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Host stars of exoplanets Matthias Ammler-von Eiff (TLS Tautenburg)

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1 Host stars of exoplanets Matthias Ammler-von Eiff (TLS Tautenburg)

2 Literature - a selection Books: – Perryman, M.: “The Exoplanet Handbook”, Cambridge, Cambridge University Press, 2011 (topic of lecture: Chap. 8) – Gray, D.F.: „The observation and analysis of stellar photospheres“, 3rd edition, Cambridge: Cambridge University Press, 2005 (advanced) – Gray, R.O.: „Stellar spectral classification“, Princeton and Oxford: Princeton University Press, 2009 (advanced) – Schrijver, C.J. & Zwaan, C.: „Solar and stellar magnetic activity“, Cambrigde: Cambridge University Press, 2000 (advanced) – Stix, M.: „The Sun - An Introduction“, 2. korr. Druck, Berlin, Heidelberg: Springer-Verlag, 1991 (advanced) – Strassmeier, K.: „Aktive Sterne - Laboratorien der Astrophysik“, Wien: Springer, 1997 (basic) – Kaler, J.B.: „Sterne und ihre Spektren“, Heidelberg: Spektrum,1994 – Unsöld, A. & Baschek, B.: „Der neue Kosmos“, 6. Auflage, Berlin: Springer, 1999 (basic)

3 Literature - a selection Research articles related to the subject: – access arXiv e-prints via http://cdsads.u- – CoRoT-7: Bruntt, H. et al. (2010), A&A 519, 51, arXiv:1005.3208 CoRoT-7: Bruntt, H. et al. (2010), A&A 519, 51, arXiv:1005.3208 – CoRoT-19: Guenther, E.W. et al. (2011), arXiv:1112.1035 – Kepler-22: Borucki, W.J. et al. (2012), ApJ 745, 120B, arXiv:1112.1640

4 Aspects - an incomplete view Planet detection Detection techniques are indirect. It is the star that is measured! Planet formation Planet frequency and properties seem to depend on star! Habitability Existence of life depends on the star! And on the location in the Galaxy? Other galaxies? Planet properties The accuracy of measured parameters depends on the knowledge of the star! Star-planet interaction Can a planet cause features on the star?

5 Contents of this lecture Determination of stellar parameters – fundamental stellar parameters and their determination Effects of metallicity – correlation of stellar metallicity and planet occurence Stellar activity – magnetic and chromospheric activity – measurement of chromospheric activity – stellar flares and their influence on habitable planets


7 Determination of stellar parameters Most planets are found indirectly by studying the light of the central star! We need to know the star in order to know its planets! Methods: – photometry – spectroscopy – asteroseismology – interferometry – astrometry

8 Example: CoRoT-19b


10 Spectroscopic analysis

11 Example: CoRoT-19b light curve

12 Example: CoRoT-19b evolutionary models

13 Stellar Parameter Network (incomplete!) Spectroscopy Photometry Asteroseismology Interferometry brightness distance luminosity angular diameter radius effective temperature age rotational period rotational velocity chemical abundances mass mean stellar density model isochrones surface gravity Astrometry

14 Stellar Parameter Network (incomplete!) huge diversity of methods and tools to determine stellar parameters choice of methods and tools depends on: – precision requirements – available resources – observational data available

15 Stellar distance Accuracy of distances of planet host stars increased substantially from ground-based (a) to Hipparcos (b). Hipparcos provides proper motions for a large sample of stars and is a major step in the study of stellar streams. … more to come with GAIA! ©Perryman 2011, fig. 8.1

16 HR diagram with planet host stars HR diagram of stars within 25 pc based on Hipparcos data Hawley & Reid (2003), Perryman (2011; fig. 8.5) planet host stars

17 Host stars of planets main-sequence stars low-mass objects: M dwarfs, brown dwarfs pulsating stars giant stars pulsars binary and single stars stellar populations: thick disk, metal-poor stars, open clusters

18 Improving precision… Angular diameters can be measured for nearby stars, give precise linear radii with Hipparcos distance Baines et al. (2008)

19 … but usually … … direct measurement not possible! Often, mass and radius are estimated from evolutionary models. Models predict radii and effective temperature at given mass and age. Comparison to observed effective temperature then gives constraints on mass and age. However, models to be used with care!

20 Effective temperature Luminosity of black body with surface 4πR 2 and temperature T: Consider star with same luminosity and same radiating surface area, defines effective temperature: Gray (2005), pp. 3, 118ff. radiative flux

21 High-resolution spectroscopy example: Hα profile of host stars of planets to derive effective temperature precisely further parameters can be derived: – surface gravity – chemical abundances – projected rotational velocity – microturbulence – macroturbulence Fuhrmann et al. (1998)

22 How to model a spectrum? Set of stellar parameters: T eff, logg, [M/H],... Temperature profile Pressure profile... Synthetic spectrum Model atmospheres: e.g. ATLAS9, MARCS, MAFAGS Line formation codes: e.g. SPECTRUM, MOOG, LINFOR

23 Metallicity and chemical abundances

24 Astronomer‘s Metals More Metals ! Even more Metals !!

25 The „Bracket“ [Fe/H] e.g. [Fe/H] = –1 → 1/10 the iron abundance of the sun unit: „dex“ (contraction of decimal exponent, indicates decimal logarithmic ratio which is in fact unitless) [Fe/H] is often used as an overall metallicity indicator, other elements then are related to Fe, e.g. [Mg/Fe].


27 Metallicity correlation host stars of planets appear on average more metal-rich than comparison stars without planets probability to find planet is higher for metal-rich star Santos et al. (2005)

28 Is it true? selection effects: – metal-rich stars show deeper absorption lines so that planets can be detected more easily by RV surveys – metal-rich stars are intrinsically brighter than metal- poor stars at same spectral type, so that more metal- rich stars are selected in magnitude-limited samples – possibly correlation of orbital radius and metallicity Selection effects cannot fully explain the metallicity correlation!

29 Metallicity correlation So far giant planets around FGK type stars! Dependence on nature of star: – giant stars: no correlation found! – metal-poor stars: more planets in thick disk (α- enhancement!) than in thin disk Correlation only for Jovian planets, not for lower-mass Neptune mass planets!

30 M dwarfs with planets M-type planet host stars tend to be more metal-rich but generally less Jovian planets around M dwarfs than around FGK dwarfs (effect of stellar mass!) Johnson et al. (2010)

31 Hypotheses primordial origin: – planet formation preferred in disks of metal-rich stars self-enrichment: – stellar atmospheres enriched with infalling planetary material or planetesimals

32 Primordial origin metallicity correlation can be reproduced with models of core accretion! models also predict lower frequency of short- period giant planets around M dwarfs and presence of Neptune-mass ice-giants around M dwarfs

33 Self-enrichment spectroscopy measures „surface“ abundances of chemical species! enrichment of surface region with: – engulfment of planet migrating inward – accretion of disk material resulting from migration – infall of planetary material or planetesimals

34 Self-enrichment accreted material is mixed in outer convection zone less atmospheric metal-enhancement when convection zone is deep Ford et al. (1999)

35 Self-enrichment of giant stars planet-bearing stars with deep convection zones (sub-giants) should have less metallicity This is not the case! Fischer & Valenti (2005) o sub-giants

36 Conclusions observational evidence for metallicity- correlation probably primordial but: accretion of planetary material and planetesimals is inevitable (Sun!)

37 Galactic origin Galactic radial metallicity gradient (0.07-0.1 dex kpc -1 ) radial mixing: most metal-rich stars migrated from inner Galactic disk to solar Galactocentric radius Wielen (1996)

38 Galactic origin planetary systems form in inner metal-rich disk (independent of metallcity there!) also Sun is more metal-rich than local average and might have formed at inner Galactic radii model: metallicity-correlation from radial mixing of different Galactic components

39 Model of metallicity-correlation frequency of giant planets: metal-poor stars: 0%; local stars: 5%; metal-rich stars: 25% Haywood (2009)

40 Conclusions metallicity range of planet-bearing stars corresponds to Galactic ring of molecular hydrogen linked to star formation low-mass planets might simply form in less dense hydrogen regions giant stars with giant planets are relative young and thus not affected by radial mixing

41 Condensation important for planet formation! refractory species: – high condensation temperature – planet hosts: Al, Ca, Ti, V volatile species: – low condensation temperature – planet hosts: C, Cu, N, Na, O, S, Zn intermediate: Co, Fe, Mg, Ni

42 Condensation primordial origin of metallicity-correlation: – expect similar occurence trends for metals other than iron self-enrichment: – expect overabundance of refractory elements

43 Refractory Elements no abundance trends beyond [Fe/H] Bodaghee et al. (2003) o stars with planetsxcomparison sample

44 Volatile Elements no abundance trend beyond [Fe/H] Ecuvillon et al. (2006)


46 Magnetic braking Gray (2005), p. 485 formation of convective envelope

47 Magnetic braking convection + rotation are thought to generate magnetic field via stellar dynamo (Gray, 2005, pp. 490-492)

48 Magnetic braking stars with convective envelopes form a magnetic field stellar wind is coupled to magnetic field lines and thus to stellar rotation therefore, stellar wind takes away angular momentum and the stellar rotation is braked Strassmeier (1997), pp. 68-70 Gray (2005), pp. 492 Strassmeier (1997), pp. 68-70 Gray (2005), pp. 492

49 Magnetic activity Active stars show magnetic phenomena Stellar dynamos are thought to produce magnetic fields Ionised stellar material couples to magnetic field lines This produces a plethora of phenomena of magnetic activity: photospheric spots, chromospheric faculae, coronal holes, loops, mass ejections,...

50 Chromospheric activity Sun in white light: photosphere Sun in Ca II K: chromosphere! Schrijver & Zwaan (2000), pp. 2,3 plages spots chromospheric network

51 Measurement principle Active star Inactive star measure flux f 50 in Willstrop‘s band (3925-3975Å; Willstrop, 1964, Mem. RAS 69, 83) for active star and inactive standard measure angular diameter of standard to get absolute flux F 50 of standard 3925Å 3975Å Strassmeier (1997), pp. 249, 250 famous: Mt. Wilson S-index, here R‘ HK index

52 Measurement principle Active star Inactive star measure flux in emission line f(H) and f(K) absolute flux in H and K of active star: Strassmeier (1997), pp. 249, 250 subtract photospheric contribution F phot based on model atmosphere or inactive standard:

53 Measurement principle Active star Inactive star absolute flux in H and K of active star: Strassmeier (1997), pp. 249, 250

54 Activity-related RV Saar et al. (1998, ApJ 498L, 153)

55 Activity-related RV largest RV scatter: active F stars dMe stars high Ca II H&K emission largest RV scatter: active F stars dMe stars high Ca II H&K emission Saar et al. (1998, ApJ 498L, 153)

56 Activity-related RV RV scatter scales with rotational speed! Young G-type (0.3 Gyr) star with vsini=8-10km/s: 20-45 m/s Good agreement with expectations from convective motions and spots! RV scatter scales with rotational speed! Young G-type (0.3 Gyr) star with vsini=8-10km/s: 20-45 m/s Good agreement with expectations from convective motions and spots! Saar et al. (1998, ApJ 498L, 153)

57 Solar and stellar flares flares may have dramatic consequences for life on planets solar outer atmospheric layers: 10 -4 of photospheric radiation on average local, short-lived explosive events may exceed average by factor 10 3 -10 4, i.e. larger than photosperic flux! Stix (1991, pp. 351-357)

58 Solar and stellar flares Stix (1991, pp. 352, 355) disk view: disappearing filament disk view: disappearing filament pre-flare maximum expanding ribbons expanding ribbons post-flare limb view: eruptive promincence

59 Solar and stellar flares Stix (1991, p. 353)

60 Solar and stellar flares Stix (1991, p. 353) thermal regime: up to approx. 10 7 K non-thermal regime: bremsstrahlung and synchrotron radiation of electrons with 10-100 keV thermal non-thermal

61 Solar and stellar flares solar flares: up to 10 31 erg, brightening not visible in disk-integrated light stellar case: only disk-integrated light measurable! stellar flares: large flares which are also visible in white light super flares: 10 33 -10 38 erg Sun: no super flares during last 2000 years Stix (1991, p. 354) Schaefer et al. (2000, ApJ 529, 1026) Stix (1991, p. 354) Schaefer et al. (2000, ApJ 529, 1026)

62 Stellar super flares Schaefer et al. (2000, ApJ 529, 1026)

63 Stellar super flares isolated slow or moderate rotators Schaefer et al. (2000, ApJ 529, 1026) close to main-sequence spectral types F8-G8

64 Stellar super flares and effects on planet energy deposited on a planet at 1 AU by a super flare with 10 35 erg: 3.5x10 7 erg cm -2 no melting of rock but of ice planet with atmosphere: – possibly temporary heating, ozone depletion – formation of organic molecules (Miller-Urey experiment) Schaefer et al. (2000, ApJ 529, 1026)

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