A survey for variable young stars with small telescopes: V - Analysis of TXOri, V505Ori, and V510Ori, the HST ULLYSES targets in the $\sigma$Ori cluster

TL;DR

This paper analyzes optical photometry of young stellar objects in the $\sigma$Ori cluster, providing a reference for UV spectra and revealing variability, accretion rates, and the effects of scattered light, with implications for understanding star formation.

Contribution

It offers a detailed optical photometric analysis of ULLYSES targets, identifying YSO populations and variability characteristics, aiding interpretation of UV spectroscopic data.

Findings

YSOs show significant variability and high accretion rates.

Optical colors indicate non-standard reddening and scattered light influence.

V505 Ori exhibits a seven-day variability period suggesting an inner disk warp.

Abstract

Investigations of the formation of young stellar objects (YSOs) and planets require the detailed analysis of individual sources as well as statistical analysis of a larger number of objects. The Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) project provides such a unique opportunity by establishing a UV spectroscopic library of young high- and low-mass stars in the local universe. Here we analyse optical photometry of the three ULLYSES targets (TXOri, V505Ori, V510Ori) and other YSOs in the $\sigma$Ori cluster taken at the time of the HST observations to provide a reference for those spectra. We identify three populations of YSOs along the line of sight to $\sigma$Ori, separated in parallax and proper motion space. The ULLYSES targets show typical YSO behaviour with pronounced variability and mass accretion rates of the order of 10$^{-8}$M$_\odot$/yr. Optical colours do not agree with standard interstellar reddening and suggest a significant contribution of scattered light. They are also amongst the most variable and strongest accretors in the cluster. V505\,Ori shows variability with a seven day period, indicating an inner disk warp at the co-rotation radius. Uncovering the exact nature of the ULLYSES targets will require improved detailed modelling of the HST spectra in the context of the available photometry, including scattered light contributions as well as non-standard reddening.