Delta Scuti

We analyzed data accumulated during 2005 and 2006 by the Taiwan-American Occultation Survey (TAOS) in order to detect short-period variable stars (periods of < 1 hour) such as δ Scuti (see Kim+ 2010). TAOS is designed for the detection of stellar occultation by small-size Kuiper Belt Objects (KBOs) and is operating four 50cm telescopes at an effective cadence of 5Hz. The four telescopes simultaneously monitor the same patch of the sky in order to reduce false positives. To detect short-period variables, we first removed trends in the TAOS light curves using PDT (Kim+ 2009). Figure 1 shows example light curves of a δ Scuti star before and after removing trends. It is clear that PDT removes trends successfully.

[ Figure 1. An example light curve of a TAOS .δ Scuti star before (top) and after de-trending (bottom). The periodic signals contaminated by unstable weather (e.g. moving clouds) are successfully recovered. In the top panel, the measurement uncertainty for each data point is shown. ]

We then used the Fast Fourier Transform algorithm (FFT) inasmuch as the data points in TAOS light-curves are evenly spaced. Using FFT, we found 41 short-period variables with amplitudes smaller than a few hundredths of a magnitude and periods of about an hour, which suggest that they are low-amplitude δ Scuti stars (LADS). Figure 2 show the spectral window and the power spectrum of one TAOS δ Scuti star.

[ Figure 2. The spectral window and the power spectrum of the star ID 054.00014. We detected three frequencies (dashed lines) using PERIOD04. The top panel: the spectral window, the middle panel: the first two frequencies, and the bottom panel: the power spectrum after whitening the two frequencies. ]

We found two peculiar spectral type δ Scuti stars which have B8 and G5 spectral types. These are the bluest and reddest spectral types of δ Scuti stars ever detected. To confirm their spectral types, we observed the two stars with two spectroscopic instruments, each of which is BOES of the 1.8-m telescope at the Bohyunsan Optical Astronomy Observatory (BOAO), South Korea, and the FAST instrument mounted at the Fred Lawrence Whipple Observatory (FLWO) 1.5m telescope, Mount Hopkins in Arizona. After checking their observed spectra, we found their spectral types are likely A5 and F0 stars, not B8 and G5 stars. Figure 3 shows the spectrum of the A5 star.

[ Figure 3. The normalized spectrum of the B8 candidate star. There are strong Ca II K line and weak metallic lines, which is typical for A type stars. The star is likely an A5 type star rather than B8. ]

The work is published in AJ (Kim+ 2010).