Variability of young stellar objects in the Orion nebula

¹ Astronomisches Institut, Ruhr–Universität Bochum, Universitätsstraße 150, 44801 Bochum, Germany
² Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, China
³ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 106, Ensenada 22860, BC, Mexico

^★ Corresponding author; sdg.noroozi@gmail.com

Received: 14 May 2024
Accepted: 24 September 2024

Abstract

Context. The Orion Nebula is one of the best-studied star-forming regions in our Galaxy and provides an ideal laboratory to study the early stages of star formation. Understanding the variability of young stellar objects (YSOs) can provide valuable insights into the underlying physical processes that occur during this crucial phase of stellar evolution. In this context, we have conducted a photometric survey with high temporal resolution of a six-square-degree region centered on NGC 1980. We used two filters (R and I) between 2012 and 2015 with the 15 cm twin telescope RoBoTT at the Universitätssternwarte Bochum in Chile.

Aims. Our goal is to identify and characterize the variable objects in this sample using a unique observational setup that allows us to obtain high-quality light curves with time sampling on the order of minutes. This is the first time such an intensive ground-based photometric campaign has been conducted in the Orion Nebula.

Methods. After careful quality control, more than 13 000 light curves were produced in either R or I; the total number of matched light curves is 11 889. The corresponding stars were subsequently matched with the 2MASS point source catalog, resulting in 8197 stars with I < 16.5 mag and AAA quality flags. We employed three methods – amplitude, standard deviation, and the Stetson J index – to identify variable objects in our sample, successfully classifying 561 stars as variables (irregular, periodic, and quasi-periodic). These stars serve as the foundation for further analysis. We confirmed 456 of these as known pre-main-sequence stars through cross-referencing with existing YSO catalogs, supported by spectroscopic and photometric data. The youth of the remaining stars was established based on their optical and infrared color signatures.

Results. The period, P, could be determined for 147 sources and ranges from 0.2 days to several weeks. The frequency of very short periods (<1 day) is two to four times higher than reported in previous studies. A significant population (10%) comes primarily from weak-line T Tauri stars, which are characterized by rotation periods of less than 1 day. These results emphasize the advantages of observations with an exceptional time sampling strategy that allows the identification of variability on scales from hours to years.