We present CCD time-series photometry of BQ Ind, which is either a Population I
large-amplitude δ Scuti star or an SX Phoenicis star

Brogt, E.

Fu, J.-N.

Sterken, C.

Zhou, A.Y.

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2002: The Journal of Astronomical Data 8, 3. c© C. Sterken et al.CCD time-series photometry of BQ Ind∗C. Sterken1, E. Brogt2, J.N. Fu1 and A.Y. Zhou3(1) University of Brussels (VUB), Pleinlaan 2, 1050 Brussels, Belgium(2) University of Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands(3) National Astronomical Observatories, CAS, 100012 Beijing, ChinaReceived 03 December 2002; accepted 19 December 2002AbstractWe present CCD time-series photometry of BQ Ind, which is either a Population Ilarge-amplitude δ Scuti star or an SX Phoenicis star.1 IntroductionBQ Ind (HD 198830 = HIP 103290) was discovered to be a pulsating star by theHipparcos satellite (ESA 1997), with P = 0.d0820,ΔV = 0.m250 (Rodr´ıguez et al.2000), but no ground-based observations of BQ Ind were so far reported in theliterature.Sterken et al. (2003) obtained new data and in a first analysis reported thepresence of a primary f1 = 12.1951 d−1 and a secondary frequency f2 = 15.7686 d−1(see Fig. 1 as an example). The period ratio P2/P1 = 0.7734 made them to suggestthat BQ Ind is a double-mode SX Phe star. (SX Phoenicis stars are pulsatingPopulation II stars with low metallicities and high spatial motions which are foundin the blue-straggler region in a globular cluster H-R diagram and within the lowestsection of the classical Cepheid instability strip).Double-mode SX Phe stars are rare, but time-series observations possibly allowto identify the pulsation modes in some cases and to infer some physical properties ofthe star from the ratio between the two periods and the main-mode period (Petersen& Christensen-Dalsgaard 1996).∗Based on observations obtained at ESO La Silla Observatory.12 C. Sterken, E. Brogt, J.N. Fu and A.Y. ZhouFigure 1: Light curves of BQ Ind in b, July 2001 (Sterken et al. 2003).2 The dataWe obtained Stro¨mgren b and v time series in July 2001 at the Dutch 0.90-m tele-scope (510 × 510 CCD detector, ESO La Silla) and another sequence of 68 framesin Johnson V at the Danish 1.54-m telescope (DFOSC camera, 1024 × 1024 CCD)in September 2001. All frames were bias-corrected and flatfielded. Figure 2 illus-trates the field (±7 arcmin diameter) surrounding BQ Ind. Tables 1 and 2 givethe coordinates of the comparison stars. Data reduction was carried out using thesoftware package MOMF (Kjeldsen & Frandsen 1992) that combines aspects of PSFand aperture photometry. We shown in Fig. 3 an example light curve (Johnson Vband).2.1 Dutch telescope CCD framesThe field of the Dutch camera being small, these frames only contain BQ Ind and theneighboring stars GSC 8800 1107, 872 and 678. These nearby stars served as com-parison stars and are much fainter than the program star, which unavoidably resultsin CCD frames that are at times overexposed. This problem has been thoroughlydiscussed in Sterken et al. (2003).In order to save disk space, the individual CCD images have been decomposed inCCD time-series photometry of BQ Ind 3Figure 2: BQ Ind field at the Danish telescope. N is up and E is left. Comparisonstars GSC 8800-1107, -0872, -0678 and -0069 are indicated.Table 1: Identification of the stars in the Dutch-telescope frames.Identif. α2000 δ2000Variable BQ Ind=HD 198830 20:55:33.67 -56:44:31.10Comp1 GSC 8800-01107 20:55:24.084 -56:44:24.91Comp2 GSC 8800-00678 20:55:21.983 -56:45:24.88Comp3 GSC 8800-00872 20:55:32.714 -56:45:21.45Comp4 GSC 8800-00005 20:55:48.444 -56:46:03.93sections of 60×60 pixels, and were recombined in a set of artificial images (see Fig. 4as an example). The b images are available in directory dutch_b, the reconstructedv frames are in directory dutch_v. The ASCII tables with the differential results(HJD, magnitude) are in files dutch_b.dat and dutch_v.dat (460 and 1156 usefulframes, respectively)1.Areas with the same size of 60 × 60 pixels around BQ Ind and around each ofthe comparison stars was combined into one artificial field in which every sub-imagewas separated from the preceding one by a 1-pixel wide column with ADU of abouttwice the background level, yielding a 304×50 array. The order of stars in the imageis, from left to right: BQ Ind, Comp1, Comp2, Comp3 and Comp4. The raw imageheader remained untouched.Note that comparison stars 1–3 are the same stars in both sets, but that the1The discrepancy between the number of b and v frames is to be ascribed to the fact that morethan 50% of the b frames have saturated images4 C. Sterken, E. Brogt, J.N. Fu and A.Y. Zhou2452174.52 2452174.54 2452174.56 2452174.58HJD-3.95-4.00-4.05-4.10-4.15-4.20-4.25Differential magnitudeBQ IndJohnson V bandFigure 3: Johnson V light curve of BQ Ind.fourth comparison star is not.2.2 Danish telescope CCD framesThe DFOSC frames also recorded GSC 8800-0069, a nearby (but very red) starof brightness comparable to BQ Ind. Areas with the same size of 50 × 50 pixelsaround BQ Ind and around each of the 4 comparison stars was combined into oneartificial field in which every sub-image was separated from the preceding one bya 1-pixel wide column with ADU amounting to about twice the background level,yielding a 254 × 60 array. The order of stars in the image is, from left to right:BQ Ind Variable, comp1, comp2, comp3, comp5. The raw image header remaineduntouched. 68 frames were reconstructed, and the rearranged frames are in directorydanish_v, the ASCII tables with the differential results are in file danish_v.dat.CCD time-series photometry of BQ Ind 5Table 2: Identification of the stars in the Danish-telescope frames.Identif. α2000 δ2000Variable BQ Ind=HD 198830 20:55:33.67 -56:44:31.10Comp1 GSC 8800-01107 20:55:24.084 -56:44:24.91Comp2 GSC 8800-00678 20:55:21.983 -56:45:24.88Comp3 GSC 8800-00872 20:55:32.714 -56:45:21.45Comp5 GSC 8800-00069 20:55:22.061 -56:48:04.01Figure 4: Reconstructed frames: Danish telescope (top), Dutch telescope (bottom).AcknowledgementsThe authors acknowledge a research grant from the Belgian Fund for Scientific Re-search (FWO) and the Vrije Universiteit Brussel. This project was supported by theFlemish Ministry for Foreign Policy, European Affairs, Science and Technology, un-der contract BIL 99/2, and by the Danish Natural Science Research Council throughthe centre for Ground-Based Observational Astronomy. This work has been sup-ported by “IUAP P5/36” Interuniversity Attraction Poles Programme of the BelgianFederal Office for Scientific, Technical and Cultural Affairs, and the Belgian Fundfor Scientific Research (FWO).ReferencesKjeldsen H., Frandsen S. 1992, PASP 104, 413Petersen J.O., Christensen-Dalsgaard J. 1996, A&A 312, 463Rodr´ıguez E., Lo´pez-Gonza´lez M.J. 2000, A&A 359, 597Sterken C., Fu J.N., Brogt E. 2003, in Interplay between periodic, cyclic and stochasticvariability in selected areas of the H-R diagram, ASP Conf. Ser. in pressSterken C., Zhou, A.Y., Fu J.N. 2003, Comm. in Asteroseismology, submitted

CCD time-series photometry of BQ Ind

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CCD time-series photometry of BQ Ind

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