13 research outputs found
Modeling TeV gamma-rays from LS 5039: An active OB star at the extreme
Perhaps the most extreme examples of "Active OB stars" are the subset of
high-mass X-ray binaries -- consisting of an OB star plus compact companion --
that have recently been observed by Fermi and ground-based Cerenkov telescopes
like HESS to be sources of very high energy (VHE; up to 30 TeV) gamma-rays.
This paper focuses on the prominent gamma-ray source, LS5039, which consists of
a massive O6.5V star in a 3.9-day-period, mildly elliptical (e = 0.24) orbit
with its companion, assumed here to be a black-hole or unmagnetized neutron
star. Using 3-D SPH simulations of the Bondi-Hoyle accretion of the O-star wind
onto the companion, we find that the orbital phase variation of the accretion
follows very closely the simple Bondi-Hoyle-Lyttleton (BHL) rate for the local
radius and wind speed. Moreover, a simple model, wherein intrinsic emission of
gamma-rays is assumed to track this accretion rate, reproduces quite well Fermi
observations of the phase variation of gamma-rays in the energy range 0.1-10
GeV. However for the VHE (0.1-30 TeV) radiation observed by the HESS Cerenkov
telescope, it is important to account also for photon-photon interactions
between the gamma-rays and the stellar optical/UV radiation, which effectively
attenuates much of the strong emission near periastron. When this is included,
we find that this simple BHL accretion model also quite naturally fits the HESS
light curve, thus making it a strong alternative to the pulsar-wind-shock
models commonly invoked to explain such VHE gamma-ray emission in massive-star
binaries.Comment: To appear in "Active OB Stars: Structure, Evolution, Mass Loss &
Critical Limits", Proceedings of IAUS 272, held July 2010 in Paris, France. 7
pages; 3 figures. This version 2 corrects an alignment error in figure
Mass and angular momentum loss of fast rotating stars via decretion disks
The spinup of massive stars induced by evolution of the stellar interior can bring the star to near-critical rotation. In critically rotating stars the decrease of the stellar moment of inertia must be balanced by a net loss of angular momentum through an equatorial decretion disk. We examine the nature and role of mass loss via such disks. In contrast to the usual stellar wind mass loss set by exterior driving from the stellar luminosity, such decretion-disk mass loss stems from the angular momentum loss needed to keep the star near and below critical rotation, given the interior evolution and decline in the star's moment of inertia. Because the specific angular momentum in a Keplerian disk increases with the square root of the radius, the decretion mass loss associated with a required level of angular momentum loss critically depends on the outer radius for viscous coupling of the disk, and can be significantly less than the spherical, wind-like mass loss commonly assumed in evolutionary calculation
High spatial resolution monitoring of the activity of BA supergiant winds
There are currently two optical interferometry recombiners that can provide
spectral resolutions better than 10000, AMBER/VLTI operating in the H-K bands,
and VEGA/CHARA, recently commissioned, operating in the visible. These
instruments are well suited to study the wind activity of the brightest AB
supergiants in our vicinity, in lines such as H or BrGamma. We present
here the first observations of this kind, performed on Rigel (B8Ia) and Deneb
(A2Ia). Rigel was monitored by AMBER in two campaigns, in 2006-2007 and
2009-2010, and observed in 2009 by VEGA; whereas Deneb was monitored in
2008-2009 by VEGA. The extension of the Halpha and BrGamma line forming regions
were accurately measured and compared with CMFGEN models of both stars.
Moreover, clear signs of activity were observed in the differential visibility
and phases. These pioneer observations are still limited, but show the path for
a better understanding of the spatial structure and temporal evolution of
localized ejections using optical interferometry.Comment: Proceedings of conf. IAUS272 - Active OB stars - Paris, July 19-23,
201
The prototype colliding-wind pinwheel WR 104
Results from the most extensive study of the time-evolving dust structure
around the prototype "Pinwheel" nebula WR 104 are presented. Encompassing 11
epochs in three near-infrared filter bandpasses, a homogeneous imaging data set
spanning more than 6 years (or 10 orbits) is presented. Data were obtained from
the highly successful Keck Aperture Masking Experiment, which can recover high
fidelity images at extremely high angular resolutions, revealing the geometry
of the plume with unprecedented precision. Inferred properties for the
(unresolved) underlying binary and wind system are orbital period 241.5 +/- 0.5
days and angular outflow velocity of 0.28 +/- 0.02 mas/day. An optically thin
cavity of angular size 13.3 +/- 1.4 mas was found to lie between the central
binary and the onset of the spiral dust plume. Rotational motion of the wind
system induced by the binary orbit is found to have important ramifications:
entanglement of the winds results in strong shock activity far downstream from
the nose of the bowshock. The far greater fraction of the winds participating
in the collision may play a key role in gas compression and the nucleation of
dust at large radii from the central binary and shock stagnation point.
Investigation of the effects of radiative braking pointed towards significant
modifications of the simple hydrostatic colliding wind geometry, extending the
relevance of this phenomena to wider binary systems than previously considered.
Limits placed on the maximum allowed orbital eccentricity of e < 0.06 argue
strongly for a prehistory of tidal circularization in this system. Finally we
discuss the implications of Earth's polar (i < 16 deg) vantage point onto a
system likely to host supernova explosions at future epochs.Comment: 35 pages, 8 figures, Accepted for publication in Astrophysical
Journa
Modeling TeV γ-rays from LS 5039: an active OB star at the extreme
Perhpas the most extreme examples of "Active OB stars" are the subset of high-mass X-ray binaries - consisting of an OB star plus compact companion - that have recently been observed by Fermi and ground-based Cerenkov telescopes like HESS to be sources of very high energy (VHE; up to 30 TeV!) γ-rays. This paper focuses on the prominent γ-ray source, LS5039, which consists of a massive O6.5V star in a 3.9-day-period, mildly elliptical (e ≈ 0.24) orbit with its companion, assumed here to be a black-hole or unmagnetized neutron star. Using 3-D SPH simulations of the Bondi-Hoyle accretion of the O-star wind onto the companion, we find that the orbital phase variation of the accretion follows very closely the simple Bondi-Hoyle-Lyttleton (BHL) rate for the local radius and wind speed. Moreover, a simple model, wherein intrinsic emission of γ-rays is assumed to track this accretion rate, reproduces quite well Fermi observations of the phase variation of γ-rays in the energy range 0.1-10 GeV. However for the VHE (0.1-30 TeV) radiation observed by the HESS Cerenkov telescope, it is important to account also for photon-photon interactions between the γ-rays and the stellar optical/UV radiation, which effectively attenuates much of the strong emission near periastron. When this is included, we find that this simple BHL accretion model also quite thus making it a strong alternative to the pulsar-wind-shock models commonly invoked to explain such VHE γ-ray emission in massive-star binaries.Facultad de Ciencias Exacta
Modeling TeV γ-rays from LS 5039: an active OB star at the extreme
Perhpas the most extreme examples of "Active OB stars" are the subset of high-mass X-ray binaries - consisting of an OB star plus compact companion - that have recently been observed by Fermi and ground-based Cerenkov telescopes like HESS to be sources of very high energy (VHE; up to 30 TeV!) γ-rays. This paper focuses on the prominent γ-ray source, LS5039, which consists of a massive O6.5V star in a 3.9-day-period, mildly elliptical (e ≈ 0.24) orbit with its companion, assumed here to be a black-hole or unmagnetized neutron star. Using 3-D SPH simulations of the Bondi-Hoyle accretion of the O-star wind onto the companion, we find that the orbital phase variation of the accretion follows very closely the simple Bondi-Hoyle-Lyttleton (BHL) rate for the local radius and wind speed. Moreover, a simple model, wherein intrinsic emission of γ-rays is assumed to track this accretion rate, reproduces quite well Fermi observations of the phase variation of γ-rays in the energy range 0.1-10 GeV. However for the VHE (0.1-30 TeV) radiation observed by the HESS Cerenkov telescope, it is important to account also for photon-photon interactions between the γ-rays and the stellar optical/UV radiation, which effectively attenuates much of the strong emission near periastron. When this is included, we find that this simple BHL accretion model also quite thus making it a strong alternative to the pulsar-wind-shock models commonly invoked to explain such VHE γ-ray emission in massive-star binaries.Facultad de Ciencias Exacta
Modeling TeV γ-rays from LS 5039: an active OB star at the extreme
Perhpas the most extreme examples of "Active OB stars" are the subset of high-mass X-ray binaries - consisting of an OB star plus compact companion - that have recently been observed by Fermi and ground-based Cerenkov telescopes like HESS to be sources of very high energy (VHE; up to 30 TeV!) γ-rays. This paper focuses on the prominent γ-ray source, LS5039, which consists of a massive O6.5V star in a 3.9-day-period, mildly elliptical (e ≈ 0.24) orbit with its companion, assumed here to be a black-hole or unmagnetized neutron star. Using 3-D SPH simulations of the Bondi-Hoyle accretion of the O-star wind onto the companion, we find that the orbital phase variation of the accretion follows very closely the simple Bondi-Hoyle-Lyttleton (BHL) rate for the local radius and wind speed. Moreover, a simple model, wherein intrinsic emission of γ-rays is assumed to track this accretion rate, reproduces quite well Fermi observations of the phase variation of γ-rays in the energy range 0.1-10 GeV. However for the VHE (0.1-30 TeV) radiation observed by the HESS Cerenkov telescope, it is important to account also for photon-photon interactions between the γ-rays and the stellar optical/UV radiation, which effectively attenuates much of the strong emission near periastron. When this is included, we find that this simple BHL accretion model also quite thus making it a strong alternative to the pulsar-wind-shock models commonly invoked to explain such VHE γ-ray emission in massive-star binaries.Facultad de Ciencias Exacta