Alexander William Fullerton

Time-Series Observations of O Stars. II. Optical Observations of zeta Ophiuchi

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.

The IUE Mega Campaign. Modulated Structure in the Wind of HD 64760 (B0.5 Ib)

We highlight systematic variability in the stellar wind of the early B type supergiant, HD 64760, whose UV line profiles were monitored for almost 16 days in 1995 January as part of the IUE MEGA Campaign. The extensive coverage reveals a pattern of rapidly evolving discrete optical depth changes which typically migrate from ~-200 km s-1 to ~-1500 km s-1 in less than 12 hr. These features coexist with more slowly evolving structures lasting several days. Time-series analysis of the Si IV, Si III, and N V profile variations presents a clear 1.2 day periodicity, which is a quarter of the estimated maximum rotation period of HD 64760. The line profile changes are consistent with an interpretation in terms of a set of corotating wind features which occult the stellar disk at least 3 times during the observing run. These data are combined with UV observations collected in 1993 March to argue in favor of rotationally modulated wind variations in HD 64760. The basic result of very regular, large-scale optical depth variations points to a clock whose origin is on the stellar surface, rather than a mechanism that is entirely intrinsic to the stellar wind.

Binary star orbits from speckle interferometry. 5: A combined speckle/spectroscopic study of the O star binary 15 Monocerotis

We report on the discovery of a speckle binary companion to the O7 V (f) star 15 Monocerotis. A study of published radial velocities in conjunction with new measurements from Kitt Peak National Observatory (KPNO) and IUE suggests that the star is also a spectroscopic binary with a period of 25 years and a large eccentricity. Thus, 15 Mon is the first O star to bridge the gap between the spectroscopic and visual separation regimes. We have used the stars membership in the cluster NGC 2264 together with the cluster distance to derive masses of 34 and 19 solar mass for the primary and secondary, respectively. Several of the He I line profiles display a broad shallow component which we associate with the secondary, and we estimate the secondarys classification to be O9.5 Vn. The new orbit leads to several important predictions that can be tested over the next few years.

Time series observations of O stars. I - IUE observations of variability in the stellar wind of Zeta Puppis

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.

The IUE Mega Campaign: Wind Variability and Rotation in Early-Type Stars

Wind variability in OB stars may be ubiquitous, and a connection between projected stellar rotation velocity and wind activity is well established. However, the origin of this connection is unknown. To probe the nature of the rotation connection, several of the attendees at the workshop on Instability and Variability of Hot-Star Winds drafted an IUE observing proposal. The goal of this program was to follow three stars for several rotations to determine whether the rotation connection is correlative or causal. The stars selected for monitoring all have rotation periods less than or equal to 5 days. They were HD 50896 (WN5), HD 64760 (B0.5 Ib), and HD 66811 [zeta Pup; O4 If(n)]. During 16 days of nearly continuous observations in 1995 January (dubbed the MEGA campaign), 444 high-dispersion IUE spectra of these stars were obtained. This Letter presents an overview of the results of the MEGA campaign and provides an introduction to the three following Letters, which discuss the results for each star.

Time-Series Observations of O Stars. III. IUE and HST Spectroscopy of zeta Ophiuchi and Implications for the ``Photospheric Connection''

Stellar-wind variability in the archetypal nonradially pulsating O star ζ 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 (≥10 3 km s −1 ≃0.8v∞) and then migrate blueward; the recurrence time scale for the phenomenon is ∼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

Propagating absorption enhancements in the wind of the extreme of supergiant HD 151804

Variations in the shape, position, and strength of the absorption trough of the He I λ5876 P Cygni profile have been detected in high-quality Reticon spectra of the extreme Of supergiant HD 151804. These changes are primarily caused by the propagation of absorption enhancements through the trough of the profile. The short time series presented here is dominated by the progressive motion of two enhancement episodes, one of which could be traced for 4 consecutive days as it accelerated blueward from an initial velocity of -180 km/s to -700 km/s

HD 53975: An O-type spectroscopic binary with a large mass ratio

We present radial velocities and line profiles of the O-type star HD 53975, which we find to be a spectroscopic binary with a period of 6.0173 days. Although there are no obvious signatures of the secondary in the line profiles, we have used the merit function of Shafter et al. (1980) to show that a secondary component is present in lines found in B-type spectra. We have compared the observed merit functions with merit functions for model profiles constructed assuming a main-sequence secondary, and this comparison consistently indicates a companion with a mass ratio of q = M(sub 2)/M(sub 1) = 0.23 +/- 0.04 and a magnitude difference Delta m = 3.3 +/- 0.4. We present a tomographic reconstruction of the spectrum of the secondary for these parameters. We suggest that other large mass ratio binaries may be found among the O stars when observed with high-resolution and high signal-to-noise ratio spectroscopy.

Detection of small-amplitude pulsations in HD 34656 (O7 II)

Radial velocity variations with period 8.18 hr and semiamplitude 3 km/s have been detected in high-quality Reticon spectra of the O7 II(f) star HD 34656. These fluctuations indicate that the star is pulsating, possibly in the radial fundamental mode. 17 refs.

Photospheric and Circumstellar Variability of the Rapidly Rotating O Star HD 93521

HD 93521 is an O9.5 V star with several well-documented peculiarities, including: an extremely large projected rotational velocity (400 km/s according to Conti & Ebbets 1977); absorption line profile variations (Fullerton 1990); weak emission at Hα (Irvine 1989); evidence for a rotationally-modified stellar wind in its UV resonance lines (Massa 1992; Howarth & Reid 1993); and stellar wind variability in the form of discrete absorption components (Prinja & Howarth 1986). Thus, HD 93521 provides an ideal laboratory for studying the interaction between rapid rotation and variability in both photospheric layers and the circumstellar environment.