Search

This report covers some of the developments in the field of Commission 34, between the summer of 1993 and 1996. My predecessor, Harm Habing, noted in the last report, published in 1994, “that the traditional way of reporting is no longer adequate and that we have to rethink the format of this periodic report”. Some limited progress towards this goal has since been made, and greater discretion is now granted to the organizing committees in the preparation of their reports. The main consequence, as far as Commission 34 is concerned, is that the present report is briefer than in the recent past and makes no pretence at being a comprehensive survey of the literature over the review period. Indeed, “Chemistry” and “Planetary Nebulae” are not covered at all, as they are the topics of recent IAU Symposia (nos. 178 and 180, respectively).

Analysis of the eclipsing (period=3.39 days) binary V342 Aquilae at visible wavelengths reveals that the system is heavily enshrouded with gas, masking the identity of the underlying stars. In the ultraviolet V342 Aquilae shows six times more attenuation at phases outside of eclipse than most Algols. It is likely, then, that V342 Aquilae is the most enshrouded short period Algol system known. Our ultraviolet spectra show very pronounced Fe II absorption lines arising from ground and metastable levels, indicating an extensive circumstellar gas envelope. The strength of this absorption shows both an orbital and a cycle-to-cycle variability. The eclipse spectra display very strong emission from lines such as C II, Si IV, and C IV, with a striking similarity to the interacting binary TT Hydrae (B9.5V+K0III). We have deduced that the system consists of a late B star with a late GIII-IV companion and a mass ratio on the order of 0.5. A detailed descriptionof the observations, as well as deduced geometrical and physical properties of the system are presented. The apparent conflict of these observations with presently understood theory of binary evolution will be discussed.

The IRAS Faint Source Survey data base and the ADDSCAN/SCANPI software are used to search for systems with circumstellar dust in two samples: all of the 62 A stars in Woolley's catalog, which lie near the main sequence and are typically within 25 pc of the sun, and all A stars in the Bright Star Catalogue with mV in the range 4-5 and v sin i not less than 100 km/s. In the first sample, 11 nearby A stars having (12)-(25) and (25)-(60) colors consistent with circumstellar dust are found. The analysis of the second sample demonstrates that the use of the FSS data base increases the likelihood of identifying A stars with circumstellar dust fainter than mV = 4. Three more new dusty systems are found in this sample. Beta Pictoris (A5 IV-V) is the only star within 25 pc of the sun that displays IR colors indicative of both warm and cool dust. It is inferred from the absence of cool dust in the vast majority of the systems that the dust disks of these stars are much less extended than that seen around Beta Pictoris.

A complete investigation of the UV characteristics of the starburst knots in the inner 3 kpc region of NGC 1068 mapped by the IUE is presented. It is noted that these knots probably represent the most luminous H II regions yet studied at optical and UV wavelengths. Comparisons suggest that the brightest knot, knot 1, is 30 times brighter than NGC 604 in M33 and has a total luminosity of not less than 10 to the 43rd ergs/s.

The emissions of the highly ionized species of N V, C IV, and He II have been used to delineate the extent of the high-ionization emission region (HIER) of NGC 1068. The observed fluxes from these features are quite large and are equal to those seen in IUE exposure of the central AGN. The HIER is extremely anisotropic about the nucleus with emission seen only in the NE quadrant within 2.5 kpc of the AGN. The correlation of strong emission in the HIER with the UV starburst knots strongly suggests that much of the observed emission is produced in the galactic plane of NGC 1068. A speculative, qualitative model is presented which supports the so-called dusty torus model and suggests that there has been preferential clearing of attenuating gas and dust both very near the nucleus and in the inner galactic disk.

Studies of the ultraviolet spectra of hot interacting binaries reveal methods that may represent important new tools for probing the physical conditions in the winds emanating from hot stars. Two systems, Mu Sagittarii and UW CMa are taken as examples of how these methods can be utilized to investigate the physics of mass loss in such objects.

We have used the plate data from An Emission Line Survey of the Milky Way by Parker, Gull and Kirschner(1979) to make a new search for ring nebulae around Wolf-Rayet stars. The Survey consists of narrow-band interference filter direct imagery centered on the emission lines of Hα + [N II] at λ6570, [0 III] at λ5007, and [S II] at λ6730. We have discovered at least five new ring nebulae. Of the fifteen ring nebulae we were able to detect on the Survey, including our new discoveries, eleven were brighter in the [0 III] bandpass than in the Hα + [N II] bandpass, and were filamentary in [0 III]. All of the nebulosities we were able to detect were rated on the basis of three criteria: 1) Sharp, filamentary structure present in any or all bandpasses. 2) Wolf-Rayet star centered in projected nebulosity, or, if off-center, the segment of the ring nearest the star proportionally brighter than the rest of the ring. 3) Absence of any O stars within the nebulosity, or O star in off-centered position not adjacent to the brightest or sharpest portion of the ring.

Single-epoch multifrequency data are presented for the radio-quiet low-redshift QSO Ton 1542, spanning radio through ultraviolet wavelengths. A 3 Sigma upper limit of 9.2 mJy is obtained for the 20.3 GHz flux density; the first ultraviolet spectrum of this object is presented and discussed. A particularly strong excess of flux density is found at about 3000 A, which is likely to be dominated by Balmer continuum emission. An L-alpha/beta-hydrogen flux ratio of about 17 is also found, which is higher than that usually observed in QSOs.

A search for accretion of wind-flare material onto the white dwarf in V471 Tauri has unexpected by revealed what appears to be shortward-shifted absorption, centered on two distinct velocities, near heliocentric velocities of -300 and -590 km/s in the line of sight toward V471 Tauri. The possibility that these features are due to instrumental effects is explored. The narrow interstellar-like appearance of the -590 km/s component and lack of variability with orbital phase indicate that the observed high-velocity absorption components are formed at a great distance from the V471 Tauri system and probably owe their origins to a discrete mass ejection event.

Archival high dispersion UV spectra obtained by the IUE satellite, together with laboratory data for the Fe peak ions, reveal more than 550 identifiable spectral features in the far-UV region in the cases of the O subdwarfs BD +75 deg 325 and BD +28 deg 4211, as well as the sharp lined main sequence star HD 46202. The wide range of effective temperatures of the selected stars makes this spectral line identification list suitable for use with all stars of spectral type O. The most numerous of the features identified are those of the lines attributed to iron atoms, with each of the spectra studied exhibiting three levels of ionization that range from Fe III to Fe V for the coolest O star, HD 46202, and Fe IV through Fe VI for the hottest O star, BD +28 4211.

Multiple stacked IUE spectra reveal the presence of interstellar C I 1657 in the trough of a corresponding photospheric feature in the nearby star, Alpha PsA (d = 7 pc). This represents the first detection of this neutral atom in the interstellar medium within the immediate vicinity of the sun. It is suggested that C I may be a much better diagnostic tool in studying the local interstellar medium than the neutral species K I and Na I, which are observable at visual wavelengths. Variations in C I column density, coupled with b-values deduced from the Mg II doublet ratio, may prove to be an important means to unravel density and temperature fluctuations in the very local interstellar medium. Comparison of the line of sight toward Alpha PsA with previous Copernicus interstellar Mg II results for that of Alpha Leo tentatively indicates that the distribution of Mg II in the local cloud is not homogeneous about the sun. Rough constraints on the ionization fraction of hydrogen toward Alpha PsA do not conflict with previous data, implying that the very local interstellar medium is significantly ionized.

Objective-prism spectra at ultraviolet wavelengths extending to 1300 A were photographed during the three Skylab missions in a survey covering 9 percent of the sky. Several spectral features, notably resonance lines of C IV and Si IV, show striking variations with stellar temperature and luminosity. Marked P Cygni profiles in C IV and Si IV, indicative of significant outflow of mass, appear in the spectra of all stars with bolometric magnitude brighter than -8.4.

Results are reported for various sets of X-ray, UV, visual, IR, millimeter, and radio observations of OJ 287. The spectral information is used to constrain nonthermal energy production models for the source, and three possible models are considered. It is found that, in order to describe all the observed flux in the radio to X-ray bands with a synchrotron self-Compton model, a high degree of relativistic beaming is required such that the equivalent Doppler factor ranges from about 20 to 165. It is concluded that submillimeter flux and X-ray spectral measurements could distinguish between the two possible models that fit the present observations.

The results of a preliminary analysis of high-resolution and low-resolution UV spectra of the edge-on extended-disk shell star Beta Pic, obtained with the IUE SWP and LWR instruments on November 5-6, 1984, are reported and compared with IRAS and ground-based observations. No selective UV extinction is detected, and the fine structure of the Fe II and C I absorption lines and the presence of metastable Fe II lines are considered consistent with a circumstellar nebula or extended envelope of density greater than 1000/cu cm and electron density (1-10) x 10 to the 8th/cu cm or less at 1-2 AU from the central star. The possibility that the extended orbiting disk is clumpy, as observed by Smith and Terrile (1985), is discussed.

Three strong asymmetric absorption features with short-wavelength wings extending 350 km/s from their respective line centers at 2061.6, 2068.2, and 2079.0 A appear in high-resolution UV spectral data (0.1 A) for the Be star Phi Per (B2 Ve4) obtained with the Balloon-borne Ultraviolet Stellar Spectrometer (BUSS) on the evening of September 16, 1976. These transitions are not resonance lines and have been identified as transitions of Fe III (UV 48) originating from a metastable state with an excitation of 5.01 eV. The relative populations of the metastable and ground states are independent of the dilution factor, allowing calculation of the mass-outflow rate for a generalized model. Thus, a mass-outflow rate of 5 hundred-billionths of a solar mass per year is obtained. These transitions are observable with IUE and represent an important tool for studying mass outflow in Be and shell stars.

We have used the plate data from An Emission Line Survey of the Milky Way by Parker, Gull and Kirschner(1979) to make a new search for ring nebulae around Wolf-Rayet stars. The Survey consists of narrow-band interference filter direct imagery centered on the emission lines of Hα + [N II] at λ6570, [O III] at λ5007, and [S II] at λ6730. We have discovered at least five new ring nebulae. of the fifteen ring nebulae we were able to detect on the Survey, including our new discoveries, eleven were brighter in the [O III] bandpass than in the Hα + [N II] bandpass, and were filamentary in [O III]. All of the nebulosities we were able to detect were rated on the basis of three criteria: 1)Sharp, filamentary structure present in any or all bandpasses.2)Wolf-Rayet star centered in projected nebulosity, or, if off-center, the segment of the ring nearest the star proportionally brighter than the rest of the ring.3)Absence of any O stars within the nebulosity, or O star in off-centered position not adjacent to the brightest or sharpest portion of the ring.

IUE low-resolution spectra of 11 OB stars in the Small Magellanic Cloud reveal considerably lower wind velocities than corresponding galactic stars. The reduced effectiveness of radiative acceleration due to lower abundances of heavy elements in the SMC is believed to be responsible for this effect. The absorption strengths of the SMC P Cygni profiles are somewhat weaker in Si IV and much weaker in C IV. Crude estimates of relative mass-loss rates suggest that the observed differences may be dominated by the differing relative abundances of Si and C, although a lower degree of wind ionization or lower mass-loss rates for the SMC stars cannot be ruled out.

It is shown that only a small fraction of the many supernovae in the Galaxy produces observable supernova remnants; this fraction, which is found to depend weakly on the lower mass limit of the SN progenitors, and more strongly on the specific characteristics of the associated interstellar medium, decreases from about 15% near the galactic center to 10% at R(gal) of about 10 kpc and drops nearly to zero for R(gal) greater than 15 kpc. Whether an SNR is detectable is determined by the density of the ambient interstellar medium in which it is embedded; it is found that SNRs are detectable only above some critical density (about 0.1 per cu cm). The presence of large low-density superbubble cavities around stellar associations due to the combined effects of stellar winds and supernova shells strongly suggests that a large portion of the detectable SNRs must have runaway stars as their progenitors.

The theory of line formation along the line of sight to the visual companions of M stars undergoing spherically symmetric mass loss is developed. The theory is then used to determine the position of the companions relative to the primary. It is found that Alpha-2 Her is 1470 AU further along the line of sight than Alpha-1 Her for an adopted distance of 150 pc; Alpha-2 Sco is 420 AU closer than Alpha-1 Sco for an adopted distance of 185 pc.

High-resolution spectra of the nearby (48 pc) white dwarf G191-B2B, obtained with the International Ultraviolet Explorer, reveal sharp resonance lines of N V, C IV, and Si IV. The origin of these features is most likely linked to the white dwarf, possibly being formed in an expanding halo around the star. Interstellar lines of C II, N I, Mg II, Si II, and Fe II are also seen in the spectrum. Analysis of these features indicates an average neutral hydrogen number density of 0.064 for this line of sight. In combination with the recent EUV and soft X-ray results, this is interpreted to mean that the interstellar medium in the most immediate solar vicinity is of the normal density n approximately equal to 0.1/cu cm of lower ionization, while just beyond it, at least in some directions, is a hot lower density plasma. These results are apparently in conflict with the model of the interstellar medium by McKee and Ostriker (1977) in its present form.

A study has been conducted of eight white dwarfs, including seven DA and one He-rich types. The study is based on high-resolution observations conducted with the aid of the International Ultraviolet Explorer. Four of the dwarfs show features related to heavy elements which are not interstellar in origin. It is tentatively suggested that, at least in the hottest low-gravity DA white dwarfs, the observed narrow-lined features are formed in expanding halos or winds associated with the white dwarfs. Theoretically, stable white dwarf halos should actually be coronae with temperatures in excess of 1,000,000 K. However, the observed narrow-lined features do not suggest such high temperatures. The observed radial velocities suggest weak stellar winds in two hot white dwarfs, namely, G191-B2B and 2111+49. It is tentatively proposed that radiative levitation can explain the appearance of the observed metallic lines in the hot DA white dwarfs.