Tübingen, Hydrogen-Deficient Stars1 O(He) Stars Thomas Rauch Elke Reiff Klaus Werner Jeffrey W. Kruk Institute for Astronomy and Astrophysics Kepler Center for Astro and Particle Physics Eberhard-Karls University Tübingen Germany

Tübingen, Hydrogen-Deficient Stars2 Overview O(He) stars spectral analyses evolutionary scenario

Tübingen, Hydrogen-Deficient Stars3 O(He) Stars spectral sub-type O(He) by Méndez et al. (1986) –spectra dominated by He II absorption lines CSPN K 1-27 CSPN LoTr 4 HS HS HS preliminary analysis NLTE analysis by Rauch et al. 1998

Tübingen, Hydrogen-Deficient Stars5 O(He) Photospheric Parameters T eff / kK log g H/He C/He N/He O/He CSPN K < 0.2 < CSPN LoTr < < HS HS < 0.2 Rauch et al. 1998, A&A 338, 651 based on optical, UV (IUE), and X-ray (ROSAT) spectra

O(He) stars found amongst PG 1159 stars two pairs of spectroscopic twins –HS LoTr 4 –HS K 1-27 no PN PN

Tübingen, Hydrogen-Deficient Stars7 O(He) CSPN construction of consistent models CS + PN –NLTE model-atmosphere fluxes used as ionizing spectra in photoionization models H  [O III ] K 1-27 LoTr 4

Tübingen, Hydrogen-Deficient Stars8 K 1-27 (PN G ) Rauch, Köppen, Werner 1994, A&A 286, 543 –O(He) CSPN T eff = 105 kK log g = 6.5 (cgs) H/He < 0.2 possible born again star! M = 0.55 M  d = 1.3 kpc –PN solar abundances M = M  possible born again PN? t exp << t evol N 54eV much too low

Tübingen, Hydrogen-Deficient Stars9 LoTr 4 (PN G ) Rauch, Köppen, Werner 1996, A&A 310, 613 –O(He) CSPN T eff = 120 kK log g = 5.5 (cgs) H/He = 0.5 possible born again star! M = 0.65 M  d = 6 kpc –PN Solar abundances M = 0.29 M  normal PN t exp >> t evol

Tübingen, Hydrogen-Deficient Stars10 Evolutionary Status of O(He) Stars AGB [WC] sdO(He) PG 1159 O(He) DA DO our picture 1998 ? ? ? ??

Tübingen, Hydrogen-Deficient Stars11 Evolution of O(He) Stars Evolutionary models (e.g. Herwig et al. 1999) –PG 1159 abundances (He:C:O=33:50:17 by mass) are result of late He-shell flash –O(He) cannot be explained

Tübingen, Hydrogen-Deficient Stars12 O(He) vs. RCrB T eff / kK log g H/He C/He N/He O/He K < 0.2 < LoTr < < HS HS < 0.2 RCrB < V 854 Cen

Tübingen, Hydrogen-Deficient Stars13 Evolution of O(He) Stars evolutionary models (e.g. Herwig et al. 1999) –PG 1159 abundances (He:C:O=33:50:17 by mass) are result of late He-shell flash –O(He) cannot be explained third post-AGB evolutionary sequence? –hydrogen-rich –hydrogen-deficient ( [WC] – PG 1159 – DO ) –hydrogen-deficient ( RCrB – O(He) – DO ) ?

Tübingen, Hydrogen-Deficient Stars14 Spectroscopy of O(He) Stars high T eff  flux maximum in the EUV precise NLTE spectral analysis needs –metal lines (of highly ionized species) ionization equilibria  T eff abundances –high S/N, high resolution UV spectra IUE ÅR < ÅR < ÅR < FUSE ÅR 

Tübingen, Hydrogen-Deficient Stars15 HST + FUSE Spectroscopy photospheric spectra characterized by a few, broad and shallow, absorption lines from highly ionized species e.g. He II, C IV, O VI, Si IV

Tübingen, Hydrogen-Deficient Stars16 UV Observations HST GHRS (Cy06) + STIS –Cy06: if C and N deficient  lines not visible –Cy07: optical analyses will answer questions –Cy08: line profiles mainly sensitive to velocity field –Cy09: data analysis not well described –Cy10: not as compelling as other proposals –Cy11: unclear how precise the abundances have to be (changed PI: Werner) –Cy12: these objects are only a small group in WDs – general interest not clear –Cy13: accepted (added “successors of RCrB stars?” to title) first observations scheduled for Aug 9, 2004 STIS failure Aug 3, 2004

September 18, 2007Hydrogen-Deficient Stars17 Longmore 4

Tübingen, Hydrogen-Deficient Stars18 UV Observations FUSE –Cy03: accepted ( 25 ksec) –Cy06: abundances of 4 stars will not fit a clear pattern (204 ksec) –Cy07: no good justification to repeat for higher S/N (204 ksec) –Cy08: accepted (only 3 stars, 204 ksec) observations scheduled for summer 2007 FUSE failure July 12, 2007

Rauch Thomas, heard about the new wheel failure of FUSE today? They have to terminate the mission.

Tübingen, Hydrogen-Deficient Stars20 FUSE resolution reduced to 7Å

Tübingen, Hydrogen-Deficient Stars21

Tübingen, Hydrogen-Deficient Stars22

Tübingen, Hydrogen-Deficient Stars23 static models

Tübingen, Hydrogen-Deficient Stars24 “wind” models radiation-driven mass-loss rates (Pauldrach et al. 1988)

Tübingen, Hydrogen-Deficient Stars25 mass-loss rates from Pauldrach X 10

Tübingen, Hydrogen-Deficient Stars26 mass-loss rates from Pauldrach X 30

Tübingen, Hydrogen-Deficient Stars27 thin: no iron-group elements thick:iron-group, solar abundances

Tübingen, Hydrogen-Deficient Stars28 red: no iron-group elements blue:iron-group, 10 X solar abundances

Tübingen, Hydrogen-Deficient Stars29 Models with Fe group lines

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HS

Tübingen, Hydrogen-Deficient Stars32 Conclusions mass-loss rates of O(He) stars are not higher than predicted by radiation-driven wind theory  change of surface composition due to wind unlikely FUSE spectra do not show isolated metal lines and thus, allow to give only upper limits for abundances iron-group abundances are (probably) solar UV spectroscopy will be performed with COS / STIS? –determination of C, N, O, and Si abundances to corroborate link to RCrBs

Tübingen, Hydrogen-Deficient Stars33 Miller Bertolami & Althaus, 2006, A&A, 454, 845 M = 0.512M ʘ post early-AGB star “numerical experiment” increased mass-loss rates  hydrogen deficiency

Tübingen, Hydrogen-Deficient Stars34 Conclusions II low-mass O(He) stars –post early-AGB stars –first thermal pulse (TP) after departure from AGB –higher mass-loss rates  hydrogen deficiency high-mass O(He) stars –“normal” born-again scenario –(V)LTP  hydrogen deficiency alternative O(He) scenario –double-degenerate merger similar H/He surface composition suggests that the O(He) stars are the progeny of RCrB stars –RCrB  O(He)  non-DA WD

KPD is a successor of high-mass O(He) stars? “Truth suffers from too many analysis.” Ancient Fremen Saying, Dune Messiah