Characterizing the i-band variability of YSOs over six orders of magnitude in time-scale

Abstract

We present an i-band photometric study of over 800 young stellar objects in the OB association Cep OB3b, which samples timescales from 1 minute to ten years. Using structure functions we show that on all timescales (τ ) there is a monotonic decrease in variability from Class I to Class II through the transition disc (TD) systems to Class III, i.e. the more evolved systems are less variable. The Class Is show an approximately power-law increase (τ 0.8 ) in variability from timescales of a few minutes to ten years. The Class II, TDs and Class III systems show a qualitatively different behaviour with most showing a power-law increase in variability up to a timescale corresponding to the rotational period of the star, with little additional variability beyond that timescale. However, about a third of the Class IIs show lower overall variability, but their variability is still increasing at 10 years. This behaviour can be explained if all Class IIs have two primary components to their variability. The first is an underlying roughly power-law variability spectrum, which evidence from the infrared suggests is driven by accretion rate changes. The second component is approximately sinusoidal and results from the rotation of the star. We suggest that the systems with dominant longer-timescale variability have a smaller rotational modulation either because they are seen at low inclinations or have more complex magnetic field geometries.We derive a new way of calculating structure functions for large simulated datasets (the "fast structure function"), based on fast Fourier transforms.

Multicolor Variability of Young Stars in the Lagoon Nebula: Driving Causes and Intrinsic Timescales

“…15 (lower panels). The typical P rot -to-τ knee ratio is 4.5 ± 1.6, consistent with what observed by Sergison et al (2020) for their sample in the Cep OB3b association.…”

“…The index β of the best power-law fit to the SF region dominated by intrinsic variability can vary broadly from case to case: measured values for our K2 sample of Lagoon Nebula members span the entire range from <0.1 to 0.9. As discussed in Sergison et al (2020) 2 , a β ∼ 0 value is expected in case of a light curve dominated by uncorrelated (white) noise, such as those of stars labeled as 'N' in the K2 sample (Sect. 4.1).…”

“…In order to probe the characteristic timescales of variability of Lagoon Nebula members, irrespective of the periodic or aperiodic nature of their variation patterns, we adopted the method of structure functions (Simonetti et al 1985;Hughes et al 1992;de Vries et al 2003), as recently implemented to study YSO variability by Sergison et al (2020). The method consists of extracting every timescale of variability τ encompassed by the time series, and, for each τ , computing the average amplitude of normalized flux variability among all pairs of points in the light curve that are spaced by that time interval τ .…”

“…From a theoretical standpoint, the behavior of SF(τ ) is expected to consist of three main separate regimes (see Fig. 7 of Sergison et al 2020). Initially, the SF is expected to be relatively flat or slowly-increasing with τ , corresponding to the short-τ regime where the observed flux variations are dominated by photometric uncertainties rather than intrinsic variability.…”

“…Venuti et al (2014) examined the comparative extent of variations in accretion luminosity exhibited by a representative sample of young stars in the NGC 2264 cluster on hour-to-weeks timescales. They concluded that such variations are dominated by accretion shock modulation along the stellar rotation period, although changes in the accretion rate throughout the disk and close to the star (e.g., Sergison et al 2020), as well as dust clumps and density perturbations within the disk (e.g., Bouvier et al 2013), may induce additional variability beyond the rotational timescales for at least a fraction of Class II YSOs.…”