X-ray Line Diagnostics of Shocked Outflows in
Eta Carinae and Other Massive Stars
M. F. Corcoran
12 Years of Chandra
May 23, 2011
Overview
• Introduction: Problems of Mass and Mass Loss
• X-ray Fine Analysis as a Probe of Mass Loss
• Mass Outflows and Shocks:
a) Embedded Wind Shocks in single stars
b) Colliding Wind Shocks in binaries
• Summary and More Questions
12 Years of Chandra
May 23, 2011
Introduction:
The Masses of Massive stars
Mass is the fundamental
stellar parameter
but as it increases it becomes
(observationally) less well
constrained
Moffat 1989
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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: Crucial impact
on Evolution
Meynet & Maeder 2003
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Problems Measuring Mass Loss
•
•
•
•
Smooth wind vs. clumped? (Mass-loss rates obtained from P V wind profiles are
systematically smaller than those obtained from fits to Hα emission profiles or radio
free-free emission by median factors of ~20–130, Fullerton, Massa & Prinja 2006)
spherical or not?
eruption: timescales & rates?
explosion: core & remnant amounts?
Beginning to End…
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X-ray Studies of Mass Loss
• All observed OB stars in the range B2V–O2I are X-ray emitters
• Thermal X-rays arise from at least 3 (non-exclusive) shock
processes:
 embedded wind shocks from unstable line driving
 wind-wind collisions in binaries
 magnetically-confined wind shocks
• X-rays sensitive to detailed mass loss process:
 Shocks depend on density and velocity as f(r)
 continuum and lines sensitive to overlying opacity
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Some Observed X-ray Properties
• For single OB stars, Lx α 10–7 Lbol (known since days of the Einstein
Observatory)
• Single WR stars very weak X-ray sources
Naze et al.2011 Chandra Carina Project
O stars
B stars
12 Years of Chandra
Broadly Speaking:
• Emission thermal; kT < 1 keV for single O
stars; lots of emission lines
• Single stars: X-ray emission non-variable (but
see q2 Ori A, Schulz et al. 2996, Mitschang et
al. 2009)
• Binaries: may be harder (kT>2 keV); brighter;
variable
May 23, 2011
Questions
• What is the spatial distribution of the shocked gas in the
stellar wind?
• What is the temperature distribution vs. radius?
• What does the shocked gas tell us about the detailed mass
loss process (mass loss rates, velocity laws, unstable regions,
large-scale vs. small-scale clumps)
12 Years of Chandra
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Tools:
High Energy High Resolution Spectrometry
• Stellar wind velocities (1000-3000 km/s) generate distribution of
emitting gas in the Chandra/XMM band (0.2-10 keV)
X-ray
• Shocked gas generates thermal X-ray line emission useful for detailed
measures of wind flow & densities
• High-energy lines particularly important since inner winds have high
optical depth at soft X-ray energies (E<1 keV)
• HETG spectroscopy provides a unique tool:
– energy band matches wind dynamics
– spectrally resolve broad lines, esp. at high energies
– spatially resolve clustered stars
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Diagnostics
Process/Property
Diagnostic
Comments/Complications
wind velocity and density profile
Line profile shape
profiles sensitive to velocity, impact
parameter, density/optical depth
ionization process (collisions vs. radiation)
G=(F+I)/R (~1 collisional; >4 photoionized)
resonance line scattering, optical depth
effects
Equilibrium vs. Non-Eq.
G > 1, R> Ro, Li-like satellites  non-eq
Abundance
Resonance line ratios of different
elements
Uncertainty due to poorly constrained ion
fractions
Location
R=F/I
sensitive both to ambient particle and UV
radiation density; blending with satellite
lines may be important
Electron Temperature
G ratio, H/He ratio
photoionization; resonance scattering;
distinct formation regions
Non-Thermal Processes
ratio of satellite to resonance lines
line broadness
see Porquet, Dubau, Grosso 2011
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a) Embedded Wind Shocks
• radiative driving force which generates winds in OB and WR stars unstable
to Doppler shadowing (Lucy & White 1980, Feldmeier 1997)
• Wind should break up into slow dense clumps embedded within a lower
density, fast wind
• collisions produce shock heated gas dependent on local velocity field
• Temperatures typically millions of K since shocks expected to occur in wind
acceleration zone close to the star
• Opacity of overlying wind beyond the acceleration zone should reduce the
red-shifted portion of the line relative to the blue shifted portion,
dependent on wind density
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Zeta vs. Zeta
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Line Profiles
• Widths: narrow to broad
• in general FWHM << 2x
wind velocity
• Centroids: zero to negative
blueshifts
• No redshifts?
• Symmetric or Asymmetric?
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Single Star Summary
• Lines broad but only 0.2 < HWHM < 0.8 (Gudel & Naze 2009)
•
EMX << EMwind
•
Profiles (apparently) symmetric; centroids show small blueshifts (exc. Zeta
Pup).
•
Opacities low; wavelength-dependent?
•
Lines apparently form rather deep in the wind (Ro~1.5-1.8 R*)
• Zeta Pup: opacity wavelength dependent, but profiles require a reduction
of a factor of 3 in mass loss rate (Cohen et al. 2010)
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b) Eta Car and WR 140:
The Thermal Spectra of Colliding Winds
Long-Period, eccentric colliding wind systems are excellent laboratories for
studying:
– the development of astrophysical shocks
– the physics of line formation
– the process of mass loss in more than 2 dimensions
Key properties:
location of X-ray emitting volume
constrained to the wind-wind shock boundary
In eccentric systems, variations of density (at
constant temperature) around the orbit
 clumping-free mass loss rates?
Pittard 2007
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Eta Carinae: LBV+?, P=5.5yr
V-Band Lightcurve
50 yrs
1820
1850
1880
1910
1940
1970
2000
“Great Eruption”
A. Damineli
5.5 yr
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Eta Car: Orbital and Wind Geometry
3D SPH model
(Okazaki et al. 2008)
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Eta Car: X-ray Variations
Sampling with HETG
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MEG Spectral Variations
Hotter than single star
Highly variable
Emission variations
Normalized here
Absorption variations
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Cycle-to-Cycle Comparison
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Line Formation
Radial Velocities: lines become more
Model showing the location of maximum
blueshifted near periastron as the shock cone emissivity of the Si XIV line along the shock
sweeps past the line of sight
boundary (Henley et al. 2008)
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F/I ratios
Zeta Pup
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Eta Car
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HETG Results
Strong changes in continuum flux and lines
lines of high ionization potential show smaller blueshifts than lines of
lower IP
•high IP lines form close to stagnation point where electron
temperature is higher
 variations in centroid velocities of Si & S lines
• probably due to changing orientation of bow shock to line of sight
• possible transient emission associated with RXTE flares? (Behar et
al. 2005)
 R=F/I ratio is above the low-density limit
• ionization from inner shell of Li-like ion in NIE plasma?
• excitation to n>3 levels followed by radiative cascades?
• Charge exchange?
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Results: Iron
 Broad, Variable Fe K fluorescence
• X-ray scattering by wind
 Fe XXV “satellite lines” which increase in strength near periastron
• cooling (via conduction?) due to the growth of dense cold instabilities
• dust formation?
 Fe XXV & Fe K profile nearly identical in 2 near periastron observations
separated by 1 cycle.
12 Years of Chandra
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WR140: Shock Physics Lab
Courtesy P.M. Williams
12 Years of Chandra
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Chandra phase-dependent grating spectra of WR140
(2009-01-25)
WC
O
2006-04-01
(,D/a,q)=(2.649,1.77,-36)
apastron
Courtesy Andy Pollock
12 Years of Chandra
2008-08-22
2000-12-29
(,D/a,q)=(2.951,0.59,+2)
O-star
(,D/a,q)=(1.987,0.23,+44)
periastron
May 23, 2011
WR140 phase-dependent MEG spectra
• T~5keV electron continuum 80%
• lines 20%
• WC abundances
periastron =1.987
O-star =2.951
apastron =2.649
Courtesy Andy Pollock
12 Years of Chandra
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Courtesy P.M.
Williams
XUVOIR : WR140 NeX MEG line profiles
Courtesy Andy Pollock
periastron =1.987
O-star =2.951
apastron =2.649
Apastron: view flow from both sides of shock cone; velocity equilibrium?
Periastron: emission from leading arm suppressed – due to changes in cooling?
O-star conjunction: emission from near side of shock cone dominates
12 Years of Chandra
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Conclusions & Questions
• Embedded shock emission from single stars suggest that most of the shocked gas
exists deep in the wind near the wind acceleration region
• Mass loss rates need to be reduced
• How general is the wavelength dependence of X-ray line opacity?
• Importance of radiative instabilities/NEI effects in X-ray line formation in binaries
• change in shock physics/cooling near periastron in Eta Car and WR 140
• (Very) small R ratios in long-period binaries vs. large R ratios in single stars
• Evidence for ionization stratification along the shock boundary in colliding-wind
systems
• No double-peaked profiles in CWBs: simple conical picture too simple?
Other Issues:
• magnetic fields & collisionless plasmas
• close, late-type companions
• satellite lines
• charge exchange
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END
12 Years of Chandra
May 23, 2011
X-ray Line Emission: Observed Trends
• Declining trend of X-ray ionization with stellar spectral
type and weakening of H- to He-like ratios (Walborn et
al. 2009)
dwarfs
supergiants
g Cas
giants
pec.
•
q1 Ori C
HD 93250
z Pup
z Oph t Sco
Walborn et al. 2009
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Summary of Grating Observations
• how many massive stars have grating
observations? (hetgs, letgs, rgs)
• how many need these observations?
Tools:
TGCAT:
XATLAS:http://cxc.cfa.harvard.edu/XATLAS
Relatively small number of stars observed at high resolution, so hard to make firm
conclusions regarding trends in line formation properties and connection to winds
and stellar properties
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Dichotomy: 1 vs 2
• Single and Binaries: different sources of emission:
I.
II.
Single stars: embedded shocks at some fraction of the wind
terminal velocity. Produced by instabilities in radiative wind driving
force; “clumping”
Binaries: Shocked gas produced by the collision of the wind of one
star and that of its companion; also (I) above
Studies of X-ray emission from single and binary stars provide
complementary information regarding stellar mass loss (and
more generally the production of X-rays in shocks)
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Examples: Zeta Pup & Zeta Ori
• Perhaps the best-studied massive star at high X-ray spectral resolution
• Zeta Pup: O4If, N overabundance compared to C, O
• Zeta Ori: O9.7Ib
• Strong thermal line emission
• Mdot ~ 10-5 M yr-1 based on H-α line
Issues:
Lines much less blueshifted and more
symmetric than expected given high Mdot
 Also high-temperature X-ray lines (S XV)
deep in wind; but how deep?
12 Years of Chandra
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Zeta Pup Line Analysis
• X-ray wind opacity: grey (large, dense clumps) vs. wavelength-dependent
(small, optically thin clumps)
• Kramer et al. (2003) analysis of HETG spectrum: X-ray optical depth nearly
independent of wavelength: large, dense clumps
• Cohen et al. (2010) re-analysis, including short-wavelength lines: X-ray
optical depth IS dependent on wavelength;
– requires ~factor of 3 reduction in Mdot
– lines form near 1.5 R*
• Short wavelength lines at high S/N key probes of the inner wind
12 Years of Chandra
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