Carbon and Oxygen Abundances in Cool Metal-Rich Exoplanet Hosts: A Case Study of the C/O Ratio of 55 Cancri

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 6300Å [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 6300Å [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 6300Å [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. Recently Madhusudhan et al. (2012) suggest an alternative and carbon-rich composition of 55 Cnc e, garnering the super-Earth popular attention as "the diamond planet." Measurements of the carbon and oxygen abundances from two C I lines (5052Å, 5135Å) and one forbidden [O I] line (6300Å) indicate a C/O 1 ratio of 1.12±0.19 (Delgado Mena et al. 2010), i.e., a highly carbon-rich star compared to the solar C/O∼0.50 (Asplund et al. 2005). If the disk shared the host star's composition, and the host star is carbon-rich, then the planetesimals accreted during the formation of 55 Cnc e were likely Fe-and C-rich (Bond et al. 2010; Madhusudhan et al. 2012). To investigate the composition of the possibly carbon-rich exoplanet, Madhusudhan et al. (2012) consider two families of carbon-rich interior models of 55 Cnc e, consisting of layers, from inner to outer, of Fe-SiC-C and Fe-MgSiO 3 -C. Included in their carbon equation of state (EOS) are the graphite EOS at low pressures, the phase transition to diamond between 10 GPa<P<1000 GPa, and the Thomas-Fermi-Dirac EOS at high pressures. Madhusudhan et al. (2012) find a wide range of compositions are possible, including extreme combinations like (Fe, SiC, C) = (33%, 0%, 67%), and the best match to 55 Cnc e's observations depends on the adopted radius measurement, and the conditions in the protoplanetary disk, e.g. temperature, at which the building blocks of the planet condense.The exact composition of 55 Cnc e depends on the primary source of accreted planetesimals...

Born dry in the photoevaporation desert: Kepler's ultra-short-period planets formed water-poor

“…Such a carbon rich mantle would allow the planet to match the current mass and radius without needing a large volatile envelope. This proposal is somewhat supported by observations by Teske et al (2013),which suggest a slightly enhanced stellar C/O of 0.78 ± 0.08, although Brewer et al (2016) found a lower value of 0.53±0.054, consistent with Solar composition. At the moment though, any definitive conclusions about the composition of 55 Cancri e are premature and further work is needed from both observers and theorists to confirm these recent observations and to further and model characterize the planet's variability and possible exosphere.…”

Mass, Radius, and Composition of the Transiting Planet 55 Cnc e: Using Interferometry and Correlations

“…Madhusudhan et al 2012). This was motivated by a high C/O ratio estimate for the star (1.12 ± 0.19, Delgado Mena et al 2010), but this ratio has been later corrected down to 0.78 ± 0.08 (Teske et al 2013), making C-rich interior models less timely for 55 Cnc e. Although Moriarty et al (2014) argue that a sequential condensation during the whole life of an evolving proto-planetary disk can favor the formation of C-rich planetesimals, they find that the planetesimals expected to form around 55 Cnc should have C/O<1, even assuming C/O=1 for this system (their figure 1). In addition, C-rich interiors are poorly understood.…”

M dwarfs and the fraction of high carbon-to-oxygen stars in the solar neighbourhood

“…Nissen (2013) measures 33 G dwarfs using high resolution spectra including a different oxygen line, and concludes that the claims of high C/O are "spurious." On the other hand, Teske et al (2013) present a case study of the 55 Cnc (G8V) planetary system and conclude that C/O ≈ 0.8.…”