Short-Period Variables in ASAS Photometry

Abstract

We present results of our study of short–period variables in ASAS photometry. We focus on slowly pulsating B stars (SPBs), most of which were discovered by the Hipparcos satellite. We discuss the problem of aliasing in the ASAS photometry and its influence on the identification of frequencies of SPBs. Finally, we show that the ASAS photometry can be used not only to confirm the Hipparcos frequencies and amplitudes but it can also give new results for selected SPBs. Introduction The ASAS (All Sky Automatic Survey) project aims for monitoring variability of all objects in the sky brighter that 14 mag. Its prototype instrument and data pipeline were developed by Dr. Pojmański (Pojmański 1997). In 1997–2000 ASAS observed in the I–band (ASAS– 2 project) and since 2001 in V (ASAS–3 project). It succeeded in discovering more than 5 500 new variables (Pojmański 2000 and 2002) and was used for studying Cepheids in the Magellanic Clouds (Pietrukowicz 2001, 2002) and the Galaxy (Beltrame & Poretti 2002). In this work, we analyze ASAS–3 photometry of stars classified as SPB in the Hipparcos Catalogue (ESA 1997). We discuss ASAS observing and spectral window and present results obtained for selected stars. Observing and Spectral Windows In the ASAS–3 observing schedule each of the preselected fields is observed once per night. However, the fields overlap so that many stars are observed several times per night. We show a typical ASAS observing window in the left panel of Fig. 2. This window (compiled for HD52057 from three separate data sets) covers a time span of 2.3 years and consists of 152 observations indicated by impulses. In this figure, Observations are gathered in three clumps inside which sampling is close to one per night. Observations sampled more frequently are rare but also present. Such sampling produces significant daily aliases in the spectral window; in the left panel of Fig. 1 we show an example – the spectral window computed for IS Lup. Windows of this shape, with daily aliases getting lower for higher frequencies, can be computed for ∼ 50% stars observed by ASAS. The other stars have spectral windows with a 2 c/d peak higher than the one at 1 c/d. We find such spectral windows for stars which were observed at the beginning and then at the end of the majority of observing nights. As an example, we show the spectral window for HD52057 in the middle panel of Fig. 1 . For few stars we find spectral windows with the 3 c/d peak higher than the 1 and 2 c/d ones. Windows of this shape occur for stars which are observed sparsely and for which observations are clumped in J. Molenda–Żakowicz 49 8.86 8.87 8.88 8.89 8.9 8.91 8.92 8.93 8.94 8.95 8.96 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 ’032101.hp’