@article{ToalaGuerreroTodtetal.2015, author = {Toala, J. A. and Guerrero, M. A. and Todt, Helge Tobias and Hamann, Wolf-Rainer and Chu, Y.-H. and Gruendl, R. A. and Sch{\"o}nberner, Detlef and Oskinova, Lida and Marquez-Lugo, R. A. and Fang, X. and Ramos-Larios, Gerardo}, title = {The born-again Planetary nebula A78: an X-RAY twin of A30}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {799}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/799/1/67}, pages = {10}, year = {2015}, abstract = {We present the XMM-Newton discovery of X-ray emission from the planetary nebula (PN) A78, the second born-again PN detected in X-rays apart from A30. These two PNe share similar spectral and morphological characteristics: they harbor diffuse soft X-ray emission associated with the interaction between the H-poor ejecta and the current fast stellar wind and a point-like source at the position of the central star (CSPN). We present the spectral analysis of the CSPN, using for the first time an NLTE code for expanding atmospheres that takes line blanketing into account for the UV and optical spectra. The wind abundances are used for the X-ray spectral analysis of the CSPN and the diffuse emission. The X-ray emission from the CSPN in A78 can be modeled by a single C VI emission line, while the X-ray emission from its diffuse component is better described by an optically thin plasma emission model with a temperature of kT = 0.088 keV (T approximate to 1.0 x 10(6) K). We estimate X-ray luminosities in the 0.2-2.0 keV energy band of L-X,L-CSPN =(1.2 +/- 0.3) x 10(31) erg s(-1) and L-X,L-DIFF =(9.2 +/- 2.3) x 10(30) erg s(-1) for the CSPN and diffuse components, respectively.}, language = {en} } @article{GvaramadzeCheneKniazevetal.2014, author = {Gvaramadze, V. V. and Chene, A.-N. and Kniazev, A. Y. and Schnurr, O. and Shenar, Tomer and Sander, Andreas Alexander Christoph and Hainich, Rainer and Langer, N. and Hamann, Wolf-Rainer and Chu, Y.-H. and Gruendl, R. A.}, title = {Discovery of a new Wolf-Rayet star and a candidate star cluster in the Large Magellanic Cloud with Spitzer}, series = {Monthly notices of the Royal Astronomical Society}, volume = {442}, journal = {Monthly notices of the Royal Astronomical Society}, number = {2}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stu909}, pages = {929 -- 945}, year = {2014}, abstract = {We report the first-ever discovery of a Wolf-Rayet (WR) star in the Large Magellanic Cloud via detection of a circular shell with the Spitzer Space Telescope. Follow-up observations with Gemini-South resolved the central star of the shell into two components separated from each other by a parts per thousand 2 arcsec (or a parts per thousand 0.5 pc in projection). One of these components turns out to be a WN3 star with H and He lines both in emission and absorption (we named it BAT99 3a using the numbering system based on extending the Breysacher et al. catalogue). Spectroscopy of the second component showed that it is a B0 V star. Subsequent spectroscopic observations of BAT99 3a with the du Pont 2.5-m telescope and the Southern African Large Telescope revealed that it is a close, eccentric binary system, and that the absorption lines are associated with an O companion star. We analysed the spectrum of the binary system using the non-LTE Potsdam WR (powr) code, confirming that the WR component is a very hot (a parts per thousand 90 kK) WN star. For this star, we derived a luminosity of log L/ L-aS (TM) = 5.45 and a mass-loss rate of 10(- 5.8) M-aS (TM) yr(- 1), and found that the stellar wind composition is dominated by helium with 20 per cent of hydrogen. Spectroscopy of the shell revealed an He iii region centred on BAT99 3a and having the same angular radius (a parts per thousand 15 arcsec) as the shell. We thereby add a new example to a rare class of high-excitation nebulae photoionized by WR stars. Analysis of the nebular spectrum showed that the shell is composed of unprocessed material, implying that the shell was swept-up from the local interstellar medium. We discuss the physical relationship between the newly identified massive stars and their possible membership of a previously unrecognized star cluster.}, language = {en} } @article{FangGuerreroMarquezLugoetal.2014, author = {Fang, X. and Guerrero, M. A. and Marquez-Lugo, R. A. and Toala, J. A. and Arthur, S. J. and Chu, Y.-H. and Blair, William P. and Gruendl, R. A. and Hamann, Wolf-Rainer and Oskinova, Lida and Todt, Helge Tobias}, title = {Expansion of hydrogen-poor knots in the born-again planetary nebulae A30 and A78}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {797}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/797/2/100}, pages = {11}, year = {2014}, abstract = {We analyze the expansion of hydrogen-poor knots and filaments in the born-again planetary nebulae A30 and A78 based on Hubble Space Telescope ( HST) images obtained almost 20 yr apart. The proper motion of these features generally increases with distance to the central star, but the fractional expansion decreases, i.e., the expansion is not homologous. As a result, there is not a unique expansion age, which is estimated to be 610-950 yr for A30 and 600-1140 yr for A78. The knots and filaments have experienced complex dynamical processes: the current fast stellar wind is mass loaded by the material ablated from the inner knots; the ablated material is then swept up until it shocks the inner edges of the outer, hydrogen-rich nebula. The angular expansion of the outer filaments shows a clear dependence on position angle, indicating that the interaction of the stellar wind with the innermost knots channels the wind along preferred directions. The apparent angular expansion of the innermost knots seems to be dominated by the rocket effect of evaporating gas and by the propagation of the ionization front inside them. Radiation-hydrodynamical simulations show that a single ejection of material followed by a rapid onset of the stellar wind and ionizing flux can reproduce the variety of clumps and filaments at different distances from the central star found in A30 and A78.}, language = {en} } @article{HenaultBrunetOskinovaGuerreroetal.2012, author = {Henault-Brunet, V. and Oskinova, Lida and Guerrero, M. A. and Sun, W. and Chu, Y.-H. and Evans, C. J. and Gallagher, J. S. and Gruendl, R. A. and Reyes-Iturbide, J.}, title = {Discovery of a Be/X-ray pulsar binary and associated supernova remnant in the wing of the small magellanic cloud}, series = {Monthly notices of the Royal Astronomical Society}, volume = {420}, journal = {Monthly notices of the Royal Astronomical Society}, number = {1}, publisher = {Wiley-Blackwell}, address = {Malden}, issn = {0035-8711}, doi = {10.1111/j.1745-3933.2011.01183.x}, pages = {L13 -- L17}, year = {2012}, abstract = {We report on a new Be/X-ray pulsar binary located in the Wing of the Small Magellanic Cloud (SMC). The strong pulsed X-ray source was discovered with the Chandra and XMM-Newton X-ray observatories. The X-ray pulse period of 1062 s is consistently determined from both Chandra and XMM-Newton observations, revealing one of the slowest rotating X-ray pulsars known in the SMC. The optical counterpart of the X-ray source is the emission-line star 2dFS 3831. Its B0-0.5(III)e+ spectral type is determined from VLT-FLAMES and 2dF optical spectroscopy, establishing the system as a Be/X-ray binary (Be-XRB). The hard X-ray spectrum is well fitted by a power law with additional thermal and blackbody components, the latter reminiscent of persistent Be-XRBs. This system is the first evidence of a recent supernova in the low-density surroundings of NGC602. We detect a shell nebula around 2dFS 3831 in H alpha and [OIII] images and conclude that it is most likely a supernova remnant. If it is linked to the supernova explosion that created this new X-ray pulsar, its kinematic age of (2-4) x 10(4) yr provides a constraint on the age of the pulsar.}, language = {en} } @article{EvansHainichOskinovaetal.2012, author = {Evans, C. J. and Hainich, Rainer and Oskinova, Lida and Gallagher, J. S. and Chu, Y.-H. and Gruendl, R. A. and Hamann, Wolf-Rainer and Henault-Brunet, V. and Todt, Helge Tobias}, title = {A rare early-type star revealed in the wing of the small megellanic cloud}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {753}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/753/2/173}, pages = {10}, year = {2012}, abstract = {Sk 183 is the visually brightest star in the N90 nebula, a young star-forming region in the Wing of the Small Magellanic Cloud (SMC). We present new optical spectroscopy from the Very Large Telescope which reveals Sk 183 to be one of the most massive O-type stars in the SMC. Classified as an O3-type dwarf on the basis of its nitrogen spectrum, the star also displays broadened He I absorption, which suggests a later type. We propose that Sk 183 has a composite spectrum and that it is similar to another star in the SMC, MPG 324. This brings the number of rare O2- and O3-type stars known in the whole of the SMC to a mere four. We estimate physical parameters for Sk 183 from analysis of its spectrum. For a single-star model, we estimate an effective temperature of 46 +/- 2 kK, a low mass-loss rate of similar to 10(-7) M-circle dot yr(-1), and a spectroscopic mass of 46(-8)(+ 9) M-circle dot (for an adopted distance modulus of 18.7 mag to the young population in the SMC Wing). An illustrative binary model requires a slightly hotter temperature (similar to 47.5 kK) for the primary component. In either scenario, Sk 183 is the earliest-type star known in N90 and will therefore be the dominant source of hydrogen-ionizing photons. This suggests Sk 183 is the primary influence on the star formation along the inner edge of the nebula.}, language = {en} } @article{OskinovaSunEvansetal.2013, author = {Oskinova, Lida and Sun, W. and Evans, C. J. and Henault-Brunet, V. and Chu, Y.-H. and Gallagher, J. S. and Guerrero, M. A. and Gruendl, R. A. and G{\"u}del, M. and Silich, S. and Chen, Y. and Naze, Y. and Hainich, Rainer and Reyes-Iturbide, J.}, title = {Discovery of x-ray emission from young suns in the small magellanic cloud}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {765}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/765/1/73}, pages = {12}, year = {2013}, abstract = {We report the discovery of extended X-ray emission within the young star cluster NGC 602a in the Wing of the Small Magellanic Cloud (SMC) based on observations obtained with the Chandra X-Ray Observatory. X-ray emission is detected from the cluster core area with the highest stellar density and from a dusty ridge surrounding the H II region. We use a census of massive stars in the cluster to demonstrate that a cluster wind or wind-blown bubble is unlikely to provide a significant contribution to the X-ray emission detected from the central area of the cluster. We therefore suggest that X-ray emission at the cluster core originates from an ensemble of low-and solar-mass pre-main-sequence (PMS) stars, each of which would be too weak in X-rays to be detected individually. We attribute the X-ray emission from the dusty ridge to the embedded tight cluster of the newborn stars known in this area from infrared studies. Assuming that the levels of X-ray activity in young stars in the low-metallicity environment of NGC 602a are comparable to their Galactic counterparts, then the detected spatial distribution, spectral properties, and level of X-ray emission are largely consistent with those expected from low-and solar-mass PMS stars and young stellar objects (YSOs). This is the first discovery of X-ray emission attributable to PMS stars and YSOs in the SMC, which suggests that the accretion and dynamo processes in young, low-mass objects in the SMC resemble those in the Galaxy.}, language = {en} } @article{ToalaGuerreroChuetal.2015, author = {Toal{\´a}, J. A. and Guerrero, M. A. and Chu, Y.-H. and Arthur, S. J. and Gruendl, R. A.}, title = {Diffuse X-ray Emission within Wolf-Rayet Nebulae}, series = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015}, journal = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-88316}, pages = {333 -- 336}, year = {2015}, abstract = {We discuss our most recent findings on the diffuse X-ray emission within Wolf-Rayet (WR) nebulae. The best-quality X-ray observations of these objects are those performed by XMM- Newton and Chandra towards S 308, NGC 2359, and NGC 6888. Even though these three WR nebulae might have different formation scenarios, they all share similar characteristics: i) the main plasma temperatures of the X-ray-emitting gas is found to be T =[1-2]×^K, ii) the diffuse X-ray emission is confined inside the [O iii] shell, and iii) their X-ray luminosities and electron densities in the 0.3-2.0 keV energy range are LX ≈10^33-10^34 erg s-1 and ne ≈0.1-1 cm^-3 . These properties and the nebular-like abundances of the hot gas suggest mixing and/or thermal conduction is taking an important r{\^o}le reducing the temperature of the hot bubble.}, language = {en} }