manuel g¼del eth z¼rich switzerland with michael meyer & hans martin schmid...
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Manuel Gdel ETH Zrich Switzerland With Michael Meyer & Hans Martin Schmid Habitable Planets: Targets and their Environments Pathways, Barcelona, 15 September 2009 http://motls.blogspot.com/2007/04/gliese-581-has-habitable-planet.html Slide 2 Not discussed here: Star and planet formation, disks & gaps/migration/zodi light : see M. Meyers talk Galactic population statistics Geophysical issues Outline THE STARS: What role for planetary habitability? (luminosity, age, metallicity, high-energy radiation and particles) Pathways, Barcelona, 15 September 2009 Slide 3 Luminosity Toward smaller HZ: less perturbation by Jupiters & companions and: low-mass stars have fewer Jupiters (Endl et al. 03, Butler et al. 07) stable orbits & conditions Spec.Luminosity HZ Type(L )radius (AU) (Unsld & Baschek) (Kasting & Catling 03) A054 4 F06.52.5 G01.51.5 K00.430.9 M00.0770.3 M50.0110.1 M G, K (Scalo et al. 2007) (Kasting & Catling 03) Pathways, Barcelona, 15 September 2009 classical definition of HZ log m Slide 4 Metallicity High-[Fe/H] stars more likely to host Jupiter-like planets Not true for Neptunes/Super-Earths (more easily found around low [Fe/H] stars; Sousa et al. 2008, Mayor et al. 2009) However: Earth-like planetary mass in solar system 2M E [Fe/H] -0.3 (Turnbull 08) requirement: stars in young disk population (Fischer & Valenti 2005) Neptunes (Sousa et al. 2008) Pathways, Barcelona, 15 September 2009 Slide 5 Age Age can be estimated from position in HRD, from rotation period, or from magnetic activity. Spec.Massmain sequence Type(M ) lifetime (Gyr) A030.39too short for biology F01.51.8 still short G01.15.1 (>30% evolutionary change in L bol ) K00.814 M00.548very slow evolution stable HZ Con-M: Evolution toward MS very slow as well: on MS with stable HZ only after 1 Gyr for 0.1M (Burrows et al. 2001) (Unsld & Baschek) Pathways, Barcelona, 15 September 2009 Slide 6 The Young Sun was a Fainter Star.... (Sackmann & Boothroyd 2003) 30% Deep freeze on young Earth and Mars? Do other wavelength matter here? Pathways, Barcelona, 15 September 2009 Slide 7 The "Young Active Sun": Non-Flaring Emission Wavelength-Dependent Evolution (Guinan & Ribas 2002)(Ribas, Guinan, Guedel 2005) age soft X EUV UV soft X EUV UV optical Luminosity decay more rapid over much larger scale in X-rays than in UV (while optical radiation is increasing) Pathways, Barcelona, 15 September 2009 Slide 8 M dwarf photospheres L U,V = 3x10 -7 -0.02 M dwarf chromosphere Irradiance Normalized to HZ Even active M dwarfs show lower UV in their HZ outside flares (Segura et al. 2005, Scalo et al. 2007) Different photochemistry: Less molecule formation (OH) or destruction (CH 4, N 2 O) Good bioindicator! (Segura et al. 2005) Greenhouse gas! HZ? Pathways, Barcelona, 15 September 2009 Slide 9 Continuous Flaring 300Myr (Audard et al. 2003) (Telleschi, Guedel et al. 2005) UV Cet M5.5 G1 Pathways, Barcelona, 15 September 2009 Slide 10 EUV flare rate (above 10 32 erg) L X (Audard, Guedel, et al. 2000) Flares: L UV L X for biologically relevant UV (Mitra-Kraev, Harra, Guedel et al. 2005) Slope 1.170.05 (2450-3200 ) Pathways, Barcelona, 15 September 2009 Slide 11 XUV flare rate above a given threshold decreases with - decreasing mass - increasing age as does the overall emission E (0.01-10 keV) 10 3 0.6 0.01 0.2 N (>E) per day (Audard, Guedel, et al. 2000) age mass GK M M G Pathways, Barcelona, 15 September 2009 Slide 12 G and M dwarf flares physically/spectrally similar, related to L X But: larger relative modulation in UV domain (Segura et al. 2005, Scalo et al. 2007) : consequence for (non-equilibrium) atmospheric photochemistry or life? Dependent on amplitudes? Sun M Dwarfs (Scalo et al. 2007) 50-70% H active (West et al. 04, see also Silvestri et al. 05, Feigelson et al. 04) normalized L X M stars stay at a relatively high (X-ray) activity level for a longer time Pathways, Barcelona, 15 September 2009 Slide 13 Evaporation of Planetary Atmospheres < 1700 heats thermosphere (by photoioniz./dissociation) mv 2 /2 > GMm/R: particle escapes: up to several bars! Exosphere: mean free path > local scale height dissociation H 2 O 2H + O (+ further reactions) Loss of large amounts of water EUV Thermosphere Exosphere T exo __ Earth Mars 500km 210km 90km 90km blow-off (Kulikov et al. 2007) Mars (eg, Watson 1981, Kasting & Pollack 1983, Chassefiere & Leblanc 2004, Kulikov et al. 2007, Tian et al. 2008) Slide 14 Semi-Empirical Mass-Loss Estimates for the Young Sun Further, Coronal Mass Ejections in active stars act like continuous wind (500 km/s, 10 3 cm -3 ) (Khodachenko et al. 2007, Lammer et al. 2007) (Wood et al. 2005) old youngyoung old age Wind mass loss decreases with age: dM/dt t -2.3 Pathways, Barcelona, 15 September 2009 Slide 15 Wind CME UV Dissociative recombination Molecule ionization, recombination fast neutrals Sputtering Ions reimpact atmosphere eject molecules Ion pickup Impact ionization + charge exchange, E and B fields atmospheric loss Interaction atmosphere environment (solar wind) Nonthermal Escape Pathways, Barcelona, 15 September 2009 (see, e.g., Lammer et al. 2003, Lundin & Barabas 2004, Lundin et al. 2007) http://www.irf.se/~rickard/Rickard_research_interest.html Slide 16 M star HZ closer to star planets may rotate synchronously (Griemeier et al. 2005) synchronous rotation weaker magnetospheric shielding smaller distance Pathways, Barcelona, 15 September 2009 Slide 17 Tidal Locking and Magnetospheres & denser stellar wind weaker magnetic shielding stronger cosmic ray flux more NO x production ozone destruction biological damage? or evolutionary driver? (Griemeier et al. 2005) Earth M dwarf planet & high activity & flares continuous CMEs EUV heating atmospheric expansion small magnetospheric standoff distance atmospheric erosion for M dwarf planets, 10s to 100s of bars (Khodachenko et al. 2007, Lammer et al. 2007) Slide 18 Pathways, Barcelona, 15 September 2009 To make a planet habitable.... Watch out for the host stars! optical spectrum and luminosity traditional HZ planetary rotation (locked?) magnetic moment of planet metallicity formation of terrestrial planets age/evolutionary scales usefulness of HZ for life XUV activity heating/ionizing upper atmosphere atmosph. photochemistry atmospheric erosion XUV variability non-equilibrium atmospheres? winds, CMEs, particles ionisation, erosion Slide 19 END