Ultraviolet Variable Sources in the Kepler Field
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Ultraviolet Variable Sources in the Kepler Field by Nestor´ Daniel Olmedo Aguilar Thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE IN ASTROPHYSICS at the Instituto Nacional de Astrof´ısica, Optica´ y Electronica´ February 2017 Tonantzintla, Puebla Under the supervision of: Ph.D. Miguel Chavez´ INAOE Ph.D. Emanuele Bertone INAOE c INAOE 2017 The author hereby grants to INAOE permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part. Abstract This work presents the UV flux catalog Multi-visit GALEX CAUSE Kepler (MGCK), created from the observations conducted over a period of about 45 days by the Galaxy Evolution Explorer (GALEX) space telescope within the Complete All-Sky Ultravio- let Survey Extension (CAUSE) in August-September 2012. The construction of the catalog was made with the Source Extraction software (SExtractor) in a dual mode us- ing GALEX CAUSE Kepler (GCK) coadded catalog as input to perform the sources detections and their photometric measurements in each tile for each visit. MGCK contains the light curves of 660,490 point sources in the near ultraviolet (NUV) de- tected in the 104 square degrees field observed by the Kepler space telescope. MGCK has 475,164 sources in common with the Kepler Input Catalog (KIC). Approximately 31,000 sources from MGCK present significant variability. The MGCK catalog should enable the UV variability study in the Kepler field of astronomical sources like variable stars, eclipsing stars, UV flares detection, and extragalactic objects, if detected, like AGNs and Quasars, as well the complementary characterization of the Visible variabil- ity. Acknowledgments I express my gratitude to: My thesis directors, Ph.D. Miguel Chavez´ and Ph.D. Emanuele Bertone, for their guidance, teachings, and patience. M.Sc. Manuel Olmedo, fellow group colleague who guided me through the creation of the MGCK catalog. M.Sc. Ricardo Lopez,´ fellow group colleague who guided me through the spectroscopic data reduction process. To my thesis reviewers, for reading it and make useful suggestions. CONACyT, for the financial support it granted me to study my Masters degree. And thus, the opportunity to do science! CONCyTEP, for the scholarship granted in the last months of this thesis. To my mother, father, and brothers, Luis and Manuel, for the encourage- ment they gave me every day. To Alejandra Jurado, Xiomara Garc´ıa and my friends, for all the support. To my generation colleagues, with whom I shared classes and many study hours. v Dedico esta tesis a mi madre, a mi padre, y a mis dos hermanos. Contents Abstract iii 1 The Kepler field in the UV 1 1.1 UV phenomena . .2 1.1.1 Thermal emission in the UV . .2 1.1.2 Stellar activity . .2 1.1.3 Non-thermal processes . .2 1.2 Variable sources in the UV . .2 1.2.1 Cepheids . .3 1.2.2 β Cephei .............................4 1.2.3 Flare Stars . .4 1.2.4 Cataclysmic Variables . .5 1.2.5 Eclipsing binaries and rotational variables . .5 1.3 Contents . .6 2 The GALEX and Kepler missions 7 2.1 Kepler Mission . .7 2.1.1 Kepler telescope . .8 2.1.2 Kepler Field of View . 10 2.1.3 Kepler Input Catalog, Kepler Objects of Interest and other cat- alogs . 10 2.1.4 Kepler ligthcurves and discoveries so far . 11 2.1.5 Kepler K2............................. 12 2.2 Galaxy Evolution Explorer ........................ 14 2.2.1 GALEX telescope . 15 2.2.2 GALEX Complete All-Sky UV Survey Extension (CAUSE) . 15 2.3 The GCK Catalog . 16 2.3.1 GALEX CAUSE proposal on the Kepler Field . 16 2.3.2 Observations . 17 2.3.3 Assembly of a catalog of NUV sources . 18 Image Co-adding . 18 Background and threshold estimations . 19 Source extraction and photometry . 19 ix Artifact Identification . 19 2.3.4 The GCK UV source catalog . 20 3 Multi-visit GALEX CAUSE Kepler Catalog (MGCK) 23 3.1 Detection and photometry . 23 3.1.1 Weighting . 26 3.1.2 Artifact flags . 26 3.1.3 Cross-identification: Association . 27 3.1.4 Photometry . 27 3.1.5 Input files . 28 3.1.6 Extra processing . 28 3.2 Catalog description . 29 3.2.1 Output files . 30 3.3 Catalog quality . 32 4 Sources with Significant Variability 37 4.1 Variability detection algorithm . 37 4.1.1 Discrimination of artifacts and systematic errors . 38 4.1.2 False positives . 40 4.2 Examples of UV variable sources . 44 4.2.1 Eclipsing binary system . 44 4.2.2 Cepheid stars . 44 4.2.3 Cataclysmic variable . 45 4.2.4 Flare events . 46 4.3 Examples of UV variable sources of unidentified nature . 52 4.3.1 One visit event source . 52 4.3.2 Outburst variable . 53 4.3.3 Sources showing flare-like events . 54 4.3.4 Sources showing transit-like events . 55 4.3.5 UV variable - Visible stable source . 57 5 Optical spectroscopic follow-up of MGCK sources of unknown nature 59 5.1 Spectroscopic data reduction . 59 5.2 Spectrum and object’s nature inference . 60 6 Conclusions 63 List of Figures 65 List of Tables 67 A MGCK’s DataFrame 69 Bibliography 75 x Chapter 1 The Kepler field in the UV To study the nature and evolution of astrophysical objects and phenomena, the multi- wavelength observations are a fundamental tool. In the particular case for stars, the observations in the space Ultraviolet (UV) range are an important complement to the optical data, infrared, X-ray, etc. The UV is particularly useful to the study of the stellar emission variability because it is more sensible to the external layers of stellar atmospheres in the case of solar-type stars. This variability is studied through light curves, this is the brightness fluctuations as a function of time. The data obtained by Kepler has revolutionized the study of exoplanets and astro- physics by providing high-precision, high-cadence, continuous lightcurves of tens of thousands of stars (Howell et al. 2014). The Kepler space telescope observed approximately 150,000 stars in the visible range between May 2009 and June 2013 searching for planets through planetary transits (see section 2.1). GALEX space telescope observed the same field in the UV range between August and September 2012, as part of the private fund Complete All-Sky Ultraviolet Survey Extension (CAUSE), in this case by the Cornell University (see subsection 2.3.1). The data were used to create the GALEX CAUSE Kepler (GCK) catalog of Ultraviolet point sources in the Kepler field. The catalog is published in Olmedo et al. (2015). The goal of this work is to create a NUV light curve catalog (multi-visit catalog) of the sources observed by GALEX in the Kepler Field, and to produce a sample of vari- able objects. This catalog will allow the UV study of variable phenomena, described below; in the case of stars and sources also observed by Kepler, a comparison between the visible and the UV ligth curves can be made. Having simultaneous observations offers an exceptional opportunity to study UV-Visible variability and study different phenomena. GALEX observations have approximately 17 visits throughout its month of observations and we expect to find a correlation between the two wavelenght ranges. To illustrate the richness of the information that this catalog can contribute, we show some examples of sources with significant variability. 1 1.1 UV phenomena Before we analyze the GALEX data it is worth to recall some of the potential sources of UV radiation and its variability. In this subsection, we briefly describe some of the physical phenomena that produce UV light in stars and galaxies. They are summarized below: 1.1.1 Thermal emission in the UV Stars radiate energy at all electromagnetic wavelengths with a flux peak defined by its temperature according to the Planck function through the Wien law. Thus, all stars radiate in the UV, but especially the spectral types O and B whose peak is in those wave- lengths. This radiation is affected when stars change their Teff , either in short periods as in the case of pulsating stars or in long timescales as a result of stellar evolution. Thermal Bremsstrahlung emission is generated by the deceleration of an electron after it passes nearly a nucleus or ion field without being captured. The interaction can change the kinetic energy of the electron producing free-free radiation. This also applies for very hot gases with T > 106 K, where hydrogen is fully ionized. 1.1.2 Stellar activity The presence of photospheric features such as spots, faculae and flares (Hall 2008) are associated with chromosphere activity that may produce variations in the UV flux. These phenomena are associated with magnetic fields and are closely related to stellar rotation. 1.1.3 Non-thermal processes This is electromagnetic radiation produced by particles due to physical processes other than their thermal energy. Synchrotron emission occurs when a magnetic field affects a free charge forcing it to accelerate in a spiral orbit. The electron is thus constantly accelerated and will emit electromagnetic radiation in the direction of its velocity vector. Depending on the energy of the electron and the strength of the magnetic field, synchrotron emission can also occur at visible, ultraviolet and X-ray wavelengths1. 1.2 Variable sources in the UV The UV variable sources we expect to find in the GALEX Kepler survey are mostly stellar. Extragalactic transients sources may be found if they undergo significant en- 1http://astronomy:swin:edu:au/cosmos/S/Synchrotron+Emission 2 hancement of UV emission.