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J. M. Pittard

Publications and source records attributed to J. M. Pittard.

At least 55 records · Page 3Linked to original sources
arXiv2011

Global X-ray properties of the O and B stars in Carina

The key empirical property of the X-ray emission from O stars is a strong correlation between the bolometric and X-ray luminosities. In the framework of the Chandra Carina Complex Project, 129 O and B stars have been detected as X-ray sources; 78 of those, all with spectral type earlier than B3, have enough counts for at least a rough X-ray spectral characterization. This leads to an estimate of the Lx/Lbol ratio for an exceptional number of 60 O stars belonging to the same region and triples the number of Carina massive stars studied spectroscopically in X-rays. The derived log(Lx/Lbol) is -7.26 for single objects, with a dispersion of only 0.21dex. Using the properties of hot massive stars listed in the literature, we compare the X-ray luminosities of different types of objects. In the case of O stars, the Lx/Lbol ratios are similar for bright and faint objects, as well as for stars of different luminosity classes or spectral types. Binaries appear only slightly harder and slightly more luminous in X-rays than single objects; the differences are not formally significant (at the 1% level), except for the Lx/Lbol ratio in the medium (1.0--2.5keV) energy band. Weak-wind objects have similar X-ray luminosities but they display slightly softer spectra compared to "normal" O stars with the same bolometric luminosity. Discarding three overluminous objects, we find a very shallow trend of harder emission in brighter objects. The properties of the few B stars bright enough to yield some spectral information appear to be different overall (constant X-ray luminosities, harder spectra), hinting that another mechanism for producing X-rays, besides wind shocks, might be at work. However, it must be stressed that the earliest and X-ray brightest amongst these few detected objects are similar to the latest O stars, suggesting a possibly smooth transition between the two processes.

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arXiv2011

The WR 140 periastron passage 2009: first results from MONS and other optical sources

We present the results from the spectroscopic follow-up of WR140 (WC7 + O4-5) during its last periastron passage in January 2009. This object is known as the archetype of colliding wind binaries and has a relatively large period (~ 8 years) and eccentricity (~ 0.89). We provide updated values for the orbital parameters, new estimates for the WR and O star masses and new constraints on the mass-loss rates.

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arXiv2010

Tails of the Unexpected: The Interaction of an Isothermal Shell with a Cloud

A new mechanism for the formation of cometary tails behind dense clouds or globules is discussed. Numerical hydrodynamical models show that when a dense shell of swept-up matter overruns a cloud, material in the shell is focussed behind the cloud to form a tail. This mode of tail formation is completely distinct from other methods, which involve either the removal of material from the cloud, or shadowing from a strong, nearby source of ionization. This mechanism is relevant to the cometary tails seen in planetary nebulae and to the interaction of superbubble shells with dense clouds.

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arXiv2010

Spiralling out of control: 3D hydrodynamical modelling of the colliding winds in $η\thinspace$Carinae

Three dimensional (3D) adaptive-mesh refinement (AMR) hydrodynamical simulations of the wind-wind collision between the enigmatic super-massive star \etacar and its mysterious companion star are presented which include radiative driving of the stellar winds, gravity, optically-thin radiative cooling, and orbital motion. Simulations with static stars with a periastron passage separation reveal that the preshock companion star's wind speed is sufficiently reduced that radiative cooling in the postshock gas becomes important, permitting the runaway growth of non-linear thin shell (NTSI) instabilities which massively distort the WCR. However, large-scale simulations which include the orbital motion of the stars, show that orbital motion reduces the impact of radiative inhibition, and thus increases the acquired preshock velocities. As such, the postshock gas temperature and cooling time see a commensurate increase, and sufficient gas pressure is preserved to stabilize the WCR against catastrophic instability growth. We then compute synthetic X-ray spectra and lightcurves and find that, compared to previous models, the X-ray spectra agree much better with {\it XMM-Newton} observations just prior to periastron. The narrow width of the 2009 X-ray minimum can also be reproduced. However, the models fail to reproduce the extended X-ray mimimum from previous cycles. We conclude that the key to explaining the extended X-ray minimum is the rate of cooling of the companion star's postshock wind. If cooling is rapid then powerful NTSIs will heavily disrupt the WCR. Radiative inhibition of the companion star's preshock wind, albeit with a stronger radiation-wind coupling than explored in this work, could be an effective trigger.

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arXiv2010

The interactions of winds from massive young stellar objects

The supersonic stellar and disk winds possessed by massive young stellar objects will produce shocks when they collide against the interior of a pre-existing bipolar cavity (resulting from an earlier phase of jet activity). The shock heated gas emits thermal X-rays which may be observable by spaceborne observa- tories such as the Chandra X-ray Observatory. Hydrodynamical models are used to explore the wind-cavity interaction. Radiative transfer calculations are performed on the simulation output to produce synthetic X-ray observations, allowing constraints to be placed on model parameters through comparisons with observations. The model reveals an intricate interplay between the inflowing and outflowing material and is successful in reproducing the observed X-ray count rates from massive young stellar objects.

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arXiv2010

Numerical heat conduction in hydrodynamical models of colliding hypersonic flows

Hydrodynamical models of colliding hypersonic flows are presented which explore the dependence of the resulting dynamics and the characteristics of the derived X-ray emission on numerical conduction and viscosity. For the purpose of our investigation we present models of colliding flow with plane-parallel and cylindrical divergence. Numerical conduction causes erroneous heating of gas across the contact discontinuity which has implications for the rate at which the gas cools. We find that the dynamics of the shocked gas and the resulting X-ray emission are strongly dependent on the contrast in the density and temperature either side of the contact discontinuity, these effects being strongest where the postshock gas of one flow behaves quasi-adiabatically while the postshock gas of the other flow is strongly radiative. Introducing additional numerical viscosity into the simulations has the effect of damping the growth of instabilities, which in some cases act to increase the volume of shocked gas and can re-heat gas via sub-shocks as it flows downstream. The resulting reduction in the surface area between adjacent flows, and therefore of the amount of numerical conduction, leads to a commensurate reduction in spurious X-ray emission, though the dynamics of the collision are compromised. The simulation resolution also affects the degree of numerical conduction. A finer resolution better resolves the interfaces of high density and temperature contrast and although numerical conduction still exists the volume of affected gas is considerably reduced. However, since it is not always practical to increase the resolution, it is imperative that the degree of numerical conduction is understood so that inaccurate interpretations can be avoided. This work has implications for the dynamics and emission from astrophysical phenomena which involve high Mach number shocks.

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arXiv2010

The Turbulent Destruction of Clouds - II. Mach Number Dependence, Mass-loss Rates, and Tail Formation

The turbulent destruction of a cloud subject to the passage of an adiabatic shock is studied. We find large discrepancies between the lifetime of the cloud and the analytical result of Hartquist et al. (1986). These differences appear to be due to the assumption in Hartquist et al. that mass-loss occurs largely as a result of lower pressure regions on the surface of the cloud away from the stagnation point, whereas in reality Kelvin-Helmholtz (KH) instabilities play a dominant role in the cloud destruction. We find that the true lifetime of the cloud (defined as when all of the material from the core of the cloud is well mixed with the intercloud material in the hydrodynamic cells) is about 6 times t_KHD, where t_KHD is the growth timescale for the most disruptive, long-wavelength, KH instabilities. These findings have wide implications for diffuse sources where there is transfer of material between hot and cool phases. The properties of the interaction as a function of Mach number and cloud density contrast are also studied. The interaction is milder at lower Mach numbers with the most marked differences occuring at low shock Mach numbers when the postshock gas is subsonic with respect to the cloud (i.e. M < 2.76). Material stripped off the cloud only forms a long "tail-like" feature if the density contrast of the cloud to the ambient medium, chi > 1e3.

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arXiv2009

3D models of radiatively driven colliding winds in massive O+O star binaries - III. Thermal X-ray emission

The X-ray emission from the wind-wind collision in short-period massive O+O-star binaries is investigated. The emission is calculated from three-dimensional hydrodynamical models which incorporate gravity, the driving of the winds, orbital motion of the stars, and radiative cooling of the shocked plasma. Changes in the amount of stellar occultation and circumstellar attenuation introduce phase-dependent X-ray variability in systems with circular orbits, while strong variations in the intrinsic emission also occur in systems with eccentric orbits. The X-ray emission in eccentric systems can display strong hysteresis, with the emission softer after periastron than at corresponding orbital phases prior to periastron, reflecting the physical state of the shocked plasma at these times. Furthermore, the rise of the luminosity to maximum does not necessarily follow a 1/D law. Our models further demonstrate that the effective circumstellar column can be highly energy dependent. We simulate Chandra and Suzaku observations, and fit these using standard XSPEC models. We find that the recovered temperatures from two or three-temperature mekal fits are comparable to those from fits to the emission from real systems with similar stellar and orbital parameters/nature. We also find that when the global abundance is thawed in the spectral fits, sub-solar values are exclusively returned, despite the calculations using solar values as input (abridged).

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arXiv2009

3D models of radiatively driven colliding winds in massive O+O star binaries - II. Thermal radio to sub-mm emission

In this work the thermal emission over cm to sub-mm wavelengths from the winds in short-period O+O-star binaries is investigated (potential non-thermal emission is presently ignored). The calculations are based on three-dimensional hydrodynamical models which incorporate gravity, the driving of the winds, orbital motion of the stars, and radiative cooling of the shocked plasma. The thermal emission arises from the stellar winds and the region where they collide. We investigate the flux and spectrum from a variety of models as a function of orbital phase and orientation of the observer, and compare to the single star case. The emission from the wind-wind collision region (WCR) is strongly dependent on its density and temperature, being optically thick in radiative systems, and optically thin in adiabatic systems. The flux from systems where the WCR is highly radiative, as investigated for the first time in this work, can be over an order of magnitude greater than the combined flux from identically typed single stars. This excess occurs over a broad range of wavelengths from cm to sub-mm. In contrast, when the WCR is largely adiabatic, a significant excess in the thermal flux occurs only below 100 GHz. Eccentric systems may show order of magnitude or greater flux variability, especially if the plasma in the WCR transitions from an adiabatic to a radiative state and vice-versa - in such cases the flux can display significant hysteresis with stellar separation. We further demonstrate that clumping can affect the variability of radio lightcurves. We investigate the spectral index of the emission, and often find indices steeper than +0.6. Our predictions are of interest to future observations with the next generation of radio and sub-mm telescopes (abridged).

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arXiv2009

Non-thermal emission from Massive Young Stellar Objects

In the young stellar object (YSO) phase of their lives, massive stars drive bi-polar molecular outflows. These outflows produce beautiful, often hourglass shaped, cavities. The central star possesses a powerful stellar wind (v ~ 2000 km s^-1), and possibly a dense equatorial disk wind (v ~ 400 km s^-1), which collide with the inner surface of the bi-polar cavity and produces hot (T ~ 10^5 - 10^8 K) shocked plasma. A reverse shock is formed at the point where the ram pressure between the preshock flow balances the thermal pressure of the postshock flow and provides a site for the acceleration of non-thermal particles to relativistic energies. Hydrodynamical models of the wind interaction, coupled with calculations of the non-thermal energy spectrum, are used to explore the observable synchrotron and gamma-ray emission from these objects.

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arXiv2009

The interactions of winds from massive young stellar objects: X-ray emission, dynamics, and cavity evolution

2D axis-symmetric hydrodynamical simulations are presented which explore the interaction of stellar and disk winds with surrounding infalling cloud material. The star, and its accompanying disk, blow winds inside a cavity cleared out by an earlier jet. The collision of the winds with their surroundings generates shock heated plasma which reaches temperatures up to ~10^8 K. Attenuated X-ray spectra are calculated from solving the equation of radiative transfer along lines-of-sight. This process is repeated at various epochs throughout the simulations to examine the evolution of the intrinsic and attenuated flux. We find that the dynamic nature of the wind-cavity interaction fuels intrinsic variability in the observed emission on timescales of several hundred years. This is principally due to variations in the position of the reverse shock which is influenced by changes in the shape of the cavity wall. The collision of the winds with the cavity wall can cause clumps of cloud material to be stripped away. Mixing of these clumps into the winds mass-loads the flow and enhances the X-ray emission measure. The position and shape of the reverse shock plays a key role in determining the strength and hardness of the X-ray emission. In some models the reverse shock is oblique to much of the stellar and disk outflows, whereas in others it is closely normal over a wide range of polar angles. For reasonable stellar and disk wind parameters the integrated count rate and spatial extent of the intensity peak for X-ray emission agree with \textit{Chandra} observations of the deeply embedded MYSOs S106 IRS4, Mon R2 IRS3 A, and AFGL 2591.(abridged)

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arXiv2009

Models of the non-thermal emission from early-type binaries

The powerful wind-wind collision in massive star binaries creates a region of high temperature plasma and accelerates particles to relativistic energies. I briefly summarize the hydrodynamics of the wind-wind interaction and the observational evidence, including recent $γ$-ray detections, of non-thermal emission from such systems. I then discuss existing models of the non-thermal emission and their application to date, before concluding with some future prospects.

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arXiv2009

3D Models of Radiatively Driven Colliding Winds In Massive O+O Star Binaries: I. Hydrodynamics

The dynamics of the wind-wind collision in massive stellar binaries is investigated using three-dimensional hydrodynamical models which incorporate gravity, the driving of the winds, the orbital motion of the stars, and radiative cooling of the shocked plasma. In this first paper we restrict our study to main-sequence O+O binaries. The nature of the wind-wind collision region is highly dependent on the degree of cooling of the shocked plasma, and the ratio of the flow timescale of the shocked plasma to the orbital timescale. The pre-shock wind speeds are lower in close systems as the winds collide prior to their acceleration to terminal speeds. Radiative inhibition may also reduce the pre-shock wind speeds. Together, these effects can lead to rapid cooling of the post-shock gas. Radiative inhibition is less important in wider systems, where the winds are accelerated to higher speeds before they collide, and the resulting collision region can be largely adiabatic. In systems with eccentric orbits, cold gas formed during periastron passage can persist even at apastron, before being ablated and mixed into its surroundings and/or accelerated out of the system.

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arXiv2009

Non-thermal processes in colliding-wind massive binaries: the contribution of Simbol-X to a multiwavelength investigation

Several colliding-wind massive binaries are known to be non-thermal emitters in the radio domain. This constitutes strong evidence for the fact that an efficient particle acceleration process is at work in these objects. The acceleration mechanism is most probably the Diffusive Shock Acceleration (DSA) process in the presence of strong hydrodynamic shocks due to the colliding-winds. In order to investigate the physics of this particle acceleration, we initiated a multiwavelength campaign covering a large part of the electromagnetic spectrum. In this context, the detailed study of the hard X-ray emission from these sources in the SIMBOL-X bandpass constitutes a crucial element in order to probe this still poorly known topic of astrophysics. It should be noted that colliding-wind massive binaries should be considered as very valuable targets for the investigation of particle acceleration in a similar way as supernova remnants, but in a different region of the parameter space.

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arXiv2009

3D modelling of the colliding winds in Eta Carinae - evidence for radiative inhibition

The X-ray emission from the super-massive star Eta Carinae is simulated using a three dimensional model of the wind-wind collision. In the model the intrinsic X-ray emission is spatially extended and energy dependent. Absorption due to the unshocked stellar winds and the cooled postshock material from the primary LBV star is calculated as the intrinsic emission is ray-traced along multiple sightlines through the 3D spiral structure of the circumstellar environment. The observable emission is then compared to available X-ray data, including the lightcurve observed by the Rossi X-ray Timing Explorer (RXTE) and spectra observed by XMM-Newton. The orientation and eccentricity of the orbit are explored, as are the wind parameters of the stars and the nature and physics of their close approach. Our modelling supports a viewing angle with an inclination of ~ 42 degrees, consistent with the polar axis of the Homunculus nebula (Smith 2006), and the projection of the observer's line-of-sight onto the orbital plane has an angle of ~ 0 - 30 degrees in the prograde direction on the apastron side of the semi-major axis. However, there are significant discrepancies between the observed and model lightcurves and spectra through the X-ray minimum. In particular, the hard flux in our synthetic spectra is an order of magnitude greater than observed. Further calculations reveal that radiative inhibition significantly reduces the preshock velocity of the companion wind. As a consequence the hard X-ray emission is quenched, but it is unclear whether the long duration of the minimum is due solely to this mechanism alone. Models incorporating a collapse/disruption of the WCR and/or reduced preshock companion wind velocities bring the predicted emission and the observations into much better agreement (abridged).

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arXiv2008

The turbulent destruction of clouds - I. A k-epsilon treatment of turbulence in 2D models of adiabatic shock-cloud interactions

The interaction of a shock with a cloud has been extensively studied in the literature, where the effects of magnetic fields, radiative cooling and thermal conduction have been considered. However, the formation of fully developed turbulence has often been prevented by the artificial viscosity inherent in hydrodynamical simulations, and a uniform post-shock flow has been assumed in all previous single-cloud studies. In reality, the flow behind the shock is also likely to be turbulent, with non-uniform density, pressure and velocity structure created as the shock sweeps over inhomogenities upstream of the cloud. To address these twin issues we use a sub-grid compressible k-epsilon turbulence model to estimate the properties of the turbulence generated in shock-cloud interactions and the resulting increase in the transport coefficients that the turbulence brings. A detailed comparison with the output from an inviscid hydrodynamical code puts these new results into context. We find that cloud destruction in inviscid and k-epsilon models occurs at roughly the same speed when the post-shock flow is smooth and when the density contrast between the cloud and inter-cloud medium is less than 100. However, there are increasing and significant differences as this contrast increases. Clouds subjected to strong ``buffeting'' by a highly turbulent post-shock environment are destroyed significantly quicker. Additional calculations with an inviscid code where the post-shock flow is given random, grid-scale, motions confirms the more rapid destruction of the cloud. Our results clearly show that turbulence plays an important role in shock-cloud interactions, and that environmental turbulence adds a new dimension to the parameter space which has hitherto been studied (abridged).

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