Search

TESS observations of pulsating hot main sequence stars paint a very different picture from what is currently accepted. There are large numbers of delta Scuti (DSCT) stars hotter than the theoretical hot edge of the instability strip, continuing to what appear to be DSCT stars of mid-B type (historically known as Maia variables). The frequencies of maximum amplitude in DSCT stars are in poor agreement with unstable frequencies from the models. There is a well-defined upper envelope in the frequencies of maximum amplitude as a function of effective temperature for DSCT and MAIA stars which requires an explanation. The gamma Doradus (GDOR) stars should be regarded as DSCT stars with suppressed high frequencies rather than a separate class. They are found mostly among the cool DSCT stars, but occur throughout the DSCT instability strip and as early A-type stars, where they merge with the SPB variables. The mixture of DSCT and GDOR stars throughout the instability strip is one example of the unexplained large variation of frequency patterns in DSCT stars. The location of beta Cephei stars in the H-R diagram agrees quite well with the models, but the observed frequencies are generally higher than predicted. There are no discernible boundaries between the traditional classes of pulsating stars. Existing pulsation models do not describe the observations at all well and a re-evaluation is required., Comment: 9 pages, 7 figures

From TESS photometry, 493 mid- to late-B stars with high frequencies (Maia variables) have been identified. The distribution of projected rotational velocities shows that the rotation rates of Maia variables are no different from those of SPB stars. Moreover, many Maia stars pulsate with frequencies exceeding 60 c/d. Rapid rotation is ruled out as a possible factor in understanding the Maia variables. There is clearly a serious problem with current pulsational models. Not only are the models unable to account for the Maia stars, but they also fail to account for the fact that SPB and gamma Dor variables form one continuous instability strip from the cool end of the delta Sct region to the hot end of the beta Cep instability strip. Likewise, there is continuity between the distributions of delta Sct, Maia, and beta Cep variables. In fact, Maia stars seem to be an extension of delta Sct stars to the mid-B type. These observations suggest an interplay between multiple driving mechanisms rather than separate dominant mechanisms for each variability group., Comment: 5 pages, 1 table, 3 figures

Visual classification of the variability classes of over 120,000 Kepler, K2 and TESS stars is presented. The sample is mainly based on stars with known spectral types. Since variability classification often requires the location of the star in the H--R diagram, a catalogue of effective temperatures was compiled. Luminosities were estimated from Gaia DR3 parallaxes. The different classes of variable found in this survey are discussed. Examples of light curves and periodograms for common variability classes are shown. A catalogue of projected rotational velocities is also included., Comment: 11 pages, 2 figures, 6 tables

Inspection of the periodograms of TESS delta Scuti stars indicates that there is little, if any, similarity between the frequencies of stars in the same region of the H-R diagram. This is difficult to understand because pulsation models predict that stars with similar physical parameters should have similar frequencies. To investigate the problem, a quantitative measure of similarity between frequency patterns is described. When applied to non-adiabatic pulsation models with similar temperatures and luminosities, a strong correlation is found between the frequency patterns, as expected. The correlation increases when rotational frequency splitting is included. When applied to observations, no correlation can be found, confirming the impression from visual inspection. It seems that each star has its own unique frequency pattern, unrelated to its position in the H-R diagram. This presents a problem in our understanding of stellar pulsation. The existence of a period-luminosity law and the effect of combination frequencies in delta Scuti stars is briefly discussed., Comment: 8 pages, 7 figures, 1 table

Classification of over 50000 TESS stars in sectors 1-18 has resulted in the detection of 766 pulsating main sequence B stars as well as over 5000 $\delta$ Scuti, 2300 $\gamma$ Doradus and 114 roAp candidates. It is found that a distinct group of $\beta$ Cephei stars, defined as B stars with frequencies greater than 2.5 d$^{-1}$ and predicted by models, does not exist. Instead, high frequencies are to be found over the whole B-star range, eventually merging with $\delta$ Scuti stars. The cool B stars pulsating in high frequencies are the Maia variables. It is shown that Maia variables are not rapidly-rotating $\beta$ Cephei or SPB stars. In the region where $\beta$ Cephei variables are found, the proportion of pulsating stars is larger and amplitudes are higher and a considerable fraction pulsate in a single mode and low rotation rate. There is no distinct SPB instability region. Stars pulsating sole in low frequencies are found among all B stars. At most, only one-third of B stars appear to pulsate. These results, as well as the fact that a large fraction of A and B stars show rotational modulation, indicate a need for a revision of current ideas regarding stars with radiative envelopes., Comment: 9 pages, 7 figures, 3 tables

Light curves of 57 classical Be stars in TESS sectors 1 - 15 are examined. In most Be stars, the periodogram shows groups at a fundamental and one or more harmonics, which we attribute to rotation. In about 40 percent of the stars, the group is just a single narrow or slightly broadened peak. In about 30 percent, it consists of a multiple, closely spaced peaks. These groups can be interpreted as non-coherent variations most likely associated with photospheric gas clouds. Approximate rotational frequencies for about 75 percent of the stars can be derived. Comparison with the projected rotational velocities shows that the photometric frequency is consistent with rotation. The first harmonic plays a prominent role in many Be stars and manifests itself in either single-wave or double-wave light curves. The reduction in amplitude of beta Cep pulsations in a few Be stars during an outburst and their subsequent recovery is most likely an obscuration effect. Other instances of possible obscuration of the photosphere are suspected. A simple model, which attempts to explain these observations and other general properties of Be stars, is proposed., Comment: 17 pages 17 figures 3 Tables

It has long been thought that starspots are not present in the A and B stars because magnetic fields cannot be generated in stars with radiative envelopes. Space observations show that a considerable fraction of these stars vary in light with periods consistent with the expected rotation periods. Here we show that the photometric periods are the same as the rotation periods and that starspots are the likely cause for the light variations. This discovery has wide-ranging implications and suggests that a major revision of the physics of hot stellar envelopes may be required., Comment: 6 pages, 2 figures

We present the results of a search for Galactic Beta Cephei stars, which are massive pulsating stars with both pressure modes and mixed modes. Thus, these stars can serve as benchmarks for seismological studies of the interiors of massive stars. We conducted the search by performing a frequency analysis on the optical light curves of known O- and B-type stars with data from the KELT exoplanet survey. We identify 113 Beta Cephei stars, of which 86 are new discoveries, which altogether represents a ~70% increase in the number presently known. An additional 96 candidates are identified. Among our targets, we find five new eclipsing binaries and 22 stars with equal frequency spacings suggestive of rotational splitting of nonradial pulsation modes. Candidates for runaway stars among our targets and a number of interesting individual objects are discussed. Most of the known and newly discovered Beta Cephei stars will be observed by the TESS mission, providing by far the most comprehensive observational data set of massive main sequence pulsating stars of sufficient quality for detailed asteroseismic studies. Future analysis of these light curves has the potential to dramatically increase our understanding of the structure of stellar interiors and the physical processes taking place therein., Comment: 40 pages, 11 figures, 9 tables

All stars in the Kepler field brighter than 12.5 magnitude have been classified according to variability type. A catalogue of {\delta} Scuti and {\gamma} Doradus stars is presented. The problem of low frequencies in {\delta} Sct stars, which occurs in over 98 percent of these stars, is discussed. Gaia DR2 parallaxes were used to obtain precise luminosities, enabling the instability strips of the two classes of variable to be precisely defined. Surprisingly, it turns out that the instability region of the {\gamma} Dor stars is entirely within the {\delta} Sct instability strip. Thus {\gamma}Dor stars should not be considered a separate class of variable. The observed red and blue edges of the instability strip do not agree with recent model calculations. Stellar pulsation occurs in less than half of the stars in the instability region and arguments are presented to show that this cannot be explained by assuming pulsation at a level too low to be detected. Precise Gaia DR2 luminosities of high-amplitude {\delta} Sct stars (HADS) show that most of these are normal {\delta} Sct stars and not transition objects. It is argued that current ideas on A star envelopes need to be revised., Comment: 10 pages, 8 figures, 4 tables

A spectroscopic and photometric analysis has been carried out for thirty-two candidate SX_Phe variable blue straggler stars in the Kepler-field (Balona & Nemec 2012). Radial velocities (RVs), space motions (U,V,W), projected rotation velocities (v sin i), spectral types, and atmospheric characteristics (T_eff, log g, [Fe/H], xi_t, zeta_RT, etc.) are presented for 30 of the 32 stars. Although several stars are metal-weak with extreme halo orbits, the mean [Fe/H] of the sample is near solar, thus the stars are more metal-rich than expected for a typical sample of Pop.II stars, and more like halo metal-rich A-type stars (Perry 1969). Two thirds of the stars are fast rotators with v sin i > 50 km/s, including four stars with v sin i > 200 km/s. Three of the stars have (negative) RVs > 250 km/s, five have retrograde space motions, and 21 have total speeds (relative to the LSR) > 400 km/s. All but one of the 30 stars have positions in a Toomre diagram consistent with the kinematics of bona fide halo stars (the exception being a thick-disk star). Observed Roemer time delays, pulsation frequency modulations and light curves suggest that at least one third of the stars are in binary (or triple) systems with orbital periods ranging from 2.3 days to more than four years., Comment: 43 pages, 23 figures, accepted MNRAS Nov.23,2016

Rotational light modulation in Kepler photometry of K - A stars is used to estimate the absolute rotational shear. The rotation frequency spread in 2562 carefully selected stars with known rotation periods is measured using time-frequency diagrams. The variation of rotational shear as a function of effective temperature in restricted ranges of rotation period is determined. The shear increases to a maximum in F stars, but decreases somewhat in the A stars. Theoretical models reproduce the temperature variation quite well. The dependence of rotation shear on rotation rate in restricted temperature ranges is also determined. The dependence of the shear on the rotation rate is weak in K and G stars, increases rapidly for F stars and is strongest in A stars. For stars earlier than type K, a discrepancy exists between the predicted and observed variation of shear with rotation rate. There is a strong increase in the fraction of stars with zero frequency spread with increasing effective temperature. The time-frequency diagrams for A stars are no different from those in cool stars, further supporting the presence of spots in stars with radiative envelopes., Comment: 11 pages 13 figures 2 tables

We consider the high mode density reported in the delta Scuti star HD 50844 observed by CoRoT. Using simulations, we find that extracting frequencies down to a given false alarm probability by means of successive prewhitening leads to a gross over-estimate of the number of frequencies in a star. This is due to blending of the peaks in the periodogram due to the finite duration of the time series. Prewhitening is equivalent to adding a frequency to the data which is carefully chosen to interfere destructively with a given frequency in the data. Since the frequency extracted from a blended peak is not quite correct, the interference is not destructive with the result that many additional fictitious frequencies are added to the data. In data with very high signal-to-noise, such as the CoRoT data, these spurious frequencies are highly significant. Continuous prewhitening thus causes a cascade of spurious frequencies which leads to a much larger estimate of the mode density than is actually the case. The results reported for HD 50844 are consistent with this effect. Direct comparison of the power in the raw periodogram in this star with that in delta Scuti stars observed by Kepler shows that HD 50844 has a typical mode density., Comment: 8 pages, 7 figures, 3 tables

Observations of the A5p star KIC 8677585 obtained during the Kepler 10-d commissioning run with 1-min time resolution show that it is a roAp star with several frequencies with periods near 10 min. In addition, a low frequency at 3.142 cycles/day is also clearly present. Multiperiodic gamma Doradus and delta Scuti pulsations, never before seen in any Ap star, are present in Kepler observations of at least three other Ap stars. Since gamma Doradus pulsations are seen in Ap stars, it is likely that the low-frequency in KIC 8677585 is also a gamma Doradus pulsation. The simultaneous presence of both gamma Doradus and roAp pulsations and the unexpected detection of delta Scuti and gamma Doradus pulsations in Ap stars present new opportunities and challenges for the interpretation of these stars., Comment: 9 pages and 5 figures

HD110432 has gained considerable attention because it is a hard, variable X-ray source similar to gamma Cas. From time-serial echelle data obtained over two weeks during 2005 January and February, we find several remarkable characteristics in the star's optical spectrum. The line profiles show rapid variations on some nights which can be most likely be attributed to irregularly occurring and short-lived migrating subfeatures. Such features have only been observed to date in gamma Cas and AB Dor, two stars for which it is believed magnetic fields force circumstellar clouds to corotate over the stellar surface. The star's optical spectrum also exhibits a number of mainly FeII and HeI emission features with profiles typical of an optically thin disk viewed edge-on. Using spectral synthesis techniques, we find that its temperature is 9800K +/-300K, that its projected area is a remarkably large 100 stellar areas, and its emitting volume resides at a distance of 1 AU from the star. We also find that the star's absorption profiles extend to +/-1000 km/s, a fact which we cannot explain. Otherwise, HD110432 and gamma Cas share similarly peculiar X-ray and optical characteristics such as high X-ray temperature, erratic X-ray variability on timescales of a few hours, optical emission lines, and submigrating features in optical line profiles. Because of these similarities, we suggest that this star is a new member of a select class of "gamma Cas analogs.", Comment: 31 pages, 9 figures, accepted by ApJ (3/20/06)