Time-dependent convection seismic study of five γ Doradus stars

We apply for the first time the time-dependent convection (TDC) treatment of Gabriel and Grigahcène et al. to the photometric mode identification in γ Doradus (γ Dor) stars. We consider the influence of this treatment on the theoretical amplitude ratios and phase differences. Comparison with the observed amplitudes and phases of the stars γ Dor, 9 Aurigae, HD 207223 = HR 8330, HD 12901 and 48501 is presented and enables us to identify the degree ℓ of the pulsation modes for four of them. We also determine the mode stability for different models of these stars. We show that our TDC models agree better with observations than with frozen convection models. Finally, we compare the results obtained with different values of the mixing-length parameter α.     

Simultaneous photometry and spectroscopy of the newly discovered γ Doradus variable HR 8330=HD 207223

We present BV photometry and simultaneous high-resolution, high signal-to-noise ratio spectroscopy of the newly-discovered γ Doradus variable HR 8330 taken during the 1997 and 1998 observing seasons. We calculate power spectra for the B - and V -band data sets and for the time series defined throughout the observing season at each point across the Fe  ii λ 4508.289 and the Ti  ii λ 4501.278 line profiles to search for periodic variability. Period analysis reveals a single, 2.6-d period in both the photometric and the spectroscopic data, with a 237° phase lag between them. Based on the location of HR 8330 in the HR diagram and the characteristics of its photometric and spectroscopic variations, we conclude that HR  8330 is a bona fide γ Doradus-type pulsating variable.     

The domain of γ Doradus variables in the Hertzsprung–Russell diagram

70 new γ Doradus candidates were identified from Hipparcos photometry, which represents more than a doubling of the presently known number. Selecting the objects with good evidence for multiperiodicity, it is found that these stars, together with bona fide members of the class, occupy a well-defined region in a colour–magnitude diagram. This domain corresponds to a range of 7200–7700 K on the zero-age main sequence (ZAMS) and 6900–7500 K one magnitude above it, which partly overlaps with the instability strip of δ Scuti stars.
For the first time, γ Doradus stars can be discussed as a group. They can be found over a significant fraction of the main sequence lifetimes for objects in the relevant temperature range. An upper limit on the surface metallicity of γ Doradus stars is apparent, which may guide the search for their pulsation driving mechanism. The importance of possible objects exhibiting both γ Doradus and δ Scuti-type pulsations is discussed.     

Spectral line profile variations in the γ Doradus variable HD 164615: non-radial pulsations versus star-spots

High-resolution spectral data of the Fe  II 5318 Å line in the γ Doradus star HD 164615 are presented. These show systematic changes in the spectral lineshapes with the photometric period of 0.8133 d which are modelled using either non-radial pulsations or cool star-spots. The non-radial modes that can fit the lineshape changes have m degree of 2–4. However, only the m = 2 mode seems to be consistent with the amplitude of the radial velocity variations measured for this star. The star-spot model, although it can qualitatively fit the lineshape changes, is excluded as a possible hypothesis on the basis of (1) poorer fits to the observed spectral line profiles, (2) an inability to account for the large changes in the spectral linewidth as a function of phase, (3) a predicted radial velocity curve that looks qualitatively different from the observed one, and (4) a predicted photometric curve that is a factor of 5 larger than the observed light curve (and with the wrong qualitative shape). Finally, a 'Doppler image' (assuming cool spots) derived from a sequence of synthetic line profiles having non-radial pulsations results in an image that is almost identical to the Doppler image derived for HD 164615. These results provide strong evidence that non-radial pulsations are indeed the explanation for the variability of HD 164615 as well as the other γ Dor variables.     

Chromospheric Ca II H and K emission among subdwarfs

Echelle spectra have been obtained of the Ca  II H and K lines for a sample of metal-poor subdwarf stars as well as for a number of nearby Population I dwarfs selected from among those included in the Mount Wilson HK survey. The main conclusion of this paper is that Ca  II H- and K-line emission does occur among subdwarfs. It is particularly notable among those subdwarfs with colours of B − V ≥0.75; all such stars observed exhibit chromospheric emission, although emission is observed among some subdwarfs bluer than this colour. The Ca  II K emission profile in most subdwarfs exhibits an asymmetry of V / R >1, similar to that seen in the integrated light of the solar disc. Two quantitative indicators of the contrast between the peaks in the emission profile and the neighbouring photospheric line profile are introduced. Measurements of these indicators show that the level of Ca  II emission among the subdwarfs is similar to that among low-activity Population I dwarfs.     

Asteroseismology of δ Scuti and γ Doradus stars

We give an overview of past and present efforts to make seismology of δ Scuti and γ Doradus stars possible. Previous work has not led to the observational detection and identification of a sufficient number of pulsation modes for these pulsators for the construction of unique seismic models. However, recent efforts including large ground-based observational campaigns, work on pre-main sequence pulsators, asteroseismic satellite missions, theoretical advances on mode identification methods, and the discovery of a star showing simultaneous self-excited δ Scuti and γ Doradus oscillations suggest that we may be able to explore the interiors of these pulsators in the very near future.     

OH masers and magnetic fields near the cometary H ii region G34.3+0.2

The mechanism responsible for exciting high-order acoustic oscillations in rapidly oscillating Ap stars is still an open issue. Recently, Balmforth et al. (hereafter BCDGV) proposed a model according to which high-frequency oscillations may be excited in roAp stars if the intensities of the magnetic fields present in these stars are sufficiently large to suppress convection at least in some region of their envelopes. Using models similar to those proposed by BCDGV, we predict the theoretical edges of the instability strip appropriate to roAp stars and compare them with the observations. Moreover, we discuss our results in the light of some of the systematic differences found between roAp stars, noAp stars and Ap stars in general. We suggest that a combination of intrinsic differences between these types of stars and a bias related to the frequencies of the unstable oscillations might hold the explanation to some of the differences observed.     

Radial velocity study of the roAp star α Circinus

We present results of 548 high-dispersion spectra of the roAp star α Circinus over a five-night period. The pulsational radial velocities measured from the rare-earth elements, Nd iii , Hα and Hβ are easily measured and occur at the photometric period. The amplitude is largest in Nd iii and Hα, is lower in the rare earths and Hβ, and cannot be measured in other metal lines. This behaviour can be understood in terms of an increase of pulsational amplitude with height in the atmosphere coupled with abundance stratification. The radial velocities show a significant variation at the rotational period of 4.463 d and a marginally significant periodicity at a frequency of 8.16 cycle d⁻¹. The latter may be a very low-amplitude δ Scuti pulsation.     

Mode switching in the nearby Mira-like variable R Doradus

We discuss visual observations spanning nearly 70 years of the nearby semiregular variable R Doradus. Using wavelet analysis, we show that the star switches back and forth between two pulsation modes having periods of 332 d and about 175 d, the latter with much smaller amplitude. Comparison with model calculations suggests that the two modes are the first and third radial overtone, with the physical diameter of the star making fundamental-mode pulsation unlikely. The mode changes occur on a time-scale of about 1000 d, which is too rapid to be related to a change in the overall thermal structure of the star and may instead be related to weak chaos.   The Hipparcos distance to R Dor is 62.4 ± 2.8 pc which, taken with its dominant 332-d period, places it exactly on the period–luminosity (P–L) relation of Miras in the Large Magellanic Cloud. Our results imply first-overtone pulsation for all Miras which fall on the P–L relation. We argue that semiregular variables with long periods may largely be a subset of Miras and should be included in studies of Mira behaviour. The semiregulars may contain the immediate evolutionary Mira progenitors, or stars may alternate between periods of semiregular and Mira behaviour.     

The pulsating λ Bootis star HD 105759

We present spectroscopic and multisite time series photometric observations of the star HD 105759 which confirm that this is a new pulsating λ Bootis star. Our spectroscopy indicates an overall metallicity of [Z] = log Z − log Z_⊙ = −1 and that T _eff = 8000 ± 300 K and log  g  = 4.0 ± 0.2. The photometric data reveal that this star pulsates with at least five frequencies: 8.62, 12.69, 20.15, 21.27 and 23.66 d⁻¹. These frequencies do not all match those found in an earlier single-site study, indicating that the pulsation spectrum changes with time.     

The β Cephei instability strip

We carried out a series of linear stability analyses of the radial and low-degree non-radial p modes for stellar models with initial masses of     . The stellar models were computed by using convective overshoot distance     , 0.25 and 0.40  H _P. Our numerical results show that the β Cephei instability strip forms a horn-shaped region pointing upwards near the main sequence on the Hertzsprung–Russell diagram (HRD). The lower part of the instability strip for the radial modes join the zero-age main-sequence (ZAMS) at     , while the top of the instability strip extends up to     . The instability strip for the non-radial modes is even wider. The overall instability strip is dominated by the radial and non-radial fundamental modes. The first overtone (the radial-order index     is also pulsationally unstable. We have shown that the β Cephei stability is almost independent of the overshoot parameter d _over used for the stellar models, while it depends critically on the metal abundance. With decreasing metal abundance, the instability region shrinks and eventually disappears for     .     

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 u v y 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  ℓ= 1  triplet and possibly three components of a rotationally split  ℓ= 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.