Numerical Study of Colliding Winds in Massive Stars

We run a numerical experiment ejecting stellar winds in a very massive binary system measuring the properties of the resulting colliding wind structure and accreted mass onto the companion under different conditions. Colliding massive binaries interact and create a colliding wind structure with a shape that depends on the momentum ratio, orbital motion, distance between the stars, and other factors. We run simulations of a static LBV-WR binary and in each simulation abruptly varying the mass loss rate of the LBV from the fiducial value. The modified wind front propagates and interacts with the previous colliding wind structure, and modifies its shape. We calculate the emitted X-ray from the interaction and investigate the proprieties of the new shape. We derive the mass accretion rate onto the secondary, and find that it depends on the momentum ratio of the winds. We then add orbital velocity that reduces the mass accretion rate, a similar behaviour as the analytical estimates based on modified Bondi–Hoyle–Lyttleton. Creating a large set of simulations like those presented here can allow constraining parameters for specific colliding wind binaries and derive their stellar parameters and orbital solution.

Hard X-Ray Irradiation Potentially Drives Negative AGN Feedback by Altering Molecular Gas Properties

To investigate the role of active galactic nucleus (AGN) X-ray irradiation on the interstellar medium (ISM), we systematically analyzed Chandra and Atacama Large Millimeter/submillimeter Array CO (J = 2–1) data for 26 hard X-ray (>10 keV) selected AGNs at redshifts below 0.05. While Chandra unveils the distribution of X-ray-irradiated gas via Fe-Kα emission, the CO (J = 2–1) observations reveal that of cold molecular gas. At high resolutions ≲1″, we derive Fe-Kα and CO (J = 2–1) maps for the nuclear 2″ region and for the external annular region of 2″–4″, where 2″ is ∼100–600 pc for most of our AGNs. First, focusing on the external regions, we find the Fe-Kα emission for six AGNs above 2σ. Their large equivalent widths (≳1 keV) suggest a fluorescent process as their origin. Moreover, by comparing the 6–7 keV/3–6 keV ratio, as a proxy of Fe-Kα, and CO (J = 2–1) images for three AGNs with the highest significant Fe-Kα detections, we find a possible spatial separation. These suggest the presence of X-ray-irradiated ISM and the change in the ISM properties. Next, examining the nuclear regions, we find that (1) the 20–50 keV luminosity increases with the CO (J = 2–1) luminosity; (2) the ratio of CO (J = 2–1)/HCN (J = 1–0) luminosities increases with 20–50 keV luminosity, suggesting a decrease in the dense gas fraction with X-ray luminosity; and (3) the Fe-Kα-to-X-ray continuum luminosity ratio decreases with the molecular gas mass. This may be explained by a negative AGN feedback scenario: the mass accretion rate increases with gas mass, and simultaneously, the AGN evaporates a portion of the gas, which possibly affects star formation.

Simulating the Negative Jet Feedback Mechanism in Common Envelope Jet Supernovae

We use the stellar evolution code MESA to study the negative jet feedback mechanism in common envelope jet supernovae (CEJSNe), in which a neutron star (NS) launches jets in the envelope of a red supergiant (RSG). We find that the feedback reduces the mass accretion rate to be χ j ≃ 0.04–0.3 times the mass accretion rate without the operation of jets. We mimic the effect of the jets on the RSG envelope by depositing the energy that the jets carry into the envelope zones outside the NS orbit. The energy deposition inflates the envelope, therefore reducing the density in the NS vicinity, which in turn reduces the mass accretion rate in a negative feedback cycle. In calculating the above values for the negative jet feedback coefficient (the further reduction in the accretion rate) χ j, we adopt the canonical ratio of jet power to actual accretion power of 0.1, and the results of numerical simulations that show the actual mass accretion rate to be a fraction of 0.1–0.5 of the Bondi–Hoyle–Lyttleton mass accretion rate.

An alternative approach to the relation between accretion luminosity and mass accretion rate

In this paper I introduce and discuss an alternative approach to the relation between accretion luminosity, Lacc, and mass accretion rate, ˙M : instead of the universally adopted Lacc = GM ˙M/R, I propose the dynamical definition Lacc = v2f˙M/2 where vf is the final velocity of the infalling matter at the surface of the accreting object of mass M and radius R. Both definitions are based on the energy conservation, but while the former assumes that matter is in free fall, the latter is valid always. By adopting the alternative form for Lacc, I show that the Eddington luminosity Led, when the outward radiation pressure wins on gravity, is never produced with a finite ˙M. Instead, Led is a limit asymptotically reached when ˙M → ¥. My argument is very simple, so I felt the need to find a possible explanation to why no one arrived to this conclusion before. To this aim, I give a brief presentation of the history of accretion, from the pioneer work of Hoyle and collaborators until the ’60s of last century, to show how the perception of the role of the radiation pressure in accretion evolved. I give also some practical applications of the formulae I derived, in the case of high-mass star formation and of the growth of super massive black holes. The study of these two processes, already complex per se, becomes more difficult to solve because of the existence of a limiting ˙M, named Eddington mass accretion rate or ˙Med, that it is supposed to generate a luminosity equal to Led, making it impossible to accrete at rate ˙M > ˙Med. Accretion rates higher than ˙Med are however necessary, as theory and observations show. My definition of Lacc takes naturally into account the work done by radiation pressure to slow down the infalling matter: as a consequence, Lacc does not increase linearly with ˙M and Led is only an asymptotic value.

The splashback boundary of haloes in hydrodynamic simulations

The splashback radius, Rsp, is a physically motivated halo boundary that separates infalling and collapsed matter of haloes. We study Rsp in the hydrodynamic and dark matter only IllustrisTNG simulations. The most commonly adopted signature of Rsp is the radius at which the radial density profiles are steepest. Therefore, we explicitly optimise our density profile fit to the profile slope and find that this leads to a $\sim 5\%$ larger radius compared to other optimisations. We calculate Rsp for haloes with masses between 1013 − 15M⊙ as a function of halo mass, accretion rate and redshift. Rsp decreases with mass and with redshift for haloes of similar M200m in agreement with previous work. We also find that Rsp/R200m decreases with halo accretion rate. We apply our analysis to dark matter, gas and satellite galaxies associated with haloes to investigate the observational potential of Rsp. The radius of steepest slope in gas profiles is consistently smaller than the value calculated from dark matter profiles. The steepest slope in galaxy profiles, which are often used in observations, tends to agree with dark matter profiles but is lower for less massive haloes. We compare Rsp in hydrodynamic and N-body dark matter only simulations and do not find a significant difference caused by the addition of baryonic physics. Thus, results from dark matter only simulations should be applicable to realistic haloes.