21 research outputs found
The Thermal Evolution of the Donors in AM CVn Binaries
(Abridged) We calculate the full stellar-structural evolution of donors in AM
CVn systems formed through the WD channel coupled to the binary's evolution.
Contrary to assumptions made in prior modelling, these donors are not fully
convective over much of the AM CVn phase and do not evolve adiabatically under
mass loss indefinitely. Instead, we identify three distinct phases of
evolution: a mass transfer turn-on phase (during which the orbital period
continues to decrease even after contact, the donor contracts, and the mass
transfer rate accelerates to its maximum), a phase in which the donor expands
adiabatically in response to mass loss, and a cooling phase beginning at
orbital periods of approximately 45--55 minutes during which the donor
contracts. The physics that determines the behaviour in the first and third
phases, both of which are new outcomes of this study, are discussed in some
detail. We find the overall duration of the turn-on phase to be between - yrs, significantly longer than prior estimates. We predict the
donor's luminosity and effective temperature. During the adiabatic expansion
phase (ignoring irradiation effects), the luminosity is approximately
-- and the effective temperature is approximately
1000--1800 K. However, the flux generated in the accretion flow dominates the
donor's intrinsic light at all times. The impact of irradiation on the donor
extends the phase of adiabatic expansion to longer orbital periods and alters
the donor's observational characteristics. Irradiated donors during the
adiabatic phase can attain a surface luminosity of up to . We argue that the turn-on and cooling phases both will leave
significant imprints on the AM CVn population's orbital period distribution.Comment: (20 pages, 20 figures, accepted to the Monthly Notices of the Royal
Astronomical Society
The Connection Between Low-Mass X-ray Binaries and (Millisecond) Pulsars: A Binary Evolution Perspective
I review the evolutionary connection between low-mass X-ray binaries (LMXBs)
and pulsars with binary companions (bPSRs) from a stellar binary evolution
perspective. I focus on the evolution of stellar binaries with end-states
consisting of a pulsar with a low-mass (<1.0 solar mass) companion, starting at
the point the companion's progenitor first initiates mass transfer onto the
neutron star. Whether this mass transfer is stable and the physics driving
ongoing mass transfer partitions the phase space of the companions's initial
mass and initial orbital period into five regions. The qualitative nature of
the mass-transfer process and the binary's final end-state differ between
systems in each region; four of these regions each produce a particular class
of LMXBs. I compare the theoretical expectations to the populations of galactic
field LMXBs with companion-mass constraints and field bPSRs. I show that the
population of accreting millisecond pulsars are all identified with only two of
the four LMXB classes and that these systems do not have readily identifiable
progeny in the bPSR population. I discuss which sub-populations of bPSRs can be
explained by binary evolution theory and those that currently are not. Finally
I discuss some outstanding questions in this field.Comment: 9 pages, 5 figures, to appear in the proceedings of the 40 Years of
Pulsars conference held at McGill University in August 200
Ultracompact Binaries as Bright X-Ray Sources in Elliptical Galaxies
Chandra observations of distant elliptical galaxies have revealed large
numbers of Low Mass X-ray Binaries (LMXBs) accreting at rates in excess of
10^{-8} solar masses per year. The majority of these LMXBs reside in globular
clusters (GCs) and it has been suggested that many of the field LMXBs also
originated in GCs. We show here that ultracompact binaries with orbital periods
of 8-10 minutes and He or C/O donors of 0.06-0.08 solar masses naturally
provide the observed accretion rates from gravitational radiation losses alone.
Such systems are predicted to be formed in the dense GC environment, a
hypothesis supported by the 11.4 minute binary 4U 1820-30, the brightest
persistent LMXB in a Galactic GC. These binaries have short enough lifetimes
(less then 3 Myr) while bright that we calculate their luminosity function
under a steady-state approximation. This yields a luminosity function slope in
agreement with that observed for luminosities in the range of 6E37 ergs/sec to
5E38 ergs/sec, encouraging us to use the observed numbers of LMXBs per GC mass
to calculate the accumulated number of ultracompact binaries. For a constant
birthrate over 8 Gyrs, the number of ultracompact binaries which have evolved
through this bright phase is nearly 4000 in a 10 million solar mass GC,
consistent with dynamical interaction calculations. Perhaps most importantly,
if all ultracompacts become millisecond radio pulsars, then the observed
normalization agrees with the inferred number of millisecond radio pulsars in
47 Tuc and Galactic GCs in general.Comment: to Appear in Astrophysical Journal Letter
Gravitational Settling of ^{22}Ne in Liquid White Dwarf Interiors--Cooling and Seismological Effects
We assess the impact of the trace element ^{22}Ne on the cooling and
seismology of a liquid C/O white dwarf (WD). Due to this elements' neutron
excess, it sinks towards the interior as the liquid WD cools. The subsequent
gravitational energy released slows the cooling of the WD by 0.25--1.6 Gyrs by
the time it has completely crystallized, depending on the WD mass and the
adopted sedimentation rate. The effects will make massive WDs or those in metal
rich clusters (such as NGC 6791) appear younger than their true age. Our
diffusion calculations show that the ^{22}Ne mass fraction in the crystallized
core actually increases outwards. The stability of this configuration has not
yet been determined. In the liquid state, the settled ^{22}Ne enhances the
internal buoyancy of the interior and changes the periods of the high radial
order g-modes by approximately 1%. Though a small adjustment, this level of
change far exceeds the accuracy of the period measurements. A full assessment
and comparison of mode frequencies for specific WDs should help constrain the
still uncertain ^{22}Ne diffusion coefficient for the liquid interior.Comment: 26 pages (11 text pages with 15 figures); to appear in The
Astrophysical Journa
White Dwarf Donors in Ultracompact Binaries: The Stellar Structure of Finite Entropy Objects
We discuss the mass-radius (M-R) relations for low-mass (M<0.1 Msun) white
dwarfs (WDs) of arbitrary degeneracy and evolved (He, C, O) composition. We do
so with both a simple analytical model and models calculated by integration of
hydrostatic balance using a modern equation of state valid for fully ionized
plasmas. The M-R plane is divided into three regions where either Coulomb
physics, degenerate electrons or a classical gas dominate the WD structure. For
a given M and central temperature, T_c, the M-R relation has two branches
differentiated by the model's entropy content. We present the M-R relations for
a sequence of constant entropy WDs of arbitrary degeneracy parameterized by M
and T_c for pure He, C, and O. We discuss the applications of these models to
the recently discovered accreting millisecond pulsars. We show the relationship
between the orbital inclination for these binaries and the donor's composition
and T_c. In particular we find from orbital inclination constraints that the
probability XTE J1807-294 can accommodate a He donor is approximately 15% while
for XTE J0929-304, it is approximately 35%. We argue that if the donors in
ultracompact systems evolve adiabatically, there should be 60-160 more systems
at orbital periods of 40 min than at orbital periods of 10 min, depending on
the donor's composition.Comment: emulateapj style, 11 pages, 12 figures. Accepted to the Astrophysical
Journal. Tables with interpolation routines of the M-R relations are
available at http://www.physics.ucsb.edu/~cjdeloye/research.htm
The Turn-On of Mass Transfer in AM CVn Binaries: Implications for RX J0806+1527 and RX J1914+2456
We report on evolutionary calculations of the onset of mass transfer in AM
CVn binaries, treating the donor's evolution in detail. We show that during the
early contact phase, while the mass transfer rate, \Mdot, is increasing,
gravity wave (GW) emission continues to drive the binary to shorter orbital
period, \Porb. We argue that the phase where \Mdot > 0 and \nudot > 0
(\nu = 1/\Porb) can last between and yrs, significantly longer
than previously estimated. These results are applied to RX J0806+1527 (\Porb =
321 s) and RX J914+2456 (\Porb=569 s), both of which have measured \nudot >
0. \emph{Thus, a \nudot > 0 does not select between the unipolar inductor
and accretion driven models proposed as the source of X-rays in these systems}.
For the accretion model, we predict for RX J0806 that \ddot{\nu} \approx
\ee{1.0-1.5}{-28} Hz s and argue that timing observations can probe
at this level with a total yr baseline. We also place
constraints on each system's initial parameters given current observational
data.Comment: 5 pages, 3 figures, accepted to ApJ
Arbitrarily Degenerate Helium White Dwarfs as Donors in AM CVn Binaries
We apply the Deloye & Bildsten (2003) isentropic models for donors in
ultracompact low-mass X-ray binaries to the AM CVn population of ultracompact,
interacting binaries. The mass-radius relations of these systems' donors in the
mass range of interest (M_2<0.1 \msun) are not single-valued, but
parameterized by the donor's specific entropy. This produces a range in the
relationships between system observables, such as orbital period, \Porb, and
mass transfer rate, \Mdot. For a reasonable range in donor specific entropy,
\Mdot can range over several orders of magnitude at fixed \Porb. We
determine the unique relation between \Mdot and in the AM CVn systems
with known donor to accretor mass ratios, . We use structural
arguments, as well as each system's photometric behavior, to place limits on
\Mdot and in each. Most systems allow a factor of about 3 variation in
\Mdot, although V803 Cen, if the current estimates of its are accurate,
is an exception and must have M_2 \approx 0.02 \msun and \Mdot \approx
10^{-10} \msun yr. Our donor models also constrain each donor's core
temperature, , range and correlate with . We examine how
variations in donor specific entropy across the white dwarf family
\citep{nele01a} of AM CVn systems affects this population's current galactic
distribution. Allowing for donors that are not fully degenerate produces a
shift in systems towards longer \Porb and higher \Mdot increasing the
parameter space in which these systems can be found. This shift increases the
fraction of systems whose \Porb is long enough that their gravity wave (GW)
signal is obscured by the background of detached double white dwarf binaries
that dominate the GW spectrum below a frequency mHz.Comment: 13 pages, 10 figures, uses emulateapj.cls. Accepted to Astrophysical
Journa