Discovery of new dipper stars with K2 : a window into the inner disc region of T Tauri stars

In recent years, a new class of young stellar object (YSO) has been defined, referred to as dippers, where large transient drops in flux are observed. These dips are too large to be attributed to stellar variability, last from hours to days and can reduce the flux of a star by 10-50 per cent. This variability has been attributed to occultations by warps or accretion columns near the inner edge of circumstellar discs. Here, we present 95 dippers in the Upper Scorpius association and ρ Ophiuchus cloud complex found in K2 Campaign 2 data using supervised machine learning with a random forest classifier. We also present 30 YSOs that exhibit brightening events on the order of days, known as bursters. Not all dippers and bursters are known members, but all exhibit infrared excesses and are consistent with belonging to either of the two young star-forming regions. We find 21.0 ± 5.5 per cent of stars with discs are dippers for both regions combined. Our entire dipper sample consists only of late-type (KM) stars, but we show that biases limit dipper discovery for earlier spectral types. Using the dipper properties as a proxy, we find that the temperature at the inner disc edge is consistent with interferometric results for similar and earlier type stars

Kepler Bonus: Light Curves of Kepler Background Sources

NASA's \textit{Kepler} primary mission observed about 116 $deg^2$ in the sky for 3.5 consecutive years to discover Earth-like exoplanets. This mission recorded pixel cutouts, known as Target Pixel Files (TPFs), of over $200,000$ targets selected to maximize the scientific yield. The Kepler pipeline performed aperture photometry for these primary targets to create light curves. However, hundreds of thousands of background sources were recorded in the TPFs and have never been systematically analyzed. This work uses the Linearized Field Deblending (LFD) method, a Point Spread Function (PSF) photometry algorithm, to extract light curves. We use Gaia DR3 as input catalog to extract $606,900$ light curves from long-cadence TPFs. $406,548$ are new light curves of background sources, while the rest are Kepler's targets. These light curves have comparable quality as those computed by the Kepler pipeline, with CDPP values $<100$ ppm for sources $G<16$. The light curve files are available as high-level science products at MAST. Files include PSF and aperture photometry, and extraction metrics. Additionally, we improve the background and PSF modeling in the LFD method. The LFD method is implemented in the \texttt{Python} library \texttt{psfmachine}. We demonstrate the advantages of this new dataset with two examples; deblending of contaminated false positive Kepler Object of Interest identifying the origin of the transit signal; and the changes in estimated transit depth of planets using PSF photometry which improves dilution when compared to aperture photometry. This new nearly unbiased catalog enables further studies in planet search, occurrence rates, and other time-domain studies.Comment: 30 pages, 16 figures, 3 appendix section

SHEDDING NEW LIGHT ON OLD DATA: FINDING NEW RESULTS FOR EXOPLANET SCIENCE IN ARCHIVAL DATA

In Chapter 2 of this thesis I present my database of molecular absorption cross sections. These were developed using public molecular transition line-lists (from the ExoMol group). I use them to find limitations in the modelling of exoplanet atmospheres due to pressure broadening. Pressure broadening, where collisions between molecules in atmospheres cause a Lorentzian broadening of molecular transitional lines, is little understood in the field. In this chapter I consider its effects on real exoplanet atmosphere observations, both with current and future instruments. I show that pressure broadening may affect future observations of exoplanets in the JWST era. Pressure broadening primarily affects cooler, small exoplanets such as Earth analogues. In Chapter 3 I present the pipeline I have developed to reduce HST WFC3 spectra of exoplanet hosts during transits to create transmission spectra. This code corrects several instrumental systematics, from varying dark signal in the detector to subpixel shifts in the target position over time. By creating a pipeline to process all targets, regardless of observing strategy, systematics are dealt with uniformly and different planets’ spectra can be meaningfully compared. I show that the height of the water feature in 30 unique exoplanets’ transmission spectra is strongly correlated with the most simplistic absorption model. I use this to predict a list of the best future targets for observations with HST WFC3 to find water. In Chapter 4 I discuss my work with the stellar spectra from WFC3, which utilise the sub-pixel shifts in target position to oversample the spectra and increase the resolution. I have compared these exoplanet host stellar spectra with stellar models to investigate how well stellar atmosphere models describe the near IR. I find a small discrepancy in temperature when WFC3 alone is used to assess the stellar temperature, particularly with cooler stars. I attribute this firstly to an error in the WFC3 sensitivity curve and secondly to an inaccuracy in models of cool, small stars due to molecular absorption. In Chapter 5 I present my work on K2 light curve data using machine learning to find young stellar objects that display unusual, transit-like behaviour. These objects are known as dipper stars due to their distinctive occultations with depths of 10-50% in flux and very fast orbital periods of a few hours to a few days. Such large occultations are difficult to explain and are currently attributed to material at the inner edge of the protoplanetary disk. This behaviour is often variable and aperiodic, suggesting that the occulting material is changing in morphology on the time scale of a single orbit. Using python’s scikit-learn I have developed a code that utilises a Random Forest algorithm to classify stars in K2 Campaign Field 2 and distinguish these objects from other types of variables, such as eclipsing binaries and pulsating stars. This method has proved very successful and has allowed me to nearly quadruple the number of known dipper candidates in the Upper Scorpius and Rho Ophiuchus regions.STFC PhD Studentshi