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Observing Winds in Collision with RXTE
Weight-loss Secrets of the Biggest Stars
Michael F. Corcoran (CRESST/USRA/NASA-GSFC)
With: B. Ishibashi (Nagoya), T. Gull (NASA-GSFC), A. Damineli (IAGUSP), K. Hamaguchi (CRESST/UMBC/NASA-GSFC), A. F. J. Moffat (U. Montreal), E. R. Parkin (ANU), A. M. T Pollock (ESA), J. M. Pittard (U. Leeds), Atsuo Okazaki (Hokkai-Gakuen University), S. P. Owocki (U. Delaware), C. M. P. Russell (U. Delaware)
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Overview
• Introduction: The complicated mass of massive stars• Colliding Wind Binaries: A shocking way to study mass loss• RXTE Breakthroughs
– WR 140 – Eta Carinae
• Conclusion
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Introduction: M ~ M
Mass is the fundamental stellar parameter, but as it increases it becomes (observationally) less well constrained; especially near the upper mass limit (~150 solar masses, Figer 2005)
Moffat 1989
R145 in 30 Dor: M sin3i= 116+48 M
(i =38° => M=300+125 M!)Schnurr et al 2009
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Problems In Measuring Mass Loss• Winds: Smooth wind vs. clumped? (factor of a few overestimate);
spherical or not?• LBV eruptions: timescales & rates?• SN explosion: core & remnant amounts?
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X-rays as a Probe of Colliding Wind Systems
Solving the X-ray Lightcurve:
• wind terminal velocities (i.e. escape velocities) through temperatures; wind acceleration
• shock conditions & adiabatic/radiative cooling; flow dynamics from X-ray lines
• density information via absorbing column measurements
• D–1 variation – orbital elements and masses
Wind velocities of 1000’s of km/s X-ray emission
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“Canonical” Examples of Colliding Wind Binaries
Two important “colliding wind binary shock laboratories”:
• WR 140, a carbon-rich Wolf-Rayet star + a “normal” O-type companion in a peculiar orbit: 8 year period, eccentricity = 0.88
• Eta Carinae: an LBV + unseen companion, in a peculiar orbit: 5.5 year period, eccentricity ~ 0.9
(Space Oddity: Both went through periastron passage within 5 days of each other in January 2009: next occurrence AD 12,620.808)
RXTE has been key in defining the X-ray emitting shock properties vs. time
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Parameter WR 140 Eta Car
Sp. Type WC7+O4-5I LBV+O4?
Period (days) 2897 2024
e 0.89 ~0.9
a (AU) 15 15
Vwinds (km/s) 2860, 3100 500, 3000
Mass loss rate (M/yr) 3.3 x 10-5, 10-6 few x 10-4, 10-5
Masses (M) 16, 41 90, 30?
References Fahed et al. 2011; Russell et al. 2011; Monnier et al. 2011; Dougherty et al 2011
Hillier et al. 2001;Corcoran et al. 2010; Damineli et al. 2008; Parkin et al. 2011; Russell et al. 2011
WR 140 + Eta Car: Dynamical Sisters
X-ray derived
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WR 140: “Canonical1” Massive Long Period Binary
• Coordinated variability at wavelengths from cm to 10-8 cm
• all driven by wind-wind collision between the two stars
P. Williams
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A Brief History of Eta Car
1820 1850 1880 1910 1940 1970 2000
50 yrs
V-Band Lightcurve
V9
75
31
–1
An Eruptive Variable – what caused the Eruption?
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Panchromatic (?) Variability: Eta Car
• Periodic spectral variations in He I 10830A (Damineli 1996) and X-rays
• strict period => gravitational dynamics
Radio variability (R. Duncan, S. White et al 1995)
X-rays (Corcoran et al. 1996)
1992 1993
Har
d
Sof
t
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WR 140 + Eta Car: 3D Hydro models
Simulations by E. R. Parkin, J. M. Pittard
WR 140 Eta Carinae
Two fast winds at different Mdots fast+slow wind at different Mdots
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Structured Mass Loss: Eta Car
Bow shock shapes primary wind
Variable (NH)
Coriolis distortions in wind near periastron passage effects photoionization of wind/nebula by the companion
Thermalization of KE at WWC produces X-rays sensitive to orbit
“Flashlight Effect”: variable illumination of the circumstellar material (See poster by T. Gull)
Density profile
wind structure in orbital plane
Okazaki et al. 2008
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WR 140: Historical X-ray Variability
EXOSAT � ROSAT + ASCA
EXOSAT: 1983 – 1986(Williams et al. 1990)
ROSAT: 1990-1999 ASCA: 1993-2000
Courtesy A. Pollock
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X-ray Spectrum: Eta Car
Hot Component 50 MK
Fe XXV
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RXTE Breakthroughs:Snapshot observations hampers detailed understanding
RXTE Observations of WR 140 and Eta Car provided:• Strongest evidence that Eta Car is a binary system• Adequate time sampling to separate phase-locked and secular trends in both systems• Detailed lightcurves to compare to models• New physical insights: clumping, radiative braking• Realistic wind structure and the “flashlight” effect • Surprises
RXTE PCU observations:Eta Car: 1500 ksec Feb 9, 1996 - Dec 28, 2011WR 140: 500 ksec Mar 3, 2001 - Dec 23, 2011
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RXTE: WR140
“Historical”
RXTE lightcurve in the 2-10 keV band
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RXTE: Eta Car
2-10 keV
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Comparison of Periastra: WR140
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Eta Car
…what happened in 2009?
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Eta Car Flare Behavior
... 1 AU Clumps?
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Color Change: WR 140
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Color Change Near Minimum: WR 140H
ardn
ess
Rat
io =
(7.5
-3 k
eV)/
(7.5
+3
keV
)
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Color Change Near Minimum: Eta Car
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3D Lightcurve Models: WR 140
Modelling by C. Russell
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=270-
Okazaki et al. 2008
Parkin et al. 2008
3-D SPH + Isothermal point source emission
3-D Hydro + extended emission via 2-D hydro
Duration of X-ray minimum suggests collapse of shock(radiative braking? radiative inhibition?)
3D Lightcurve Models: Eta Car
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Eta Car: Extended Emission?
Post-Periastron “Hot Bubble” due to fast companion wind colliding with swept-up wall of the LBV wind reduces minimum depth, duration... (Russell et al. 2011)
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What Shortened the 2009 Minimum?
Minimum depends on: intrinsic 2-10 keV emission of shock near periastron (radiative instabilities important) amount of absorption around the “bubble”
A decrease in the LBV’s mass loss rate could make the shock less radiative (more stable) and less absorbed shorter X-ray Minimum?
Or something Else?
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SummaryStudies of the colliding winds in systems like WR140 and Eta Car in X-rays (and UV, optical, IR & radio) provide unique information regarding mass loss in extremely massive stars, and information on behavior of astrophysical shocks
• Detailed modeling of the wind-shock boundary shape and flow
• Relative mass-loss pressure ratio (in-situ density diagnostics; clumping independent?)
• Dynamical studies (and periastron triggers?)
• Dust generation from shocked gas
• Detailed lab shock-physics experiments with varying conditions/parameters
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Thanks to the RXTE Team for 16 Great Years!
esp. Evan Smith, heroic scheduler
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Colliding Winds in WR 140Wind-Wind collisions in WR 140 allow us to probe time-variable shock physics under conditions of densities and temperatures which are difficult to reproduce on Earth
WR140: Our Shock Physics Laboratory
Courtesy P. Williams
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Initial X-ray Results • Bright, variable X-ray source (unusual for a single massive star, even more unusual for a single WR star)
• Variable X-ray spectrum: Changes in emission measure of the hot gas, absorption to the hot gas
• Hard source: kT ~ 3-4 keV (also unusual for single massive star)
Need Detailed Monitoring: the Rossi X-ray Timing Explorer
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Direct Imaging
VLBA imaging courtesy S Dougherty
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What RXTE Sees:
Optical: Crowded Stellar Field
Dominated by WR140 CW emission above 2 keV
RASS 2x2 degree image of WR 140, 0.5-2 keV(with RXTE 5%, 30% & 80% contours)
X-ray: WR 140 dominant source
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RXTE InstrumentationRXTE has 3 instruments:
• The Proportional Counter Array (PCA): • a set of five collimated Xenon-filled proportional counter units• 2-70 keV; 1600 cm2
• Most useful for WR 140
• The High Energy X-ray Timing Experiment (HEXTE)• two clusters of 4 NaI/CsI scintillator detectors• 15-250 keV; 1600 cm2
• The All-Sky Monitor (ASM)• 3 shadow cameras; 90 cm2
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Outstanding Issues• Cause of the “Event” (eclipse; cooling/“discombobulation”; phase-
dependent mdot? Jet formation?)
• Stability of the bow shock
• Interactions near periastron
• Geometry of the inner/outer wind
• Density profile of the inner wind of Eta Car
• Radial velocities & mass ratio
Goals: 3-D Reconstruction of mass outflows, ejecta, photon fields Use the orbit of the companion as a probe of the fundamental parameters of Eta Car
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RXTE ObservationsRXTE started observing 2-10 keV emission from Eta Car shortly after launch in Feb 1996
Continued monitoring with daily/weekly/monthly cadence since then
Daily monitoring near X-ray minima
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The Campaign
P=2024 d; e~0.9; a~15 AU; i~50o
Ishibashi et al. 1998; Corcoran et al. 2001
Swift
Simulations by A. Okazaki
The Event
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A (Sea) Change in Mass Loss RateSTIS, Gemini reveal weakening of emission lines in 2009 (Mehner et al. 2010)Large decrease in Mdot from LBV?
Decrease in Mdot may also explain the decrease in duration of X-ray minimum ( See C. Russell, Poster P5.20; Kashi & Soker 2009)
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3-D Spectro-Models: Geometry of Mass Loss
Courtesy T. Madura, Phd thesis
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Synthetic Slit Spectra from 3-D SPH Sims
ω = 270°
ω = 90°
ω = 180°
ω = 0°
STISMODEL
Courtesy Tom Madura
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A Coupled Problem:• Evolution effects mass loss• Mass loss effects evolution
Vink et al. 2001(rotation? magnetic fields?)
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LBV Nebulae
Post RSG?
Smith & Owocki 2006
Brandner et al.
D. F. Figer (UCLA) et al., NICMOS, HST, NASA,
Barlow et al. 1994
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Weight Loss Secrets of the Stars• Mass is lost due to
– radiatively driven stellar winds – Transfer/Roche Lobe leaks– Eruptions– Explosions…
Mass Loss History
For the most massive star,mass loss makes it difficult to measure masses
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Colliding Winds: A Simplified Approach
Early Work: Cherepashchuk (1976), Prilutskii & Usov (1976), Usov (1992), Steven, Blondin, Pollock (1992)
for Adiabatic shock, Lx 1/D(a, e)
Temps of 107K: Thermal X-ray Emission
cooling parameter
>1:adiabatic<1:radiative
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Colliding Winds as a Means to Mdot
Stellar winds will hit something:• Interstellar medium
• another star
• wind from another star
Colliding wind Binaries provide:
– in-situ probe of mass loss
– “Clumping-free” estimator? (Pittard 2007)
– way to probe the stellar parameters
– “laboratory” physics of astrophysical hypersonic shocks
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“Reality is Complicated”– Hideki Yukawa
Importance of radiative instabilities at wind-wind interface (thin shell, etc, Parkin et al. 2009, Pittard & Corcoran 2002)
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Results
• Point source approximation near periastron can reproduce minimum
• More realistic distribution of hot gas along the shock boundary (“extended emission model”) does not reproduce the X-ray minimum as well…
• radiative cooling near periastron vs. adiabatic cooling near apastron?
• shift in X-ray temperature near periastron?
X-ray Value Non-X-ray ValueMass Loss Rate M/yr 1.2x10-6 & 3.8x10-5 same
Terminal Velocity (km/s) 3200 & 2860 same
Escape Velocity (km/s)§ 1280 & 1144 same
§assuming V∞=2.5 Vesc
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Eta Carinae
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Car is one of the most luminous (massive) stars in the Galaxy (most luminous, massive star within 3 kpc.)
Hubble Mosaic of the Carinae Nebula optical & X-ray
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Results: Eta Car
• Point source approximation near periastron can reproduce depth of minimum
• More realistic distribution of hot gas along the shock boundary (“extended emission model”) does not reproduce the X-ray minimum as well…
• radiative cooling near periastron vs. adiabatic cooling near apastron?
• shift in X-ray temperature near periastron?
X-ray Value Non-X-ray Value
Mass Loss Rate 2.5x10-4 & 10-5 10-3 & ?
Terminal Velocity 500 & 3000 500 & ?
Escape Velocity 200 & 1200 200 & ?