4,429 research outputs found
The Kinematic and Plasma Properties of X-ray Knots in Cassiopeia A from the Chandra HETGS
We present high-resolution X-ray spectra from the young supernova remnant Cas
A using a 70-ks observation taken by the Chandra High Energy Transmission
Grating Spectrometer (HETGS). Line emission, dominated by Si and S ions, is
used for high-resolution spectral analysis of many bright, narrow regions of
Cas A to examine their kinematics and plasma state. These data allow a 3D
reconstruction using the unprecedented X-ray kinematic results: we derive
unambiguous Doppler shifts for these selected regions, with values ranging
between -2500 and +4000 km/s. Plasma diagnostics of these regions, derived from
line ratios of resolved He-like triplet lines and H-like lines of Si, indicate
temperatures largely around 1 keV, which we model as O-rich reverse-shocked
ejecta. The ionization age also does not vary considerably over these regions
of the remnant. The gratings analysis was complemented by the non-dispersed
spectra from the same dataset, which provided information on emission measure
and elemental abundances for the selected Cas A regions. The derived electron
density of X-ray emitting ejecta varies from 20 to 200 cm^{-3}. The measured
abundances of Mg, Si, S and Ca are consistent with O being the dominant element
in the Cas A plasma. With a diameter of 5 arcmin, Cas A is the largest source
observed with the HETGS to date. We, therefore, describe the technique we use
and some of the challenges we face in the HETGS data reduction from such an
extended, complex object.Comment: 26 pages, 16 figures, evised version (minor changes), accepted for
publication in ApJ (Oct 20 2006
Stellar mass-loss near the Eddington limit. Tracing the sub-photospheric layers of classical Wolf-Rayet stars
Towards the end of their evolution hot massive stars develop strong stellar
winds and appear as emission line stars, such as WR stars or LBVs. The
quantitative description of the mass loss in these important pre-SN phases is
hampered by unknowns such as clumping and porosity due to an in-homogeneous
wind structure, and by an incomplete theoretical understanding of optically
thick stellar winds. In this work we investigate the conditions in deep
atmospheric layers of WR stars to find out whether these comply with the theory
of optically thick winds, and whether we find indications of clumping in these
layers. We use a new semi-empirical method to determine sonic-point optical
depths, densities, and temperatures for a large sample of WR stars of the
carbon (WC) and oxygen (WO) sequence. Based on an artificial model sequence we
investigate the reliability of our method and its sensitivity to uncertainties
in stellar parameters. We find that the WR stars in our sample obey an
approximate relation with P_rad/P_gas~80 at the sonic point. This 'wind
condition' is ubiquitous for radiatively driven, optically thick winds, and
sets constraints on possible wind/envelope solutions affecting radii, mass-loss
rates, and clumping properties. Our results suggest that the presence of an
optically thick wind may force many stars near the Eddington limit to develop
clumped, radially extended sub-surface zones. The clumping in these zones is
most likely sustained by the non-linear strange-mode instability, and may be
the origin of the observed wind clumping. The properties of typical late-type
WC stars comply with this model. Solutions without sub-surface clumping and
inflation are also possible but demand for compact stars with comparatively low
mass-loss rates. These objects may resemble the small group of WO stars with
their exceptionally hot stellar temperatures and highly ionized winds.Comment: accepted by A&
Narrow He II emission in star-forming galaxies at low metallicity. Stellar wind emission from a population of Very Massive Stars
In a recent study star-forming galaxies with HeII emission between redshifts
2 and 4.6 have been found to occur in two modes, distinguished by the width of
their HeII emission lines. Broad HeII emission has been attributed to stellar
emission from a population of evolved Wolf-Rayet (WR) stars while narrow HeII
emission has been attributed to nebular emission excited by a population of
very hot PopIII stars formed in pockets of pristine gas at moderate redshifts.
In this work we propose an alternative scenario for the origin of the narrow
HeII emission, namely very massive stars (VMS) at low metallicity (Z) which
form strong but slow WR-type stellar winds due to their proximity to the
Eddington limit. We estimate the expected HeII line fluxes and equivalent
widths based on wind models for VMS and population synthesis models, and
compare the results with recent observations of star-forming galaxies at
moderate redshifts. The observed HeII line strengths and equivalent widths are
in line with what is expected for a population of VMS in one or more young
super-clusters located within these galaxies. In our scenario the two observed
modes of HeII emission originate from massive stellar populations in distinct
evolutionary stages at low Z. If this interpretation is correct there is no
need to postulate the existence of PopIII stars at moderate redshifts to
explain the observed narrow HeII emission. An interesting possibility is the
existence of self-enriched VMS with similar WR-type spectra at extremely low Z.
Stellar HeII emission from such very early generations of VMS may be detectable
in future studies of star-forming galaxies at high redshifts with the James
Webb Space Telescope. The fact that the HeII emission of VMS is largely
neglected in current population synthesis models will generally affect the
interpretation of the integrated spectra of young stellar populations.Comment: 4 pages, 1 figure, A&A letters (accepted
Implications of the metallicity dependence of Wolf-Rayet winds
Aims: Recent theoretical predictions for the winds of Wolf-Rayet stars
indicate that their mass-loss rates scale with the initial stellar metallicity
in the local Universe.We aim to investigate how this predicted dependence
affects the models of Wolf-Rayet stars and their progeny in different chemical
environments. Methods: We compute models of stellar structure and evolution for
Wolf-Rayet stars for different initial metallicities, and investigate how the
scaling of the Wolf-Rayet mass-loss rates affects the final masses, the
lifetimes of the WN and WC subtypes, and how the ratio of the two populations
vary with metallicity. Results: We find significant effects of metallicity
dependent mass-loss rates for Wolf-Rayet stars. For models that include the
scaling of the mass-loss rate with initial metallicity, all WR stars become
neutron stars rather than black holes at twice the solar metallicity; at lower
, black holes have larger masses. We also show that our models that include
the mass-loss metallicity scaling closely reproduce the observed decrease of
the relative population of WC over WN stars at low metallicities.Comment: 8 pages, 9 figures, accepted by Astronomy & Astrophysic
Capillary Waves in a Colloid-Polymer Interface
The structure and the statistical fluctuations of interfaces between
coexisting phases in the Asakura-Oosawa (AO) model for a colloid--polymer
mixture are analyzed by extensive Monte Carlo simulations. We make use of a
recently developed grand canonical cluster move with an additional constraint
stabilizing the existence of two interfaces in the (rectangular) box that is
simulated. Choosing very large systems, of size LxLxD with L=60 and D=120,
measured in units of the colloid radius, the spectrum of capillary wave-type
interfacial excitations is analyzed in detail. The local position of the
interface is defined in terms of a (local) Gibbs surface concept. For small
wavevectors capillary wave theory is verified quantitatively, while for larger
wavevectors pronounced deviations show up. For wavevectors that correspond to
the typical distance between colloids in the colloid-rich phase, the
interfacial fluctuations exhibit the same structure as observed in the bulk
structure factor. When one analyzes the data in terms of the concept of a
wavevector-dependent interfacial tension, a monotonous decrease of this
quantity with increasing wavevector is found. Limitations of our analysis are
critically discussed.Comment: 12 pages, 15 figure
Internal entrainment and the origin of jet-related broad-band emission in Centaurus A
Date of Acceptance: 14/11/2014The dimensions of Fanaroff-Riley class I jets and the stellar densities at galactic centres imply that there will be numerous interactions between the jet and stellar winds. These may give rise to the observed diffuse and 'knotty' structure of the jets in the X-ray, and can also mass load the jets. We performed modelling of internal entrainment from stars intercepted by Centaurus A's jet, using stellar evolution- and wind codes. From photometry and a codesynthesized population of 12 Gyr (Z = 0.004), 3 Gyr (Z = 0.008) and 0-60 Myr (Z = 0.02) stars, appropriate for the parent elliptical NGC 5128, the total number of stars in the jet is ∼8 × 108. Our model is energetically capable of producing the observed X-ray emission, even without young stars. We also reproduce the radio through X-ray spectrum of the jet, albeit in a downstream region with distinctly fewer young stars, and recover the mean X-ray spectral index.We derive an internal entrainment rate of ∼2.3 × 10-3M yr-1 which implies substantial jet deceleration. Our absolute nucleosynthetic yields for the Asymptotic Giant Branch stellar population in the jet show the highest amounts for 4He, 16O, 12C, 14N and 20Ne. If some of the events at ≥55 EeV detected by the Pierre Auger Observatory originate from internal entrainment in Centaurus A, we predict that their composition will be largely intermediate-mass nuclei with 16O, 12C and 14N the key isotopes.Peer reviewe
The loss-limited electron energy in SN 1006: effects of the shock velocity and of the diffusion process
The spectral shape of the synchrotron X-ray emission from SN 1006 reveals the
fundamental role played by radiative losses in shaping the high-energy tail of
the electron spectrum. We analyze data from the XMM-Newton SN 1006 Large
Program and confirm that in both nonthermal limbs the loss-limited model
correctly describes the observed spectra. We study the physical origin of the
observed variations of the synchrotron cutoff energy across the shell. We
investigate the role played by the shock velocity and by the electron
gyrofactor. We found that the cutoff energy of the syncrotron X-ray emission
reaches its maximum value in regions where the shock has experienced its
highest average speed. This result is consistent with the loss-limited
framework. We also find that the electron acceleration in both nonthermal limbs
of SN 1006 proceeds close to the Bohm diffusion limit, the gyrofactor being in
the range 1.5-4. We finally investigate possible explanations for the low
values of cutoff energy measured in thermal limbs.Comment: Accepted for publication in Astronomische Nachrichten. Proceedings of
the XMM-Newton Science Workshop 201
Accelerated Electrons in Cassiopeia A: An Explanation for the Hard X-ray Tail
We propose a model for the hard X-ray (> 10 keV) emission observed from the
supernova remnant Cas A. Lower hybrid waves are generated in strong (mG)
magnetic fields, generally believed to reside in this remnant, by shocks
reflected from density inhomogeneities. These then accelerate electrons to
energies of several tens of keV. Around 4% of the x-ray emitting plasma
electrons need to be in this accelerated distribution, which extends up to
electron velocities of order the electron Alfven speed, and is directled along
magnetic field lines. Bremsstrahlung from these electrons produces the observed
hard x-ray emission. Such waves and accelerated electrons have been observed in
situ at Comet Halley, and we discuss the viability of the extrapolation from
this case to the parameters relevant to Cas A.Comment: 20 pages, 3 figures, aasTeX502, accepted in Ap
Probing the evolving massive star population in Orion with kinematic and radioactive tracers
We assemble a census of the most massive stars in Orion, then use stellar
isochrones to estimate their masses and ages, and use these results to
establish the stellar content of Orion's individual OB associations. From this,
our new population synthesis code is utilized to derive the history of the
emission of UV radiation and kinetic energy of the material ejected by the
massive stars, and also follow the ejection of the long-lived radioactive
isotopes 26Al and 60Fe. In order to estimate the precision of our method, we
compare and contrast three distinct representations of the massive stars. We
compare the expected outputs with observations of 26Al gamma-ray signal and the
extent of the Eridanus cavity. We find an integrated kinetic energy emitted by
the massive stars of 1.8(+1.5-0.4)times 10^52 erg. This number is consistent
with the energy thought to be required to create the Eridanus superbubble. We
also find good agreement between our model and the observed 26Al signal,
estimating a mass of 5.8(+2.7-2.5) times 10^-4 Msol of 26Al in the Orion
region. Our population synthesis approach is demonstrated for the Orion region
to reproduce three different kinds of observable outputs from massive stars in
a consistent manner: Kinetic energy as manifested in ISM excavation, ionization
as manifested in free-free emission, and nucleosynthesis ejecta as manifested
in radioactivity gamma-rays. The good match between our model and the
observables does not argue for considerable modifications of mass loss. If
clumping effects turn out to be strong, other processes would need to be
identified to compensate for their impact on massive-star outputs. Our
population synthesis analysis jointly treats kinematic output and the return of
radioactive isotopes, which proves a powerful extension of the methodology that
constrains feedback from massive stars.Comment: Accepted for publication in A&A, 10 page
- …