268 research outputs found
The dwarf nova SS Cygni: what is wrong?
Since the Fine Guiding Sensor (FGS) on the Hubble Space Telescope (HST) was
used to measure the distance to SS Cyg to be pc, it became apparent
that at this distance the disc instability model fails to explain the absolute
magnitude during outburst. It remained, however, an open question whether the
model or the distance have to be revised. Recent observations led to a revision
of the system parameters of SS Cyg and seem to be consistent with a distance of
d\gta 140 pc. We re-discuss the problem taking into account the new binary
and stellar parameters measured for SS Cyg. We confront not only the
observations with the predictions of the disc instability model but also
compare SS Cyg with other dwarf novae and nova-like systems. We assume the disc
during outburst to be in a quasi stationary state and use the black-body
approximation to estimate the accretion rate during outburst as a function of
distance. Using published analysis of the long term light curve we determine
the mean mass transfer rate of SS Cyg as a function of distance and compare the
result with mass transfer rates derived for other dwarf novae and nova-like
systems. At a distance of d\gta 140 pc, both the accretion rate during
outburst as well as the mean mass transfer rate of SS Cyg contradict the disc
instability model. More important, at such distances we find the mean mass
transfer rate of SS Cyg to be higher or comparable to those derived for
nova-like systems. Our findings show that a distance to SS Cyg \gta 140 pc
contradicts the main concepts developed for accretion discs in cataclysmic
variables during the last 30 years. Either our current picture of disc
accretion in these systems must be revised or the distance to SS Cyg is pcComment: 6 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
Reversing the verdict: Cataclysmic variables could be the dominant progenitors of AM CVn binaries after all
Context. AM CVn binaries are potential progenitors of thermonuclear
supernovae and strong sources of persistent gravitational wave radiation. For a
long time, it has been believed that these systems cannot descend from
cataclysmic variables (CVs), at least not in large numbers, because the initial
conditions need to be fine-tuned and, even worse, the resulting surface
hydrogen abundance would be high enough to be detected which contradicts a
defining feature of AM CVn binaries.
Aims. Here we show that both claimed weaknesses of the CV formation channel
for AM CVn binaries are model-dependent and rely on poorly constrained
assumptions for magnetic braking.
Methods. We performed binary evolution simulations with the MESA code for
different combinations of post-common-envelope white dwarf and companion masses
as well as orbital periods assuming the CARB model for strong magnetic braking.
Results. We found that AM CVn binaries with extremely-low surface hydrogen
abundances are one natural outcome of CV evolution if the donor star has
developed a non-negligible helium core prior to the onset of mass transfer. In
this case, after hydrogen envelope exhaustion during CV evolution, the donor
becomes degenerate and its surface hydrogen abundance substantially drops and
becomes undetectable. Our simulations also show that the CV formation channel
is able to explain the observed AM CVn binaries with very low mass and bloated
donor stars (Gaia14aae and ZTF J1637+49).
Conclusions. CVs with evolved donors are likely the progenitors of at least a
fraction of AM CVn binaries.Comment: Accepted for publication in A&
Formation and Evolution of Accreting Compact Objects
Accreting compact objects are crucial to understand several important
astrophysical phenomena such as Type Ia supernovae, gravitational waves, or
X-ray and -ray bursts. In addition, they are natural laboratories to
infer fundamental properties of stars, to investigate high-energy phenomena and
accretion processes, to test theories of stellar and binary evolution, to
explore interactions between high-density plasma and very strong magnetic
fields, to examine the interplay between binary evolution and dynamical
interactions (in the case they belong to dense star clusters), and they can
even be used as a probe for the assembling process of galaxies over cosmic
time-scales. Despite the fundamental importance of accreting compact objects
for astrophysics and recent progress with the comprehension of these
fascinating objects, we still do not fully understand how they form and evolve.
In this chapter, we will review the current theoretical status of our knowledge
on these objects, and will discuss standing problems and potential solutions to
them.Comment: Invited chapter for the Handbook of X-ray and Gamma-ray Astrophysics
(Editors: Cosimo Bambi, Andrea Santangelo; Publisher: Springer Singapore
Cold giant planets evaporated by hot white dwarfs
Atmospheric escape from close-in Neptunes and hot Jupiters around Sun-like stars driven by extreme ultraviolet (EUV) irradiation plays an important role in the evolution of exoplanets and in shaping their ensemble properties. Intermediate and low mass stars are brightest at EUV wavelengths at the very end of their lives, after they have expelled their envelopes and evolved into hot white dwarfs. Yet the effect of the intense EUV irradiation of giant planets orbiting young white dwarfs has not been assessed. We show that the giant planets in the solar system will experience significant hydrodynamic escape caused by the EUV irradiation from the white dwarf left behind by the Sun. A fraction of the evaporated volatiles will be accreted by the solar white dwarf, resulting in detectable photospheric absorption lines. As a large number of the currently known extrasolar giant planets will survive the metamorphosis of their host stars into white dwarfs, observational signatures of accretion from evaporating planetary atmospheres are expected to be common. In fact, one-third of the known hot single white dwarfs show photospheric absorption lines of volatile elements, which we argue are indicative of ongoing accretion from evaporating planets. The fraction of volatile contaminated hot white dwarfs strongly decreases as they cool. We show that accretion from evaporating planetary atmospheres naturally explains this temperature dependence if more than 50% of hot white dwarfs still host giant planets
Period bouncers as detached magnetic cataclysmic variables
The general prediction that more than half of all CVs have evolved past the
period minimum is in strong disagreement with observational surveys, which show
that the relative number of these objects is just a few per cent. Here, we
investigate whether a large number of post-period minimum CVs could detach
because of the appearance of a strong white dwarf magnetic field potentially
generated by a rotation- and crystallization-driven dynamo. We used the MESA
code to calculate evolutionary tracks of CVs incorporating the spin evolution
and cooling as well as compressional heating of the white dwarf. If the
conditions for the dynamo were met, we assumed that the emerging magnetic field
of the white dwarf connects to that of the companion star and incorporated the
corresponding synchronization torque, which transfers spin angular momentum to
the orbit. We find that for CVs with donor masses exceeding 0.04 Msun, magnetic
fields are generated mostly if the white dwarfs start to crystallize before the
onset of mass transfer. It is possible that a few white dwarf magnetic fields
are generated in the period gap. For the remaining CVs, the conditions for the
dynamo to work are met beyond the period minimum, when the accretion rate
decreased significantly. Synchronization torques cause these systems to detach
for several Gyrs even if the magnetic field strength of the white dwarf is just
one MG. If the rotation- and crystallization-driven dynamo - which is currently
the only mechanism that can explain several observational facts related to
magnetism in CVs and their progenitors - or a similar temperature-dependent
mechanism is responsible for the generation of magnetic field in white dwarfs,
most CVs that have evolved beyond the period minimum must detach for several
Gyrs at some point. This reduces the predicted number of semi-detached period
bouncers by up to 60-80 per cent.Comment: A&A letters, in pres
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