x-rays from young massive stars david cohen swarthmore college
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X-rays from Young Massive Stars
David CohenSwarthmore College
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O Stars are the brightest X-ray sources in young clusters
In addition to the X-ray and UV radiation from O stars
Prodigious matter, momentum, and kinetic energy input into the cluster environment via their winds
Tr 14 in Carina: Chandra
The winds are the site and energy source of the X-rays
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Carina: ESO
Tr 14 in Carina: Chandra
HD 93129A (O2If*)
The Carina Complex
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Chandra Carina Complex Project (Townsley)
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Radiation-driven O star winds
Prinja et al. 1992, ApJ, 390, 266
Velocity (km/s)
v∞=2350 km/s
UV spectrum: C IV 1548, 1551 Å
z Pup (O4 supergiant): M ~ few 10-6 Msun/yr
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Three mechanisms for massive star x-ray emission
1. Instability driven shocks
3. Magnetically channeled wind shocks
2. Wind-wind interaction in close binaries
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Three mechanisms for massive star x-ray emission
1. Instability driven shocks
3. Magnetically channeled wind shocks
2. Wind-wind interaction in close binaries
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Owocki, Cooper, Cohen 1999
Radiation-driven winds are inherently unstable: shocks, X-rays
Self-excited instability Excited by turbulence imposed at the wind base
Feldmeier,Puls, Pauldrach 1997
numerical simulations of the line-driving instability
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Numerous shock structures, distributed above ~1.5 R*
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Shocked plasma is moving ~few 1000 km/s
Emission lines should be Doppler broadened
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Only ~1% of the wind is shock heated at any given time
Bound-free absorption in the other ~99% of the wind
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z Pup – as prototypical (and nearby at ~400 pc) – O star X-ray source
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Chandra HETGS Pup (O4 If)
Capella (G5 III) – coronal source – for comparison
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Morphology – line widths Pup (O4 If)
Capella (G5 III) – coronal source – for comparison
Ne X Ne IX Fe XVII
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Morphology – line widths Pup (O4 If)
Capella (G5 III) – coronal source – for comparison
Ne X Ne IX Fe XVII
~2000 km/s ~ vinf
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Pup (O4 If)
Capella (G5 III) – unresolved
Vinf
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Pup (O4 If)Vinf
Kinematics conclusions: consistent with X-rays arising in the stellar wind
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Pup (O4 If)Vinf
What about the distinctive profile shape?
blue shiftasymmetry
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continuum absorption in the bulk wind preferentially absorbs red shifted photons from
the far side of the wind
Contours of constant optical depth (observer is on the left)
wavelength
redblue
bri
ghtn
ess
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Wind opacity due to bound-free transitions
Chandra HETGS
Opacity from partially ionized metals
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We fit these x-ray line profile models to each line in the Chandra data
Fe XVIIFe XVII
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And find a best-fit t*
Fe XVIIFe XVII t* = 2.0
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z Pup: three emission lines
Mg Lya: 8.42 Å Ne Lya: 12.13 Å O Lya: 18.97 Å
t* = 1 t* = 2 t* = 3
Recall:
M
4Rv
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Results from the 3 line fits shown previously
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Fits to 16 lines in the Chandra spectrum of zPup
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Fits to 16 lines in the Chandra spectrum of zPup
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M
4Rv
t*(l) trend consistent with k(l)
Fits to 16 lines in the Chandra spectrum of zPup
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Fits to 16 lines in the Chandra spectrum of zPup CNO processed
Solar
t*(l) trend consistent with k(l)
M
4Rv
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M
4Rv
t*(l) trend consistent with k(l)
M becomes the free parameter of the fit to the t*(l) trend
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M
4Rv
t*(l) trend consistent with k(l)
M becomes the free parameter of the fit to the t*(l) trend
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Traditional mass-loss rate: 8.3 X 10-6 Msun/yrFrom Ha, ignoring clumping
Our best fit: 3.5 X 10-6 Msun/yr
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Fe XVIITraditional mass-loss rate: 8.3 X 10-6 Msun/yr
Our best fit: 3.5 X 10-6 Msun/yr
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Mass-loss rate conclusions
The trend of t* value with l is consistent with :
Mass-loss rate of 3.5 X 10-6 Msun/yr
Factor of ~3 reduction w.r.t. unclumped H-alpha mass-loss rate diagnostics
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z Pup mass-loss rate < 4.2 x 10-6 Msun/yr
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Three mechanisms for massive star x-ray emission
1. Instability driven shocks
3. Magnetically channeled wind shocks
2. Wind-wind interaction in close binaries
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The embedded wind shock (EWS) mechanism should occur in all O stars
But other mechanisms can dominate, especially in young clusters/SFRs
Like colliding wind shocks (CWS) in h Car
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Corcoran et al. 2005
h Car RXTE X-ray light curve
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Parkin et al. 2011
Hydrodynamics simulations of the colliding wind shock mechanism explain much of the observed X-ray properties
• hard emission (~5 keV)• Lx ~ 1035 erg/s• orbital modulation of X-rays
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Carina: ESO
Tr 14: Chandra
HD 93129A (O2If*) is the 2nd brightest X-ray source in Tr 14
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HD93129A – O2 If*
Extremely massive (120 Msun), luminous O star (106.1 Lsun)
Strongest wind of any Galactic O star (2 X 10-5 Msun/yr; vinf = 3200 km/s)
From H-alpha, assuming a smooth
wind
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There is an O3.5 companion with a separation of ~100
AU
Non-thermal radio measurements indicate wind-wind interactionsBut the vast majority of the X-rays
come from embedded wind shocks in the O2If* primary
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kT = 0.6 keV *wind_abs*ismadd 5% kT = 2.0 keV*ism
Typical of O stars like zPup
small contribution from colliding wind
shocks
Chandra ACIS (low-res, CCD) spectrum of HD 93129A
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Its X-ray spectrum is hard
Mg XII Mg XISi XIIISi XIVz Pup
HD 93129A
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Its X-ray spectrum is hard
H-like vs. He-like
Mg XII Mg XISi XIIISi XIV
HD 93129A
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Its X-ray spectrum is hard
low H/He
But the plasma temperature is low:little plasma with kT > 8 million K
MgSi HD 93129A
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Its X-ray spectrum is hard
low H/He
Bound-free absorption in the wind is the cause of the observed X-ray hardness
MgSi
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X-ray line profiles show same characteristic shape
Ro = 1.8 R*
t* = 1.4M-dot ~ 8 x 10-6 Msun/yr
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HD 93129A (O2 If*): Mg XII Lya 8.42 Å
Ro = 1.8 R*
t* = 1.4
M-dot ~ 2 x 10-5 Msun/yr from unclumped Ha
Vinf ~ 3200 km/s
M-dot ~ 8 x 10-6 Msun/yr
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Low-resolution Chandra CCD spectrum of HD93129AFit: thermal emission with wind + ISM absorption
plus a second thermal component with just ISM
kT = 0.6 keV *wind_abs*ismadd 5% kT = 2.0 keV*ism
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kT = 0.6 keV *wind_abs*ismadd 5% kT = 2.0 keV*ism
Typical of O stars like zPup
t*/k = 0.03 (corresp. ~ 8 x 10-6 Msun/yr)
small contribution from colliding wind
shocks
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Three mechanisms for massive star x-ray emission
1. Instability driven shocks
3. Magnetically channeled wind shocks
2. Wind-wind interaction in close binaries
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~7’
Orion Nebula Cluster: age ~ 1Myr; d ~ 450pc
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Color coding of x-ray energy: <1keV, 1keV < E < 2.5keV, >2.5keV
Chandra ~106 seconds, COUP (Penn. St.)
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q1 Ori C (O7 V)
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z Pup
1 Ori C
Chandra HETGS
1 Ori C: hotter plasma, narrower emission lines
z Pup (O4 I): cooler plasma, broad emission lines
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z Pup
1 Ori C
Si XIII
Si XIVMg XIMg XII
H-like/He-like ratio is temperature sensitive
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Differential emission measure (temperature distribution)
q1 Ori C:
peak near 30 million K
Non-magnetic O stars, peak at a few million K
Wojdowski & Schulz (2005)
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Dipole magnetic field (> 1 kG) measured on 1 Ori C
Magnetic field obliquity, ~ 45o
Wade et al. (2006)
Zeeman magnetic field measurements
R. Townsend
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temperature emission measure
MHD simulations of magnetically channeled wind
Channeled collision is close to head-on – at 1000+ km s-1 : T = 107+ K
simulations by A. ud-Doula; Gagné et al. (2005)
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Emission measure
contour encloses T > 106 K
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MHD simulations show multi-106 K plasma, moving slowly, ~1R* above photosphere
contour encloses T > 106 K
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Differential emission measure (temperature distribution)
MHD simulation of 1 Ori C reproduces the observed differential emission measure
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0.0
0.5
1.0
1.5
Sim
ulat
ion
EM
(10
56 c
m-3)
0.0
0.1
0.2
0.3
0.4
θ1 Ori
C A
CIS
-I c
ount
rat
e (s
-1)
0.0 0.2 0.4 0.6 0.8 1.0Rotational phase (P=15.422 days)
Chandra broadband count rate vs. rotational phase
Model from MHD simulation
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g.s. 1s2 1S
1s2s 3S1s2p 3P
1s2p 1P
resonance (r)
intercombination (i)forbidden (f)
10-20 eV
1-2 keV
Helium-like ions (e.g. O+6, Ne+8, Mg+10, Si+12, S+14) – schematic energy level diagram
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1s2s 3S
1s2p 3P1s2p 1P
resonance (r)
intercombination (i)forbidden (f)
g.s. 1s2 1S
The f/i ratio is thus a diagnostic of the strength of the local UV radiation field.
UV
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1s2s 3S
1s2p 3P1s2p 1P
resonance (r)
intercombination (i)forbidden (f)
g.s. 1s2 1S
If you know the UV intensity emitted from the star’s surface, it thus becomes a diagnostic of the distance
that the x-ray emitting plasma is from the star’s surface.
UV
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Rfir=1.2 R*
Rfir=4.0 R*
Rfir=2.1 R*
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Conclusions
• Shock processes in O star winds convert kinetic energy to heat and X-rays
• Three different mechanisms can operate
• Harder and stronger emission from CWS and MCWS
• But significant and sometimes moderately hard X-ray emission from EWS too
• Wind absorption effects are significant and can be used as a clumping-independent mass-loss rate diagnostic: mass lass rates are lower (factors of 3 to 5) than previously thought