The Whole Earth Telescope (WET) and similar global telescope networks are discussed. In particular this work focuses on the recent contribution of such networks to the study of magnetic A-type stars with particular attention given to pulsating variable stars. In addition, telescopes that are part of such networks have the ability to provide similar observations for the study of multi-planetary systems

Holdsworth, Daniel Luke

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ArticleWET stars and planets: telescope network observations of mCP stars and exoplanetsHoldsworth, Daniel LukeAvailable at http://clok.uclan.ac.uk/28978/Holdsworth, Daniel Luke ORCID: 0000­0003­2002­896X (2019) WET stars and planets: telescope network observations of mCP stars and exoplanets. Contributions of the Astronomical Observatory Skalnate Pleso, 49 (2). pp. 475­483.  It is advisable to refer to the publisher’s version if you intend to cite from the work.For more information about UCLan’s research in this area go to http://www.uclan.ac.uk/researchgroups/ and search for <name of research Group>.For information about Research generally at UCLan please go to http://www.uclan.ac.uk/research/ All outputs in CLoK are protected by Intellectual Property Rights law, includingCopyright law.  Copyright, IPR and Moral Rights for the works on this site are retained by the individual authors and/or other copyright owners. Terms and conditions for use of this material are defined in the http://clok.uclan.ac.uk/policies/CLoKCentral Lancashire online Knowledgewww.clok.uclan.ac.ukContrib. Astron. Obs. Skalnate´ Pleso 49, 475 – 483, (2019)WET stars and planets: telescope networkobservations of mCP stars and exoplanetsD.L.Holdsworth1,21 Jeremiah Horrocks Institute, University of Central Lancashire,Preston PR1 2HE, UK (E-mail: dlholdsworth@uclan.ac.uk)2 Center for Space Research, North-West University, Mafikeng Campus,Private Bag X2046, Mmabatho 2745, South AfricaReceived: December 1, 2018; Accepted: January 14, 2019Abstract. The Whole Earth Telescope (WET) and similar global telescopenetworks are discussed. In particular this work focuses on the recent contribu-tion of such networks to the study of magnetic A-type stars with particularattention given to pulsating variable stars. In addition, telescopes that are partof such networks have the ability to provide similar observations for the studyof multi-planetary systems.Key words: asteroseismology – stars: chemically peculiar – planets and satel-lites: general1. IntroductionThe Whole Earth Telescope (WET) is an international collaboration between45 observatories which aims to to provide near-continuous photometric observa-tions of variable stars. The broad longitudinal coverage of the network, a shownin Fig. 1, allows a star to be monitored by one site in the east, then picked up bya more westerly observatory as it starts to set at the first location. This system,in principle, allows for uninterrupted observations leading to a 100 % duty cyclewhich would otherwise be unobtainable from the ground.The WET was founded in the 1980s with the primary goal of observingpulsating white dwarf stars. Although multi-site campaigns had successfully ob-served variable stars before the foundation of the WET, the WET is uniqueas it works as a single instrument. During observations at each site, informa-tion is continuously fed-back to headquarters (HQ) at Mt. Cuba Observatoryand the University of Delaware (see http://www.physics.udel.edu/gp/darc/2015new/index.html for details) so that if an observatory site monitoring theprimary star is clouded over, a second over-lapping site can switch from a sec-ondary target to the primary to obtain continuous coverage.At the end of each night, the data are sent back to HQ for uniform reductionand analysis with the in-house data reduction packages MAESTRO and WQED(with details available on the WET web pages). Standard data reduction stepsare applied, as well as correction for primary extinction. The final light curvesconsist of differential photometry, with time stamps in BJD.476 D.L.HoldsworthFigure 1. The locations of the observatories, marked by red circles, that have agreedto contribute to Whole Earth Telescope observations.For a more detailed discussion of the WET, I refer the reader to Sullivan(2001) and Provencal et al. (2014).2. Observations of magnetic chemically peculiar starsThe magnetic chemically peculiar (mCP) stars span the spectral types of late Bto early F. They are characterised by slow rotation, strong global magnetic fieldsand over-abundances of elements such as Sr, Eu, Cr and rare earth elementswhose abundances can be up to one million times that of the Sun (Ryabchikovaet al., 2004). The magnetic field in these stars confines the chemical abundancesin spots, typically around the magnetic poles, which leads to a light curve mod-ulated with the rotation period of the star. This modulation is evident as themagnetic axis is misaligned with the rotational axis resulting in oblique rotation(Stibbs, 1950).The rotation periods of the mCP stars have a broad range, from less than aday to, potentially, hundreds of years (Mathys, 2017). The cause for these shortrotation periods, when compared to their non-magnetic counterparts, is thoughtto be magnetic breaking (Abt & Morrell, 1995; Ste¸pien´, 2000).In the late 1970s, Kurtz (1978) observed rapid oscillations in the very pe-culiar Przybylski’s star (HD 101065). With further exploration of other mCPstars, he was able to define the class of rapidly oscilllating Ap (roAp) stars(Kurtz, 1982). The pulsations in these stars are thought to be driven by theκ-mechanism acting in the H i ionisation zone causing the excitation of high-overtone, low degree, pressure (p-) modes (Balmforth et al., 2001; Saio, 2005).However, it is also possible that turbulent pressure in the atmospheres of thesestars can also excite pulsations (Cunha et al., 2013), which helps to explain theWET stars and planets: telescope network observations of mCP stars and exoplanets 477observed frequencies which cannot be driven by the κ-mechanism. The pulsa-tion periods of the roAp range from 5 − 24 min, with peak-to-peak amplitudesup to 34 mmag in B-band observations (Smalley et al., 2015; Joshi et al., 2016;Holdsworth et al., 2018a). It is thought that the magnetic field suppresses low-frequency pulsations in Ap stars (Saio, 2005), although recent K2 observationsanalysed by Bowman et al. (2018) suggest that this might not be the case.The pulsation axis in the roAp stars is closely aligned with the magneticone which leads to a modulation of the pulsation amplitude as the star rotates.This is evident in Fig. 2. This modulation in the amplitude causes a frequencymultiplet in the amplitude spectrum of the light curve, and holds informationon the geometry of the mode. For a non-distorted mode, a modulated dipolepulsation (i.e. ` = 1) results in a frequency triplet, and a quadrupole (` = 2)mode shows a quintuplet in the amplitude spectrum.Figure 2. K2 observations of the roAp star HD 24355. The top panel shows the lightcurve. Evident is the rotational modulation due to surface spots (with a period of about28 d) and also the ∼ 6 h drift in the spacecraft pointing. The lower panel shows thelight curve with the low-frequency signals removed. The modulation seen is that of thepulsation envelope which coincides with the spot modulation. Figure from Holdsworthet al. (2016).If the amplitude of the pulsation goes to zero over the rotation cycle, the lineof sight to the star coincides with a pulsation node. When this occurs, we expecta phase change of the pulsation by pi-rad as we are now observing the oppositepulsation pole. This is, again, the expected case for a non-distorted mode. Fig. 3shows examples of the non-distorted case (lower three panels) along side thatof a distorted quadrupole pulsation (top panel; HD 24355).478 D.L.HoldsworthFigure 3. A comparison of non-distorted pulsations in roAp stars (bottom 4 panels),and the distorted mode in HD 24355. The amplidue modulation is as expected, butthe lack of a pi-rad phase change allows us to conclude that the mode in HD 24355 isdistorted. Figure from Holdsworth et al. (2016).2.1. WET observations of roAp starsAlthough the Whole Earth Telescope was primarily designed for the observa-tions of pulsating white dwarf stars (e.g. Kepler et al., 1995; Sullivan et al.,2008; Provencal et al., 2012), it is well suited for the study of pulsations in Apstars. The need for continuos observations to reduce (or ideally remove) theeffects of aliases in an amplitude spectrum is key to identifying real frequencieswhich are not perturbed by cross talk with the spectral window. The roAp starscan have a rich spectrum of closely spaced modes (cf. HR 1217; Kurtz et al.,2005) representing the large and small frequency separation, thus allowing fora full asteroseismic model of the star.The most extensively studied (from the ground) roAp star is HD 1217. Thisstar was the first roAp star targeted with the WET in 2000 after observationin 1986 showed this to be a promising star to perform asteroseismology on. Thecampaign lasted a total of 35 d, achieved a duty cycle of 36 % and providedthe most precise ground-based measurements of the amplitudes in roAp stars,reaching a precision of 14µmag (see Fig. 4 Kurtz et al., 2005). With such precisedata, predictions by Cunha (2001) of a missing mode in the 1986 data wereWET stars and planets: telescope network observations of mCP stars and exoplanets 479proven to be true, allowing for a complete asteroseismic model of the star to becomputed.Figure 4. The amplitude spectrum of WET data for HR 1217. The high duty cycle(for ground-based observations) led to a suppression of aliases, and the data lengthprovided high frequency precision. Most of the structure seen in the frequency range2600 − 2800µmag is real. The amplitude unit, milli-modulation amplitude, convertsto milli-magnitude as 1 mma=1.086 mmag. Figure from the WET team.More recently, the WET targeted the largest amplitude roAp star known:J1940 (Holdsworth et al., 2014, 2018a). Although only achieving a duty cycle of21 %, the multi-site observations revealed that J1940 pulsates in a very distortedquadrupole mode. Through modelling the pulsational phase variations (after themethod of Saio 2005), which are sensitive to mass and the polar magnetic fieldstrength, it was shown that the pulsation frequency is much higher than thetheoretical cut-off frequency i.e. the pulsation frequency cannot be recreatedwith the κ-mechanism alone.Other multi-site campaigns of roAp stars have also unveiled stars whichshow a suppression of the pulsation phase. Holdsworth et al. (2018b) collecteddata from three observatory sites (Siding Springs, Australia; Sutherland, SouthAfrica; Cerro Tololo Inter-American Observatory, Chile) to study J1640, whileHoldsworth et al. (2018c) utilised the Las Cumbres Observatory’s 0.4-m tele-scopes to observe J0855. Both projects aimed to obtain continuous observations480 D.L.Holdsworthin the same way as the WET, but through independent means. These multi-sitecampaigns, coupled with results from the Kepler Space Telescope, have identi-fied many roAp stars that pulsate with frequencies greater than their respectiveacoustic cut-off frequencies (Fig. 5). All of these stars show suppressed phasevariations, with the modes being strongly distorted by the star’s magnetic field.Figure 5. The logTeff − νL/M plane showing the positions of the roAp stars. Thesquares represent the principal pulsation frequency, with vertical bars showing therange of frequencies for multi-periodic stars. The hatched region represents wherehigh-order p modes are excited by the κ-mechanism in the H-ionization zone in non–magnetic models. Acoustic cutoff frequencies are represented by the dashed lines. Thestars shown in red are those which show suppressed phase variations. The numericallabels correspond to the stars in table A1 of Holdsworth et al. (2018b). Figure fromHoldsworth et al. (2018c), after Saio (2014).WET stars and planets: telescope network observations of mCP stars and exoplanets 4813. Observations of exoplanetsIt is not just asteroseismology that benefits from continuous observations; thestudy of transiting exoplanets can make good use of uninterrupted data sets.A prime example of this is the TRAPPIST-1 planetary system (Gillon et al.,2017). The presence of three transiting planets were well established not longafter discovery of the system, however the precise period of the (then) outer mostplanet was not known. A total of 10 telescopes were used to try and confirm theparameters of the system through near-continuous observations. However, it wasnot until the Spitzer space telescope observed the system, with complementarydata from the ground, that the full extent of the planetary system was known.Uninterrupted data provides the opportunity to probe the full period range ofplanets without diurnal gaps.A further point that should be mentioned is the power of multi-site spec-troscopy. Although this is by far not a new concept, the Las Cumbres Observa-tory is installing 6 identical high-resolution spectrographs at a selection of theirsites. With the use of identical instruments and telescopes, instrumental profilesbecome less of an issue so that the data require less processing to be analysedtogether. Since spectroscopy is also more sensitive to smaller planets (from theground at least), continuous spectroscopic observations have the potential toprovide many planetary systems with many terrestrial planets.Acknowledgements. The author acknowledges financial support from the STFCvia grant ST/M000877/1 and the National Research Foundation of South Africa.Thanks go to Judith Provencal for assistance in compiling the original talk.ReferencesAbt, H. A. & Morrell, N. I., The Relation between Rotational Velocities and Spec-tral Peculiarities among A-Type Stars. 1995, Astrophys. J., Suppl., 99, 135, DOI:10.1086/192182Balmforth, N. J., Cunha, M. S., Dolez, N., Gough, D. O., & Vauclair, S., On theexcitation mechanism in roAp stars. 2001, Mon. Not. R. Astron. Soc., 323, 362,DOI: 10.1046/j.1365-8711.2001.04182.xBowman, D. M., Buysschaert, B., Neiner, C., et al., K2 space photometry revealsrotational modulation and stellar pulsations in chemically peculiar A and B stars.2018, Astron. Astrophys., 616, A77, DOI: 10.1051/0004-6361/201833037Cunha, M. S., The sixth frequency of roAp star HR 1217. 2001, Mon. Not. R. Astron.Soc., 325, 373, DOI: 10.1046/j.1365-8711.2001.04434.xCunha, M. S., Alentiev, D., Branda˜o, I. 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WET stars and planets: telescope network observations of mCP stars and exoplanets

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WET stars and planets: telescope network observations of mCP stars and exoplanets

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