13,035 research outputs found
A Binary Scenario for the Formation of Strongly Magnetized White Dwarfs
Since their initial discovery, the origin of isolated white dwarfs (WDs) with
magnetic fields in excess of 1 MG has remained a mystery. Recently, the
formation of these high-field magnetic WDs has been observationally linked to
strong binary interactions incurred during post-main-sequence evolution.
Planetary, brown dwarf or stellar companions located within a few AU of
main-sequence stars may become engulfed during the primary's expansion off the
main sequence. Sufficiently low-mass companions in-spiral inside a common
envelope until they are tidally shredded near the natal white dwarf. Formation
of an accretion disk from the disrupted companion provides a source of
turbulence and shear which act to amplify magnetic fields and transport them to
the WD surface. We show that these disk-generated fields explain the observed
range of magnetic field strengths for isolated, high-field magnetic WDs.
Additionally, we discuss a high-mass binary analogue which generates a
strongly-magnetized WD core inside a pre-collapse, massive star. Subsequent
core-collapse to a neutron star may produce a magnetar.Comment: To appear in the Proceedings of the 2nd International Symposium on
Strong Electromagnetic Fields and Neutron Stars, Varadero, Cub
SwSt 1: an O-rich planetary nebula around a C-rich central star
The hydrogen-deficient carbon-rich [WCL] type central star HD167362 and its
oxygen-rich planetary nebula (PN) SwSt~1 are investigated. The nebular
chemistry might indicate a recent origin for the carbon-rich stellar spectrum.
Its stellar and nebular properties might therefore provide further
understanding of the origin of the [WCL] central star class. The UV-IR stellar
spectra are modelled with state of the codes and show ~40kK central star with a
wind and a C/O~3, indicative of efficient third dredge-up. The synthetic
stellar flux distribution is used to model the high density, compact PN, which
has a solar C/O ratio, is still enshrouded by 1200K and 230K dust shells and,
reported here for the first time, in molecular hydrogen. Although it appears
that the change in C/O ratio has been recent, the published spectroscopy since
1895 has been re-examined and no clear spectral change is seen. If an event
occurred that has turned it into a hydrogen-deficient central star, it did not
happen in the last 100 years.Comment: 31 pages, 19 figures (some are gif files), MNRAS in pres
Wolf-Rayet nebulae as tracers of stellar ionizing fluxes: I. M1-67
We use WR124 (WN8h) and its associated nebula M1-67, to test theoretical
non-LTE models for Wolf-Rayet (WR) stars. Lyman continuum ionizing flux
distributions derived from a stellar analysis of WR124, are compared with
nebular properties via photo-ionization modelling. Our study demonstrates the
significant role that line blanketing plays in affecting the Lyman ionizing
energy distribution of WR stars, of particular relevance to the study of HII
regions containing young stellar populations.
We confirm previous results that non-line blanketed WR energy distributions
fail to explain the observed nebular properties of M1-67, such that the
predicted ionizing spectrum is too hard. A line blanketed analysis of WR124 is
carried out using the method of Hillier & Miller (1998), with stellar
properties in accord with previous results, except that the inclusion of
clumping in the stellar wind reduces its wind performance factor to only
approx2. The ionizing spectrum of the line blanketed model is much softer than
for a comparable temperature unblanketed case, such that negligible flux is
emitted with energy above the HeI 504 edge. Photo-ionization modelling,
incorporating the observed radial density distribution for M1-67 reveals
excellent agreement with the observed nebular electron temperature, ionization
balance and line strengths. An alternative stellar model of WR124 is
calculated, following the technique of de Koter et al. (1997), augmented to
include line blanketing following Schmutz et al. (1991). Good consistency is
reached regarding the stellar properties of WR124, but agreement with the
nebular properties of M1-67 is somewhat poorer than for the Hillier & Miller
code.Comment: 12 pages, 5 figures, latex2e style file, Astronomy & Astrophysics
(accepted
A close look into the carbon disk at the core of the planetary nebula CPD-568032
We present high spatial resolution observations of the dusty core of the
Planetary Nebula with Wolf-Rayet central star CPD-568032. These observations
were taken with the mid-infrared interferometer VLTI/MIDI in imaging mode
providing a typical 300 mas resolution and in interferometric mode using
UT2-UT3 47m baseline providing a typical spatial resolution of 20 mas. The
visible HST images exhibit a complex multilobal geometry dominated by faint
lobes. The farthest structures are located at 7" from the star. The mid-IR
environment of CPD-568032 is dominated by a compact source, barely resolved by
a single UT telescope in a 8.7 micron filter. The infrared core is almost fully
resolved with the three 40-45m projected baselines ranging from -5 to 51 degree
but smooth oscillating fringes at low level have been detected in spectrally
dispersed visibilities. This clear signal is interpreted in terms of a ring
structure which would define the bright inner rim of the equatorial disk.
Geometric models allowed us to derive the main geometrical parameters of the
disk. For instance, a reasonably good fit is reached with an achromatic and
elliptical truncated Gaussian with a radius of 97+/-11 AU, an inclination of
28+/-7 degree and a PA for the major axis at 345+/-7 degree. Furthermore, we
performed some radiative transfer modeling aimed at further constraining the
geometry and mass content of the disk, by taking into account the MIDI
dispersed visibilities, spectra, and the large aperture SED of the source.
These models show that the disk is mostly optically thin in the N band and
highly flared.Comment: Paper accepted in A&
Evolution of the disc atmosphere in the X-ray binary MXB 1659-298, during its 2015-2017 outburst
We report on the evolution of the X-ray emission of the accreting neutron
star (NS) low mass X-ray binary (LMXB), MXB 1659-298, during its most recent
outburst in 2015-2017. We detected 60 absorption lines during the soft state
(of which 21 at more than 3 ), that disappeared in the hard state
(e.g., the Fe xxv and Fe xxvi lines). The absorbing plasma is at rest, likely
part of the accretion disc atmosphere. The bulk of the absorption features can
be reproduced by a high column density () of highly
ionised () plasma. Its disappearance during the
hard state is likely the consequence of a thermal photo-ionisation instability.
MXB 1659-298's continuum emission can be described by the sum of an absorbed
disk black body and its Comptonised emission, plus a black body component. The
observed spectral evolution with state is in line with that typically observed
in atoll and stellar mass black hole LMXB. The presence of a relativistic Fe
K disk-line is required during the soft state. We also tentatively
detect the Fe xxii doublet, whose ratio suggests an electron density of the
absorber of , hence, the absorber is likely located at
from the illuminating source, well inside the Compton and
outer disc radii. MXB 1659-298 is the third well monitored atoll LMXB
showcasing intense Fe xxv and Fe xxvi absorption during the soft state that
disappears during the hard state.Comment: MNRAS in pres
A Spitzer view of protoplanetary disks in the gamma Velorum cluster
We present new Spitzer Space Telescope observations of stars in the young ~5
Myr gamma Velorum stellar cluster. Combining optical and 2MASS photometry, we
have selected 579 stars as candidate members of the cluster. With the addition
of the Spitzer mid-infrared data, we have identified 5 debris disks around
A-type stars, and 5-6 debris disks around solar-type stars, indicating that the
strong radiation field in the cluster does not completely suppress the
production of planetesimals in the disks of cluster members. However, we find
some evidence that the frequency of circumstellar primordial disks is lower,
and the IR flux excesses are smaller than for disks around stellar populations
with similar ages. This could be evidence for a relatively fast dissipation of
circumstellar dust by the strong radiation field from the highest mass star(s)
in the cluster. Another possibility is that gamma Velorum stellar cluster is
slightly older than reported ages and the the low frequency of primordial disks
reflects the fast disk dissipation observed at ~5 Myr.Comment: Accepted for publication in the Astrophysical Journal: 32 pages; 11
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