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

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_ ̊m b =1.286^ +0.043 _ -0.044 ,R_⊕ and an orbital period of ̊m 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 = 130.0^ slowly rotating (P_ ̊m rot +16.9 _ -13.4 ,d) M5.0 dwarf with M_⋆ = 0.151 =6.95^ at a distance of 15,pc (V=15.2,mag, K=9.2,mag). 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_ ̊m b +0.55 _ -0.60 ,m,s^-1, implying a planetary mass of m_ ̊m b = 2.37 = 6.15^ and a bulk density of ρ_ ̊m b +0.60 _ -0.61 ,g,cm^-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 $ 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 (ESM) 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.