Giant Planet Occurrence in the Stellar Mass-Metallicity Plane (1005.3084v2)

Abstract: Correlations between stellar properties and the occurrence rate of exoplanets can be used to inform the target selection of future planet search efforts and provide valuable clues about the planet formation process. We analyze a sample of 1194 stars drawn from the California Planet Survey targets to determine the empirical functional form describing the likelihood of a star harboring a giant planet as a function of its mass and metallicity. Our stellar sample ranges from M dwarfs with masses as low as 0.2 Msun to intermediate-mass subgiants with masses as high as 1.9 Msun. In agreement with previous studies, our sample exhibits a planet-metallicity correlation at all stellar masses; the fraction of stars that harbor giant planets scales as f \propto 10^{1.2 [Fe/H]}. We can rule out a flat metallicity relationship among our evolved stars (at 98% confidence), which argues that the high metallicities of stars with planets are not likely due to convective envelope "pollution." Our data also rule out a constant planet occurrence rate for [Fe/H]< 0, indicating that giant planets continue to become rarer at sub-Solar metallicities. We also find that planet occurrence increases with stellar mass (f \propto Mstar), characterized by a rise from 3.5% around M dwarfs (0.5 Msun) to 14% around A stars (2 Msun), at Solar metallicity. We argue that the correlation between stellar properties and giant planet occurrence is strong supporting evidence of the core accretion model of planet formation.

• The paper demonstrates that giant planet occurrence sharply increases with stellar metallicity, following a quantifiable relation of f ∝ 10^(1.2[Fe/H]).
• The paper reveals that planet occurrence rises with stellar mass, from 3% around M dwarfs to 14% around A-type stars at solar metallicity.
• The findings reinforce the core accretion model and refute the polluted star theory, guiding future exoplanet surveys for target selection.

Analysis of Giant Planet Occurrence in the Stellar Mass-Metallicity Plane

The paper "Giant Planet Occurrence in the Stellar Mass-Metallicity Plane" provides a detailed empirical investigation into the relationship between stellar characteristics—specifically, mass and metallicity—and the likelihood of a star hosting a giant planet. Utilizing a substantial sample from the California Planet Survey (CPS), comprising 1,266 stars ranging from M dwarfs to intermediate-mass subgiants, the research aims to elucidate the underlying distribution patterns related to planet occurrence.

Key Findings

1. Planet-Metallicity Correlation (PMC): Consistent with earlier findings, the paper uncovers a strong planet-metallicity correlation across all stellar masses in the sample. It is quantified that the likelihood of a star harboring a giant planet increases sharply with stellar metallicity, following the relation $f \propto 10^{1.2 \, \text{[Fe/H]}}$.
2. Dependence on Stellar Mass: A noteworthy outcome is the positive correlation between stellar mass and giant planet occurrence. The paper identifies a trend where planet occurrence increases from 3% around low-mass M dwarfs (0.5 M⊙) to 14% around A-type stars (2 M⊙) at solar metallicity—described by the function $f \propto M_\star$. This highlights a significant rise in giant planet occurrence with increasing stellar mass.
3. Rule Out of Polluted Star Theory: The data contravene any significant effect of superficial metallicity pollution, a concept where metal enrichment is restricted to a star's outer layers. The uniformity of the planet-metallicity correlation across evolved stars disputes the "pollution" hypothesis, reinforcing that the observed stellar metallicities reflect conditions during the planet formation epoch.
4. Inferences on Planet Formation Theories: The findings further bolster the core accretion model of planet formation, implying that both higher metallicities and stellar masses lead to the increased likelihood of planet formation. Such correlation challenges the disk instability model, which posits independence from stellar properties for giant planet formation.

Implications and Future Directions

The paper’s results have profound implications for both theoretical and observational astrophysics, particularly in refining models of planet formation. The clear empirical relationship between stellar mass, metallicity, and giant planet occurrence can guide future exoplanetary surveys in target selection, especially in direct imaging and microlensing techniques, where high-mass stars or those with substantial metallicity promise richer yields.

Moreover, the implications underscore the importance of stellar composition in shaping planetary system architectures, which might also extend to terrestrial-type planets. Given that the dataset employed spans a wide range of stellar masses, future research could benefit from extending these methods to broader stellar environments, including clusters with varying initial conditions.

Conclusion

Through robust statistical techniques, this paper provides a pivotal contribution to the understanding of exoplanet demographics. It explicitly aligns empirical data with theoretical predictions about planet formation, particularly augmenting the core accretion model. As new survey data become available, the methodologies employed here will prove essential in further dissecting the complex interplay of factors dictating planet formation across the galaxy.