Clumps In Stellar Wind Research Articles

Non-thermal radiation from a pulsar wind interacting with an inhomogeneous stellar wind

Binaries hosting a massive star and a non-accreting pulsar are powerful non-thermal emitters due to the interaction of the pulsar and the stellar wind. The winds of massive stars are thought to be inhomogeneous, which could have an impact on the non-thermal emission. We study numerically the impact of the presence of inhomogeneities or clumps in the stellar wind on the high-energy non-thermal radiation of high-mass binaries hosting a non-accreting pulsar. We compute the trajectories and physical properties of the streamlines in the shocked pulsar wind without clumps, with a small clump, and with a large one. This information is used to compute the synchrotron and inverse Compton emission from the non-thermal populations, accounting also for the effect of gamma-ray absorption through pair creation. A specific study is done for PSR B1259-63/LS2883. When stellar wind clumps perturb the two-wind interaction region, the associated non-thermal radiation in the X-ray band,of synchrotron origin, and in the GeV-TeV band, of inverse Compton origin, is affected by several effects: (i) strong changes in the the plasma velocity direction that result in Doppler boosting factor variations, (ii) strengthening of the magnetic field that mainly enhances the synchrotron radiation, (iii) strengthening of the pulsar wind kinetic energy dissipation at the shock, potentially available for particle acceleration, and (iv) changes in the rate of adiabatic losses that affect the lower energy part of the non-thermal particle population. The radiation above 100 GeV detected, presumably, during the post-periastron crossing of the Be star disc in PSR B1259-63/LS2883, can be roughly reproduced assuming that the crossing of the disc is modeled as the encounter with a large inhomogeneity.

X-ray diagnostics of massive star winds

AbstractObservations with powerful X-ray telescopes, such as XMM-Newton and Chandra, significantly advance our understanding of massive stars. Nearly all early-type stars are X-ray sources. Studies of their X-ray emission provide important diagnostics of stellar winds. High-resolution X-ray spectra of O-type stars are well explained when stellar wind clumping is taking into account, providing further support to a modern picture of stellar winds as non-stationary, inhomogeneous outflows. X-ray variability is detected from such winds, on time scales likely associated with stellar rotation. High-resolution X-ray spectroscopy indicates that the winds of late O-type stars are predominantly in a hot phase. Consequently, X-rays provide the best observational window to study these winds. X-ray spectroscopy of evolved, Wolf-Rayet type, stars allows to probe their powerful metal enhanced winds, while the mechanisms responsible for the X-ray emission of these stars are not yet understood.

Clumpy stellar winds and high-energy emission in high-mass binaries hosting a young pulsar

High-mass binaries hosting young pulsars can be powerful gamma-ray emitters. The stellar wind of the massive star in the system is expected to be clumpy. Since the high-energy emission comes from the pulsar-star wind interaction, the presence of clumps can affect the spectrum and variability of this radiation. We look for the main effects of the clumps on the two-wind interaction region and on the non-thermal radiation. A simple analytical model for the two-wind interaction dynamics was developed accounting for the lifetime of clumps under the pulsar-wind impact. This time plays a very important role with regard to the evolution of the clump, the magnetic field in the clump-pulsar wind interaction region, and the non-radiative and radiative cooling of the non-thermal particles. We also computed the high-energy emission produced at the interaction of long-living clumps with the pulsar wind. For reasonable parameters, the clumps will induce small variability on the X-ray and gamma-ray radiation. Sporadically, large clumps can reach closer to the pulsar increasing the magnetic field, triggering synchrotron X-ray flares and weakening other emission components like inverse Compton. The reduction of the emitter size induced by clumps also makes non-radiative losses faster. Stellar wind clumps can also enhance instability development and matter entrainment in the shocked pulsar wind when it leaves the binary. Growth limitations of the clumps from the wind acceleration region may imply that a different origin for the largest clumps is required. The large-scale wind structures behind the observed discrete absorption components in the UV may be the source of these large clumps. The presence of structure in the stellar wind can produce substantial energy-dependent variability and should not be neglected when studying the broadband emission from high-mass binaries hosting young pulsars.

Three-dimensional radiative transfer in clumped hot star winds

The true mass-loss rates from massive stars are important for many branches of astrophysics. For the correct modeling of the resonance lines, which are among the key diagnostics of stellar mass-loss, the stellar wind clumping turned out to be very important. In order to incorporate clumping into radiative transfer calculation, 3-D models are required. Various properties of the clumps may have strong impact on the resonance line formation and, therefore, on the determination of empirical mass-loss rates. We incorporate the 3-D nature of the stellar wind clumping into radiative transfer calculations and investigate how different model parameters influence the resonance line formation. We develop a full 3-D Monte Carlo radiative transfer code for inhomogeneous expanding stellar winds. The number density of clumps follows the mass conservation. For the first time, realistic 3-D models that describe the dense as well as the tenuous wind components are used to model the formation of resonance lines in a clumped stellar wind. At the same time, non-monotonic velocity fields are accounted for. The 3-D density and velocity wind inhomogeneities show very strong impact on the resonance line formation. The different parameters describing the clumping and the velocity field results in different line strengths and profiles. We present a set of representative models for various sets of model parameters and investigate how the resonance lines are affected. Our 3-D models show that the line opacity is reduced for larger clump separation and for more shallow velocity gradients within the clumps. Our new model demonstrates that to obtain empirically correct mass-loss rates from the UV resonance lines, the wind clumping and its 3-D nature must be taken into account.

Orbital parameters of supergiant fast X-ray transients

Supergiant fast X-ray transient (SFXT) is a new class of the high mass X-ray binary that shows short X-ray flares. The physical mechanism of SFXT short flares is still open for discussion. The accretion process of dense clumps in stellar wind onto neutron star (NS) has been proposed as the origin of such short flares. In order to examine the applicability of the clumpy wind scenario, we focus on the accretion mode that depends on orbital parameters. Our goal is to impose restrictions on the orbital parameters of SFXT. Assuming a simple analytic model of clumpy wind, we investigate the condition where the size of accretion cylinder overcomes the clump size. The allowed parameter region for SFXT is restricted in a relatively narrow window in $P_{\rm{orb}} - e$ diagram. Binary systems with large eccentricities ($e \ga 0.4$) and moderate orbital periods ($P_{\rm{orb}} \sim 10$ d) are prone to show periodic X-ray outbursts which are characteristic for SFXT. We confirm that systems with a long orbital period of more than 100 days cannot produce bright X-ray flares in the simple clumpy wind scenario.

Neglecting the porosity of hot-star winds can lead to underestimating mass-loss rates

Context: The mass-loss rate is a key parameter of massive stars. Adequate stellar atmosphere models are required for spectral analyses and mass-loss determinations. Present models can only account for the inhomogeneity of stellar winds in the approximation of small-scale structures that are optically thin. This treatment of ``microclumping'' has led to reducing empirical mass-loss rates by factors of two and more. Aims: Stellar wind clumps can be optically thick in spectral lines. We investigate how this ``macroclumping'' impacts on empirical mass-loss rates. Methods: The Potsdam Wolf-Rayet (PoWR) model atmosphere code is generalized in the ``formal integral'' to account for clumps that are not necessarily optically thin. Results: Optically thick clumps reduce the effective opacity. This has a pronounced effect on the emergent spectrum. Our modeling for the O-type supergiant zeta Puppis reveals that the optically thin H-alpha line is not affected by wind porosity, but that the PV resonance doublet becomes significantly weaker when macroclumping is taken into account. The reported discrepancies between resonance-line and recombination-line diagnostics can be resolved entirely with the macroclumping modeling without downward revision of the mass-loss rate. Conclusions: Mass-loss rates inferred from optically thin emission, such as the H-alpha line in O stars, are not influenced by macroclumping. The strength of optically thick lines, however, is reduced because of the porosity effects. Therefore, neglecting the porosity in stellar wind modeling can lead to underestimating empirical mass-loss rates.

Stagnation and Infall of Dense Clumps in the Stellar Wind of τ Scorpii

Observations of the B0.2 V star q Scorpii have revealed unusual stellar wind characteristics: redshifted absorption in the far-ultraviolet O VI resonance doublet up to D)250 km s~1 and extremely hard X-ray emission implying gas at temperatures in excess of 107 K. We describe a phenomenological model to explain these properties. We assume the wind of q Sco consists of two components: ambient gas in which denser clumps are embedded. The clumps are optically thick in the UV resonance lines primarily responsible for accelerating the ambient wind. The reduced acceleration causes the clumps to slow and even infall, all the while being con—ned by the ram pressure of the out—owing ambient wind. We calcu- late detailed trajectories of the clumps in the ambient stellar wind, accounting for a line radiation driving force and the momentum deposited by the ambient wind in the form of drag. We show that these clumps will fall back toward the star with velocities of several hundred km s~1 for a broad range of initial conditions. The velocities of the clumps relative to the ambient stellar wind can approach 2000 km s~1, producing X-rayemitting plasmas with temperatures in excess of (1¨6) ) 107 K in bow shocks at their leading edge. The infalling material explains the peculiar redshifted absorption wings seen in the O VI doublet. Of order 103 clumps with individual masses g are needed to explain the m c D 1019¨1020 observed X-ray luminosity and also to explain the strength of the O VI absorption lines. These values correspond to a mass-loss rate in clumps of to 10~8 yr~1, a small fraction of the total M0 c D 10~9 M _ mass-loss rate yr~1). We discuss the position of q Sco in the H-R diagram, conclud- (M0 D 3 ) 10~8 M _ ing that q Sco is in a crucial position on the main sequence. Hotter stars near the spectral type of q Sco have too powerful winds for clumps to fall back to the stars, and cooler stars have too low mass-loss rates to produce observable eUects. The model developed here can be generally applied to line-driven out—ows with clumps or density irregularities. Subject headings: stars: early-typestars: individual (q Scorpii) ¨ stars: winds, out—ows ¨ ultraviolet: starsX-rays: stars