Robert Szabo

Publications (94) View all

• Article: Asteroseismology of KIC 11754974: a high-amplitude SX Phe pulsator in a 343-day binary system

  S. J. Murphy, A. Pigulski, D. W. Kurtz, J. C. Suarez, G. Handler, L. A. Balona, B. Smalley, K. Uytterhoeven, R. Szabo, A. O. Thygesen, V. Elkin, M. Breger, A. Grigahcene, J. A. Guzik, J. M. Nemec, J. Southworth
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  ABSTRACT: The candidate SX Phe star KIC 11754974 shows a remarkably high number of combination frequencies in the Fourier amplitude spectrum: 123 of the 166 frequencies in our multi-frequency fit are linear combinations of independent modes. Predictable patterns in frequency spacings are seen in the Fourier transform of the light curve. We present an analysis of 180 d of short-cadence Kepler photometry and of new spectroscopic data for this evolved, late A-type star. We infer from the 1150-d, long-cadence light curve, and in two different ways, that our target is the primary of a 343-d, non-eclipsing binary system. According to both methods, the mass function is similar, f(M)=0.0207 +/- 0.0003 Msun. The observed pulsations are modelled extensively, using separate, state-of-the-art, time-dependent convection (TDC) and rotating models. The models match the observed temperature and low metallicity, finding a mass of 1.50-1.56 Msun. The models suggest the whole star is metal-poor, and that the low metallicity is not just a surface abundance peculiarity. This is the best frequency analysis of an SX Phe star, and the only Kepler \delta\ Sct star to be modelled with both TDC and rotating models.
  04/2013;
• Article: Period and light curve fluctuations of the Kepler Cepheid V1154 Cyg

  A. Derekas, Gy. M. Szabo, L. Berdnikov, R. Szabo, R. Smolec, L. L. Kiss, L. Szabados, M. Chadid, N. R. Evans, K. Kinemuchi, J. M. Nemec, S. E. Seader, J. C. Smith, P. Tenenbaum
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  ABSTRACT: We present a detailed period analysis of the bright Cepheid-type variable star V1154 Cygni (V =9.1 mag, P~4.9 d) based on almost 600 days of continuous observations by the Kepler space telescope. The data reveal significant cycle-to-cycle fluctuations in the pulsation period, indicating that classical Cepheids may not be as accurate astrophysical clocks as commonly believed: regardless of the specific points used to determine the O-C values, the cycle lengths show a scatter of 0.015-0.02 days over the 120 cycles covered by the observations. A very slight correlation between the individual Fourier parameters and the O-C values was found, suggesting that the O - C variations might be due to the instability of the light curve shape. Random fluctuation tests revealed a linear trend up to a cycle difference 15, but for long term, the period remains around the mean value. We compare the measurements with simulated light curves that were constructed to mimic V1154 Cyg as a perfect pulsator modulated only by the light travel time effect caused by low-mass companions. We show that the observed period jitter in V1154 Cyg represents a serious limitation in the search for binary companions. While the Kepler data are accurate enough to allow the detection of planetary bodies in close orbits around a Cepheid, the astrophysical noise can easily hide the signal of the light-time effect.
  07/2012;
• Source

  Available from: S. Hekker

  Article: Asteroseismology of old open clusters with Kepler: direct estimate of the integrated red giant branch mass‐loss in NGC 6791 and 6819

  A. Miglio, K. Brogaard, D. Stello, W. J. Chaplin, F. D’Antona, J. Montalbán, S. Basu, A. Bressan, F. Grundahl, M. Pinsonneault, [......], B. Mosser, P. Ventura, A. Bonanno, A. Noels, V. Silva Aguirre, R. Szabo, J. Li, S. McCauliff, C. K. Middour, H. Kjeldsen
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  ABSTRACT: Mass-loss of red giant branch (RGB) stars is still poorly determined, despite its crucial role in the chemical enrichment of galaxies. Thanks to the recent detection of solar-like oscillations in G–K giants in open clusters with Kepler, we can now directly determine stellar masses for a statistically significant sample of stars in the old open clusters NGC 6791 and 6819. The aim of this work is to constrain the integrated RGB mass-loss by comparing the average mass of stars in the red clump (RC) with that of stars in the low-luminosity portion of the RGB [i.e. stars with L≲L(RC)]. Stellar masses were determined by combining the available seismic parameters νmax and Δν with additional photometric constraints and with independent distance estimates. We measured the masses of 40 stars on the RGB and 19 in the RC of the old metal-rich cluster NGC 6791. We find that the difference between the average mass of RGB and RC stars is small, but significant [ (random) ±0.04 (systematic) M⊙]. Interestingly, such a small does not support scenarios of an extreme mass-loss for this metal-rich cluster. If we describe the mass-loss rate with Reimers prescription, a first comparison with isochrones suggests that the observed is compatible with a mass-loss efficiency parameter in the range 0.1 ≲η≲ 0.3. Less stringent constraints on the RGB mass-loss rate are set by the analysis of the ∼2 Gyr old NGC 6819, largely due to the lower mass-loss expected for this cluster, and to the lack of an independent and accurate distance determination. In the near future, additional constraints from frequencies of individual pulsation modes and spectroscopic effective temperatures will allow further stringent tests of the Δν and νmax scaling relations, which provide a novel, and potentially very accurate, means of determining stellar radii and masses.
  Monthly Notices of the Royal Astronomical Society 01/2012; 419(3):2077 - 2088. · 4.90 Impact Factor
• Source

  Available from: ArXiv

  Article: Ground-Based BVRI Follow-Up Observations of the Cepheid V1154 Cyg in Kepler's Field

  C. -C. Ngeow, R. Szabo, L. Szabados, A. Henden, M. A. T. Groenewegen, the Kepler Cepheid Working Group
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  ABSTRACT: The almost un-interrupted observations from Kepler Space Telescope can be used to search for Earth-size and larger planets around other stars, as well as for stellar variability and asteroseismological study. However, the Kepler's observations are carried out with a single broad-band filter, and ground-based follow-up observations are needed to complement Kepler's light curves to fully characterize the properties of the target stars. Here we present ground-based optical (BVRI) followup observations of V1154 Cyg, the only Cepheid in the Kepler field of view, and deriving basic properties of this star.
  11/2011;
• Source

  Available from: Francesca D'Antona

  Article: Asteroseismology of old open clusters with Kepler: direct estimate of the integrated RGB mass loss in NGC6791 and NGC6819

  A. Miglio, K. Brogaard, D. Stello, W. J. Chaplin, F. D'Antona, J. Montalban, S Basu, A. Bressan, F. Grundahl, M. Pinsonneault, [......], B. Mosser, P. Ventura, A. Bonanno, A. Noels, V. Silva Aguirre, R. Szabo, J. Li, S. McCauliff, C. K. Middour, H. Kjeldsen
  [show abstract] [hide abstract]
  ABSTRACT: Mass loss of red giant branch (RGB) stars is still poorly determined, despite its crucial role in the chemical enrichment of galaxies. Thanks to the recent detection of solar-like oscillations in G-K giants in open clusters with Kepler, we can now directly determine stellar masses for a statistically significant sample of stars in the old open clusters NGC6791 and NGC6819. The aim of this work is to constrain the integrated RGB mass loss by comparing the average mass of stars in the red clump (RC) with that of stars in the low-luminosity portion of the RGB (i.e. stars with L <~ L(RC)). Stellar masses were determined by combining the available seismic parameters numax and Dnu with additional photometric constraints and with independent distance estimates. We measured the masses of 40 stars on the RGB and 19 in the RC of the old metal-rich cluster NGC6791. We find that the difference between the average mass of RGB and RC stars is small, but significant (Delta M=0.09 +- 0.03 (random) +- 0.04 (systematic) Msun). Interestingly, such a small DeltaM does not support scenarios of an extreme mass loss for this metal-rich cluster. If we describe the mass-loss rate with Reimers' prescription, a first comparison with isochrones suggests that the observed DeltaM is compatible with a mass-loss efficiency parameter in the range 0.1 <~ eta <~ 0.3. Less stringent constraints on the RGB mass-loss rate are set by the analysis of the ~ 2 Gyr-old NGC6819, largely due to the lower mass loss expected for this cluster, and to the lack of an independent and accurate distance determination. In the near future, additional constraints from frequencies of individual pulsation modes and spectroscopic effective temperatures, will allow further stringent tests of the Dnu and numax scaling relations, which provide a novel, and potentially very accurate, means of determining stellar radii and masses.
  09/2011;