Fourier analysis of non‐Blazhko ab‐type RR Lyrae stars observed with the <i>Kepler</i> space telescope

Nemec, J. M.,Smolec, R.,Benkő,, J. M.,Moskalik, P.,Kolenberg, K.,Szabó,, R.,Kurtz, D. W.,Bryson, S.,Guggenberger, E.,Chadid, M.,Jeon, Y.‐,B.,Kunder, A.,Layden, A. C.,Kinemuchi, K.,Kis Blackwell Publishing Ltd 2011 Monthly notices of the Royal Astronomical Society Vol.417 No.2

<P><B>ABSTRACT</B></P><P>Nineteen of the ∼40 RR Lyr stars in the <I>Kepler</I> field have been identified as candidate non‐Blazhko (or unmodulated) stars. In this paper we present the results of Fourier decomposition of the time‐series photometry of these stars acquired during the first 417 d of operation (Q0–Q5) of the <I>Kepler</I> telescope. Fourier parameters based on ∼18 400 long‐cadence observations per star (and ∼150 000 short‐cadence observations for FN Lyr and for AW Dra) are derived. None of the stars shows the recently discovered ‘period‐doubling’ effect seen in Blazhko variables; however, KIC 7021124 has been found to pulsate simultaneously in the fundamental and second overtone modes with a period ratio <I>P</I><SUB>2</SUB>/<I>P</I><SUB>0</SUB>∼ 0.593 05 and is similar to the double‐mode star V350 Lyr. Period change rates are derived from O − C diagrams spanning, in some cases, over 100 years; these are compared with high‐precision periods derived from the <I>Kepler</I> data alone. Extant Fourier correlations by Kovács, Jurcsik et al. (with minor transformations from the <I>V</I> to the <I>Kp</I> passband) have been used to derive underlying physical characteristics for all the stars. This procedure seems to be validated through comparisons of the <I>Kepler</I> variables with Galactic and Large Magellanic Cloud (LMC) RR Lyr stars. The most metal‐poor star in the sample is NR Lyr, with [Fe/H] =−2.3 dex; and the four most metal‐rich stars have [Fe/H] ranging from −0.6 to +0.1 dex. Pulsational luminosities and masses are found to be systematically smaller than <I>L</I> and <IMG src='/wiley-blackwell_img/equation/MNR_19317_mu1.gif' alt ='inline image'/> values derived from stellar evolution models, and are favoured over the evolutionary values when periods are computed with the Warsaw linear hydrodynamics code. Finally, the Fourier parameters are compared with theoretical values derived using the Warsaw non‐linear convective pulsation code.</P>

<i>Kepler</i> photometry of the prototypical Blazhko star RR Lyr: an old friend seen in a new light

Kolenberg, K.,Bryson, S.,Szabó,, R.,Kurtz, D. W.,Smolec, R.,Nemec, J. M.,Guggenberger, E.,Moskalik, P.,Benkő,, J. M.,Chadid, M.,Jeon, Y.‐,B.,Kiss, L. L.,Kopacki, G.,Nuspl, J.,Still, M Blackwell Publishing Ltd 2011 Monthly notices of the Royal Astronomical Society Vol.411 No.2

<P><B>ABSTRACT</B></P><P>We present our analysis of the long‐cadence <I>Kepler</I> data for the well‐studied Blazhko star RR Lyr, gathered during the first two quarters of the satellite’s observations and covering a total of 127 d. Besides being of great importance for our understanding of RR Lyrae stars in general, these RR Lyr data can be regarded as a case study for observations of bright stars with <I>Kepler</I>. <I>Kepler</I> can perform high‐precision photometry on targets like RR Lyr, as the saturated flux is conserved to a very high degree. The <I>Kepler</I> data on RR Lyr are revolutionary in several respects. Even with long‐cadence sampling (one measurement per 29.4 min), the unprecedented precision (< mmag) of the <I>Kepler</I> photometry allows the study of the star’s extreme light‐curve variations in detail. The multiplet structures at the main frequency and its harmonics, typical for Blazhko stars, are clearly detected up to the quintuplets. For the first time, photometric data of RR Lyr reveal the presence of half‐integer frequencies, linked to a period‐doubling effect. This phenomenon may be connected to the still unexplained Blazhko modulation. Moreover, with three observed Blazhko cycles at our disposal, we observe that there is no exact repetition in the light‐curve changes from one modulation cycle to the next for RR Lyr. This may be due to additional periodicities in the star, or to transient or quasi‐periodic changes.</P>

The pulsating hot subdwarf Balloon 090100001: results of the 2005 multisite campaign

Baran, A.,Oreiro, R.,Pigulski, A.,Herná,ndez, F. Pé,rez,Ulla, A.,Reed, M. D.,Rodrí,guez-Ló,pez, C.,Moskalik, P.,Kim, S.-L.,Chen, W.-P.,Crowe, R.,Siwak, M.,Armendarez, L.,Binder Blackwell Publishing Ltd 2009 MONTHLY NOTICES- ROYAL ASTRONOMICAL SOCIETY Vol.392 No.3

<P>ABSTRACT</P><P>We present the results of a multisite photometric campaign on the pulsating B-type hot subdwarf star Balloon 090100001 (Bal09). The star is one of the two known hybrid hot subdwarfs with both long- and short-period oscillations, theoretically attributed to <I>g</I> and <I>p</I> modes. The campaign involved eight telescopes with three obtaining <I>UBVR</I> data, four <I>B</I>-band data and one Strömgren <I>uvby</I> photometry. The campaign covered 48 nights, providing a temporal resolution of 0.36 μHz with a detection threshold of about 0.2 mmag in <I>B</I>-filter data.</P><P>Bal09 has the richest pulsation spectrum of any known pulsating subdwarf B star, and our analysis detected 114 frequencies including 97 independent and 17 combination ones. Most of the 24 <I>g</I>-mode frequencies are between 0.1 and 0.4 mHz. Of the remaining 73, presumably <I>p</I> modes, 72 group into four distinct regions near 2.8, 3.8, 4.7 and 5.5 mHz. The density of frequencies requires that some modes must have degrees ℓ larger than 2. The modes in the 2.8 mHz region have the largest amplitudes. The strongest mode (<I>f</I><SUB>1</SUB>) is most likely radial, while the remaining ones in this region form two nearly symmetric multiplets: a triplet and quintuplet, attributed to rotationally split ℓ= 1 and 2 modes, respectively. We find clear increases of splitting in both multiplets between the 2004 and 2005 observing campaigns, amounting to ∼15 per cent on average. The observed splittings imply that the rotational rate in Bal09 depends on stellar latitude and is the fastest on the equator. We also speculate on the possible reasons for the changes of splitting. The only plausible explanation we find is torsional oscillation. This hypothesis, however, needs to be verified in the future by detailed modelling. In this context, it is very important to monitor the splittings on a longer time-scale as their behaviour may help to explain this interesting phenomenon.</P><P>The amplitudes of almost all terms detected in both 2004 and 2005 were found to vary. This is evident even during one season; for example, amplitudes of modes <I>f</I><SUB>8</SUB> and <I>f</I><SUB>C</SUB> were found to change by a factor of 2–3 within about 50 d during 2005.</P><P>We use a small grid of models to constrain the main mode (<I>f</I><SUB>1</SUB>), which most likely represents the radial fundamental pulsation. The groups of <I>p</I>-mode frequencies appear to lie in the vicinity of the consecutive radial overtones, up to the third one. Despite the large number of <I>g</I>-mode frequencies observed, we failed to identify them, most likely because of the disruption of asymptotic behaviour by mode trapping. The observed frequencies were not, however, fully exploited in terms of seismic analysis which should be done in the future with a larger grid of reliable evolutionary models of hot subdwarfs.</P>

Whole Earth Telescope observations of the subdwarf B star KPD 1930+2752: a rich, short‐period pulsator in a close binary

Reed, M. D.,Harms, S. L.,Poindexter, S.,Zhou, A.‐,Y.,Eggen, J. R.,Morris, M. A.,Quint, A. C.,McDaniel, S.,Baran, A.,Dolez, N.,Kawaler, S. D.,Kurtz, D. W.,Moskalik, P.,Riddle, R.,Zola, S.,Østense Blackwell Publishing Ltd 2011 Monthly notices of the Royal Astronomical Society Vol.412 No.1

<P><B>ABSTRACT</B></P><P>KPD 1930+2752 is a short‐period pulsating subdwarf B (sdB) star. It is also an ellipsoidal variable with a known binary period of 2.3 h. The companion is most likely a white dwarf and the total mass of the system is close to the Chandresekhar limit. In this paper, we report the results of Whole Earth Telescope (WET) photometric observations during 2003 and a smaller multisite campaign of 2002. From 355 h of WET data, we detect 68 pulsation frequencies and suggest an additional 13 frequencies within a crowded and complex temporal spectrum between 3065 and 6343 μHz (periods between 326 and 157 s). We examine pulsation properties including phase and amplitude stability in an attempt to understand the nature of the pulsation mechanism. We examine a stochastic mechanism by comparing amplitude variations with simulated stochastic data. We also use the binary nature of KPD 1930+2752 for identifying pulsation modes via multiplet structure and a tidally induced pulsation geometry. Our results indicate a complicated pulsation structure that includes short‐period (≈16 h) amplitude variability, rotationally split modes, tidally induced modes and some pulsations which are geometrically limited on the sdB star.</P>