The Search for Dipper Stars in the Orion a Molecular Cloud Complex: Analysis of Variable YSOs From NGTS Photometric Time-Series Data
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CitationMoulton, Tyler. 2020. The Search for Dipper Stars in the Orion a Molecular Cloud Complex: Analysis of Variable YSOs From NGTS Photometric Time-Series Data. Bachelor's thesis, Harvard College.
AbstractContext: Dipper stars are young, low-mass stellar variables which exhibit drops in brightness by as much as 10 to 50 percent for periods as long as hours to days. The dipping phenomenon is currently attributed to accretion occultation by circumstellar disks. Having only been discovered in the past decade, much work remains to catalogue and analyse dipper stars.
Aims: I seek to identify dippers and other variable young stellar objects (YSOs) of interest in the Orion A molecular cloud complex. Using this I seek to characterize dipper population statistics in young clusters and investigate the relationship between circumstellar discs and dipper phenomenon.
Methods: I prepare light curves from Next Generation Transit Survey (NGTS) data for the Orion field observed August 2017 to March 2018, and identify 2052 sources with membership analysis confirmed by Kounkel et al 2018. After building a set of quantitative features and classifying the variable type of a subset of these stars by eye, I implement a random forest (RF) classifier towards identifying the morphology types of the remainder of my population. Separate from this identification process, I obtain information on disk-bearing status for 700 of my stars from Yao et al 2018 and, using Gaia G mag and relevant colour excesses as a proxy, run an additional RF classifier to determine the disk-bearing status of all my stars. Results: I find 128 dippers out of the 1268 stars flagged as members by Kounkel, or 11.9 per cent of the cluster. I find a disc fraction of 27.2 ± 1.5 per cent for Orion A, and confirm an inverse relationship between age and disc fraction in the Orion groups represented in my data. Combining my variable classification and disc identification results, I find that 85 per cent of my dippers bear discs and that of the disc-bearing stars in Orion, 37.3 ± 3.3 per cent. This dipper fraction is twice that found in all previously dipper studies in clusters. I point to possible issues with my disc fraction causing this metric to be inflated, and otherwise conclude that this result points out shortcomings in current understandings of disc characteristics, stellar evolution, and dipper phenomena.
Citable link to this pagehttps://nrs.harvard.edu/URN-3:HUL.INSTREPOS:37364679
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