Institute for Astronomy and Astrophysics, University of Tübingen
X-ray Photospheres
Klaus Werner
Institute for Astronomy and Astrophysics, University of Tübingen, Germany
13 July 2007
X-ray Grating Spectroscopy
Cambridge, USA
1
Institute for Astronomy and Astrophysics, University of Tübingen
Outline
• Introduction: Thermal soft X-ray emission from stellar photospheres
• Chandra spectroscopy of the hot DA white dwarf LB1919:
Implications for vertical chemical stratification in WDs
• Chandra spectroscopy of the PG1159 star PG1520+525:
Constraining the GW Vir instability strip in the HRD
• Chandra spectroscopy of the naked C/O stellar core H1504+65:
The hottest known and chemically most extreme white dwarf
13 July 2007
X-ray Grating Spectroscopy
Cambridge, USA
2
Introduction
Only evolved compact stars are hot enough to be able to emit
thermal (soft) X-radiation from their photosphere:
• Neutron stars (not covered in this talk)
• (Pre-) white dwarfs (WDs), (some are central stars of PNe)
WDs come in two flavors:
• Hydrogen-rich (DA WDs)
• Helium and/or C-O-rich (non-DAs),
relevant here: PG1159 stars, the hottest non-DA WDs
Introduction
Hydrogen-rich WDs:
• Photospheres of hot DAs are almost completely ionized, hence,
very low opacity. Observed X-rays stem from very deep, hot
layers.
• Soft X-rays are detected from objects with Teff >30,000 K
• Famous example: Sirius B
He-C-O-rich (non-DA) WDs:
• Opacities of He and metals prevent leakage of X-rays from
deep hot layers, except for hottest objects, where these
species are highly ionised and more transparent:
• Soft X-rays are detected from objects with Teff >140,000 K
• Famous example: H1504+65
Outline
• Introduction: Thermal soft X-ray emission from stellar photospheres
• Chandra spectroscopy of the hot DA white dwarf LB1919:
Implications for vertical chemical stratification in WDs
• Chandra spectroscopy of the PG1159 star PG1520+525:
Constraining the GW Vir instability strip in the HRD
• Chandra spectroscopy of the naked C/O stellar core H1504+65:
The hottest known and chemically most extreme white dwarf
Metals as sensitive regulators of X-rays from DA
white dwarfs
• Hydrogen in hot DAs almost completely ionized, EUV/soft X-ray
opacity strongly reduced → DAs with Teff >30,000 K can emit
thermal soft X-rays from deep hot layers
• However, ROSAT All-Sky Survey revealed that X-ray emission is
the exception rather than the rule → additional absorbers present
• ROSAT and EUVE revealed that metals are the origin, EUV
spectra are strongly determined by Fe and Ni through large
number of absorption lines
• Radiative levitation keeps traces of metals in the atmosphere
(e.g. Chayer et al. 1995): Generally, metal abundances increase
with increasing Teff.
• Consequently, only very few DAs with Teff>60,000 K were detected
in EUVE and ROSAT All-Sky Surveys.
• Breakthrough in understanding DA atmospheres:
development of self-consistent models for equilibrium of
gravitational settling / radiative levitation, yield vertical
abundance stratification
• Generally, good agreement between observed and
computed EUV flux distributions (e.g. Schuh et al. 2002)
• However, several exceptions are known. Some DAs show
much larger metal abundances than expected from theory,
reason: ISM accretion or wind-accretion from unseen
companion
• More difficult to explain: objects with metallicity smaller
than expected
The problem of metal-poor DA white dwarfs
• Prominent example: HZ43 (Teff = 49,000 K), virtually metal free,
shows no EUV or X-ray absorption features
• Even more surprising: low metallicity of two DAs with even
higher Teff. Two of hottest known DAs have extraordinarily low
metal abundances: LB1919 (69,000 K) & MCT0027-6341 (60,000 K)
• These stars could hold the key to understanding metal-poor DAs
as a class
• We concentrate on LB1919, it is brighter in EUV/X-rays
• LB1919: hottest of the 90 DAs detected in EUVE all-sky survey
(Vennes et al. 1997). Hottest of the 20 DAs whose EUVE spectra
were analyzed in detail by Wolff et al. (1998)
• Chemical composition unknown: EUVE resolution insufficient to
resolve individual lines; metallicity of fainter DAs usually
determined relative to G191-B2B (56,000 K) that is well studied by
UV spectroscopy.
The problem of metal-poor DA white dwarfs
• Our stratified models successfully describe the EUV spectrum of
G191-B2B. EUVE spectra of other DAs could also be fitted by
simply scaling G191´s relative metal abundance pattern.
• But: the EUV spectrum of LB1919 cannot be described by
chemically homogeneous models scaled to G191 relative
abundances. Also, radiatively stratified models fail spectacularly.
The problem of metal-poor DA white dwarfs
Four processes can disturb equilibrium between gravitation and
levitation; potentially responsible for metal-poor hot DAs:
• Mass-loss tends to homogenize chemical stratification. However,
M-dot drops below critical limit (10-16 M/yr) for Teff<70,000 K
(Unglaub & Bues 1998). So, LB1919 should not be affected.
• Wind-accretion calculations (MacDonald 1992) show that ISM
accretion is prevented for LB1919 since its L>1L. Instead, massloss rate of 10-18 M/yr will be sustained
• Convection not expected in LB1919
• Rotation could lead to meridional mixing, however, WDs are
generally slow rotators. In particular, LB1919 shows sharp Lyman
line cores (FUSE), ruling out high rotation rate.
Currently there is no explanation for the low metallicity in LB1919
and similar DAs
Chandra observation of LB1919
• Aim: Empirical determination of abundance stratification using
individual lines of different ionization stages of Fe, Ni ...
• IF metals are stratified, then they are in diffusion/levitation
equilibrium. The low metallicity might originate in earlier
evolutionary phases (selective radiation driven wind?)
• IF metals are homogeneous, then one of the above
mechanisms is at work, contrary to our understanding
Simulated Chandra observations of LB1919
LETG+HRC-S, 120 ksec
Parameters of LB1919:
Teff=70,000 K logg=8.2 Fe/H=7.5∙10-7 Ni/H=5∙10-8
(EUVE analysis of Wolff et al. 1998 with homogeneous models)
Two models shown:
a) nickel enhanced by 1 dex (black line)
b) iron and nickel enhanced by 1 dex (red line)
Strong sensitivity of the spectrum against abundance variations
Chandra observation of LB1919
• Low Energy Transmission Grating (LETG+HRC-S)
• Integration time 111 ksec, Feb. 02, 2006
Chandra observation of LB1919
• Low Energy Transmission Grating (LETG+HRC-S)
• Integration time 111 ksec, Feb. 02, 2006
Line features in model too strong, analysis is on-going, no results yet
Outline
• Introduction: Thermal soft X-ray emission from stellar photospheres
• Chandra spectroscopy of the hot DA white dwarf LB1919:
Implications for vertical chemical stratification in WDs
• Chandra spectroscopy of the PG1159 star PG1520+525:
Constraining the GW Vir instability strip in the HRD
• Chandra spectroscopy of the naked C/O stellar core H1504+65:
The hottest known and chemically most extreme white dwarf
The PG1159 spectroscopic class, a group of 40 stars
• Very hot hydrogen-deficient (pre-) WDs
Teff
= 75,000 – 200,000 K
log g = 5.5 – 8
M/M
= 0.51 – 0.89 (mean: 0.62)
log L/L = 1.1 – 4.2
• Atmospheres dominated by C, He, O, and Ne, e.g. prototype
PG1159-035:
He=33%, C=48%, O=17%, Ne=2% (mass fractions)
• = chemistry of material between H and He burning shells in
AGB-stars (intershell abundances)
late He-shell flash
causes return to AGB
Evolutionary tracks for a 2 M star. Born-again track offset for clarity.
(Werner & Herwig 2006)
• Loss of H-rich envelope consequence of (very) late thermal
pulse during post-AGB phase (LTP) or WD cooling phase (VLTP)
(like Sakurai’s object and FG Sge)
• Hydrogen envelope (thickness 10-4 M) is ingested and burned
(VLTP) or diluted (LTP) in He-rich intershell (thickness 10-2 M)
• In any case, composition of He/C/O-rich intershell region
dominates complete envelope on top of stellar C/O core
Late He-shell flash
+CO core material
(dredged up)
Pulsating (filled circles) and non-pulsating PG1159 stars
PG1159-035
PG1520+525
0.6 M track
(Wood & Faulkner 1986)
Blue (Gautschy et al. 2005) and red (Quirion et al. 2004)
edges of GW Vir instability strip
The pulsator/non-pulsator pair
PG 1159-035 and PG 1520+525
• Do both stars confine the blue edge of the instability strip?
• To what accuracy is their Teff known?
• PG1159-035: 140,000 +/- 5,000 K
from HST/STIS high-resolution UV spectrum, Jahn et al. (2007)
• PG1520+525: 150,000 +/- 15,000 K
from HST/GHRS medium-resolution UV spectrum, Dreizler & Heber (1998)
• Need a more precise Teff estimate for PG1520+525
• Try soft X-ray region; PG1520+525 is the soft X-ray brightest
PG1159 star after H1504+65.
• First attempt with EUVE suggested Teff around 150,000 K,
however, poor-S/N spectrum
Werner et al. (1996)
Soft X-ray spectral modeling of PG 1520+525
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•
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Grid of non-LTE models (hydrostatic, radiative equilibrium)
Ions included: He I-III, C III-V, O IV-VII, Ne IV-IX, Mg IV-IX
Particular model shown here tailored to PG1520+525
For comparison with Chandra observation: model flux used to
simulate Chandra count rate spectrum including expected S/N
Chandra observation of PG 1520+525
• Low Energy Transmission Grating (LETG+HRC-S)
• Integration time 142 ksec, April 04-06, 2006
Chandra observation of PG 1520+525
• Low Energy Transmission Grating (LETG+HRC-S)
• Integration time 142 ksec, April 04-06, 2006
Detail of PG 1520+525 Chandra spectrum, 100-123 Å
model
observation
Detail of PG 1520+525 Chandra spectrum, 100-123 Å
NeVII OVI
OVI
NeVI
model
observation
NeVII NeVI
Identification of OVI and NeVI / VII lines
140,000 K model too cool, good fit with 150,000 K model.
Pulsating (filled circles) and non-pulsating PG1159 stars
PG1159-035
PG1520+525
0.6 M track
(Wood & Faulkner 1986)
PG1159-035 and PG1520+525 indeed confine
the blue edge of the GW Vir instability strip
Outline
• Introduction: Thermal soft X-ray emission from stellar photospheres
• Chandra spectroscopy of the hot DA white dwarf LB1919:
Implications for vertical chemical stratification in WDs
• Chandra spectroscopy of the PG1159 star PG1520+525:
Constraining the GW Vir instability strip in the HRD
• Chandra spectroscopy of the naked C/O stellar core H1504+65:
The hottest known and chemically most extreme white dwarf
Properties of H1504+65
1983 – H1504 is the 7th brightest X-ray source in the 0.25 keV band
(HEAO1 survey, Nugent et al.)
1986 – Optical identification: Extremely hot white dwarf, lacking H
and He lines (Nousek et al.)
1991 – NLTE analysis of optical spectra (Werner):
It is the hottest WD known (Teff close to 200 000 K)
H1504 is devoid of hydrogen and helium
Dominant photospheric species: C and O (50:50)
1999 – Analysis of EUVE & Keck data (Werner & Wolff)
High neon abundance: 2-5% (>20 times solar)
H1504 is an extreme member of the PG1159 spectroscopic class
• Chandra LETG+HRC-S observation of H1504+65:
Sept. 27, 2000, integration time 7 hours
• Richest absorption line spectrum ever recorded from a stellar
photosphere
(Werner et al. 2004, A&A 421, 1169)
• Examples for spectral fitting:
Relative flux
Model fit to H1504+65 Chandra spectrum 80-110 Å
Wavelength / Å
Relative flux
Model fit to H1504+65 Chandra spectrum 80-110 Å
Wavelength / Å
Relative flux
Model fit to H1504+65 Chandra spectrum 80-110 Å
Wavelength / Å
Relative flux
Model fit to H1504+65 Chandra spectrum 110-140 Å
Wavelength / Å
Relative flux
Model fit to H1504+65 Chandra spectrum 110-140 Å
Wavelength / Å
Relative flux
Model fit to H1504+65 Chandra spectrum 110-140 Å
Wavelength / Å
• Origin of unique C/O/Ne surface composition of H1504 remains
unknown. Obviously, H1504 is a bare C/O core of a former AGB
giant.
• Detection of Mg2% in Chandra spectrum even suggests:
H1504 could be a bare O/Ne/Mg white dwarf, i.e. first
observational proof for existence of such objects
• Approved HST UV-spectroscopy (2005): Search for Na, but:
failure of STIS just before observations should be done
Summary
• Hottest WDs have detectable photospheric soft X-ray emission
• X-ray grating spectroscopy important (and often essential) to
derive stellar parameters and details of photospheric processes
• Results are relevant for our understanding of late phases of stellar
evolution
• In detail: Chandra spectroscopy of a hot DA white dwarf and of
two PG1159 stars
• Analysis of LB1919 will provide clues to answer the question why
some hot DAs show a lower metallicity than expected from
radiative levitation theory
• PG1520+525 confines the blue edge of the GW Vir instability strip
(Teff=140,000—150,000 at logg=7)
• H1504+65 could turn out to be the first definitive proof for the
existence of (single) O/Ne/Mg white dwarfs