Lighting up Stars: New X-ray Diagnostics of Stellar

and Young Stellar Atmospheres

Rachel OstenHotSci@STScIJune 2, 2010

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

Stellar X-ray Spectragenerally well-described by plasma in collisional ionization equilibrium

Iron Kα line seen in a variety of astrophysical

objectsfluorescence

in a solar flare;

Parmar et al. 1984

K shell ionization edge is at 7.11 keV

Iron Kα line seen in a variety of astrophysical

objects

galactic microquasar;

Miller et al. 2002

relativistic broadening

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

Osten et al. 2007

Testa et al. 2008

Recent detections of Iron Kα emission line in

flares from nearby active stars confirm the

relatively compact nature of flares deduced from hydrodynamic

modellingII Peg (Osten et al. 2007): h/R★ ≤ 0.5

HR 9024 (Testa et al. 2008): h/R★ ≤ 0.3

II Peg: K2IV +dM

HR 9024: G1 III

first few minutes of EV Lac (M3.5V) flare decay

Osten et al. 2010, ApJ submitted

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

effect of nonthermal electrons in soft X-ray spectrum?

consistent parameter consistent parameter set can explain both set can explain both excess Kα emission excess Kα emission and the lack of and the lack of detection in hard X-detection in hard X-ray spectrumray spectrum

have constraints on have constraints on hard X-ray fluxhard X-ray flux

Osten et al. 2010, ApJ submitted

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

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

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

• 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?

Sizes of X-ray emitting loops on young stars are

large

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

Tsujimoto et al. 2005

Kα emission in young stars

•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?)

Giardino et al. 20076.4 keV emission from the young star Elias 29

Kα emission in young stars

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

importance of accelerated particles in stellar

atmospheres & environments

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

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