SN 2022ngb: A faint, slowly evolving Type IIb supernova with a low-mass envelope

Context. Type IIb supernovae (SNe IIb) are stellar explosions whose spectra reveal transitional features between hydrogen-rich (Type II) and helium-rich (Type Ib) SNe. Their progenitors are massive stars that were mostly stripped of their hydrogen envelope, likely through binary interaction and/or strong stellar winds. This makes such stars key tools in studies of the late stages of the evolution of massive stars. Aims. We present an extensive photometric and spectroscopic follow-up campaign of the Type IIb SN 2022ngb. Through the detailed modeling of this dataset, we aim to constrain the key physical parameters of the explosion, infer the nature of the progenitor star and its environment, and probe the dynamical properties of the ejecta. Methods. We analyzed photometric and spectroscopic data of SN 2022ngb. By constructing and modeling the bolometric light curve with semi-analytic models, we were able to estimate the primary explosion parameters. The spectroscopic data were compared with those of well-studied SNe IIb and NLTE models to constrain the properties of the progenitor and the structure of the resulting ejecta. Results. SN 2022ngb is a low-luminosity SN IIb with a peak bolometric luminosity of $ L_{\mathrm{Bol}} = 7.76^{+1.15}_{-1.00} \times 10^{41} \, \mathrm{erg \, s^{-1}} $ and a V-band rising time of 24.32 ± 0.50 days. The light curve modeling indicates an ejecta mass of ∼2.9 − 3.2 M⊙, an explosion energy of ∼1.4 × 1051 erg, and a low synthesized 56Ni mass of ∼0.045 M⊙. The nebular phase spectra exhibit asymmetric line profiles, pointing to a nonspherical explosion and an anisotropic distribution of radioactive material. Our analysis reveals a relatively compact stripped-envelope progenitor with a pre-SN mass of approximately 4.7 M⊙ (corresponding to a 15–16 M⊙ ZAMS star). Conclusions. Our analysis suggests that SN 2022ngb originated from the explosion of a moderate-mass relatively compact, stripped-envelope star in a binary system. The asymmetries inferred from the nebular phase spectral line features indicate the occurrence of a nonspherical explosion.