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THE UBC-OAN PHOTOMETRIC SURVEY by FRANCOIS

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THE UBC-OAN PHOTOMETRIC SURVEY by FRANCOIS SEARCHING FOR NORTHERN ROAP STARS: THE UBC-OAN PHOTOMETRIC SURVEY By FRANCOIS CHAGNON B. Sc. (Specialise en Physique) Universite de Montreal (1996) A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF PHYSICS & ASTRONOMY We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September 1998 © FRANCOIS CHAGNON, 1998 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Physics & Astronomy The University of British Columbia 129-2219 Main Mall Vancouver, Canada V6T 1Z4 Date: Abstract Because they pulsate in multiple high-overtone p-modes, rapidly oscillating Ap stars (roAp) represent a very powerful tool to apply the techniques of asteroseismology, which can lead to the global properties and internal structure of stars. The majority of roAp stars are in the Southern Hemisphere, beyond the reach of northern observatories like CFHT and DAO, which have superb coude spectrographs that can help reveal additional modes and clues to the pulsation dynamics. To try to correct this imbalance, we began a systematic search for roAp stars in the Northern Hemisphere. The UBC-OAN roAp star Survey was conducted with the 84-cm and 1.5-m tele• scopes at the Observatorio Astronomico Nacional (OAN) in Mexico, equipped with a single-channel photoelectric photometer and a Johnson B filter. During 28 nights in July/August 1997 and 16 nights in May/June 1998, we monitored about 50 cool A-Fp stars with time resolution of less than a minute. The reduced data were Fourier analysed to search for periodic signals and we were generally sensitive to amplitudes as small as 0.3 millimagnitude in the period range of 4-15 min. We have made one detection, HD 10088, which shows two periods near 9.3 and 10.6 min, and amplitudes of about 1.6 and 1.3 mmag respectively, at the 99% confidence level. A tentative candidate, HD 3883, with period near 9.2 min and amplitude around 1.35 mmag is highlighted for further study. We also observed the known roAp star 10 Aql (HD 176232) and detected its main pulsation at a period near 11.5 min, the largest amplitude yet observed for this star. Finally, we independently confirm the newly discovered roAp star HD 122970 (Handler & Paunzen 1998) and combine our data with those of a global campaign (Handler et al. 1998) to infer some of the characteristics of this star. u Table of Contents Abstract ii List of Tables v List of Figures vi Acknowledgements viii 1 INTRODUCTION 1 1.1 The Ap stars 1 1.2 The roAp stars 3 1.2.1 Driving Mechanism 7 1.2.2 Oblique Pulsator Model 8 1.3 Asteroseismology of roAp stars . 12 1.4 Searching for roAp stars 16 1.4.1 The Cape rapidly oscillating Ap star Survey 17 1.4.2 The Lowell-Wisconsin Northern Survey for rapid variability. ... 17 1.4.3 Why look for the northern roAp stars? 18 2 THE UBC-OAN roAp star SURVEY 20 2.1 Instrumentation and observing technique 21 2.2 The Survey sample 23 2.3 Observing runs 25 m 3 REDUCTION AND ANALYSIS 34 3.1 Photometric Reduction 34 3.2 Flexure Problems 36 3.3 Frequency Analysis 40 4 THE UBC-OAN SURVEY RESULTS 45 4.1 HD 10088 45 4.2 HD 3883 (HR 178): roAp or not roAp, that is the question 51 4.3 HD 176232 (10 Aql) 55 4.4 HD 122970 57 5 DISCUSSION 62 5.1 The Discoveries 62 5.2 HD 122970: a demonstration of asteroseismology 63 5.3 The effectiveness of the UBC-OAN Survey 79 References 83 Appendices 93 A Reduction procedure 93 A.l Dead-Time Correction 93 A. 1.1 A value for the dead-time constant 94 A.1.2 How good is the approximation and is it important? 95 A.2 Dark and Sky Subtraction 96 A.3 Airmass extinction 96 A.4 Heliocentric Correction 98 iv List of Tables 1.1 The Chemically Peculiar stars 3 2.1 Diaphragm Size 23 2.2 UBC-OAN Survey Sample 28 2.2 UBC-OAN Survey Sample (continued) 29 2.2 UBC-OAN Survey Sample (continued) 30 2.3 Journal of Observations 31 2.3 Journal of Observations (continued) 32 2.3 Journal of Observations (continued) 33 4.1 Characteristics of the main peak in the Fourier spectrum of HD 122970 on each of the three nights of observations 59 5.1 The roAp stars and candidates detected in the UBC-OAN Survey .... 63 5.2 Frequency Identification of HD 122970 71 5.3 Frequency spacings in the Fourier spectrum of HD 122970 71 A.l Comparison of 2 dead-time constant values 95 A.2 Validity of the approximation (ip = 100 ns) 95 A.3 Importance of the dead-time correction (tp = 100 ns) 96 v List of Figures 1.1 Light curve of roAp star HR 1217 4 1.2 Amplitude spectrum of HR 1217 5 1.3 Phase and amplitude of pulsation in HR 3831 vs. the rotation phase. 9 1.4 The sound speed squared as a function of the radius in the Sun 15 2.1 Colour-colour diagram of the. sample and survey stars 27 3.1 Flexure problems: The relative magnitude Am vs. the airmass X 38 3.2 Bouguer's plot for a linear variation with the horizontal distance of the extinction coefficient. 41 3.3 Bouguer's plot for a linear variation with time of the extinction coefficient. 41 3.4 Bouguer's plot for an instrumental drift linear with time 42 4.1 A null detection for the star HD 204411 46 4.2 Light curve of HD 10088 on 1997 August 1 48 4.3 Fourier spectra (amplitude and power) of HD 10088 on 1997 August 1. 49 4.4 Successive filtering of the two periods detected in HD 10088 50 4:5 Power spectra of HD 3883 on two different nights 53 4.6 Fourier amplitude spectra for the star HD 3883 on 1997 August 5. ... 54 4.7 Detection of roAp-type pulsation in the known roAp star 10 Aql 56 4.8 Light curve of HD 122970 on 1998 June 12 58 4.9 Fourier spectrum of the combined three nights of data for HD 122970. 60 4.10 Successive filtering of the principal frequencies in HD 122970 61 vi 5.1 Data string for HD 122970 from the global campaign 64 5.2 Window function for the frequency vx 65 5.3 Fourier Spectrum of the campaign data set of HD 122970 67 5.4 Fourier spectrum of HD 122970 in the frequency range of interest. ... 68 5.5 Successive filtering of the main frequencies in HD 122970's eigenspectrum. 69 5.6 Subtraction of the window pattern of v\ from the eigenspectrum of HD 122970 72 5.7 Constraints on i and $ for Hd 122970 75 5.8 The asteroseismological H-R diagram showing HD 122970 78 5.9 Line of sight inclination vs projected rotation speed for HD 122970. ... 80 vii Acknowledgements I wish to take time to thank many people who have helped me throughout this work or during my experience in Astronomy. Je veux particulierement remercier Nicole St-Louis et Gilles Fontaine, professeurs a l'Universite de Montreal, ainsi que Rene Doyon, attache de recherche a l'U. de M., pour m'avoir donne mes premieres chances en astronomic I gratefully acknowledge the contribution of the Astronomy group at UBC in the development of this work through a few and insightful comments. I especially want to thank Greg Fahlman for the various and helpful comments he provided me after reading this work. Also, I want to acknowledge the help received from Gerry Grieve on computer matters. Part of this work (Flexure Problems) was enlightened by Gordon Walker's comments: it was much appreciated, as much as the various exchanges on translation for Cassiopeia. Many thanks to the other graduate students (and post-docs) in Astronomy, especially Alexei Razoumov, David Woods, Stephen Holland and Pat Durrell for many discussions and for their companionship. I would like to offer my sincerest gratitude to Philippe Eenens (Universidad de Gua- najato, Mexico) who made possible our observing at the Observatorio Astronomico Na• cional, Baja California, Mexico. Quisiera exprimir mi gratitud por su ayuda y su collab- oracion. Tambien, quisiera agradecer la Comision de Asignacion de Tiempo de Telescopio del Observatorio de San Pedro Martir por habernos asignado los tiempos de observacion deseados. Su comprension en cuanto a las tempestades de El Nino fue muy apreciada. Muchas Gracias al equipo de apoyo tecnico a SPM por su ayuda, a veces tarde en la noche, al laboratorio de Electronica y a todos los que encontre en la montafia. Final- mente, quiero agradecer a William Schuster (Instituto de Astronomia, Ensenada, Mexico) vm por sus commentarios. Philippe Eenens and Petr Harmanec (Ondfejov Observatory, Czech Republic) are following up on our promising roAp star candidate, HD 10088, at the moment of writing these lines. This effort is really appreciated. I owe many thanks to Gerald Handler (University of Vienna, Austria) for various discussions and suggestions, and especially for sharing the campaign data on HD 122970 with us.
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