A Structural and Spectroscopy Study of the Clusters Surrounding the Orion Nebula: NGC 1977 and NGC 1981 Braz, P.H.F.B.1, Corradi, W.J.B.2,1, Katime Santrich, O.J.3, Santos Jr., J.F.1, Roman-Lopes, A.4, Ferreira, F.A.1, Angelo, M.S.5, Maia, F.F.S.6, Andrés Piatti7 Universidade Federal de Minas Gerais - Brazil1, Laboratório Nacional de Astrofísica - MCTI - Brazil2, Universidade Estadual de Santa Cruz - Brazil3, Universidad de La Serena - Chile4, Centro de Federal de Educação Tecnológica - MG - Brazil5, Universidade Federal do Rio de Janeiro - Brazil6, Institutuo Interdisciplinario de Ciencias Básicas - Argentina7 Introduction The study of young open clusters (OCs) provides constraints to models of star formation and evolution as well as to the properties of the Galactic disc (Cantat-Gaudin et al. 2018; Ferreira et al. 2020; Holanda et al. 2021). The surrounding OCs of the Orion Nebula (ON) stands out as an interesting science cases to investigate star and cluster formation and evolution, because: • NGC 1981 presently inhabits a relatively dust-free field, while NGC 1977, located about 20’ southward of it, remains embedded within its progenitor cloud. • A sequential star formation scenario for the OCs in the ON region was suggested by Maia et al. (2010); • A hypothesis that the feedback from the massive and earlier formed OCs played a crucial role in triggering the subsequent star formation events in the ON and its surrounding younger populations was suggested by Bouy et al. (2014) (Fig. 1). 3.5pc (30’) NGC 1981 NGC 1977 OMC-2/3 M42 NGC 1980 M43 L1641-N V380 Ori iota Ori Figure 1: Optic photography of the region sorrounding the ON, where NGC 1981, NGC 1977, NGC 1980, M42 and other nearby populations are highlighted (Alves & Bouy 2012). Goals • Determine the astrophysical parameters and evaluate the origin and connection of NGC 1977 and NGC 1981; • Deepen the understanding of the star formation history of the clusters surrounding the ON; Methodology In order to characterize NGC 1977 and NGC 1981 we have performed the following steps: Astrophysical Parameters Determination • To separate the members of a clusters from the field stars we have used the data form Gaia DR3 (Gaia Collaboration et al. 2023) together with the decontamination method developed by our group, described in detail in Angelo et al. (2019) e Ferreira et al. (2020). Structural Parameters • We have performed a radial density profiles (RDP) analysis and the King’s profile fit (King 1962) to determine the limit radii (Rlim), the core radius (Rc) and the central density (σ0). Spectroscopy Analysis • The spectroscopic data of the brightest stars of NGC 1981, collected at CASLEO - Argentina (2015) and OPD - Brazil (2022), together with the spectra and the stellar parameters from the APOGEE data (Abdurro’uf et al. 2022) were used to analyze the radial velocity (Vrad) and metallicity ([Fe/H]). Astrophysical and Structural Parameters Table 1: Astrophysical parameters of NGC 1977 and NGC 1981. NGC 1977 NGC 1981 RA (◦) 83.85 83.83 DEC (◦) -4.81 -4.35 µα (mas/yr) 1.40 1.2 µδ (mas/yr) -0.75 0.6 ϖ (mas) 2.56 2.53 Rlim (”) 1000 ± 57 1062 ± 57 E(B-V) (mag) 0.07 ± 0.04 0.03 ± 0.04 (M-m)o (pc) 417 ± 54 381 ± 50 Age (Myrs) 4 ± 1 8 ± 1 [Fe/H] (dex) 0.04 ± 0.40 0.04 ± 0.57 Vrad (km s−1) - 41.6 ± 8.3 0 500 1000 1500 2000 2500 R (arcsec) 5.0•10-5 1.0•10-4 1.5•10-4 2.0•10-4 2.5•10-4 3.0•10-4 σ (stars.arcsec-2) BIN SIZE 225" 300" 375" 450" 525" 600" NGC1977_1_5_flags Sky fit 1-σ uncert. σbg = ( 0.000065 ± 0.000010) ( " )-2 0 500 1000 1500 2000 2500 R (arcsec) 2•10-5 4•10-5 6•10-5 σ (stars.arcsec-2) BIN SIZE 225" 300" 375" 450" 525" 600" NGC1981_1_5_flags Sky fit 1-σ uncert. σbg = ( 0.000031 ± 0.000006) ( " )-2 Figure 2: RDP of NGC 1977 (top panel) and NGC 1981 (bottom panel). The bins used on the fit are shown in arcsec on the panel. The limit radii (Rlim) and its error are represented by the solid and dashed vertical lines, respectively. The RDPs and King’s profile present fluctuations in