Pre-main-sequence populations and young stellar variability in the Large Magellanic Cloud

Abstract

Detailed studies of intermediate- and low-mass pre-main-sequence (PMS) stars outside the Galaxy have so far been conducted only for small targeted regions harbouring known star formation complexes. This work presents a comprehensive analysis of the PMS population in a ∼ 1.5 deg2 area located in the gas-rich Large Magellanic Cloud (LMC). The identification and classification of the populations was achieved by statistical analysis of the Ks/(Y − Ks) colour−magnitude distribution of stars using a PSF photometric source catalogue of the VISTA Survey of the Magellanic Clouds (VMC). The analysis reveals ∼ 2260 PMS candidates with ages ≲ 10 Myr and masses ≲ 4 M. The young populations exhibit non-uniform spatial distributions and appear to be hierarchically organized with large structures containing smaller and denser substructures. Regions containing only intermediate/low-mass PMS stars are more scattered and usually located in the outskirts of star forming complexes. The young populations are clustered along ridges and filaments where dust emission in the far-infrared (FIR) (70 µm – 500 µm) is bright. At 70 µm and 100 µm we report a strong dust emission increase in regions hosting young massive stars, which is less pronounced in regions populated only by less massive (≲ 4 Mʘ) PMS stars. In addition, this thesis presents the first systematic variability study of massive young stellar objects (YSOs) in the LMC. By using a χ 2 -analysis on multi-epoch observations obtained by the VMC and my open time programme, stellar variability is identified. 173 high-reliability YSO candidates are selected based on several Spitzer studies, out of which 39 displayed variability. They have been classified into eruptive, fader, dipper, short-term variable and long period variable-YSO based on the appearance of their Ks-band lightcurves. The majority of YSO variables are aperiodic; for five YSOs the lightcurves indicate a possible periodicity, identified using a Lomb-Scargle periodogram analysis. The lightcurve shapes and colour shifts can be mostly associated with unsteady accretion or (grey) extinction. Overall, the observed amplitudes are moderate with only two YSOs exhibiting a ∆Ks > 1 mag. Eruptive variables tend to have the largest amplitudes amongst all classes. Compared with similar Galactic studies the amplitudes tend to be smaller, which could be attributed to smaller extinction variations due to the larger gas-to-dust ratio of the LMC.