ABSTRACT
X-ray-emitting symbiotic stars exhibit a variety of spectral shapes classified as α, β, γ, δ, and β/δ types, which have been attributed to different phenomena such as thermonuclear burning ...on the surface of the white dwarf component, shocks between winds and jets with the red giant companion’s extended atmosphere, the presence of heavily extinguished hot plasma from the inner region from an accretion disc, and/or a combination of these. However, there is observational evidence that this classification scheme is not definite and, for example, some sources change from one type to another within months or years. In this work, it is proposed that a simple disc-like model can be used to explain the X-ray properties observed from reflection-dominated symbiotic stars. For this purpose, we use the Stellar Kinematics Including Radiative Transfer (skirt) code, which has been recently upgraded to include radiative transfer from X-ray photons. It is found that the properties of the accretion disc (geometry and density) in combination with the viewing angle can be invoked to explain the spectral properties of β, δ, and β/δ X-ray-emitting symbiotic stars. Spectral variations and type swaps observed for some X-ray-emitting sources can also be explained by variations in the disc properties.
We investigate the collapse of non-spherical substructures, such as sheets and filaments, which are ubiquitous in molecular clouds. Such non-spherical substructures collapse homologously in their ...interiors but are influenced by an edge effect that causes their edges to be preferentially accelerated. We analytically compute the homologous collapse timescales of the interiors of uniform-density, self-gravitating filaments and find that the homologous collapse timescale scales linearly with the aspect ratio. The characteristic timescale for an edge-driven collapse mode in a filament, however, is shown to have a square-root dependence on the aspect ratio. For both filaments and circular sheets, we find that selective edge acceleration becomes more important with increasing aspect ratio. In general, we find that lower dimensional objects and objects with larger aspect ratios have longer collapse timescales. We show that estimates for star formation rates, based upon gas densities, can be overestimated by an order of magnitude if the geometry of a cloud is not taken into account.
ABSTRACT
The superbubbles (SBs) carved in the interstellar medium by stellar winds and supernovae (SNe) are filled with hot (T > 106 K) gas that produces soft X-ray emission (0.3–2.0 keV). Models ...that assume a constant density medium and central SNe events fail to reproduce the soft X-ray luminosity that is observed in some SBs. We address this problem by generating models that trace the history of SNe in the SB, and produce off-centre SNe, and account for the missing soft X-ray emission. We test the models against archival, radio, optical, and X-ray observations of the SB DEM L50 located in the Large Magellanic Cloud. The soft X-ray properties of DEM L50, including its high luminosity, make it a perfect candidate to test our models. Furthermore, the multiple wave-band observations of this object will help us assess how well our models can reproduce other SB properties beside its soft X-ray properties. We find that a configuration where DEM L50 forms at the edge of a filament reproduces the observed soft X-ray luminosity, optical morphology, shell velocity, and swept-up mass of neutral gas. This configuration is supported by IR observations of the LMC. In addition, we find that off-centre SNe, which enhance soft X-ray emission, naturally occur for all of the initial ambient conditions we tested in our models. Finally, we show that an off-centre SN can explains the observed soft X-ray luminosity of DEM L50, and that the resulting luminosity is consistent with a plasma in non-equilibrium ionization.
ABSTRACT Spectroscopic observations have shown for decades that the Wolf–Rayet (WR) phenomenon is ubiquitous among stars with different initial masses. Although much effort to understand the winds ...from massive WR stars has been presented in the literature, not much has been done for such type of stars in the low-mass range. Here we present an attempt to understand the winds from WR-type stars using results from spectral analyses with the full non-local thermodynamic equilibrium stellar atmosphere code PoWR. These results are put into context with the properties of massive WR stars. We found that WC + WC stars and WO + WO stars create independent sequences in the mass-loss rate ($\dot{M}$) and modified wind momentum (Dmom) versus luminosity (L) diagrams. Our analysis indicates that even when the winds of WR and WR stars become optically thin, there is no breakdown of the general mass-loss trend, contrary to the observed ‘weak wind phenomenon’ in OB stars. We report that all WR-type stars studied here broadly define single sequences in the wind efficiency (η) versus transformed mass-loss rate ($\dot{M}_\mathrm{t}$), the $\dot{M}_\mathrm{t}$–Teff diagram, and the $(L, T_\mathrm{eff}, \dot{M})$ space, which suggest these to be fundamental properties of the WR phenomenon (regardless of the mass range), at least for WR-type stars of the O and C sequences. Our analytical estimations could drive computations of future stellar evolution models for WR-type stars.
We present the discovery of extended X-ray emission from the planetary nebula (PN) NGC 5189 around the WO1-type WD 1330−657 with XMM-Newton. The X-ray-emitting gas fills the cavities detected in the ...Hubble Space Telescope O iii narrowband image and presents a limb-brightened morphology toward the outer edges of the east and west lobes. The bulk of the X-ray emission is detected in the soft (0.3-0.7 keV) band with the XMM-Newton EPIC spectra dominated by the C vi Ly line at 0.37 keV (=33.7 ). Spectral analysis resulted in carbon and neon abundances 38 and 6 times their solar values, with a plasma temperature of kT = 0.14 0.01 keV (T = 1.6 × 106 K) and X-ray luminosity of LX = (2.8 0.8) × 1032 erg s−1. NGC 5189 is an evolved and extended PN ( 0.70 pc in radius), thus, we suggest that the origin of its X-ray emission is consistent with the born-again scenario in which the central star becomes carbon-rich through an eruptive very late thermal pulse, subsequently developing a fast, carbon-rich wind powering the X-ray emission as suggested for A 30 and A 78.
Dissecting the Hot Bubbles in LMC-N57 with XMM-Newton Ramírez-Ballinas, Isidro; Reyes-Iturbide, Jorge; Toalá, Jesús A. ...
Astrophysical journal/The Astrophysical journal,
11/2019, Volume:
885, Issue:
2
Journal Article
Peer reviewed
Open access
We present a study of the diffuse X-ray emission from the star-forming region LMC-N 57 in the Large Magellanic Cloud. We use archival XMM-Newton observations to unveil in detail the distribution of ...hot bubbles in this complex. X-ray emission is detected from the central superbubble (SB) DEM L 229, the supernova remnant (SNR) 0532−675, and the Wolf-Rayet (WR) bubble DEM L 231 around the WR star Br 48. Comparison with infrared (IR) images unveils the powerful effect of massive stars in destroying their nurseries. The distribution of the hot gas in the SNR and the SB display their maxima in regions in contact with the filamentary cold material detected by IR images. Our observations do not reveal extended X-ray emission filling DEM L 231, although several pointlike sources are detected in the field of view of this WR nebula. The X-ray properties of Br 48 are consistent with a binary WN4+O as proposed by other authors. We modeled the X-ray emission from the SB and found that its X-ray emission can be simply explained by pressure-driven wind model-that is, there is no need to invoke the presence of an SN explosion as previously suggested. The pressure calculations of the hot gas confirms that the dynamical evolution of SB DEM L 229 is dominated by the stellar winds from the star cluster LH 76.
We present high-angular-resolution observations of Sakurai’s object using the Atacama Large Millimeter Array, shedding new light on its morpho-kinematical structure. The millimetre continuum emission ...observed at an angular resolution of 20 milliarcsec (corresponding to 70 AU) reveals a bright compact central component whose spectral index indicates that it is composed of amorphous carbon dust. Based on these findings, we conclude that this emission traces the previously suggested dust disc observed in mid-infrared observations, and therefore our observations provide the first direct imaging of this disc. The H
12
CN(
J
= 4 → 3) line emission observed at an angular resolution of 300 milliarcsec (corresponding to 1000 AU) displays a bipolar structure with a north–south velocity gradient. From the position–velocity diagram of this emission, we identify the presence of an expanding disc and a bipolar molecular outflow. The inclination of the disc is determined to be
i
= 72°. The derived values for the de-projected expansion velocity and the radius of the disc are
v
exp
= 53 km s
−1
and
R
= 277 AU, respectively. On the other hand, the de-projected expansion velocity of the bipolar outflow detected in the H
12
CN(
J
= 4 → 3) emission is of approximately 1000 km s
−1
. We propose that the molecular outflow has an hourglass morphology with an opening angle of around 60°. Our observations unambiguously show that an equatorial disc and bipolar outflows formed in Sakurai’s object during the 30 years following the occurrence of the born-again event, providing important constraints for future modelling efforts of this phenomenon.
The central star of NGC 2392 shows the hardest X-ray emission among central stars of planetary nebulae (CSPNe). The recent discovery of a spectroscopic companion with an orbital period of 1.9 days ...could provide an explanation for its hard X-ray emission, as well as for the collimation of its fast outflow. Here, we analyze the available Chandra and XMM-Newton X-ray observations to determine accurately the spectral and temporal variation properties of the CSPN of NGC 2392. The X-ray emission can be described by an absorbed thermal plasma model with temperature MK and X-ray luminosity (8.7 1.0) × 1030 erg s−1. No long-term variability is detected in the X-ray emission level, but the Chandra light curve is suggestive of short-term variations with a period ∼0.26 days. The possible origins of this X-ray emission are discussed. X-ray emission from the coronal activity of a companion or shocks in the stellar wind can be ruled out. Accretion of material from an unseen main-sequence companion onto the CSPN or from the CSPN wind onto a white dwarf companion are the most plausible origins for its hard X-ray emission, although the mismatch between the rotational period of the CSPN and the modulation timescale of the X-ray emission seems to preclude the former possibility.
Abstract
We present the analysis of archival XMM-Newton observations of the symbiotic stars HM Sge, NQ Gem, and PU Vul. The EPIC-pn spectra hint at the presence of emission lines, which are further ...confirmed in the first-order RGS spectra of the three sources. Spectral modeling of the EPIC-pn data discloses unprecedented characteristics; for instance, the best fit to the EPIC-pn spectrum of the
β
-type symbiotic star PU Vul reveals the presence of two-plasma components. We report the discovery of an extremely soft spectral component in the EPIC-pn spectrum of the
β
-type symbiotic star HM Sge, which we suggest is produced by periodic mass ejections such as jets. Consequently, we suggest that a simple
β
-type classification no longer applies to HM Sge. Finally, the spectrum of the
β
/
δ
-type symbiotic star NQ Gem cannot be fitted by a two-temperature plasma model as performed by previous authors. The model requires extra components to fit the 1.0–4.0 keV energy range. More sophisticated models to
β
/
δ
-type symbiotic stars are needed in order to peer into the accretion process from such systems.
Abstract
We present the analysis of archival XMM-Newton European Photon Imaging Camera (EPIC) X-ray observations of the symbiotic star R Aquarii. We used the Extended Source Analysis Software package ...to disclose diffuse soft X-ray emission extending up to 2.′2 (≈0.27 pc) from this binary system. The depth of these XMM-Newton EPIC observations reveals in unprecedented detail the spatial distribution of this diffuse emission, with a bipolar morphology spatially correlated with the optical nebula. The extended X-ray emission shares the same dominant soft X-ray-emitting temperature as the clumps in the jet-like feature resolved by Chandra in the vicinity of the binary system. The harder component in the jet might suggest that the gas cools down; however, the possible presence of nonthermal emission produced by the presence of a magnetic field collimating the mass ejection cannot be discarded. We propose that the ongoing precessing jet creates bipolar cavities filled with X-ray-emitting hot gas that feeds the more extended X-ray bubble as they get disrupted. These EPIC observations demonstrate that the jet feedback mechanism produced by an accreting disk around an evolved, low-mass star can blow hot bubbles, similar to those produced by jets arising from the nuclei of active galaxies.
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