We carried out a multicolour time-series photometric study of six stars claimed as ‘hybrid’ p and g mode pulsators in the literature. γ Peg was confirmed to show short-period oscillations of the β ...Cep type and simultaneous long-period pulsations typical of Slowly Pulsating B (SPB) stars. From the measured amplitude ratios in the Strömgren uvy passbands, the stronger of the two short period pulsation modes was identified as radial; the second is ℓ= 1. Three of the four SPB-type modes are most likely ℓ= 1 or 2. Comparison with theoretical model calculations suggests that γ Peg is either a ∼8.5 M⊙ radial fundamental mode pulsator or a ∼9.6 M⊙ first radial overtone pulsator. HD 8801 was corroborated as a ‘hybrid’δ Sct/γ Dor star; four pulsation modes of the γ Dor type were detected, and two modes of the δ Sct type were confirmed. Two pulsational signals between the frequency domains of these two known classes of variables were confirmed and another was newly detected. These are either previously unknown types of pulsation or do not originate from HD 8801. The O-type star HD 13745 showed small-amplitude slow variability on a time-scale of 3.2 d. This object may be related to the suspected new type of supergiant SPB stars, but a rotational origin of its light variations cannot be ruled out at this point. 53 Psc is an SPB star for which two pulsation frequencies were determined and identified with low spherical degree. Small-amplitude variability was formally detected for 53 Ari but is suspected not to be intrinsic. The behaviour of ι Her is consistent with non-variability during our observations, and we could not confirm light variations of the comparison star 34 Psc previously suspected. The use of signal-to-noise criteria in the analysis of data sets with strong aliasing is critically discussed.
Tidally trapped pulsations in binary stars Fuller, J; Kurtz, D W; Handler, G ...
Monthly notices of the Royal Astronomical Society,
11/2020, Letnik:
498, Številka:
4
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ABSTRACT
A new class of pulsating binary stars was recently discovered, whose pulsation amplitudes are strongly modulated with orbital phase. Stars in close binaries are tidally distorted, so we ...examine how a star’s tidally induced asphericity affects its oscillation mode frequencies and eigenfunctions. We explain the pulsation amplitude modulation via tidal mode coupling such that the pulsations are effectively confined to certain regions of the star, e.g. the tidal pole or the tidal equator. In addition to a rigorous mathematical formalism to compute this coupling, we provide a more intuitive semi-analytic description of the process. We discuss three resulting effects: (1) Tidal alignment, i.e. the alignment of oscillation modes about the tidal axis rather than the rotation axis; (2) Tidal trapping, e.g. the confinement of oscillations near the tidal poles or the tidal equator; (3) Tidal amplification, i.e. increased flux perturbations near the tidal poles where acoustic modes can propagate closer to the surface of the star. Together, these phenomena can account for the pulsation amplitude and phase modulation of the recently discovered class of ‘tidally tilted pulsators.’ We compare our theory to the three tidally tilted pulsators HD 74423, CO Cam, and TIC 63328020, finding that tidally trapped modes that are axisymmetric about the tidal axis can largely explain the first two, while a non-axisymmetric tidally aligned mode is present in the latter. Finally, we discuss implications and limitations of the theory, and we make predictions for the many new tidally tilted pulsators likely to be discovered in the near future.
Rotational modulation in TESS B stars Balona, L A; Handler, G; Chowdhury, S ...
Monthly notices of the Royal Astronomical Society,
05/2019, Letnik:
485, Številka:
3
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Abstract
Light curves and periodograms of 160 B stars observed by the Transiting Exoplanet Survey Satellite (TESS) space mission and 29 main-sequence B stars from Kepler and K2 were used to classify ...the variability type. There are 114 main-sequence B stars in the TESS sample, of which 45 are classified as possible rotational variables. This confirms previous findings that a large fraction (about 40 per cent) of A and B stars may exhibit rotational modulation. Gaia DR2 parallaxes were used to estimate luminosities, from which the radii and equatorial rotational velocities can be deduced. It is shown that observed values of the projected rotational velocities are lower than the estimated equatorial velocities for nearly all the stars, as they should be if rotation is the cause of the light variation. We conclude that a large fraction of main-sequence B stars appear to contain surface features which cannot likely be attributed to abundance patches.
ABSTRACT
Eclipsing binaries with (a) pulsating component(s) are remarkable objects to investigate the evolution and structure of stellar systems. Detailed studies of such systems are also important ...to reveal their pulsation properties. The largest sample of pulsating eclipsing binaries is the one containing δ Scuti variables. In this study, we present a comprehensive spectroscopic and photometric study of three such systems, GQ Dra, RR Lep, and TYC 683-640-1. Their orbital parameters were derived from their large-amplitude radial velocity variations. All systems were found to be single-lined eclipsing binaries. We determined the atmospheric parameters of the primary components, which are the more luminous ones. By using the results of the spectroscopic analysis, the binarity of the systems was modelled and the fundamental stellar parameters (mass, radius) of each system were obtained. In addition, the pulsations of the systems were analysed. Both GQ Dra and RR Lep exhibit frequencies that are modulated with the orbital period. Therefore, the pulsating components of these systems are identified as candidate tidally tilted pulsators.
The single-sided pulsator CO Camelopardalis Kurtz, D W; Handler, G; Rappaport, S A ...
Monthly notices of the Royal Astronomical Society,
06/2020, Letnik:
494, Številka:
4
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ABSTRACT
CO Cam (TIC 160268882) is the second ‘single-sided pulsator’ to be discovered. These are stars where one hemisphere pulsates with a significantly higher amplitude than the other side of the ...star. CO Cam is a binary star comprised of an Am δ Sct primary star with Teff = 7070 ± 150 K, and a spectroscopically undetected G main-sequence secondary star. The dominant pulsating side of the primary star is centred on the L1 point. We have modelled the spectral energy distribution combined with radial velocities, and independently the TESS light curve combined with radial velocities. Both of these give excellent agreement and robust system parameters for both stars. The δ Sct star is an oblique pulsator with at least four low radial overtone (probably) f modes with the pulsation axis coinciding with the tidal axis of the star, the line of apsides. Preliminary theoretical modelling indicates that the modes must produce much larger flux perturbations near the L1 point, although this is difficult to understand because the pulsating star does not come near to filling its Roche lobe. More detailed models of distorted pulsating stars should be developed. These newly discovered single-sided pulsators offer new opportunities for astrophysical inference from stars that are oblique pulsators in close binary stars.
Planet Hunters IX. KIC 8462852 – where's the flux? Boyajian, T. S; LaCourse, D. M; Rappaport, S. A ...
Monthly notices of the Royal Astronomical Society,
04/2016, Letnik:
457, Številka:
4
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Over the duration of the Kepler mission, KIC 8462852 was observed to undergo irregularly shaped, aperiodic dips in flux of up to ∼20 per cent. The dipping activity can last for between 5 and 80 d. We ...characterize the object with high-resolution spectroscopy, spectral energy distribution fitting, radial velocity measurements, high-resolution imaging, and Fourier analyses of the Kepler light curve. We determine that KIC 8462852 is a typical main-sequence F3 V star that exhibits no significant IR excess, and has no very close interacting companions. In this paper, we describe various scenarios to explain the dipping events observed in the Kepler light curve. We confirm that the dipping signals in the data are not caused by any instrumental or data processing artefact, and thus are astrophysical in origin. We construct scenario-independent constraints on the size and location of a body in the system that are needed to reproduce the observations. We deliberate over several assorted stellar and circumstellar astrophysical scenarios, most of which have problems explaining the data in hand. By considering the observational constraints on dust clumps in orbit around a normal main-sequence star, we conclude that the scenario most consistent with the data in hand is the passage of a family of exocomet or planetesimal fragments, all of which are associated with a single previous break-up event, possibly caused by tidal disruption or thermal processing. The minimum total mass associated with these fragments likely exceeds 10−6 M⊕, corresponding to an original rocky body of >100 km in diameter. We discuss the necessity of future observations to help interpret the system.
HD 187547 was the first candidate that led to the suggestion that solar-like oscillations are present in delta Scuti stars. Longer observations, however, show that the modes interpreted as solar-like ...oscillations have either very long mode lifetimes, longer than 960 days, or are coherent. These results are incompatible with the nature of "pure" stochastic excitation as observed in solar-like stars. Nonetheless, one point is certain: the opacity mechanism alone cannot explain the oscillation spectrum of HD 187547. Here we present new theoretical investigations showing that convection dynamics can intrinsically excite coherent pulsations in the chemically peculiar delta Scuti star HD 187547. More precisely, it is the perturbations of the mean Reynold stresses (turbulent pressure) that drives the pulsations and the excitation takes place predominantly in the hydrogen ionization zone.
ABSTRACT
We present a seismic study of δ Scuti based on a mode identification from multicoulor photometry. The dominant frequency can be associated only with a radial mode and the second frequency ...is, most probably, a dipole mode. The other six frequencies have more ambiguous identifications. The photometric mode identification provided also some constraints on the atmospheric metallicity m/H ≈ +0.5 and microturbulent velocity ξt ≈ 4 km s−1. For models reproducing the dominant frequency, we show that only the fundamental mode is possible and the first overtone is excluded. However, the location of δ Scuti near the terminal age main sequence requires the consideration of three stages of stellar evolution. For the star to be on the main sequence, it is necessary to include overshooting from the convective core with a parameter of at least αov = 0.25 at the metallicity greater than Z = 0.019. It turned out that the value of the relative amplitude of the bolometric flux variations (the non-adiabatic parameter f) is mainly determined by the position of the star in the HR diagram, i.e. by its effective temperature and luminosity, whereas the effect of the evolutionary stage is minor. On the other hand, the convective efficiency in the subphotospheric layers has a dominant effect on the value of the parameter f. Comparing the theoretical and empirical values of f for the radial dominant mode, we obtain constraints on the mixing length parameter αMLT which is less than about 1.0, independently of the adopted opacity data and chemical mixture. This value of αMLT is substantially smaller than for a calibrated solar model indicating rather low to moderately efficient convection in the envelope of δ Scuti.
Abstract
In this paper we introduce the MOBSTER collaboration and lay out its scientific goals. We present first results based on the analysis of 19 previously known magnetic O, B, and A stars ...observed in 2-min cadence in sectors 1 and 2 of the Transiting Exoplanet Survey Satellite (TESS) mission. We derive precise rotational periods from the newly obtained light curves and compare them to previously published values. We also discuss the overall photometric phenomenology of the known magnetic massive and intermediate-mass stars and propose an observational strategy to augment this population by taking advantage of the high-quality observations produced by TESS.
ABSTRACT
Several studies have shown that a number of stars pulsating in p modes lie between the β Cep and δ Sct instability strips in the Hertzsprung–Russell (HR) Diagram. At present, there is no ...certain understanding of how p modes can be excited in this Teff range. The goal of this work is to disprove the conjecture that all stars pulsating in p modes and lying in this Teff range are the result of incorrect measurements of Teff, contamination, or the presence of unseen cooler companions lying in the δ Sct instability strip (given the high binary fraction of stars in this region of the HR Diagram). Using TESS data, we show that the A0Vnne star HD 42477 has a single p mode coupled to several r modes and/or g modes. We rule out a contaminating background star with a pixel-by-pixel examination, and we essentially rule out the possibility of a companion δ Sct star in a binary. We model the pulsations in HD 42477, and suggest that the g modes are excited by overstable convective core modes. We also conjecture that the single p mode is driven by coupling with the g modes, or that the oblateness of this rapidly-rotating star permits driving by He ii ionization in the equatorial region.
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![Magnetic OB[A] Stars with T...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0035-8711_1365-2966/small "Magnetic OB[A] Stars with TESS: probing their Evolutionary and Rotational properties (MOBSTER) – I. First-light observations of known magnetic B and A stars")