The formation of stars above about twenty solar masses and their apparently high multiplicity remain heavily debated subjects in astrophysics. We have performed a vast high-resolution radial velocity spectroscopic survey of about 250 O- and 540 B-type stars in the southern Milky Way which indicates that the majority of stars (> 82%) with masses above 16 solar masses form close binary systems while this fraction rapidly drops to 20% for stars of 3 solar masses. The binary fractions of O- type stars among different environment classes are: clusters (72 \pm 13%), associations (73 \pm 8%), field (43 \pm 13%), and runaways (69 \pm 11%). The high frequency of close pairs with components of similar mass argues in favour of a multiplicity originating from the formation process rather than from a tidal capture in a dense cluster. The high binary frequency of runaway O stars that we found in our survey (69% compared to 19-26% in previous surveys) points to the importance of ejection from young star clusters and thus supports the competitive accretion scenario.Comment: 6 pages, 4 figures, accepted by MNRA

FEROS is a new fiber-fed bench-mounted prism-crossdispersed echelle spectrograph which has been recently cornmissioned at the ESO 1.52-rn telescope at La Silla. The opto-rnechanical concept and performance predictions have been presented by Kaufer & Pasquini.1 In this contribution we present the test results as obtained during two commissioning runs in October and December 1998. Special emphasis is given to the measured performances in efficiency, spectral resolution, straylight contamination, and spectral stability. The definite highlight of the FEROS instrument performance is the high peak detection quantum efficiency of 17% at 550nm (1%©360nm, 16%©440nm, 11%©790nm, 6%@9OOnm). These measured numbers include the 2-mirror telescope, the fiber link, the instrument, and the detector while the whole wavelength range is covered by a single exposure on a thinned EEV 2kx4k 15 micron pixel CCD and a constant resolving power of R = 48.000. In addition the FEROS instrument proved its high spectral stability by radial-velocity observations as carried out on the known radial-velocity standard star 7 Ceti over a time base of 2 months. By recording a calibration-lamp spectrum in parallel with the object spectrum and by the use of a simple cross-correlation technique, a rms of 21 rn/s has been obtained for a data set of 130 individual measurements. FEROS has been made available to the ESO community in January 1999.

We report the recovery of a spectroscopic event in h Carinae in 1997/1998 after a prediction by Damineli in 1996. A true periodicity with days (0.2% uncertainty) is obtained. The line intensities and the P = 2020 ‫ע‬ 5 radial velocity curve display a phase-locked behavior, implying that the energy and dynamics of the event repeat from cycle to cycle. This rules out S Doradus oscillation or multiple shell ejection by an unstable star as the explanation of the spectroscopic events. A colliding-wind binary scenario is supported by our spectroscopic data and by X-ray observations. Although deviations from a simple case exist around periastron, intensive monitoring during the next event (mid-2003) will be crucial to our understanding of the system.

Extensive spectral observations of η Carinae over the last cycle, and particularly around the 2003.5 low‐excitation event, have been obtained. The variability of both narrow and broad lines, when combined with data taken from two earlier cycles, reveal a common and well‐defined period. We have combined the cycle lengths derived from the many lines in the optical spectrum with those from broad‐band X‐rays, optical and near‐infrared observations, and obtained a period length of Ppres= 2022.7 ± 1.3 d. Spectroscopic data collected during the last 60 yr yield an average period of Pavg= 2020 ± 4 d, consistent with the present‐day period. The period cannot have changed by more than ΔP/P= 0.0007 since 1948. This confirms the previous claims of a true, stable periodicity, and gives strong support to the binary scenario. We have used the disappearance of the narrow component of He i 6678 to define the epoch of the Cycle 11 minimum, T0= JD 245 2819.8. The next event is predicted to occur on 2009 January 11 (±2 d). The dates for the start of the minimum in other spectral features and broad‐bands are very close to this date, and have well‐determined time‐delays from the He i epoch.

Abstract.We have extensively monitored the Luminous Blue Variable AG Car (HD 94910) spectroscopically. Our data cover the years 1989 to 1999. In this period, the star underwent almost a full S Dor cycle from visual minimum to maximum and back. Over several seasons, up to four months of almost daily spectra are available. Our data cover most of the visual spectral range with a high spectral resolution (λ/∆λ ≈ 20 000). This allows us to investigate the variability in many lines on time scales from days to years. The strongest variability occurs on a time scale of years. Qualitatively, the variations can be understood as changes of the effective temperature and radius, which are in phase with the optical light curve. Quantitatively, there are several interesting deviations from this behaviour, however. The Balmer lines show P Cygni profiles and have their maximum strength (both in equivalent width and line flux) after the peak of the optical light curve, at the descending branch of the light curve. The line-width during maximum phase is smaller than during minimum, but it has a local maximum close to the peak of the visual light curve. We derive mass-loss rates over the cycle from the Hα line and find the highest mass loss rates (logṀ /(M yr −1 ) ≈ −3.8, about a factor of five higher than in the minimum, where we find logṀ/(M yr −1 ) ≈ −4.5) after the visual maximum. Line-splitting is very commonly observed, especially on the rise to maximum and on the descending branch from maximum. The components are very long-lived (years) and are probably unrelated to similar-looking line-splitting events in normal supergiants. Small apparent accelerations of the components are observed. The change in radial velocity could be due to successive narrowing of the components, with the absorption disappearing at small expansion velocities first. In general, the linesplitting is more likely the result of missing absorption at intermediate velocities than of excess absorption at the velocities of the components. The Hei lines and other lines which form deep in the atmosphere show the most peculiar variations. The Hei lines show a central absorption with variable blue-and red-shifted emission components. Due to the variations of the emission components, the Hei lines can change their line profile from a normal P Cyg profile to an inverse P Cyg-profile or double-peak emission. In addition, very broad (±1500 km s −1 ) emission wings are seen at the strongest Hei lines of AG Car. At some phases, a blue-shifted absorption is also present. The central absorption of the Hei lines is blue-shifted before and red-shifted after maximum. Possibly, we directly see the expansion and contraction of the photosphere. If this explanation is correct, the velocity of the continuum-forming layer is not dominated by expansion but is only slightly oscillating around the systemic velocity.