Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userTwo Wolf-Rayet stars at the heart of colliding-wind binary Apep
95
0
0.0
(
0
)
Added by
Joseph Callingham Dr
Publication date
2020
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
Infrared imaging of the colliding-wind binary Apep has revealed a spectacular dust plume with complicated internal dynamics that challenges standard colliding-wind binary physics. Such challenges can be potentially resolved if a rapidly-rotating Wolf-Rayet star is located at the heart of the system, implicating Apep as a Galactic progenitor system to long-duration gamma-ray bursts. One of the difficulties in interpreting the dynamics of Apep is that the spectral composition of the stars in the system was unclear. Here we present visual to near-infrared spectra that demonstrate that the central component of Apep is composed of two classical Wolf-Rayet stars of carbon- (WC8) and nitrogen-sequence (WN4-6b) subtypes. We argue that such an assignment represents the strongest case of a classical WR+WR binary system in the Milky Way. The terminal line-of-sight wind velocities of the WC8 and WN4-6b stars are measured to be $2100 pm 200$ and $3500 pm 100$ km s$^{-1}$, respectively. If the mass-loss rate of the two stars are typical for their spectral class, the momentum ratio of the colliding winds is expected to be $approx$ 0.4. Since the expansion velocity of the dust plume is significantly smaller than either of the measured terminal velocities, we explore the suggestion that one of the Wolf-Rayet winds is anisotropic. We can recover a shock-compressed wind velocity consistent with the observed dust expansion velocity if the WC8 star produces a significantly slow equatorial wind with a velocity of $approx$530 km s$^{-1}$. Such slow wind speeds can be driven by near-critical rotation of a Wolf-Rayet star.
rate research
Read More
The recent discovery of a spectacular dust plume in the system 2XMM J160050.7-514245 (referred to as Apep) suggested a physical origin in a colliding-wind binary by way of the Pinwheel mechanism. Observational data pointed to a hierarchical triple-star system, however several extreme and unexpected physical properties seem to defy the established physics of such objects. Most notably, a stark discrepancy was found in the observed outflow speed of the gas as measured spectroscopically in the line-of-sight direction compared to the proper motion expansion of the dust in the sky plane. This enigmatic behaviour arises at the wind base within the central Wolf-Rayet binary: a system that has so far remained spatially unresolved. Here we present an updated proper motion study deriving the expansion speed of Apeps dust plume over a two-year baseline that is four times slower than the spectroscopic wind speed, confirming and strengthening the previous finding. We also present the results from high-angular-resolution near-infrared imaging studies of the heart of the system, revealing a close binary with properties matching a Wolf-Rayet colliding-wind system. Based on these new observational constraints, an improved geometric model is presented yielding a close match to the data, constraining the orbital parameters of the Wolf-Rayet binary and lending further support to the anisotropic wind model.
The Wolf-Rayet (WR) phenomenon is widespread in astronomy. It involves classical WRs, very massive stars (VMS), WR central stars of planetary nebula CSPN [WRs], and supernovae (SNe). But what is the root cause for a certain type of object to turn into an emission-line star? In this contribution, I discuss the basic aspects of radiation-driven winds that might reveal the ultimate difference between WR stars and canonical O-type stars. I discuss the aspects of (i) self-enrichment via CNO elements, (ii) high effective temperatures Teff, (iii) an increase in the helium abundance Y, and finally (iv) the Eddington factor Gamma. Over the last couple of years, we have made a breakthrough in our understanding of Gamma-dependent mass loss, which will have far-reaching consequences for the evolution and fate of the most massive stars in the Universe. Finally, I discuss the prospects for studies of the WR phenomenon in the highest redshift Ly-alpha and He II emitting galaxies.
WR 125 is considered as a Colliding Wind Wolf-rayet Binary (CWWB), from which the most recent infrared flux increase was reported between 1990 and 1993. We observed the object four times from November 2016 to May 2017 with Swift and XMM-Newton, and carried out a precise X-ray spectral study for the first time. There were hardly any changes of the fluxes and spectral shapes for half a year, and the absorption-corrected luminosity was 3.0e+33 erg/s in the 0.5 - 10.0 keV range at a distance of 4.1 kpc. The hydrogen column density was higher than that expected from the interstellar absorption, thus the X-ray spectra were probably absorbed by the WR wind. The energy spectrum was successfully modeled by a collisional equilibrium plasma emission, where both the plasma and the absorbing wind have unusual elemental abundances particular to the WR stars. In 1981, the Einstein satellite clearly detected X-rays from WR 125, whereas the ROSAT satellite hardly detected X-rays in 1991, when the binary was probably around the periastron passage. We discuss possible causes for the unexpectedly low soft X-ray flux near the periastron.
Vigorous mass loss in the classical Wolf-Rayet (WR) phase is important for the late evolution and final fate of massive stars. We develop spherically symmetric time-dependent and steady-state hydrodynamical models of the radiation-driven wind outflows and associated mass loss from classical WR stars. The simulations are based on combining the opacities typically used in static stellar structure and evolution models with a simple parametrised form for the enhanced line-opacity expected within a supersonic outflow. Our simulations reveal high mass-loss rates initiated in deep and hot optically thick layers around Tapprox 200kK. The resulting velocity structure is non-monotonic and can be separated into three phases: i) an initial acceleration to supersonic speeds ii) stagnation and even deceleration, and iii) an outer region of rapid re-acceleration. The characteristic structures seen in converged steady-state simulations agree well with the outflow properties of our time-dependent models. By directly comparing our dynamic simulations to corresponding hydrostatic models, we demonstrate explicitly that the need to invoke extra energy transport in convectively inefficient regions of stellar structure and evolution models is merely an artefact of enforcing a hydrostatic outer boundary. Moreover, the dynamically inflated inner regions of our simulations provide a natural explanation for the often-found mismatch between predicted hydrostatic WR radii and those inferred from spectroscopy. Finally, we contrast our simulations with alternative recent WR wind models based on co-moving frame radiative transfer for computing the radiation force. Since CMF transfer currently cannot handle non-monotonic velocity fields, the characteristic deceleration regions found here are avoided in such simulations by invoking an ad-hoc very high degree of clumping.
Wolf-Rayet stars are advanced evolutionary stages of massive stars. Despite their large mass-loss rates and high wind velocities, none of them display a bow shock, although a fraction of them are classified as runaway. Our 2.5-D numerical simulations of circumstellar matter around a 60Mo runaway star show that the fast Wolf-Rayet stellar wind is released into a wind-blown cavity filled with various shocks and discontinuities generated throughout the precedent evolutionary phases. The resulting fast-wind slow-wind interaction leads to the formation of spherical shells of swept-up dusty material similar to those observed in near infrared 24 micron with Spitzer, and which appear to be co-moving with the runaway massive stars, regardless of their proper motion and/or the properties of the local ambient medium. We interpret bright infrared rings around runaway Wolf-Rayet stars in the Galactic plane, like WR138a, as indication of their very high initial masses and a complex evolutionary history. Stellar-wind bow shocks become faint as stars run in diluted media, therefore, our results explain the absence of detected bow shocks around Galactic Wolf-Rayet stars such as the high-latitude, very fast-moving objects WR71, WR124 and WR148. Our results show that the absence of a bow shock is consistent with a runaway nature of some Wolf-Rayet stars. This questions the in-situ star formation scenario of high-latitude Wolf-Rayet stars in favor of dynamical ejection from birth sites in the Galactic plane.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
