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Автор: Grafiati
Опубліковано: 6 вересня 2023

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1

Agrawal, Manindra, Rohit Gurjar, Arpita Korwar, and Nitin Saxena. "Hitting-Sets for ROABP and Sum of Set-Multilinear Circuits." SIAM Journal on Computing 44, no. 3 (January 2015): 669–97. http://dx.doi.org/10.1137/140975103.

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2

Balona, L. A. "Rapidly oscillating TESS A–F main-sequence stars: are the roAp stars a distinct class?" Monthly Notices of the Royal Astronomical Society 510, no. 4 (January 8, 2022): 5743–59. http://dx.doi.org/10.1093/mnras/stac011.

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Анотація:
ABSTRACT From sector 1–44 TESS observations, 19 new rapidly oscillating Ap (roAp) stars, 103 ostensibly non-peculiar stars with roAp-like frequencies, and 617 δ Scuti stars with independent frequencies typical of roAp stars were found. Examination of all chemically peculiar stars observed by TESS resulted in the discovery of 199 Ap stars that pulsate as δ Sct or γ Dor variables. The fraction of pulsating Ap stars is the same as the fraction of pulsating chemically normal stars. There is no distinct separation in frequency or radial order between chemically peculiar δ Sct stars and roAp stars. In fact, all the features that originally distinguished roAp from δ Sct stars in the past have disappeared. There is no reason to assume that the high frequencies in roAp stars are driven by a different mechanism from the high frequencies in chemically normal stars. However, chemically peculiar stars are far more likely to pulsate with high frequencies. The term ‘roAp’ should be dropped: all roAp stars are normal δ Scuti stars.
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3

Holdsworth, D. L., M. S. Cunha, D. W. Kurtz, V. Antoci, D. R. Hey, D. M. Bowman, O. Kobzar, et al. "TESS cycle 1 observations of roAp stars with 2-min cadence data." Monthly Notices of the Royal Astronomical Society 506, no. 1 (May 31, 2021): 1073–110. http://dx.doi.org/10.1093/mnras/stab1578.

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Анотація:
ABSTRACT We present the results of a systematic search for new rapidly oscillating Ap (roAp) stars using the 2-min cadence data collected by the Transiting Exoplanet Survey Satellite (TESS) during its Cycle 1 observations. We identify 12 new roAp stars. Amongst these stars we discover the roAp star with the longest pulsation period, another with the shortest rotation period, and six with multiperiodic variability. In addition to these new roAp stars, we present an analysis of 44 known roAp stars observed by TESS during Cycle 1, providing the first high-precision and homogeneous sample of a significant fraction of the known roAp stars. The TESS observations have shown that almost 60 per cent (33) of our sample of stars are multiperiodic, providing excellent cases to test models of roAp pulsations, and from which the most rewarding asteroseismic results can be gleaned. We report four cases of the occurrence of rotationally split frequency multiplets that imply different mode geometries for the same degree modes in the same star. This provides a conundrum in applying the oblique pulsator model to the roAp stars. Finally, we report the discovery of non-linear mode interactions in α Cir (TIC 402546736, HD 128898) around the harmonic of the principal mode – this is only the second case of such a phenomenon.
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4

Kurtz, D., and P. Martinez. "roAp stars." Highlights of Astronomy 10 (1995): 338–40. http://dx.doi.org/10.1017/s1539299600011461.

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Анотація:
Among the A stars there is a subclass of peculiar stars, the Ap stars, which show strongly enhanced spectral lines of the Fe peak, rare earth and lanthanide elements. These stars have global surface magnetic fields several orders of magnitude larger than that of the Sun, 0.3 to 30 kGauss is the measured range. For stars with the strongest magnetic fields, the spectral lines are split by the Zeeman Effect and the surface magnetic field strength can be measured. Generally, though, the magnetic fields are not strong enough for the magnetic splitting to exceed other sources of line broadening. In these cases residual polarization differences between the red and blue wings of the spectral lines give a measure of the effective magnetic field strength - the integral of the longitudinal component of the global magnetic field over the visible hemisphere, weighted by limb-darkening. In the Ap stars the effective magnetic field strengths vary with rotation. This is well understood in terms of the oblique rotator model in which the magnetic axis is oblique to the rotation axis, so that the magnetic field is seen from varying aspect with rotation.
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5

Matthews, Jaymie M. "Seismology of Pulsating Ap Stars: Results From The Past Decade, Prospects For The Next." International Astronomical Union Colloquium 139 (1993): 122–31. http://dx.doi.org/10.1017/s0252921100117087.

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Анотація:
AbstractSince the discovery of the first rapidly oscillating Ap (roAp) star in 1978 by Kurtz, this class of magnetic chemically-peculiar pulsators has grown to over two dozen. The eigenfrequency spectra of roAp stars (with periods of ∼ 6 – 15 min) are consistent with nonradial p- modes of low degree and high overtone n, not unlike the Sun's five-minute oscillations seen in integrated light. However, unlike the Sun, the strong global dipole fields of roAp stars significantly affect the pulsations.Although much of the effort in the last decade has been towards detecting new roAp candidates and refining the frequencies of known variables, initial “seismic” analyses have already yielded important results. Measurements of fundamental frequency spacings constrain the luminosities and radii of some roAp stars. In addition, mode splitting provides: (1) an independent determination of rotation period, even in the absence of longer-term light variations; (2) limits on the rotational inclination i and magnetic obliquity β; and (3) an indication of the relative internal field strengths of certain roAp stars. Very recently, the temperature - optical depth structure of the atmosphere of HR 3831 was inferred from optical and IR photometry of its oscillations.Judging from current developments, the next decade promises exciting results on both observational and theoretical fronts. Several roAp stars have now been monitored for over a decade, allowing us to investigate long-term period changes due to evolution, binarity, etc. Eigenfrequency models for stars in the mass and radius range appropriate for Ap stars are becoming available, as well as explicit treatments of the perturbations due to magnetic fields. Armed with these, we may be able to place some roAp stars on a theoretical (or “asteroseismological H-R“) diagram to derive independently their masses and main-sequence ages.
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6

Paris, Sophie, Jean-Paul Debeaupuis, Reto Crameri, Marilyn Carey, Franck Charlès, Marie Christine Prévost, Christine Schmitt, Bruno Philippe, and Jean Paul Latgé. "Conidial Hydrophobins of Aspergillus fumigatus." Applied and Environmental Microbiology 69, no. 3 (March 2003): 1581–88. http://dx.doi.org/10.1128/aem.69.3.1581-1588.2003.

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ABSTRACT The surface of Aspergillus fumigatus conidia, the first structure recognized by the host immune system, is covered by rodlets. We report that this outer cell wall layer contains two hydrophobins, RodAp and RodBp, which are found as highly insoluble complexes. The RODA gene was previously characterized, and ΔrodA conidia do not display a rodlet layer (N. Thau, M. Monod, B. Crestani, C. Rolland, G. Tronchin, J. P. Latgé, and S. Paris, Infect. Immun. 62:4380-4388, 1994). The RODB gene was cloned and disrupted. RodBp was highly homologous to RodAp and different from DewAp of A. nidulans. ΔrodB conidia had a rodlet layer similar to that of the wild-type conidia. Therefore, unlike RodAp, RodBp is not required for rodlet formation. The surface of ΔrodA conidia is granular; in contrast, an amorphous layer is present at the surface of the conidia of the ΔrodA ΔrodB double mutant. These data show that RodBp plays a role in the structure of the conidial cell wall. Moreover, rodletless mutants are more sensitive to killing by alveolar macrophages, suggesting that RodAp or the rodlet structure is involved in the resistance to host cells.
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7

Balona, L. A., D. L. Holdsworth, and M. S. Cunha. "High frequencies in TESS A–F main-sequence stars." Monthly Notices of the Royal Astronomical Society 487, no. 2 (May 23, 2019): 2117–32. http://dx.doi.org/10.1093/mnras/stz1423.

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Анотація:
Abstract The driving mechanism for high-frequency oscillations in some chemically peculiar Ap stars, the rapidly oscillating Ap stars (roAp stars), is not understood. The Transiting Exoplanet Survey Satellite mission (TESS) data provide an ideal opportunity to extend the number of roAp stars that might provide further clues to address this problem. From an examination of over 18 000 stars in TESS sectors 1–7, we have discovered high-frequency pulsations in 14 A–F stars, of which only 3 are classified as Ap stars. In addition to these new discoveries, we discuss the frequencies in nine previously known roAp stars. In one of these stars, HD 60435, we confirm a previous finding that the pulsations have lifetimes of only a few days. In another known roAp star, HD 6532, the relative amplitudes of the rotationally modulated sidelobes, which are generally used to estimate the inclination of the magnetic axis relative to the rotational axis, are significantly different from those found in ground-based B-band photometric observations. We also discuss four δ Scuti stars that appear to have independent frequencies similar to those of roAp stars.
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8

Martinez, Peter, and D. W. Kurtz. "Rapid Pulsations in Ap Stars." International Astronomical Union Colloquium 155 (1995): 58–69. http://dx.doi.org/10.1017/s0252921100036782.

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Анотація:
AbstractThe rapidly oscillating Ap (roAp) stars are upper mainsequence stars that pulsate in non-radial p modes of high overtone. We present an observer’s overview of the roAp phenomenon and discuss significant developments since the last major observational reviews of Kurtz (1990a) and Matthews (1991).
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9

Cunha, M. S., J. M. M. B. Fernandes, and M. J. P. F. G. Monteiro. "Seismic Tests of Theoretical Models of HR 1217." International Astronomical Union Colloquium 185 (2002): 302–3. http://dx.doi.org/10.1017/s0252921100016316.

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Анотація:
A way in which we might learn about the physics of the interior of roAp stars is by studying their oscillations. To that aim we have initiated a study in order to investigate what additional constraints might be imposed to the physics of the interior of roAp stars from the study of their oscillation spectra. We have chosen to start this work by studying the well known multi-periodic roAp star HR 1217. The observed data used for HR1217 were L/L⊙=7.8±0.7 and Teff=7400±100 K, from Matthews et al. (1999), Zs=0.009 estimated from Ryabchikova et al. (1997), and Δv=67.91±0.12μHz from Kurtz et al. (1989).
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10

Holdsworth, Daniel L., Hideyuki Saio, and Donald W. Kurtz. "HD 42659: the only known roAp star in a spectroscopic binary observed with B photometry, TESS, and SALT." Monthly Notices of the Royal Astronomical Society 489, no. 3 (September 2, 2019): 4063–71. http://dx.doi.org/10.1093/mnras/stz2419.

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Анотація:
ABSTRACT We present a multi-instrument analysis of the rapidly oscillating Ap (roAp) star HD 42659. We have obtained B photometric data for this star and use these data, in conjunction with the Transiting Exoplanet Survey Satellite (TESS) observations, to analyse the high-frequency pulsation in detail. We find a triplet that is split by the rotation frequency of the star (νrot = 0.3756 d−1; Prot = 2.66 d) and present both distorted dipole and distorted quadrupole mode models. We show that the pulsation frequency, 150.9898 d−1 (Ppuls = 9.54 min), is greater than the acoustic cut-off frequency. We utilize 27 high-resolution ($R\simeq 65\, 000$), high signal-to-noise ratio (∼120) spectra to provide new orbital parameters for this, the only known roAp star to be in a short-period binary (Porb = 93.266 d). We find the system to be more eccentric than previously thought, with e = 0.317, and suggest the companion is a mid-F to early-K star. We find no significant trend in the average pulsation mode amplitude with time, as measured by TESS, implying that the companion does not have an effect on the pulsation in this roAp star. We suggest further photometric observations of this star, and further studies to find more roAp stars in close binaries to characterize how binarity may affect the detection of roAp pulsations.
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11

Weiss, W. W., T. A. Ryabchikova, F. Kupka, T. R. Lueftinger, I. S. Savanov, and V. P. Malanushenko. "Spectroscopic Survey of Rapidly Oscillating Ap Stars." International Astronomical Union Colloquium 176 (2000): 487–88. http://dx.doi.org/10.1017/s0252921100058565.

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Анотація:
AbstractA spectroscopic survey of roAp stars has been initiated in Vienna in order to determine their fundamental astrophysical parameters and abundances. We report here on our attempt to confirm and elaborate an atmospheric peculiarity recently discovered (Ryabchikova et al. 1999) which should allow to identify roAp stars with a single spectrum.
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12

Hubrig, S., N. Kharchenko, and G. Mathys. "The Single Life of Rapidly Oscillating Ap Stars." Symposium - International Astronomical Union 185 (1998): 311–12. http://dx.doi.org/10.1017/s0074180900238837.

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Анотація:
Rapidly oscillating Ap (roAp) stars are cool magnetic Ap SrCrEu stars which pulsate in high-overtone (n ≫ l), low-degree (ℓ ≤ 3) p-modes, with periods from 6 to 15 minutes and typical amplitudes of a few millimagnitudes. 28 such stars are currently known. The roAp phenomenon is confined to a well-defined region of the Strömgren photometry parameter space. However, this region also contains other Ap stars, in which no pulsation could be detected, despite sometimes thorough searches. These apparently constant Ap stars (non-oscillating Ap stars, or noAp stars) appear remarkably similar to the roAp stars in many respects (e.g. colour indices, abundances, magnetic fields). Here we present recently found indications for differences between both groups.
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13

Audard, N., F. Kupka, W. W. Weiss, P. Morel, and J. Provost. "The Acoustic Cut-Off Frequency of roAp Stars." Symposium - International Astronomical Union 185 (1998): 299–300. http://dx.doi.org/10.1017/s0074180900238801.

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Анотація:
For 5 out of 28 known rapidly oscillating magnetic chemically peculiar (roAp) stars, the largest observed frequency seems to exceed the theoretical acoustic cutoff frequency, which is determined by the outermost stellar regions. We show that a better modelling of the atmosphere reconciles the theory with the observations for at least the roAp star α Cir.
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14

Matthews, Jaymie M., William H. Wehlau, John Rice, and Gordon A. H. Walker. "Atmospheric Structure of the Pulsating Ap [CP2] Star HR 3831 from Rapid Multicolour Photometry." International Astronomical Union Colloquium 137 (1993): 199–201. http://dx.doi.org/10.1017/s0252921100017784.

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Анотація:
Rapidly oscillating Ap (roAp) stars are cool magnetic CP2 stars which vary with periods of a few minutes and amplitudes less than 0.01 mag and 1 km/s in light and velocity. Analysis of their p-mode eigenfrequency patterns and splittings gives information about evolutionary state, rotation rate, magnetic field geometry and internal field strength (see Kurtz 1990; Matthews 1991). We present here an example of how roAp pulsations can be used to obtain an estimate of the temperature structure of an Ap atmosphere.The pulsation amplitudes of roAp stars decline more rapidly with increasing wavelength than other known pulsators. Matthews et al. (1990) explained this by the wavelength dependence of limb darkening and its weighting effect on the integrated amplitude of an (l, m) = (1,0) mode.
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15

Balmforth, N. J., M. S. Cunha, N. Dolez, D. O. Gough, and S. Vauclair. "Excitation Mechanism in roAp Stars." International Astronomical Union Colloquium 176 (2000): 453–54. http://dx.doi.org/10.1017/s0252921100058395.

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Анотація:
AbstractIn the present work we develop a theoretical model for roAp stars characterized by the suppression of convection around the magnetic poles. When calculating the growth rates of acoustic oscillations in models of this type we find that most models whose positions in the HR diagram coincide with that of the observed roAp stars are unstable against high-order pulsations.
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16

Saio, Hideyuki. "Pulsation of magnetic stars." Proceedings of the International Astronomical Union 9, S301 (August 2013): 197–204. http://dx.doi.org/10.1017/s1743921313014324.

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Анотація:
AbstractSome Ap stars with strong magnetic fields pulsate in high-order p modes; they are called roAp (rapidly oscillating Ap) stars. The p-mode frequencies are modified by the magnetic fields. Although the large frequency separation is hardly affected, small separations are modified considerably. The magnetic field also affects the latitudinal amplitude distribution on the surface. We discuss the properties of axisymmetric p-mode oscillations in roAp stars.
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17

Hubrig, S., S. P. Järvinen, I. Ilyin, K. G. Strassmeier та M. Schöller. "The rapidly oscillating Ap star γ Equ: linear polarization as an enhanced pulsation diagnostic?" Monthly Notices of the Royal Astronomical Society: Letters 508, № 1 (24 серпня 2021): L17—L21. http://dx.doi.org/10.1093/mnrasl/slab101.

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Анотація:
ABSTRACT We present the first short time-scale observations of the rapidly oscillating Ap (roAp) star γ Equ in linear polarized light obtained with the Potsdam Echelle Polarimetric and Spectroscopic Instrument installed at the Large Binocular Telescope. These observations are used to search for pulsation variability in Stokes Q and U line profiles belonging to different elements. The atmospheres of roAp stars are significantly stratified with spectral lines of different elements probing different atmospheric depths. roAp stars with strong magnetic fields, such as γ Equ with a magnetic field modulus of 4 kG and a pulsation period of 12.21 min, are of special interest because the effect of the magnetic field on the structure of their atmospheres can be studied with greatest detail and accuracy. Our results show that we may detect changes in the transversal field component in Fe i and rare earth element lines possessing large second-order Landé factors. Such variability can be due to the impact of pulsation on the transverse magnetic field, causing changes in the obliquity angles of the magnetic force lines. Further studies of roAp stars in linear polarized light and subsequent detailed modelling are necessary to improve our understanding of the involved physics.
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18

Cunha, M. S. "Pulsations in roAp Stars." International Astronomical Union Colloquium 185 (2002): 272–79. http://dx.doi.org/10.1017/s0252921100016237.

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Анотація:
AbstractThe current status of the research concerning rapidly oscillating Ap stars is reviewed, paying particular attention to the interplay between recent developments in the theoretical and observational aspects of these stars.
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19

Medupe, R., J. Christensen-Dalsgaard, and D. W. Kurtz. "Applications of Non-Adiabatic Radial Pulsation Equations to roAp Stars." International Astronomical Union Colloquium 185 (2002): 296–99. http://dx.doi.org/10.1017/s0252921100016298.

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Анотація:
AbstractWe apply a code that solves grey radial non-adiabatic pulsation equations with consistent treatment of radiative transfer to the roAp frequency regime. We find evidence for the existence of chromospheric modes at frequencies 2.5 mHz, 3.7 mHz, 4.6 mHz and 5.7 mHz in the equilibrium model we used. We also find that, since the oscillations are not adiabatic at the surfaces of roAp stars, better surface boundary conditions need to be considered in future studies.
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20

Shibahashi, H. "Theoretical Aspects of the Rapidly Oscillating AP Stars." Highlights of Astronomy 11, no. 2 (1998): 686–88. http://dx.doi.org/10.1017/s1539299600018463.

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Анотація:
Until the rapid oscillations in Przybylski’s star, HD 101065, were discovered by Kurtz in 1978, chemical peculiarity and pulsation were thought to be mutually exclusive. Though the location of the Ap stars in the HR diagram overlaps with that of the Delta Scuti stars, no Ap stars were known to pulsate, and the Delta Scuti type pulsating stars, with a few exceptions, were not claimed to reveal chemical peculiarity. The striking impact of the discovery of the rapid oscillations of Ap stars is that the basic conception of the exclusiveness of the chemical peculiarity and the pulsation was broken. So far, twenty-nine Ap stars have been discovered to be rapidly oscillating Ap (roAp) stars (Kurtz 1997). The observed pulsations of Ap stars are, however, different from those of the Delta Scuti stars in various aspects. The pulsation periods of roAp stars are typically 10.minutes and are much shorter than those of the Delta Scuti stars, which are typically 2 hrs. In some cases, the amplitudes are modulated with the same period and phase as the magnetic strength variation. The amplitudes of some of the roAp stars are very stable, while some others show a fairly short-term variation of a time scale of a day. Some of the roAp stars show a long-term variation of the frequency with a time scale of years.
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21

Kurtz, D. W., and Peter Martinez. "Determination of Luminosity, Atmospheric Structure, and Magnetic Geometry from Studies of the Pulsation in roAp Stars." International Astronomical Union Colloquium 138 (1993): 561–76. http://dx.doi.org/10.1017/s0252921100021035.

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Анотація:
AbstractFor several of the rapidly oscillating Ap stars the best luminosity estimates available come from the asteroseismological interpretation of their pulsational frequency spectra. We give a list of the 23 currently known roAp stars along with their Strömgren photometric indices, Teff estimated from the Hβ index, and luminosity estimated asteroseismologically. In one case, ϒ Equ, we have an asteroseismological luminosity and a parallax luminosity which are in good agreement. Some of the roAp stars pulsate with frequencies greater than the critical frequency calculated for standard A-star models. This plus multi-colour high-speed photometry of HR 3831 indicate that the temperature gradients in the atmospheres of these stars are substantially steeper than in standard A-star models. We advocate a fine analysis of HR 3831 to see if there is consistency with the pulsational conclusions about T-τ in this star. Further fine analyses and multi-colour pulsational analyses on other roAp stars are then called for. The pulsation mode in HR 3831 can be decomposed into primarily an axisymmetric dipole mode with small radial, quadrupole and octupole perturbations. If the magnetic field is governing the distortion of this mode from a purely dipole mode, then the pulsation can be used to infer the magnetic field geometry. Comments on our current knowledge of all 23 roAp stars are made.
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22

Hatzes, A. P., A. Kanaan, and D. Mkrtichian. "Radial Velocity Observations of Non-radially Pulsating Stars." International Astronomical Union Colloquium 170 (1999): 166–76. http://dx.doi.org/10.1017/s025292110004851x.

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Анотація:
AbstractThe rapidly oscillating Ap stars (roAp) are a class of nonradially pulsating stars oscillating in low-degree modes with periods of 4–15 minutes. We have started a program to study the oscillations on several roAp stars using precise radial velocity (RV) measurements. The typical mean RV amplitude for the roAp stars we have observed is 50–400 ms−1, but this amplitude depends on the spectral region used for the measurement of the RV amplitude. A detailed line-by-line analysis reveals that the pulsational amplitude depends not only on atomic species, but on the line strength as well. For a given atomic species weak spectral lines exhibit a pulsational amplitude 10–100 times higher than for strong lines. The elemental effect can be understood in the context of the inhomogeneous distribution of elements known to occur on these stars and that is believed to result from the global dipole magnetic fields that are present. For instance, if an element is concentrated near the magnetic pole then it may have a higher RV amplitude than one that is distributed about the magnetic equator. The line strength effect is interpreted as arising from vertical structure to the pulsations since weaker lines are formed, on average, deeper in the atmosphere than stronger lines. Precise RV measurements may prove to be a powerful tool for probing both the vertical and horizontal structure of the pulsations in roAp stars.
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23

Kupka, F., M. Gelbmann, U. Heiter, R. Kuschnig, W. W. Weiss, and T. A. Ryabchikova. "Fine Analysis of Pulsating CP Stars." International Astronomical Union Colloquium 155 (1995): 317–18. http://dx.doi.org/10.1017/s0252921100037283.

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24

Engelbrecht, C. A., F. A. M. Frescura, B. S. Frank, and P. R. Nicol. "Multi-season photometry of the newly-discovered roAp star HD75445." Proceedings of the International Astronomical Union 6, S271 (June 2010): 377–78. http://dx.doi.org/10.1017/s174392131101787x.

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Анотація:
AbstractHD75445 was recently announced by Kochukhov et al. (2009) to be a low-amplitude roAp star, based on spectroscopic measurements. We present putative pulsation frequencies of HD75445 determined from 22 hours of Johnson B photometry obtained in 2008, 2009 and 2010. We present the first photometric periodicities detected in this star. We make a marginal detection of one of Kochukhov et al. (2009)'s spectroscopic periods, along with a range of confidently detected periodicities covering the low-frequency end of the roAp instability spectrum and the high-frequency end of the Delta Scuti instability spectrum.
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25

Gautschy, Alfred, Hideyuki Saio, and Housi Harzenmoser. "How to drive roAp stars." Monthly Notices of the Royal Astronomical Society 301, no. 1 (November 1998): 31–41. http://dx.doi.org/10.1046/j.1365-8711.1998.01960.x.

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26

Mkrtichian, David E., and Artie P. Hatzes. "Echelle-Diagrams for roAp Stars." International Astronomical Union Colloquium 176 (2000): 455–56. http://dx.doi.org/10.1017/s0252921100058401.

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27

Zita, E. J. "Magnetic Oscillations in Radiative Stars." International Astronomical Union Colloquium 176 (2000): 386–87. http://dx.doi.org/10.1017/s0252921100058140.

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AbstractWhat drives acoustic modes in magnetic, radiative stars, such as the rapidly oscillating Ap (roAp) stars? We propose that oscillations in the shape of the magnetic field can drive magnetosonic waves, which are observed as p modes. The force-free atmosphere minimizes magnetic energy. If the field is strong enough, the magnetic configuration can oscillate about its equilibrium state. Initial calculations showed that the wavenumbers, frequencies, and available energy are consistent with observations of roAp stars (Zita 1997). We present the results of new calculations and proposed tests of our model. We also discuss the resultant <υ×b> dynamo, which may shed light on observations in B stars.
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28

Kochukhov, O., and T. Ryabchikova. "Time-Resolved High-Resolution Spectroscopy of roAp Stars." International Astronomical Union Colloquium 185 (2002): 284–87. http://dx.doi.org/10.1017/s0252921100016250.

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AbstractWe report results of Spectroscopic monitoring of the roAp stars γ Equ, α Cir and HR 3831 with the ESO 3.6-meter telescope. Series of very high-resolution and high S/N spectra allowed to resolve changes of line profiles due to the pulsations. We found that pulsational behaviour of all three roAp stars is dominated by the variations of the doubly ionized rare-earth lines. Detailed analysis of the pulsational changes of Nd III and Pr III spectral features allowed us to identify the pulsational mode of γ Equ and to study rotational modulation of the pulsational pattern in the spectra of α Cir and HR 3831.
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29

Matthews, Jaymie M., William H. Wehlau, John Rice та Gordon A. H. Walker. "An Empirical T – τ Curve for the Roap Star HR 3831: Atmospheric Structure from Pulsation Amplitudes". International Astronomical Union Colloquium 138 (1993): 617–22. http://dx.doi.org/10.1017/s0252921100021096.

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The atmospheric structures of magnetic CP2 (Ap) stars are notoriously difficult to model: line blanketing is severe, surface gravities are extremely uncertain, and the surface abundance inhomogeneities lead to different atmospheric properties as a function of position on the star. Seismology of the p-modes of rapidly oscillating Ap (roAp) stars (Kurtz 1990), which vary with periods of a few minutes and amplitudes below 0.01 mag and 1 km/s in light and velocity, has already helped constrain the luminosities – and hence, the logg values – of some cool CP2 stars (Kurtz 1992, these proceedings). We show here that the pulsations of an roAp star can also directly probe the temperature structure of a CP2 atmosphere.
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30

Ghosal, Purnata, and B. V. Raghavendra Rao. "On Proving Parameterized Size Lower Bounds for Multilinear Algebraic Models." Fundamenta Informaticae 177, no. 1 (December 18, 2020): 69–93. http://dx.doi.org/10.3233/fi-2020-1980.

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We consider the problem of obtaining parameterized lower bounds for the size of arithmetic circuits computing polynomials with the degree of the polynomial as the parameter. We consider the following special classes of multilinear algebraic branching programs: 1) Read Once Oblivious Branching Programs (ROABPs), 2) Strict interval branching programs, 3) Sum of read once formulas with restricted ordering. We obtain parameterized lower bounds (i.e., nΩ(t(k)) lower bound for some function t of k) on the size of the above models computing a multilinear polynomial that can be computed by a depth four circuit of size g(k)nO(1) for some computable function g. Further, we obtain a parameterized separation between ROABPs and read-2 ABPs. This is obtained by constructing a degree k polynomial that can be computed by a read-2 ABP of small size such that the rank of the partial derivative matrix under any partition of the variables is large.
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31

Cunha, Margarida S. "Magnetoacoustic oscillations in Ap stars." Proceedings of the International Astronomical Union 5, H15 (November 2009): 362–63. http://dx.doi.org/10.1017/s1743921310009816.

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32

Bigot, L. "Asteroseismology of the Rapidly Oscillating Ap Stars." Symposium - International Astronomical Union 203 (2001): 90–93. http://dx.doi.org/10.1017/s0074180900218834.

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33

Kochukhov, O. "Observations of pulsations in roAp stars." Communications in Asteroseismology 150 (2007): 39–47. http://dx.doi.org/10.1553/cia150s39.

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34

Ryabchikova, T., G. A. Wade, M. Aurière, S. Bagnulo, J. F. Donati, S. V. Jeffers, N. Johnson, et al. "Rotational periods of four roAp stars." Astronomy & Astrophysics 429, no. 3 (January 2005): L55—L58. http://dx.doi.org/10.1051/0004-6361:200400112.

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35

Ryabchikova, T., N. Nesvacil, W. W. Weiss, O. Kochukhov, and Ch Stütz. "The spectroscopic signature of roAp stars." Astronomy & Astrophysics 423, no. 2 (August 2004): 705–15. http://dx.doi.org/10.1051/0004-6361:20041012.

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36

Paunzen, E., M. Netopil, M. Rode-Paunzen, G. Handler, H. Božić, D. Ruždjak, and D. Sudar. "The Hvar survey for roAp stars." Astronomy & Astrophysics 542 (June 2012): A89. http://dx.doi.org/10.1051/0004-6361/201118752.

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37

Dupret, M.-A., S. Théado, and A. Noels. "The driving mechanism of roAp stars." Journal of Physics: Conference Series 118 (October 1, 2008): 012052. http://dx.doi.org/10.1088/1742-6596/118/1/012052.

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38

Paunzen, E., M. Netopil, M. Rode-Paunzen, G. Handler, and H. Božić. "The Hvar survey for roAp stars." Astronomy & Astrophysics 575 (February 13, 2015): A24. http://dx.doi.org/10.1051/0004-6361/201425281.

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39

Kurtz, D. W., V. G. Elkin, G. Mathys, J. Riley, M. S. Cunha, H. Shibahashi, and E. Kambe. "Some recent discoveries in roAp stars." Proceedings of the International Astronomical Union 2004, IAUS224 (July 2004): 343–52. http://dx.doi.org/10.1017/s1743921304004740.

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40

Riley, J. D., D. W. Kurtz, and M. S. Cunha. "Long period oscillations in roAp stars." Proceedings of the International Astronomical Union 2004, IAUS224 (July 2004): 767–69. http://dx.doi.org/10.1017/s1743921305009713.

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41

Hatzes, A. P., A. Kanaan, and D. Mkrtichian. "Radial Velocity Variations of the Rapidly Oscillating Ap Star 33 Lib." International Astronomical Union Colloquium 170 (1999): 183–86. http://dx.doi.org/10.1017/s0252921100048533.

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AbstractWe present precise stellar radial velocity (RV) measurements of the rapidly oscillating Ap (roAp) star 33 Lib taken in rapid succession over a 3-hr time span. A Fourier analysis of the data clearly shows the 8.2 min. pulsation period found previously by photometric investigations and gives a peak-to-peak (2K) amplitude of about 80 ms−1. We find, like in other roAp stars we have studied, that the RV amplitude depends on the spectral region used for measuring the pulsational RV amplitude and is as high as 57 ± 4.7 ms−1 in the region 5411–5500 Å and as low as 7 ± 3 ms−1 in the 5877–5976 Å region. An analysis of individual spectral lines show considerable scatter in the RV amplitude, ranging as high as 320 ms−1 and as low as 7 m−1. There is an overall trend of increasing RV amplitude with decreasing line strength. We also found that spectral lines due to nickel have a higher mean RV amplitude than chromium lines. We believe that the line strength variations result from the vertical atmospheric structure of the pulsations and that the elemental differences are related to the inhomogeneous distribution of elements known to occur on Ap stars. Precise stellar radial velocity studies of roAp stars may be a powerful tool for studying both the spatial (surface) and vertical structure to the pulsational velocity field.
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42

Martinez, Peter. "The Cape Rapidly Oscillating Ap Star Survey." International Astronomical Union Colloquium 155 (1995): 345–46. http://dx.doi.org/10.1017/s0252921100037428.

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AbstractThe Cape rapidly oscillating Ap star survey is a systematic search for rapidly oscillating Ap stars in the southern hemisphere. To date, 12 new roAp stars have been discovered. This paper describes the current state of the survey.
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43

Jayaraman, Rahul, Donald W. Kurtz, Gerald Handler, Saul Rappaport, and George Ricker. "Two New roAp Stars Discovered with TESS." Research Notes of the AAS 5, no. 11 (November 22, 2021): 268. http://dx.doi.org/10.3847/2515-5172/ac3a8a.

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Abstract We present two new rapidly oscillating Ap (roAp) stars, TIC 198781841 and TIC 229960986, discovered in TESS photometric data. The periodogram of TIC 198781841 has a large peak at 166.506 day−1 (1.93 mHz), with two nearby peaks at 163.412 day−1 (1.89 mHz) and 169.600 day−1 (1.96 mHz). These correspond to three independent high-overtone pressure modes, with alternating even and odd ℓ values. TIC 229960986 has a high-frequency triplet centered at 191.641 day−1 (2.218 mHz), with sidebands at 191.164 day−1 (2.213 mHz) and 192.119 day−1 (2.224 mHz). This pulsation appears to be a rotationally split dipole mode, with sideband amplitudes significantly larger than that of the central peak; hence, both pulsation poles are seen over the rotation cycle. Our photometric identification of two new roAp stars underscores the remarkable ability of TESS to identify high-frequency pulsators without spectroscopic observations.
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44

Mathys, G. "Magnetic fields of rapidly oscillating Ap stars." International Astronomical Union Colloquium 139 (1993): 132. http://dx.doi.org/10.1017/s0252921100117099.

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Magnetic field appears to play a major role in the pulsations of rapidly oscillating Ap (roAp) stars. Understanding of the behaviour of these objects thus requires knowledge of their magnetic field. Such knowledge is in particular essential to interpret the modulation of the amplitude of the photometric variations (with a frequency very close to the rotation frequency of the star) and to understand the driving mechanism of the pulsation. Therefore, a systematic programme of study of the magnetic field of roAp stars has been started, of which preliminary (and still very partial) results are presented here.Magnetic fields of Ap stars can be diagnosed from the Zeeman effect that they induced in spectral lines either from the observation of line-splitting in high-resolution unpolarized spectra (which only occurs in favourable circumstances) or from the observation of circular polarization of the lines in medium- to high-resolution spectra.
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45

Nelson, Matthew J., and Tobias J. Kreidl. "New Limits on the Instability Strip of the HRD: Observations from a Northern Sky Survey of Ap Stars for Rapid Variability." International Astronomical Union Colloquium 137 (1993): 727–29. http://dx.doi.org/10.1017/s0252921100018728.

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AbstractThe search for pulsational variables that occupy the hotter realms of the instability strip in the Hertzsprung-Russel diagram (HRD) have been traditionally restricted to the regions where models indicated that pulsation is tractable. Recent surveys of stars have revealed, however, that stars that were thought not to lie in the instability strip do indeed pulsate. This raises questions about the extent of the instability strip.While this has implications for all hot pulsating stars, we use here primarily our survey results of Ap stars to compare with the models. We report results here from a high-speed photometric survey of 120 Ap stars that was conducted between 1985 and 1991 to search for rapid variability. The absence of pulsations in the hotter Ap stars (roughly B8-A3) is noted and deemed significant on the basis of the number of stars observed in this temperature range, as well as the overall quality of the data. This, however, does not preclude their existence, especially since HD 218495 was recently discovered to be a rapidly oscillating Ap (roAp) star (Martinez, Kurtz and Kaufmann 1991), and has a spectral type of about A3.Color-magnitude diagrams of the survey stars are presented, with the known roAp stars included for reference. The diagrams are presented in the Strömgren and Geneva systems. The color-magnitude diagrams demonstrate the completeness of the survey in covering Ap stars at a wide range of temperatures. We find no obvious means of using color indices to differentiate roAp stars from non-pulsating Ap stars.
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46

Weiss, W. W., T. A. Ryabchikova, I. Savanov, N. Piskunov, V. Tsymbal, P. Mittermayer, P. Martinez, O. Kochukhov, and N. Nesvacil. "Spectroscopy of Rapidly Oscillating Ap Stars." International Astronomical Union Colloquium 185 (2002): 280–83. http://dx.doi.org/10.1017/s0252921100016249.

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AbstractWe identify Spectroscopic differences between roAp stars and Ap stars with no observational evidence for pulsation, but with otherwise similar Teff and log g values. These differences concern the abundance pattern, Hydrogen line profile anomalies, evidence for stratification, and effects of pulsation on spectral lines.
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47

Kreidl, T. J. "Chaos in Pulsating Variable Stars: Preliminary Analysis of Photometric Photometry and Observational Constraints of Detection." International Astronomical Union Colloquium 139 (1993): 133. http://dx.doi.org/10.1017/s0252921100117105.

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AbstractChaos theory has been applied to a variety of variable stars, but few convincing candidates for chaos have been identified. Here, well-established analysis methods have been applied to some very extensive data sets of rapidly oscillating Ap (roAp) stars and one white dwarf. It it shown that in spite of the amount of data, the signal-to-noise ratio makes positive detection of chaos extremely difficult, especially due to scintillation noise. A new form of dimension computation is presented and discussed. Simple models were constructed to show what noise levels can be tolerated before the detection of chaos is no longer possible and comparisons are drawn with data that could be obtained in the future from space. The lack of phase and amplitude stability in HD 134214 and mode switching in HD217522 and HD 137949 are pointed out as the possible results of chaos, making frequent monitoring of roAp stars desirable.
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48

Sachkov et al., M. "Vertical structure of pulsations in roAp stars." Communications in Asteroseismology 150 (2007): 81–82. http://dx.doi.org/10.1553/cia150s81.

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49

Kurtz, D. W. "Discussion on δ Scuti and roAp star". Communications in Asteroseismology 150 (2007): 87–90. http://dx.doi.org/10.1553/cia150s87.

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50

Balona, L. A., та C. D. Laney. "Radial velocity study of the roAp starαCircinus". Monthly Notices of the Royal Astronomical Society 344, № 1 (вересень 2003): 242–46. http://dx.doi.org/10.1046/j.1365-8711.2003.06819.x.

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