September 28 th, 20068 th EVN Symposium, Torun Winds in collision: high energy particles in massive...

September 28th, 20068th EVN Symposium, Torun

Winds in collision: high energy particles in massive binary

systems

Sean M. Dougherty (NRC)

In collaboration with: Julian M. Pittard (Leeds)

Evan O’Connor (PEI)Nick Bolingbroke (Victoria)

Perry M. Williams (IfA, Edinburgh)Tony Beasley (ALMA)

Observations of Massive stars• Most massive stars (O or B-type)

– positive spectra from IR to radio – brightness temperature ~ 104 K– Thermal emission – expected!

• in a few systems – “flat” or negative spectra in the radio– brightness temperature ~ >106 K– Non-thermal radio emission – where from?

– Also thermal/non-thermal X-rays, -rays(?)


Moran et al. 1989Williams et al. 1997

• WR147• High resolution observations

- MERLIN @ 5GHz: • 50 mas = 77AU @ 650pc

The radio structure of a colliding-wind binary

– two components - one thermal + one non-thermal

• IR obs resolve two stars– WR+ O/B type

• Position of NT emission: consistent with position of wind-wind collision region


D

What is a wind-collision region?

• Two massive stars with stellar winds• Contact discontinuity where ram pressures are

equal

ratiomomentumwind

2/1

2/1

1D

rOB

• Standing shocks on either side of the CD• Thermal X-ray emission from shock-heated gas in

collision region• Particle acceleration in wind-collision region

– at the shocks – and/or through reconnection at the CD

-> non-thermal emission• radio, X-ray etc.

• Relativistic particles • Higher magnetic, particle & radiation densities than

in SNR – Good particle acceleration laboratory


– radially symmetric, isothermal winds, collide at terminal velocity, axis-symmetric– constrained by radio spectrum and images

• Radiative transfer – Assume cylindrical symmetry, ideal gas, adiabatic index=5/3– Treatment of non-thermal emission

• Urel = Uthermal

• tangled magnetic field– assumption of shock acceleration

• power-law energy distribution at the shocks • p = 2 electron momentum spectrum accelerated at shocks

– Electron energy spectrum evolves downstream due to IC cooling.– Thermal/non-thermal emission & absorption - determined from 2D hydro grid

Modelling radio emission from CWB systems

• Early models of CWB systems tended to be simple.

– Point source non-thermal emission, radially symmetric winds – maintains analytic solutions

– No consideration cooling mechanisms (e.g. Compton cooling – important - even for wide systems c.f. 146, 147) or other absorption.

• Hydro-modelling of CWBs

ff

eSSS ntthermalobs

Hydro model of a CWB – spatial density distribution

WR star – dense stellar wind

O star – less dense wind

Wind-collision region

Wind-collision region

hot plasma + NT particles


Intrinsic NT+ IC cooling

+ Razin effect &

ff absorption

(max=1000)

Typical radio spectra


1.6 GHz

22 GHz

No IC cooling With IC cooling


NT emission does not dominate

Modelling the radio spectrum of WR147

Thermal flux

NT flux

– poor data constraints for modelling

Total flux


Simulated MERLIN 4.8 GHz

Spatial distribution in WR147


WR146

WR146

• MERLIN obs - spatially resolved thermal and NT components (cf. WR147)• Brightest radio CWB – NT emission dominates• VLBI imaging

=> Excellent data constraints


WR146 (2)

(O’Connor et al. 2005, 2007)

Greyscale - EVN 5 GHzContours – VLA+PT 43 GHzCrosses denote stellar positions - HST

43-GHz model4.8-GHz EVN model (contours)

EVN 5 GHz


NT emission in massive stars : binary required?

• In spatially resolved WR-systems, NT emission is from a wind-collision region c.f. WR146, 147, 140

• Are all systems with NT emission “binary” systems?– 25 WR stars - mixture of both single and binary that have measured radio

spectra– 11 systems have spectra identifying non-thermal emission

• WR 11, 48, 98a, 104, 105, 112, 125, 137, 140, 146, 147 • 10 of these WR stars have OB-binary companions

WR stars with NT radio emission ARE BINARYARE BINARY


And O+O star systems?:

• Cyg OB2 #5 • Eclipsing O6+O6 binary – 6-day period!• VLA 5GHz obs

– thermal + non-thermal sources

• Binary coincident with thermal emission

• B-type star & NT emission

(Contreras et al. 1999)

• Wind-collision region?

• Radio-detected O-stars – 60% exhibit non-thermal

emission– “large” fraction are binary

VLA 5 GHz

O stars with NT radio emission ARE BINARY?ARE BINARY?

State of Play:

• Wind-collision regions are laboratories for investigating particle acceleration

• Non-thermal emission in massive stars required a binary/companion– Certainly true for WR stars– Starting to look like the case for O stars

• Successful models of the both radio spectrum and spatial distribution of emission


• A major reason why non-thermal emission is clearly seen in WR147 + WR146– the systems are very wide– free-free opacity along l.o.s. to the wind-collision zone is small

• But --- “static” systems – families of satisfactory models – Ill-defined system parmeters = ill-constrained models

• Shorter period, eccentric systems– possibility of well-specified orbit parameters– variable radiation density – IC cooling variable high energy emission– variable ion density variable circumstellar ff opacity to WCR

• WR 140 is the best studied WR+OB binary– WR + O in a 7.9 year, eccentric (e=0.88) orbit - orbit size ~ 15 AU– Radio-bright – dramatic variations in radio emission as orbit progresses– WCR resolved by VLBI

-> good data constraints.– IC cooling important

– Flow time ~ ROB/vWR ~ 100 hrs– IC Cooling tIC ~12 hrs @apastron @periastron ~250 times shorter!– considerably shorter than flow time– at all radio frequencies under consideration

– High eccentricity + good data excellent lab for studying wind-collision phenomena

WR 140 - the CWB laboratory


Cartoon of the colliding-wind region in WR140

Orbit parameters from Williams et al. 1990 - interaction region based on Eichler & Usov 1993


The radio light curve of WR140

8 years of VLA observations (White & Becker 1995) + WSRT data (Williams p.c.)

2cm

6cm

21cm


VLBA imaging of WR140

• 23 epochs @ 3.6 cm • phase~ 0.74 -> 0.93 (from Jan 1999 to Nov 2000)• Resolution ~ 2 mas• Linear res ~ 4 AU

• Non-thermal emission (Tb~107 K)

• Resolved – “curved” emission region=> wind-collision region

• Observe rotation & pm of emission region– Full orbit definition – particularly inclination– Distance independent of stellar parameters=> Much needed modelling constraints


• “resolved” the binary components– 12.7 mas @ 151.7 degrees at phase 0.297

• Combined with other known orbit parms families of solutions for a,• Orbit definition could wait for more IOTA observation, but in the meanwhile…..

IOTA observation – Monnier et al. 2004


• VLBA obs – assume axis of symmetry along line-of-centres– Rotation of WCR as orbit progresses => O star moves from SE to E of WR star during

observations => derive inclination.

Orbit inclination


• Orbit solution– a = 9.0+/-0.5 mas; = 353+/- 3 degrees ; =122+/-5 degrees

Orbit & distance of WR 140

• Distance – NOT from stellar parameters!– a sin i = 14.10 +/- 0.5 AU => a = 16.6 +/- 1.1 AU for i = 122 deg.– a = 9.0 +/- 0.5 mas Distance = 1.85 +/- 0.16 kpc

• O supergiant

• All important system parms now defined!!!– Stellar types– Distance– All orbit parameters (including inclination)

– ALL VERY IMPORTANT to modelling


Modelling the spectra

A: =0.22, e=1.38x10-3, B=0.05B: =0.02, e=5.36x10-3, B=0.05

Using these orbit parms…

• Constrain (& mass-loss) with thermal X-ray observations - independent of wind-clumping.

• Successfully model individual orbit phases – good!

• Most importantly, establish a value for B, the magnetic field strength

Phase 0.837


• Possible to constrain models with VLBI obs

Modelling 8 GHz VLBI observations of WR140

- demands good observations


1.6

5

8.3

15

22/43

Radiometry

• New multi-frequency VLA observations• Repeat fluxes from previous orbit(s)

– Suggests that emission arises from a “well-behaved” process – Similar behavior seen in O+O star binary systems


First stab at modelling WR140

Looking good

But…Relationship from one to another is UNCLEAR –

badContinues as a work in progress


Modelling the spectra

Models give all radio emission components Most important -- intrinsic Lsyn, the non-thermal radio power

• Now have estimate of B and intrinsic Lsyn

And why are these so important?

Phase 0.837

Thermal stellar wind

Lsyn


WR140 lies within the error box of 3EG J2022+4317

EGRET (100MeV – 20 GeV)

From Benaglia & Romero (2003)

Is WR140 a gamma-ray source?


NT bremsstrahlung

Pion decay

Emission from WR140 at phase 0.8

Photon pair production opacity Inverse Compton

Radio ASCA

INT

EG

RA

L

GL

AST

VE

RIT

AS


Looks like a duck, quacks like a duck – it’s a duck!

• Cyg OB2 #9 • Not a spectroscopic binary

– Apparently single!

• VLBA obs– looks like a WCR

• Other evidence of companion?e.g. WCR rotate on plane-of-sky?

• Variable radio emission – 2.4-yr period

• Radio obs => binary• Is there a WCR?


Summary

• Colliding winds in early-type binaries are useful laboratories for investigating particle acceleration– New insights into particle acceleration – at higher mass, B-field, and energy densities than in

SNRs• Excellent data on a number of systems

– Radiometry and imaging – WR140 and WR146 – more recently Cyg OB2 #9– WR140 has well-constrained system parms from high-resolution imaging – very important for

modelling– WR140 and Cyg OB2 #9 – similar flux orbit-to-orbit - emission arises from well-behaved

process(es)• Hydro models of plasma distribution

– Successful models of spectrum and spatial distribution of emission. – Some issues revealed in models of WR146 – better data constraints

• high-frequency spectrum & spatial extent of emission

• Models lead to intrinsic synchrotron radio emission and magnetic energy density – used to estimate the non-thermal X-ray and -ray emission

• Insight into particle (ions & electrons) acceleration efficiencies, and the B-field• Exciting period with respect to new data from INTEGRAL, GLAST, HESS, VERITAS, etc.

– Constrain models (e.g. pion decay signature of relativistic ion production).

Dougherty, Beasley, Claussen, Zauderer, Bolingbroke, 2005, ApJ 623, 447Pittard, Dougherty, Coker, O’Connor, Bolingbroke, 2006, A&A 446,1001

Pittard & Dougherty, 2006, MNRAS, in pressO’Connor, Dougherty, Pittard, Williams, 2007, in prep

September 28 th, 20068 th EVN Symposium, Torun Winds in collision: high energy particles in massive...

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