Spots, multiplets, and new frequencies: new MOST observations of roAp stars M. Gruberbauer1,2, D. Huber3, R. Kuschnig2, W. W. Weiss2, D.B. Guenther1, J.M. Matthews4, 5 6 7 8 A. F. J. Moffat , S. Rucinski , D. Sasselov , G. A. H. Walker 1 - Saint Mary’s University, Halifax, Canada; 2 - University of Vienna, Austria; 3 - University of Sydney, Australia; 4 - University of British Columbia, Vancouver, Canada; 5 - University of Montreal, Canada; 6 - Harvard Smithsonian Center for Astrophysics, U.S.A; 7 - University of Toronto, Canada; 8 - University of Victoria, Canada
Abstract
HD 134214
What’s new?
• roAp stars are chemically peculiar, pulsating stars with global magnetic fields ( ~ several kG) and spectral types ranging from A-F. They usually show a small number of excited p-modes with low amplitudes (< 13 mmag) and short periods (5 - 20 minutes, high radial order). The oscillations are often observed to exhibit modulation with the stellar rotation period, which can be explained by the oblique pulsator model introduced by Kurtz et al. (1982). • MOST is a Canadian micro-satellite in low-Earth orbit built for high-precision visual band photometry (Walker 2003). It has delivered data for over 1,100 stars since its launch in June 2003. MOST has several different modes of observation. • All stars presented here have been observed in Direct Imaging (DI) mode, and the data has been reduced using the DI pipeline initially developed by Huber (see Huber & Reegen). The frequency analysis has been performed using the SigSpec 2.0 software (Reegen 2007), which fits a multi-cosine model to a time series according to a robust false-alarm probability statistic.
• Only the primary pulsation frequency reported by Kurtz (1994) can be confirmed, but 5 new pulsation frequencies were identified that constitute two different, multiplets (see Fig. 2).
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ν2
HD 99563
1.5
ν2 = ν1 − νrot ν3 = ν1 + νrot ν4 = ν1 − 2νrot ν5 = ν1 + 2νrot ν6 = ν1 − 3νrot ν7 = ν1 + 3νrot ν8 ∼ ν1 − 0.5νrot
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HD 9289 ν1
ν2 = ν1 − 2νrot ν3 = ν1 + 2νrot ν5 = ν4 − νrot ν6 = ν4 + 3νrot
ν2 ν5 ν4
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instrumental magnitude
• The new MOST data set was heavily influenced by instrumental artifacts, but careful analysis reveals that most of the Kurtz et al. (2007)-frequencies can be confirmed. • We also detect the first harmonic to the primary pulsation frequency. We also find 4 new candidate frequencies (see Fig. 3). This results in 9 frequencies never before seen in photometry, for which reliable values could be established. The semi-regular spacing (see the right-most 3 frequencies in Fig. 3) suggests Δν ≈ 68 μHz, similar to the roAp star HD 24712.
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Fig.2: Schematic pulsation multiplets of HD 99563 and HD 9289 superimposed on the residual spectrum (solid lines) to SigSpecʼs multi-cosine fit. We find a new candidate frequency, ν8, in the spectrum of HD99563. HD 9289 reveals two multiplets, both of which are most likely quintuplets with missing components or components below the detection threshold.
What was already known: • Well-studied mono-periodic oblique pulsator with distorted dipole (Handler et al. 2006, Freyhammer et al. 2009)
-4.255
What’s new?
0
HD 99563 was observed in 2008 for 27.53 days, and 62261 stacked images were obtained. The final light curve after data reduction consists of 55176 data points spanning 26.68 days.
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•A single, prominent frequency at 2949.6 μHz with a high amplitude as observed using ground based photometry (Kreidl & Kurtz 1986) • The detection of 5 - 7 additional frequencies in highquality spectroscopic data was claimed, and a new kind of upper atmospheric variability was proposed to be the cause (Kurtz et al. 2007). • A previous, short 10.27 hour test run of MOST photometry in 2006 (Cameron et al., 2006) could not detect any of these new frequencies.
ν3
ν6
HD 99653 -4.265
What was already known:
0.2
amplitude [mmag]
Introduction
amplitude [mmag]
Here we present new results based on recent MOST observations of the roAp stars HD 9289, HD 99563, and HD 134214.
• Data shows strong evidence for rotational modulation due to spots. • We employ a Bayesian nested sampling technique (Feroz et al. 2009) to fit circular spots (Dorren 1987) to the data. We find that models with at least 3 dark spots outperform models with fewer components by far (see Fig. 1), and we can also reliably determine the rotation period of HD 9289 for the first time to be 8.567 ± 0.005 days. Using vsini ~ 10.5 km/s (Ryabchikova et al. 2007) then gives a lower limit for the radius of R > 1.78 R⊙.
amplitude [mmag]
The MOST (Microvariability and Oscillations of STars) microsatellite (Walker et al. 2003) is in a unique position to obtain longtimebase, high-precision space photometry for a substantial number of well-known rapidly oscillating Ap (roAp) stars. Several previous studies based on MOST data have improved our understanding of this enigmatic and sparsely populated class of rapidly pulsating, chemically peculiar stars (e.g., Gruberbauer et al. 2008, Huber et al. 2008).
In 2008 MOST observed HD 134214 for over 26 days and obtained 35099 stacked images of HD 134214. The final light curve after reduction and outlier removal consists of 31244 data points spanning almost 24 days.
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Fig.1: Fitting spot models to HD 9289 to reproduce the mean light variation: 3 spots (red line) or more are vastly preferred over 2 spot (green) models by the Bayesian nested sampling analysis. There are at least three significant intensity dips in the light curve.
HD 9289 MOST observed HD 9289 for a total of 26.27 days in 2007. 41884 stacked images, composed of multiple exposures, were obtained. After data reduction and outlier correction 37635 data points remained, spanning a total of 26.06 days.
What was already known: • Discovery as a roAp star: 3 frequencies published by Kurtz et al. (1994)
• MOST detected the well known multiplet and its harmonics. We find the components ν4 and ν5, which could not be detected in ground based photometry. There is also evidence for a previously unknown additional frequency ν8, and possibly long term modulation of ν2 and ν3 (see Fig.2). •We do not find the nonuplet components ν6 - νrot and ν7 + νrot which Freyhammer et al. reported to be significant in their data. • There is evidence for rotational modulation of the mean light in the form a double sine wave as already reported in Handler et al. (2006) but it is not prominent enough to perform spot modeling due to the broad MOST passband.
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Fig.3: P-mode spectrum of HD 134214 as found by MOST (black lines). The uncertainties of the MOST frequencies are of the order of the thickness of the black lines. The residual spectrum to the SigSpec solution is shown as well. Red dotted lines indicate the frequencies found in spectroscopic data as reported by Kurtz et al. The red symbols provide an estimated upper limit for their uncertainties. The frequency next to the arrow is the primary frequency and has actually an amplitude of 2.20 mmag - almost seven times as large as the axis range shown.
-4.25
What’s new?
2900
References Cameron, C., et al., 2006, CoAst, 148, 57 Dorren, J.D., 1987, ApJ, 320, 756 Feroz, F., et al., 2009, MNRAS, 398, 1601 Freyhammer, L. M., et al., 2009, MNRAS, 396, 325 Gruberbauer, M., et al., 2008, A&A, 480, 223 Handler, G., et al., 2006, MNRAS, 366, 257 Huber, D., et al., 2008, A&A, 483, 239 Huber, D. & Reegen, P., 2008, CoAst, 152, 77 Kreidl, T. & Kurtz D.W., 1986, MNRAS, 220, 313 Kurtz, D.W., et al., 1994, MNRAS, 271, 421 Kurtz, D.W., et al., 2007, MNRAS, 381, 1301 Reegen P., 2007, A&A, 467, 1353 Ryabchikova et al., 2007, A&A, 473, 907 Walker, G. A. H., et al., 2003, PASP, 115, 102
