The formation and measurement of Cepheid photospheric spectral lines are explored to understand better the systematic effects involved in radial and pulsation velocity determination, and hence to help secure Cepheid distance scale calibrations that depend on Cepheid velocities (e.g., the Baade-Wesselink method). Using high-resolution optical and infrared spectra and synthetic line profiles, we examine techniques for measuring the position of line center and the amount of asymmetry of Cepheid absorption line profiles. The line asymmetry is observed to be a specific function of pulsation phase and to correlate with line center measurements, thus (1) the conversion factor between radial and pulsation velocity (p-factor) may not be assumed constant with phase and (2) there is a systematic offset of about 1 km s(-1) in the center-of-mass velocity. Our Cepheid models, which employ non-LTE radiative hydrodynamics, reproduce and describe (as an opacity effect) the observed unequal line asymmetry magnitudes during contraction and expansion stages. The models also allow us to study the phase dependence of the p-factor and its influence on Baade-Wesselink radius calculations. We conclude that in order to reach an accuracy of better than 7% (0.15 mag) in the zero point of the Cepheid distance scale, these systematic effects should be taken into account.

Some investigators have attributed the photometric and spectral line profile variations (lpv) that are common among the B stars to nonradial pulsation while others have attempted to explain them by rotation of photospheric or circumstellar structures with respect to our line of sight. One of the problems in resolving this debate has been our lack of knowledge of how these variations depend on fundamental stellar characteristics. Surveys of lpv have covered the Be and a few Bn stars (D. Penrod, unpublished), the O stars (Fullerton 1990), and B8-B9.5 main sequence stars (Baade 1989), but noone has carried out a systematic search for lpv among the near main sequence, non-emission line early-and mid-B stars. This paper describes preliminary results from such a survey.

The B type star 53 Persei was discovered in 1977 by Smith (1977) as the prototype of a separate group of B-type variables showing light and line profile variability. The physical cause of the variability was thought to be nonradial pulsation (NRP) (see, e.g. Smith et al. 1984). However, the NRP model for this star has been questioned by Balona (1986) who suggested the rotational modulation (RM) model to explain the variability. In order to resolve the long lasting debate about 53 Persei, a campaign was initiated to organize coordinated optical photometry and spectroscopy from the ground, and Far-UV photometry from Voyager in 1991 January. This paper presents the results of period analysis on the groundbased UBV data. In another paper, Smith & Huang (1994) report the new identification of pulsation modes using Voyager Far-UV photometry combined with the results from optical observations. Some preliminary results from APT uvby observations taken at a single site are also cited for comparison.

New UBV observations of 53 Persei obtained during an international photometric campaign in 1991 January are presented. Techniques of observation and data reduction employed at each of the four participating observatories are described in detail, followed by a discussion of the observational uncertainties and systematic errors. The formal standard deviations determined from check-star measurements are 0.011, 0.012, and 0.020 mag for V, B, and U, respectively.Multiple frequency analysis shows that the light variations of 53 Persei during the campaign can be fitted very well by two sinusoids of frequencies 0.462 and 0.603 cycles per day (c/d) which are practically identical to those found by others from 1977-1983 photometric and spectroscopic data. The detection of the same frequencies in the data obtained more than 10 yr apart has convincingly confirmed the stable multiperiodicity in 53 Persei and extended the stability duration from 5.5 yr reported in an earlier investigation to about 13 yr. This pivotal result strongly supports the nonradial pulsation as the physical cause responsible for light and line-profile variations in this prototype star.Definite changes have been detected in amplitudes and amplitude ratios of the light variations. Since 1981, the amplitude ratio of the two modes mentioned above has increased from approximately 0.9 to > 2.5. The single mode of 0.462 c/d has become the dominant component in the two-mode pulsation of 53 Persei.The deviations of data from the two-frequency fits may imply the existence of additional frequencies. But we cannot prove this inference with the present data.

Stellar-wind variability in the archetypal nonradially pulsating O star zeta Oph (O9.5 V) is discussed on the basis of new time-series IUE and HST spectroscopy and archival results. Time-variable ''discrete absorption components'' are first observed at high velocities (greater than or similar to 10(3) km s-1 congruent-to 0.8v(infinity)) and then migrate blueward; the recurrence time scale for the phenomenon is approximately 20 hr. This is the first record of this type of variability in a luminosity-class V star and provides support for the previously inferred ubiquity of such behavior across the O-star regime. The accelerations are slower than predicted by steady state wind models and are unlikely to represent the time-averaged velocity law of the outflow. The UV results are discussed in the context of simultaneous optical observations (Paper II); no evidence is found to suggest that nonradial pulsations have any direct role in determining the gross observational characteristics of discrete absorption components, on either long (years) or short (hours) time scales. The line-driven instability provides a promising mechanism to account for variability observed in the UV P Cygni profiles of zeta Oph, and, by extension, in all other O stars.

AbstractWe have undertaken a multi-site, multi-wavelength observing campaign on the archetypal O stars ζ Puppis (O4 I(n)f) and ζ Ophiuchi (O9.5 V). Both stars are well known for the strength of their line profile variations (lpv's), and represent extremes of O spectral type and luminosity class. UV time-series spectroscopy of ζ Pup and ζ Oph is described by Prinja et al. (Ap.J. 1992, 390, 266), and Howarth et al. (Ap.J. 1992, submitted) respectively. The optical spectroscopic results of ζ Oph are reported by Reid et al. (1992, ApJ submitted), of which some of the principal results are given here.During late April, and early May, 1989, we obtained high-resolution, high signal-to-noise optical spectra of the late O-type, rapid rotator ζ Oph. Time-series analysis, using the CLEAN algorithm, has shown that the characteristic lpv seen in HeI λ447lÅ, Si III λλ4552, 4567, 4575Å, and MgII λ448lÅ can be satisfactorily represented as a set of 4 sinusoids. No substantial variation is observed in HeII λ4541, or NIII λ4517Å. We attribute this behaviour to a combination of equatorial gravity-darkening and a latitudinally-confined origin for the lpv.The phase changes over the line profiles indicate repetitive patterns of axial symmetry, rotating prograde in the co-rotating frame of the star. The periods are 3.339 hours (-m = 4), 2.435 hours (-m = 5 or -m = 6), 1.859 hours (-m = 9 ± 1), and either 1.366 hours or 1.292 hours (-m = 11 ± 1); -m represents the spatial frequency around the stellar equator. The first three periods confirm those found at earlier epochs, and we conclude that some lpv characteristics are reproduced over at least a 2-year interval.Since no commensurate superperiod (|m|P) exists, and since the super-periods are less than our estimated minimum rotation period for ζ Oph (> 18 hours), we reject a rotational modulation origin for the lpv and conclude that the star is undergoing multi-mode, sectorial, non-radial pulsations.

We present photopolarimetry and extensive high-resolution, high-signal-to-noise optical spectroscopy of the rapidly rotating late O-type star zeta Ophiuchi (HD 149757). The polarimetric data show no substantial variability over 1 week (sigma(p) < 0.04%), while the spectroscopic data show characteristic line-profile variations in the form of ''bumps'' migrating from blue to red on typical time scales of several hours. These variations are relatively strong (amplitude approximately 1% of the continuum level) in He I lambda4471, and Si III lambdalambda4552, 4567, 4575. They are seen at lower amplitudes in Mg II lambda4481, but are almost undetectable (amplitude less than or similar to 0.3%) in He II lambdalambda4541, 4686 and N III lambdalambda4511, 4515, 4518. We attribute this to a combination of equatorial gravity darkening and a latitudinally confined origin of the variations.Using a Fourier CLEAN technique, it is shown that the line-profile variations can be adequately represented as a set of sinusoids. The periods, combined with an estimate of the rotational period, lead us to rule out rotational modulation as the origin of the variability. The phase changes of the sinusoids across the line-profiles indicate a repetitive pattern, consistent with sectorial (l = -m) nonradial pulsation. We find four periods: P = 3.339 hr (Absolute value of m = 4), 2.435 hr (Absolute value of m = 5 or 6), 1.859 hr (Absolute value of m = 9 +/- 1), and either 1.366 or 1.292 hr (Absolute value of m = 11 +/- 1). The ''superperiods,'' P Absolute value of m, may be nearly commensurate (at approximately 13-14 hr), but that commensurability is not exact.

Stellar wind variability in zeta-Pup (O4 I(n)f) is described based on 31 high-resolution IUE observations secured over 5 1/2 days in 1989 April. Extensive changes are evident in the absorption regions of Si IV lambda-lambda-1393.76, 1402.77 and N IV lambda-1718.55 P Cygni profiles. Both lines exhibit similar patterns of variability, which are characterized by the development (at about -1000 km s-1), and subsequent blueward migration, of discrete absorption components. The formation of four discrete features is identified over approximately 2.2 days of intensive observations, with a recurrence time of approximately 15 hr. The time scales, velocities, and accelerations of the progressive absorption enhancements are determined. These changes are accompanied by fluctuations of up to approximately 200 km s-1 in the maximum observed blue edge velocities in saturated C IV and N V P Cygni profiles.

Un modele d'atmosphere a blanketing de raie, a geometrie spherique etendue, est developpe pour les etoiles de temperature intermediaire. Une comparaison entre les modeles avec blanketing et sans blanketing, et entre les modeles spheriques et a plans paralleles est realisee

view Abstract Citations (19) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Infrared Spectroscopy of Cepheids: Peculiar Velocity Structure and Its Effect on Radii and Distances Sasselov, Dimitar D. ; Lester, John B. ; Fieldus, Michael S. Abstract High-resolution spectra in the 1.1 micron region covering one complete pulsational period of the classical Cepheids X Sgr and Eta Aql show that all photospheric lines exhibit variable line asymmetries or doubling. This behavior resembles previously reported effects in the optical spectra of Cepheids, which suggest a peculiar velocity structure common to these stars. If the line asymmetries are not taken into account, the velocity curve has a systematically lower amplitude, which leads to a systematic underestimate of the radii and distance derived by the Baade-Wesselink method of 20-40 percent. Publication: The Astrophysical Journal Pub Date: February 1989 DOI: 10.1086/185371 Bibcode: 1989ApJ...337L..29S Keywords: Cepheid Variables; Infrared Spectroscopy; Peculiar Stars; Radial Velocity; Stellar Spectrophotometry; Line Spectra; Stellar Atmospheres; Stellar Magnitude; Visible Spectrum; Astrophysics; STARS: CEPHEIDS; STARS: DIAMETERS; STARS: PULSATION full text sources ADS | data products SIMBAD (2)