Resumo:
M dwarf stars constitute the most numerous stellar class in the Milky Way. However, due to the complexity of their spectra,
which are rich in molecular lines, they remain the least studied spectral type in the literature. In this work, we
perform a detailed spectroscopic analysis of 19 M dwarf stars belonging to the Praesepe open cluster, using spectra
from the Galactic survey SDSS/APOGEE. The spectral modeling was conducted in local thermodynamic equilibrium, focusing
on molecular lines of OH and water, through a previously validated methodology that derives atmospheric
parameters and metallicities with high precision. We obtained a mean metallicity for Praesepe of
<[M/H]> = 0.14 ± 0.07 dex, in agreement with high-resolution optical studies of G and K dwarf stars,
as well as red giants, and consistent with the premise of chemical homogeneity in open clusters. We determined that
the Praesepe open cluster has <[O/M]> = -0.02 ± 0.03 dex, a result consistent with expectations
for an open cluster belonging to the Galactic thin disk. When comparing our results with the parameters derived by the
APOGEE ASPCAP pipeline, we find that the uncalibrated values of log g are physically implausible, and that ASPCAP
systematically underestimates [M/H] relative to our results. Furthermore, when comparing the stellar parameters obtained
in this work with those derived by the SP and RVS pipelines from the DESI Milky Way Survey, we note that, especially for SP,
the estimates deviate significantly from values predicted by theoretical models. We identify systematic differences
between the RVS results and ours for all analyzed parameters. The SP pipeline derives systematically higher values of
[M/H] and log g compared to our determinations, however, its Teff values are consistent with those we determined.
We also find that the RVS pipeline retrieves [M/H] values that are independent of the signal-to-noise ratio (SNR) of the
DESI spectra in our sample, whereas the SP pipeline shows a trend with SNR, yielding more consistent [M/H] values and
lower dispersions for spectra with higher SNR. This work not only provides a precise reference for M dwarfs, but also
demonstrates that there are significant issues in the stellar parameter results for M dwarfs in the publicly available
APOGEE and DESI data releases. These findings highlight important limitations in current parameter derivation pipelines
for this stellar class. Our results may serve as a benchmark for future improvements in the DESI and APOGEE pipelines.
