The X-Ray Universe. The X-Ray Universe

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1 The X-Ray Universe The X-Ray Universe Potsdam University Dr. Lidia Oskinova Wintersemester astro.physik.uni-potsdam.de/~lida/x-ray.html Chandra X-ray, HST optical, Spitzer IR NGC602 in the SMC d=60pc

2 X-rays from Stars 10 NASA/EIT/W.Waldron, J.Cassinelli

3 Brief reminder: Stars 11 Low-mass stars on main sequence: pp-cycle ε~t 6 Radiative equilibrium core Large temperature gradient outwards Outer convection envelopes Weak stellar winds Massive stars on main sequence: CNO-cycle ε~t 15 Convective core Outer radiative envelopes Strong stellar winds Sp T eff Hβ Other Fetures M/M R/R L/L T(MS) O >33000 weak He +, mb. emission >20 >10 90, , Myr B 10,500-30,000K med. HeI absorption , Myr A 7,500-10,000K strong H lines Gyr Myr F 6,000-7,200 K med Gy G 5,500-6,000 K weak, Ca+ H K, Na D Gy K 4,000-5,250 K v. weak a+, Fe, CH,CN Gy M 2,600-3,850 K TiO, mol Gy L Brown dwarfs <0.08

4 Magnetic fields The Sun by Soft X-Ray Telescope on the Yohkoh spacecraft 12 After 5 billion years, the Sun is still popping and boiling, unable to settle down into the decadent middle age that simple theoretical considerations would suggest. (...) It appears that the radical element for the continuing thread of cosmic unrest is the magnetic field. (E N Parker, Cosmical Magnetic Fields, 1979) Direct measurments of magnetic fileds: photosphere Magnetic filed plays a minor role in the photosphere Magnetic field dominates in the corona: X-rays coronal heating, flares, CMEs, wind acceleration

5 Dynamo theory 13 Cosmic Magnetic Fields are omnipresent. Seeded in Big Bang?? Dynamo: mechanism for flux generation starting from a small initial seed field A fully consistent and predictive theory is still lacking The basics of modern dynamo theories (Parker 1970) Dynamo problem: solution of Maxwell eqs. + Ohm s law + hydrodynamic eqs. Finite kinetic energy and gradients, no magentic filed at infinity Two anti-dynamo theorems: Cowling s theorem: rotationally symmetric magnetic field cannot be maintained by dynamo; Elsasser s theorem: differential rotation alone cannot drive dynamo. Thus, working dynamo requires a complex 3-D and non-axisymmetric structure of both the generated magnetic filed and the driving velocity field. In the Sun, differential rotation and convective flows are the most importnat ingridients. Parker loop: Convective up- and down flows in rotating system generate meridional flux lops through twisting of the field lines due to the action of Coriolis force. Located in the layer between convection and radial zones tacholine

6 αω-dynamo 14 The Parker mechanism has been mathematically formalized and generalized by mean-field theory (Krause, Rädler, Steenbeck 1966). Evolution of a suitably averaged magnetic field in a turbulent flow of an electrically conducting fluid. B t = ( u B +α B ) [(η+β) B α B drives a mean current parallel or anti-parallel to the mean magnetic field. α-effect generates a meridional field from an azimuthal and vs. Ω-effect describels differential rotation, Ω is angular velocity α Ω r describes dynamo waves Dynamo Waves propagate latitudinally, equatorwards or polewards Mean-field αω-dynamo reproduces many of the key features of the solar cycle: the periodic filed reversals, the polarity of sun-spot groups, the equatorwards drift of the activity zones. However the equatorwards drift requires radially inward fast increase of the angular velocity. This is in direct conflict with the results of helioseismology.

7 Ω-effect 15 Differential rotation - the change in rotation rate as a function of latitude and radius within the Sun. Magnetic filed is stretched out and wound around the Sun Latitudal differential rotation can wrap magnetic field line around the Sun in about 8 months.

8 α-effect 16 Effect of rotation on the rising "tubes" of magnetic field- loops look like letter α α-effect governs tilt of spot groups and polarity reversal Similar effects are observed in solar type stars, so Sun is an average star. Dynamo operates in all low- and solar-mass stars. Activity scales with rotation rate.

9 Solar dynamo 17 Observational laws: 1) 11-year period; 2) butterfly diagram; 3) tilt of sunspot group; 4) 22-year magnetic cycle

10 The rotation of the Sun 18 Solar rotation and polar flows of the Sun as deduced from measurements by SOHO. The cutaway reveals rotation speed inside the Sun. The left side of the image represents the difference in rotation speed between various areas on the Sun. Red-yellow is faster than average and blue is slower than average. The light orange bands are zones that are moving slightly faster than their surroundings. Light-orange bands down to km

11 Magnetic fields in and out 19 Convection brings the magnetic field lines on the surface. Lines are always closed, the relative role of gas and magnetic filed pressure changes with height

12 Hinode image of the Chromosphere 20 D=2500 km thick, ρ(r) decreases, T(r) from 4500 to K.

13 Lower corona: now and then 21 SOHO images in Fe IX/X 171A o line. Lower corona, plasma temperature 1MK /12/10 13:00

14 Flares 22 Solar flares are an explosions in the solar atmosphere: Sudden bursts of particle acceleration, plasma heating, and bulk mass motion. Release of energy stored in the magnetic fields that thread the corona. In the largest flares >10 32 ergs can be released in a few minutes. Largest flares observed around solar maximum There is a continious spectrum of flare sizes dn/de=ke -α, α= No consistent theory of flares Reconnection, 3D-topologies

15 RHESSI 3keV-17MeV 23 Significant fraction of released energy accelerating paricles Flare 23 July keV e-p annihilation MeV neutron capture

16 How corona is heated? 24 Coronal activity scales with magnetic activity, 11 yr cycle Corona never disappears: L X =10 26 erg/s in min activity, erg/s in max activity (+ very strong flares) The energy release mechanism is magnetic in origin - but, what specific heat input is dominating in a given coronal feature throughout the solar cycle? Leading theories: * * Sound wave dissipation (Walsh & Ireland 2003) Differential heating by plasmakinetic processes (Vocks & Marsch 2002) Nanoflare heating: small scale reconnection events (Parker 1988) * Model shall describe physical conditions in the corona: density, temperature, velocity Corona heating and dynamo theory should be consistent Sun offeres only a limited range of parameters.

17 Solar-Stellar connections 25 All low- and solar-type stars posses magnetic fileds and are X-ray active Is Sun an average star? The Sun in time? How dynamo and heating processes differ among stellar types? In some cases, the magnetic filed can be directly measured through the Zeeman effect, but in general X-ray emission is used to infer stellar activity. In accordance with dynamo theory the magnetic activity of cool stars is related to their rotation rate: faster spinning stars are more active (α-effect increases with rotation rate). For solar type stars (M< 1.5Msun) and ages of > 100 Myr, angular momentum loss by a stellar wind brakes rotation: rotation nearly uniquely determined by the stellar age. Late type stars, M and brown-dwarfs are fully convective. How their activity level differ from solar-type stars?

18 Coronal activity cycles in solar analog stars The Sun 11-yr cycle is the global manifestation of its activity. The behavior of HD (G2 + G9) is a simple extension of the solar case. Two more studied stars show cyclic activity as well. 26 Favata etal A& A 490, 1121 Fig.4 The evolution of the coronal X-ray temperature and luminosity along the cycle in both the Sun (crosses) and HD (triangles).

19 High-Resolution X-ray spectroscopy 27 CIE plasma: I q /I q-1 depends on T collisional ionization is balanced by radiative recombination from Güdel 2004, A& AR, 12, 71

20 General properties 28 Age-luminosity correlation for M~1Msun L X t 1.5 erg/s, [t]=gyr Sun and its near-twin α Cen A behave very much alike Sun is a star! Low-mass stars stay active for a longer time. Saturation limit of L X /L bol =10-3 Dynamo rules it all! Pleiades (100 Myr), an open stellar cluster. The image is false-coloured: soft (0.2-1 kev), medium (1-1.3 kev), hard ( kev).

21 Young stars (solar type) 29 Feigelson & Montmerle ARAÄ 37, 363

22 Possible sites of X-ray generation 30 X-rays from collapsing extended envelope X-rays from inner disk and outflow X-rays from star-disk magnetic-interaction region Feigelson & Montmerle ARA& A 37, 363

23 Observations 31 DG Tau M=1M sun, Age=1Myr Jets, disk absorption, flares Accretion and corona (?) Importance for planet formation

24 Accretion disks 32 T Tau star flares are importnat to induce turbulens in the disks MHD simulation turbulent disks prevent planet infall to the central star X-rays may be needed to initiate chemical reaction neccesasry for planet formation X-ray activity declines with age

25 HST & Chandra: Pillars of Creation in M2 33

26 Chandra s Orion Nebula 34 see twinkling version

27 Chandra s Orion Trapezium 35 Massive stars in the center of Orion nebula Young (1Myr) O type stars that ionize the nebula... Are X-ray sources and they are hard Massive stars (earlier than A-type) are fully radiative Solar type coronae powered by αω-dynamo cannot operate there Massive stars posses strong stellar winds

28 Massive Stars and Stellar Winds M > 8M 36 Live Fast, Die Young (~ few Myr) T eff > K surface brightness high Light: momentum (+ energy) force to the scattering atoms Light force > gravitational force STELLAR WIND Radiative driving is by line scattering Moving media: Doppler: line width ν v Feedback: radiative driving force depends on acceleration

29 The evolution of (very) massive stars 37 log L / L WR OB T eff Planetary Nebula LBV 40 M Evolution stellar wind (!) O and B type stars Luminous Blue Variables Wolf-Rayet (WR) stars According to dominant spectral lines WN (nitrogen) WC (carbon) WO (oxygen) 1 White Dwarf ZAMS log T eff / kk 1 M

30 How X-rays are generated in O stars? Leading theories. 38 Bow shocks around blobs (Lucy & White 80, Cassinelli etal. 08) Magnetically confined loops at the stellar base (Cassinelli & Swank 83) Wind shocks from the instabilities of radiation driving (Owocki etal. 83) Collisions of dense shells in deep wind regions (Feldmeier etal. 97) Log(Temperature [K]) Density [g/cm 3 ] Distance [R * ] Distance [R * ]

31 High-Resolution X-ray Spectra 39 SiXIII MgXII MgXI NeX NeX NeIX FeXVIII FeXVII OVIII FeXVII FeXVII OVIII OVII NVII NVI CVI XMM RGS enhnaced N ζ Pup O4I XMM RGS ζ Ori O9.7I Wavelength (A o ) * Overall spectral fitting plasma model, abundunces * Line ratios T X (r), spatial distribution * Line profiles velocity field, wind opacity

32 Why it matters: mass-loss from massive stars 40 Ṁ - key feedback agent Ṁ - key parameter of stellar evolution Empirical determinations are model dependant Spectral analisis is hampered by unknown degree of wind clumping Literature values differ by 100 times LMC 30 Dor Chandra+Spitzer Brandl etal. 05 X- rays measure wind opacity -> Ṁ

33 Conclusions: Intrinsic wind emission from O stars 41 X-rays originate close to the stellar core. Hot plasma fills some space between clumps. There are some indications that hottest plasma located close to the core. "Hybrid" model? Loop-like structures at the surface, shocks around blobs due to the wind instability? Stellar wind is clumped untill proven otherwise. RT in clumped wind is not the same Clumping explains shape of X-ray emission line profiles. Consitent Ṁ estimates ranging from radio to X-ray

34 X-Ray from stars 42 Low and solar type stars: magnetic fields Magnetic fields reconnection: coronal heating X-ray Young forming stars (T Tauri stars): coronae and accretion High-mass stars: stellar wind shocks