Planetesimal Compositions in Exoplanet...
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Planetesimal Compositions in Exoplanet Systems Torrence V. Johnson 1 , J. I. Lunine 2 , and O. Mousis 3 (1)Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109, USA; (2) Dipartimento di Fisica, Università degli Studi di Roma "Tor Vergata", 00133 Rome, Italy (3) Institut UTINAM, CNRS-UMR 6213, Observatoire de Besançon, Besançon, France. 4. Summary Acknowledgements. TVJ’s work has been conducted at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. Government sponsorship acknowledged. • Exoplanet systems around stars with different compositions from the Sun may have planetesimals formed beyond the ‘’snow line’’ which have a greater range of rock and metal, carbon and ice proportions than solar system planetesimals. • The fraction of rock plus metal in extrasolar planetesimals is most strongly dependent on the C/O ratio in their stellar nebula rather than metallicity, [Fe/H]. • These characteristics may be investigated for extrasolar systems in the future through their impact on the refractory and volatile content of extrasolar planetesimal belts and the amount of heavy element enrichment of extrasolar giant planets (e.g. Mousis et al. 2009, [9]). [1] Wong, M. H., et al., in Oxygen in the Solar System, G. J. MacPherson, Ed. (Mineralogical Society of America, Chan-tilly, VA, 2008), vol. Reviews in Mineralogy and Geochemi-stry Vol. 68, pp. 241-246. [2] Prinn, R. G. and B. Fegley (1981) Astrophys. J. 249, 308-317. [3] Gonzales, G., et al. (2001) Astron. J. 121, 432-452. [4] Takeda, Y., et al. (2001) Publications of the Astronomi-cal Society of Japan 53, 1211-1221. [5] Bond, J. C., et al. (2010) Astrophys. J. 715, 1050-1070. [6] Fossati, L., et al. (2010) Astrophys. J. 720, 872-886. [7] Johnson, T. V. and J. I. Lunine (2005) Nature 435, 69-71. [8] Asplund, M., et al. (2009) Annual Review of Astronomy and Astrophysics 47, 481-522. [9] Mousis, O., et al. (2009) Astron. Astrophys. 507, 1671-1674. [10] Mousis, O., et al. (2011) Astrophys. J. 727:77 (7pp) [11] Mousis, O., et al. (2009) Astrophys. J. 696, 1348-1354. (a) (b) Figure 1 Rock-metal fraction f(r-m) vs C/O. [C/O] is expressed as logarithmic units, dex, relative to the Sun = 0, [C/O] = log (C/O) star – log (C/O) Sun . (a) Oxidizing: All carbon in gaseous phase as CO (b) Reducing: All carbon in gaseous phase as CH 4 . Solar values are from Asplund et al. (2009) [8]. 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 ‐0.3 ‐0.2 ‐0.1 0 0.1 0.2 0.3 0.4 0.5 Fraction Rock + Metal [C/O ] dex, Sun =0 Stellar Nebula Condensates: Reducing, CH 4 - rich Gonzales et al.; Takeda et al. Sun C/O Uncertainty 51 Peg Gonzales 51 Peg Takeda wasp 12 Bond et al 2010 Linear (Gonzales et al.; Takeda et al.) C/O ratio = 0.8 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 Fraction Rock + Metal [C/O ] dex, Sun =0 Stellar Nebula Condensates: Oxidizing, CO - rich Gonzales et al; Takeda et al. Sun C/O Uncertainty 51 Peg Gonzales 51 Peg Takeda wasp 12 Bond et al 2010 Linear (Gonzales et al; Takeda et al.) C/O ratio = 0.8 1. The Solar System In our solar system, the composition of planetesimals formed beyond the ‘snow line’ is believed to depend strongly on the abundances of C and O in the solar nebula, the redox state of C (i.e. CO rich vs. CH 4 rich regions), and the amount of carbon in the form of solid organics. These factors largely determine the fraction of rock plus metal, f(r–m), versus volatile ice in condensates, and therefore the material (uncompressed) density of the condensates [1]. For “warm” nebula models where mid-plane temperatures are >~50K, CO and CH 4 remain in the gaseous state and water ice (and some hydrates) is the primary condensed volatile ice. In the outer solar nebula, CO is expected to be the dominant C-bearing gas, resulting in less O available to form water ice, and therefore higher values of f(r- m). In warmer circumplanetary environments with higher density, kinetic considerations are expected to produce more reducing conditions, with CH 4 being the primary carbon species [2], resulting in more water ice and lower values of f(r-m). In “cool” nebula models, with lower mid-plane temperatures, more volatile ices and clathrates, including CO, CH 4 and, H 2 S, are added to the water ice and f(r-m) is lower. 2. Exoplanet Systems Exoplanet host stars have observed metallicities that cluster around the solar value or higher. However, the detailed compositions for ma- jor planet forming elements are observed to differ significantly. Assuming this reflects equivalent differences in the protoplanetary nebulae from which planets formed in these systems, we can estimate the range of possible f(r-m) in systems for which stellar composition data are available (the most important solid forming elements being C, O, Si, S, Fe and Ni for these purposes). Stellar data are from two surveys [3, 4], with a total of 25 exoplanet host stars, and ten stars compiled for a study of diversity in extrasolar terrestrial planets [5]. Also, an example of a host star of a transiting planet, WASP-12 was included [6]. We calculated f(r-m) for these compositions, assuming water is the only condensed volatile, following the solar composition scheme developed in [1,7]. As expected, the condensate rock plus metal fraction is generally correlated with metallicity, [Fe/H]. However, it is even more strongly influenced by the C/O ratio. Figure 1 shows f(r- m) vs. [C/O], in dex with Sun=0, for both the oxidizing and reducing nebula cases. Note that for C/O > ~0.8 both cases become ‘oxygen’ starved, with little or no water ice for the most carbon-rich stars. The total range of f(r-m) possible in these systems is considerably greater than the solar case (~ 0.47-0.76). 3. Composition of Planetesimals vs Stellar C/O To explore the range of planetesimal composition as function of the C/O ratio of the host star, we combined the f(r-m) calculation from [1] with a more complete thermodynamic treatment of the volatile ices [9,10,11]. We used the set of stars compiled by Bond et al. [5], covering a wide range of C/O stellar values and calculated the mass fraction of each condensed species for both “warm” and “cool” nebular cases. Silicate plus metal fractions for the “warm” case are similar to the values in Fig. 1, reduced by the addition of small amounts of ices containing CO 2 (oxidizing) and NH 3 hydrates (reducing). For the “cool” cases, larger amounts of CO and CH 4 ices reduce f(r-m) still further . Fig. 2 illustrates the relative proportions of the condensed phases for oxidizing conditions over a range of stellar C/O. Note that for the extreme carbon-rich star HD 4203, lack of O produces a shift to reducing conditions. References Fig. 2 Planetesimal Composition Warm Nebula - Oxidizing Cool Nebula - Oxidizing Warm Nebula - Reducing Cool Nebula - Reducing Silicate+Metal 39% H2O 35% CH4 0% CH3OH 1% CO 15% CO2 5% N2 3% NH3 1% H2S 1% PH3 0% HD19994 Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S PH3 Silicate+Metal 39% H2O 17% CH4 0% CH3OH 2% CO 25% CO2 11% N2 3% NH3 1% H2S 2% PH3 0% Sun Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S PH3 Silicate+Metal 41% H2O 0% CH4 3% CH3OH 3% CO 34% CO2 16% N2 2% NH3 0% H2S 1% PH3 0% HD108874 Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S PH3 Silicate+Metal 30% H2O 42% CH4 0% CH3OH 1% CO 18% CO2 7% N2 1% NH3 0% H2S 1% PH3 0% HD177830 Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S PH3 Silicate+Metal 48% H2O 11% CH4 0% CH3OH 2% CO 25% CO2 11% N2 2% NH3 0% H2S 1% PH3 0% HD213240 Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S PH3 Silicate+Metal 40% H2O 5% CH4 0% CH3OH 3% CO 33% CO2 15% N2 2% NH3 0% H2S 2% PH3 0% Gl777A Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S PH3 Silicate+Metal 35% H2O 4% CH4 0% CH3OH 3% CO 38% CO2 17% N2 2% NH3 0% H2S 1% PH3 0% HD72659 Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S PH3 Silicate+Metal 53% H2O 38% CH4 0% CH3OH 1% CO 0% CO2 6% N2 0% NH3 1% H2S 1% PH3 0% HD19994 Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S Silicate+Metal 65% H2O 18% CH4 0% CH3OH 2% CO 0% CO2 12% N2 0% NH3 1% H2S 2% PH3 0% Sun Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S PH3 Silicate+Metal 77% H2O 0% CH4 0% CH3OH 3% CO 0% CO2 18% N2 0% NH3 0% H2S 2% PH3 0% HD108874 Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S PH3 Silicate+Metal 42% H2O 48% CH4 0% CH3OH 1% CO 0% CO2 8% N2 0% NH3 0% H2S 1% PH3 0% HD177830 Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S Silicate+Metal 75% H2O 11% CH4 0% CH3OH 2% CO 0% CO2 11% N2 0% NH3 0% H2S 1% PH3 0% HD213240 Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S Silicate+Metal 73% H2O 5% CH4 0% CH3OH 3% CO 0% CO2 17% N2 0% NH3 0% H2S 2% PH3 0% Gl777A Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S Silicate+Metal 70% H2O 5% CH4 0% CH3OH 4% CO 0% CO2 20% N2 0% NH3 0% H2S 1% PH3 0% HD72659 Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S Silicate+Metal 66% H2O 11% CH4 19% CH3OH 0% CO 0% CO2 0% N2 0% NH3 3% H2S 1% PH3 0% HD4203 Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S Silicate+Metal 66% H2O 11% CH4 19% CH3OH 0% CO 0% CO2 0% N2 0% NH3 3% H2S 1% PH3 0% HD4203 Silicate+Metal H2O CH4 CH3OH CO CO2 N2 NH3 H2S STAR C/O 0.32 STAR C/O 0.35 STAR C/O 0.51 STAR C/O 0.55 STAR C/O 0.63 STAR C/O 0.68 STAR C/O 0.71 STAR C/O 1.51
Transcript of Planetesimal Compositions in Exoplanet...
Planetesimal Compositions in Exoplanet SystemsTorrence V. Johnson1, J. I. Lunine2, and O. Mousis3
(1)Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109, USA; (2) Dipartimento di Fisica, Università degli Studi di Roma "Tor Vergata", 00133 Rome, Italy
(3) Institut UTINAM, CNRS-UMR 6213, Observatoire de Besançon, Besançon, France.
4. Summary
Acknowledgements. TVJ’s work has been conducted at the Jet PropulsionLaboratory, California Institute of Technology, under a contract with NASA.Government sponsorship acknowledged.
• Exoplanet systems around stars with different compositions from theSun may have planetesimals formed beyond the ‘’snow line’’ whichhave a greater range of rock and metal, carbon and ice proportionsthan solar system planetesimals.• The fraction of rock plus metal in extrasolar planetesimals is moststrongly dependent on the C/O ratio in their stellar nebula rather thanmetallicity, [Fe/H].• These characteristics may be investigated for extrasolar systems inthe future through their impact on the refractory and volatile content ofextrasolar planetesimal belts and the amount of heavy elementenrichment of extrasolar giant planets (e.g. Mousis et al. 2009, [9]).
[1] Wong, M. H., et al., in Oxygen in the Solar System, G. J. MacPherson, Ed. (Mineralogical Society of America, Chan-tilly, VA, 2008), vol. Reviews in Mineralogy and Geochemi-stry Vol. 68, pp. 241-246. [2] Prinn, R. G. and B. Fegley (1981) Astrophys. J. 249, 308-317. [3] Gonzales, G., et al. (2001) Astron. J. 121, 432-452. [4] Takeda, Y., et al. (2001) Publications of the Astronomi-cal Society of Japan 53, 1211-1221. [5] Bond, J. C., et al. (2010) Astrophys. J. 715, 1050-1070. [6] Fossati, L., et al. (2010) Astrophys. J. 720, 872-886. [7] Johnson, T. V. and J. I. Lunine (2005) Nature 435, 69-71. [8] Asplund, M., et al. (2009) Annual Review of Astronomy and Astrophysics 47, 481-522. [9] Mousis, O., et al. (2009) Astron. Astrophys. 507, 1671-1674. [10] Mousis, O., et al. (2011) Astrophys. J. 727:77 (7pp)[11] Mousis, O., et al. (2009) Astrophys. J. 696, 1348-1354.
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Figure 1 Rock-metal fraction f(r-m) vs C/O. [C/O] is expressed as logarithmicunits, dex, relative to the Sun = 0, [C/O] = log (C/O)star – log (C/O)Sun. (a)Oxidizing: All carbon in gaseous phase as CO (b) Reducing: All carbon ingaseous phase as CH4. Solar values are from Asplund et al. (2009) [8].
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1. The Solar SystemIn our solar system, the composition of planetesimals formed beyondthe ‘snow line’ is believed to depend strongly on the abundances of Cand O in the solar nebula, the redox state of C (i.e. CO rich vs. CH4rich regions), and the amount of carbon in the form of solid organics.These factors largely determine the fraction of rock plus metal, f(r–m),versus volatile ice in condensates, and therefore the material(uncompressed) density of the condensates [1].
For “warm” nebula models where mid-plane temperatures are >~50K,CO and CH4 remain in the gaseous state and water ice (and somehydrates) is the primary condensed volatile ice. In the outer solarnebula, CO is expected to be the dominant C-bearing gas, resulting inless O available to form water ice, and therefore higher values of f(r-m). In warmer circumplanetary environments with higher density,kinetic considerations are expected to produce more reducingconditions, with CH4 being the primary carbon species [2], resulting inmore water ice and lower values of f(r-m). In “cool” nebula models,with lower mid-plane temperatures, more volatile ices and clathrates,including CO, CH4 and, H2S, are added to the water ice and f(r-m) islower.
2. Exoplanet Systems
Exoplanet host stars have observed metallicities that cluster aroundthe solar value or higher. However, the detailed compositions for ma-jor planet forming elements are observed to differ significantly.Assuming this reflects equivalent differences in the protoplanetarynebulae from which planets formed in these systems, we can estimatethe range of possible f(r-m) in systems for which stellar compositiondata are available (the most important solid forming elements being C,O, Si, S, Fe and Ni for these purposes). Stellar data are from twosurveys [3, 4], with a total of 25 exoplanet host stars, and ten starscompiled for a study of diversity in extrasolar terrestrial planets [5].Also, an example of a host star of a transiting planet, WASP-12 wasincluded [6]. We calculated f(r-m) for these compositions, assumingwater is the only condensed volatile, following the solar compositionscheme developed in [1,7]. As expected, the condensate rock plusmetal fraction is generally correlated with metallicity, [Fe/H]. However,it is even more strongly influenced by the C/O ratio. Figure 1 shows f(r-m) vs. [C/O], in dex with Sun=0, for both the oxidizing and reducingnebula cases. Note that for C/O > ~0.8 both cases become ‘oxygen’starved, with little or no water ice for the most carbon-rich stars.
The total range of f(r-m) possible in these systems isconsiderably greater than the solar case (~ 0.47-0.76).
3. Composition of Planetesimals vs Stellar C/OTo explore the range of planetesimal composition as function of theC/O ratio of the host star, we combined the f(r-m) calculation from [1]with a more complete thermodynamic treatment of the volatile ices[9,10,11]. We used the set of stars compiled by Bond et al. [5],covering a wide range of C/O stellar values and calculated the massfraction of each condensed species for both “warm” and “cool”nebular cases. Silicate plus metal fractions for the “warm” case aresimilar to the values in Fig. 1, reduced by the addition of smallamounts of ices containing CO2 (oxidizing) and NH3 hydrates(reducing). For the “cool” cases, larger amounts of CO and CH4 icesreduce f(r-m) still further . Fig. 2 illustrates the relative proportions ofthe condensed phases for oxidizing conditions over a range of stellarC/O. Note that for the extreme carbon-rich star HD 4203, lack of Oproduces a shift to reducing conditions.
References
Fig. 2 Planetesimal Composition Warm Nebula - Oxidizing Cool Nebula - Oxidizing
Warm Nebula - Reducing Cool Nebula - Reducing
Silicate+Metal39%
H2O35%
CH40%
CH3OH1%
CO15%
CO25%
N23%
NH31% H2S
1%PH30%
HD19994
Silicate+Metal
H2O
CH4
CH3OH
CO
CO2
N2
NH3
H2S
PH3
Silicate+Metal39%
H2O17%
CH40%
CH3OH2%
CO25%
CO211%
N23%
NH31%
H2S2%
PH30%
Sun
Silicate+Metal
H2O
CH4
CH3OH
CO
CO2
N2
NH3
H2S
PH3
Silicate+Metal41%
H2O0%
CH43%CH3OH
3%
CO34%
CO216%
N22%
NH30%
H2S1%
PH30%
HD108874
Silicate+Metal
H2O
CH4
CH3OH
CO
CO2
N2
NH3
H2S
PH3
Silicate+Metal30%
H2O42%
CH40%
CH3OH1%
CO18%
CO27%
N21%
NH30%
H2S1%
PH30%
HD177830
Silicate+Metal
H2O
CH4
CH3OH
CO
CO2
N2
NH3
H2S
PH3
Silicate+Metal48%
H2O11%
CH40%
CH3OH2%
CO25%
CO211%
N22%
NH30%
H2S1%
PH30%
HD213240
Silicate+Metal
H2O
CH4
CH3OH
CO
CO2
N2
NH3
H2S
PH3
Silicate+Metal40%
H2O5%
CH40%CH3OH
3%
CO33%
CO215%
N22%
NH30%
H2S2%
PH30%
Gl777A
Silicate+Metal
H2O
CH4
CH3OH
CO
CO2
N2
NH3
H2S
PH3
Silicate+Metal35%
H2O4%CH4
0%
CH3OH3%
CO38%
CO217%
N22%
NH30% H2S
1%PH30%
HD72659
Silicate+Metal
H2O
CH4
CH3OH
CO
CO2
N2
NH3
H2S
PH3
Silicate+Metal53%
H2O38%
CH40%
CH3OH1%
CO0%
CO26%
N20%
NH31% H2S
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PH30%
HD19994
Silicate+Metal
H2O
CH4
CH3OH
CO
CO2
N2
NH3
H2S
Silicate+Metal65%
H2O18%
CH40%
CH3OH2% CO
0%CO212%
N20%
NH31% H2S
2% PH30%
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Silicate+Metal
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CO
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Silicate+Metal77%
H2O0%
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CO218%
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H2S2% PH3
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Silicate+Metal
H2O
CH4
CH3OH
CO
CO2
N2
NH3
H2S
PH3
Silicate+Metal42%
H2O48%
CH40%
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CO0% CO2
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NH30% H2S
1% PH30%
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Silicate+Metal
H2O
CH4
CH3OH
CO
CO2
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NH3
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Silicate+Metal75%
H2O11%
CH40%
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CO211%
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NH30%
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Silicate+Metal73%
H2O5%
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NH3
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Silicate+Metal70%H2O
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NH30% H2S
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Silicate+Metal66%
H2O11%
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