Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Burnet, M.

Sterken, C.

Veen, P.M.

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The long-term behaviour of the Be star HD163868

Leiden University Scholary Publications

Astron. Astrophys. 332, 605–609 (1998) ASTRONOMYANDASTROPHYSICSThe long-term behaviour of the Be star HD 163868 ?C. Sterken1,??, P. Veen2, and M. Burnet31 University of Brussels (VUB), Pleinlaan 2, B-1050 Brussels, Belgium2 Leiden Observatory, Postbus 9513, 2300 RA Leiden, The Netherlands3 Observatoire de Genève, Sauverny, SwitzerlandReceived 21 August 1997 / Accepted 5 January 1998Abstract. In this paper we discuss the light variations ofHD 163868 on the basis of all available photometric data origi-nating from photometric monitoring during the last two decades.We suggest that one explanation of the on-and-off type vari-ability can be a binary configuration with a compact companionand a period of the order of 850 days, though the observed ef-fects could also be seen in the context of positive interferenceof NRPs. The bright stages are associated with strong redden-ing consistent with an increase in circumstellar material. Wedraw the attention to the similarity with cyclically-recurrentmild S Dor phases of LBVs.Key words: stars: individual: HD 163868, HD 164152,HD 164270 – stars: emission-line, Be – stars: variables: other1. IntroductionIn this paper we discuss the light variations of HD 163868(V =7.4, B5Ve) on the basis of all available photometric dataoriginating from photometric monitoring during the last 23years.1.1. Strömgren uvby dataHD 163868 was used for some time as one of the comparisonstars for monitoring WR 103 (HD 164270, see Veen et al. 1997)in the framework of the “Long-term Photometry of Variables”(LTPV) project (Sterken 1983, 1994). During the analysis of theWR 103 data, strong variability of HD 163868 in all bands wasnoticed. Though Massey et al. (1984) found the star constant“to a few hundreths of a magnitude” (during a time intervalof 10 days in June 1980), the discovery of the variability ofSend offprint requests to: C. Sterken? Based on observations obtained at the European Southern Obser-vatory at La Silla, Chile (observing proposals 55D-0317, 56D-0249and 57D-0133) and on data obtained with the Observatoire de GenèveSwiss Telescope at La Silla?? Belgian Fund for Scientific Research (FWO)HD 163868 was already announced by Woodward (1975). Thisauthor assigned a lower limit of 20 days as a possible period ofvariation, and pointed out that the star is possibly variable on atime scale of several hours.About 150 uvby measurements have been collected at ESO;the photometric magnitudes and colour indices in the standardphotometric system were published by Manfroid et al. (1994)and by Sterken et al. (1993, 1995), and we refer to these papersfor more details on the observing strategy and on the reductionprocedure. Fig. 1 displays the y ≡ V light curve together withthe V light curve from data of Woodward (1975). Table 1 givesthe average magnitudes and colour indices for HD 163868 andthe (constant) comparison star B9007 = HD 164152. The data inTable 1 are based on data from “System 7” (see Sterken 1993)only, and the B star mean errors give a general impression ofthe photometric accuracy of the LTPV program. The standarddeviations of A9007=HD 163868 by large exceed those of thecomparison star.1.2. Geneva UBB1B2V1G dataHD 163868 has been observed in the Geneva photometric sys-tem at two occasions in 1989 and once in 1991. The Geneva databeing non-differential, the homogeneisation of Geneva mV andy photometry is not always guaranteed. Sheer fortune providedus with four measurements of our comparison star HD 164152obtained during one single night in 1983 when one Geneva ob-server erroneously measured HD 164152 by misidentification.This average value, mV = 8.873 ± 0.004–thanks to the highphotometric fidelity of the Geneva photometric system–allowsus to bring the more recent measurements of HD 163868 to acommon scale with the LTPV data. Table 1 gives the Geneva vi-sual magnitude mV and the colour indices U, V,B1, B2, V1, G.It is obvious that the scatter of the measured quantities plainlyconfirms the variability in the visual magnitude and the visualcolour indices, with even stronger variability in theU index dueto the larger amplitude of the variations in the ultraviolet. Theindex βc is the Geneva emission-free β index calculated fromthe photometric data (see Cramer 1994). The deviance by 0.m1of βc from the observed Strömgren β index (β = 2.523, Klare606 C. Sterken et al.: The long-term behaviour of the Be star HD 163868Table 1. HD 163868 and comparison star HD 164152: average y(V ), b− y,m1, c1 and standard deviations σ (in millimag.). N denotes the totalnumber of observations of each star. Note that the results are solely based on data belonging to System 7 (Sterken et al. 1993, see also Sterken1993). Geneva photometry: mV is the visual magnitude, U, V,B1, B2, V1, G are the colour indices with respect to the B band. The index βc isthe Geneva emission-free β index calculated from the colour indices. The first column gives HJD−2440000LTPV HD MK type y(V ) b− y m1 c1 N σy σb−y σm1 σc1A9007 163868 B5Ve 7.365 0.056 0.045 0.140 153 45 12 7 43B9007 164152 B9V 8.892 0.060 0.126 0.950 150 8 4 6 8HJD mV U V B1 B2 V1 G βc7672.820 7.268 0.496 0.955 0.817 1.550 1.655 2.116 2.6257756.594 7.319 0.505 0.971 0.819 1.556 1.667 2.138 2.6308494.585 7.332 0.532 0.966 0.814 1.554 1.666 2.146 2.6349100 9200 9300 9400 9500 9600 9700 9800 9900H JD -24000007.27 .37 .48100 8200 8300 8400 8500 8600 8700 8800 89007 .27 .37 .47 .57100 7200 7300 7400 7500 7600 7700 7800 79007 .27 .37 .41994 19951991 1992 19931988 1989 19902100 2200 2300 2400 2500 2600 2700 2800 29007 .27 .37 .41974 1975 1976Fig. 1. All availableV and y data. Top panel:V data from Woodward (1975), other pan-els: LTPV. The triangles represent Genevavisual magnitude data, the crosses indicate afew measurements of HD 163868 for whichno comparison star measurements are avail-ableC. Sterken et al.: The long-term behaviour of the Be star HD 163868 6070.00 0.01 0.02 0.03 0.04 0.05frequency0.000.010.020.030.000.200.400.600.80Fig. 2. Spectral window (top) and amplitudespectrum (bottom) of HD 163868 (y) in thefrequency range 0.0-0.05 cd−170 00 75 00 80 00 85 00 90 00 95 00H JD -2 44 000 07.207 .307 .4 0y7 .307 .407 .5 0b7 .507 .607 .7 0v7 .707 .807 .9 0uFig. 3. LTPV uvby light curves andleast-squares sine fits. The triangles repre-sent Geneva visual magnitude data& Neckel 1977) is the signature of very strong emission in Hβ,an emission that also seems to be variable, as seen in columnβc in Table 1.2. PeriodicitiesA period search was carried out using Fourier analysis on thedifferential A9007 minus B9007 y ≡ V LTPV data in the fre-quency range 0.0-0.2 cd−1. The spectral window (Fig. 2, top) isdominated by a strong peak at 0.00276 cycles per day (cd−1),and a further 1 cycle per year alias at 0.005525 cd−1due to the an-nual cycle of our observations. The amplitude spectrum (Fig. 2,bottom) shows the strongest peak at f = 0.0013 cd−1with aweaker peak for the 1 cycle per year alias 0.00405 cd−1. Aleast-squares sine fit with this frequency reduces the O − Cstandard deviation by little more than 0.m01 in all bands, lead-ing to residuals that are still a factor of four larger than ex-pected. A least-squares sine fit in each band leads to an averagef = 0.00120 cd−1(P = 833 d, for the LTPV data).Besides the light curve published by Woodward (1975), theliterature contains only one strongly deviant visual magnitudeof HD 163868, that is, Kozok (1985) who reports V = 7.144on HJD 2444025. Though we should not attach too much im-portance to this number in absolute terms, it is notwithstandingthe brightest V -band measure ever reported, so we feel con-fident when identifying this bright phase with the maximumof a brightening event as the one seen by Woodward (1975).With our tentative period of the order of 830 days, we can con-struct a cycle-count scheme in which the maximum assigned tothe measurement of Kozok (1985) occurs two cycles after thevery first maximum. Then, four cycles would elapse till the sup-posed maximum around JD 2447600-7700 (no data available),after which brightness peaks around JD 2448500 and 2449200each follow at one cycle interval. This scheme then leads to apreliminary linear ephemerisTmax = 244 2244 + 884E±71 ± 7We stress that this “period” should not be regarded as rigid inthe true sense of the word, but that we rather deal with semi-periodic behaviour. As such, the mean errors quoted with theephemeris only represent the uncertainties in the mathematicalsolution.608 C. Sterken et al.: The long-term behaviour of the Be star HD 1638683. DiscussionThe light curve presented in Fig. 3 reveals several patterns ofvariability, viz.:1. the long wave on a time scale of the order of 850 days. Fig. 4gives the phase diagram forP = 833 d in y, b, v, u and in thecolour index b−y; the system is visibly redder during max-imum light, as was already remarked by Woodward (1975)and as is illustrated in Fig. 52. variability of a semi-regular or even irregular character ontime scales of tens of days or more3. burst-like events that occur around the maxima of the longwave. They rise the light level in each band by more than0.m1 in Woodward’s light curve, and on two occasions inour data. The well-covered event around JD2448400-8500has a double structure (see also Fig. 5), and the dip preced-ing maximum light has been observed by two independentobservers. We draw special attention to the fact that burst-like events of a smaller amplitude seem to occur at epochsof minima too, see Fig. 1 (JD 2448800-8900) and Fig. 4,where large error bars accompany data taken during mini-mum (phases 0.1-0.2)–we stress, though, that our data arenot numerous enough to preclude the existence of flares atother phases in the 830-day variability cycle4. during some time intervals (like JD 2448060-8160) the staris almost non-variable (confirming the statement by Masseyet al. 1984).The most prominent variability aspect, clearly, is the 830-day variability cycle. The interpretation of this on-and-off vari-ability is not straightforward. HD 163868 may simply be an in-teracting binary including an emission disk, so that the∼ 850 dcycle is due to changing aspects of a precessing envelope (like,e.g. HR 2142, which also shows rising/fading events as dis-cussed by Mennickent et al. 1997).But the variability of HD 163868 does also resemble thecase of κ CMa, µ Cen and HR 2517, three Be stars showingrecurrent flaring. Balona (1990) ascribes the flaring of κ CMa(a multiple-peak outburst amounting to 0.m1 suddenly appears,and fades in several days) to the abrupt appearance of a brightarea–thus ruling out NRP to explain Be outbursts–but Riviniuset al. (1997) found in µ Cen a correlation between beat periodsin NRP and the increases of Hα emission strength, thus givingnew support to the notion of pulsationally driven outbursts ofBe stars.Sterken & Manfroid (1996) offer an alternative explanationfor HR 2517 in the sense that this star may be an eccentric,massive close binary system, where the primary has lost a largeamount of mass so that the secondary obtains an internal struc-ture very similar to that of a single star of that mass. A subsequentsupernova explosion makes a compact remnant orbit the massgainer, and a high-mass X-ray binary (HMXB, see Kaper 1994)is observed. In HMXBs where the secondary star is a Be star,often a transient X-ray source is found, and this is explainedby the fact that a detectable X-ray luminosity is only expectedwhen the compact star crosses the dense equatorial disk of theBe star (Kaper 1994, van den Heuvel & Rappaport 1987). About0.2 0.4 0.6 0.8 1.0                                         pha se7.37 .4y 7 .4b  7 .5v 7 .77 .8u  0.2 0.4 0.6 0.8b-yFig. 4. LTPV uvby magnitudes in function of phase in the P = 833 dperiod (f = 0.001201). Phase 0 (JD 2447000) was chosen arbitrarily,phase bins correspond to about 30 days. The diagram at the top is thephase diagram of the colour index b− y. The error bars represent m.e.Triangles in the y graph are based on Geneva mV magnitudes1000 to 10000 such Be/X systems may exist in our own galaxy,but only about two dozen of them are observed, all close tothe galactic plane (Giovanelli et al. 1992), and they have longorbital periods. HD 163868 and HR 2517 are on the galacticplane; the low observed X-ray luminosity of HR 2517 (Grillo etal. 1992) fits the explanation, but for HD 163868 no informationon X-ray luminosity is available. Such systems need not go intooutburst every orbital cycle, but can remain dormant for manyorbital periods (Stevens 1988), a situation that may also apply toHR 2517, where the flaring has variable (even vanishing) am-plitude, and does not contradict the fact that X-rays have notbeen detected in HD 163868. An objection against this inter-pretation remains with the spectral type B5Ve of HD 163868, avery unusual spectrum for such a system, and rather pointing tothe possible existence of a white-dwarf companion.We also draw the attention to the resemblance with the de-scription by Roche et al. (1997) of X Per, a (BeXRB) mas-sive X-ray binary. These authors describe an extended lowstate (ELS) bracketed by similar flare-like events as we ob-C. Sterken et al.: The long-term behaviour of the Be star HD 163868 6092180 2200 2220 2240HJD - 24000007.207 .257 .307 .357 .40VB-V8420 8440 8460 8480 8500 8520 8540HJD  - 24000007.207 .257 .307 .357 .40Vb-yFig. 5. Woodward’s light and colour curve during the 1974 maximum,together with the flare event that occurred in 1991 (theB−V and b−yscales are the same as the V ≡ y scale, but the colour indices are notcalibrated)served in HD 163868, including the reddening during light max-imum consistent with an increase in circumstellar material. TheHD 163868 system, though, shows interesting differences ascompared to the behaviour of X Per. The evolution of the shapeof the 1974 maximum, and the one covered in 1991 is remark-able: Fig. 5 shows both maxima and their colour variation onthe same graphical scale, and illustrates that the 1974 event risesand falls much more steeply–and at the same time has a strongerreddening aspect–than does the more contemporaneous occur-rence, thus pointing to a denser and less extended slab in thepast. The double structure seen in the latter (and also duringthe following minimum) may be explained by a ring or disk ofmatter of non-uniform composition, though we must stress thatWoodward’s (1975) data do not exclude the existence of such adouble peak. Another difference to be stressed is that our dataindicate that the occurrence of an ELS is not a random, but acyclic (or quasi-periodic) phenomenon.One cannot help seeing a striking analogy between the shapeof the light and colour curves of HD 163868, and the occur-rences of S Dor phases (SD phases, see van Genderen et al.1997) in Luminous Blue Variables (LBVs): except for the scaleof events–that is, the magnitude of the brightening/reddening–the SD phases in LBVs are also cyclic, and display a similarreddening behaviour. Recently, Marlborough (1997) has out-lined the evidence supporting the possible connection betweenthe LBV variations of 1-2 magnitudes on time scales of tens ofyears (i.e. SD phases) and the shell phases of the Be star γ Cas.It is evident that the reported behaviour of HD 163868 is muchgentler than that of γ Cas–where the events led to a completedisruption of the disk with a subsequent slow recovery–and thatit is (so far) not at all certain that the long-term behaviour ofHD 163868 is related to mild S Dor variability as seen in someLBVs (like ζ1 Sco and R 40, see Sterken et al. 1997, 1998).It is clear that for a star like HD 163868 that was successivelyclaimed to be variable on a short time scale, then constant andfinally variable on a time scale of 2-3 years, continuous photo-metric and spectrographic monitoring is of urgent need.Acknowledgements. C.S. acknowledges financial support from theBelgian Fund for Scientific Research (FWO). This research has profitedfrom the SIMBAD database operated at Observatoire de Strasbourg,France. We thank the referee, L.B.F.M. Waters, for useful comments.ReferencesBalona L.A., 1990, MNRAS 245, 92Cramer N., 1994, Applications de la photométrie de Genève aux étoilesB et à l’extinction interstellaire, Ph. D. Thesis, Observatoire deGenéveGiovanelli F., Sabau Graziati L., 1992, SSR, 59, 1Grillo F., Sciortino S., Micela G. et al., 1992, ApJS 81, 195van Genderen A.M., Sterken C., de Groot M., 1997, A&A 318, 81Kaper L., 1994, in Wolf-Rayet Stars: Binaries, Colliding Winds, Evo-lution, IAU Symp. 163, K.A. van der Hucht & P.M. Williams (eds),Kluwer Ac. 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Sterken, M. de Groot, NATOASI Series C, 436, 1, Kluwer Academic PublishersSterken C., Manfroid J., Anton K., et al., 1993, A&AS, 102, 79Sterken C., Manfroid J., Beele D., et al., 1995, A&AS 113, 31Sterken C., Manfroid J., 1996, A&A 305, 481Sterken C., de Groot M., van Genderen A.M., 1997, A&A 326, 640Sterken C., de Groot M., van Genderen A.M., 1998, A&A submittedStevens I.R., 1988, MNRAS 232, 199van den Heuvel E.P.J., Rappaport S.A., 1987, in Physics of Be Stars,IASU Coll. 92, A. Slettebak & P. Snow (eds.), 291Veen P.M., van Genderen A.M., van der Hucht K.A. et al., 1997, A&Ain pressWoodward E.J., 1975, IBVS 993This article was processed by the author using Springer-Verlag LaTEXA&A style file L-AA version 3.

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The long-term behaviour of the Be star HD163868

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