R. R. Shobbrook

On the relationship between the δ Scuti and γ Doradus pulsators

We searched for 8 Scuti-type pulsations amongst known and candidate y Doradus stars. The motivations for such a project come from the need to understand the relationship of these two classes of pulsator better, from the present poor knowledge of the hot border of the y Doradus phenomenon, and from the exciting prospects for asteroseismology should stars be found which have both types of pulsation excited. We acquired 270 h of observations and monitored a total of 26 stars. One target, HD 209295, turned out to be a member of both classes of pulsating star, but this object is peculiar in the sense that it is a close binary. We classify six of our targets as new bona fide y Doradus stars, whereas nine more are good γ Doradus candidates, and three turned out to be ellipsoidal variables. One of our programme stars was found to be a δ Scuti star, with no additional y Doradus variations. Furthermore, one star was already known to be a bona fide y Doradus star, and we could not find an unambiguous explanation for the variability of five more stars. The analysis of our data together with improved knowledge of stars from the literature enabled us to revise the blue border of the y Doradus phenomenon towards cooler temperatures. This new blue edge is much better defined than the previous one and extends from a temperature of about 7550 K on the ZAMS to 7400 K one magnitude above it. Five bona fide y Doradus stars we observed are located inside the 8 Scuti instability strip, but none of them exhibited observable δ Scuti pulsations. We therefore suggest that y Doradus stars are less likely to be 8 Scuti pulsators compared with other normal stars in the same region of the lower instability strip. In addition, we show that there is a clear separation between the pulsation constants Q of 8 Scuti and y Doradus stars. The y Doradus stars known to date all have Q > 0.23 d.

Asteroseismology of the β Cephei star ν Eridani – II. Spectroscopic observations and pulsational frequency analysis

Asteroseismology of the β Cephei star ν Eridani : II. Spectroscopic observations and pulsational frequency analysis

Detection of 75+ pulsation frequencies in the delta Scuti star FG Virginis

Extensive photometric multisite campaigns of the 6 Scuti variable FG Vir are presented. For the years 2003 and 2004, 926 h of photometry at the millimag precision level were obtained. The combinations with earlier campaigns lead to excellent frequency resolution and high signal/noise. A multifrequency analysis yields 79 frequencies. This represents a new record for this type of star. The modes discovered earlier were confirmed. quencies. This represents a new Pulsation occurs over a wide frequency band from 5.7 to 44.3 c/d with amplitudes of 0.2 mmag or larger Within this wide band the frequencies are not distributed at random, but tend to cluster in groups. A similar feature is seen in the power spectrum of the residuals after 79 frequencies are prewhitened. This indicates that many additional modes are excited. The interpretation is supported by a histogram of the photometric amplitudes, which shows an increase of modes with small amplitudes. The old question of the missing modes may be answered now: the large number of detected frequencies as well as the large number of additional frequencies suggested by the power spectrum of the residuals confirms the theoretical prediction of a large number of excited modes. FG Vir shows a number of frequency combinations of the dominant mode at 12.7162 c/d (m = 0) with other modes of relatively high photometric amplitude. The amplitudes of the frequency sums are higher than those of the difference;. A second mode (20.2878 c/d) also shows combinations. This mode of azimuthal order m = -1 is coupled with two other modes of m = + 1.

Asteroseismology of the β Cephei star ν Eridani – IV. The 2003–2004 multisite photometric campaign and the combined 2002–2004 data

We report on the second multisite photometric campaign devoted to ν Eridani (ν Eri). The campaign, carried out from 2003 September 11 to 2004 February 16, was very nearly a replica of the first campaign, 2002–2003: the five telescopes and photometers we used were the same as those in the first campaign, the comparison stars and observing procedure were identical, and the numbers and time baselines of the data were comparable. For ν Eri, analysis of the new data adds four independent frequencies to the nine derived previously from the 2002–2003 data: three in the range 7.20–7.93 d−1 and a low one, equal to 0.614 d−1. Combining the new and the old data results in two further independent frequencies, equal to 6.7322 and 6.2236 d−1. Altogether, the oscillation spectrum is shown to consist of 12 high and two low frequencies. The latter have u amplitudes about twice as large as the v and y amplitudes, a signature of high radial-order g modes. Thus, we confirm the suggestion, put forward on the basis of the data of the first campaign, that ν Eri is both a β Cephei and a slowly pulsating B (SPB) star. Nine of the 12 high frequencies form three triplets, of which two are new. The triplets represent rotationally split l= 1 modes, although in case of the smallest-amplitude one this may be questioned. Mean separations and asymmetries of the triplets are derived with accuracy sufficient for meaningful comparison with models. The first comparison star, μ Eri, is shown to be an SPB variable with an oscillation spectrum consisting of six frequencies, three of which are equidistant in period. The star is also found to be an eclipsing variable. The eclipse is a transit, probably total, the secondary is fainter than the primary by several magnitudes, and the system is widely detached. The second comparison star, ξ Eri, is confirmed to be a δ Scuti variable. To the frequency of 10.8742 d−1 seen already in the data of the first campaign, another, equal to 17.2524 d−1, is added.

Pushing the ground-based limit: 14-μmag photometric precision with the definitive Whole Earth Telescope asteroseismic data set for the rapidly oscillating Ap star HR 1217

HR 1217 is one of the best-studied rapidly oscillating Ap (roAp) stars, with a frequency spectrum of alternating even- and odd-� modes that are distorted by the presence of a strong, global magnetic field. Several recent theoretical studies have found that within the observable

CONSTRAINING THE EVOLUTION OF ZZ CETI

We report our analysis of the stability of pulsation periods in the DAV star (pulsating hydrogen atmosphere white dwarf) ZZ Ceti, also called R548. On the basis of observations that span 31 years, we conclude that the period 213.13 s observed in ZZ Ceti drifts at a rate dP/dt ≤ (5.5 ± 1.9) × 10-15 s s-1, after correcting for proper motion. Our results are consistent with previous values for this mode and an improvement over them because of the larger time base. The characteristic stability timescale implied for the pulsation period is P/ ≥ 1.2 Gyr, comparable to the theoretical cooling timescale for the star. Our current stability limit for the period 213.13 s is only slightly less than the present measurement for another DAV, G117-B15A, for the period 215.2 s, establishing this mode in ZZ Ceti as the second most stable optical clock known, comparable to atomic clocks and more stable than most pulsars. Constraining the cooling rate of ZZ Ceti aids theoretical evolutionary models and white dwarf cosmochronology. The drift rate of this clock is small enough that we can set interesting limits on reflex motion due to planetary companions.

Discovery of the ‘missing’ mode in HR 1217 by the Whole Earth Telescope

ABSTRACT HR1217 is a prototypical rapidly oscillating Ap star that has presented a test tothe theory of nonradial stellar pulsation. Prior observations showed a clear patternof five modes with alternating frequency spacings of 33.3µHz and 34.6µHz, with asixth mode at a problematic spacing of 50.0µHz (which equals 1.5×33.3µHz) to thehigh-frequency side. Asymptotic pulsation theory allowed for a frequency spacing of34µHz, but hipparcos observations rule out such a spacing. Theoretical calculationsof magnetoacoustic modes in Ap stars by Cunha (2001) predicted that there shouldbe a previously undetected mode 34µHz higher than the main group, with a smallerspacing between it and the highest one. In this Letter, we present preliminary resultsfrom a multi-site photometric campaign on the rapidly oscillating Ap star HR1217using the “Whole Earth Telescope”. While a complete analysis of the data will appearin a later paper, one outstanding result from this run is the discovery of a newlydetected frequency in the pulsation spectrum of this star, at the frequency predictedby Cunha (2001).Keywords: Stars: oscillations – stars: variables – stars: individual (HR1217) – stars:magnetic.

Asteroseismological studies of three β Cephei stars: IL Vel, V433 Car and KZ Mus

We have acquired between 127 and 150 h of time-resolved multicolour photometry for each of the three β Cephei stars IL Vel, V433 Car and KZ Mus over a time-span of 4 months from two observatories. All three objects are multiperiodic with at least three modes of pulsation. Mode identification from the relative colour amplitudes is performed. We obtain unambiguous results for the two highest-amplitude modes of IL Vel (both are l = 1) and the three strongest modes of KZ Mus (l = 2, 0 and 1), but none for V433 Car. Spectroscopy shows the latter star to be a fast rotator (υ sin i = 240 km s - 1 ), whereas the other two have moderate υ sin i (65 and 47 km s - 1 , respectively). We performed model calculations using the Warsaw-New Jersey stellar evolution and pulsation code. We find that IL Vel is an object of about 12 M O . in the second half of its main-sequence evolutionary track. Its two dipole modes are most probably rotationally split components of the mode originating as p 1 on the zero-age main sequence; one of these modes is m = 0. V433 Car is suggested to be an unevolved 13-M O . star just entering the β Cephei instability strip. KZ Mus seems less massive (12.7 M O .) and somewhat more evolved, and its radial mode is probably the fundamental one. In this case its quadrupole mode would be the one originating as g 1 , and its dipole mode would be p 1 . Two of our photometric comparison stars also turned out to be variable. HD 90434 is probably a new slowly pulsating B-type star, the dominant mode of which is a dipole, whereas the variability of HD 89768 seems to be a result of binarity. It is suggested that mode identification of slowly rotating β Cephei stars based on photometric colour amplitudes is reliable; we estimate that a relative accuracy of 3 per cent in the amplitudes is sufficient for unambiguous identifications. Owing to the good agreement of our theoretical and observational results we conclude that the prospects for asteroseismology of multiperiodic slowly rotating β Cephei stars are good.

An asteroseismic study of the β Cephei star θ Ophiuchi: photometric results

We have carried out a three-site photometric campaign for the β Cephei star θ Oph from 2003 April to August. 245 h of differential photoelectric uvy photometry were obtained during 77 clear nights. The frequency analysis of our measurements has resulted in the detection of seven pulsation modes within a narrow frequency interval between 7.116 and 7.973 c d −1 . No combination or harmonic frequencies have been found. We have performed a mode identification of the individual pulsations from our colour photometry that shows the presence of one radial mode, one rotationally split � = 1 triplet and possibly three components of a rotationally split � = 2 quintuplet. We discuss the implications of our findings and point out the similarity of the pulsation spectrum of θ Oph to that of another β Cephei star, V836 Cen. Ke yw ords: techniques: photometric ‐ stars: early-type ‐ stars: individual: θ Oph ‐ stars: oscillations ‐ stars: variables: other.

On the Period-Luminosity-Colour-Metallicity relation and the pulsational characteristics of λ Bootis type stars

Generally, chemical peculiarity found for stars on the upper main sequence excludes Scuti type pulsation (e.g. Ap and Am stars), but for the group of Bootis stars it is just the opposite. This makes them very interesting for asteroseismological investigations. The group of Bootis type stars comprises late B- to early F-type, Population I objects which are basically metal weak, in particular the Fe group elements, but with the clear exception of C, N, O and S. The present work is a continuation of the studies by Paunzen et al. (1997, 1998), who presented first results on the pulsational characteristics of the Bootis stars. Since then, we have observed 22 additional objects; we found eight new pulsators and confirmed another one. Furthermore, new spectroscopic data (Paunzen 2001) allowed us to sort out misidentified candidates and to add true members to the group. From 67 members of this group, only two are not photometrically investigated yet which makes our analysis highly representative. We have compared our results on the pulsational behaviour of the Bootis stars with those of a sample of Scuti type objects. We find that at least 70% of all Bootis type stars inside the classical instability strip pulsate, and they do so with high overtone modes (Q< 0.020 d). Only a few stars, if any, pulsate in the fundamental mode. Our photometric results are in excellent agreement with the spectroscopic work on high-degree nonradial pulsations by Bohlender et al. (1999). Compared to the Scuti stars, the cool and hot borders of the instability strip of the Bootis stars are shifted by about 25 mmag, towards smaller (b y)0. Using published abundances and the metallicity sensitive indices of the Geneva 7-colour and Stromgren uvby systems, we have derived (Z) values which describe the surface abundance of the heavier elements for the group members. We find that the Period- Luminosity-Colour relation for the group of Bootis stars is within the errors identical with that of the normal Scuti stars. No clear evidence for a statistically significant metallicity term was detected.