Cloud Condensation Chemistry. in Low-Mass Objects. Katharina Lodders Washington University, St. Louis

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1 Cloud Condensation Chemistry in Low-Mass Objects Katharina Lodders Washington University, St. Louis

2 For a recent review on this topic, see Lodders & Fegley 2006, Astrophysics Update 2, Springer, p. 1 ff. Several figures from this talk and more information on chemistry can be found in our papers available at: For reference, a list of minerals and their formulas can be found at the end.

3 Why are there clouds in the atmospheres of low-mass objects? Which minerals are expected in the condensate clouds of low-mass object atmospheres? How many mineralogically different cloud layers do we expect? Why are some clouds made of different minerals than one may expect from the condensation sequence in protoplanetary disks (e.g., solar nebula; meteorites) or winds from giant stars? In absence of direct cloud observations in T, L, and M dwarfs, how do we know that certain clouds are present or not present?

4 Why are there clouds in the atmospheres of low-mass objects? Many low-mass objects are expected to have near-solar overall composition, as are most real dwarfs stars low temperatures favor molecular and condensation chemistry: Hot stars M dwarfs late M, L, & T ionized atoms neutral atoms gas molecule formation condensate formation This explains condensates, but not yet the clouds protoplanetary disks (e.g., solar nebula) or stellar winds (M giants) are also near-solar in composition and produce condensates, but these environs are not cloudy Atmospheres are strongly gravitationally bound and gravity causes condensates to settle into cloud layers

5 How many cloud layers? Uppermost Cloud Decks: ammonia ice salt rock (ol,px) refractory liquid iron ceramics NH NH 4 HS H 2 O RbCl CH 4 gas CsCl CsCl RbCl KCl CH 4 gas CH 4 gas CO gas Mg-silicates iron metal liquid perovskite CO gas CO gas corundum KCl LiF Na 2 S Li 2 S CO gas LiF Na 2 S Li 2 S Mg-silicates CO gas Ca-Ti-oxides Ca-Al-oxides CO gas iron metal liquid Mg-silicates deeper iron metal liquid Ca-Ti-oxides Ca-Al-oxides hotter denser CO gas Ca-Ti-oxides Ca-Al-oxides Jupiter CO gas T dwarfs L dwarfs L- to M dwarf transition The coolest objects are expected to have the highest number of cloud layers From Lodders 2004, Brown Dwarfs Faint at Heart, Rich in Chemistry, Science 0, 2

6 Why are some clouds in low-mass objects made of different minerals than one may expect from the condensation sequence in protoplanetary disks (e.g., solar nebula; meteorites) or winds from giant stars? Atmospheres have higher total pressure than disks or stellar winds. Condensate mineralogy and condensation temperatures depend on total pressure. Condensate settling changes the chemistry above the clouds Consider primary and secondary condensates, and check reactions for making them In absence of direct cloud observations in T, L, and M dwarfs, how do we know that certain clouds are present or not present? The gas and condensation chemistry is coupled, use chemical gas tracers for cloud diagnostics

7 What condensates make the clouds in low-mass object atmospheres? Use thermodynamic calculations to find out Application of equilibrium condensation calculations is supported experimentally Condensation experiments at high T and low total pressure ( K, ~0.004 bar) yield many expected condensates in crystalline form; steady-state attained within an hour Equilibrium thermodynamics also explains presolar grain mineralogy and their trace element contents In low pressure winds of giant stars, so should work well in high P atmospheres

8 What condensates make the clouds in low-mass object atmospheres? Use thermodynamic calculations to find out Composition of gas and condensates under thermodynamic equilibrium depends on: overall elemental abundances (metallicity, C/O ratio) temperature total pressure 8 stable elements many possibilities Use solar abundances, focus on more abundant and/or observed elements C, N, O, S, P, Fe, Mg, Si, Ca, Al, Ti, alkalis, Use P-T conditions to encompass atmospheric conditions in giant planets, T, L, and early M dwarfs K, bars use nominal pressure of 1 bar to illustrate trends

9 Condensation Temperature refers to the temperature where a condensate starts forming, e.g., Fe-metal starts condensing at 184 K (at 1 bar) 50% Condensation Temperature refers to the temperature where an element is distributed evenly between gas and condensate(s) e.g., 50% of all Fe is condensed as Fe-metal at 179 K (at 1 bar) Iron Condensation at 1 bar gases: Fe, FeH,... fraction Fe-metal alloy 179 K, 50% Fe condensed 184 K, Fe condensation starts

10 Primary condensates directly condense out of the gas phase e.g., Fe (g) = Fe (metal) Ca (g) +TiO (g) + H 2 O (g) = CaTiO (perovskite) + H 2 (g) Ca (g) + 12 AlOH (g) + 7 H 2 O (g) = CaAl 12 O 19 (hibonite) + 1 H 2 (g) Secondary condensates form by gas-solid reactions from pre-existing, primary condensates e.g., Fe (metal) + H 2 S (g) = FeS (troilite) + H 2 (g) Ca (g) + 6 Al 2 O (corundum) + H 2 O (g) = CaAl 12 O 19 (hibonite) + H 2 (g) Mg 2 SiO 4 (forsterite) + SiO (g) + H 2 O (g) = 2 MgSiO (enstatite) + H 2 (g)

11 Secondary condensates can only form if gas-solid equilibrium is maintained to temperatures where the secondary condensates are thermodynamically stable Iron Distribution at 1 bar gases: Fe, FeH,... fraction Fe-metal alloy FeS Fe P In a solar composition gas, primary Fe metal condensation starts at 184 K, and some of the metal reacts to Fe P at 1487 K and to FeS at 704 K. This is more applicable to disks and stellar winds, but not atmospheres where metal sediments down More examples

12 A Sampler of Primary and Secondary Condensates from a Solar-Composition Gas proto planetary disk chemistry log (total pressure, bar) H 2 O FeS 50% Na in feldspar Fe P Ca-Ti-ox. Fe metal enstatite Ca-Al-oxides forsterite anorthite Fe metal to Fe P and FeS Forsterite to enstatite Ca-Al-oxides to anorthite (a feldspar), which serves as host for Na condensation in solidsolution Not all of this condensation chemistry can happen in substellar atmospheres! Also: Condensation temperatures decrease with decreasing total pressure. Exceptions: Fe P, FeS Low P << 1 bar apply to proto-planetary disks (solar nebula) & stellar winds, higher P >0.1 to atmospheres

13 0 "proto planetary disk chemistry" Secondary condensates may not form in atmospheres if primary condensates are more refractory and settle into clouds H 2 O 500 FeS FeS formation requires presence of Fe-metal at 700 K enstatite anorthite forsterite 50% Na in feldspar Fe P Ca-Ti-ox. Fe metal Ca-Al-oxides However, condensate settling clears the upper, cooler atmosphere of Femetal and a metal cloud layer appears with a base near the P-T of the metal condensation curve. High T High P log (total pressure, bar) - More examples from other elements

14 All primary condensates participate in low temperature gas-solid reactions * Not all primary condensates participate in low temperature gas-solid reactions 0 "proto planetary disk chemistry" Common condensates "cloudy atmosphere chemsitry" NH 4 SH 0 H 2 O Water H 2 O 500 NH 4 H 2 PO FeS Jupiter 50% Na in Na 2 S Fe P enstatite anorthite forsterite 50% Na in feldspar Ca-Ti-ox. Fe metal Ca-Al-oxides Enstatite Forsterite Fe-metal Ca-Ti-oxides Ca-Al-oxides Gl229B forsterite enstatite Fe metal dm Na 2 S Ca-Ti-ox. Ca-Al-oxides log (total pressure, bar) - FeS (troilite) Differences NH 4 SH log (total pressure, bar) - Fe P (schreibersite) NH 4 H 2 PO 4 Na dissolves in feldspar Na 2 S *if reaction kinetics and timescales permit this

15 Consequences for gas chemistry on Fe-metal cloud formation Protoplanetary Disk Chemistry H 2 S is the major S-gas FeS (troilite) formation removes all H 2 S gas at T below 700K. Iron is twice as abundant as sulfur, so no H 2 S gas can remain. Cloudy Atmosphere Chemistry H 2 S is the major S-gas H 2 S remains in atmosphere until NH 4 SH condenses below ~200K H 2 S is present in Jupiter s deep atmosphere (Galileo probe); expect H 2 S in T dwarfs PH, PH 2, P 2, PO are major P-gases Fe P (schreibersite) formation consumes all P gases. Cooler atmospheres (i.e., <1500K; above the Fe P cloud) would be free of P gases PH,PH 2, P 2, (P 4 O 6 ) are major P-gases NH 4 H 2 PO 4 condenses around K and removes PH and other P-gases at lower T. Above T = K, all P should be in the gas. PH gas is present in deep giant planet atms. Expect PH and other P gases in M, L, and T dwarfs. For brown dwarf P and S chemistry, see Visscher, Lodders, & Fegley 2006, ApJ 648, 1181

16 Abundant elements are responsible for forming major massive clouds Solar abundance Selection of by number Gas Abundance(s) Affected O in water H 2 O N in ammonia NH Mg in magnesium silicates Mg, MgH Si in magnesium silicates Si, SiO, SiS Fe in iron metal Fe, FeH S in NH 4 SH H 2 S Somewhat less abundant elements form less massive clouds, cloudlets Ca in ceramics Ca, CaH Al in Ca-Al-oxides Al, AlH Ti in Ca-Ti-oxides, TiN Ti, TiO Na in Na 2 S Na K in KCl K Cr in Cr metal or Cr 2 O (dep. P tot ) CrH Mn in MnS Mn P in NH 4 H 2 PO 4 PH

17 Mg & Si condensation I. solar composition, 1 bar 1.0 Major condensates Forsterite: Mg-olivine, Mg 2 SiO 4 Enstatite: Mg-pyroxene, MgSiO Other minor condensates: Mg & Si in melilite, diopside Mg spinel; Si anorthite Forsterite reacts to Enstatite: Fraction of Total Abundance enstatite diopside forsterite Mg gases: Mg, MgOH, MgH, MgO,... spinel & melilite Mg 2 SiO 4 + SiO + H 2 O = 2 MgSiO + H 2 Mg and Si are not completely removed from the gas when enstatite starts condensing. Enstatite condensation starts ~90 K lower than forsterite (P-dep.) More Fraction of Total Abundance enstatite diopside forsterite anorthite gases: SiO, SiS, Si,... melilite Si

18 Mg & Si condensation II. solar composition, 1 bar Forsterite reacts to Enstatite: Mg 2 SiO 4 + SiO + H 2 O = 2 MgSiO + H 2 Solar composition has Mg:Si = 1:1 In principle, forsterite Mg 2 SiO 4, can accommodate all solar Mg but only half of all solar Si Fraction of Total Abundance enstatite diopside forsterite Mg gases: Mg, MgOH, MgH, MgO,... spinel & melilite Forsterite condensation leaves major silicon gases Si, SiO, & SiS behind. Enstatite with Mg:Si = 1:1 can accommodate all Mg and Si. What happens if enstatite does not form? Fraction of Total Abundance enstatite diopside forsterite anorthite gases: SiO, SiS, Si,... melilite Si

19 Mg & Si condensation III. solar composition, 1 bar 1.0 If forsterite does not react with the gas, enstatite does not form. Forsterite takes all Mg, and about 50% of all Si (~10% Si is in diopside and anorthite) The 40% Si left in the gas would condense as SiO 2. Fraction of Total Abundance diopside forsterite Mg gases: Mg, MgOH, MgH, MgO,... spinel & melilite Example: Tcond. Forsterite: 1690 K Enstatite: 1600 K SiO 2 : 1540 K (if enstatite is absent) SiO 2 would condense ~60 K lower than enstatite does. Important difference: Enstatite forms by gas-solid reaction SiO 2 condenses directly from gas Fraction of Total Abundance SiO 2 diopside forsterite anorthite gases: SiO, SiS, Si,... melilite Si

20 Refractory Ca-Al-Ti-Ceramic Clouds: The mineralogy of the initial Ca-Al-condensate depends on total pressure. Melilite = (solid-) solution of gehlenite Ca 2 Al 2 SiO 7 and akermanite Ca 2 MgSi 2 O Ca-Al-condensates melilite 2 grossite CaAl 4 O 7 hibonite CaAl 12 O log (total pressure, bar) Condensation temperatures and Ca:Al ratios of the initial condensate increase with P tot Ca:Al Corundum Al 2 O 0 : 1 Hibonite CaAl 12 O 19 1 : 12 Grossite CaAl 4 O 7 1 : 4 Gehlenite Ca 2 Al 2 SiO 7 1 : 1 corundum Al 2 O - -4 Ca & Al have about the same solar abundance Lodders 2002, ApJ 577, 974

21 Refractory Ceramic Clouds: Ca, Al, & Ti condensation 1600 The composition of the initial Ca-Ti-oxide condensate depends on total pressure. Ca:Ti ratios increase with P tot (like Ca:Al) Osbornite TiN Ca Ti 2 O 7 Ca 4 Ti O 10 perovskite CaTiO Ca-Ti-condensates The chemistry of Ti is coupled to that of Al through the Ca chemistry. Total Ti is ~40x less abundant than Al log (total pressure bar) Ca-Al-condensates CaTiO Ca 4 Ti O 10 Ca Ti 2 O 7 CaAl 12 O 19 melilite CaAl 4 O 7 - Al 2 O TiN Ca-Ti-condensates Lodders 2002, ApJ 577, log (total pressure bar) - -4

22 Refractory Ceramic Clouds: Ca, Al, & Ti condensation constant total pressure Atom-Fraction of All Titanium Atom-Fraction of All Calcium (a) Ti 4 O K (b) Ti O 5 CaMgSi 2 O 6 (s) CaAl 2 Si 2 O 8 (s) Ti2 O CaTiO Ca 4 Ti O 10 Ca Ti 2 O 7 CaAl 4 O 7 TiO 2 (g) TiO (g) Ca 4 Ti O 10 Ti (g) Temperature (K) Lodders 2002, ApJ 577, 974 Ca 2 MgSi 2 O 7 (s) CaAl 12 O 19 CaTiO Ca 2 Al 2 SiO 7 (s,l) Ca4 Ca4 Ca CaOH (g) CaH (g) Ca (g) CaAl 12 O 19 Example: 1 bar At lower T than their condensation temperatures, the given initial Ca-Ti- and Ca-Al condensates can react with Ca still in the gas. Near1700 K, essentially all Ca and Ti are out of the gas. TiO gas is depleted, similarly VO gas becomes depleted: Happens at M/L transition Settling of condensates may prevent formation of the relatively low temperature Ca-Alsilicates and Ti-oxides here. i.e.,no diopside, no anorthite, no Ti O 5,Ti 4 O 7.

23 proto planetary disk Fe P enstatite anorthite forsterite FeS 50% Na in feldspar H 2 O Ca-Ti-ox. - Fe metal Ca-Al-oxides "cloudy atmosphere chemsitry" Jupiter Gl229B NH 4 H 2 PO 4 forsterite enstatite Fe metal dm NH 4 SH H 2 O 50% Na in Na 2 S Na 2 S Ca-Ti-ox. Ca-Al-oxides alkalis (Na, K, Rb, Cs) Less abundant; may condense into (solid) solution with major element host phases, e.g., (solid) solutions of feldspars Anorthite CaAl 2 Si 2 O 8 Albite NaAlSi O 8 Orthoclase KAlSi O log (total pressure, bar) log (total pressure, bar) - Anorthite is the most refractory feldspar, and significant alkali incorporation into feldspar requires much lower temperatures than for initial anorthite formation. If anorthite never forms, alkalis cannot dissolve in it. If anorthite forms from other Ca-Al oxides, it sits in a cloud layer at high temperatures where essentially no alkalis enter it. In low-mass object atmospheres, alkalis condense as sulfides or chlorides

24 More alkalis Lithium-test Presence of monatomic Li line may be used as indicator for the substellar nature of an object Li-burning occurs in objects with >65M Jup. But Li-test only works in hotter objects where Li is mainly in the form of atomic Li gas T (K) Jupiter LiOH LiF (s) Gl 229B LiCl LiCl Li 2 S (s) dm LiF /T (K) 8 Lower temperature favor gaseous Li molecules (LiOH, LiCl, LiF), and Li-condensates LiF, Li 2 S Li Absence of monatomic Li in Jupiter does not mean that Jupiter is a star Total pressure, log (P, bar) Lodders 1999, ApJ 519, 79

25 Chemistry of the elements is coupled CO+ H 2 = CH 4 + H 2 O equilibrium influences molecular chemistry of many elements, e.g., LiOH appears in methane and more H 2 O-rich field Condensation of Na 2 S removes Na from the gas and frees chlorine that was in NaCl (g). Then KCl and RbCl gas become more abundant and KCl and RbCl condense at lower T T (K) CsCl (s) Jupiter LiOH 2 RbCl (s) NH N 2 Li Cs Cs Gl 229B CsCl Cs Mg 2 SiO 4 (s) Fe (l,s) -1 KCl (s) LiF (s) LiCl Li 2 S (s) Na 2 S (s) Rb Rb + -2 dm - CO CH 4 K = KCl Rb = RbCl Ca-Ti-cond. VO V LiF Ca-Al-ox /T (K) Total pressure, log (P, bars) Lodders 1999, ApJ 519, 79

26 Summary Gas and condensation chemistry in low-mass object atmospheres is different than in proto-planetary disks or stellar winds because condensates settle into cloud layers. Cloud formation can be tracked by the depletion of gases (weakening spectral features) that contain the elements sequestered into clouds (e.g., TiO, VO, FeH, CrH, Li, Na, K) The elements removed into the refractory clouds are not available to react with atmospheric gases at lower temperatures (higher up in the atm.) e.g., Ca, Al, Ti are in refractory ceramics, Fe in Fe-metal This means different types of condensates form in atmospheres at lower temperatures than in disks or stellar winds (e.g., alkali sulfides & chlorides, NH 4 SH). It also means that certain elements are removed from the gas into clouds at lower temperatures in atmospheres than one would expect from chemistry in disks or stellar winds (e.g., sulfides & chlorides of Na, K, Rb, Cs)

27 From Lodders & Fegley 2006, Astrophysics Update, Vol. 2, p. 1ff.

28 Minerals & Their Formulas Oxides Corundum Al 2 O Periclase MgO Rutile TiO 2 Quartz SiO 2 Calcium Aluminates Grossite CaAl 4 O 7 Hibonite CaAl 12 O 19 Calcium Titanates Perovskite CaTiO No name Ca Ti 2 O 7 No name Ca 4 Ti O 10 Solid-Solution Series Melilite Gehlenite Ca 2 Al 2 SiO 7 Akermanite Ca 2 MgSi 2 O 7 Olivines Forsterite Mg 2 SiO 4 Fayalite Fe 2 SiO 4 Pyroxenes Enstatite MgSiO Wollastonite CaSiO Ferrosilite FeSiO Diopside CaMgSi 2 O 6 Sulfides Troilite FeS Feldspars Anorthite CaAl 2 Si 2 O 8 Salts Halite NaCl Sylvite KCl Albite NaAlSi O 8 Orthoclase KAlSi O 8

1. Chemistry of Low Mass Substellar Objects

1. Chemistry of Low Mass Substellar Objects Katharina Lodders 1 & Bruce Fegley, Jr. 1 Abstract: Brown dwarfs is the collective name for objects more massive than giant planets such as Jupiter but less