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Stars are the building blocks of planetary systems, clusters, associations, and galaxies. The evolution of stars is driven by physical processes in their interiors making theory of stellar interior structure and evolution an important ingredient of contemporary astrophysics. Despite its importance, this theory contains major shortcomings starting from the early stages of stellar evolution which significantly impact all subsequent evolutionary phases. Studying the pulsations of young intermediate-mass stars, i.e., conducting pre-main sequence (pre-MS) asteroseismology, has the potential to contribute to a better understanding of the processes acting during the earliest phases of stellar evolution. With ultra-precise observational data obtained from space and from ground in combination with improvements of our theoretical models for pre-MS stars, the field of pre-MS asteroseismology will advance in the future and provide important constraints for the input physics of early stellar evolution.

A new perspective of pulsation in stars within the δ Scuti instability strip has recently emerged as a result of Kepler observations. The majority of stars within the instability strip do not pulsate and practically all δ Scuti stars contain low frequencies. Because γ Doradus stars co-exist with δ Sct stars in the same region of the instability strip, it follows that γ Dor stars are driven by the same mechanism as δ Sct stars. The difference must be due to different mode selection processes. The search for an unknown damping factor which is missing from the models will be essential for further progress. Maia variables and hot γ Dor stars are briefly discussed. Luminosities of roAp stars obtained from Gaia DR2 parallaxes and spectroscopic effective temperatures show that the roAp stars are slightly evolved with temperatures in the range 6,300–8,300 K, considerably cooler than predicted by the models. The roAp stars and stars with solar-like oscillations share the same mass—temperature—luminosity relation, but with frequencies which are about 50 percent higher. This suggests that roAp frequencies are determined by the critical acoustic frequency, but this frequency is larger than in standard models, perhaps as a result of a temperature inversion in the atmosphere.

In this study, pulsational and physical characteristics of two δ Scuti stars, V775 Tau and V483 Tau, are analysed by using four-year high-precision photometric data of the STEREO satellite. Thus, it is aimed to gain new insights into behaviours of these pulsators and evolution of δ Scuti, γ Dor and Am type stars. The data are taken between 2007-2011 and examined with the help of the Lomb-Scargle method. The detection precision in the four-year combined data is around 10−5 cd−1 in frequency and 10−5 mag in amplitude. It is revealed that V775 Tau exhibits weak pulsation characteristic which is interpreted as the existence of the interaction between the helium loss in the partial ionization zone and pulsation intensities. It is also considered that the absence of strong pulsations is also related to the evolution status of the star. Further, its periodogram shows low-frequency peaks. If these oscillations are g-modes, V775 Tau can be thought to be one of the rare stars that show all γ Dor, δ Scuti and Am type variations. V483 Tau is comparatively more luminous, hotter and has higher rotational velocity. Therefore, although it shares the same region with V775 Tau in the H-R diagram, it is not considered to be an Am star. Yet, it exactly overlaps with the γ Dor stars. These clues as well as g-modes detected in its periodogram indicate that V483 Tau is a hybrid star. Finally, both V775 Tau and V483 Tau display period changes whose rates are between 10−3 and 10−4 yr−1. Considering the δ Scuti nature, it may be speculated that these changes are non-evolutionary. [ABSTRACT FROM AUTHOR]

The Fourier analyses are performed for photoelectric observations of two δ Scuti stars V567 Oph and KU Cen. It is shown that V567 Oph is a multiperiodic star and KU Cen has one main period.

By using a non-local and time-dependent theory of convection, the linear non-adiabatic oscillations of radial and low-degree non-radial F-p8 modes are calculated for the stellar evolutionary models in the mass range of 1.4∼3.0 M ʘ , with three helium abundances of convective envelope ( Y =0.28, 0.13, 0.00), and the solar metal abundance ( Z =0.02). The numerical results show that the red edge of theoretical δ Scuti instability strip almost does not change with the helium abundance. With the decrease of the helium abundance, the blue edge of the δ Scuti instability strip moves toward the direction of low temperatures, the high-temperature stars on the hot side of instability strip become more stable, while the low-temperature stars on the cold side of instability strip become more instable. It seems to be impossible to explain the non-variable stars in the δ Scuti instability strip by using the diffusion of helium. However, the ratios of non-variable stars to variable stars in the hot and cold sides of the δ Scuti strip may be taken as the observational evidence of helium diffusion. [ABSTRACT FROM AUTHOR]

VZ Cnc is a population I double mode high amplitude δ Scuti variable. We observed the star and collected the data from January 2007 to January 2009 to determine 14 times of maximum light. We also searched its times of maximum light from other papers and from IBVS and got 57 values. We collected a total of 194 times of maximum light and used them to perform an (O-C) analysis and concluded that there may be no tendency of binary orbital light time effect. But by parabola tendency, it shows a continuous period increasing at the rate of 1.4×10−8 per year; this is compatible with the stellar evolution model calculation both in direction and size. [ABSTRACT FROM AUTHOR]

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 . Three of the four SPB-type modes are most likely or 2. Comparison with theoretical model calculations suggests that γ Peg is either a radial fundamental mode pulsator or a 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. [ABSTRACT FROM AUTHOR]

Only a fraction of the theoretically predicted non-radial pulsation modes have so far been observed in δ Scuti stars. Nevertheless, the large number of frequencies detected in recent photometric studies of selected δ Scuti stars allow us to look for regularities in the frequency spacing of modes. Mode identifications are used to interpret these results. Statistical analyses of several δ Scuti stars (FG Vir, 44 Tau, BL Cam and others) show that the photometrically observed frequencies are not distributed at random, but that the excited non-radial modes cluster around the frequencies of the radial modes over many radial orders. The observed regularities can be partly explained by modes trapped in the stellar envelope. This mode selection mechanism was proposed by Dziembowski & Królikowska and shown to be efficient for modes. New pulsation model calculations confirm the observed regularities. We present the s– f diagram, which compares the average separation of the radial frequencies ( s) with the frequency of the lowest frequency unstable radial mode ( f). This provides an estimate for the value of the observed star, if we assume that the centres of the observed frequency clusters correspond to the radial mode frequencies. This assumption is confirmed by examples of well-studied δ Scuti variables in which radial modes were definitely identified. [ABSTRACT FROM AUTHOR]

We have carried out a photometric and spectroscopic survey of bright high-amplitude δ Scuti (HADS) stars. The aim was to detect binarity and multiperiodicity (or both) in order to explore the possibility of combining binary star astrophysics with stellar oscillations. Here, we present the first results for 10, predominantly southern, HADS variables. We detected the orbital motion of RS Gru with a semi-amplitude of ∼6.5 km s−1 and 11.5 d period. The companion is inferred to be a low-mass dwarf star in a close orbit around RS Gru. We found multiperiodicity in RY Lep from both photometric and radial velocity data and detected orbital motion in the radial velocities with hints of a possible period of 500–700 d. The data also revealed that the amplitude of the secondary frequency is variable on the time-scale of a few years, whereas the dominant mode is stable. Radial velocities of AD CMi revealed cycle-to-cycle variations, which might be due to non-radial pulsations. We confirmed the multiperiodic nature of BQ Ind, while we obtained the first radial velocity curves of ZZ Mic and BE Lyn. The radial velocity curve and the O–C diagram of CY Aqr are consistent with the long-period binary hypothesis. We took new time series photometry on XX Cyg, DY Her and DY Peg, with which we updated their O–C diagrams. [ABSTRACT FROM AUTHOR]

We present a new catalogue of variable stars compiled from the data taken for the University of New South Wales Extrasolar Planet Search. From 2004 October to 2007 May, 25 target fields were each observed for one to four months, resulting in ∼87 000 high-precision light curves with 1600–4400 data points. We have extracted a total of 850 variable light curves, 659 of which do not have a counterpart in the General Catalogue of Variable Stars, the New Suspected Variables catalogue or the All Sky Automated Survey southern variable star catalogue. The catalogue is detailed here, and includes 142 Algol-type eclipsing binaries, 23 β Lyrae-type eclipsing binaries, 218 contact eclipsing binaries, 53 RR Lyrae stars, 26 Cepheid stars, 13 rotationally variable active stars, 153 uncategorized pulsating stars with periods <10 d, including δ Scuti stars, and 222 long period variables with variability on time-scales of >10 d. As a general application of variable stars discovered by extrasolar planet transit search projects, we discuss several astrophysical problems which could benefit from carefully selected samples of bright variables. These include (i) the quest for contact binaries with the smallest mass ratio, which could be used to test theories of binary mergers; (ii) detached eclipsing binaries with pre-main-sequence components, which are important test objects for calibrating stellar evolutionary models and (iii) RR Lyrae-type pulsating stars exhibiting the Blazhko effect, which is one of the last great mysteries of pulsating star research. [ABSTRACT FROM AUTHOR]

Abstract: We present V band light curves of δ Scuti type variable CC And, obtained between August and December 2005 at the Ankara University Observatory (AUG) and the TÜBİTAK National Observatory (TUG). By application of multiple-frequency analyses using Period04 (V 1.0) to photometric observations of CC And, a seven-frequency solution was found to be well fitted to the data. In accordance with the computed Q values for CC And, it was found that this star has p-mode and g-mode oscillations. [Copyright &y& Elsevier]

We report the discovery of the first high-amplitude δ Scuti star in an eclipsing binary, which we have designated UNSW-V-500. The system is an Algol-type semi-detached eclipsing binary of maximum brightness . A best-fitting solution to the binary light curve and two radial velocity curves is derived using the Wilson–Devinney code. We identify a late-A spectral-type primary component of mass 1.49 ± 0.02 M⊙ and a late-K spectral-type secondary of mass 0.33 ± 0.02 M⊙, with an inclination of , and a period of 5.350 4751 ± 0.000 0006 d. A Fourier analysis of the residuals from this solution is performed usingperiod04 to investigate the δ Scuti pulsations. We detect a single pulsation frequency of , and it appears that this is the first overtone radial mode frequency. This system provides the first opportunity to measure the dynamical mass for a star of this variable type; previously, masses have been derived from stellar evolution and pulsation models. [ABSTRACT FROM AUTHOR]

High-resolution spectroscopic observations of the rapidly rotating δ Scuti star γ Bootis have been carried out in 2005, over six consecutive nights, in order to search for line-profile variability. Time-series, consisting of flux measurements at each wavelength bin across the Ti ii 4571.917 Å line profile as a function of time, have been Fourier analysed. The results confirm the early detection reported by Kennelly et al. of a dominant periodic component at frequency 21.28 cycles d−1 in the observer's frame, probably due to a high-azimuthal-order sectorial mode. Moreover, we found other periodicities at 5.06, 12.02 cycles d−1, probably present but not secure, and at 11.70 and 18.09 cycles d−1, uncertain. The latter frequency, if present, should be identifiable as another high-azimuthal-order sectorial mode and the three additional terms as low- l modes as proved by the analysis of the first three moments of the line. Owing to the short time baseline and the one-site temporal sampling we consider our results only preliminary but encouraging for a more extensive multisite campaign. A refinement of the atmospheric physical parameters of the star has been obtained from our spectroscopic data and adopted for preliminary computations of evolutionary models of γ Bootis. [ABSTRACT FROM AUTHOR]

A comparison between linear-stability analysis and observations of pulsation modes in five δ Scuti stars, belonging to the same cluster, is presented. The study is based on a work by Michel et al., in which such a comparison was performed for a representative set of model solutions obtained independently for each individual star considered. In this paper, we revisit that work by Michel et al. following, however, a new approach which consists of the search for a single, complete, and coherent solution for all the selected stars, in order to constrain and test the assumed physics describing these objects. To do so, refined descriptions for the effects of rotation on the determination of the global stellar parameters and on the adiabatic oscillation frequency computations are used. In addition, a crude attempt is made to study the role of rotation on the prediction of mode instabilities. The present results are found to be comparable with those reported by Michel et al. Within the temperature range , agreement between observations and model computations of unstable modes is restricted to values for the mixing-length parameter . This indicates that for these stars a smaller value for is required than that suggested from a calibrated solar model. We stress the point that the linear-stability analysis used in this work still assumes stellar models without rotation and that further developments are required for a proper description of the interaction between rotation and pulsation dynamics. [ABSTRACT FROM AUTHOR]

We have made an asteroseismic analysis of the variable blue stragglers in the open cluster M67. The data set consists of photometric time-series from eight sites using nine 0.6–2.1 m telescopes with a time-baseline of 43 d. In two stars, EW Cnc and EX Cnc, we detect the highest number of frequencies (41 and 26) detected in δ Scuti stars belonging to a stellar cluster, and EW Cnc has the second highest number of frequencies detected in any δ Scuti star. We have computed a grid of pulsation models that take the effects of rotation into account. The distribution of observed and theoretical frequencies shows that in a wide frequency range a significant fraction of the radial and non-radial low-degree modes are excited to detectable amplitudes. Despite the large number of observed frequencies we cannot constrain the fundamental parameters of the stars. To make progress we need to identify the degrees of some of the modes from either multicolour photometry or spectroscopy. [ABSTRACT FROM AUTHOR]

We report on an ambitious multisite campaign aimed at detecting stellar variability, particularly solar-like oscillations, in the red giant stars in the open cluster M67 (NGC 2682). During the six-week observing run, which comprised 164 telescope nights, we used nine 0.6-m to 2.1-m class telescopes located around the world to obtain uninterrupted time series photometry. We outline here the data acquisition and reduction, with emphasis on the optimization of the signal-to-noise ratio of the low-amplitude (50–500 μmag) solar-like oscillations. This includes a new and efficient method for obtaining the linearity profile of the CCD response at ultrahigh precision (∼10 parts per million). The noise in the final time series is 0.50 mmag per minute integration for the best site, while the noise in the Fourier spectrum of all sites combined is 20 μmag. In addition to the red giant stars, this data set proves to be very valuable for studying high-amplitude variable stars such as eclipsing binaries, W UMa systems and δ Scuti stars. [ABSTRACT FROM AUTHOR]

The Rapid Temporal Survey (RATS) is a survey to detect objects whose optical intensity varies on time-scales of less than ∼70 min. In our pilot data set taken with the Isaac Newton Telescope and the Wide Field Camera in 2003 November, we discovered nearly 50 new variable objects. Many of these varied on time-scales much longer than 1 h. However, only four objects showed a modulation on a time-scale of 1 h or less. This paper presents followup optical photometry and spectroscopy of these four objects. We find that RAT J0455 + 1305 is a pulsating (on a period of 374 s) subdwarf B star of the EC 14026 type. We have modelled its spectrum and determine and which locates it on the cool edge of the EC 14026 instability strip. It has a modulation amplitude which is one of the highest of any known EC 14026 star. Based on their spectra, photometric variability and their infrared colours, we find that and J0807 + 1510 are likely to be SX Phe stars – dwarf δ Sct stars. Our results show that our observing strategy is a good method for finding rare pulsating stars. [ABSTRACT FROM AUTHOR]

A catalogue of close eclipsing binary systems (detached and semidetached) with at least one of the components located in the δ Scuti region of the Cepheid instability strip is presented. The positions of the stars in the instability strip are determined by their accurate temperatures and luminosities. Observationally detected binaries (20 semidetached, four detached and one unclassified) with oscillating components were included in the catalogue as a separate table. The primaries of the oscillating Algols tend to be located near the blue edge of the instability strip. Using reliable luminosities and temperatures determined by recent photometric and spectroscopic studies, we have found that at least one or two components of 71 detached and 90 semidetached systems are located in the δ Scuti region of the Cepheid instability strip. In addition, 36 detached or semidetached systems discovered by the Hipparcos satellite were also given as a separate list. One of their components is seen in the δ Scuti region, according to their spectral type or colours. They are potential candidate binaries with the δ Scuti-type pulsating components which need further photometric and spectroscopic studies in better precision. This catalogue covers information and literature references for 25 known and 197 candidate binaries with pulsating components. [ABSTRACT FROM AUTHOR]

The nature of the observed amplitude variability of several modes in the δ Scuti star FG Vir is examined. This is made possible by the extensive photometry obtained during 2002–04, as well as the long photometric time-base starting in 1992. In this star, three frequencies show strong amplitude and phase variations. In the power spectrum, these frequencies also show up as frequency doublets. However, since true amplitude variability of a single frequency can also lead to (false) frequency doublets in the power spectrum, a specific test examining in detail the observed amplitude and the phase variations of an assumed single frequency is applied. For the frequencies at 12.15 and 23.40 cycle d−1, it is shown that the amplitude variability of a single mode can be ruled out. In particular, an important property of beating between two modes is fulfilled: the amplitude and the phase vary synchronously with a phase shift close to 90°. The origin of the amplitude variability of a third mode, namely, near 19.86 cycle d−1, is not clear due to the long beat period of 20+ yr, for which the amplitude–phase test suffers from gaps in the coverage. However, even for this frequency, the amplitude variations can be expressed well by a mathematical two-mode model. If we examine these three close frequency pairs together with other (usually more widely separated) close frequencies in FG Vir, 18 pairs of frequencies with separations closer than 0.10 cycle d−1 have been detected. It is shown that the majority of the pairs occur near the theoretically expected frequencies of radial modes. Mode identifications are available for only a few modes: the only detected radial mode at 12.15 cycle d−1 is part of a close pair. It is shown that accidental agreements between the frequencies of excited modes can be ruled out because of the large number of detected close frequency doublets. [ABSTRACT FROM AUTHOR]

Abstract: We present the results of spectroscopy and B, V CCD photometry carried out from 2003 to 2005 of GSC4619-450, a high-amplitude pulsating variable discovered by Zhang et al., [Zhang, X.B., Deng, L., Zhou, X., Xin, Y., 2004. MNRAS, 355, 1369]. From this study, the star is classified as a high-amplitude δ Scuti star. A Fourier analysis based on the B and V photometric data indicates that GSC 4619-450 is a mono-periodic radial pulsator, with a period of 0.13341195 days. From a short time line we find that the pulsating period of this star is increasing rapidly with a rate of 9.1×10−2 s/yr, and is thus suggested to be highly evolved and undergoing rapid evolving at the present time. A spectral type of F0 is assigned to the variable considering the results from both the spectroscopy and photometry. [Copyright &y& Elsevier]

Considering a sample of 20 eclipsing binary systems with δ Scuti type primaries, we discovered that there is a possible relation among the pulsation periods of the primaries and the orbital periods of the systems. According to this empirical relation, the longer the orbital period of a binary, the longer the pulsation period of its pulsating primary. Among the sample, the masses of the secondaries and the separations between the components are known for eight systems for which a versus log F (the gravitational pull exerted per gram of the matter on the surface of the primaries by the secondaries) diagram also verifies such an interrelation between the periods. So, as the gravitational force applied by the secondary component onto the pulsating primary increases, its pulsation period decreases. The detailed physics underlying this empirical relation between the periods needs further confirmation, especially theoretically. However, one must also consider the fact that the present sample does not contain a sufficiently large sample of longer period binaries. [ABSTRACT FROM AUTHOR]

This thesis is on the framework of the "NainiTal-Cape Survey" programme for searching photometric variability in chemically peculiar stars, initiated in 1997 at the ARIES, NainiTal, India, in collaboration with the South African Astronomical Observatory, Cape Town and ISRO, Bangalore. This paper presents the corresponding results. The Am stars HD 98851, HD 102480, HD 13079 and HD 113878 were discovered to exhibit a δ Scuti type of variability. Photometric variability was also discovered in HD 13038, for which the type of peculiarity and variability is not yet fully explained. The null results obtained in this survey are also presented and discussed, as they provide an interesting data reference for upcoming studies. [ABSTRACT FROM AUTHOR]

We apply for the first time the time-dependent convection (TDC) treatment of Gabriel and Grigahcène et al. to the mode identification and seismic study of δ Sct stars. We consider the influence of this treatment on the photometric amplitude ratios and phase differences, and compare our TDC results to frozen convection (FC) results. We also compare the results obtained with different values of the mixing-length (ML) parameter α. Finally, we identify the modes and perform a seismic study of the stars V784 Cassiopeae (Cas), 1 Monocerotis (Mon) and 28 Andromedae (And), and show that our TDC models agree better with observations than FC models. [ABSTRACT FROM AUTHOR]

A search for pulsating pre-main-sequence (PMS) stars was performed in the young open cluster NGC 6383 using CCD time-series photometry in JohnsonBandVfilters. With an age of only∼1.7 Myr all cluster members later than spectral type A0 have not reached the ZAMS yet, hence being ideal candidates for investigating PMS pulsation among A- and F-type stars. In total 286 stars have been analysed using classical Fourier techniques. From about a dozen stars within the boundaries of the classical instability strip, two stars were found to pulsate: NGC 6383#170, with five frequencies simultaneously, and NGC 6383#198, with a single frequency. In addition, NGC 6383#152 is a suspected PMS variable star, but our data remain inconclusive. Linear, non-adiabatic models assuming PMS evolutionary phase and purely radial pulsation were calculated for the two new PMS pulsators. NGC 6383#170 appears to pulsate radially in third and fifth overtones, while the other three frequencies seem to be of non-radial nature. NGC 6383#198 pulsates monoperiodically, most probably in the third radial overtone. Magnitudes andcolours were available in the literature for only one third of all stars and we used them for calibrating the remaining. [ABSTRACT FROM AUTHOR]

Reports on the discovery of unusual pulsations in the cool, evolved Am stars HD 98851 and HD 102480. Time series photometric and spectroscopic observations; Pulsation frequencies; H-line spectral classes of the stars.

An investigation on the subject of evolution of propagation diagrams and obtaining the theoretical frequency spectrum of some δ Scuti star models with masses around 2 M[sub ⊙] has been presented. The variations of some eigenfunctions throughout the star, the variation of ω[sup 2] with logT[sub e] and the distribution of nodes of eigenfunctions of some modes along with the evolution of the star have been plotted and examined. For the models at the ZAMS phase, pure acoustic modes in the outer stellar layers and pure gravity modes in the interiors were calculated. A mixed character of low degree nonradial modes was found in evolved models. These modes behave like p modes in the envelope and like g modes in the interior. [ABSTRACT FROM AUTHOR]

We present the results of a three-year photometric study of the δ Scuti star UV Trianguli. Our data sets consist of 9378 differential measurements in Johnson V together with a few data collected into the Strömgren uvbyβ system. UV Tri is at least a biperiodic variable. The two best-fitting frequencies, f[SUB1] = 9.3298 d[SUP-1] and f[SUB2] = 10.8513 d[SUP-1], are still not the complete set of pulsation frequencies representing the light variations of the start A suspected third frequency might present in the star. Several 'anomalous cycles' are observed in the light curves. They seem real, but are aperiodic. We derive the colour indices and physical parameters for the variable and conclude that it is a Population I δ Sct star with normal metal abundance ([Me/H] = 0.0 ± 0.1 dex) evolving on its main-sequence stage at an early evolutionary phase before the turn-off point. Finally, we compare the observed oscillation frequencies with theoretical models. The two pulsation modes of UV Tri are likely to be non-radial gravity modes. [ABSTRACT FROM AUTHOR]

The results of a multisite photometric campaign of θ[SUP2] Tau are reported. This binary system consists of an evolved and a main-sequence A star, both situated inside the instability strip. The 12th Delta Scuti Network campaign included 152 h of high-precision photometry obtained at four observatories. This leads to the derivation of 11 frequencies of pulsation in the 10.8 to 14.6 cycle d[SUP-1] range. These frequencies confirm the results from previous Earth-based (1982-1986) as well as satellite (2000) photometry, although amplitude variability on a time-scale of several years is present. We show that at least two high frequencies (26.18 and 26.73 cycle d[SUP-1]) are also present in θ[SUP2] Tau. Arguments are given that these high frequencies originate in the main-sequence companion and are not combination frequencies, fi + fi, from the primary. Models for both the primary and the secondary components were checked for the instability against radial and non-radial oscillations. All hot models of the primary with T[SUBeff] > 8000 K are stable in the observed frequency range. The best fit between the theoretical and observed frequency ranges is achieved for models with T[SUBeff] ≈ 7800 K (or slightly higher), in agreement with photometric calibrations. The instability range spans two or three radial orders in the range p[SUB4] to p[SUB6] for radial modes. Post-main-sequence models (with or without overshooting) are preferable for the primary, but main-sequence models with overshooting cannot be excluded. For the less-luminous secondary component the instability range is wider and spans 5 to 7 radial orders from p[SUB2] to p[SUB8]. The observed frequencies lie around radial modes p[SUB5] to p[SUB6]. The main uncertainties of these results are caused by a simple treatment of the convective flux in the hydrogen ionization zone. [ABSTRACT FROM AUTHOR]

We present 318 high-dispersion echelle spectra and 450 UBVR[sub C]I[sub C] observations of the pre-main-sequence star V351 Orionis. These data were obtained simultaneously over a two-week period. The star is unusual in that it no longer shows the frequent sharp fadings that occurred prior to 1986. We find features characteristic of Herbig Ae stars: Hα emission, sharp and broad Na iiabsorption components and weak He iabsorption. The Hα profile is strongly variable with a time-scale of 1 d and normally is an inverse P Cygni profile, but on occasions overt emission disappears altogether. The equivalent widths of the sharp Na iiabsorption lines have decreased quite markedly since 1995, indicating a decrease in the column density of gas in the line of sight. We also detect very weak forbidden [O i] emission. There is no evidence for veiling of the star, but a possible 1.0-d or 0.5-d period in the equivalent widths of the lines could indicate the presence of an accretion hotspot. The star is of particular interest because it is pulsating. We detect at least two periods in the light variation and a further period from the radial velocity variations. Attempts at photometric mode identification were not successful, but from the morphology of the line profiles we can identify one mode as non-radial. In addition, we see on some nights several residual absorption subfeatures moving across the line profile from blue to red. These are almost certainly caused by one or more modes of high degree (ℓ≈8) . [ABSTRACT FROM AUTHOR]

We present simultaneous UBVRI photometry and high-dispersion spectroscopy of the δ Scuti star QQ Tel. At least seven periodicities are detected in the light curve, but there are likely to be many more. The line profile variations suggest that some of the observed frequencies may be due to modes of moderately high degree (ℓ&gsim;4) . [ABSTRACT FROM AUTHOR]

Time-series photometry of the Hipparcos variable stars HD 199434 and 21190 is reported. Both stars are pulsators of the δ Scuti type. Reclassifications of the MK types of the stars, based on new spectrograms, are given. HD 21190 is found to be F2III SrEuSi:, making it the most evolved Ap star known. Its Strömgren photometric indices support the peculiar spectral type. It is also one of the most evolved δ Scuti stars known. Its combined Ap–δ Scuti nature makes it an important test of models of pulsation in peculiar stars recently developed by Turcotte et al., although it is more extreme than any model they examined. Physical parameters of both stars are estimated from Strömgren and Hβ photometry, and Hipparcos absolute magnitudes. We attempt mode identifications based on amplitude ratios and phase differences from our photometry. The dominant pulsation of HD 21190 may be an overtone radial mode. The model fits for HD 199434 are even less satisfactory, but favour an ℓ=2 mode. Given the good quality and wavelength coverage of our data, the poor results from the application of the photometric theory of mode identification may call into question the use of that technique. [ABSTRACT FROM AUTHOR]

We analyse 147 h of single-site CCD time series photometry of the multiperiodic low-amplitude δ Scuti star XX Pyx with the aim of investigating variability at low frequencies. Part of the data were obtained in the context of the 1998 multisite campaign on XX Pyx, the results of which were described by Handler et al. We find that periodic low-frequency variations are present in the XX Pyx light curves, and we detect two frequencies at f[sub A]=0.8695 cycle d[sup -1] and f[sub B]=1.7352 cycle d[sup -1], respectively, with amplitudes of 4.5 and 5.4 mmag. The low-frequency variability is intrinsic to XX Pyx, and cannot be attributed to instrumental or atmospheric effects. The near 2:1 ratio of the frequencies leads us to suggest that XX Pyx is a δ Scuti star in a binary system, with a possible binary period of 27.6 h. This is strongly supported by the detection of radial velocity variations from the re-analysis of echelle spectra obtained by Handler et al. However, in the absence of a spectroscopic period, alternative explanations of the photometric variability involving pulsation are also possible; the variations occur close to possible combination frequencies of the short-period (δ Scuti) variations, but high Q values of 0.57 and 0.28 d suggest that the variations are not a result of normal (p-)modes. They could possibly be due to g-modes excited to observable amplitudes as a result of resonance effects. Surface features (spots) are unlikely to be the cause of the variations. We searched for combination frequencies ( f[sub i]-f[sub j]), f being the normal δ Scuti frequencies detected by Handler et al., but failed to detect any. [ABSTRACT FROM AUTHOR]

We present new multicolour photometry and simultaneous high-dispersion spectroscopy for the δ Scuti star 1 Mon. The two main periodicities in the star are still present, but the third known period is not directly detected in the new radial velocities or light variations. However, it is detected in the periodogram of the second moment of the line profile variations. We use the cross-correlation function as an approximation for the line profile variations. By computing theoretical profiles for a given mode and comparing them with phased cross-correlation profiles, we are able to determine a goodness-of-fit criterion and estimate the most probable spherical harmonic degree, ℓ, the azimuthal order, m, of the pulsation and also the angle of inclination. We then compare the relative amplitudes and phases of the photometric variations in five wavebands and obtain the best estimates of ℓ for the two visible periodicities. We confirm the earlier determinations that the main periodicity is a radial mode and that the other periodicity is probably ℓ= 1 , m[sup -] =- 1. We show that the line profile variations and light variations give consistent results. We point out the importance of a long wavelength range when using the photometric mode identification technique. Finally, we attempt to match the two periods with unstable modes from linear, non-adiabatic calculations. We are able to show that the principal period is well matched by either the fundamental or first overtone radial mode, but could not find a satisfactory fit to the ℓ= 1 mode. We discuss implications for mode identification of δ Scuti stars based on what we have learned from this star. [ABSTRACT FROM AUTHOR]

We report multisite observations devoted to the main-sequence δ Scuti star XX Pyx, conducted as the 17th run of the Delta Scuti Network. Over 125 nights a total of 550 h of usable time-series photometric B- and V-filter data were acquired involving both photoelectric and CCD measurements at eight observatories spread around the world, which represents the most extensive single time-series for any pulsating star other than the Sun obtained so far. We describe our observations and reduction methods, and present the frequency analysis of our new data. First, we detect six new pulsation and five new combination frequencies in the star's light curves. We also discover evidence for amplitude and/or frequency variations of some of the modes during the observations. These can occur on time-scales as short as 20 d and show quite diverse behaviour. To take them into account in the frequency analysis, a so-called non-linear frequency analysis method was developed, allowing us to quantify the temporal variability of the modes and to compensate for it. Following that we continue the frequency search and we also incorporate published multisite observations. In this way, we reveal three more pulsation and two more combination frequencies. In the end, we report a total of 30 significant frequencies – 22 of which correspond to independent pulsation modes. This is the largest number of independent modes ever detected in the light curves of a δ Scuti star. The frequencies of the modes show preferred separations as already suggested by previous work on this star; they are also arranged in clear patterns. These results lead to a refinement of the stellar mean density [equation] and to a new constraint on the rotation rate of XX Pyx (ν[sub rot]=1.1±0.3 d[sup -1]). However, our attempts to identify the modes by pattern recognition failed. Moreover, mode identification from multicolour photometry failed as well because the high pulsation frequencies make this method unfavourable. The diverse behaviour of the amplitude and frequency variations of some of the modes leaves resonances as the only presently known possibility for their explanation. [ABSTRACT FROM AUTHOR]

ABSTRACT The discovery of very low-amplitude periodic brightness variations of β Pictoris is reported. Three frequencies in the approximate range 47–53 d[sup -1] could be extracted from four observing runs spanning a week. The character of the variations, together with the high incidence of δ Scuti pulsations in mid A type stars, suggests that β Pic is a member of this class of variable stars. [ABSTRACT FROM AUTHOR]

HD 34282 has been found to pulsate during a systematic search for short-term photometric variability in Herbig Ae/Be stars with the goal of determining the position and size of the pre-main-sequence instability strip. Simultaneous Strömgren photometry is used in the frequency analysis, yielding two frequencies with values of and cycle d−1. The light curve with the largest amplitude is that of the u band. This behaviour, which is not common for δ Scuti stars, is explained as pulsation in a high radial order in stars near the blue edge of the instability strip. The main period, with a value of 18.12 min, represents the shortest period observed so far for a δ Scuti-type pulsator. A seismic modelling, including instability predictions and rotation effects, has been attempted. It is found that both main-sequence and pre-main-sequence models predict modes in the range of 56 to 82 cycle d−1 (between 648 and 949 μHz), corresponding to oscillations of radial order n from 6 to 8. The highest of the observed frequencies only becomes unstable for models of low metallicity, in agreement with results from spectroscopic measurements. [ABSTRACT FROM AUTHOR]