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TOI-7169 b: A Hot Jupiter Transiting a Metal-Poor Star

Joshua D. Simon, Joseph E. Rodriguez, Jhon Yana Galarza, David W. Latham, Victoria DiTomasso, Karen A. Collins, Jack Schulte, Anirudh Chiti, Samuel N. Quinn, Mohammad K. Mardini, Shubham Kanodia, Johanna K. Teske, Peter S. Ferguson, Samuel W. Yee, T. G. Tan, Khalid Alsubai, Khalid Barkaoui, Zouhair Benkhaldoun, Krzysztof Bernacki, Jaikrit Bhattacharya, Jerome P. de Leon, Sarah J. Deveny, Mark E. Everett, Izuru Fukuda, Akihiko Fukui, Michael Gillon, Arvind F. Gupta, Steve B. Howell, Emmanuel Jehin, Gaia Lacedelli, Adam Lark, Colin Littlefield, Felipe Murgas, Norio Narita, Enric Palle, Hannu Parviainen, Adam Popowicz, Richard P. Schwarz, Avi Shporer, Abderahmane Soubkiou, Francis P. Wilkin

Abstract

Most known planets are found around metal-rich host stars, which has made it difficult to determine whether a lower metallicity limit for planet formation exists and how the properties of planets born in low-metallicity environments may differ from those with metal-rich origins. We present the discovery and characterization of TOI-7169 b (TIC 372048733 b), a hot Jupiter that is orbiting a spectroscopically-confirmed metal-poor ([Fe/H] = -0.72 +/- 0.05) host star. Based on photometry from TESS and follow-up ground-based imaging, we measure an orbital period of 3.4373125 d and a planetary radius of 1.475 +/- 0.029 R_Jup. We use TRES spectroscopy to determine a mass for TOI-7169 b of 0.41 +/- 0.14 M_Jup. The planet is therefore inflated, with a low density of 0.159 +0.055/-0.054 g/cm^3. We also characterize the host star, showing that TOI-7169 is ancient (12.3 +/- 0.6 Gyr) and alpha-enhanced ([alpha/Fe] ~ 0.3), but with a Galactocentric orbit that is confined to the thin disk. TOI-7169 is perhaps the oldest and most metal-poor star currently known to host a transiting giant planet. Future transmission spectroscopy probing the atmosphere of TOI-7169 b may provide insight into the effect of metallicity on the physical properties of giant planets.

TOI-7169 b: A Hot Jupiter Transiting a Metal-Poor Star

Abstract

Most known planets are found around metal-rich host stars, which has made it difficult to determine whether a lower metallicity limit for planet formation exists and how the properties of planets born in low-metallicity environments may differ from those with metal-rich origins. We present the discovery and characterization of TOI-7169 b (TIC 372048733 b), a hot Jupiter that is orbiting a spectroscopically-confirmed metal-poor ([Fe/H] = -0.72 +/- 0.05) host star. Based on photometry from TESS and follow-up ground-based imaging, we measure an orbital period of 3.4373125 d and a planetary radius of 1.475 +/- 0.029 R_Jup. We use TRES spectroscopy to determine a mass for TOI-7169 b of 0.41 +/- 0.14 M_Jup. The planet is therefore inflated, with a low density of 0.159 +0.055/-0.054 g/cm^3. We also characterize the host star, showing that TOI-7169 is ancient (12.3 +/- 0.6 Gyr) and alpha-enhanced ([alpha/Fe] ~ 0.3), but with a Galactocentric orbit that is confined to the thin disk. TOI-7169 is perhaps the oldest and most metal-poor star currently known to host a transiting giant planet. Future transmission spectroscopy probing the atmosphere of TOI-7169 b may provide insight into the effect of metallicity on the physical properties of giant planets.

Paper Structure

This paper contains 17 sections, 9 figures.

Table of Contents

  1. Introduction
  2. Identification of Candidates
  3. Observations
  4. TESS
  5. Ground-based Photometry
  6. Speckle Imaging
  7. Spectroscopy
  8. Analysis
  9. Host Star Characterization
  10. Derivation of Planetary Properties
  11. Discussion
  12. Comparison to Exoplanet Population and Occurrence Rates
  13. Is a Low Host Star Metallicity Associated with Low Planet Density?
  14. Orbit in the Milky Way
  15. Spectroscopy Metrics
  16. ...and 2 more sections

Figures (9)

  • Figure 1: Flattened TESS light curve of TOI-7169 from Sectors 83 and 84. Sector 83 used an integration time of 120 s and the Sector 84 integration time was 200 s. The 15 transits of TOI-7169 b that occur across these two sectors are indicated by red hash marks.
  • Figure 2: Results from speckle imaging of TOI-7169. (Left) Gemini/'Alopeke detection limits. The blue and red curves show the $5\sigma$ contrast limits in magnitudes reached in the 562 nm and 832 nm filters, respectively. The inset in the upper right presents the 832 nm image of the field, with no sources other than the host star detected. (Right) WIYN/NESSI $5\sigma$ detection limits. The blue and red curves show the contrast limits reached in the 562 nm and 832 nm filters and the insets show the two reconstructed images.
  • Figure 3: Comparison between the spectrum of TOI-7169 (black) and the Sun (blue). The temperatures of the two stars are quite similar, so the differences in line strength are primarily due to abundance differences (with the exception of the very strong Mg line at 5185 Å). Spectral lines of iron-peak (Fe and Cr) and $\alpha$ elements (Mg and Ti) are shaded as indicated in the legend.
  • Figure 4: Derived absolute magnitude and effective temperature of TOI-7169 from the q$^{2}$ package, compared to Yonsei-Yale isochrones. The red isochrone represents the best-fitting age, which does not exactly match the derived stellar properties because it is a marginalized Bayesian posterior in a higher-dimensional parameter space. The tight constraints on both parameters, and hence on the age, are consequences of the small error bars on the stellar parallax and $G$-band magnitude from Gaia.
  • Figure 5: Abundance pattern of TOI-7169. In each panel, the derived abundance for TOI-7169 is plotted as a red circle, the solar abundance is indicated by the black sun symbol, and abundances of main-sequence stars from GALAH DR4 buder25 are displayed as small gray dots. The TOI-7169 abundances generally agree with the extrapolated trends from GALAH, with the exception of oxygen, which has a significantly lower abundance than expected for a star of its metallicity.
  • ...and 4 more figures