Theoretical astrophysics and plasma physics at RPC

Abstract:

Fragmentation often occurs in disk-like structures, both in the early Universe and in the context of present-day star formation. Supermassive black holes (SMBHs) are astrophysical objects whose origin is not well understood; they weigh millions of solar masses and reside in the centers of galaxies. An important formation scenario for SMBHs is based on collisions and mergers of stars in a massive cluster with a high stellar density, in which the most massive star moves to the center of the cluster due to dynamical friction. This increases the rate of collisions and mergers since massive stars have larger collisional cross sections. This can lead to a runaway growth of a very massive star which may collapse to become an intermediate-mass black hole. Here we investigate the dynamical evolution of Miyamoto-Nagai models that allow us to describe dense stellar clusters, including flattening and different degrees of rotation. We find that the collisions in these clusters depend mostly on the number of stars and the initial stellar radii for a given radial size of the cluster. By comparison, rotation seems to affect the collision rate by at most 20%. For flatness, we compared spherical models with systems that have a scale height of about 10% of their radial extent, in this case finding a change in the collision rate of less than 25%. Overall, we conclude that the parameters only have a minor effect on the number of collisions. Our results also suggest that rotation helps to retain more stars in the system, reducing the number of escapers by a factor of 2-3 depending on the model and the specific realization. After two million years, a typical lifetime of a very massive star, we find that about 630 collisions occur in a typical models with N = 104, R = 100 Rpdbl and a half-mass radius of 0.1 pc, leading to a mass of about 6.3 × 103 Mpdbl for the most massive object. We note that our simulations do not include mass loss during mergers or due to stellar winds. On the other hand, the growth of the most massive object may subsequently continue, depending on the lifetime of the most massive object.

Abstract:

We search for an indirect signal of dark matter through very high-energy γ rays from the Wolf-Lundmark-Melotte (WLM) dwarf irregular galaxy. The pair annihilation of dark matter particles would produce Standard Model particles in the final state such as γ rays, which might be detected by ground-based Cherenkov telescopes. Dwarf irregular galaxies represent promising targets as they are dark matter dominated objects with well-measured kinematics and small uncertainties on their dark matter distribution profiles. In 2018, the five-telescopes of the high energy stereoscopic system observed the dwarf irregular galaxy WLM for 18 hours. We present the first analysis based on data obtained from an imaging atmospheric Cherenkov telescope for this subclass of dwarf galaxy. As we do not observe any significant excess in the direction of WLM, we interpret the result in terms of constraints on the velocity-weighted cross section for dark matter pair annihilation «σv» as a function of the dark matter particle mass for various continuum channels, as well as the prompt γγ emission. For the τ+τ- channel, the limits reach a «σv» value of about 4×10-22 cm3 s-1 for a dark matter particle mass of 1 TeV. For the prompt γγ channel, the upper limit reaches a «σv» value of about 5×10-24 cm3 s-1 for a mass of 370 GeV. These limits represent an improvement of up to a factor 200, with respect to previous results for the dwarf irregular galaxies for TeV dark matter search.

Abstract:

The observations of high-redshifts quasars at z ≳ 6 have revealed that supermassive black holes (SMBHs) of mass ∼109M⊙∼109M⊙ were already in place within the first ∼Gyr after the big bang. Supermassive stars (SMSs) with masses 103−5M⊙103−5M⊙ are potential seeds for these observed SMBHs. A possible formation channel of these SMSs is the interplay of gas accretion and runaway stellar collisions inside dense nuclear star clusters (NSCs). However, mass-loss due to stellar winds could be an important limitation for the formation of the SMSs and affect the final mass. In this paper, we study the effect of mass-loss driven by stellar winds on the formation and evolution of SMSs in dense NSCs using idealized N-body simulations. Considering different accretion scenarios, we have studied the effect of the mass-loss rates over a wide range of metallicities Z* = [.001–1]Z⊙ and Eddington factors fEdd=L∗/LEdd=0.5,0.7,and0.9fEdd=L∗/LEdd=0.5,0.7,and0.9⁠. For a high accretion rate of 10−4M⊙yr−110−4M⊙yr−1⁠, SMSs with masses ≳103M⊙yr−1≳103M⊙yr−1 could be formed even in a high metallicity environment. For a lower accretion rate of 10−5M⊙yr−110−5M⊙yr−1⁠, SMSs of masses ∼103−4M⊙∼103−4M⊙ can be formed for all adopted values of Z* and fEdd, except for Z* = Z⊙ and fEdd = 0.7 or 0.9. For Eddington accretion, SMSs of masses ∼103M⊙∼103M⊙ can be formed in low metallicity environments with Z* ≲ 0.01 Z⊙. The most massive SMSs of masses ∼105M⊙∼105M⊙ can be formed for Bondi–Hoyle accretion in environments with Z* ≲ 0.5 Z⊙. An intermediate regime is likely to exist where the mass-loss from the winds might no longer be relevant, while the kinetic energy deposition from the wind could still inhibit the formation of a very massive object.