53 research outputs found
Quark-Novae in Low-mass X-ray Binaries II: Application to G87-7 and to GRB 110328A
We propose a simple model explaining two outstanding astrophysical problems
related to compact objects: (1) that of stars such as G87-7 (alias EG 50) that
constitute a class of relatively low-mass white dwarfs which nevertheless fall
away from the C/O composition and (2) that of GRB 110328A/Swift J164449.3+57345
which showed spectacularly long-lived strong X-ray flaring, posing a challenge
to standard GRB models. We argue that both these observations may have an
explanation within the unified framework of a Quark-Nova occurring in a
low-mass X-ray binary (neutron star- white dwarf). For LMXBs where the binary
separation is sufficiently tight, ejecta from the exploding Neutron Star
triggers nuclear burning in the white dwarf on impact, possibly leading to
Fe-rich composition compact white dwarfs with mass 0.43M_sun < M_WD <
0.72M_sun, reminiscent of G87-7. Our results rely on the assumption, which
ultimately needs to be tested by hydrodynamic and nucleosynthesis simulations,
that under certain circumstances the WD can avoid the thermonuclear runaway.
For heavier white dwarfs (i.e. M_WD > 0.72M_sun) experiencing the QN shock,
degeneracy will not be lifted when Carbon burning begins, and a
sub-Chandrasekhar Type Ia Supernovae may result in our model. Under slightly
different conditions, and for pure He white dwarfs (i.e. M_WD < 0.43M_sun), the
white dwarf is ablated and its ashes raining down on the Quark star leads to
accretion-driven X-ray luminosity with energetics and duration reminiscent of
GRB 110328A. We predict additional flaring activity towards the end of the
accretion phase if the Quark star turns into a Black Hole.Comment: Accepted for publication in ApJ. Extended paper size to 6 journal
pages (from 4). Table is extended and more detailed. Related animations at:
http://quarknova.ucalgary.ca/media/ (find paper I of the series here:
http://adsabs.harvard.edu/abs/2011ApJ...729...60O
Disk wind feedback from high-mass protostars
We perform a sequence of 3D magnetohydrodynamic (MHD) simulations of the
outflow-core interaction for a massive protostar forming via collapse of an
initial cloud core of . This allows us to characterize the
properties of disk wind driven outflows from massive protostars, which can
allow testing of different massive star formation theories. It also enables us
to assess quantitatively the impact of outflow feedback on protostellar core
morphology and overall star formation efficiency. We find that the opening
angle of the flow increases with increasing protostellar mass, in agreement
with a simple semi-analytic model. Once the protostar reaches
the outflow's opening angle is so wide that it has blown
away most of the envelope, thereby nearly ending its own accretion. We thus
find an overall star formation efficiency of , similar to that
expected from low-mass protostellar cores. Our simulation results therefore
indicate that the MHD disk wind outflow is the dominant feedback mechanism for
helping to shape the stellar initial mass function from a given prestellar core
mass function.Comment: Accepted for publication in Ap
Outflow-Confined HII regions. II. The Early Break-Out Phase
In this series of papers, we model the formation and evolution of the
photoionized region and its observational signatures during massive star
formation. Here we focus on the early break out of the photoionized region into
the outflow cavity. Using results of 3-D magnetohydrodynamic-outflow
simulations and protostellar evolution calculations, we perform post-processing
radiative-transfer. The photoionized region first appears at a protostellar
mass of 10Msun in our fiducial model, and is confined to within 10-100AU by the
dense inner outflow, similar to some observed very small hypercompact HII
regions. Since the ionizing luminosity of the massive protostar increases
dramatically as Kelvin-Helmholz (KH) contraction proceeds, the photoionized
region breaks out to the entire outflow region in <10,000yr. Accordingly, the
radio free-free emission brightens significantly in this stage. In our fiducial
model, the radio luminosity at 10 GHz changes from 0.1 mJy kpc2 at m=11Msun to
100 mJy kpc2 at 16Msun, while the infrared luminosity increases by less than a
factor of two. The radio spectral index also changes in the break-out phase
from the optically thick value of 2 to the partially optically thin value of
0.6. Additionally, we demonstrate that short-timescale variation in free-free
flux would be induced by an accretion burst. The outflow density is enhanced in
the accretion burst phase, which leads to a smaller ionized region and weaker
free-free emission. The radio luminosity may decrease by one order of magnitude
during such bursts, while the infrared luminosity is much less affected, since
internal protostellar luminosity dominates over accretion luminosity after KH
contraction starts. Such variability may be observable on timescales as short
10-100 yr, if accretion bursts are driven by disk instabilities.Comment: 9 pages, 5 figures, accepted for publication in Ap
Quark-Novae Ia in the Hubble diagram: Implications For Dark Energy
The accelerated expansion of the Universe was proposed through the use of
Type-Ia SNe as standard candles. The standardization depends on an empirical
correlation between the stretch/color and peak luminosity of the light curves.
The use of Type Ia SN as standard candles rests on the assumption that their
properties (and this correlation) do not vary with red-shift. We consider the
possibility that the majority of Type-Ia SNe are in fact caused by a Quark-Nova
detonation in a tight neutron-star-CO-white-dwarf binary system; a Quark-Nova
Ia. The spin-down energy injected by the Quark Nova remnant (the quark star)
contributes to the post-peak light curve and neatly explains the observed
correlation between peak luminosity and light curve shape. We demonstrate that
the parameters describing Quark-Novae Ia are NOT constant in red-shift.
Simulated Quark-Nova Ia light curves provide a test of the stretch/color
correlation by comparing the true distance modulus with that determined using
SN light curve fitters. We determine a correction between the true and fitted
distance moduli which when applied to Type-Ia SNe in the Hubble diagram
recovers the Omega_M = 1 cosmology. We conclude that Type-Ia SNe observations
do not necessitate the need for an accelerating expansion of the Universe (if
the observed SNe-Ia are dominated by QNe-Ia) and by association the need for
Dark Energy.Comment: 22 pages, 6 figures. Accepted for publication in Research in
Astronomy and Astrophysic
Quark deconfinement in neutron star cores: The effects of spin-down
We study the role of spin-down in driving quark deconfinement in the high
density core of isolated neutron stars. Assuming spin-down to be solely due to
magnetic braking, we obtain typical timescales to quark deconfinement for
neutron stars that are born with Keplerian frequencies. Employing different
equations of state (EOS), we determine the minimum and maximum neutron star
masses that will allow for deconfinement via spin-down only. We find that the
time to reach deconfinement is strongly dependent on the magnetic field and
that this time is least for EOS that support the largest minimum mass at zero
spin, unless rotational effects on stellar structure are large. For a fiducial
critical density of for the transition to the quark phase
(g/cm is the saturation density of nuclear
matter), we find that neutron stars lighter than cannot reach a
deconfined phase. Depending on the EOS, neutron stars of more than
can enter a quark phase only if they are spinning faster than
about 3 milliseconds as observed now, whereas larger spin periods imply that
they are either already quark stars or will never become one.Comment: 4 pages, 4 figures, submitted to ApJ
Hydrodynamic Simulations of the Interaction between an AGB Star and a Main Sequence Companion in Eccentric Orbits
The Rotten Egg Nebula has at its core a binary composed of a Mira star and an
A-type companion at a separation >10 au. It has been hypothesized to have
formed by strong binary interactions between the Mira and a companion in an
eccentric orbit during periastron passage ~800 years ago. We have performed
hydrodynamic simulations of an asymptotic giant branch star interacting with
companions with a range of masses in orbits with a range of initial
eccentricities and periastron separations. For reasonable values of the
eccentricity, we find that Roche lobe overflow can take place only if the
periods are <<100 years. Moreover, mass transfer causes the system to enter a
common envelope phase within several orbits. Since the central star of the
Rotten Egg nebula is an AGB star, we conclude that such a common envelope phase
must have lead to a merger, so the observed companion must have been a tertiary
companion of a binary that merged at the time of nebula ejection. Based on the
mass and timescale of the simulated disc formed around the companion before the
common envelope phase, we analytically estimate the properties of jets that
could be launched. Allowing for super-Eddington accretion rates, we find that
jets similar to those observed are plausible, provided that the putative lost
companion was relatively massive.Comment: accepted for publication in MNRA
The role of dredge-up in double white dwarf mergers
We present the results of an investigation of the dredge-up and mixing during
the merger of two white dwarfs with different chemical compositions by
conducting hydrodynamic simulations of binary mergers for three representative
mass ratios. In all the simulations, the total mass of the two white dwarfs is
. Mergers involving a CO and a He white dwarf have
been suggested as a possible formation channel for R Coronae Borealis type
stars, and we are interested in testing if such mergers lead to conditions and
outcomes in agreement with observations. Even if the conditions during the
merger and subsequent nucleosynthesis favor the production of , the merger must avoid dredging up large amounts of , or
else it will be difficult to produce sufficient to explain
the oxygen ratio observed to be of order unity. We performed a total of 9
simulations using two different grid-based hydrodynamics codes using fixed and
adaptive meshes, and one smooth particle hydrodynamics (SPH) code. We find that
in most of the simulations, of is
indeed dredged up during the merger. However, in SPH simulations where the
accretor is a hybrid He/CO white dwarf with a layer of
helium on top, we find that no is being dredged up, while in
the simulation of has been
brought up, making a WD binary consisting of a hybrid CO/He WD and a companion
He WD an excellent candidate for the progenitor of RCB stars.Comment: Accepted for publication in Ap
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