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The X-Ray Life of Stars: Low-Mass and Pre-Main Sequence

The X-Ray Life of Stars: Low-Mass and Pre-Main Sequence. Manuel Güdel University of Vienna. Outline. On the main sequence: from the Sun back to ZAMS Younger still: Protoplanetary disks and accretion More massive: Herbig stars More embedded: Jets and outflows Eruptive variables

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The X-Ray Life of Stars: Low-Mass and Pre-Main Sequence

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  1. The X-Ray Life of Stars: Low-Mass and Pre-Main Sequence Manuel Güdel University of Vienna

  2. Outline • On the main sequence: from the Sun back to ZAMS • Younger still: Protoplanetary disks and accretion • More massive: Herbig stars • More embedded: Jets and outflows • Eruptive variables • Summary

  3. The Sun Among Stars YEAR • α Cen AB: solar-like behavior (Ayres+ 2009) • cycles • rotational modulation, • slow changes in coronal T The Sun behaves like a normal X-ray G2V star and therefore as an example and anchor for stellar X-ray astronomy.

  4. Line Shifts: Contact Binary Rapidly Rotating Giant (NeX 12.14A centroids, HEG) (composite, 15 lines in MEG) VV Cep FK Com at rest redshift 60-140 km/s (Drake+ 2008) • line shift, • broadening, • Doppler im., • near-polar • region, ≤ 1R* • (Drake+ 2008) • no eclipse • near-polar • on primary • height 0.06-0.2 R* • (Huenemoerder+ 2006)

  5. (Flaring) Coronal Structure from Fluorescence Monte-Carlo modeling of fluorescent efficiency for different source heights (depending on flux > 7.11 keV): h < 0.3R* (Testa+ 2008; see also Osten et al. 2007: alternatively electron impact but inefficient)

  6. Beyond the Limit of Stars: Brown Dwarfs LX/Lbol log LX stars ----------------BDs--------------- (Preibisch+ 2005) log L(Hα) * Young BDs (M6-M9): like stars (M6-M9): coronal activity depends on Teff, not mass! * Old BDs: X-ray faint – but not radio faint! Magnetic activity persists, but coronal heating declines. log LR (Berger+ 2010)

  7. Toward Forming Stars: Accretion and the „High-Energy“ Environment Shocks in accretion streams: vff • T = 3mHv2 / 16k • v  vff = (2GM/R)1/2 • T = a few MK dM/dt = 4R2fvffnemp ne  1012-1014 cm-3 f (Günther+ 2008) r ne = 1012 cm-3 (Günther+ 2006) i f Dense, cool plasma in accretion shocks? (Kastner+ 2002, Stelzer & Schmitt 2004, Schmitt+ 2005, Günther+ 2006, Argiroffi+ 2007, Robrade & Schmitt 2006/07, Huenemoerder+ 2007, etc)

  8. hot OVIII 3-4 MK OVII 2 MK cool 10-30 MK non-accreting accreting 1-2 MK X-Ray Soft Excess (Güdel 2006; Telleschi+ 2007; G&T 2007) L(OVII) CTTS WTTS MS stars Related to accretion AND coronal activity L(OVIII)

  9. But Shocks are Complex... 2.5 dex 0.3 dex • (Curran+ 2011) . . . • MX << Mopt: need right conditions: • too fast: chromospheric absorption • too slow: T too low, no X-rays; • mixture of structured flows • (Sacco+2010) • MX nearly constant! • Not correl. with Mopt. • (Curran+ 2011): optical-depth effects • increasing with accretion rate?

  10. Post-shock Problems with Cooling.... MgXI NeIX • Post-shockcoolswith distance. • O VII should show • higherdensities • higherabsorption • than Ne IX or Mg XII OVII shock (Günther+ 2007) • Observations: (TW Hya, Brickhouse+ 2010) • OVII lower density (Brickhouse+ 2010) • OVII lowerabsorption: • OVII NH = 4.1x1020 cm-2 • NeIX NH = 1.8x1021 cm-2

  11. Shock-heated gas channeled back to the corona? „Accretion-fed corona“ (Brickhouse+ 2010) (2D simulations by Orlando+ 2010; By=1 G, plasma-β >> 1) denser Or fibril-structured accretion streams, dense core developing shock deeper in chromosphere: NeIX from deep, dense layers, OVII only from outer, low-dens layer (Sacco+ 2010) accretion flow OVII, NeIX chromos- phere shock NeIX more NH

  12. Disk Ionization by Stellar X-Rays: Fluorescence (flaring protostar in COUP) Fe K6.4 keV Fluorescence of cool disk material (COUP, Tsujimoto+ 2005) Fe XXV 6.7 keV (30 – 100 MK)

  13. Orion YSO (COUP): 6.4 keV line during impulsive phase like HXR or radio: EW = 1.4 keV!! theoretical disks: <150 eV (George & Fabian 1991, Drake+ 2008) 6.4 keV EWcalc source height (Drake+ 2008) SXR flare K shell electron ejection by nonthermal electrons? (Osten et al. 2007, Czesla & Schmitt 2007) rather inefficient 6.4 keV Irradiating hard source hidden: suppressed continuum: (Czesla & Schmitt 2007)

  14. Herbig Ae/Be Stars CTTS AB Aur (HAe) (Telleschi et al. 2007) • Soft spectrum • low density • high Teff (10 kK) h (≈1600Å) photoexcitation  X-ray source at R > 1.7R*

  15. Magnetically Confined Winds in AB Aur? Accretion in HD 104237A? NeIX: 1012 cm-3 (Testa+ 2008) Jets in HD 163296? (Günther+ 2009) And low-mass companions in many others? (Stelzer+ 2009)

  16. Jets, Accretion Flows • L1551 IRS 5: (observations 2001, 2005, 2009) • Star absorbed, but inner jet X-ray strong • Cooling jet, dominated by expansion • standing structure at 0.5-1” • (Schneider+ 2011) cooling

  17. Jets, Accretion Flows TAX spectrum during 1 week DG Tau DG Tau: observations 2004, 2005/06, 2010 (Chandra LP; Güdel+ 2011) hard/hot: variable soft/cool: constant hard/hot low NH high NH >> NH(AV) time Hard emission: coronal excessively absorbed by dust-depleted accretion flows.

  18. ACIS-S image similar spectrum: soft “stellar” component and jet

  19. 1pixel = 0.0615” Deconvolution of SER-treated ACIS data (Güdel+ 2011) 0.17” (40 AU along jet) 2-8 keV ≈20AU n = 105 cm-3 0.3-1.5 keV Offset in 2010 identical to 2005/06 (Schneider+): standing structure; collimation region? (Günther+ 2009)

  20. Eruptive Variables FU Ori stars (FUors): disk dominates optical spectrum; large amplitude; yrs-decades EX Lup stars (EXors): star dominates optical spectrum; less energetic, shorter • FUOrs not in immediate outburst: (Skinner+ 2009, 2011) • very hard spectra • excess absorption: due to accretion streams, winds, puffed-up inner disk? • possibly X-ray overluminous for known/estimated masses companion FU Ori (Skinner+ 2011)

  21. Z CMa: no X-ray change (Stelzer+ 2009) EX Lup: correlated, accretion-funnel absorbed hard spectrum + accretion- related soft spectrum (Grosso+ 2010) X-rays X-rays V1647 Ori: strong X-ray increase; correlated with accretion rate? induced magnetic reconnnection between star and disk? (Kastner+ 2006) V1118 Ori: moderate X-ray increase; disk inner radius 0.4 to 0.2 AU due to increased accretion post-outburst: X-rays low disruption of magnetosphere by narrower disk? (Audard+ 2010)

  22. Summary • Unexpected diversity of emission mechanisms and heating processes revealed in cool/pre-main sequence stars: • Coronal radiation, T = 1-100 MK • accretion shocks ear photosphere in T Tauri stars • internalor bow shocks in jets of T Tauris and Herbigs • standing shocks (?) in jet collimation regions • magnetically confined winds in Herbig stars • protoplanetary disk ionisation and fluorescence • disk-magnetosphere interactions in eruptive variables • Rich field that has helped diagnose basic physical processes especially around young, pre-main sequence stars.

  23. NGC 2071 IRS1 Extreme Fluorescence? XMM: NGC 2071 IRS 1: - EW(6.4 keV) = 2.4 keV (Skinner+ 2007,10) - constant over years - not accompanied by flares - no Fe XXV contribution! Chandra: (Skinner et al. 2007, 2010) similar: Hamaguchi et al. (1.5 keV) Irradiating hard source hidden (behind star, disk, etc): suppressed continuum, strong fluorescent line

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