Lighting up Stars: New X-ray Diagnostics of Stellar and Young Stellar Atmospheres

1. Lighting up Stars: New X-ray Diagnostics of Stellar and Young Stellar Atmospheres • Rachel Osten • HotSci@STScI • June 2, 2010

2. Outline • “New” discoveries of “old” spectral diagnostics • Iron Kα 6.4 keV emission line and its use in nearby active stellar coronae • Iron Kα 6.4 keV emission line in young stellar objects, and what we can learn

3. Stellar X-ray Spectra • generally well-described by plasma in collisional ionization equilibrium

4. Iron Kα line seen in a variety of astrophysical objects fluorescence in a solar flare; Parmar et al. 1984 K shell ionization edge is at 7.11 keV

5. Iron Kα line seen in a variety of astrophysical objects galactic microquasar; Miller et al. 2002 relativistic broadening

6. Utility of this Line in Stellar Coronae • strength of line depends on height of source above photosphere: larger solid angle for smaller h • Bai (1979) pointed out that this line could give information on the height of the X-ray source and photospheric iron abundance Testa et al. 2008

7. II Peg: K2IV +dM Recent detections of Iron Kα emission line in flares from nearby active stars confirm the relatively compact nature of flares deduced from hydrodynamic modelling Osten et al. 2007 Testa et al. 2008 II Peg (Osten et al. 2007): h/R★ ≤ 0.5 HR 9024 (Testa et al. 2008): h/R★ ≤ 0.3 HR 9024: G1 III

8. first few minutes of EV Lac (M3.5V) flare decay Osten et al. 2010, ApJ submitted

9. production mechanism may be more complicated • some impulsive solar flares show additional Iron Kα flux beyond that produced by thermal photoionization: collisional ionization of the K shell electron • Osten et al. (2010) show that variability of Iron Kα line flux in two flares produces an excess Kα flux above thermal photoionization, find plausible parameters for collisional ionization from a beam of electrons to produce this excess

10. effect of nonthermal electrons in soft X-ray spectrum? • consistent parameter set can explain both excess Kα emission and the lack of detection in hard X-ray spectrum • have constraints on hard X-ray flux Osten et al. 2010, ApJ submitted

11. consistent flare picture • a simultaneously observed white light flare gives optical area constraints A>2x1019 cm2, footpoint radius of 109 cm • implies beam fluxes of 1011-1014 erg cm-2 s-1 • aspect ratio α= r/2l of 0.1 Osten et al. 2010, ApJ submitted

12. X-rays from young stars • coronal emission from magnetic reconnection, but possible contribution from an accretion disk • studies have shown the existence of large flaring loops which may connect the star to the disk

13. Sizes of X-ray emitting loops on young stars are large • Favata et al. (2005) performed hydrodynamic modelling of large flares seen on young stars in ONC during COUP, derived loop semi-lengths • 3/4 of the flares had loop sizes >1 R★ • ≈ 12 YSOs show Iron Kα emission line • use Iron Kα to give size constraints?

14. Kα emission in young stars Tsujimoto et al. 2005 Geometry for young stars with disks (from Camenzind 1990); 6.4 keV fluorescent line seen during some X-ray flares implies a geometry due to reflection of stellar X-rays off disk material

15. Kα emission in young stars Giardino et al. 20076.4 keV emission from the young star Elias 29 • high equivalent width of Iron Kα line • no observed variability in X-ray spectrum (<10 keV) • equivalent widths larger than can be produced by photospheric fluorescence or reflection off a centrally illuminated disk • alternate formation mechanism: collisional ionization from accelerated particles (but also: obscuration?)

16. importance of accelerated particles in stellar atmospheres & environments in solar flares, energetics of accelerated particles >>those of heated plasma stellar X-ray astronomy is only reaching the tip of the iceberg Emslie et al. 2003

17. x-rays bathe the disk, play a role in photoionizing circumstellar material collisional ionization interpretation of Kα line supports the role of nonthermal particles in young stars, impact on the disk ⇒solar system evidence suggesting MeV particles in flash heating of chondrules from Feigelson 2003 the role of accelerated particles needs to be tested against better understood conditions, as obtained in nearby stellar atmospheres

18. Conclusions • study of Iron Kα emission in nearby stellar flares provides constraints on size scales, complementary to hydrodynamic modelling • variability on short timescales points to additional mechanism producing Iron Kα • geometry is more complicated in young stellar objects, but can use results from the Sun and nearby stars (well-characterized environments) to investigate constraints on the nature of accelerated electrons