Model fitting of high-resolution
X-ray Spectroscopy of the
Planetary Nebula BD+30◦3639
Young Sam Yu, Joel Kastner
Rochester Institute of Technology, USA
John Houck
Massachusetts Institute of Technology, USA
Ehud Behar, Raanan Nordon, Noam Soker
Technion-Israel Institute of Technology, Israel
What are planetary nebulae (PNe)?
• Last stages of evolution for stars of initial
mass(1-8M⊙)
• Unveiled central star has terminated its
asymptotic giant branch (AGB) evolution
• Ejected AGB envelope is ionized by
central stellar UV radiation and has yet to
disperse entirely
• The remnant carbon-oxygen core will
become a white dwarf
Various morphologies of PNe
Optical images in
red/green (mostly from
HST)
X-ray images in blue
(XMM & Chandra)
X-ray/visual image
overlays by M. Guerrero
Montage by B. Balick
Stellar evolution along the HR
diagram
From Herwig
(2005, Annu. Rev. Astron. Astrophys.)
X-ray emission from PNe
• Photoionized gas in PNe is far too cool to emit X-rays
• The central stars are at best source of very soft X-rays
• Theorists have long predicted “X-ray emitting gas (hot
bubble) comes from wind interactions”
Possible scenarios:
- Shocked fast wind (~700-2000 km/s) from central star rams into
the ejected red giant envelope that is coasting outward at
~10km/s
- Collimated fast winds or jets blown in conjunction with the
companion to the central star (e.g. NGC 7027)
=> X-ray observations are essential to verify &
distinguish scenarios
The planetary nebula BD+30°3639
Basic data:
Distance from Earth : 1.2 kpc
Dimensions: 4 X 5’’ (~0.02 pc)
Dynamical age : less than 1000yr
Central Star:
Carbon-rich Wolf-Rayet type
Temp : 30,000K
Mass loss rate : ~10-6 Msolar/ yr
Wind speed: ~700 km/s
Top left : Optical (HST WFPC2 in [S III] at
9532A) (Arnaud et al. 1996)
Top right : infrared (Gemini at 2.2 microns (K
band) )
Bottom left : Chandra Image (ACIS-S3)
Bottom right : Chandra spectrum (ACISS3) (Kastner et al. 2000)
Previous results from X-ray CCD spectroscopy:
Fitting models to BD+30°3639 spectra
telescope
(instrument)
NH (1021
cm-2)
TX (106 K)
abundances
reference
ROSAT (PSPC)
1.4*
2.5
…
Kreysing et al
(1992)
Arnaud et al.
(1996)
ASCA (SIS)
1.2*
3.0
C greatly enhanced;
N, Ne enhanced;
Fe depleted
Chandra (ACIS)
1.0
2.7
C, Ne enhanced;
Fe depleted
Kastner et al.
(2000)
Maness et al.
(2003)
Chandra (ACIS)
2.5
2.1
C, Ne greatly enhanced;
N, O enhanced;
Fe depleted
Chandra (ACIS)
2.0
2.4
No useful constraints!
SUZAKU
2.1
2.2
C, N, Ne greatly
enhanced;
Fe depleted
* NH constrained by AV for PSPC and SIS model fits
Georgiev et al.
(2006)
Murashima et al.
(2006)
Chandra observation of BD+30°3639
• BD+30°3639 is the best target for getting
high resolution X-ray grating spectroscopy
due to its large X-ray flux at Earth.
We decided to use
LETG/ACIS-S
combination (rather than
HETG/ACIS-S)
after conducting MARX
simulations
Chandra observation of BD+30°3639
We were granted 300ks observing time for Chandra Cycle 6.
Observation
ID
Date
Instrument
Exp
(ks)
1st order
counts
5409
7278
Feb. 13. 2006
Mar. 22. 2006
LETG/ACIS-S
LETG/ACIS-S
85.4
61.8
1742
1231
5410
8495
8498
Dec. 21. 2006
Dec. 22. 2006
Dec. 24. 2006
LETG/ACIS-S
LETG/ACIS-S
LETG/ACIS-S
53.9
77.1
19.9
1043
1480
386
298.1
5882
Total:
High resolution X-ray spectroscopy
of BD+30°3639
Extracted spectrum result from 5
split datasets
Model fitting result with APED
NeIX FeXVII OVIII OVII
CVI
Model fitting result with APED
CVI
Results
• Temperature of shocked plasma: T ~ 2.25x106 K
• Hydrogen column density : log NH (cm-2) = 21.4
• Plasma abundances, relative to solar:
– C/O ~ 49.3 (yes, C is very overabundant!)
– Ne/O ~ 4.5 (yes, Ne is overabundant)
– Fe/O ~ 0.3 (yes, Fe is quite depleted!)
– N/O ~ 0.3 (yes, N is quite depleted)
The inner layers
of a highly
evolved star
AGB stars are
carbon factories!
From Herwig
(2005, Ann. Rev.
Astron. Astrophys.)
Interpretation
•
X-ray temperature (Tx)
– lower than expected from a simple adiabatic shock model (700 km/s wind should
produce a few x 10 million degree plasma)
– Possible scenarios
• Heat conduction moderates Tx
• Shocked wind presently seen in X-rays was ejected at earlier epoch, when the
“fast wind” was slower (~400km/s)
•
C/O overabundance
– He shell burning and subsequent dredge-up (AGB “third dredge-up”) brings intershell
products (C, s-process elements) to the surface
•
Ne/O over abundance
– α capture process(α : He)
• 14N + α -> 18O(Burnt in He shell burning) -> 22Ne
18F
•
Fe/O, N/O depletion
– s-process within pulse-driven convection zone(PDCZ) ), associated He shell burning
– 22Ne is neutron source in s-process and make Fe depletion
=> non-solar composition of shocked gas originated from intershell region of
AGB star
Summary
• Chandra provides unique means for studying
origin of X-ray-emitting gas in Planetary nebulae
• Sharply non-solar composition of shocked gas
originated from intershell region of AGB star
• Our preliminary results can provide strong
constraints on shock and AGB interior models
• To further understand morphologies and density
structures of X-ray-emitting plasmas in PNe, we
need to develop 3D volumetric model based on
these results