The O-type ionizing cluster in IC 2944
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Observational data on IC 2944 are compiled and analyzed. It is shown that, contrary to some recent discussions (Ardeberg and Maurice, 1980; Perry and Landolt, 1986), the O stars in the field of the H II region IC 2944 constitute a significant physical cluster.Keywords:
O-type star
Previous studies have demonstrated that putatively single nitrogen-type Wolf-Rayet stars (WN stars) without known companions are X-ray sources. However, almost all WN star X-ray detections so far have been of earlier WN2 - WN6 spectral subtypes. Later WN7 - WN9 subtypes (also known as WNL stars) have proved more difficult to detect, an important exception being WR 79a (WN9ha). We present here new X-ray detections of the WNL stars WR 16 (WN8h) and WR 78 (WN7h). These new results, when combined with previous detections, demonstrate that X-ray emission is present in WN stars across the full range of spectral types, including later WNL stars. The two WN8 stars observed to date (WR 16 and WR 40) show unusually low X-ray luminosities (Lx) compared to other WN stars, and it is noteworthy that they also have the lowest terminal wind speeds (v_infty). Existing X-ray detections of about a dozen WN stars reveal a trend of increasing Lx with wind luminosity Lwind = (1/2) M_dot v_infty^2, suggesting that wind kinetic energy may play a key role in establishing X-ray luminosity levels in WN stars.
Wolf–Rayet star
O-type star
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Anomalous ionization (C IV and Si IV) is seen in IUE spectra of Be stars as late as B8, and occurs also in standard stars of similar spectral type. Asymmetrical lines suggesting mass loss are present in all the Be stars and several of the standard stars as well, with no obvious correlation with v sin i. Emission shoulders are present in the Mg II lines of two B5e stars but not in Be stars of later type. Again there is no correlation with v sin i. The A-F shell stars show rich Fe II absorption spectra in the ultraviolet, in one case with velocity structure.
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Ultraviolet
Flare star
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We present the results of a survey of stellar winds in 62 Be and 43 normal B stars in spectral types B0 to B5 and luminosity classes V-III. We find that in general the wind absorption seen in the resonance lines of C IV and Si IV in Be stars is the result of the superposed absorption from multiple shortward-shifted discrete absorption components. These components are observed in 63% of our Be stars, and are not observed in the normal B stars, confirming the result of Henrichs (1984).
O-type star
Stellar wind
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My intention here is to discuss the ‘high temperature’ portion of this symposium and call attention to those stars that are called Of. There are some similarities in spectral appearance to WR stars, e.g. emission lines. I should first like to define what I think are the essential differences among four groups of hot stars; O stars : Stars that have only absorption lines in the visible spectrum. Type O is distinguished from type B by the presence of He ii 4541 at MK dispersion. It may be that some (supergiants) O stars will have emission lines in the rocket UV region but this description will be primarily concerned with ground based observations. Of stars : These are O type stars that also have λλ 4634,40 N iii in emission above the continuum. In addition to normal O star absorption lines and N iii emission, they may also have other lines in emission. I will discuss this further below. Oe stars : These are O type stars that have emission in the hydrogen lines (or at least at H α ), but with no emission in N iii or in other lines. I personally think that this small class of objects is related to the Be stars in their evolutionary status and in their emission mechanism. WR stars : These stars are primarily characterized by emission lines. The only absorption lines seen are violet shifted (P Cyg type). Although in some cases emission lines appear which are similar to those found in some Of stars, the latter types always have some unshifted absorption lines present. Several Of stars have P Cyg profiles in some lines.
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An updated classification scheme for transition O2-3.5If*/WN5-7 stars is presented, following recent revisions to the spectral classifications for O and WN stars. We propose that O2-3.5If*, O2-3.5If*/WN5-7 and WN5-7 stars may be discriminated using the morphology of Hbeta to trace increasing wind density as follows: purely in absorption for O2-3.5If* stars in addition to the usual diagnostics from Walborn et al.; P Cygni for O2-3.5If*/WN5-7 stars; purely in emission for WN stars in addition to the usual diagnostics from Smith et al. We also discuss approximate criteria to discriminate between these subtypes from near-IR spectroscopy. The physical and wind properties of such stars are qualitatively discussed together with their evolutionary significance. We suggest that the majority of O2-3.5If*/WN5-7 stars are young, very massive hydrogen-burning stars, genuinely intermediate between O2-3.5If* and WN5-7 subtypes, although a minority are apparently core helium-burning stars evolving blueward towards the classical WN sequence. Finally, we reassess classifications for stars exhibiting lower ionization spectral features plus Hbeta emission.
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Classification scheme
Herbig Ae/Be star
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Abstract Star clusters are often born as star-cluster systems, which include several stellar clumps. Such star-cluster complexes could have formed from turbulent molecular clouds. Since Gaia Data Release 2 provided us high quality velocity data of individual stars in known star-cluster complexes, we now can compare the velocity structures of the observed star-cluster complexes with simulated ones. We performed a series of N -body simulations for the formation of star-cluster complexes starting from turbulent molecular clouds. We measured the inter-cluster velocity dispersions of our simulated star-cluster complexes and compared them with the Carina region and NGC 2264. We found that the Carina region and NGC 2264 formed from molecular clouds with a mass of ∼4 × 10 5 M ⊙ and ∼4 × 10 4 M ⊙ , respectively. In our simulations, we also found that the maximum cluster mass ( M c,max ) in the complex follows ${M_{{\rm{c}},{\rm{max}}}} = 0.{\rm{2}}0M_g^{0.76}$ , where M g is the initial gas mass.
Star (game theory)
Velocity dispersion
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Aims. Having established the binary status of nineteen O-type stars located in four Cygnus OB associations, we now determine their fundamental parameters to constrain their properties and their evolutionary status. We also investigate their surface nitrogen abundances, which we compare with other results from the literature obtained for galactic O-type stars. Methods. Using optical spectra collected for each object in our sample and some UV data from the archives, we apply the CMFGEN atmosphere code to determine their main properties. For the binary systems, we have disentangled the components to obtain their individual spectra and investigate them as if they were single stars. Results. We find that the distances of several presumably single O-type stars seem poorly constrained because their luminosities are not in agreement with the standard luminosities of stars with similar spectral types. The ages of these O-type stars are all less than 7 Myrs. Therefore, the ages of these stars agree with those, quoted in the literature, of the four associations, except for CygOB8 for which the stars seem older than the association itself. However, we point out that the distance of certain stars is debatable relative to values found in the literature. The N content of these stars put in perspective with N contents of several other galactic O-type stars seems to draw the same five groups as found in the Hunter diagram for the O and B-type stars in the LMC even though their locations are obviously different. We determine mass-loss rates for several objects from the Halpha line and UV spectra. Finally, we confirm the mass discrepancy especially for O stars with masses smaller than 30 Msun. .
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Star (game theory)
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In the course of a large-scale multi-wavelength survey of bright B and Be stars we have discovered five hitherto unknown Be stars and one new Oe star: HR 1037 (B6Vn), 2806 (O9V), 5953 (B0.3IV), 6427 (B9.5V + A6III), 6747 (B0IIIn) and 8967 (B9V). We present Hα spectra and equivalent widths for these stars. We find that one known Be star, HR 6621 (B4IVe), probably has been misclassified as such because due to its binary nature its spectrum mimics Hα emission. From our results we infer―with 99% confidence―that the Be stars constitute at least 15% of all non-supergiant B stars in the Bright Star Catalogue
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O-type star
Herbig Ae/Be star
Be star
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Attracted by the simplicity of the recently published by Pri nja (1994) method of determination of terminal wind velocities in hot stars from low resolution IU E spectra we investigate its application to WR stars. With a large sample of low resolution IUE spectra of WR stars we found even simpler, that is linear instead of square, empirical relation between as defined by Prinja (1994) and terminal wind velocity ‐ . Using this new empirical relation we present for a sample of 85 galactic and LMC stars, 19 of them determined for the first time. We almost tripled the number of terminal velocity determinations for LMC WR stars. The comparison with other determinations shows that this simple method is accurate to within 10‐20%. We confirm the correlation between terminal velocity and WC subtype. We also show that terminal velocities of WN stars are lower than that of WCE. A comparison between galactic and LMC stars shows that the LMC WN stars have slower winds in most of WN subtypes.
Wolf–Rayet star
Terminal velocity
O-type star
Radial velocity
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