Electron density in the emission-line region of Wolf-Rayet stars

The Inglis-Teller relation, generalized for a hydrogen-like or alkali-like ion with an arbitrary core charge, is used to estimate the electron density in the emission-line region of Wolf-Rayet stars. It is found that the electron density in the region which gives rise to Heii emission lines is 4×10¹⁴ cm^–3.     

Oxygen abundance in giant H II galaxies

A sample of 93 emission-line high luminosity galaxies from the Sloan Digital Sky Survey (SDSS) has been investigated. Line intensities have been measured in 116 SDSS spectra. Oxygen abundance has been determined in the studied galaxies. Since the auroral line of twice ionized oxygen [O III] λ 436.3 nm cannot be detected in the spectra of the sample, the intensity ratio of nebular to auroral lines needed to determine the electron temperature is calculated using the ff-relation. The oxygen abundances obtained in SDSS high luminosity galaxies are 0.2–0.5 dex lower than the maximum attainable value. This is caused by the fact that the sample includes only the gas-rich galaxies in which intense bursts of star formation occur. The equivalent number of O7 V stars which are responsible for excitation of luminescent H II regions in the studied sample is two or three orders of magnitude more than the number of stars which cause the luminescence of the brightest H II regions in nearby galaxies, and it exceeds by one order of magnitude the number of stars which cause gas in SBS 0335-052 E to glow.     

Study of the nature of He II nebular emission in the spectra of low-metallicity H II regions from the Sloan Digital Sky Survey

We studied a sample of galaxies from the Sloan Digital Sky Survey Data Release 7 that exhibited the He II λ 468.6 nm nebular line (an indicator of hard radiation in H II regions) in their spectra. The intensity of this line in the spectra of H II regions from our sample increased with decreasing metallicity, thus confirming the results of earlier studies. However, the theoretical models of population synthesis predict that the He II line intensity must decrease with decreasing metallicity. A possible connection between hard UV radiation and Wolf-Rayet stars was investigated. Only 30% of spectra from our sample exhibited both the nebular emission and the broad He II emission of Wolf-Rayet stars. This fact does not rule out the possibility that Wolf-Rayet stars serve as sources of hard ionizing radiation in some H II regions. However, other possible sources, such as the fast radiative shock waves, seem to be more likely to produce this hard ionizing radiation.     

Sample of Wolf-Rayet galaxies from the SLOAN digital sky survey

We have analyzed the spectra of blue compact dwarf galaxies from the SLOAN Digital Sky Survey (SDSS) Data Release 7 and created a sample of 271 galaxies with Wolf-Rayet (WR) spectral features produced by high-velocity stellar winds. A blue WR feature (bump) is a blend of the N V λλ 460.5 and 462.0 nm, N III λλ 463.4 and 464.0 nm, C III λ 465.0 nm, C IV λ 465.8 nm, and He II λ 468.6 nm emission lines. A red WR feature (bump) is the broad C IV λ 580.8 nm emission. The blue WR bump is mainly due to emissions of nitrogen WR (WN) stars, while the red bump is fully produced by emissions of carbon WR (WC) stars. All the sample spectra show the blue WR bumps, whereas the red WR bumps are only identified in 50% of sample spectra. We have derived the numbers of early-type WC stars (WCE) and late-type WN stars (WNL) in the galaxies using the luminosities of single WC and WN stars in the red and blue bumps, respectively. The number of O stars is estimated using the H_β luminosity. The ratio of the overall number of WR stars of different types to the number of all massive stars N(WR)/N(O + WR) decreases with decreasing metallicity, corresponding to the evolution population synthesis models.     

The ionizing radiation field in the galactic centre and the nature of LINERs

The Galactic centre contains a low-ionization nebula that has been previously interpreted as plasma photoionized by relatively cool O stars. We consider the possibility that this material is instead ionized by more energetic continua and described by a relatively low ionization parameter (ratio of ionizing photon to nucleon density). We find that the predicted spectrum is more sensitive to ionization parameter and cloud density than to details of the ionizing continuum shape. The Galactic centre spectrum can be generated by stars with a range of temperatures irradiating clouds with multiple components of differing densities. The present calculations also show that optical emission from many LINERs (low-ionization nuclear emission-line regions) can be generated by relatively hot, yet normal O stars embedded in gas clouds similarly characterized by a range of densities.     

How are intergalactic filaments made?

Computer simulations are performed which suggest that narrow intergalactic filaments can be produced by tidal forces. A fixed potential perturber passes a self-gravitating disk of stars in a nearly parabolic orbit. Due to their large velocity dispersion, the stars which are pulled out by the tide form only an amorphous cloud. The gas clouds are simulated by a disk of points of zero mass, zero velocity dispersion superimposed on the stellar disk. The ‘gas clouds’ form remarkably long, narrow filaments; but not so narrow as the narrowest filaments observed.     

Symbiotic stars with Wolf-Rayet spectra as protoplanetary nebulae?

Symbiotic systems, in particular symbiotic novae, have been suggested to be very early stages of planetary nebulae. Some of them have been described as going through a Wolf-Rayet phase. We argue that there may be a direct relation between symbiotic objects and planetary nebulae, and that the Wolf-Rayet phase is connected to an active spell of the hot companion. Symbiotic stars could lead to planetary nebulae with two central stars with different radiation temperatures and luminosities, where each has the power to ionize a planetary nebula on its own.     

A new component of cosmic rays?

Recent direct measurements of the energy spectra of the major mass components of cosmic rays have indicated the presence of a ‘kink’ in the region of 200 GeV per nucleon. The kink, which varies in magnitude from one element to another, is much sharper than predicted by our cosmic ray origin model in which supernova remnants are responsible for cosmic ray acceleration and it appears as though a new, steeper component is responsible.The component amounts to about 20 percent of the total at 30 GeV/nucleon for protons and helium nuclei and its magnitude varies with nuclear charge; the unweighted fraction for all cosmic rays being 36%.The origin of the new component is subject to doubt but the contenders include O, B, A, supergiant and Wolf-Rayet stars, by way of their intense stellar winds. Another explanation is also in terms of these particles as the sources but then being trapped, and even further accelerated, in the Local Bubble.     

Searching for hidden Wolf–Rayet stars in the Galactic plane – 15 new Wolf–Rayet stars

We report the discovery of 15 previously unknown Wolf–Rayet (WR) stars found as part of an infrared (IR) broad-band study of candidate WR stars in the Galaxy. We have derived an empirically based selection algorithm which has selected ∼5000 WR candidate stars located within the Galactic plane drawn from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (mid-IR) and Two-Micron All-Sky Survey (near-IR) catalogues. Spectroscopic follow-up of 184 of these reveals 11 nitrogen-rich (WN) and four carbon-rich (WC) WR stars. Early WC subtypes are absent from our sample and none shows evidence for circumstellar dust emission. Of the candidates which are not WR stars, ∼120 displayed hydrogen emission-line features in their spectra. Spectral features suggest that the majority of these are in fact B supergiants/hypergiants, ∼40 of these are identified Be/B[e] candidates.
Here, we present the optical spectra for six of the newly detected WR stars, and the near-IR spectra for the remaining nine of our sample. With a WR yield rate of ∼7 per cent and a massive star detection rate of ∼65 per cent, initial results suggest that this method is one of the most successful means for locating evolved, massive stars in the Galaxy.     

After the Supernova: Runaway Stars and Massive X-ray Binary Populations with Metallicity

One of the main pathways by which massive runaways are thought to be produced is by the disruption of a binary system after the supernova (SN) of one of its components. Under such a scenario, the populations of runaway stars in different phases should reflect the input binary population and its evolution. Conversely, if the system stays together after the SN, a High Mass X-Ray Binary (HMXB) may result. We present simulations exploring the behaviour of such runaway and HMXB populations with metallicity, and compare them to observations. As many as two-thirds of massive runaway stars may be produced by supernovae in binaries. Decreasing metallicity lowers the fraction of O stars which are runaway, but increases the Wolf-Rayet runaway fractions and the number of potential HMXBs.     

Interactions between the Abell 2597 central radio source and dense gas in its host galaxy

We present a detailed HST/WFPC2 study of the complex network of emission-line filaments and blue continuum emission associated with the lobes of the radio source at the center of the nearby strong cooling-flow cluster Abell 2597. This object is a prototypical “blue-lobed” cluster radio galaxy. We discuss ways in which the radio source is interacting with the cool dense gas around it, and derive detailed constraints on the physical properties of the gas. We also resolve the blue continuum emission, which is most likely due to young stars, and discuss possible relationships between the radio source, the supply of gas and the triggering mechanisms for the active nucleus and the star formation.     

Analyses of Wolf-Rayet ultraviolet spectra

Ultraviolet spectra of population I WR stars obtained from IUE archive are used to determine fundamental stellar parameters. Terminal velocities for 85 galactic and LMC Wolf-Rayet stars were obtained by means of the empirical relation between spectral quantities easily measured in low resolution and high-resolution terminal velocity measurements. Temperatures and so-called transformed radii were derived based on available contour plots of spectral characteristics for a grid of NLTE models. The effect of the reddening law on stellar far ultraviolet continua is emphasized and the revised extinction curve towards WR stars is used for dereddening. For the sample of stars attributed to open clusters or associations we construct the stellar distance scale and adopt it for the other WR stars. The remaining fundamental parameters are derived and HR diagram for population I WR stars is presented.     

Colliding Winds in Binaries: Observations

Colliding winds in binaries are discussed mainly from an observational point of view. Collisions are especially energetic in the case of hot, luminous stars, which drive strong, fast winds. Emphasis is therefore devoted to binaries containing Wolf-Rayet stars. The subject is divided up into (1) continuum radiation (X-ray and non-thermal radio from the hot bow shock head, IR from dust formed in some WC + O binaries far downstream in the collision shock cone) and (2) line radiation (optical and UV, both from various regions downstream from the bow shock head). The latter is particularly useful in providing constraints on the velocity field and hence ultimately the geometry of the wind collision and the binary system itself. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Wolf-Rayet stars and mass loss

Wolf-Rayet stars are defined, a summary is given of the properties of Wolf-Rayet stars, and a qualitative model of a Wolf-Rayet star is sketched. It is incontrovertible that Wolf-Rayet stars are losing mass, a typical rate of mass loss being near 10^–5 M per year. The outward directed velocity of the expanding shell has been estimated for 10 stars. The largest value found is 2500 km/sec; most values lie between 1000 and 1500 km/sec. Two outstanding problems are to understand how the observed high velocities are generated and to demonstrate quantitatively the effect of these velocities on the observed spectrum. Five questions raised by the fact that mass loss is observed to take place from Wolf-rayet stars are discussed briefly in Section 5.Presented at the Trieste Colloquium on Mass Loss from Stars, September 12–16, 1968.     

Are QSOs local objects?

The similarities of the spectra of QSOs with those of Wolf-Rayet stars are pointed out. The emission spectrum of the earliest discovered QSO, 3C 273, in the ultraviolet and visible regions is interpreted as that of an object deficient in hydrogen like Wolf-Rayet stars but havingno redshift. The visible emission spectra of two other QSOs, 3C 48 and 3C 280.1, are also similarly interpreted. It is further assumed that the absorption lines of QSOs are produced in an expanding atmosphere so that they are violet shifted as in Wolf-Rayet stars. Fifty-four out of 55 narrow absorption lines of the QSO Q 1246-057 are interpreted on the assumption that the average velocity of the absorbing ions is 500 km s^–1, although the redshift theory can explain only 23 lines by invoking six different redshifts: Four of the five emission lines of the same object can be identified assuming no shift. Since the QSOs are here assumed to be comparatively local objects, the problems of energy supply, superluminal velocities, etc., raised by the conventional explanation do not arise in this case.Presently at the Institut für Physikalische und Theoretische Chemie, Universität Erlangen, F.R.G.     

Dust properties of [WCL] CSPNS

The rather rare class of central stars of planetary nebulae thatshow Wolf-Rayet spectra have been a subject of great interest,particularly in the infrared, since their discovery in the late1960s. I will focus on further peculiarities found within thepast 1-2 years with the advent of infrared spectroscopy fromISO. Notably, these stars simultaneously betray the presenceof regions of carbon-rich and oxygen-rich dust chemistry. Icompare and contrast complete ISO spectra from 2 to 200 micronsof a small sample of [WC9] to [WC11] central stars.     

Far-infrared emission from ring nebulae

Ring nebulae are known to form around stars like the Wolf-Rayet and the Of stars. The dust in these nebulae is heated by the central star and, therefore, provides a positive clue to the origin of the nebulae, complementing the optical techniques. A systematic search has been carried out to study the infra-red emission from these nebulae based on the IRAS data. The influence of the local interstellar material properties on the formation of nebular dust is studied.     

Laser action in stellar envelopes

It is shown that in high-temperature stars in which high speed mass loss is occurring, the rapidly recombining plasma in the stellar envelope can act as an amplifying medium. Model calculations for laser action in Heii 4686, using the collisional-radiative model, are presented. Menzel's hypothesis of laser action in distended stellar atmospheres is shown to be fully substantiated. The relevance of these results in resolving the problem of intensity anomalies in the spectra of Wolf-Rayet stars is pointed out.     

Massive stellar populations in Wolf-Rayet galaxies

The analysis of the long-slit spectral observations of 40 Wolf-Rayet (WR) galaxies with heavy element mass fraction ranging over two orders of magnitudes from Z_⊙/50 to 2Z_⊙ are presented. We derive the number of O stars from the luminosity of the Hβ emission line, the number of early carbon Wolf-Rayet stars (WCE) from the luminosity of the red bump (broad CIV λ5808 emission) and the number of late nitrogen Wolf-Rayet stars (WNL) from the luminosity of the blue bump (broad emission near λ4650). We identified some of weak WR emission lines, most often the N III λ4512 and Si III λ4565 lines, which have very rarely or never been seen and discussed before in WR galaxies. A new technique for deriving the number of WNL stars (WN7–WN8) from the N III λ4512 and the number of WN9–WN11 from Si III λ4565 emission lines has been proposed. This technique is potentially more precise than the blue bump method because it does not suffer from contamination of WCE and early WN (WNE) stars and nebular gaseous emission. We find that the fraction of WR stars relative to all massive stars increases with increasing metallicity, in agreement with predictions of evolutionary synthesis models. The relative number ratios N(WC)/N(WN) and the equivalent widths of the blue and red bumps derived from observations are also in satisfactory agreement with theoretical predictions, except for the most metal-deficient WR galaxies. A possible source of disagreement is too low a line emission luminosity adopted for a single WCE star in low-metallicity models.