Revista Mexicana de Astronom´ıa y Astrof´ısica, 57, 1–14 (2021) A STUDY OF THE SECULAR VARIATION OF THE HIGH-AMPLITUDE DELTA SCUTI STAR AD CMI1 J. H. Pe˜na2,3,4, D. S. Pi˜na2,4, H. Huepa3, S. B. Ju´arez4, C. Villarreal4, J. Guill´en4, A. A. Soni2,4, A. Renter´ıa2,3,4, J. M. Donaire5, R. R. Mu˜noz5, T. Benadalid5, J. D. Paredes6, E. D. Orozco4, I. Soberanes4, H. Posadas4, C. Castro4, J. Briones4, M. Romero4, F. Martinez4, A. L. Zu˜niga4, J. L. Carrillo4, B. Ch´avez4, D. Navez4 & C. Garc´ıa4 Received June 1 2020; accepted April 20 2021 ABSTRACT We determined the nature of the Delta Scuti star AD CMi and its physical parameters from newly determined times of maximum light and other times from the literature, as well as from uvby β photoelectric photometry. − RESUMEN A partir de los reci´en determinados tiempos de m´aximo en fotometr´ıa de la estrella Delta Scuti AD CMi y una recopilaci´on de tiempos de m´aximo de la literatura, as´ıcomo datos con fotom´etria uvby β hemos determinado la naturaleza de esta estrella y sus par´ametros f´ısicos. − Key Words: δ Scuti stars — HADS stars — Photometry — Variable stars 1. INTRODUCTION dedicated to δ Scuti stars have begun changing their observational strategies. After the Canadian satel- The study of δ Scuti stars has undergone many lite MOST was successfully launched to study as- changes since its inception. Soon after the pioneering teroseismology from space, future missions were de- works of Eggen (1957) and Millis (1966, 1968), sys- signed to determine adequate frequency resolutions. tematic studies like those of Breger (1979) and the This satellite and COROT were the first to study δ early considerations about the nature of their vari- Scuti stars from space. CoRoT (Convection, Rota- ability by the French group led by Le Contel et al. tions, and Planetary Transits), led by the CNES in (1974), as well as the later coordinated longitudinal association with French laboratories with large inter- campaigns designed to discover the true nature of national participation, was a satellite built to study the pulsational variability, altered the nature of our the internal structure of stars and to detect extraso- knowledge of these stars. During these years some re- lar planets. It continuously observed star fields in the searchers such as Zessewitch (1950) and Abhyankar Milky Way for periods of up to 6 months. Over the (1959) among others recognized the value of using course of 2,729 days, CoRoT collected some 160,000 the old data sets which provided, as in the present light curves showing variations in star brightness study, time bases of fifty years of more leading more over time. This mission ended in June 2014. precise interpretations of the variability. However, the attainable limits of ground-based observations There have been several other space projects. were finally reached and efforts were made to study One of the most important was that of NASA’s these variables from space. Kepler satellite, which was launched on March 6, As Garrido and Poretti (2004) pointed out, teams 2009 with the primary mission of searching for ex- oplanets via planetary transits using high-precision, 1Based on observations collected at the Tonantzintla and long time-series CCD photometry with a secondary San Pedro M´artir Observatories, M´exico. mission of studying stellar variability (Guzik et al., 2Instituto de Astronom´ıa, Universidad Nacional Aut´onoma de M´exico, Cd. M´exico. 2019). In 2013 the K2 mission was developed to 3Observatorio Astron´omico Nacional, Tonantzintla observe 19 new fields along the ecliptic plane. δ Puebla, M´exico. 4 Scuti variables have been included in these cam- Facultad de Ciencias, Universidad Nacional Aut´onoma de paigns since they are a promising type of star for M´exico. 5Universidad Mayor de San Andr´es, La Paz, Bolivia. research using pulsations to infer interior structure 6Universidad Central de Venezuela, Venezuela. and, as a result, can be used to test the input of 1 2 PENA,˜ ET AL. physics and methods of solar and stellar modelling. (2020) carried out a study in a close binary system, Multiple papers have utilized the Kepler data, and HD 74423, discovered by TESS. They claim that it as Guzik et al. (2019) pointed out, the lists of can- might be the prototype of a new class of obliquely didate stars observed for this data could be used to pulsating stars in which the interactions of stellar discover targets for asteroseismic research. pulsations and tidal distortion can be studied. Recent studies of δ Scuti stars using data from In the present Paper we utilize both the obser- the Kepler mission include Bowman and Kurtz vations of the AD CMi star obtained decades ago to (2018) who demonstrated that an ensemble-based determine its secular behavior as well as uvby β to approach using space photometry from the Kepler determine its physical parameters. The HADS− star mission was not only possible, but it was also valu- AD CMi has the following characteristics: right as- able as a method to identify stars for mode identifi- cension and declination of 07:52:47.18, +01:35:50.50 cation and asteroseismic modelling. This work was (epoch 2000), magnitude of 9.38, a spectral type of extended by Antoci et al. (2019) for hot young δ F0IV, a period of 0.12297443 d (2.9 hr) and an ampli- Scuti which were lacking in the Kepler mission data tude of variation of 0.3 mag. All these features make set. it an easy target for the small telescopes provided Kepler’s database, combined with other satellite with CCD cameras that are used in our astronomi- results such as Gaia DR2 that provided the paral- cal courses. laxes, has served to derive precise luminosities (Mur- With respect to the background of the star, ac- phy et al., 2019) for stars in and around the δ Scuti cording to Anderson and McNamara (1960) AD CMi instability strip for a sample over 15 000 A and F was discovered by Hoffmeister (1934) and Zessewitch stars into δ and non δ Sctuti stars. Murphy et al., (1950) who described it as an eclipsing variable of the (2019) determined a new empirical instability strip Algol type with a period of 1.276d. It was Abhyankar based on the observed pulsator fraction that is sys- (1959) who determined that AD CMi was an intrinsic tematically hotter than theoretical strips. Ziaali et variable with a period of only 3 h. From their analy- al. (2019) examined the period-luminosity (P-L) re- sis they found the following ephemerides for this star: lation for δ Sctuti stars with the same Gaia DR2 Hel.max. = JD2436601.8228+ 0.122972E. Breger parallaxes for the sample of Rodriguez et al. (2000) (1975) carried out an extensive study of AD CMi as well as 1124 stars observed in the Kepler mis- with uvby β photometry. He determined a Wes- − sion and determined absolute magnitudes. Barcelo selink radius of 3R⊙ which leads to a Population I Forteza et al. (2020) remarked that thanks to the mass. He stated that except for the large amplitude, high-precision photometric data legacy from space the star is virtually indistinguishable from a normal telescopes like CoRoT and Kepler, the scientific com- δ Scuti variable. Dean et al. (1977) considered AD munity was able to detect and characterize the power CMi to be an RR Lyrae star and reported V and V-I spectra of hundreds of thousands of stars. Thanks to photometry from one night. Later, in 1983, Balona long-duration high-cadence TESS light curves, they and Stobie reported simultaneous BVRI photometry could analyze more than two thousand of δ Scuti and photoelectric radial velocities of several dwarf classical pulsators and proposed the frequency at Cepheids, AD CMi among them, but no conclusions maximum power as a proper seismic index since it is were reached. It was Rodr´ıguez and the Spanish directly related with the intrinsic temperature, mass group who conducted several studies on this star. and radius of the star. The first (Rodriguez et al., 1988) used uvby β pho- − Extending the study of large satellite data sets, tometry and proposed the possibility of a quadratic Jayasinghe et al. (2020) characterized an all-sky cat- ephemerides. They derived physical parameters of alogue of 8400 δ Scuti variables in ASAS-SN, which Te = 7550 K, log g = 3.83 and MV = 1.68 mag. includes 3300 new discoveries which combined with Through the Wesselink method they derived a ra- the Gaia DR2 derived period-luminosity relation- dius of 2.77 R⊙ and a mass of 1.9 M⊙. In a later ships for both the fundamental mode and overtone paper (Rodr´ıguez et al., 1990) they proposed a pe- pulsators. The determined results are indicative of riod change slightly different from that determined a Galactocentric radial metallicity gradient. In their independently by Jiang (1987). sample, Jayasinghe et al. (2020) identified two new The last comprehensive study of AD CMi was δ Scuti eclipsing binaries, ASASSN-V J071855.62- done by Khokhuntod et al. (2007). With obser- 434247.3 and ASASSN-V J170344.20-615941.2 with vations carried out in 2005 and 2006, they ana- short orbital periods of 2.6096 and 2.5347 d, respec- lyzed the pulsation of the star with the derived tively. In studies of particular stars Handler et al. multiple frequencies fitted to their data. Besides AD CMI 3 the dominant frequency and its harmonics, one low way that an f adjustment function was found so that frequency (2.27402 c/d) was discovered which, they in the open selected interval, the derivative equal to stated, provides a reasonable interpretation for the zero was a minimum (if it was convex) or maximum long-noticed luminosity variation at maximum and (if it was concave) relative to f.