Carbon and oxygen abundances in cool metal-rich exoplanet hosts: A case study of the C/O ratio of 55 cancri

Johanna K. Teske, Katia Cunha, Simon C. Schuler, Caitlin Griffith, Verne V. Smith

Research output: Contribution to journalArticle

25 Citations (Scopus)

Abstract

The super-Earth exoplanet 55 Cnc e, the smallest member of a five-planet system, has recently been observed to transit its host star. The radius estimates from transit observations, coupled with spectroscopic determinations of mass, provide constraints on its interior composition. The composition of exoplanetary interiors and atmospheres are particularly sensitive to elemental C/O ratio, which to first order can be estimated from the host stars. Results from a recent spectroscopic study analyzing the 6300 Å [O I] line and two C I lines suggest that 55 Cnc has a carbon-rich composition (C/O = 1.12 ± 0.09). However, oxygen abundances derived using the 6300 Å [O I] line are highly sensitive to a Ni I blend, particularly in metal-rich stars such as 55 Cnc ([Fe/H] =0.34 ± 0.18). Here, we further investigate 55 Cnc's composition by deriving the carbon and oxygen abundances from these and additional C and O absorption features. We find that the measured C/O ratio depends on the oxygen lines used. The C/O ratio that we derive based on the 6300 Å [O I] line alone is consistent with the previous value. Yet, our investigation of additional abundance indicators results in a mean C/O ratio of 0.78 ± 0.08. The lower C/O ratio of 55 Cnc determined here may place this system at the sensitive boundary between protoplanetary disk compositions giving rise to planets with high (>0.8) versus low (<0.8) C/O ratios. This study illustrates the caution that must applied when determining planet host star C/O ratios, particularly in cool, metal-rich stars.

Original languageEnglish (US)
Article number132
JournalAstrophysical Journal
Volume778
Issue number2
DOIs
StatePublished - Dec 1 2013

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extrasolar planets
planet
stars
oxygen
carbon
metal
planets
metals
transit
protoplanetary disks
atmosphere
atmospheres
radii
estimates

Keywords

  • planets and satellites: formation
  • planets and satellites: individual (55 Cnc)
  • stars: abundances
  • stars: atmospheres

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

Carbon and oxygen abundances in cool metal-rich exoplanet hosts : A case study of the C/O ratio of 55 cancri. / Teske, Johanna K.; Cunha, Katia; Schuler, Simon C.; Griffith, Caitlin; Smith, Verne V.

In: Astrophysical Journal, Vol. 778, No. 2, 132, 01.12.2013.

Research output: Contribution to journalArticle

Teske, Johanna K. ; Cunha, Katia ; Schuler, Simon C. ; Griffith, Caitlin ; Smith, Verne V. / Carbon and oxygen abundances in cool metal-rich exoplanet hosts : A case study of the C/O ratio of 55 cancri. In: Astrophysical Journal. 2013 ; Vol. 778, No. 2.
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abstract = "The super-Earth exoplanet 55 Cnc e, the smallest member of a five-planet system, has recently been observed to transit its host star. The radius estimates from transit observations, coupled with spectroscopic determinations of mass, provide constraints on its interior composition. The composition of exoplanetary interiors and atmospheres are particularly sensitive to elemental C/O ratio, which to first order can be estimated from the host stars. Results from a recent spectroscopic study analyzing the 6300 {\AA} [O I] line and two C I lines suggest that 55 Cnc has a carbon-rich composition (C/O = 1.12 ± 0.09). However, oxygen abundances derived using the 6300 {\AA} [O I] line are highly sensitive to a Ni I blend, particularly in metal-rich stars such as 55 Cnc ([Fe/H] =0.34 ± 0.18). Here, we further investigate 55 Cnc's composition by deriving the carbon and oxygen abundances from these and additional C and O absorption features. We find that the measured C/O ratio depends on the oxygen lines used. The C/O ratio that we derive based on the 6300 {\AA} [O I] line alone is consistent with the previous value. Yet, our investigation of additional abundance indicators results in a mean C/O ratio of 0.78 ± 0.08. The lower C/O ratio of 55 Cnc determined here may place this system at the sensitive boundary between protoplanetary disk compositions giving rise to planets with high (>0.8) versus low (<0.8) C/O ratios. This study illustrates the caution that must applied when determining planet host star C/O ratios, particularly in cool, metal-rich stars.",
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