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Image-tube spectra (with dispersions of 40-90 A/mm) and UBV photometry of OB+ stars have been obtained at Cerro Tololo and Kitt Peak. The survey is complete to galactic latitudes of + or - 30 deg for galactic longitudes of between 300 and 60 deg. Most of the stars are found to be O-type and B-type supergiants, and 378 new O-type and B-type detections have been identified.
The variability in soft X-ray emission of 12 OB stars is studied. Two different methods of analysis, one more suitable for detecting short-term variations, the other aimed at detecting long time-scale variations, are applied to all stars in the sample. The long-term variability analysis shows that Cyg-OB2 8A Zeta Pup and Delta Ori exhibit significant count rate variations between different data sections. Similar variations are marginally detected in 15 Mon; the count rate variations for the other eight stars are consistent with statistical fluctuations. The light curve of Cyg-OB2 8A suggests the existence of two different emission levels. The short-term variability analysis detects marginal variability in Tau Sco with an effective amplitude of about 30 percent and a time scale of about 50 s. The upper limits to the effective short-term variability amplitude for all other sample stars are in the 10-30 percent range.
This paper presents measurements of terminal velocities of OB stars which are rapid rotators, based on archival high-dispersion IUE spectra of the C IV resonance doublet. The terminal velocities of the most rapidly rotating stars appear to be systematically lower than those of the less rapidly rotating stars (at least for the cooler stars), although the number of very rapid rotators is only three. The modified line-radiation driven wind model of Friend and Abbott, which takes into account the finite size of the star as well as its rotation, predicts that the terminal velocity should drop with increasing rotational velocity. However, when a smaller but very homogeneous subset of the data is used (BO giants only), the correlation between terminal velocity and rotational velocity disappears.
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars in the regions of five nearby southern OB associations: Scorpio Centaurus (Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2. Derived V and (B - V) in the Johnson system are included.
The Ultraviolet Imaging Telescope (UIT) obtained UV images of the giant M31 OB association NGC 206. Magnitudes in bands at 1520 and 2490 A were obtained for 30 massive stars, which demonstrate the effectiveness of UIT for photometry of moderately crowded hot stars to V about 21. The UV colors and magnitudes observed for stars in NGC 206 place them in the region of the color magnitude diagram occupied by evolutionary models for 30-60 solar mass stars, after correcting for extinction. The brighter stars are systematically redder than the fainter stars, indicating that they are supergiants of age about 4 Myr, while the fainter, bluer stars are nearer age zero. The relative numbers of probable supergiants measured by us and the number of probable main-sequence O stars measured from optical images are in agreement with the relative lifetimes. Calculated UIT colors are presented for a library of standard star spectra constructed from IUE and ground-based observations.
UBV photometry is presented for 2400 stars in the OB associations of 30 Doradus in the Large Magellanic Cloud, and I discuss new spectroscopic observations of 54 stars including 24 O stars. The CCD fields cover an area of 50 square arcminutes in the central region. The entire catalog (exclusive of the dense core cluster R 136) is photometrically complete to V = B = 18 mag and U = 17 mag, although the completeness magnitudes are fainter for regions with less nebular contamination.
The study obtains near-UV and FUV magnitudes for 76 massive stars in 24 OB associations in the central and southern portions of M31 from images obtained by the Ultraviolet Imaging Telescope (UIT) during the Astro 1 spacelab mission. A comparison is made with the previous UIT photometry of 30 stars in the giant association NGC 206. Extinctions are estimated from the relation between E(B - V) and the distance from the center of M31 derived by Hodge and Lee (1988) from topical CCD stellar photometry. From evolutionary models, lower limits to the maximum stellar mass are estimated at about 60-100 solar masses in NGC 206, A29, A61, A63, A130, and A132. For other associations, the limits are in the range of about 20-55 solar masses.
Using the point spread function photometry program DAOPHOT, we have used UBV CCD photometry to construct color-magnitude diagrams and luminosity functions for three OB associations in the Large Magellanic Cloud. The region LH 76 appears to be completely coeval; the region LH 13 shows some evidence for noncoevality which will need to be checked with spectra of the stars in question. The region LH 105, which lies on the southern edge of 30 Doradus, shows significant contamination by an underlying older population, possibly from previous star forming events. The luminosity functions, which serve as the first step toward determining the initial mass function in these regions, are calculated.
We present charge coupled device (CCD) photometry and spectroscopy for stars in Lucke-Hodge 58, an isolated OB association in the Large Magellanic Cloud (LMC) northwest of 30 Doradus. The photometric catalog contains 839 stars with UBV magnitudes complete to V approximately = 19. We have obtained spectra and classified 35 stars; combined with previous published spectral types, we find 22 O-type stars. The earliest type is O3-4 V, and there are three Wolf-Rayet (WR) stars in the association. The slope of the initial mass function, Gamma = -1.7 +/- 0.3, is in good agreement with other LMC associations. The presence of several evolved supergiants with masses about 15-25 solar mass suggests that some star formation took place as early as 10 million years ago, but the majority of stars formed coevally within the past few million years.
UBV charge coupled device (CCD) photometry has been obtained for 14 OB associations in the Magellanic Clouds using the University of Toronto's 0.6 m telescope and the Carnegie Institution of Washington's 1.0 m reflector, both on Las Campanas, Chile. The data are presented and used to construct color-magnitude diagrams for the purposes of investigating the massive-star content of the associations.
We have used UBV CCD photometry to determine the interstellar reddening toward 14 OB associations in the Magellanic Clouds. The two reddening-free indices available in the UBV system were used to obtain the reddening estimates. The mean color excesses of the associations range from E(B-V) = 0.01-0.26 mag in the Large Magellanic Cloud (LMC) and from E(B-V) = 0.06-0.25 mag in the Small Magellanic Cloud (SMC). We have modeled the observed scatter in the color excesses of individual stars within the associations and find that statisically significant differential reddening exists in at least some of the associations.
Images of the central and southern parts of the local group Sb spiral galaxy M31 were obtained by the Ultraviolet Imaging Telescope during the Astro-1 Spacelab mission. Stellar photometry is obtained for 611 stars in 59 van den Bergh associations in the near-UV A1 band (central wavelngth 2490 A), and for 130 of these stars in the far UV B1 band (central wavlength 1520 A). In addition, B-band magnitudes for 456 of the stars in approximately 30 associations, together with U- and R-band photometry for approximately 290 stars in approximately 15 associations are determined from CCD images. B-magnitudes from the catalog of Magnier et al. (1992) are given for 137 stars and R-magnitudes for 233 stars. Stellar positions are obtained from the ground based CCD images and from UIT images. Total fluxes in the UV bands are measured for 63 associations . Finding charts are presented in the NUV and FUV bands, as well as, one optical band. Tables give the images utilized in this investigation, the associations observed, with positions, total UV fluxes, extinctions estimated from color-magnitude diagrams, and stellar positions with photometry in up to five bands. UV color magnitude diagrams are presented with extinctions estimated by comparing the observed association color magnitude diagrams with colors and magnitudes of early-type stars computed from the IUE spectral atlas of Fanelli et al. (1992). Comparison with evolutionary models suggests a maximum initial mass approx. 85 solar mass. Ultraviolet extinctions within most M31 OB associations is found to follow an extinction curve similar to that of Hutchings et al. (1992). The variation in the fraction of near-UV stars also measured in the far-UV among the associations is probably caused by variations in age, extinction, and the degree of foreground contamination.
There is strong evidence that the epoch of massive-star formation in OB associations is spread over intervals of 10-20 Myr as a result of subclustering, continuous or sequential star formation, or delayed collapse of the most massive stars. The growth of supershells is strongly influenced by this star formation history and by mass deposition to the hot bubble interior ('radiative poisoning'). Even in cases of coeval or continuous star formation, the rising energy input from massive stars, as they evlolve off the main sequence, may be observed as a 'starburst' with peak energy deposition 3-10 times that of the late-time rate due to supernovae and an excess of Wolf-Rayet stars (WR/O ratio approximately equal 0.1). The peak in energy deposition results in faster shell growth and velocity evolution than in constant-luminosity models. At finite bubble ages, this mechanism can alter the covering factor of superbubbles in the disk by factors of 2-3 and can accelerate the dynamical effects of galactic 'superwinds.' In contrast, if sufficient matter is entrained in the hot bubble, radiative poisoning will stall its growth.
As part of its efforts to determine environmental and phenological states from radar imagery, the Boreal Ecosystem-Atmosphere Study (BOREAS) Remote Sensing Science (RSS)-17 team collected in situ tree xylem flow measurements for one growing season on five Picea mariana (black spruce) trees. The data were collected to obtain information on the temporal and spatial variability in water uptake by trees in the Southern Study Area-Old Black Spruce (SSA-OBS) stand in the BOREAS SSA. Temporally, the data were collected in 30-minute intervals for 120 days from 31 May 1994 until 27 September 1994. The data are stored in tabular ASCII files. The xylem flux data are available from the Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). The data files are available on a CD-ROM (see document number 20010000884).
The BOREAS TE-1 team collected various data to characterize the soil-plant systems in the BOREAS SSA. Particular emphasis was placed on nutrient biochemistry, the stores and transfers of organic carbon, and how the characteristics were related to measured methane fluxes. The overall transect in the Prince Albert National Park (Saskatchewan, Canada) included the major plant communities and related soils that occurred in that section of the boreal forest. Soil physical, chemical, and biological measurements along the transect were used to characterize the static environment, which allowed them to be related to methane fluxes. Chamber techniques were used to provide a measure of methane production/uptake. Chamber measurements coupled with flask sampling were used to determine the seasonality of methane fluxes. This particular data set contains carbon dioxide and methane flux values from the SSA-OBS site. The data were collected from 09-Jun to 04-Sep-1994. The data are stored in tabular ASCII files. The data files are available on a CD-ROM (see document number 20010000884), or from the Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC).
The BOREAS TF-9 team collected energy, carbon dioxide, and water vapor flux data at the BOREAS SSA-OBS site during the growing season of 1994 and most of the year for 1996. From the winter of 1995 to 1996, soil temperature data were also collected and provided. The data are available in tabular ASCII files.
The BOREAS TF-9 team collected data that describe carbon dioxide and water vapor fluxes from foliage at the BOREAS SSA-OBS site from 07-April through 23-November-1996. The data are available in tabular ASCII files.
The BOReal Ecosystem-Atmosphere Study Tower Flux (BOREAS TF-3) team collected tower flux, surface meteorological, and soil temperature data at the BOREAS Northern Study Area-Old Black Spruce (NSA-OBS) site continuously from the March 1994 through October 1996. The data are available in tabular ASCII files.