1. from Central Stars of Planetary Nebulae
Hard X-ray Emission
from Central Stars of Planetary Nebulae
Martín A. Guerrero
Instituto de Astrofísica de Andalucía, IAA-CSIC, Spain
You-Hua Chu & Robert A. Gruendl
University of Illinois at Urbana-Champaign, USA
The X-ray Universe 2011
Berlin, June 29, 2011

2. Talk Outline  X-ray emission expected from
central stars of planetary nebulae (CSPNe)
 Photospheric X-ray emission.
 Coronal emission from a companion.
 Detection of unexpected hard X-ray emission from CSPNe
 Possible mechanisms for hard X-ray production:
 Pros and cons.
 Implications for PN formation and evolution.
Martín A. Guerrero, Hard X-ray Emission from CSPNe 2

3. PNe descend from low- and intermediate-mass stars:
0.8 M < Mi < 10 M 3

4. Photospheric X-ray Emission from Hot Central Stars
Photospheric emission from hot (Teff > 100,000 K) CSPNe have
high-energy tails that may extend into the X-ray domain.
T. Rauch NLTE model
Martín A. Guerrero, Hard X-ray Emission from CSPNe 4

5. Photospheric X-ray Emission from Hot Central Stars
Einstein, EXOSAT and ROSAT detections of soft X-rays:
A 36, K 1-16, K 1-27, LoTr 5, NGC 246, NGC 1360, NGC 3587, NGC 4361, NGC 6853, NGC 7293
(de Korte et al. 1985; Tarafdar & Apparao 1988; Apparao & Tarafdar 1989; Kreysing et al. 1992;
Rauch et al. 1994; Hoare et al. 1995)
Search of the entire ROSAT archive for X-ray emission from PNe
(Guerrero, Chu & Gruendl 2000)
 Point source (at the PSPC spatial resolution of 30”)
 Soft X-rays peaking below 0.2 keV (BB or photospheric models)
Martín A. Guerrero, Hard X-ray Emission from CSPNe 5

6. Photospheric X-ray Emission from Hot Central Stars
Einstein, EXOSAT and ROSAT detections of soft X-rays:
A 36, K 1-16, K 1-27, LoTr 5, NGC 246, NGC 1360, NGC 3587, NGC 4361, NGC 6853, NGC 7293
(de Korte et al. 1985; Tarafdar & Apparao 1988; Apparao & Tarafdar 1989; Kreysing et al. 1992;
Rauch et al. 1994; Hoare et al. 1995)
Search of the entire ROSAT archive for X-ray emission from PNe
(Guerrero, Chu & Gruendl 2000)
 Ideal conditions for detection
Small distance
Low ISM extinction
 Nebulae and central stars at late evolutionary stage
High Teff
Low photospheric metal content
Large and optically thin nebulae
Martín A. Guerrero, Hard X-ray Emission from CSPNe 6

7. Unexpected Hard X-ray Emission at CSPNe
[N II] NGC 6543 H
X-rays
NGC 6543 (Chu et al. 2001)
kT1 = 0.1 keV, kT2 = 0.8 keV
Lx = 1.0×1030 erg s-1
NGC 2392 (Guerrero et al. in prep.)
kT = 2.8 keV
Lx= 3.0×1030 erg s-1
X-rays
[N II] H
[N II] NGC 2392
keV
keV
KeV
Martín A. Guerrero, Hard X-ray Emission from CSPNe 7

8. Does Hard X-ray Emission Implies a Companion?
 Coronal emission from a late type companion.
 Accretion:
 From a companion (as for quiescent novae or cataclysmic variables).
 From a debris disk.
Martín A. Guerrero, Hard X-ray Emission from CSPNe 8

9. GISW Density Gradient and Collimated Outflows
The (Generalized) Interacting-Stellar Winds (G)ISW model of PN formation
Symmetric AGB mass loss
U Cam (Oloffson et al. 2010)
U Cam
Origin of the density gradient in the GISW model?
Companion: preferential AGB mass equator
 Collimated outflows
Accretion disk and launch of fast
collimated outflows
Effects on nebular morphology
Martín A. Guerrero, Hard X-ray Emission from CSPNe
M 2-48

10. LoTr 5: When a galaxy group meets a PN
Serendipitous Chandra and XMM-Newton observations (RX J )
X-rays peaking at ≈1 keV with secondary peak at ≈1.5 keV and high energy tail
kT1=0.61 keV, kT2=3.1 keV Lx= 8.0 × 1030 ergs s-1
Notable X-ray variability (Montez et al. 2010; Guerrero et al., in prep.)
XMM observations give similar spectral shape (kT1=0.6 keV, kT2=2.3 keV), but Lx = 2.2×1030 ergs s-1 and no clear variability 10

11. LoTr 5: a bipolar PN with a multiple central star
Multiple system IN Comae including a G5 III star
Rotation speed close to break-up velocity:
vrot sin(i) ≈ 67 km s-1
Saturated coronal activity from a G5 III star:
Lx/Lbol ≈ 10-3
LoTr 5 is an almost pole-on bipolar PNe (Graham et al )
Bipolar axis and binary orbital plane are orthogonal
Martín A. Guerrero, Hard X-ray Emission from CSPNe 11

12. Two post-common envelope close binary CSPNe
Montez et al. (2010) has detected X-ray emission from the O-type subdwarf + late-type companion CSPNe of HFG 1 and DS 1
kT ≈ keV
Lx ≈ 1030 ergs s-1
Saturated coronal activity
Lx/Lbol ≈ 10-3
DS 1
DS 1 (Miszalski et al. 2009)
Martín A. Guerrero, Hard X-ray Emission from CSPNe 12

13. PNe with Collimated Outflows and Hard X-ray CSPNe
NGC 2392
Mz 3
NGC 6543
NGC 6826 13

14. Late Type Stars Coronal Activity and Age
In late type stars, Lx is related to rotation and declines with age !!!
Lx≈ v2rot (Pallavicini et al. 1981)
Lx≈ t-β < β < 2.0
(Güedel et al. 1997)
Mi=3M
Mi=1.5M
Enhanced X-ray emission
(Lx/Lbol = 10-3 – ):
 Massive PN progenitor for companions to stay active longer enough
 Evolution towards the PN phase sped up by the companion
 Angular momentum transfer from the PN progenitor to the companion
 Rotation of companion abnormally fast: LoTr 5
 Giants in symbiotic stars rotate faster (Zamanov et al. 2006)
 Rapid-rotating companions of WDs: 2RE J , 2RE J
 Tidal synchronization (close binaries), wind accretion (wide binaries)
(Jeffries & Stevens 1996)
 F7-M4 can be spun up to P ≈ 3 days (Soker & Kastner 2002)
for AGB and RGB 14

15. Hard X-ray Emission from the Helix Central Star
ROSAT: Hard (> 0.5 keV) component (Leahy et al. 1994)
- not a known binary system; spectrophotometric standard !
not a powerful stellar wind to produce a hot bubble
Spatial coincidence? Background source?
Diffuse emission?

16. Chandra and XMM-Newton Observations of the Helix
A point source at the central star
X-ray emission peaking at 0.9 keV and decline of X-ray emission:
kT1 = 0.65 keV, kT2 = 1.3 keV, Lx = (6.4 – 7.7)×1029 ergs s-1
(Guerrero et al. 2001)
Serendipitous XMM-Newton observations: even lower Lx
KT1 = 0.65 keV, KT2 = 1.3 keV, LX= 5.2×1029 ergs s-1
(Guerrero et al., in prep.)
Martín A. Guerrero, Hard X-ray Emission from CSPNe 16

17. Hard X-rays from the Helix CSPN: Not What You Expected
Variability of stellar H and [N II]: possible evidence for dMe companion (Gruendl et al. 2001)
Spitzer detection of a point source in all IRAC and MIPS bands up to 70 μm
 IRAC 3.6, 4.5, and 5.8 μm emission consistent with the CSPN Rayleigh- Jeans tail, but no sign of the dwarf companion!
 IRAC 8.0 μm and MIPS emission: cold, K thin debris disk of dust distributed between AU
 Disk origin: Collisions of Kuiper Belt- like objects, or to the breakup of comets from an Oort-like cloud? (Su et al. 2007)
X-ray emission and H and [N II] variability product of intermittent accretion of material from the disk
Martín A. Guerrero, Hard X-ray Emission from CSPNe 17

18. Enigmatic Hard X-ray Emission from WDs
PG 1159 and KPD
(O’Dwyer et al 2003; Chu et al 2004)
 Among the two hottest WDs
 PG 1159 is a pulsator -> no binary
 KPD not a background
source: O VIII lines (Werner et al. 1996)
Chandra observations (Chu et al., in prep.)
 Very low metal content:
- X-ray transparent atmospheres
- tight constrain on accretion
 Very small fraction of single WDs
shows hard X-rays (Bilíková et al. 2010)
Leakage of the Wien high-energy tail
from deep in the stellar atmosphere?
Detailed ad-hoc NLTE models by T. Rauch at the Teff and with precise surface abundances have failed so far to reproduce it. 18

19. Shock-in Winds: OB and WR stars
NGC 2392*, NGC 6543 and NGC 6826 also have fast stellar winds
DACs (Discrete Absorption Components) moving
outwards in the wind of NGC 6543 with
~0.17 days modulation (Prinja et al. 2007)
Variability of the P Cygni profiles of
high-excitation UV lines in NGC 2392 and
NGC 6826 similar to those seen in OB stars (Guerrero & De Marco, in prep.)
v ~ 1,100 km s-1
Martín A. Guerrero, Hard X-ray Emission from CSPNe 19

20. Shock-in Winds: OB and WR stars
Instabilities of the driving mechanism of the stellar wind can lead to
shocks in the wind of single O, early B, and WR stars.
(Lucy & White 1980; Gayley & Owocki 1995)
Lx = 1031 – 1033 ergs s Tx ≈ a few 106 K (kT = keV)
log Lx/Lbol = ± log LX = 0.574×log Lw
(Sana et al. 2006)
Martín A. Guerrero, Hard X-ray Emission from CSPNe 20

21. Shock-in Winds: OB and WR stars
The fast stellar winds of NGC 2392*, NGC 6543, and NGC 6826
WR stars: Lw ≈ ergs s CSPNe: Lw ≈ ergs s-1
NGC 6543 and NGC 6826 have lower Lx/Lw:
log Lx/Lw = -4.1 – -4.9
OB stars log Lx/Lw = -2.5 – -3.1
NGC 2392:
kTshock = (3/16) m v2
v∞ = 420 km s-1  kT = 1.3 [v/1,000 km s-1]2 keV = keV !!
Martín A. Guerrero, Hard X-ray Emission from CSPNe 21

22. Other Possible Origins of the Hard X-ray Emission
 Accretion:
 From a companion (quiescent novae or cataclysmic variables).
 From a debris disk.
CVs and quiescent recurrent novae have large optical/IR/UV and X- ray variability.
Accretion is inhibited by fast stellar winds: dMac/dT > 10-5 M yr-1
 Stellar winds:
 Interacting winds (WR+OB binary systems or symbiotic stars).
Orbital radius dependence, but basically noticeable wind and unseen companion -> <-
 Wind ablation of a stellar or sub-stellar companion.
Orbital radius changes with stellar evolution: the lower the mass of the companion, the stronger dependence
Martín A. Guerrero, Hard X-ray Emission from CSPNe 22

23. Possible Origins of the Hard X-ray Emission
CSPN Coronal Wind WD Accretion
shock-in colliding ablation companion dust
LoTr ✓ ✗ ✗ ✗ ✓ ✗ ✗
Mz ✓ ? ? ? ✗ ✗ ✗
NGC ✓ ✗ ✗ ? ✗ ✗ ✗
NGC ✓ ✓ ✓wb ✓cb ✗ ✗ ✗
NGC ✓ ✓ ✓wb ✓cb ✗ ✗ ✗
NGC ✗ ✗ ✗ ✗ ✓ ✗ ✓
DS ✓
HFG ✓
Martín A. Guerrero, Hard X-ray Emission from CSPNe 23

24. Conclusions  Coronal emission from a late-type companion
Enhanced coronal activity and rotation late in the star life
 Shock-in winds as in OB stars
Scaled down versions of OB winds: Lx/Lbol, Lx/Lw
Wind variability
 Other wind interactions
Dwarf stellar (no sub-stellar or giant planet) companion
 Leaking from underneath the atmosphere as in hot WDs
Unlikely due to “relatively” low Teff and surface abundances
 Accretion
Not a viable mechanism in general
Accretion from a debris disk as a last resource to explain Helix
CSPN X-ray emission: abundances, variability
Martín A. Guerrero, Hard X-ray Emission from CSPNe 24