We have analyzed the photometric and spectral variations of the classical T Tauri star DI Cep
for the last 50 years. Currently the star is at its faintest state and possesses an emission spectrum
in the visual range. Synchronous spectroscopy and UBV R photometry show that the higher the
brightness, the stronger were the intensities of hydrogen Hα, Hβ emission lines and of FeII, HeI
λ5876 ˚A emissions. For the first time, we detected, with a high probability, quasi-periodic variations
of the star’s brightness and of its spectrum with the period P = 2020 ± 200 days

Efendieva, F.

Grankin, Konstantin Nikolaevich

Ismailov, N. Z.

Khazar University Institutional Repository

Peremennye Zvezdy (Variable Stars) 28, No. 5, 2008Received 20 March; accepted 10 April.LONG-TERM PHOTOMETRIC AND SPECTRAL VARIATIONSOF DI CEPHEIN.Z. Ismailov1,2, K.N. Grankin3, F.N. Efendieva11 Shamakha Astrophysical Observatory, National Academy of Sciences of Azerbaijan,Shamakha, Azerbaijan, e-mail: Box1955n@yahoo.com2 Astrophysics Department of Baku State University, Baku, Azerbaijan3 Crimean Astrophysical Observatory, Nauchny, UkraineWe have analyzed the photometric and spectral variations of the classical T Tauri star DI Cepfor the last 50 years. Currently the star is at its faintest state and possesses an emission spectrumin the visual range. Synchronous spectroscopy and UBV R photometry show that the higher thebrightness, the stronger were the intensities of hydrogen Hα, Hβ emission lines and of FeII, HeIλ5876 Å emissions. For the first time, we detected, with a high probability, quasi-periodic variationsof the star’s brightness and of its spectrum with the period P = 2020± 200 days.1 INTRODUCTIONMore than 60 years of photometric and spectroscopic observations of young T Tauristars made it possible to collect a large amount of observations for individual membersof this class. Nevertheless, many aspects of their observable physical characteristics stillremain unexplained. Thus, it is necessary to undertake a detailed analysis of the availableobservations for individual stars to specify their common physical properties.The question of long-term variability of young stars deserves special attention. Short-term brightness variations with an amplitude of 0.m1–0.m3 per day or during an individualobserving season are characteristic of many TTSs (T Tauri stars), while the characterof long-term brightness and spectrum variations seems very individual for different stars.For example, some stars, observed for dozens of years, unexpectedly show a brightness riseand flare activity (for example, see Holtzmann et al., 1986), others reveal monotonousvariations, with the light variability amplitude as large as 1m−3m (Herbst, 1986). Athorough analysis of all the aspects of long-term observations can help to reveal thereasons for such variations, to generalize them, and to get a better understanding ofTTSs.DI Cep belongs to classical T Tauri stars (CTTSs). The spectrum of the star in theoptical range is characterized by strong emission lines of hydrogen (Balmer series), H andK CaII, as well as of FeII, FeI, etc. (Grinin et al., 1980; Krasnobabtsev, 1982; Gahmand Petrov, 1983; Ismailov, 1987; Hessman and Guenther, 1997). Bastian and Mundt(1979) found the star to show fast spectral and photometric variations within one hour.In the red part of the spectrum, the CaII λλ8498, 8542 ÅÅ emission lines and a weakOIλλ 7773, 8446 ÅÅ emission are observed (Hamann and Persson, 1992). In the UVpart of the spectrum, SiIV, CIV, HeII, [SiIII], [CIII] emission lines and the MgII λ2800 Ådoublet were detected (Gómez de Castro and Fernández, 1996). During the recent years,2 N.Z. Ismailov et al.: DI Cepregular spectroscopy of the star revealed a 9-day quasi-periodic variability of the spectrumand brightness (Ismailov, 2004). From numerous observations, significant fluctuations ofemission-line intensities in the spectrum on time scales from several minutes to hours anddays were found.Photometric observations were regularly carried out in different observatories, theywere published by Grinin et al. (1980), Kardopolov and Filip’ev (1985), Kolotilov etal. (2004) and included into the data base (Herbst and Shevchenko, 1999). Duringseveral rather short-term time intervals, photometric observations of the star were alsoperformed by Keleman (1985) and by Ismailov (1988, 1997, and this paper). AccurateUBV R photometry of DI Cep of the largest duration were carried out in Mt. MaidanakObservatory by the staff of the Variable Stars Department of the Astronomical Institute(Uzbekistan Academy of Sciences). On 17 observing seasons, they obtained 1200 UBV Rmeasurements of DI Cep (Grankin et al., 2007).The purpose of our study is to analyze the results of long-term spectroscopy andphotometry of DI Cep.2 SPECTROSCOPYMost of our spectroscopic observations were carried out in the Cassegrain focus of theechelle spectrometer at the 2-m telescope of the Shamakha Astrophysical Observatoryusing a 530×580-pixel CCD detector. The spectral resolution near the Hα line was 14000,with the average signal-to-noise ratio of 60. The spectral range covered was 4700−6700 Å.Our observations were performed during the summer and autumn in 2004–2006.A part of the CCD spectral data on hydrogen emission lines were obtained by Dr.J.F. Gameiro (Portugal) and kindly sent to us. Besides, in our analysis of the star’s spec-tral variations, we use the spectroscopy from Grinin et al. (1980) and Ismailov (1987),where observations were made using photographic plates. These observations were ob-tained with rather similar characteristics of detectors and have virtually the same lineardispersion (100 Å per mm and 93 Å per mm at Hγ, respectively).Uncertainties of radial-velocity measurements for CCD spectra did not exceed 2 km/s,and those of equivalent widths, about 3–4%. For spectrograms on photographic plates,the equivalent-width uncertainties amounted to 15%–20%.We measured equivalent widths, line widths at half intensity (FWHM), displacementsof individual components of the Hα and Hβ emissions as well as of the HeI λ5876 Å, NaIλλ5889, 5896 ÅÅ (D2, D1), FeII lines. The lines of FeII, FeI, and HeI are observed as weakemissions. Equivalent widths of such lines are within 0.1−1.0 Å, they vary synchronouslywith the hydrogen Hα and Hβ lines. The λ5876 Å line is frequently observed with twopeaks and a central drop. Better-defined similar structures are observed also for the NaID1, D2 lines (Fig. 1).The two top panels of Fig. 2 display time variations of the equivalent widths of thehydrogen Hα and Hβ emission lines. The time range marked “1” on the first panelcorresponds to the observations performed in June – December, 1975 (Grinin et al., 1980).The group of data points marked “2” are from observations by Ismailov (1987). These datacover the time interval of 1975–1987. No information on the Hα emission line is availablein this case. The third time interval contains CCD observations by Gameiro acquired in1988–1999 and the fourth interval, Ismailov’s CCD observations made in 2004–2006. Thelast two intervals also have a comparable uncertainty of measured equivalent widths, atthe level of 3–4%.N.Z. Ismailov et al.: DI Cep 3Figure 1. The region of the HeI λ5876 Å and NaI λ5889, 5896 ÅÅ (D2, D1) lines in the spectrum ofDI Cep.It appears from Fig. 2 that, while photographic observations show a relatively largescatter about the mean, the general tendency displayed by the curves is a monotonedecrease of emission equivalent widths. Over 30 years of observations, the mean equivalentwidths of the emission lines decreased more than twofold.3 PHOTOMETRYA historical light curve of the star was recently presented by Kolotilov et al. (2004). Fromthis curve, it appears that, according to early photographic observations, the absoluteminimum of the star’s light, mpg = 13.m6, was observed in 1940. During the wholehistory of its observations, the star was again in a similar condition only in 1953 and1983. The second deep photographic minimum was detected in 1953–1956. Photoelectricobservations started in 1970s and revealed the first minimum in 1983–1984. The combinedlight curve in Kolotilov et al. (2004) gives a good general description of the brightnessvariations but is insufficiently complete after 1980.In 2006, we obtained 13 new BV brightness estimates at Shamakha Observatory. Forfurther analysis, we add all observations available in the literature and the data covering17 years we obtained in Uzbekistan (Grankin et al., 2007). We have collected the totalof 1960 V -band brightness estimates and plotted the combined photoelectric light curve.It is shown in the bottom panel of Fig. 2. It can be seen that a deep minimum occurred4 N.Z. Ismailov et al.: DI CepFigure 2. The time variability of Hα and Hβ emission equivalent widths and combined light of DI Cepfor 30 years of observations. The marked intervals correspond to data from: 1 — Grinin et al. (1980),2 — Ismailov (1987), 3 — Gameiro, 4 — our unpublished data obtained in 2006.on JD 2447041 (1987), then the star’s brightness increased rather quickly, reaching amaximum at JD 2448155 (1990), and then a slower decline started, which continues tillnow. There also happened a small fading on JD 2451087 (1998), and then the brightnesswas quickly restored.The spectral data show a many-year monotone decrease of mean emission equivalentwidths (Fig. 2).From available observations, we selected the nights when the star was simultaneouslyobserved photometrically and spectroscopically. From a small amount of synchronousobservations, we earlier demonstrated (Ismailov and Grankin, 2007) that that equivalentwidths of emission lines in the spectrum of DI Cep were increasing during brightness in-creases. To check this conclusion on a large number of observations, we found, for differentfilters of the UBV R system, from 6 to 17 nights of synchronous observations. Figure 3presents a plot of the star’s brightness versus equivalent width of hydrogen emission lines.It appears that an increase of the Hα and Hβ emission was always accompanied with abrightening of the star. In all cases, the correlation coefficient was r ≥ 70%.The combined photoelectric light curve in Fig. 2 shows wave-like variations, with wavesof 5–7-year duration, interrupted with rather deep fadings. Each compact group of pointsin the light curve of Fig. 2 consists of observations of one year. It can be seen that thetotal amplitude of V -band brightness variations is about 0.m6, while the largest variationN.Z. Ismailov et al.: DI Cep 5Figure 3. Equivalent widths of Hα and Hβ emission lines versus photoelectric UBV R magnitudes. Foreach panel, the corresponding correlation coefficient is indicated.amplitude within one year is 0.m3. The photometric light curve, like in the case of theprototype, T Tau, corresponds to type IV of the classification by Ismailov (2004) — boththe amplitude of yearly variations and the year-average brightness vary. Grankin et al.(2007) also note that the character of long-term variations is very similar for DI Cep andT Tau. We can suppose that two different mechanisms are simultaneously responsible forbrightness variations of these stars. One of them, possibly related to local inhomogeneitiesof physical conditions on the stellar surface, probably causes the seasonal variability. Asecond mechanism operates at longer time scales.4 SEARCH FOR THE LONG-TERM PERIODTo search for long-term cyclic variations, we applied the PERIOD software package foranalysis of time series. This software package was developed under the Council for theCentral Laboratory of the Research Councils in the framework of the StarLink project.We used two methods incorporated in the package, the Lomb–Scargle method and theχ2 technique (Chisq). The Lomb–Scargle method (Lomb, 1976; Scargle, 1982) is a mod-ification of the classical Fourier periodogram technique making it invariant for unevenlyspaced times of observations. It is a very powerful method for unevenly spaced and noisydata. Later, the method was further modified by Horne and Baliunas (1986), and thenPress and Rybicki (1989) proposed the algorithm used in the PERIOD package.6 N.Z. Ismailov et al.: DI CepThe χ2 technique is a relatively simple method where input data are folded with anumber of trial periods. For each trial period, the data are described with a sinusoid.The resulting χ2 values are plotted as a function of trial frequencies, and minima in thesediagrams are considered probable periods (Horne et al., 1986).Both methods showed a significant peak at the frequency 0.000495 d−1. This frequencycorresponds to the period P = 2020± 200 days.We estimated the significance of the period by repeated modeling of white noise. Thisprocedure is implemented in the SIG routine of the PERIOD package, which appliesthe so-called Fisher’s method of randomization, or the Monte Carlo method (Nemec andNemec, 1985). Randomization of time series and computation of periodograms in the SIGroutine are repeated for hundreds of permutations. The period we obtained for DI Cephas a very high probability, in excess of 95%. Thus, we have reasons to believe that theperiod P = 2020± 200 days is real.Figure 4 presents the power spectrum and the spectral window obtained using theLomb–Scargle for the interval of frequencies 0− 0.006d−1. Phases correspond to the lightelements:MinV = JD2447041 + 2020d · E.Figure 4. The power spectrum and spectral window from the Lomb–Scargle method.Figure 5 presents the phase diagrams for V magnitudes (bottom) and equivalent widthsof the Hα and Hβ emission (top), plotted for the above elements. Only the CCD obser-vations of equivalent widths obtained by Gameiro and Ismailov are shown. These ob-N.Z. Ismailov et al.: DI Cep 7servations have a good accuracy and were made more frequently, which is important forsearches of a long-term period.Figure 5. The phase diagrams for V magnitudes (bottom) and equivalent widths of the Hα and Hβemissions (top), folded with the 2020-day period.It appears from Fig. 5 that the variations of brightness and of the emission spectrumare correlated.CONCLUSIONSOur many-year observations of the CTTS DI Cep show that the star’s visual brightnesscorrelates with equivalent widths of emission lines. The variations of the mean brightnessand spectrum are probably related to the same activity mechanism.Photometric observations within a year do not reveal V -band brightness changes inexcess of 0.m3, and the total amplitude of brightness variations during 30 years is 0.m6.The mean equivalent widths of emission lines decreased more than twofold in 35 years.These amplitudes correspond to maximal changes of flux with respect to its value atminimum by a factor of 1.32 and 1.74, respectively. Brightness variations within 0.m3during individual observing seasons are quite characteristic of many T Tauri stars (see,for example, Petrov et al., 1999). It is possible to explain such variability range withinseveral days by spotted structure causing modulations of brightness and spectrum due to8 N.Z. Ismailov et al.: DI Cepaxial rotation. Variations of brightness and spectrum with the period 9.d24 were found forDI Cep by Ismailov (2004) and by Kolotilov et al. (2004).Long-term variations of the mean brightness have a completely different character,they cannot be explained in the model of spotted surface. The reasons for such long-termperiodic variations can be (1) the existence of an invisible companion of the star, likea protostar or massive protoplanets, or (2) a long-term cyclic activity on the surface,similar to the 11-year cycle of the solar activity. In our opinion, the first option is moreprobable, taking into account that many T Tauri stars, including T Tau itself, haveinfrared components (Dyck et al., 1982; Koresko, 2000). It is interesting that a longperiod quite close to that of DI Cep was recently found for T Tau (Ismailov and Samus,2003; Mel’nikov and Grankin, 2005). Gameiro et al. (2006), Ismailov and Grankin (2007)suspected variability of the center-of-mass radial velocity of DI Cep, also favoring binarityof DI Cep.Ismailov and Grankin (2007) present observational evidence for magnetosphere accre-tion model of DI Cep. If such a reliable mass source as a stellar companion is present inthe system, with an extended gas and dust disk, then many difficulties connected withmagnetic fields in accretion-disk models are removed. Note that many TTSs do not showstrong magnetic fields, while the accretion disk theory demands the presence of a mag-netic field at the level of 1 kGs or more (Johns-Krull et al., 1999). It is possible that manyCTTSs have a structure similar to DI Cep.Let us summarize the main conclusions of this study.1. The level of activity revealed by the emission spectrum depends on brightness ofDI Cep: the brighter the star, the larger are equivalent widths of emission lines.2. Currently the star is at the lowest activity level of the spectrum and near its absolutebrightness minimum. 30 years of spectroscopy and photometry testify to slow fading ofthe star since 1990 till present.3. Possible quasi-cyclic brightness variations with a period of 2020 days are observed,indicating the existence of a companion near the young star DI Cep.Acknowledgements: We wish to thank Dr. J.F. Gameiro for kindly making hismeasurements of hydrogen Hα and Hβ emission available to us and Dr. N.N. Samus forcareful editing of the manuscript.References:Bastian, U., Mundt, R., 1979, Astron. & Astrophys., 78, 181Dyck, H.M., Simon, T., Zuckerman, B., 1982, Astrophys. J., 255, L103Gahm, G.P., Petrov, P.P., 1983, Activity in Red-Dwarf Stars, J. Reidel Publ. Comp., p.497Gameiro, J.F., Folha, D.F.M., Petrov, P.P., 2006, Astron. & Astrophys., 445, 323Gómez de Castro, A.I., Fernández, M., 1996, MNRAS, 283, 55Grankin, K.N., Melnikov, S.Yu., Bouvier, J., et al., 2007, Astron. & Astrophys., 461, 183Grinin, V.P., Efimov, J.S., Krasnobabtsev, V.I., et al., 1980, Peremennye Zvezdy, 21, 247Hamann, F., Persson, S.E., 1992, Astrophys. J. Suppl., 82, 247Herbst, W., 1986, Publ. Astron. Soc. Pacific, 98, 1088Hessman, F.V., Guenther, E.W., 1997, Astron. & Astrophys., 321, 497Holtzman, J.A., Herbst, W., Booth, J., 1986, Astron. J., 92, 1387Horne, J.H., Baliunas, S.L., 1986, Astrophys. J., 302, 757N.Z. Ismailov et al.: DI Cep 9Horne, K., Wade, R.A., Szkody, P., 1986, MNRAS, 219, 791Ismailov, N.Z., 1987, Peremennye Zvezdy, 22, 489Ismailov, N.Z., 1988, Peremennye Zvezdy, 22, 892Ismailov, N.Z., 1997, IBVS, No. 4470Ismailov, N.Z., 2004, Astron. 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Long-Term Photometric and Spectral Variations of DI Cephei

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Long-Term Photometric and Spectral Variations of DI Cephei

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