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(Abridged) We characterize a series of neutral vanadium atomic absorptionlines in the 800--910nm wavelength region of high signal-to-noise,high-resolution, telluric-corrected M-dwarf spectra from the CARMENES survey.Many of these lines are prominent and exhibit a distinctive broad andflat-bottom shape, which is a result of hyperfine structure (HFS). Weinvestigate the potential and implications of these HFS split lines forabundance analysis of cool stars. With standard spectral synthesis routines, asprovided by the spectroscopy software iSpec and the latest atomic data(including HFS) available from the VALD3 database, we modeled these strikingline profiles. We used them to measure V abundances of cool dwarfs. Wedetermined V abundances for 135 early M dwarfs (M0.0V to M3.5V) in the CARMENESguaranteed time observations sample. They exhibit a [V/Fe]-[Fe/H] trendconsistent with that derived from nearby FG dwarfs. The tight ($\pm$ 0.1 dex)correlation between [V/H] and [Fe/H] suggests the potential application of V asan alternative metallicity indicator in M dwarfs. We also show hints thatneglecting to model HFS could partially explain the temperature correlation inV abundance measurements observed in previous studies of samples involvingdwarf stars with $T_{\rm eff} \lesssim 5300$K. Our work suggests that HFS canimpact certain absorption lines in cool photospheres more severely than inSun-like ones. Therefore, we advocate that HFS should be carefully treated inabundance studies in stars cooler than $\sim 5000$K. On the other hand, strongHFS split lines in high-resolution spectra present an opportunity for precisionchemical analyses of large samples of cool stars. The V-to-Fe trends exhibitedby the local M dwarfs continue to challenge theoretical models of V productionin the Galaxy.

The CARMENES search for exoplanets around M dwarfs: Not-so-fine hyperfine-split vanadium lines in cool star spectra