The DESIRED temperature–metallicity relations in star-forming regions: probing the Galactic radial and azimuthal metallicity distributions
Year: 2026
Type: article
Abstract: ABSTRACT We analyse a sample of 225 star-forming regions from the DESIRED-E project, each with simultaneous determinations of the electron temperature from ionized nitrogen and oxygen, $T_{\rm e}$([N ii]) and $T_{\rm e}$([O iii]), respectively. We derive new empirical relations connecting the gas-phase metallicity to the global electron temperature, $T_{\rm e}$(H$^+$), as determined via radio observations. We establish two calibrations: one assuming a homogeneous temperature distribution ($t^2 = 0$, the ‘direct method’), and another accounting for internal temperature fluctuations ($t^2 \,\gt\, 0$). Applying these calibrations to 460 radio observations of Galactic H ii regions spanning Galactocentric distances from ${\sim} 0.1$ to 16 kpc, we determine the radial O/H gradient in the Milky Way under both assumptions. We further compare these nebular gradients to independent metallicity estimates from young O- and B-type stars and Cepheid variables. We find that the $t^2 \,\gt\, 0$ calibration yields a gradient in excellent agreement with stellar-based determinations, whereas the $t^2 = 0$ method underestimates metallicities by up to $\sim$0.3 dex. This discrepancy cannot be reconciled by invoking oxygen depletion on to dust grains or nucleosynthetic processing via the C-N-O cycle in massive stars. We also find that one widely used relation in the literature, assuming $t^2 = 0$, produces an excessively steep gradient – likely due to the use of outdated atomic data and pre-CCD observations. Finally, we explore potential azimuthal variations in the Galactic metallicity distribution driven by the presence of the spiral arms, finding no evidence for variations larger than $\sim$0.1 dex with respect to the general radial gradient.
Authors: I Rafael Martínez-Hernández, J Eduardo Méndez-Delgado, César Esteban, Jorge García-Rojas, Leticia Carigi, Luis F Rodríguez, Luis A Zapata, F Fabián Rosales-Ortega, Maialen Orte-García, Elena Reyes-Rodríguez, Karla Z Arellano-Córdova, Kathryn Kreckel, Natascha Sattler, Christophe Morisset, Manuel Peimbert, Silvia Torres-Peimbert, Miriam Peña, Žofia Chrobáková, Eleonora Zari, David A Espinoza-Galeas
Institutions: National Autonomous University of Honduras, Universidad Nacional Autónoma de México, Universidad de La Laguna, Instituto de Astrofísica de Canarias, National Institute of Astrophysics, Optics and Electronics, Royal Observatory, University of Edinburgh, Heidelberg University, Slovak Academy of Sciences, Institute of Construction and Architecture of the Slovak Academy of Sciences, University of Florence
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $3,046
Funders: Albert Ellis Institute, Ministerio de Ciencia, Innovación y Universidades, Deutsche Forschungsgemeinschaft, Ministerio de Ciencia e Innovación, Universidad Nacional Autónoma de México, Dirección General de Asuntos del Personal Académico, Universidad Nacional Autónoma de México, European Regional Development Fund, Agencia Estatal de Investigación, H2020 European Research Council
OpenAlex ID: W7125146701