 3D simulations. We have calculated various observables which will be useful to understand high energy activity in star clusters.

Paper I: We develop a two-fluid model of an interstellar bubble (ISB) where cosmic rays are considered as the second fluid. We show that diffusive shock acceleration can be an effective process in changing the thermal properties of an ISB. Movie 2 shows the time evolution of thermal and CR pressure profiles in a two-fluid interstellar bubble. The enhancement of CR pressure at the reverse shock (wind termination shock) represents the diffusive shock acceleration. Once the CR pressure becomes larger than thermal pressure, the shock becomes very smooth representing the globally smooth solution of the two-fluid model. This simulation is performed using a code which is written by me. Details of the code can be found here.

Paper II: In continuation with Paper I, we estimate gamma-ray, x-ray, and radio (thermal and nonthermal) luminosities and compare them with observations. In this study, we show how a compact star cluster can produce diffuse gamma-ray. We also discuss the role of thermal and non-thermal radio, and X-rays in constraining the high energy activity in young star clusters. Movie 3 displays the surface brightness map of an ISB in multi-waveband.
Ref:
1. Gupta et al. (2018) MNRAS, 473, 1537; [NASA/ADS]; MNRAS; arXiv
2. Gupta et al (2018) MNRAS 479, 5220; [NASA/ADS]; MNRAS; arXiv

Description to be added.
Ref.
1. Gupta et al (2020) MNRAS 493, 3159; [NASA/ADS]; MNRAS; arXiv

Description to be added.
Ref.
1. Gupta et al (2020); [NASA/ADS]; MNRAS; arXiv

My MSc thesis was focused on the applications of a modified Newtonian gravity in various astrophysical objects. This study involved a phenomenological approach which combines strong field and weak field gravities. With my advisor, Prof. Sayan Kar at IIT Kgp, we investigated the non-singular collapse of gas cloud and galaxy rotation curve.
Ref.
1. Gupta, S. 2014, MScThesis, IIT Kharagpur.