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Mass determination of the ultra-short-period planet LHS 3844 b. First K-band radial velocity measurements with CRIRES+

E. Nagel, J. Köhler, M. Zechmeister, A. D. Rains, U. Seemann, A. Hatzes, A. Reiners, N. Piskunov, L. Boldt-Christmas, P. Bristow, P. Chaturvedi, D. Cont, S. Czesla, R. J. Dorn, E. Guenther, Y. Jung, O. Kochukhov, F. Lesjak, F. Lucertini, T. Marquart, L. Nortmann, M. Rengel, F. Rodler, J. V. Smoker

Abstract

We present the first planet mass measurement obtained with CRIRES+ radial velocity (RV) observations using the K-band gas cell. Our target, LHS 3844 b (TOI-136), is a transiting super-Earth with radius $R_b=1.286^{+0.043}_{-0.044}R_\oplus$ and an orbital period of $P_b = 0.462929709^{+0.000000044}_{-0.000000042}$d, placing it in the class of ultra-short-period (USP) planets. The host star LHS 3844 is an old ($7.8\pm1.6$Gyr), slowly rotating ($P_{rot} = 130.0^{+16.9}_{-13.4}$d) M5.0 dwarf with $M_\star = 0.151\pm0.014M_\odot$ at a distance of 15pc (V=15.2mag, K=9.2mag). Combining our CRIRES+ RVs with archival ESPRESSO spectra, and confirming the signal in each dataset independently, we detected periodic RV variations with a semi-amplitude $K_b=6.95^{+0.55}_{-0.60}$m/s, implying a planetary mass of $m_b = 2.37\pm0.25M_\oplus$ and a bulk density of $ρ_b = 6.15^{+0.60}_{-0.61}$gcm$^{-3}$, consistent with a predominantly rocky composition. We further found excess RV variability that may be attributed to stellar jitter or to an additional planetary signal, for which we identified a tentative super-Earth candidate with a period of $\approx6.88$d. Owing to its proximity to its M-dwarf host, LHS 3844 b experiences intense irradiation and is unlikely to retain a substantial H/He envelope. Interior modeling places an upper limit on the iron-core mass fraction, which is consistent with an Earth-like rocky composition. With an emission spectroscopy metric of 28, LHS 3844 b is a prime JWST target for atmospheric and surface characterization and the most promising surface-characterization target known. Phase-curve spectroscopy may reveal its surface mineralogy and enable the first robust detection of exoplanet surface spectral features. Our results demonstrate that near-infrared RVs obtained with CRIRES+ enable robust mass measurements of super-Earths orbiting late M dwarfs.

Mass determination of the ultra-short-period planet LHS 3844 b. First K-band radial velocity measurements with CRIRES+

Abstract

We present the first planet mass measurement obtained with CRIRES+ radial velocity (RV) observations using the K-band gas cell. Our target, LHS 3844 b (TOI-136), is a transiting super-Earth with radius and an orbital period of d, placing it in the class of ultra-short-period (USP) planets. The host star LHS 3844 is an old (Gyr), slowly rotating (d) M5.0 dwarf with at a distance of 15pc (V=15.2mag, K=9.2mag). Combining our CRIRES+ RVs with archival ESPRESSO spectra, and confirming the signal in each dataset independently, we detected periodic RV variations with a semi-amplitude m/s, implying a planetary mass of and a bulk density of gcm, consistent with a predominantly rocky composition. We further found excess RV variability that may be attributed to stellar jitter or to an additional planetary signal, for which we identified a tentative super-Earth candidate with a period of d. Owing to its proximity to its M-dwarf host, LHS 3844 b experiences intense irradiation and is unlikely to retain a substantial H/He envelope. Interior modeling places an upper limit on the iron-core mass fraction, which is consistent with an Earth-like rocky composition. With an emission spectroscopy metric of 28, LHS 3844 b is a prime JWST target for atmospheric and surface characterization and the most promising surface-characterization target known. Phase-curve spectroscopy may reveal its surface mineralogy and enable the first robust detection of exoplanet surface spectral features. Our results demonstrate that near-infrared RVs obtained with CRIRES+ enable robust mass measurements of super-Earths orbiting late M dwarfs.

Paper Structure

This paper contains 33 sections, 3 equations, 21 figures, 11 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. TESS photometry
  4. MEarth photometry
  5. High-resolution spectroscopy
  6. CRIRES$^+$
  7. ESPRESSO
  8. RV data quality
  9. Host star properties
  10. Stellar rotation period
  11. Stellar parameters
  12. Analysis and results
  13. Photometric analysis
  14. Detrending TESS light curves
  15. Transit signal search
  16. ...and 18 more sections

Figures (21)

  • Figure 1: Target pixel file image for LHS 3844 from TESS Sector 1, with electron counts represented through color coding. The aperture mask applied by the SPOC pipeline for SAP extraction is highlighted by red squares. Nearby objects cataloged in Gaia DR3 are denoted by red circles, the sizes of which scale according to the magnitude contrast with LHS 3844. The central position of LHS 3844 is marked by a white cross, and the arrows denote proper motion directions.
  • Figure 2: Top panels: Radial velocity measurements of LHS 3844 obtained with CRIRES$^+$. The solid black line indicates the best-fit Keplerian model for a single planet on a circular orbit (Sect. \ref{['section:joint']}), with the gray shaded region marking the 68 % confidence interval. The red solid line shows the GP component. Each panel covers exactly one orbital period of LHS 3844 b and displays data from the 11 observing runs, color-coded accordingly. The large uncertainties in the rightmost panel are due to poor weather conditions. Bottom panels: ${\rm O} - {\rm C}$ residuals.
  • Figure 3: ESPRESSO RV time series of LHS 3844. Upper panels: Radial velocity measurements shown before (blue squares, top row) and after (red squares, bottom row) the instrumental intervention. The solid black line represents the best-fit Keplerian model for a single planet on a circular orbit (Sect. \ref{['section:joint']}), with the gray shaded region marking the 68 % confidence interval. The red solid line shows the GP component. Error bars are smaller than the plotted symbols. Bottom panels: ${\rm O} - {\rm C}$ residuals corresponding to the top panels.
  • Figure 4: MEarth photometry of LHS 3844 modeled with a quasi-periodic GP. Black points show the nightly binned MEarth light curve. The solid orange curve denotes the GP predictive mean, and the shaded band indicates the 68 % credibility interval.
  • Figure 5: Detrended TESS PDCSAP light curve of LHS 3844 for Sectors 1, 27, 28, 67, and 68 (blue dots). Overplotted is the best-fit single-planet transiting model (black line). The median photometric uncertainty is illustrated in the upper left corner of each panel.
  • ...and 16 more figures