Astron. Astrophys. 334, 873-894 (1998)

2. Photometry: the rotational period

Photometric variability in P1724 was first reported by Cutispoto et al. (1996) ¹, based on a total of eleven UBVRI observations obtained over a period of 13 nights. They estimated the amplitude of the variations in the V band to be mag with a period of days interpreted as due to rotational modulation by spots. Although the Cutispoto et al. light curve appears regular and well-defined, their observations do not allow one to rule out completely the possibility that the true, shorter period has been missed due to undersampling. It is sometimes found that this is the case when much finer sampling is subsequently obtained (e.g., Bouvier et al. 1993).

To investigate this possibility, we monitored P1724 extensively for six weeks with several photometric observations per night in order not to miss possibly very short periods. The star was observed using the Schmidt-Cassegrain telescope located on the roof of the Department of Earth and Space Sciences building at the State University of New York at Stony Brook. The `Mount Stony Brook' (MTSB) telescope is equipped with a thermoelectrically cooled SBIG ST-6 CCD (cf. Wolk 1996 for more information). Observations were made on 17 nights between 20 Sep 1995 and 11 Nov 1995. In total, 68 observations were made in each of the Johnson V, R, and I filters, four per night.

Additional data were taken from CTIO using the 0.9m telescope, between 30 Jan and 3 Feb 1996. The same field was observed a total of fifteen times in each filter. While the CTIO data had coarse temporal coverage, they had much higher S/N. We also expected that by comparing the results from the independent data sets our conclusions would be strengthened.

Three field stars were used for comparison in both cases. After the data were debiased and flat fielded, fluxes within a ( for CTIO) radius of P1724 and the three comparison stars were summed. An annulus with an inner radius of and an outer radius of was used for background subtraction. The star JW 242, located four arc sec south of P1724, is fainter than P1724 by several magnitudes (see Sect. 10), and therefore it does not influence the photometry.

Two independent numerical period searching routines were used: a phase dispersion minimization method (Stellingwerf 1978) and a fast Lomb periodogram (Press & Rybicki 1989). The period searches returned three main periods: 0.85 days, 1.21 days, and 5.7 days. Other weaker peaks in the power spectrum are artifacts due to the window function. In order to assess the reality of each of these periods, we experimented by fitting a variety of functions to the data and examining the fits for any residual trends during each night. The 1.2-day periodicity is easily ruled out in this way, but the situation is more ambiguous with the other two. We then applied the CLEAN algorithm (Roberts et al. 1987), which is very effective in discriminating real peaks from aliases in the power spectrum. The result in the V band for the MTSB data set is shown in Fig. 1a, where the single peak corresponds to a period of about 5.7 days. The formal false alarm probability is below . Thus, the result of Cutispoto et al. (1996) is confirmed. Phased MTSB V -band data are shown in Fig. 1b.

Fig. 1. V -band variation of P1724. a Cleaned power spectrum showing one peak with a period of days, power units are arbitrary. b V -band observations of MTSB folded with the 5.7-day period starting at the epoch of maximum brightness (see text)

To produce our best estimate of the rotational period we combined the MTSB and CTIO data sets. We fitted sine curves with a fixed offset and amplitude independently to the V, R, and I curves, and iterated on the phase shift and period until the residuals were minimized. Although the light curve is probably not strictly sinusoidal, inspection of the fits revealed that a sine curve is a sufficiently good approximation for our purposes. In all passbands the result is similar, and the average period is days, with the first maximum occurring at JD 2449984.828. The formal uncertainty in the period is probably an underestimate, due to shortcomings in the model and the very nature of the variation, which is not likely to be strictly periodic. We discuss this in more detail in Sect. 4.

Aside from the MTSB and CTIO observations, one of us (RW) obtained additional photometry in the V and (Cousins R) bands at three different sites; we refer to this as the RW data set. In Dec 1995 observations were made at the 1.23m telescope on Calar Alto Observatory, equipped with the TEK CCD #6 ( pixels, FOV arcmin). Between Jan and Mar 1996, observations were made at the 70cm telescope of Landessternwarte Heidelberg-Königstuhl (Germany), using a CCD camera with the GEC P8603 CCD ( pixels, FOV arcmin). Finally, observations with the 91cm Dutch telescope at ESO were made during a run from 1 to 11 May 1996. The instrument was a CCD camera with the ESO #33 TEK CCD ( pixels, FOV arcmin). During this run P1724 was observed once per night, at the beginning of the night. No standard stars were observed during these runs, and the photometry is therefore strictly differential. Instrumental magnitudes were obtained using PSF fitting routines from the IRAF ² package daophot. We combined the data obtained at these three sites into our RW data set, and searched for periodicities as above. The same 5.7-day period is found (see Fig. 3e).

© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998

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