# Atmospheric parameters and chemical abundances of young stars with APOGEE. I. Orion star-forming region

## TL;DR

This study analyzes atmospheric parameters and chemical abundances in young stars within the Orion star-forming region using infrared APOGEE-2 spectra, revealing chemical homogeneity and insights into Galactic chemical evolution.

## Contribution

First detailed chemical abundance analysis of young Orion stars using infrared spectra, accounting for circumstellar material and rotation effects.

## Key findings

- Orion stars show sub-solar [X/H] ratios, indicating chemical homogeneity.
- Median [$oldsymbol{ ext{alpha}}$/Fe] is lower than nearby main-sequence stars, aligning with Galactic evolution models.
- C abundance matches previous ionized gas measurements in Orion nebula.

## Abstract

We derive atmospheric parameters and chemical abundances in young G-, K-, and M-type stars (temperatures between 6500 and 3100 K) using infrared APOGEE-2 spectra. Atmospheric parameters were determined for 548 young stars in the Orion complex (Orion A, B, OB1, and $\lambda$ Ori) using the TONALLI code. For 340 slow rotators v sini $\leq$ 30 km s$^{-1}$), we derived C, Mg, Si, K, Ti, and Fe abundances using 19 atomic lines, MARCS model atmospheres, and BACCHUS. To mitigate the impact of circumstellar material, we excluded stars with infrared excess identified via 2MASS and WISE photometry. We find sub-solar [X/H] abundance ratios, consistent across elements and among all four groups, suggesting a chemically homogeneous Orion complex. We computed [$\alpha$/Fe] from [Mg/Fe], [Si/Fe], and [Ti/Fe], obtaining a median of $-0.14 \pm 0.04$, about 0.10 dex lower than the value for nearby main-sequence stars ($-0.04 \pm 0.04$) at similar [Fe/H]. This result aligns with predictions from Galactic chemical evolution models. Furthermore, the median [C/H] abundance we derived for Orion agrees with previous estimations based on the analysis of the ionized gas of the Orion nebula. This work sets the stage for extending the analysis to stars with circumstellar material and higher rotational velocities, which will not only improve our understanding of Orion, but also provide critical insight into the formation and evolution of young stars, as well as the chemical evolution of the Milky Way.