paper

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

arXiv:2603.28238

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 $R_{b} = 1.28 6_{- 0.044}^{+ 0.043} R_{\oplus}$ and an orbital period of $P_{b} = 0.46292970 9_{- 0.000000042}^{+ 0.000000044}$ d, placing it in the class of ultra-short-period (USP) planets. The host star LHS 3844 is an old ( $7.8 \pm 1.6$ Gyr), slowly rotating ( $P_{r o t} = 130. 0_{- 13.4}^{+ 16.9}$ d) M5.0 dwarf with $M_{⋆} = 0.151 \pm 0.014 M_{⊙}$ 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 $K_{b} = 6.9 5_{- 0.60}^{+ 0.55}$ m/s, implying a planetary mass of $m_{b} = 2.37 \pm 0.25 M_{\oplus}$ and a bulk density of $I ¨ _{b} = 6.1 5_{- 0.61}^{+ 0.60}$ 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 $\approx 6.88$ 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.

Accepted for publication in A&A