 |
| Titel |
Effect of stellar composition on the rock/ice composition of condensates in exoplanet systems |
| VerfasserIn |
Torrence Johnson, Olivier Mousis, Jonathan Lunine |
| Konferenz |
EGU General Assembly 2011
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250048849
|
|
|
|
|
|
| Zusammenfassung |
| In our solar system, the composition of solid material condensing beyond the ‘snow-line’
(where solar nebula pressure and temperature conditions permit the condensation of water
ice) is affected by several major factors. Starting with the ‘solar abundance’ of elements in
the nebula, derived from the current solar photospheric abundances, meteorites, and estimates
of protosolar values in the early solar nebula, the most important things affecting the
composition of the condensates are 1. The carbon and oxygen abundances, (C/O)solar = 0.55
2. The redox state of C (i.e. CO vs. CH4 rich conditions) and 3. The amount of carbon in
solid form. These factors largely determine the refractory to volatile proportions in the
expected condensates. For the solar system, we characterize the composition by the fraction
of silicates, oxides and metals in the overall condensate, fr-m. Calculated fr-m
varies from ~0.47 to 0.76 in regions of the solar nebula where water is the major
condensed volatile and CO and CH4 are non-condensable (cf. Wong et al., Oxygen in the
Solar System, G. J. MacPherson, ed, 2008). In colder regions of the nebula, other
volatiles, hydrates, clathrates and pure ices such as CO and N2 add to the condensed
volatile fraction depending on the nebula conditions – cf. Mousis et al. ApJ 696, 1348
(2009).
Recent surveys of the stellar abundances of solid forming elements in a sample of
exoplanet host stars have shown that there are significant differences from the Sun which
affect the expected rock/ice values in condensates in these systems, particularly a
wide range of C/O values. We have used the abundances of O, C. Si, S, Fe, and Ni
from these surveys to calculate fr-m for condensates in these systems for various
conditions, using the methods outlined in Wong et al. The volatile ice compositions
have been calculated following the methods described in Mousis et al. ApJ 727
(2011).
The results of our study are that fr-m in these systems may range from ~0.25 to 1.0. The
calculated fr-m values are weakly correlated with the metallicity (Fe/H) but strongly
correlated with (C/O), as expected from the solar case. The following cases illustrate the
range of possible outcomes: 1. HD17783 (C/O) =0.35 – fr-m from 0.33-0.46 (below the
solar range for all redox conditions), with the ice composition ranging from 0.58 H2O with
0.23 CO (if temperatures permit), for oxidizing conditions to 0.78 H2O with 0.15 CH4(for
low temperatures) for reducing conditions; 2. HD10887 (C/O) = 0.71 – for the
oxidizing case, little O is available for water ice and the condensates are all rock
fr-m =~1,
until temperatures allow condensation of CO2and CO ices. For the reducing case fr-m is
0.53 and the ices are primarily water and CH4 (if T permits); 3. HD4203 (C/O) = 1.5 – This
high carbon composition results in reducing conditions (all O is taken up by silicates
and oxides) and condensates are rocky (fr-m = 0.86) with water ice the major
volatile (some methane clathrate possible) until temperatures are low enough for CH4
ice.
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.
2011).
Acknowledgements: Part of this work has been conducted at the Jet Propulsion
Laboratory, California Institute of Technology, under a contract with NASA. Government
sponsorship acknowledged. |
|
|
|
|
|
|