P. G. Beck

University of Birmingham, Birmingham, England, United Kingdom

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Publications (36)159.39 Total impact

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    ABSTRACT: Red giants are evolved stars which exhibit solar-like oscillations. Although a multitude of stars have been observed with space telescopes, only a handful of red-giant stars were targets of spectroscopic asteroseismic observing projects. We search for solar-like oscillations in the two bright red-giant stars $\gamma$ Psc and $\theta^1$ Tau from time series of ground-based spectroscopy and determine the frequency of the excess of oscillation power $\nu_{max}$ and the mean large frequency separation $\Delta\nu$ for both stars. The radial velocities of $\gamma$ Psc and $\theta^1$ Tau were monitored for 120 and 190 days, respectively. Nearly 9000 spectra were obtained. To reach the accurate radial velocities, we used simultaneous thorium-argon and iodine-cell calibration of our optical spectra. In addition to the spectroscopy, we acquired VLTI observations of $\gamma$ Psc for an independent estimate of the radius. Also 22 days of observations of $\theta^1$ Tau with the MOST-satellite were analysed. The frequency analysis of the radial velocity data of $\gamma$ Psc revealed an excess of oscillation power around 32 $\mu$Hz and a large frequency separation of 4.1$\pm$0.1$\mu$Hz. $\theta^1$ Tau exhibits oscillation power around 90 $\mu$Hz, with a large frequency separation of 6.9$\pm$0.2$\mu$Hz. Scaling relations indicate that $\gamma$ Psc is a star of about $\sim$1 M$_\odot$ and $\sim$10 R$_\odot$. $\theta^1$ Tau appears to be a massive star of about $\sim$2.7 M$_\odot$ and $\sim$11 R$_\odot$. The radial velocities of both stars were found to be modulated on time scales much longer than the oscillation periods. While the mass of $\theta^1$ Tau is in agreement with results from dynamical parallaxes, we find a lower mass for $\gamma$ Psc than what is given in the literature. The long periodic variability agrees with the expected time scales of rotational modulation.
    07/2014;
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    ABSTRACT: More than 40 years of ground-based photometric observations of the delta Sct star 4CVn revealed 18 independent oscillation frequencies, including radial as well as non-radial p-modes of low spherical degree l<=2. From 2008 to 2011, more than 2000 spectra were obtained at the 2.1-m Otto-Struve telescope at the McDonald Observatory. We present the analysis of the line-profile variations, based on the Fourier-parameter fit method, detected in the absorption lines of 4CVn, which carry clear signatures of the pulsations. From a non-sinusoidal, periodic variation of the radial velocities, we discovered that 4CVn is an eccentric binary system, with an orbital period Porb = 124.44 +/- 0.03 d and an eccentricity e = 0.311 +/- 0.003. We firmly detect 20 oscillation frequencies, 9 of which are previously unseen in photometric data, and attempt mode identification for the two dominant modes, f1 = 7.3764 c/d and f2 = 5.8496 c/d, and determine the prograde or retrograde nature of 7 of the modes. The projected rotational velocity of the star, vsini ~ 106.7 km/s, translates to a rotation rate of veq/vcrit >= 33%. This relatively high rotation rate hampers unique mode identification, since higher-order effects of rotation are not included in the current methodology. We conclude that, in order to achieve unambiguous mode identification for 4CVn, a complete description of rotation and the use of blended lines have to be included in mode-identification techniques.
    07/2014;
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    ABSTRACT: The NASA Kepler mission has observed more than 190,000 stars in the constellations of Cygnus and Lyra. Around 4 years of almost continuous ultra high-precision photometry have been obtained reaching a duty cycle higher than 90% for many of these stars. However, almost regular gaps due to nominal operations are present in the light curves at different time scales. In this paper we want to highlight the impact of those regular gaps in asteroseismic analyses and we try to find a method that minimizes their effect in the frequency domain. To do so, we isolate the two main time scales of quasi regular gaps in the data. We then interpolate the gaps and we compare the power density spectra of four different stars: two red giants at different stages of their evolution, a young F-type star, and a classical pulsator in the instability strip. The spectra obtained after filling the gaps in the selected solar-like stars show a net reduction in the overall background level, as well as a change in the background parameters. The inferred convective properties could change as much as 200% in the selected example, introducing a bias in the p-mode frequency of maximum power. When global asteroseismic scaling relations are used, this bias can lead up to a variation in the surface gravity of 0.05 dex. Finally, the oscillation spectrum in the classical pulsator is cleaner compared to the original one.
    05/2014;
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    ABSTRACT: Kepler ultra-high precision photometry of long and continuous observations provide a unique dataset in which surface rotation and variability can be studied for thousands of stars. Because many of these old field stars also have independently measured asteroseismic ages, measurements of rotation and activity are particularly interesting in the context of age-rotation-activity relations. These relations generally lack good calibrators at old ages, a problem that this Kepler sample of stars is uniquely suited to address. We study the surface rotation and the photometric magnetic activity of a subset of 540 solar-like stars on the main-sequence and the subgiant branch for which stellar pulsations have been measured. The rotation period is determined by comparing the results from two different sets of calibrated data and from two complementary analyses. Global photometric levels of magnetic activity in this sample of stars are also extracted by using a photometric activity index, which takes into account the rotation period of the stars. Out of the 540 solar-like pulsating stars in our sample, we successfully measured the rotation period of 310 stars (excluding known binaries and candidate planet host stars). The rotation periods lay between 1 and 100 days. The remaining stars are classified into two categories: those not showing any surface rotation (6 stars), and those in which the four analyses did not converge to a single and robust rotation period (213). The photometric magnetic activity levels were computed and for 61.5% of the dwarfs, its value is comparable to the solar one. We then extract an age-rotation relation only for the dwarfs with very precise asteroseismic age estimations, highlighting the necessity of excluding the hot stars and the subgiants when inferring such relations. We also studied age-activity-rotation relations with a hint of correlation for the subgiants.
    03/2014;
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    ABSTRACT: About 1% of giant stars have been shown to have large surface Li abundances, which is unexpected according to standard stellar evolution models. Several scenarios for lithium production have been proposed, but it is still unclear why these Li-rich giants exist. A missing piece in this puzzle is the knowledge of the exact stage of evolution of these stars. Using low-and-high-resolution spectroscopic observations, we have undertaken a survey of lithium-rich giants in the Kepler field. In this letter, we report the finding of the first confirmed Li-rich core-helium-burning giant, as revealed by asteroseismic analysis. The evolutionary timescales constrained by its mass suggest that Li-production most likely took place through non-canonical mixing at the RGB-tip, possibly during the helium flash.
    02/2014; 784(1).
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    ABSTRACT: We carried out an extensive observational study of the Slowly Pulsating B (SPB) star, HD 25558. The ~2000 spectra obtained at different observatories, the ground-based and MOST satellite light curves revealed that this object is a double-lined spectroscopic binary with an orbital period of about 9 years. The observations do not allow the inference of an orbital solution. We determined the physical parameters of the components, and found that both lie within the SPB instability strip. Accordingly, both show line-profile variations due to stellar pulsations. Eleven independent frequencies were identified in the data. All the frequencies were attributed to one of the two components based on Pixel-by-pixel variability analysis of the line profiles. Spectroscopic and photometric mode identification was also performed for the frequencies of both stars. These results suggest that the inclination and rotation of the two components are rather different. The primary is a slow rotator with ~6 d period, seen at ~60 deg inclination, while the secondary rotates fast with ~1.2 d period, and is seen at ~20 inclination. Spectropolarimetric measurements revealed that the secondary component has a magnetic field with at least a few hundred Gauss strength, while no magnetic field can be detected in the primary.
    12/2013;
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    ABSTRACT: The unparalleled photometric data obtained by NASA's Kepler space telescope led to an improved understanding of red giant stars and binary stars. Seismology allows us to constrain the properties of red giants. In addition to eclipsing binaries, eccentric non-eclipsing binaries, exhibiting ellipsoidal modulations, have been detected with Kepler. We aim to study the properties of eccentric binary systems containing a red giant star and derive the parameters of the primary giant component. We apply asteroseismic techniques to determine masses and radii of the primary component of each system. For a selected target, light and radial velocity curve modelling techniques are applied to extract the parameters of the system. The effects of stellar on the binary system are studied. The paper presents the asteroseismic analysis of 18 pulsating red giants in eccentric binary systems, for which masses and radii were constrained. The orbital periods of these systems range from 20 to 440days. From radial velocity measurements we find eccentricities between e=0.2 to 0.76. As a case study we present a detailed analysis of KIC5006817. From seismology we constrain the rotational period of the envelope to be at least 165 d, roughly twice the orbital period. The stellar core rotates 13 times faster than the surface. From the spectrum and radial velocities we expect that the Doppler beaming signal should have a maximum amplitude of 300ppm in the light curve. Through binary modelling, we determine the mass of the secondary component to be 0.29$\pm$0.03\,$M_\odot$. For KIC5006817 we exclude pseudo-synchronous rotation of the red giant with the orbit. The comparison of the results from seismology and modelling of the light curve shows a possible alignment of the rotational and orbital axis at the 2$\sigma$ level. Red giant eccentric systems could be progenitors of cataclysmic variables and hot subdwarf B stars.
    12/2013;
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    ABSTRACT: The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (muchfuss) aims to discover subdwarf-B stars with massive compact companions such as overmassive white dwarfs (M > 1.0 M_sun), neutron stars or black holes. From the 127 subdwarfs with substantial radial-velocity variations discovered in the initial survey, a number of interesting objects have been selected for extensive follow-up. After an initial photometry run with BUSCA revealed that FBS 0117+396 is photometrically variable both on long and short timescales, we chose it as an auxiliary target during a 6-night multi-color photometry run with Ultracam. Spectroscopy was obtained at a number of observatories in order to determine the binary period and obtain a radial-velocity amplitude. After establishing an orbital period of P = 0.252 d, and removing the signal associated with the irradiated hemisphere of the M-dwarf companion, we were able to detect ten pulsation periods in the Fourier spectrum of the light curve. Two pulsation modes are found to have short periods of 337 and 379 s, and at least eight modes are found with periods between 45 minutes and 2.5 hours. This establishes that FBS 0117+396 is an sdB+dM reflection binary, in which the primary is a hybrid pulsator, and the first one found with this particular melange of flavours.
    Astronomy and Astrophysics 10/2013; · 5.08 Impact Factor
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    ABSTRACT: The three tables contain the photometry from different observatories as plotted with different colours in Figure 6 of the paper. (3 data files).
    VizieR Online Data Catalog. 10/2013;
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    ABSTRACT: We comment on the potential for continuing asteroseismology of solar-type and red-giant stars in a 2-wheel Kepler Mission. Our main conclusion is that by targeting stars in the ecliptic it should be possible to perform high-quality asteroseismology, as long as favorable scenarios for 2-wheel pointing performance are met. Targeting the ecliptic would potentially facilitate unique science that was not possible in the nominal Mission, notably from the study of clusters that are significantly brighter than those in the Kepler field. Our conclusions are based on predictions of 2-wheel observations made by a space photometry simulator, with information provided by the Kepler Project used as input to describe the degraded pointing scenarios. We find that elevated levels of frequency-dependent noise, consistent with the above scenarios, would have a significant negative impact on our ability to continue asteroseismic studies of solar-like oscillators in the Kepler field. However, the situation may be much more optimistic for observations in the ecliptic, provided that pointing resets of the spacecraft during regular desaturations of the two functioning reaction wheels are accurate at the < 1 arcsec level. This would make it possible to apply a post-hoc analysis that would recover most of the lost photometric precision. Without this post-hoc correction---and the accurate re-pointing it requires---the performance would probably be as poor as in the Kepler-field case. Critical to our conclusions for both fields is the assumed level of pointing noise (in the short-term jitter and the longer-term drift). We suggest that further tests will be needed to clarify our results once more detail and data on the expected pointing performance becomes available, and we offer our assistance in this work.
    09/2013;
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    ABSTRACT: We present the first binary modelling results for the pulsating eclipsing binary KIC 11285625, discovered by the Kepler mission. An automated method to disentangle the pulsation spectrum and the orbital variability in high quality light curves, was developed and applied. The goal was to obtain accurate orbital and component properties, in combination with essential information derived from spectroscopy. A binary model for KIC 11285625 was obtained, using a combined analysis of high-quality space-based Kepler light curves and ground-based high-resolution HERMES echelle spectra. The binary model was used to separate the pulsation characteristics from the orbital variability in the Kepler light curve in an iterative way. We used an automated procedure to perform this task, based on the JKTEBOP binary modelling code, and adapted codes for frequency analysis and prewhitening of periodic signals. Using a disentangling technique applied to the composite HERMES spectra, we obtained a higher signal-to-noise mean component spectrum for both the primary and the secondary. A model grid search method for fitting synthetic spectra was used for fundamental parameter determination for both components. Accurate orbital and component properties of KIC 11285625 were derived, and we have obtained the pulsation spectrum of the gamma Dor pulsator in the system. Detailed analysis of the pulsation spectrum revealed amplitude modulation on a time scale of a hundred days, and strong indications of frequency splittings at both the orbital frequency, and the rotational frequency derived from spectroscopy.
    Astronomy and Astrophysics 06/2013; · 5.08 Impact Factor
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    ABSTRACT: Context: OB stars are important in the chemistry and evolution of the Universe, but the sample of targets well understood from an asteroseismological point of view is still too limited to provide feedback on the current evolutionary models. Our study extends this sample with two spectroscopic binary systems. AIMS. Our goal is to provide orbital solutions, fundamental parameters and abundances from disentangled high-resolution high signal-to-noise spectra, as well as to analyse and interpret the variations in the Kepler light curve of these carefully selected targets. This way we continue our efforts to map the instability strips of beta Cep and SPB stars using the combination of high-resolution ground-based spectroscopy and uninterrupted space-based photometry. Methods: We fit Keplerian orbits to radial velocities measured from selected absorption lines of high-resolution spectroscopy using synthetic composite spectra to obtain orbital solutions. We use revised masks to obtain optimal light curves from the original pixel-data from the Kepler satellite, which provided better long term stability compared to the pipeline processed light curves. We use various time-series analysis tools to explore and describe the nature of variations present in the light curve. Results: We find two eccentric double-lined spectroscopic binary systems containing a total of three main sequence B-type stars (and one F-type component) of which at least one in each system exhibits light variations. The light curve analysis (combined with spectroscopy) of the system of two B stars points towards the presence of tidally excited g modes in the primary component. We interpret the variations seen in the second system as classical g mode pulsations driven by the kappa mechanism in the B type primary, and explain the unexpected power in the p mode region as a result of nonlinear resonant mode excitation.
    Astronomy and Astrophysics 04/2013; 553:127. · 5.08 Impact Factor
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    ABSTRACT: Fourier parameters (frequencies (fj), amplitudes (Aj& (thetaj)) of the selected peaks having a signal-to-noise ratio (S/N) above 4 for KIC 4931738 and KIC 6352430 when computed in a 1d-1 window after prewhitening. The displayed S/N values are calculated in a window of 1d-1 or 3d-1 centred on the given frequency, or from the full periodogram (from 0d-1 to 24.47d-1). A complete list of all significant frequencies is available upon request. (2 data files).
    VizieR Online Data Catalog. 04/2013;
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    ABSTRACT: We present an asteroseismic approach to study the dynamics of the stellar interior in red-giant stars by asteroseismic inversion of the splittings induced by the stellar rotation on the oscillation frequencies. We show preliminary results obtained for the red giant KIC4448777 observed by the space mission Kepler.
    12/2012;
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    ABSTRACT: We used high-quality Kepler photometry and recent spectroscopic data to investigate the Kepler target KIC 005988140 considering three possible scenario's: (1) binarity, (2) co-existence of δ Scuti- and γ Doradus-type oscillations, and (3) rotational modulation with spots over the stellar surface. Using spectrum synthesis, we derived improved atmospheric parameters from high-resolution, high signal-to-noise spectra. The star is of spectral type A7.5 IV-III with a metallicity slightly lower than that of the Sun. The Fourier analyses of both the photometric and the spectroscopic data reveal two dominant frequencies - with an integer ratio equal to two - in the low frequency regime. Nine more frequencies located in the typical δ Scuti range were also detected. The mean light and radial velocity curves which are not exactly in anti-phase, show a similar double-wave shape. We therefore suggest on a preliminary basis that the physical cause of the observed light and radial velocity variations is the simultaneous occurrence of (δ Sct-type) pressure modes and stellar rotation coupled to inhomogeneities on the surface of this late A-type star. Based on observations obtained with the HERMES spectrograph and the Mercator Telescope, operated by the Flemish Community on the island of La Palma, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, and with the 2-m Alfred-Jensch telescope of the Thüringer Landessternwarte Tautenburg.
    Astronomische Nachrichten 12/2012; · 1.40 Impact Factor
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    ABSTRACT: Numerous high-resolution spectra of the \delta Scuti star 4 CVn were gathered at McDonald Observatory in Texas, USA. They were prepared for interpretation using standard IRAF routines for a subsequent frequency analysis and mode identification with the software FAMIAS. After the reduction process a clear trend in the first moment of the line profile (radial velocity) remained, indicating the signature of binary motion. This was corrected for by fitting a Keplerian orbit, followed by a detailed analysis of the data in the reference frame of the primary. The results of the frequency analysis are in good agreement with previous photometric studies. This paper presents thirteen pulsation modes that were detected in the data sets for the years 2010 and/or 2011. Future work involves spectroscopic mode identification and subsequent seismic modeling. Data from McDonald Observatory, Texas, USA.
    Astronomische Nachrichten 12/2012; · 1.40 Impact Factor
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    ABSTRACT: A total of 40 optical spectra taken in 2010 with the HERMES spectrograph attached to the 1.2-m Mercator telescope (Observatorio del Roque de los Muchachos, Spain) in the range 380-900nm and in 2011 with the Coude-Echelle spectrograph attached to the 2-m telescope at the Thueringer Landessternwarte Tautenburg (Germany) in the range 472-740nm is being presented. The spectra were reduced by automated and standard data reduction pipeline including cosmic ray, heliocentric and telluric corrections as well as normalization to the local continuum. (2 data files).
    VizieR Online Data Catalog. 11/2012;
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    ABSTRACT: Rotation plays a key role in stellar structure and its evolution. Through transport processes which induce rotational mixing of chemical species and the redistribution of angular momentum, internal stellar rotation influences the evolutionary tracks in the Hertzsprung-Russell diagram. In turn, evolution influences the rotational properties. Therefore, information on the rotational properties of the deep interior would help to better understand the stellar evolution. However, as the internal rotational profile cannot be measured directly, it remains a major unknown leaving this important aspect of models unconstrained. We can test for nonrigid rotation inside the stars with asteroseismology. Through the effect of rotational splitting of non-radial oscillation modes, we investigate the internal rotation profile indirectly. Red giants have very slow rotation rates leading to a rotational splitting on the level of a few tenth of a \mu Hz. Only from more than 1.5 years of consecutive data from the NASA Kepler space telescope, these tiny variations could be resolved. A qualitative comparison to theoretical models allowed constraining the core-to-surface rotation rate for some of these evolved stars. In this paper, we report on the first results of a large sample study of splitting of individual dipole modes.
    Astronomische Nachrichten 11/2012; · 1.40 Impact Factor
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    ABSTRACT: We used high-quality Kepler photometry and spectroscopic data to investigate the Kepler binary candidate KIC 5988140. Using the spectrum synthesis method, we derived the fundamental parameters Teff, log g, [M/H], and v.sini and the abundances. Frequency analyses of both the photometric and the spectroscopic data were performed, revealing the same two dominant frequencies (F_1=0.688 and F_2=0.344 c/d). We also detected in the photometry the signal of nine more, significant frequencies located in the typical range of Delta Scuti pulsation. The light and radial velocity curves follow a similar, stable double-wave pattern which are not exactly in anti-phase but show a relative phase shift of about 0.1 period between the moment of minimum velocity and that of maximum light. We considered three different scenarios: binarity, co-existence of both Gamma Doradus and Delta Scuti pulsations and rotation of the stellar surface with an axisymmetric intensity distribution. However, none of these scenarios is capable of explaining all of the characteristics of the observed variations. We confirm the occurrence of various independent Delta Scuti-type pressure modes in the Kepler light curve. With respect to the low-frequency content, however, we argue that the physical cause of the remaining light and radial velocity variations of this late A-type star remains unexplained by any of the presently considered scenarios.
    Astronomy and Astrophysics 10/2012; · 5.08 Impact Factor
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    ABSTRACT: The space mission Kepler provides us with long and uninterrupted photometric time series of red giants. We are now able to probe the rotational behaviour in their deep interiors using the observations of mixed modes. We aim to measure the rotational splittings in red giants and to derive scaling relations for rotation related to seismic and fundamental stellar parameters. We have developed a dedicated method for automated measurements of the rotational splittings in a large number of red giants. Ensemble asteroseismology, namely the examination of a large number of red giants at different stages of their evolution, allows us to derive global information on stellar evolution. We have measured rotational splittings in a sample of about 300 red giants. We have also shown that these splittings are dominated by the core rotation. Under the assumption that a linear analysis can provide the rotational splitting, we observe a small increase of the core rotation of stars ascending the red giant branch. Alternatively, an important slow down is observed for red-clump stars compared to the red giant branch. We also show that, at fixed stellar radius, the specific angular momentum increases with increasing stellar mass. Ensemble asteroseismology indicates what has been indirectly suspected for a while: our interpretation of the observed rotational splittings leads to the conclusion that the mean core rotation significantly slows down during the red giant phase. The slow-down occurs in the last stages of the red giant branch. This spinning down explains, for instance, the long rotation periods measured in white dwarfs
    Astronomy and Astrophysics 09/2012; · 5.08 Impact Factor