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X-ray Photospheres. Klaus Werner Institute for Astronomy and Astrophysics, University of Tübingen, Germany. Outline. Introduction: Thermal soft X-ray emission from stellar photospheres

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X-ray Photospheres

Klaus Werner

Institute for Astronomy and Astrophysics, University of Tübingen, Germany

X-ray Grating Spectroscopy

Cambridge, USA


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

X-ray Grating Spectroscopy

Cambridge, USA


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 metallicitysmallerthan 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, mass-loss 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 LB1919LETG+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

10-4 M

10-2 M

+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

  • 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:


Model fit to H1504+65 Chandra spectrum 80-110 Å

Relative flux

Wavelength / Å


Model fit to H1504+65 Chandra spectrum 80-110 Å

Relative flux

Wavelength / Å


Model fit to H1504+65 Chandra spectrum 80-110 Å

Relative flux

Wavelength / Å


Model fit to H1504+65 Chandra spectrum 110-140 Å

Relative flux

Wavelength / Å


Model fit to H1504+65 Chandra spectrum 110-140 Å

Relative flux

Wavelength / Å


Model fit to H1504+65 Chandra spectrum 110-140 Å

Relative flux

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 dwarfandof 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


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