Period Determination Study of Selected RV Tauri Stars

Mark Kelly, Dr. Pamela Gay (Advisor)Centre for Astrophysics and Supercomputing

Swinburne University of Technology

Figure 1. Period determination for AC Her (74.6 +/- 0.2d) Phase Diagram over two cycles. (Mean amplitude=0.636, Range+6.8-9.0).

References[1] AAVSO web: American Association of Variable Star Observers

homepage, http://www.aavso.org/ (accessed 28 August)

[2] Foster G., 1995, AJ, 109, 4

[3] Kholopov, P., Samus, N., Frolov, M., Goranskij, V., Gorynya, N., Karitskaya, E., et al. 1998, General Catalogue of Variable Stars, 4.1, via the SIMBAD database.

[4] Takeuti, M., Peterson, J., 1983, A&A, 117, 352

[5] Jacoby, G., Hunter, D., & Christian, C., 1984, AJSS, 56, 275

[6] Preston, G. W., Krzeminski, W., Smak, J., Williams, J. A., 1963, ApJ, 137, 401

Methodology:The photometric analysis in this project used archival

visual and CDD (V) data from the American Association of Variable Star Observers (AAVSO web) [1]. Spectral data was obtained with cooperation from the NOAO educational outreach program, ARBSE. The maximum and minimum brightness was measured using statistical analysis of the phase diagrams (Figure 1). The periods for varying brightness were determined using the analysis of variance (ANOVA) statistic and compared to the Lomb-Scargle periodogram. Period determination was also performed using Foster’s Fourier/CLEANest time-series analysis [2].

SummaryFour stars, AC Her, R Sct, V Vul, and SS Gem, show no

variation of mean magnitude over successive cycles through at least one decade of data. These stars belong to the RVa class of stars. AC Her and V Vul show complex patterns possibly due to the presence of two or more oscillating modes with a ratio about 2:1. Only one star, U Mon shows significant change in average brightness and belongs to the RVb class of stars. One star, TT Oph displays few or no RV Tauri characteristics.

Results & DiscussionFour of the six program stars have consistent mean

magnitudes over many cycles, suggesting that they belong to the RVa class. Sufficient archival photometric data for all the stars allowed time-series analysis and spectral data was available for analysis on three of these stars. Only one program star, U Mon, had a varying mean magnitude (RVb).

Table 1: Time-series period determinations using the ANOVA statistic, Lomb-Scargle periodogram, and the Fourier transform (Foster Method). Periods found in the GCVS SIMBAD database [3] are also shown for comparison.

Example RVa type star (AC Her): In the time-series analysis for the well-known RV Tauri star, AC Her, the mean magnitude between deep minima and shallow minima does not vary and is consistent with its RVa classification (Figure 1). There are no obvious long-term variations over the 18+ years of archival data. The transform model yields a sinusoidal curve based on one harmonic frequency shown based on 75 days and a beat frequency of 136 days (Figure 2). This seems consistent with the resonance hypothesis proposed by Takeuti and Peterson [4] suggesting that alternating minima might be caused by stars pulsating at cycles whose ratios are close to 2:1.

Figure 2 . A model function using Fourier time-series analysis (red) for AC Her and a polynomial best fit curve (blue) showing a consistent mean magnitude between cycles, indicative of a RVa type.

Abstract: Using photometric data on magnitudes from RV Tauri stars, I studied the dominant dynamical characteristics for various stars within this type of variable star. Time-series analysis was conducted on six bright RV Tauri stars, namely AC Her, R Sct, TT Oph, V Vul, SS Gem, and U Mon over a time span of at least 4,700 days . The research goal was to add to our understanding of the varying brightness of RV Tauri stars, and how they fit into the scheme of stellar evolution. Two stars are examined in detail. AC Her, an example of a RVa sub-type, shows no obvious long-term variations. In contrast, the RVb sub-type star, U Mon, shows very pronounced long-term variations over the 10+ years of examined archival data.

Name ANOVA Lomb-Scargle Fourier GCVS Observations Time Span

AC Her 74.6 +/- 0.2d 75.4 +/-0.1d75.4d +/- 0.02d 136.4 +/- 0.4d 75.01 17,120 7,243d

R Sct138.6 +/-

0.7d 138.4 +/- 0.7d 139.41 +/- 0.08d 146.5 60,945 8,054d

TT Oph60.9d +/-

0.1d 60.9 +/- 0.07d 61.2 +/- 0.2d 61.08 1,331 4,742d

SS Gem 89.2 +/- 0.4d 89.2 +/- 0.4d89.16 +/- 0.02d50.73 +/- 0.01d 89.31 8,324 9,737d

V Vul 76.3 +/- 0.2d 76.5 +/- 0.2d76.50 +/- 0.01d155.3 +/- 3.6d 75.72 9,668 23,107d

U Mon 91.0 +/-0.3d 94.2d+/00.1d

90.1d+/-0.1d2,403.9d+/-

305.6d 91.32 14,149 4,741d

The spectral data for AC Her to the Jacoby Atlas [5] yields a F4III classification and closely matches the F2p classification found in the GCVS (Figure 3). The star was near its maximum when this spectral data was collected (JD2453549). The CH band at λ 3880 and CN Band at λ 3883 are of normal strength indicating that this star could be sub-classified spectroscopically as a type B using Preston’s 1963 study of RV Tauri spectra [6]..

Figure 3. Spectrum for AC Her. (data reduced from spectral images provided by the Astronomy Research Based Science Education (ARSE) team at NOAO).