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1.
We review existing ROSAT detections of single Galactic Wolf–Rayet (WR) stars and develop wind models to interpret the X-ray emission. The ROSAT data, consisting of bandpass detections from the ROSAT All-Sky Survey (RASS) and some pointed observations, exhibit no correlations of the WR X-ray luminosity ( L X) with any star or wind parameters of interest (e.g. bolometric luminosity, mass-loss rate or wind kinetic energy), although the dispersion in the measurements is quite large. The lack of correlation between X-ray luminosity and wind parameters among the WR stars is unlike that of their progenitors, the O stars, which show trends with such parameters. In this paper we seek to (i) test by how much the X-ray properties of the WR stars differ from the O stars and (ii) place limits on the temperature T X and filling factor f X of the X-ray-emitting gas in the WR winds. Adopting empirically derived relationships for T X and f X from O-star winds, the predicted X-ray emission from WR stars is much smaller than observed with ROSAT . Abandoning the T X relation from O stars, we maximize the cooling from a single-temperature hot gas to derive lower limits for the filling factors in WR winds. Although these filling factors are consistently found to be an order of magnitude greater than those for O stars, we find that the data are consistent (albeit the data are noisy) with a trend of in WR stars, as is also the case for O stars.  相似文献   

2.
We report the discovery of five massive Wolf–Rayet (WR) stars resulting from a programme of follow-up spectroscopy of candidate emission-line stars in the Anglo-Australian Observatory United Kingdom Schmidt Telescope (AAO/UKST) Southern Galactic Plane Hα survey. The 6195–6775 Å spectra of the stars are presented and discussed. A WC9 class is assigned to all five stars through comparison of their spectra with those of known late-type WC stars, bringing the known total number of Galactic WC9 stars to 44. Whilst three of the five WC9 stars exhibit near-infrared (NIR) excesses characteristic of hot dust emission (as seen in the great majority of known WC9 stars), we find that two of the stars show no discernible evidence of such excesses. This increases the number of known WC9 stars without NIR excesses to seven. Reddenings and distances for all five stars are estimated.  相似文献   

3.
The central stars of two of the new planetary nebulae found during scans of the AAO/UKST H α Survey of the Milky Way have been found to exhibit Wolf–Rayet (WR) emission features. One (PMR 1) is an early-type star of class either [WO4] or [WC4]. The other (PMR 2) is a late [WC] star which, depending on the classification scheme used, is either intermediate in class between [WC9] and [WC10] or the sole member of the [WC10] class. Both stars exhibit unusual spectral features which may be attributed to enhanced nitrogen in their atmospheres and could be indicative of unusual stellar evolution.  相似文献   

4.
We study the evolution of the circumstellar medium of massive stars. We pay particular attention to Wolf-Rayet stars that are thought to be the progenitors of some long gamma-ray bursts (GRBs). We detail the mass-loss rates we use in our stellar evolution models and how we estimate the stellar wind speeds during different phases. With these details we simulate the interactions between the wind and the interstellar medium to predict the circumstellar environment around the stars at the time of core-collapse. We then investigate how the structure of the environment might affect the GRB afterglow. We find that when the afterglow jet encounters the free-wind/stalled-wind interface, rebrightening occurs and a bump is seen in the afterglow light curve. However, our predicted positions of this interface are too distant from the site of the GRB to reach while the afterglow remains observable. The values of the final wind density,   A *  , from our stellar models are of the same order (≲1) as some of the values inferred from observed afterglow light curves. We do not reproduce the lowest   A *  values below 0.5 inferred from afterglow observations. For these cases, we suggest that the progenitors could have been a WO-type Wolf–Rayet (WR) star or a very low-metallicity star. Finally, we turn our attention to the matter of stellar wind material producing absorption lines in the afterglow spectra. We discuss the observational signatures of two WR stellar types, WC and WO, in the afterglow light curve and spectra. We also indicate how it may be possible to constrain the initial mass and metallicity of a GRB progenitor by using the inferred wind density and wind velocity.  相似文献   

5.
A spectroscopic search for luminous companions to WC9-type Wolf–Rayet stars making circumstellar dust reveals the presence of absorption lines attributable to companions in the blue spectra of WR 69 (HD 136488) and WR 104 (Ve2–45). Comparison of spectra of WR 104 observed in 1995 and 1997 showed the absorption lines to be more conspicuous in the latter observation and the emission lines weaker, suggesting a selective eclipse of the WC9 star similar to that observed by Crowther in 1996. The WC9 emission-line spectra are shown to be less uniform than previously thought, showing a significant range of O  ii line strengths. The only two WC9 stars in the observed sample that do not make circumstellar dust, WR 81 (He3–1316) and WR 92 (HD 157451), are found to have anomalously weak O  ii and strong He  ii lines. We suggest that these spectroscopic differences may reflect a compositional difference that plays a role in determining which of the WC9 stars make dust.  相似文献   

6.
A nearly complete sample of 24 Magellanic Cloud WC/WO subclass Wolf–Rayet stars is studied spectroscopically and photometrically to determine its binary frequency. Theory predicts the Roche lobe overflow produced Wolf–Rayet binary frequency to be 52±14 per cent in the Large Magellanic Cloud and 100 per cent in the Small Magellanic Cloud, not counting non-Roche lobe overflow Wolf–Rayet binaries. Lower ambient metallicity ( Z ) leads to lower opacity, preventing all but the most massive (hence luminous) single stars from reaching the Wolf–Rayet stage. However, theory predicts that Roche lobe overflow even in binaries of modest mass will lead to Wolf–Rayet stars in binaries with periods below approximately 200 d, for initial periods below approximately 1000 d, independent of Z . By examining their absolute continuum magnitudes, radial velocity variations, emission-line equivalent widths and full widths at half-maximum, a WC/WO binary frequency of only 13 per cent, significantly lower than the prediction, is found in the Large Magellanic Cloud. In the unlikely event that all of the cases with a less certain binary status actually turn out to be binary, current theory and observation would agree. (The Small Magellanic Cloud contains only one WC/WO star, which happens to be a binary.) The three WC+O binaries in the Large Magellanic Cloud all have periods well below 1000 d. The large majority of WC/WO stars in such environments apparently can form without the aid of a binary companion. Current evolutionary scenarios appear to have difficulty explaining either the relatively large number of Wolf–Rayet stars in the Magellanic Clouds, or the formation of Wolf–Rayet stars in general.  相似文献   

7.
Following on from our recent Paper I, we present theoretical models of Wolf–Rayet (WR) stars for non-solar metallicities from   Z = 0.03  to 0.0001 by mass fraction with different mass-loss rate assumptions. We find that some single WR stars may still form even at the lowest metallicities, but whether this occurs or not depends critically on the upper cut-off point of the initial mass function used. As at solar metallicity, a population of binaries is required to fully reproduce WR star observations. For most scenarios, these binaries dominate the low-metallicity WR population but probably not the enrichment. We find comparable carbon enrichment from single WR stars to that from asymptotic giant branch stars at all metallicities for which data are available, but which of them is the dominant source of carbon depends strongly on the set of asymptotic giant branch yields adopted and the assumed initial mass function. We find an increase in carbon enrichment with increasing metallicity but a decrease in oxygen enrichment, as confirmed by observation.  相似文献   

8.
A search for evidence of colliding winds is undertaken among the four certain Magellanic Cloud WC/WO spectroscopic binaries found in the companion Paper I, as well as among two Galactic WC/WO binaries of very similar subtype. Two methods of analysis, which allow the determination of orbital inclination and parameters relating to the shock cone from spectroscopic studies of colliding winds, are attempted. In the first method, Lührs' spectroscopic model is fitted to the moderately strong C  iii 5696-Å excess line emission arising in the shock cone for the stars Br22 and WR 9. The four other systems show only very weak C  iii 5696-Å emission. Lührs' model follows well the mean displacement of the line in velocity space, but is unable to reproduce details in the line profile and fails to give a reliable estimate of the orbital inclination. In the second method, an alternative attempt is also made to fit the variation of more global quantities, full width at half-maximum and radial velocity of the excess emission, with phase. This method also gives satisfactory results in a qualitative way, but shows numerical degeneracy with orbital inclination. Colliding wind effects on the very strong C  iv 5808-Å Wolf–Rayet emission line, present in all six binaries, are also found to behave qualitatively as expected. After allowing for line enhancement in colliding wind binaries, it now appears that all Magellanic Cloud WC/WO stars occupy a very narrow range in spectral subclass: WC4/WO3.  相似文献   

9.
Do some Wolf–Rayet stars owe their strong winds to something else besides radiation pressure? The answer to this question is still not entirely obvious, especially in certain Wolf–Rayet subclasses, mainly WN8 and WC9. Both of these types of Wolf–Rayet stars are thought to be highly variable, as suggested by observations, possibly due to pulsations. However, only the WN8 stars have so far been vigorously and systematically investigated for variability. We present here the results of a systematic survey during three consecutive weeks of 19 Galactic WC9 stars and one WC8 star for photometric variability in two optical bands, V and I . Of particular interest are the correlated variations in brightness and colour index in the context of carbon dust formation, which occurs frequently in WC9 and some WC8 stars. In the most variable case, WR76, we used this information to derive a typical dust grain size of  ∼ 0.1 μm  . However, most photometric variations occur at surprisingly low levels, and in fact almost half of our sample shows no significant variability at all above the instrumental level (  σ∼ 0.005– 0.01  mag).  相似文献   

10.
The recent VIIth Catalogue of Galactic Wolf-Rayet Stars lists 227 Population I WR stars, comprising 127 WN, 87 WC, 10 WN/WC and 3 WO stars. Additional discoveries bring the census to 234 WR stars. A re-determination of the optical photometric distances and the galactic distribution of WR stars shows in the solar neighbourhood a projected surface density of 2.7 WR stars per kpc2, a N WC/N WN number ratio of 1.3, and a WR binary frequency of 40 %.The galactocentric distance (R WR) distribution per subtype showsR WN and R WC decreasing with WN and WC subtypes. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

11.
In this paper we present numerical results on the decay of small stellar systems under different initial conditions (multiplicity 3 ≤  N  ≤ 10, and various mass spectra, initial velocities and initial configurations). The numerical treatment uses the CHAIN1 code (Mikkola &38; Aarseth). Particular attention is paid to the distribution of high-velocity escapers: we define these as stars with velocity above 30 km s−1. These numerical experiments confirm that small N -body systems are dynamically unstable and produce cascades of escapers in the process of their decay. It is shown that the fraction of stars that escape from small dense stellar systems with an escape velocity greater than 30 km s−1 is ∼1 per cent for all systems treated here. This relatively small fraction must be considered in relation to the rate of star formation in the Galaxy in small groups: this could explain some moderately high-velocity stars observed in the Galactic disc and possibly some young stars with relatively high metallicity in the thick disc.  相似文献   

12.
We present results from an ongoing X-ray survey of Wolf–Rayet (WR) galaxies, a class of objects believed to be very young starbursts. This paper extends the first X-ray survey of WR galaxies by Stevens &38; Strickland by studying WR galaxies identified subsequent to the original WR galaxy catalogue of Conti.   Out of a sample of 40 new WR galaxies a total of 10 have been observed with the ROSAT PSPC, and of these seven have been detected (NGC 1365, NGC 1569, I Zw 18, NGC 3353, NGC 4449, NGC 5408 and a marginal detection of NGC 2366). Of these, all are dwarf starbursts except for NGC 1365, which is a barred spiral galaxy possibly with an active nucleus. We also report on observations of the related emission-line galaxy IRAS 0833+6517.   The X-ray properties of these galaxies are broadly in line with those found for the original sample; they are X-ray overluminous compared with their blue luminosity and have thermal spectra with typically kT  ∼ 0.4 − 1.0 keV. There are some oddities: NGC 5408 is very overluminous in X-rays, even compared with other WR galaxies; I Zw 18 has a harder X-ray spectrum; NGC 1365, although thought to contain an active nucleus, has X-ray properties that are broadly similar to other WR galaxies, and we suggest that the X-ray emission from NGC 1365 is due to starburst activity.   A good correlation between X-ray and blue luminosity is found for the WR galaxy sample as a whole. However, when just dwarf galaxies are considered there is little evidence of correlation. We discuss the implications of these results on our understanding of the X-ray emission from WR galaxies and suggest that the best explanation for the X-ray activity is starburst activity from a young starburst region.  相似文献   

13.
We present FOcal Reducer/low dispersion Spectrograph-1 spectra (from the European Southern Observatory's Very Large Telescope) of a sample of 34 faint  20.0 < g * < 21.1  A-type stars selected from the Sloan Digital Sky Survey Early Data Release, with the goal of measuring the velocity dispersion of blue horizontal branch (BHB) stars in the remote Galactic halo,   R ∼ 80 kpc  . We show that colour selection with  1.08 < u *− g * < 1.40  and  −0.2 < g *− r * < −0.04  minimizes contamination of the sample by less luminous blue stragglers. In classifying the stars we confine our attention to the 20 stars with spectra of signal-to-noise ratio >15 Å−1. Classification produces a sample of eight BHB stars at distances  65–102 kpc  from the Sun (mean 80 kpc), which represents the most distant sample of Galactic stars with measured radial velocities. The dispersion of the measured radial component of the velocity with respect to the centre of the Galaxy is  58 ± 15 km s−1  . This value is anomalously low in comparison with measured values for stars at smaller distances, as well as for satellites at similar distances. Seeking an explanation for the low measured velocity dispersion, further analysis reveals that six of the eight remote BHB stars are plausibly associated with a single orbit. Three previously known outer halo carbon stars also appear to belong to this stream. The velocity dispersion of all nine stars relative to the orbit is only  15 ± 4 km s−1  . Further observations along the orbit are required to trace the full extent of this structure on the sky.  相似文献   

14.
We consider the contribution to the Galactic diffuse γ-ray emission from unresolved γ-ray pulsars. Based on the thick outer gap model, Monte Carlo methods are used to simulate the properties (period, distance, magnetic field, etc.) of the Galactic population of rotation-powered pulsars the gamma-ray flux of which is lower than the threshold sensitivity of the EGRET detector on the Compton Gamma-Ray Observatory . Furthermore, the contribution to the Galactic diffuse γ-ray spectrum from the unresolved γ-ray pulsars is calculated. Our results indicate that the unresolved γ-ray pulsars contribute ∼5 to ∼10 per cent to the measured Galactic diffuse γ-ray emission if the birth rate of neutron stars in the Galaxy is 1 to 2 per century, and that these pulsars contribute significantly to the observed Galactic diffuse γ-ray emission above 1 GeV. Comparing the model spectrum with the observed spectrum, we show that the unresolved γ-ray pulsars contribute very little to the diffuse emission at lower energies but can account for ∼50 per cent of the observed spectrum above 1 GeV if the product of the birth rate of neutron stars and the γ-ray beaming fraction is about unity. Such a large pulsar contribution can explain the difference (∼60 per cent) between the intensity of the Galactic diffuse emission as measured by EGRET above 1 GeV and model predictions based on cosmic ray–matter interaction only.  相似文献   

15.
We collected almost all Galactic Wolf-Rayet (hereafter WR) stars found so far from the literature. 578 WR stars are gathered in this paper. 2MASS counterparts with good quality magnitudes in all JHK bands are listed for 364 WR stars. In addition, WISE counterparts for these sources are also identified. It is found that free-free emission is the main dominant source for the infrared excess in most WR stars up to 3.4 μm. However at the longer wavelengths the thermal radiation is dominant. In addition, WR stars in Clusters of the Galactic center region have the strong infrared excess in the near infrared due to the dust thermal emission from the strong star forming activity in the Galactic center region. For some WR stars with the WC spectral type, in particular, with WCd type, the dust thermal emission is important radiation source while many WR stars with the WC spectral type have the near infrared flux enhancement from the broad line emission in the K band. It is also shown that many single WC stars with different spectral sub-types have different locations in the near infrared two-color diagram, in particular, WC6 and WC9d stars can be separated respectively from other spectral type stars while single WN stars with different spectral sub-types can not be separated in the near infrared two-color diagram.  相似文献   

16.
We present a study of optical spectra of the Wolf–Rayet star AzV 336a (=SMC WR7) in the Small Magellanic Cloud. Our study is based on data obtained at several Observatories between 1988 and 2001. We find SMC WR7 to be a double-lined WN+O6 spectroscopic binary with an orbital period of 19.56 d. The radial velocities of the He absorption lines of the O6 component and the strong He  ii emission at λ 4686 Å of the WN component describe anti-phased orbital motions. However, they show a small phase shift of ∼1 d. We discuss possible explanations for this phase shift. The amplitude of the radial velocity variations of He  ii emission is twice that of the absorption lines. The binary components have fairly high minimum masses, ∼18 and 34 M⊙ for the WN and O6 components, respectively.  相似文献   

17.
A sample of 1497 carbon stars in the Large Magellanic Cloud (LMC) has been observed in the red part of the spectrum with the 2dF facility on the Anglo-Australian Telescope. Of these, 156 have been identified as J-type (i.e. 13C-rich) carbon stars using a technique which provides a clear distinction between J stars and the normal N-type carbon stars that comprise the bulk of the sample, and yields few borderline cases. A simple two-dimensional classification of the spectra, based on their spectral slopes in different wavelength regions, has been constructed and found to be related to the more conventional c and j indices, modified to suit the spectral regions observed. Most of the J stars form a photometric sequence in the   K − ( J − K )  colour–magnitude diagram, parallel to and 0.6 mag fainter than the N-star sequence. A subset of the J stars (about 13 per cent) are brighter than this J-star sequence; most of these are spectroscopically different from the other J stars. The bright J stars have stronger CN bands than the other J stars and are found strongly concentrated in the central regions of the LMC. Most of the rather few stars in common with Hartwick and Cowley's sample of suspected CH stars are J stars. Overall, the proportion of carbon stars identified as J stars is somewhat lower than has been found in the Galaxy. The Na D lines are weaker in the LMC J stars than in either the Galactic J stars or the LMC N stars, and do not seem to depend on temperature.  相似文献   

18.
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 Hβ 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.  相似文献   

19.
Large samples of field horizontal branch (FHB) stars make excellent tracers of the Galactic halo; by studying their kinematics, one can infer important physical properties of our Galaxy. Here we present the results of a medium-resolution spectroscopic survey of 530 FHB stars selected from the Hamburg/ESO survey. The stars have a mean distance of ∼7 kpc and thus probe the inner parts of the Milky Way halo. We measure radial velocities from the spectra in order to test the model of Sommer-Larsen et al., who suggested that the velocity ellipsoid of the halo changes from radially dominated orbits to tangentially dominated orbits as one proceeds from the inner to the outer halo. We find that the present data are unable to discriminate between this model and a more simple isothermal ellipsoid; we suggest that additional observations towards the Galactic Centre might help to differentiate them.  相似文献   

20.
Magnetars, neutron stars with ultrastrong magnetic fields  ( B ∼ 1014−1015G)  , manifest their exotic nature in the form of soft gamma-ray repeaters and anomalous X-ray pulsars. This study estimates the birthrate of magnetars to be ∼0.22 per century with a Galactic population comprising ∼17 objects. A population synthesis was carried out based on the five anomalous X-ray pulsars detected in the ROSAT All Sky Survey by comparing their number to that of massive OB stars in a well-defined volume. Additionally, the group of seven X-ray dim isolated neutron stars detected in the same survey were found to have a birthrate of ∼2 per century with a Galactic population of ∼22 000 objects.  相似文献   

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