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تسجيل مستخدم جديدOn the optical counterpart of NGC300 X-1 and the global Wolf-Rayet content of NGC300
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Paul A Crowther
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(Conext:) Surveys of Wolf-Rayet (WR) populations in nearby galaxies provide tests of evolutionary models plus Type Ib/c supernova progenitors. This spectroscopic study complements the recent imaging survey of the spiral galaxy NGC 300 by Schild et al. (Aims): Revisions to the known WR content of NGC 300 are presented. We investigate the WR nature of candidate #41 from Schild et al. which is spatially coincident with the bright X-ray point source NGC 300 X-1; (Methods:) VLT/FORS2 multi-object spectroscopy of WR candidates in NGC 300 is obtained; (Results:) We establish an early-type WN nature of #41, i.e. similar to the optical counterpart of IC 10 X-1, which closely resembles NGC 300 X-1. We confirm 9 new WR stars, bringing the current WR census of the inner disk to 31, with N(WC)/N(WN)~0.9. (Conclusions:) If #41 is the optical counterpart for NGC 300 X-1, we estimate a WR mass of 38 Msun based upon ground-based photometry, from which a black hole mass of > 10 Msun results from the 32.8 hr period of the system and WR wind velocity of 1250 km/s. We estimate an 95% completeness among WC stars and 70% among WN stars, such that the total WR content is ~40, with N(WC)/N(WN)~0.7. From the Halpha-derived star formation rate of the inner galaxy, we infer N(WR)/N(O)~0.04
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[ABRIDGED] IC10 X-1 has recently been confirmed as a black hole (BH) + Wolf-Rayet (WR) X-ray binary, and NGC300 X-1 is thought to be. IC10 X-1 and NGC300 X-1 have similar X-ray properties, with luminosities ~10^38 erg/s, and orbital periods ~30 hr. W
e investigate similarities between these two, as well as differences between them and the known Galactic BH binary systems. We have examined XMM-Newton observations of NGC300 X-1 and IC10 X-1. We extracted lightcurves and spectra; power density spectra (PDS) were constructed from the lightcurves, and the X-ray emission spectra were modeled. Each source exhibits PDS that are characteristic of disc-accreting X-ray binaries (XBs) in the high state. In this state, Galactic XBs with known BH primaries have soft, thermal emission; however the emission spectra of our targets are predominantly non-thermal. Furthermore, the Observation 1 spectrum of NGC300 X-1 is strikingly similar to that of IC10 X-1. The remarkable similarity between the behaviour of NGC300 X-1 in Observation 1 and that of IC10 X-1 lends strong evidence for NGC300 X-1 being a (BH+WR) binary. The unusual spectra of NGC300 X-1 and IC10 X-1 may be due to these systems existing in a persistently high state, whereas all known BH LMXBs are transient. BH XBs in a persistent high state could retain their corona, and hence exhibit a large non-thermal component. LMC X-1 is a BH XB that has only been observed in the high state, and its spectrum is remarkably similar to those of our targets. We therefore classify NGC300 X-1, IC10 X-1 and perhaps LMC X-1 as a new breed of BH XB, defined by their persistently high accretion rates and consequent stable disc configuration and corona. This scenario may also explain the lack of ultraluminous X-ray sources in the canonical soft state.
Wolf-Rayet stars (WRs) represent the end of a massive stars life as it is about to turn into a supernova. Obtaining complete samples of such stars across a large range of metallicities poses observational challenges, but presents us with an exacting
way to test current stellar evolutionary theories. A technique we have developed and refined involves interference filter imaging combined with image subtraction and crowded-field photometry. This helps us address one of the most controversial topics in current massive star research: the relative importance of binarity in the evolution of massive stars and formation of WRs. Here we discuss the current state of the field, including how the observed WR populations match with the predictions of both single and binary star evolutionary models. We end with what we believe are the most important next steps in WR research.
SN2008S and the 2008 NGC300-OT were explosive transients of stars self-obscured by very dense, dusty stellar winds. An explosive transient with an un-observed shock break-out luminosity of order 10^10 Lsun is required to render the transients little
obscured and visible in the optical at their peaks. Such a large break-out luminosity then implies that the progenitor stars were cool, red supergiants, most probably ~9 Msun extreme AGB (EAGB) stars. As the shocks generated by the explosions propagate outward through the dense wind, they produce a shock luminosity in soft X-rays that powers the long-lived luminosity of the transients. Unlike typical cases of transients exploding into a surrounding circumstellar medium, the progenitor winds in these systems are optically thick to soft X-rays, easily absorb radio emission and rapidly reform dust destroyed by the peak luminosity of the transients. As a result, X-rays are absorbed by the gas and the energy is ultimately radiated by the reformed dust. Three years post-peak, both systems are still significantly more luminous than their progenitor stars, but they are again fully shrouded by the re-formed dust and only visible in the mid-IR. The high luminosity and heavy obscuration may make it difficult to determine the survival of the progenitor stars for ~10 years. However, our model indicates that SN2008S, but not the NGC300-OT, should now be a detectable X-ray source. SN2008S has a higher estimated shock velocity and a lower density wind, so the X-rays begin to escape at a much earlier phase.
We present the results of a detailed optical and near-IR study of the nearby star-forming dwarf galaxy NGC4214. We discuss the stellar content, drawing particular attention to the intermediate-age and/or old field stars, which are used as a distance
indicator. On images obtained with the Hubble Space Telescope WFPC2 and NICMOS instruments in the equivalents of the V, R, I, J and H bands, the galaxy is well resolved into stars. We achieve limiting magnitudes of F814W ~27 in the WF chips and F110W ~25 in the NIC2. The optical and near-infrared color-magnitude diagrams confirm a core-halo galaxy morphology: an inner high surface-brightness young population within ~1.5 (~1 kpc) from the center of the galaxy, where the stars are concentrated in bright complexes along a bar-like structure; and a relatively low-surface-brightness, field-star population extending out to at least 8 (7 kpc). The color-magnitude diagrams of the core region show evidence of blue and red supergiants, main-sequence stars, asymptotic giant branch stars and blue loop stars. We identify some candidate carbon stars from their extreme near-IR color. The field-star population is dominated by the red tangle, which contains the red giant branch. We use the I-band luminosity function to determine the distance based on the tip-of-the-red-giant-branch method: 2.7pm0.3 Mpc. This is much closer than the values usually assumed in the literature, and we provide revised distance dependent parameters such as physical size, luminosity, HI mass and star-formation rate.
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D. M.-A. Meyer (Universitaet Potsdam
, Institut fuer Physik undn Astronomie
, Karl-Liebknecht-Strasse 24/25
2021
Wolf-Rayet stars are amongst the rarest but also most intriguing massive stars. Their extreme stellar winds induce famous multi-wavelength circumstellar gas nebulae of various morphologies, spanning from circles and rings to bipolar shapes. This stud
y is devoted to the investigation of the formation of young, asymmetric Wolf-Rayet gas nebulae and we present a 2.5-dimensional magneto-hydrodynamical toy model for the simulation of Wolf-Rayet gas nebulae generated by wind-wind interaction. Our method accounts for stellar wind asymmetries, rotation, magnetisation, evolution and mixing of materials. It is found that the morphology of the Wolf-Rayet nebulae of blue supergiant ancestors is tightly related to the wind geometry and to the stellar phase transition time interval, generating either a broadened peanut-like or a collimated jet-like gas nebula. Radiative transfer calculations of our Wolf-Rayet nebulae for dust infrared emission at 24 $mu$m show that the projected diffuse emission can appear as oblate, bipolar, ellipsoidal or ring structures. Important projection effects are at work in shaping observed Wolf-Rayet nebulae. This might call a revision of the various classifications of Wolf-Rayet shells, which are mostly based on their observed shape. Particularly, our models question the possibility of producing pre-Wolf-Rayet wind asymmetries, responsible for bipolar nebulae like NGC 6888, within the single red supergiant evolution channel scenario. We propose that bipolar Wolf-Rayet nebulae can only be formed within the red supergiant scenario by multiple/merged massive stellar systems, or by single high-mass stars undergoing additional, e.g. blue supergiant, evolutionary stages prior to the Wolf-Rayet phase.
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