Thermal pressure vs. magnetic pressure

The Earth The auroral oval: bremsstrahlung and lines induced by electrons precipitating through the Earth s magnetic field (LX ~ 10 14 erg/s ~ 2 kg TNT equivalent/s) The sunlit crescent: fluorescent scattering of solar X-rays in the atmosphere (LX ~ 4x10 14 erg/s) The geocorona: solar wind charge exchange (SWCX) induced line emission EUV images with DE-1 SAI

Lecture 5 X-rays from nuclear burning stars

Stars on and off the main sequence Hertzsprung-Russell diagram

Optical X-ray (ROSAT) Betelgeuse Rigel Orion Nebula Sirius Sirius B Stars around the Orion Constellation

Early observations of stellar X-rays A Sun-like star at 10 pc would produce an X-ray flux of ~10-13 erg/s/cm 2 (Lx ~ 10 27 erg/s) X-rays from Capella (Lx ~ 10 31 erg/s) first detected by Catura et al. using rocket experiment in 1974 Einstein detected nearly all types of stars across the H-R diagram Systematic surveys (e.g., volumelimited samples in the solar neighborhood) were performed by ROSAT

ROSAT volume-limited samples Giants Pre-MS Güdel 2004

Stellar dynamo and magnetic activity For Sun-like stars (F, G, K dwarfs) magnetic fields are generated by the aω-dynamo, which develops at the bottom of the convection zone Early-type stars (A, B, O) have convective cores and radiative exteriors (weak surface B-field) M3 and later-type stars are fully convective; B-field generated differently Their X-ray properties may vary significantly

Sun-like stars High detection rates in the solar neighborhood (where distances are well determined) A substantial scatter in Lx (hence in Lx/Lbol), indicating solar-type flares and long-term variability

Sun-like stars: X-ray activity vs. age X-ray activity anti-correlates with rotation period or Rossby number, with a saturation limit Rx=Lx/Lbol 10-3 at R0=Prot/τconv 0.1 (larger τconv means deeper convection zone and hence stronger B-field) Suggests magnetic heating of the corona, as in the Sun Also suggests, for a given star, Lx should decrease with age due to magnetic braking -- profound impact on planets and the interplanetary environment Sun Randich et al. 2004

Sun-like stars: X-ray activity-age relation Open clusters: coeval, homogeneous metallicity Earlier-type (bluer) stars have faster decay in X-rays, because of their higher spin-down rates 100 Myr 750 Myr 昴星团毕星团 F G K F G K

Sun-like stars: X-ray activity-age relation Lx 3x10 28 erg/s (t/gyr) -1.5 Güdel 2004

X-ray spectra of sun-like stars Güdel 2004

Optically-thin thermal plasma in CIE continuum + lines lines dominate the total intensity when T~10 5-7 K L = ne ni (T, Z)V EM = ne ni V V: emitting volume (often requires assumption) EM: emission measure (T, Z): cooling function cooling timescale: E/L ne -1 Sutherland & Dopita 1993

Measurements of temperature and density line or continuum flux (observable) The differential emission measure The geometry of the X-ray emitting plasma is usually an assumption; the temperature distribution can be modelled or directly measured Drake et al. 2000

Sun-like stars: Lx-T relation Coronal mean temperature increases with X-ray luminosity Coronal mean temperature decreases with stellar age Güdel 2004

Flare spectra: fluorescence fluorescent lines (Fe-Kα ~6.4 kev) from the photosphere/chromosphere Osten et al. 2010

Evolved stars: red giants Deepening of convective zone vs. wind-driven spin-down The dividing line: late-type (K, M) giants are X-ray-dark X-ray-emitting hybrid giants: hot coronae and cool winds

Coronally-active binaries Rapid rotation due to tidal coupling: Porb ~ days to 2 weeks RS CVn: G or K giant + latetype MS luminosity function of the solar neighborhood ABs and CVs Algol: G or K giant + earlytype MS BY Dra: two late-type MS W Uma: two late-type MS in contact Sazonov et al. 2006

ROSAT volume-limited samples Güdel 2004

Late M-stars and brown dwarfs M5 and later-type stars (mass < 0.35 M ) are fully convective -- aω dynamo not at work; mostly detected during X-ray flares Brown dwarfs (mass < 0.07 M ) cannot stably burn hydrogen; when detected, Lx/Lbol 10-4 -10-3 ; X-ray radiation mechanism remains a mystery

A- and late B-stars Purely radiative interior; weak surface B-field; thus expected to be weak X-ray-emitters Existing X-ray detections might be contaminated by a late-type companion Schmitt & Stelzer 2008

ROSAT volume-limited samples Güdel 2004

Pre-MS stars (protostars) Feigelson & Montmerle 1999

Pre-MS stars (protostars) Absorption by the accretion disk may affect Classes 0 and 1 Flaccomio et al. 2003

Pre-MS stars (protostars) Their X-ray emission may be dominated by magnetic coronae, although pre-ms stars should be fully convective Orion Nebular Cluster YSOs compared with field stars (Preibisch et al. 2005)

Pre-MS stars (protostars) Stassun et al. 2004 Hamaguchi et al. 2005

The role of accretion disks around protostars The accretion shock-heated material can produce thermal X-ray emission The accretion disk irradiated by the central star can produce fluorescent lines Feigelson 2010

Chandra Very Large Programs of star-forming regions 30 Doradus (PI: Townsley) Cygnus OB2 (PI: J. Drake) Carina (PI: L. Townsley) Orion (PI: Feigelson) Getman et al. 2005

ROSAT volume-limited samples Güdel 2004

Massive (O and B) stars have convective cores and radiative exteriors -- magnetic coronae not expected Strong radiatively driven winds: mass-loss rate ~10-6 -10-5 M /yr; velocity ~10 3 km/s -- profound impact on the environment Often reside in binary/multiple systems: orbital modulation Eta Carinae Show soft, thermal X-rays from colliding and shocked winds: Lx/Lbol 10-7 -10-6

Example spectra kt ~ 0.2 kev Naze & Rauw 2008

Single compact stellar objects End products of MS stars; numerous in the Galaxy White dwarfs: not hot enough to produce X-rays Neutron stars: various classes identified (e.g., Central Compact Object, magnetar, Pulsar Wind Nebula) Black holes: so far not identified; can in principle accrete from the ISM to produce X-rays

Summary Most nuclear burning stars produce X-rays at a varied fraction of their bolometric luminosity Magnetic coronae play a central role in the X-ray emission for late-type stars The X-ray emission from pre-main sequence stars is affected by their accretion disks Early-type, massive stars do not have a magnetic corona, but they produce strong X-ray emission from the interaction of their stellar wind with the environment A- and late B-stars do not harbor a magnetic corona or wind, but some of them are detected in X-rays, the nature of which remains uncertain

Read more about... Stellar coronae in X-rays X-ray astronomy of stellar coronae by Güdel M., 2004, Astronomy & Astrophysics Reviews, 12, 71 X-ray spectroscopy of stars by Güdel M. & Nazé Y. 2009, Astronomy & Astrophysics Reviews, 17, 309 The origin of T Tauri X-ray emission: New Insights from the Chandra Orion Ultradeep Project, Preibisch et al., 2005, ApJS, 160, 401 Stellar structure and evolution An Introduction to the Theory of Stellar Structure and Evolution, 2nd edition, by Prialnik D., 2010

Problems Set 1 NAME: 1. The sun launches a wind at an average rate of ~2x10-14 M /yr. Suppose a comet arrives at a distance of 0.5 AU from the sun and produces X-rays due to the solar wind charge exchange (SWCX) process. Please estimate the SWCX luminosity for typical comet size and composition and provides your reasoning. Hint: the charge exchange emissivity follows

2. Main-sequence (MS) stars produce X-rays at varied luminosities. Please estimate the total X-ray luminosity of main-sequence stars in the Milky Way galaxy and gives your reasoning. Hint: You need to know the number density distribution of MS stars at a given mass (i.e., the mass function).