Abstract
One of the main features of interest in analysing the light curves of stars is the underlying periodic behaviour. The corresponding observations are a complex type of time series with ...unequally spaced time points. The main tools for analysing these type of data rely on the periodogram-like functions constructed with a desired feature so that the peaks indicate the presence of a potential period. We explore a particular periodogram for the irregularly observed time series. We identify the potential periods by implementing the saddlepoint approximation, as a faster and more accurate alternative to the simulation based methods that are currently used. The power analysis of the testing methodology is reported together with applications using light curves from the Hunting Outbursting Young Stars citizen science project.
This thesis deals with the problem of period estimation of irregularly sampled time series, and specifically light curves of variable young stars. Knowing the period of these objects can provide ...important information about the stars' formation and other characteristics. The light curves are measurements of brightness over time conducted in multiple astronomical filters. We examine this problem from three different points of view. First, we need to obtain an accurate period estimate. For that purpose we introduce a weighted t-process regression model for period estimation as a flexible alternative to Gaussian process regression, since it is common for such data to exhibit a fat-tail behaviour, and we extend these models in order to include measurements from multiple astronomical filters. Secondly, we need to accompany our estimates with some credibility as to whether they represent a real periodic signal. This is usually addressed through hypothesis testing. To that end, we introduce a flexible testing scheme using saddlepoint approximation, that can be applied on a range of periodic models including Gaussian process regression. These tests are also extended for data contaminated with red noise, a type of correlated noise that usually appears in such data. Finally, we explore the asymptotic properties of simple harmonic models with additional red noise and show that this estimates are consistent and asymptotically normal. We test our results through extensive simulation studies which are reported along with an application on some real light curves from the Hunting Outbursting Young Stars citizen science project.
ABSTRACT
Studying rotational variability of young stars is enabling us to investigate a multitude of properties of young star-disc systems. We utilize high cadence, multiwavelength optical time ...series data from the Hunting Outbursting Young Stars citizen science project to identify periodic variables in the Pelican Nebula (IC 5070). A double blind study using nine different period-finding algorithms was conducted and a sample of 59 periodic variables was identified. We find that a combination of four period finding algorithms can achieve a completeness of 85 per cent and a contamination of 30 per cent in identifying periods in inhomogeneous data sets. The best performing methods are periodograms that rely on fitting a sine curve. Utilizing Gaia EDR3 data, we have identified an unbiased sample of 40 periodic young stellar objects (YSOs), without using any colour or magnitude selections. With a 98.9 per cent probability, we can exclude a homogeneous YSO period distribution. Instead, we find a bi-modal distribution with peaks at 3 and 8 d. The sample has a disc fraction of 50 per cent, and its statistical properties are in agreement with other similarly aged YSOs populations. In particular, we confirm that the presence of the disc is linked to predominantly slow rotation and find a probability of 4.8 × 10−3 that the observed relation between period and presence of a disc has occurred by chance. In our sample of periodic variables, we also find pulsating giants, an eclipsing binary, and potential YSOs in the foreground of IC 5070.
One of the main features of interest in analysing the light curves of stars is the underlying periodic behaviour. The corresponding observations are a complex type of time series with unequally ...spaced time points and are sometimes accompanied by varying measures of accuracy. The main tools for analysing these type of data rely on the periodogram-like functions, constructed with a desired feature so that the peaks indicate the presence of a potential period. In this paper, we explore a particular periodogram for the irregularly observed time series data, similar to Thieler et. al. (2013). We identify the potential periods at the appropriate peaks and more importantly with a quantifiable uncertainty. Our approach is shown to easily generalise to non-parametric methods including a weighted Gaussian process regression periodogram. We also extend this approach to correlated background noise. The proposed method for period detection relies on a test based on quadratic forms with normally distributed components. We implement the saddlepoint approximation, as a faster and more accurate alternative to the simulation-based methods that are currently used. The power analysis of the testing methodology is reported together with applications using light curves from the Hunting Outbursting Young Stars citizen science project.
Studying rotational variability of young stars is enabling us to investigate a multitude of properties of young star-disk systems. We utilise high cadence, multi-wavelength optical time series data ...from the Hunting Outbursting Young Stars citizen science project to identify periodic variables in the Pelican Nebula (IC5070). A double blind study using nine different period-finding algorithms was conducted and a sample of 59 periodic variables was identified. We find that a combination of four period finding algorithms can achieve a completeness of 85% and a contamination of 30% in identifying periods in inhomogeneous data sets. The best performing methods are periodograms that rely on fitting a sine curve. Utilising GaiaEDR3 data, we have identified an unbiased sample of 40 periodic YSOs, without using any colour or magnitude selections. With a 98.9% probability we can exclude a homogeneous YSO period distribution. Instead we find a bi-modal distribution with peaks at three and eight days. The sample has a disk fraction of 50%, and its statistical properties are in agreement with other similarly aged YSOs populations. In particular, we confirm that the presence of the disk is linked to predominantly slow rotation and find a probability of 4.8\(\times\)10\(^{-3}\) that the observed relation between period and presence of a disk has occurred by chance. In our sample of periodic variables, we also find pulsating giants, an eclipsing binary, and potential YSOs in the foreground of IC5070.
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