ET And is a binary system with a B9p(Si) star as the main component. We report on the photometric observing campaigns in 1988, 1989 and 1994 which confirmed the rotation period of 1(.)(d)618875 for ET And while refuting other published values. Furthermore, the controversial issue of pulsational stability of ET And is resolved since we have discovered pulsation for HD 219891, which was the main comparison star and sometimes exclusively used. The frequency of 10.0816 d(-1), a semi-amplitude of 2.5 mmag, T(eff) and M(v) suggest this comparison star to be a delta Scuti variable close to the blue border of the instability strip. The pulsational stability of ET And could be clearly established and hence no need exists to derive new driving mechanisms for stars between the classical instability strip and the region of slowly pulsating B-type (SPB) stars

Alvarez, M.

Bedolla, S. G.

Bus, S. J.

Dorokhov, N. I.

Dorokhova, T. N.

Goranskij, V. P.

Guo, Z.

Hao, J.

Hiesberger, F.

Huang, L.

Kreidl, T. J.

Kusakin, A. V.

Kuschnig, R.

Mironov, A. V.

Mittermayer, P.

Mkrtichian, D. E.

Osip, D. J.

Polosukhina, N.

Sareyan, J. P.

Schneider, H.

Weiss, W. W.

Ziznovsky, J.

Zverko, J.

Zwintz, K.

OpenKnowledge@NAU

Astron. Astrophys. 338, 919–922 (1998) ASTRONOMYANDASTROPHYSICSResearch NotePhotometry of ET Andromedae and pulsation of HD 219891?,??W.W. Weiss1, R. Kuschnig1, D.E. Mkrtichian2, A.V. Kusakin3, T.J. Kreidl4,???, S.J. Bus4, D.J. Osip4, Z. Guo5, J. Hao5,L. Huang5, J.-P. Sareyan6, M. Alvarez7, S.G. Bedolla8,†, J. Zverko9, J. ˇZizˇnˇovsky´9, P. Mittermayer1, K. Zwintz1,N. Polosukhina10, A.V. Mironov3, N.I. Dorokhov2, V.P. Goranskij3, T.N. Dorokhova2, H. Schneider11, and F. Hiesberger11 Institut fu¨r Astronomie der Universita¨t Wien, Tu¨rkenschanzstrasse 17, A-1180 Wien, Austria (e-mail: weiss@galileo.ast.univie.ac.at)2 Astronomical Observatory, Odessa State University, Shevchenko Park, 270014 Odessa, Ukraine3 Sternberg State Astronomical Institute, Universitetskij prospect, 13, 119899 Moscow, Russia4 Northern Arizona University, Box 5100, Flagstaff, AZ 86011, USA5 Beijing Astronomical Observatory, Zong-guan-cun 100080, Beijing, P.R. China6 Observatoire de la Cote d’Azur, Le Mont Gros, F-06003 Nice, France7 Observatorio Astronomico Nacional – UNAM, Apdo. Postal 877, Ensenada, B.C., 22800, Mexico8 Instituto de Astronomia – UNAM, Apdo. Postal 70-264, 04510 Mexico, Mexico9 Astronomical Institute of the Slovak Academy of Science, SK-05960 Tatranska Lomnica, Slovakia10 Crimean Astrophysical Observatory, p/o Nauchny, 334413 Crimea, Ukraine11 Universita¨tssternwarte, Geismarlandstrasse 11, D-37083 Go¨ttingen, GermanyReceived 8 June 1998 / Accepted 23 July 1998Abstract. ET And is a binary system with a B9p(Si) star as themain component. We report on the photometric observing cam-paigns in 1988, 1989 and 1994 which confirmed the rotationperiod of 1.d618875 for ET And while refuting other publishedvalues. Furthermore, the controversial issue of pulsationalstability of ET And is resolved since we have discovered pulsa-tion for HD 219891, which was the main comparison star andsometimes exclusively used. The frequency of 10.0816 d−1,a semi–amplitude of 2.5 mmag, Teff and Mv suggest thiscomparison star to be a δ Scuti variable close to the blue borderof the instability strip. The pulsational stability of ET Andcould be clearly established and hence no need exists to derivenew driving mechanisms for stars between the classical insta-bility strip and the region of slowly pulsating B-type (SPB) stars.Key words: stars: binaries: spectroscopic – stars: chemicallypeculiar – stars: individual: ET And – stars: oscillations – stars:rotationSend offprint requests to: W.W. Weiss? Based on observations obtained at the Bulgarian National Observa-tory, Crimean Astrophysical Observatory (Ukraine), Lowell Observa-tory (USA), Mauna Kea (USA), Mt. Dushak-Erekdak (Turkmenistan),San Pedro (Mexico), Skalnate Pleso (Slovakia), Tien Shan (Kaza-khstan) and Wise Observatory (Israel)?? Dedicated to our esteemed colleague S.G. Bedolla whose untimelydeath greatly saddened us??? Current address: Northern Arizona University, Box 5100,Flagstaff, AZ 86011, USA1. IntroductionThe B9p(Si) star ET And is peculiar and controversial in severalaspects:• It is a hot Si star and a member of an excentric binary sys-tem (e = 0.46) with an orbital period of P = 48.d308 (Ouhrabka& Grygar 1979, Ouhrabka 1981).• ET And is a photometric variable with amplitudes of0.02 mag(V ), 0.03 mag(B), and 0.05 mag(U ) and light minimaat JD = 2441204.d54 + 1.d61887·E (Hildebrandt & Hempelmann1981, Scholz et al. 1985).• Evidence for short time scale variations was reported by Panov(1978) with a period of the order of 140 mn and an amplitudeof about 0.01 mag in V and B, and of 0.02 mag in U , using HD219891 as the comparison star. This was confirmed by Gerth,Scholz & Panov (1984). An additional surprising result was thedetection of radial velocity variations with a semi-amplitude ofabout 4 km s−1 and a period of 0.d1989. Photometry obtainedby Hildebrandt et al. (1985, same comparison star) corroboratethe existence of short periodic variations and the authors claimto have evidence for three more periods between 120 mn and160 mn. Hempelmann & Hildebrandt (1990) published yet an-other short period of 180 mn. No such periods, however, weredetectable in time series of high resolution spectra (Piskunov etal. 1994). It was also speculated that the amplitudes of the shortperiodic variations depend on rotation and orbital phase (Gerth,Scholz & Panov 1984).• Rapid light oscillations with periods between 7 mn and 16 mnand with amplitudes of several millimagnitudes were claimedby Panov (1984).920 W.W. Weiss et al.: (RN) Photometry of ET Andromedae and pulsation of HD 219891Table 1. Synopsis of the photometric observations in 1988–1994.Observers: Al– M. Alvarez, Be– S.G. Bedolla, Do– N.I. Dorokhov,Da– T.N. Dorokhova, Go– A.P. Goranskij, Gu– Z. Guo, Ha– J. Hao,Hi– F. Hiesberger, Hu– L. Huang, Kr– T.J. Kreidl,Ku– A.V. Kusakin, Mi– A.V. Mironov, Mk– D.E. Mkrtichian,Pa– K. Panov, Po– N. Polosukhina, Sa– J.P. Sareyan,Sc– H. Schneider, We– W.W. Weiss, Zi– J. Ziznovsky, Zv– J. ZverkoObserver Observatory Filtersperiod: Oct. ’88 Nov. ’88Kr Lowell 2.–3. 4.–7. ∼BWe MKO 2.–10. B, (V)Zv, Zi Skalnate 3.–22. 5–11. B, (V)Pa BNAO 11.–12. BPo CrAO 4. UBVperiod: Sept. ’89 Oct. ’89Gu, Ha, Hu Xinglong 4.–16. B, (V)Al, Be, Sa San Pedro 7.–14. u,v,b,yKr Lowell 22.–23. 9. ∼BZi, Zv Skalnate 20.–27. 17.–24. B, (V)period: Nov. ’90Ku, Mi Tien Shan 28. B, (V)period: Feb. ’91Ku Tien Shan 21.–22. B, (V)period: Aug. ’94 Sept. ’94Al, Be, Sa San Pedro 13.–17. u,v,b,yDa, Do, Mk Mt. Dushak 10.–18 ∼vGo, Ku Tien Shan 10.–16. 9.–16. B, (V)Hi, Sc Wise 9.–13. B,VThe proof of pulsation for ET And would pose a seriouschallenge to theoretical astrophysics related to stellar modelsand opacities, because this star would clearly fall outside thehot border of the instability strip.In conclusion, the situation concerning the photometricproperties of ET And was very confusing in 1988 when wedecided to focus our attention on this strange object by orga-nizing international spectroscopic and photometric observingcampaigns in 1988, 1989 and 1994 with results to be discussedin this paper.2. Observations and data reductionThe first campaign in 1988 was based on the photometric evi-dence published by Hildebrand et al. (1985) and the same com-parison star (C1 = HD 219891) was chosen. For the 1989 cam-paign A.V.K. & A.V.M. at Tien Shan Observatory (Kazakhstan)added C2 (see Table 2) as a second comparison star and foundindications for C1 being variable with a period of about 0.1days. The hypothesis of a variable primary comparison star wastested further at Tien Shan in 1990 and 1991. For a test of thevariability of the comparison star and the stability of ET Andtowards pulsation a multisite campaign was organized in 1994using HD 218525 as a third comparison star.Table 2. Programme and comparison starsStar HD HR V TypeET And 219749 8861 6.48 B9p(Si)C1 219891 8870 6.50 A5vnC2 219668 8857 6.43 K0ivC3 218525 8806 6.56 A2ivThe main problem with the data of the 1988-89 observationsclearly was the use of a single comparison star (C1), whichhappened to be located in the instability strip.The following describes the sequence for the data reduc-tion. First, we determined the rotation light curve, which is thedominant signal in our photometry. For this purpose we neededto filter all the higher frequency signals and, in particular, theclaimed 140 mn variations which would contribute to ‘noise’ inthe rotation light curve. Hence, the data sets were averaged over140 mn, which resulted in up to two independent data pointsper night. The main period of 1.d61887 (Scholz et al. 1985) wasconfirmed, a phase plot generated, and a rotational light curvecomputed with a cubic spline fit. In a next step, the residuals ofour photometric data were determined relative to this smoothlight curve which resulted in the data set used for the investiga-tion of short time scale variations (Weiss et al. 1989, Kuschniget al. 1990).3. Rotation of ET AndThe rotation period of ET And was discussed by Hildebrandt& Hempelmann (1981) and slightly improved to 1.d61887 byScholz et al. (1985). We have independently confirmed thisvalue on the grounds of our 1988-1989 data which settles anydispute about other rotation periods.Scholz (1986) speculated as to whether the period of 1.d6should not be attributed to g-mode pulsation because of thesimilarity of the period length to what is expected for SPB stars.For two reasons this interpretation appears unlikely to us. First,the shape of the 1.d6 light curve depends strongly on the filtersused, much more than could be accounted for by temperatureand luminosity variations due to radial and non-radial pulsation.Second, from our simultaneous multi-filter photometry we de-rive a null-wavelength at about 3700A˚. Such a null-wavelengthinherently is linked to photospheric inhomogeneities causingbackwarming effects which vary with the visibility of differentparts of the stellar atmosphere, and hence, become detectableonly due to stellar rotation. Indeed, such photospheric inhomo-geneities were determined – for Si and He – with the Dopplerimaging technique (Piskunov et al. 1994).4. Pulsation of HD 219891The 143 mn peak at f1 = 10.084 d−1 is clearly evident in the am-plitude spectrum of C1 (HD 219891) – C2 (HD 219668) magni-tude differencies (Fig. 1, middle panel). A second frequency inthe range from 8 to 12 d−1 might also be present, but the noiseW.W. Weiss et al.: (RN) Photometry of ET Andromedae and pulsation of HD 219891 921Fig. 1. Spectral window and amplitude spectra of 1994 B and v magni-tude differences (C1:HD 219891–C2:HD 219668). Upper panel: spec-tral window, middle panel: original data, lower panel: amplitude spec-trum after substraction of the 10.08 d−1variabilitylevel at these low frequencies is clearly larger than above 20 d−1,which probably is caused by zero-point variations which are dif-ficult to correct in this inhomogeneous data set.The frequency analysis of theB and v data from 1994 showsthat the amplitude at f1 was similar during the previous yearsas was derived from ET And relative to C1 after subtractionof the rotational modulation due to ET And. As can be seenin Fig. 1 (lower panel) the amplitude spectrum of the residualsafter prewhitening the data with f1 does not contain any furthersignificant peaks in the 0 to 100 d−1 frequency domain.Using the Stro¨mgren indices for C1 (HD 219891): (b-y) = 0.094,m1 = 0.184, c1 = 1.051 and β = 2.834, and the empir-ical calibration of Breger (1989) we can estimate Teff = 8270,Mv = 0.83 and log g = 3.777, which locates HD 219891 slightlyabove the ZAMS near the blue edge, but still inside the insta-bility strip of δ Scuti stars. The pulsation constant Q = 0.0269,together with the grid of theoretical pulsation models of A- toF- type stars (Fitch 1981) allows us to estimate the pulsationmode for f1 to be the first overtone of a low degree mode.From our simultaneous Stro¨mgren photometry obtained atSan Pedro we determined the amplitude ratios and phase shiftsfor different filters and we applied the mode identification tech-nique described by Garrido et al. (1990) based on Watson(1988). All phase shifts are negative and range from −1.5◦ to−5.3◦, confirming non-radial pulsation modes. The high rota-Fig. 2. Amplitude spectrum of ET And – C2 (HD 219668) magnitudedifferences of merged 1994 B and v data. The rotational modulationdue to ET And is removed.tional velocity (v sin i = 175 km s−1) and non-radial pulsationwould be consistent with low photometric amplitudes in thevisible, analogous to the rapidly rotating non-radially pulsatingδ Sct stars (Breger 1979).The final proof for the primary comparison star C1(HD 219891) being the pulsating variable is given in Fig. 2where we plot the amplitude spectrum of the magnitude dif-ferences of ET And and C2 (HD 219668) for the B and v dataafter removal of the rotational modulation. The absence of a10.08 d−1variability is clearly evident, as well as the lack ofsignificant peaks in the 0 d−1 to 100 d−1 frequency domain. Aslight increase of amplitudes below 10 d−1 can again be relatedto zero point problems when merging inhomogeneous data.A similar procedure applied to a data set which includes theobservations with the highest intrinsic accuracy, largest dutycycle and frequency resolution (Lowell & MKO in 1988, Xin-glong, San Pedro & Lowell in 1989) resulted in a mean noiselevel of 0.3 mmag(B) from 10 d−1 up to the Nyquist frequencyof about 200 d−1. No amplitude spike exceeds the 99% sig-nificance level, which is 3.6 times the mean noise amplitude(Kuschnig et al. 1997). Hence, we do not find in our data from1988 to 1994 variability for ET And with periods between 7 mnand 13 mn and amplitudes of several millimagnitudes, as weresuspected by Panov (1985).5. ConclusionsThe results from our photometric campaigns on ET And can besummarized as follows:• Spectroscopy (Teff = 12 000 K ± 250 K, log g = 3.65 ±0.25: Piskunov et al., 1994, Kuschnig et al., 1995) as wellas photometry (Teff = 12 000 K, log g = 3.90: Renson et al.1991) put ET And in the domain of late B stars and outside theclassical instability strip.• ET And is stable against pulsation, hence no problems existconcerning the pulsational stability found in this region of theHR-diagram by Dziembowski et al. (1993).• The period and shape of the rotation light curve of ET And isconsistent with observations during the last three decades.• HD 219891 (C1) is slightly evolved and the Teff and Mvvalues put this star inside and close to the blue edge of theδ Scuti instability strip.922 W.W. Weiss et al.: (RN) Photometry of ET Andromedae and pulsation of HD 219891• HD 219891 is probably a δ Sct type pulsating star withthe principal frequency of 10.082 d−1 and a (peak-to-peak)amplitude of close to 5 mmag(B). This variability (attributedpreviously to ET And) seems to have been constant in amplitudeand frequency since its discovery in 1977.Acknowledgements. This research was performed within the workinggroup Asteroseismology along the main sequence – AMS. The authorsthank Don Kurtz, Cape Town, for identifying the problem of an in-sufficient comparison star. RK and WWW express their gratitude tothe Austrian Fonds zur Fo¨rderung der wissenschaftlichen Forschung,grants No. 6927, 8776 and S-7303, and TJK acknowledges supportfrom the National Science Foundation, grant No. AST-8716971 andthe Lowell Observatory endowment. ZG, JH and LH were supportedfrom the National Natural Science Foundation of China, grant No.1870609, and KP from the Bulgarian Ministery of Education and Sci-ence, grant No. F-108. We also wish to thank A. Hempelmann formaking his published PZR data accessible to us in computer readableform.ReferencesBalona L. 1984, MNRAS 211, 973Breger M. 1979, PASP 91, 5Breger M. 1989, Communications in Astroseismology 6, 1Cramer N., Maeder A. 1979, A&A 78, 305Dziembowski W.A., Mosalik P., Pamyatnykh A.A 1993, MNRAS 265,588Fitch W.S. 1981, ApJ 249, 218Garrido R., Garcia-Lobo E., Rodriguez E. 1990, A&A 234, 262Gerth E., Scholz G., Panov K.P. 1984, Astron. Nachr. 305, 79Hauck B., North P. 1993, A&A 269, 403Hildebrandt G., Hempelmann A. 1981, Astron. Nachr. 302, 155Hildebrandt G., Scho¨neich W., Lange D., Zelwanowa E., HempelmannA. 1985, Publ. Astrophys. Obs. Potsdam, Nr. 112Hempelmann A., Hildebrandt G. 1990, Astron. Nachr. 311, 295Kuschnig R., Weiss W. W., Kreidl T. J., et al., 1990, in Confrontationbetween stellar pulsation and evolution, ASP Conference Series,Carla Cacciani & Gisella Clementini edts., Vol. 11, pg.348Kuschnig R., Ryabchikova T., Piskunov N., Weiss W.W., Le ContelJ.M. 1995, A&A 294, 757Kuschnig R., Weiss W.W., Gruber R., Bely P.Y., Jenkner H. 1997, A&A328, 544Me´gessier C. 1988, A&ASS 72, 551North P., Nicolet B., 1990, A&A 228, 78Ouhrabka M., Grygar J., 1979, Inf. Bull. Var. Stars, No. 1600Ouhrabka M., 1981, Publ.Special Aph. Obs, Acad.Sciences USSR,No.32, 45Panov K.P., 1978, Publ. Astron. Inst. Czechoslov. Acad. Sciences,No.54, 19Panov K. P., 1984, in Proc. Magnetic Stars, 6-th Scientific Conference,Institute of Physics, Latvian SSR Academy of Sciences, p. 77Panov K.P., 1985, Inf. Bull. Var. Stars, No. 2825Piskunov N.E., Ryabchikova T.A., Kuschnig R., Weiss W.W. 1994,A&A 291, 910Rakosch K.D., Fiedler W. 1978, A&AS 31, 83Renson P., Kobi D., North P. 1991, A&ASS 89, 61Schneider H., Kreidl T.J., Weiss W.W. 1992, A&A 257, 130Scholz G., Gerth E., Panov K.P. 1985, Astron. Nachr. 306, 329Scholz G. 1986, Astron. Nachr. 307, 21Scho¨neich W., Hildebrandt G., Fu¨rtig W., 1976, Astron. Nachr. 297, 39Stepien, K., Dominiczak, N. 1989, A&A 219, 197Watson R.D. 1988, Ap&SS 140, 255Weiss W.W., Kuschnig R., Kreidl T.J., Zverko J., ˇZizˇnˇovsky´ J., PanovK., Polosukhina N., Hempelmann A., Matthews J., Yang S.,Schneider H., Gerth E., Scholz G. 1989, Astron. Gesell. AbstractSeries 3, 79Weiss W.W., Schneider H. 1984, A&A 135, 148

Photometry of ET Andromedae and pulsation of HD 219891

Astron. Astrophys. 338, 919–922 (1998) ASTRONOMY
AND
ASTROPHYSICS
Research Note
Photometry of ET Andromedae and pulsation of HD 219891?,??
W.W. Weiss1, R. Kuschnig1, D.E. Mkrtichian 2, A.V. Kusakin3, T.J. Kreidl 4,???, S.J. Bus4, D.J. Osip4, Z. Guo5, J. Hao5,
L. Huang5, J.-P. Sareyan6, M. Alvarez7, S.G. Bedolla8,†, J. Zverko9, J. Ži žňovský9, P. Mittermayer1, K. Zwintz 1,
N. Polosukhina10, A.V. Mironov 3, N.I. Dorokhov2, V.P. Goranskij3, T.N. Dorokhova2, H. Schneider11, and F. Hiesberger1
1 Institut für Astronomie der Universität Wien, T̈urkenschanzstrasse 17, A-1180 Wien, Austria (e-mail: weiss@galileo.ast.univie.ac.at)
2 Astronomical Observatory, Odessa State University, Shevchenko Park, 270014 Odessa, Ukraine
3 Sternberg State Astronomical Institute, Universitetskij prospect, 13, 119899 Moscow, Russia
4 Northern Arizona University, Box 5100, Flagstaff, AZ 86011, USA
5 Beijing Astronomical Observatory, Zong-guan-cun 100080, Beijing, P.R. China
6 Observatoire de la Cote d’Azur, Le Mont Gros, F-06003 Nice, France
7 Observatorio Astronomico Nacional – UNAM, Apdo. Postal 877, Ensenada, B.C., 22800, Mexico
8 Instituto de Astronomia – UNAM, Apdo. Postal 70-264, 04510 Mexico, Mexico
9 Astronomical Institute of the Slovak Academy of Science, SK-05960 Tatranska Lomnica, Slovakia
10 Crimean Astrophysical Observatory, p/o Nauchny, 334413 Crimea, Ukraine
11 Universiẗatssternwarte, Geismarlandstrasse 11, D-37083 Göttingen, Germany
Received 8 June 1998 / Accepted 23 July 1998
Abstract. ET And is a binary system with a B9p(Si) star as the
main component. We report on the photometric observing cam-
paigns in 1988, 1989 and 1994 which confirmed the rotation
period of 1.d618875 for ET And while refuting other published
values. Furthermore, the controversial issue of pulsational
stability of ET And is resolved since we have discovered pulsa-
tion for HD 219891, which was the main comparison star and
sometimes exclusively used. The frequency of 10.0816 d−1,
a semi–amplitude of 2.5 mmag,Teff and Mv suggest this
comparison star to be aδ Scuti variable close to the blue border
of the instability strip. The pulsational stability of ET And
could be clearly established and hence no need exists to derive
new driving mechanisms for stars between the classical insta-
bility strip and the region of slowly pulsating B-type (SPB) stars.
Key words: stars: binaries: spectroscopic – stars: chemically
peculiar – stars: individual: ET And – stars: oscillations – stars:
rotation
Send offprint requests to: W.W. Weiss
? Based on observations obtained at the Bulgarian National Observa-
tory, Crimean Astrophysical Observatory (Ukraine), Lowell Observa-
tory (USA), Mauna Kea (USA), Mt. Dushak-Erekdak (Turkmenistan),
San Pedro (Mexico), Skalnate Pleso (Slovakia), Tien Shan (Kaza-
khstan) and Wise Observatory (Israel)
?? Dedicated to our esteemed colleague S.G. Bedolla whose untimely
death greatly saddened us
??? Current address: Northern Arizona University, Box 5100,
Flagstaff, AZ 86011, USA
1. Introduction
The B9p(Si) star ET And is peculiar and controversial in several
aspects:
• It is a hot Si star and a member of an excentric binary sys-
tem (e = 0.46) with an orbital period of P = 48.d308 (Ouhrabka
& Grygar 1979, Ouhrabka 1981).
• ET And is a photometric variable with amplitudes of
0.02 mag(V ), 0.03 mag(B), and 0.05 mag(U ) and light minima
at JD = 2441204.d54 + 1.d61887·E (Hildebrandt & Hempelmann
1981, Scholz et al. 1985).
• Evidence for short time scale variations was reported by Panov
(1978) with a period of the order of 140 mn and an amplitude
of about 0.01 mag inV andB, and of 0.02 mag inU , using HD
219891 as the comparison star. This was confirmed by Gerth,
Scholz & Panov (1984). An additional surprising result was the
detection of radial velocity variations with a semi-amplitude of
about 4 km s−1 and a period of 0.d1989. Photometry obtained
by Hildebrandt et al. (1985, same comparison star) corroborate
the existence of short periodic variations and the authors claim
to have evidence for three more periods between 120 mn and
160 mn. Hempelmann & Hildebrandt (1990) published yet an-
other short period of 180 mn. No such periods, however, were
detectable in time series of high resolution spectra (Piskunov et
l. 1994). It was also speculated that the amplitudes of the short
periodic variations depend on rotation and orbital phase (Gerth,
Scholz & Panov 1984).
• Rapid light oscillations with periods between 7 mn and 16 mn
and with amplitudes of several millimagnitudes were claimed
by Panov (1984).
920 W.W. Weiss et al.: (RN) Photometry of ET Andromedae and pulsation of HD 219891
Table 1.Synopsis of the photometric observations in 1988–1994.
Observers:Al– M. Alvarez, Be– S.G. Bedolla, Do– N.I. Dorokhov,
Da– T.N. Dorokhova, Go– A.P. Goranskij, Gu– Z. Guo, Ha– J. Hao,
Hi– F. Hiesberger, Hu– L. Huang, Kr– T.J. Kreidl,
Ku– A.V. Kusakin, Mi– A.V. Mironov, Mk– D.E. Mkrtichian,
Pa– K. Panov, Po– N. Polosukhina, Sa– J.P. Sareyan,
Sc– H. Schneider, We– W.W. Weiss, Zi– J. Ziznovsky, Zv– J. Zverko
Observer Observatory Filters
period: Oct. ’88 Nov. ’88
Kr Lowell 2.–3. 4.–7. ∼B
We MKO 2.–10. B, (V)
Zv, Zi Skalnate 3.–22. 5–11. B, (V)
Pa BNAO 11.–12. B
Po CrAO 4. UBV
period: Sept. ’89 Oct. ’89
Gu, Ha, Hu Xinglong 4.–16. B, (V)
Al, Be, Sa San Pedro 7.–14. u,v,b,y
Kr Lowell 22.–23. 9. ∼B
Zi, Zv Skalnate 20.–27. 17.–24. B, (V)
period: Nov. ’90
Ku, Mi Tien Shan 28. B, (V)
period: Feb. ’91
Ku Tien Shan 21.–22. B, (V)
period: Aug. ’94 Sept. ’94
Al, Be, Sa San Pedro 13.–17. u,v,b,y
Da, Do, Mk Mt. Dushak 10.–18 ∼v
Go, Ku Tien Shan 10.–16. 9.–16. B, (V)
Hi, Sc Wise 9.–13. B,V
The proof of pulsation for ET And would pose a serious
challenge to theoretical astrophysics related to stellar models
and opacities, because this star would clearly fall outside the
hot border of the instability strip.
In conclusion, the situation concerning the photometric
properties of ET And was very confusing in 1988 when we
decided to focus our attention on this strange object by orga-
nizing international spectroscopic and photometric observing
campaigns in 1988, 1989 and 1994 with results to be discussed
in this paper.
2. Observations and data reduction
The first campaign in 1988 was based on the photometric evi-
dence published by Hildebrand et al. (1985) and the same com-
parison star (C1 = HD 219891) was chosen. For the 1989 cam-
paign A.V.K. & A.V.M. at Tien Shan Observatory (Kazakhstan)
added C2 (see Table 2) as a second comparison star and found
indications for C1 being variable with a period of about 0.1
days. The hypothesis of a variable primary comparison star was
tested further at Tien Shan in 1990 and 1991. For a test of the
variability of the comparison star and the stability of ET And
towards pulsation a multisite campaign was organized in 1994
using HD 218525 as a third comparison star.
Table 2.Programme and comparison stars
Star HD HR V Type
ET And 219749 8861 6.48 B9p(Si)
C1 219891 8870 6.50 A5vn
C2 219668 8857 6.43 K0iv
C3 218525 8806 6.56 A2iv
The main problem with the data of the 1988-89 observations
clearly was the use of a single comparison star (C1), which
happened to be located in the instability strip.
The following describes the sequence for the data reduc-
tion. First, we determined the rotation light curve, which is the
dominant signal in our photometry. For this purpose we needed
to filter all the higher frequency signals and, in particular, the
claimed 140 mn variations which would contribute to ‘noise’ in
the rotation light curve. Hence, the data sets were averaged over
140 mn, which resulted in up to two independent data points
per night. The main period of 1.d61887 (Scholz et al. 1985) was
confirmed, a phase plot generated, and a rotational light curve
computed with a cubic spline fit. In a next step, the residuals of
our photometric data were determined relative to this smooth
light curve which resulted in the data set used for the investiga-
tion of short time scale variations (Weiss et al. 1989, Kuschnig
et al. 1990).
3. Rotation of ET And
The rotation period of ET And was discussed by Hildebrandt
& Hempelmann (1981) and slightly improved to 1.d61887 by
Scholz et al. (1985). We have independently confirmed this
value on the grounds of our 1988-1989 data which settles any
dispute about other rotation periods.
Scholz (1986) speculated as to whether the period of 1.d6
should not be attributed to g-mode pulsation because of the
similarity of the period length to what is expected for SPB stars.
For two reasons this interpretation appears unlikely to us. First,
the shape of the 1.d6 light curve depends strongly on the filters
used, much more than could be accounted for by temperature
and luminosity variations due to radial and non-radial pulsation.
Second, from our simultaneous multi-filter photometry we de-
rive a null-wavelength at about 3700Å. Such a null-wavelength
inherently is linked to photospheric inhomogeneities causing
backwarming effects which vary with the visibility of different
parts of the stellar atmosphere, and hence, become detectable
only due to stellar rotation. Indeed, such photospheric inhomo-
geneities were determined – for Si and He – with the Doppler
imaging technique (Piskunov et al. 1994).
4. Pulsation of HD 219891
The 143 mn peak at f1 = 10.084 d−1 is clearly evident in the am-
plitude spectrum of C1 (HD 219891) – C2 (HD 219668) magni-
tude differencies (Fig. 1, middle panel). A second frequency in
the range from 8 to 12 d−1 might also be present, but the noise
W.W. Weiss et al.: (RN) Photometry of ET Andromedae and pulsation of HD 219891 921
Fig. 1.Spectral window and amplitude spectra of 1994 B and v magni-
tude differences (C1:HD 219891–C2:HD 219668). Upper panel: spec-
tral window, middle panel: original data, lower panel: amplitude spec-
trum after substraction of the 10.08 d−1variability
level at these low frequencies is clearly larger than above 20 d−1,
which probably is caused by zero-point variations which are dif-
ficult to correct in this inhomogeneous data set.
The frequency analysis of theB andv data from 1994 shows
that the amplitude at f1 was similar during the previous years
as was derived from ET And relative to C1 after subtraction
of the rotational modulation due to ET And. As can be seen
in Fig. 1 (lower panel) the amplitude spectrum of the residuals
after prewhitening the data with f1 does not contain any further
significant peaks in the 0 to 100 d−1 frequency domain.
Using the Str̈omgren indices for C1 (HD 219891): (b-
y) = 0.094,m1 = 0.184,c1 = 1.051 andβ = 2.834, and the empir-
ical calibration of Breger (1989) we can estimateTeff = 8270,
Mv = 0.83 andlog g = 3.777, which locates HD 219891 slightly
above the ZAMS near the blue edge, but still inside the insta-
bility strip of δ Scuti stars. The pulsation constant Q = 0.0269,
together with the grid of theoretical pulsation models of A- to
F- type stars (Fitch 1981) allows us to estimate the pulsation
mode for f1 to be the first overtone of a low degree mode.
From our simultaneous Strömgren photometry obtained at
San Pedro we determined the amplitude ratios and phase shifts
for different filters and we applied the mode identification tech-
nique described by Garrido et al. (1990) based on Watson
(1988). All phase shifts are negative and range from−1.5◦ to
−5.3◦, confirming non-radial pulsation modes. The high rota-
Fig. 2. Amplitude spectrum of ET And – C2 (HD 219668) magnitude
differences of merged 1994B andv data. The rotational modulation
due to ET And is removed.
tional velocity (v sin i = 175 km s−1) and non-radial pulsation
would be consistent with low photometric amplitudes in the
visible, analogous to the rapidly rotating non-radially pulsating
δ Sct stars (Breger 1979).
The final proof for the primary comparison star C1
(HD 219891) being the pulsating variable is given in Fig. 2
where we plot the amplitude spectrum of the magnitude dif-
ferences of ET And and C2 (HD 219668) for theB andv data
after removal of the rotational modulation. The absence of a
10.08 d−1variability is clearly evident, as well as the lack of
significant peaks in the 0 d−1 to 100 d−1 frequency domain. A
slight increase of amplitudes below 10 d−1 can again be related
to zero point problems when merging inhomogeneous data.
A similar procedure applied to a data set which includes the
observations with the highest intrinsic accuracy, largest duty
cycle and frequency resolution (Lowell & MKO in 1988, Xin-
glong, San Pedro & Lowell in 1989) resulted in a mean noise
level of 0.3 mmag(B) from 10 d−1 up to the Nyquist frequency
of about 200 d−1. No amplitude spike exceeds the 99% sig-
nificance level, which is 3.6 times the mean noise amplitude
(Kuschnig et al. 1997). Hence, we do not find in our data from
1988 to 1994 variability for ET And with periods between 7 mn
and 13 mn and amplitudes of several millimagnitudes, as were
suspected by Panov (1985).
5. Conclusions
The results from our photometric campaigns on ET And can be
summarized as follows:
• Spectroscopy (Teff = 12 000 K ± 250 K, log g = 3.65 ±
0.25: Piskunov et al., 1994, Kuschnig et al., 1995) as well
s photometry (Teff = 12 000 K, log g = 3.90: Renson et al.
1991) put ET And in the domain of late B stars and outside the
classical instability strip.
• ET And is stable against pulsation, hence no problems exist
concerning the pulsational stability found in this region of the
HR-diagram by Dziembowski et al. (1993).
• The period and shape of the rotation light curve of ET And is
consistent with observations during the last three decades.
• HD 219891 (C1) is slightly evolved and theTeff and Mv
values put this star inside and close to the blue edge of the
δ Scuti instability strip.
922 W.W. Weiss et al.: (RN) Photometry of ET Andromedae and pulsation of HD 219891
• HD 219891 is probably aδ Sct type pulsating star with
the principal frequency of 10.082 d−1 and a (peak-to-peak)
amplitude of close to 5 mmag(B). This variability (attributed
previously to ET And) seems to have been constant in amplitude
and frequency since its discovery in 1977.
Acknowledgements.This research was performed within the working
groupAsteroseismology along the main sequence – AMS. The authors
thank Don Kurtz, Cape Town, for identifying the problem of an in-
sufficient comparison star. RK and WWW express their gratitude to
the Austrian Fonds zur F̈orderung der wissenschaftlichen Forschung,
grants No. 6927, 8776 and S-7303, and TJK acknowledges support
from the National Science Foundation, grant No. AST-8716971 and
the Lowell Observatory endowment. ZG, JH and LH were supported
from the National Natural Science Foundation of China, grant No.
1870609, and KP from the Bulgarian Ministery of Education and Sci-
ence, grant No. F-108. We also wish to thank A. Hempelmann for
making his published PZR data accessible to us in computer readable
form.
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Photometry of ET Andromedae and pulsation of HD 219891

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