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X-ray Photospheres

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

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  1. X-ray Photospheres Klaus Werner Institute for Astronomy and Astrophysics, University of Tübingen, Germany X-ray Grating Spectroscopy Cambridge, USA

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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.

  7. 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

  8. 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.

  9. 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.

  10. 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

  11. 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

  12. 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

  13. Chandra observation of LB1919 • Low Energy Transmission Grating (LETG+HRC-S) • Integration time 111 ksec, Feb. 02, 2006

  14. 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

  15. 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

  16. 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)

  17. late He-shell flash causes return to AGB Evolutionary tracks for a 2 M star. Born-again track offset for clarity. (Werner & Herwig 2006)

  18. 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

  19. Late He-shell flash 10-4 M 10-2 M +CO core material (dredged up)

  20. 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

  21. 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

  22. Werner et al. (1996)

  23. 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

  24. Chandra observation of PG 1520+525 • Low Energy Transmission Grating (LETG+HRC-S) • Integration time 142 ksec, April 04-06, 2006

  25. Chandra observation of PG 1520+525 • Low Energy Transmission Grating (LETG+HRC-S) • Integration time 142 ksec, April 04-06, 2006

  26. Detail of PG 1520+525 Chandra spectrum, 100-123 Å model observation

  27. 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

  28. 140,000 K model too cool, good fit with 150,000 K model.

  29. 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

  30. 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

  31. 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

  32. 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:

  33. Model fit to H1504+65 Chandra spectrum 80-110 Å Relative flux Wavelength / Å

  34. Model fit to H1504+65 Chandra spectrum 80-110 Å Relative flux Wavelength / Å

  35. Model fit to H1504+65 Chandra spectrum 80-110 Å Relative flux Wavelength / Å

  36. Model fit to H1504+65 Chandra spectrum 110-140 Å Relative flux Wavelength / Å

  37. Model fit to H1504+65 Chandra spectrum 110-140 Å Relative flux Wavelength / Å

  38. Model fit to H1504+65 Chandra spectrum 110-140 Å Relative flux Wavelength / Å

  39. 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

  40. 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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