12 years of chandramay 23, 2011 x-ray line diagnostics of shocked outflows in eta carinae and other...

12 Years of Chandra May 23, 2011

X-ray Line Diagnostics of Shocked Outflows in Eta Carinae and Other Massive Stars

M. F. Corcoran


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 starsb) Colliding Wind Shocks in binaries

• Summary and More Questions


Introduction: The Masses of Massive stars

Mass is the fundamental stellar parameter

but as it increases it becomes (observationally) less well constrained

Moffat 1989


Weight Loss Secrets of the Stars

• Mass is lost due to– radiatively driven stellar winds – Transfer/Roche Lobe leaks– Eruptions– Explosions…

Meynet & Maeder 2003

Mass Loss: Crucial impact on Evolution


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…


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


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 starsB stars

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


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)


Tools: High Energy High Resolution Spectrometry

• Stellar wind velocities (1000-3000 km/s) generate distribution of X-ray emitting gas in the Chandra/XMM band (0.2-10 keV)

• 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


DiagnosticsProcess/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


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


Zeta vs. Zeta


Line Profiles• Widths: narrow to broad

• in general FWHM << 2x wind velocity

• Centroids: zero to negative blueshifts • No redshifts?

• Symmetric or Asymmetric?


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)


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 boundaryIn eccentric systems, variations of density (at

constant temperature) around the orbit clumping-free mass loss rates?

Pittard 2007


Eta Carinae: LBV+?, P=5.5yr

1820 1850 1880 1910 1940 1970 2000

50 yrs

V-Band Lightcurve

“Great Eruption”

A. Damineli

5.5 yr


Eta Car: Orbital and Wind Geometry

3D SPH model (Okazaki et al. 2008)


Eta Car: X-ray Variations

Sampling with HETG


MEG Spectral Variations

Normalized here

Hotter than single starHighly variable

Absorption variations

Emission variations


Cycle-to-Cycle Comparison


Line Formation

Radial Velocities: lines become more blueshifted near periastron as the shock cone sweeps past the line of sight

Model showing the location of maximum emissivity of the Si XIV line along the shock boundary (Henley et al. 2008)


F/I ratios

Zeta Pup Eta Car


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?


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.


WR140: Shock Physics Lab

Courtesy P.M. Williams


2000-12-29(,D/a,)=(1.987,0.23,+44)

periastron

2006-04-01(,D/a,)=(2.649,1.77,-36)

apastron

2008-08-22(,D/a,)=(2.951,0.59,+2)

O-star

Chandra phase-dependent grating spectra of WR140

WC

O

(2009-01-25)

Courtesy Andy Pollock


WR140 phase-dependent MEG spectra

• T~5keV electron continuum 80%• lines 20%• WC abundances

periastron =1.987O-star =2.951

apastron =2.649



XUVOIR : WR140 NeX MEG line profiles

periastron =1.987O-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

Courtesy P.M. Williams


Conclusions & Questions

Other Issues:• magnetic fields & collisionless plasmas• close, late-type companions• satellite lines • charge exchange

• 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?


END


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

giants

supergiants

pec.

g Cas

q1 Ori C

t Scoz Oph

HD 93250

z Pup

Walborn et al. 2009


Summary of Grating Observations

• how many massive stars have grating observations? (hetgs, letgs, rgs)

• how many need these observations?

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

Tools: TGCAT:XATLAS:http://cxc.cfa.harvard.edu/XATLAS


Dichotomy: 1 vs 2• Single and Binaries: different sources of emission:

I. Single stars: embedded shocks at some fraction of the wind terminal velocity. Produced by instabilities in radiative wind driving force; “clumping”

II. 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)


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?


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 chandramay 23, 2011 x-ray line diagnostics of shocked outflows in eta carinae and other...

Documents

years of chandramay

mass loss smooth wind

massloss rates

xray studies of mass

problems of mass

mass loss xray fine

masses of massive stars

embedded wind shocks