1,396 research outputs found
Gravitational waves from double white dwarfs
Double white dwarfs could be important sources for space based gravitational
wave detectors like OMEGA and LISA. We use population synthesis to predict the
current population of double white dwarfs in the Galaxy and the gravitational
waves produced by this population. We simulate a detailed power spectrum for an
observation with an integration time of 10^6 s. At frequencies below ~3 mHz
confusion limited noise dominates. At higher frequencies a few thousand double
white dwarfs are resolved individually. Including compact binaries containing
neutron stars and black holes in our calculations yields a further few hundred
resolved binaries and some tens which can be detected above the double white
dwarf noise at low frequencies. We find that binaries in which one white dwarf
transfers matter to another white dwarf are rare, and thus unimportant for
gravitational wave detectors. We discuss the uncertainties and compare our
results with other authors.Comment: 6 pages, to appear in the proceedings of the XXXIVth Rencontres de
Moriond on "Gravitational Waves and Experimental Gravity", January 23-30,
199
Fun for Two
We performed populations synthesis calculations of single stars and binaries
and show that binary evolution is extremely important for Galactic astronomy.
We review several binary evolution models and conclude that they give quite
different results. These differences can be understood from the assumptions
related to how mass is transfered in the binary systems. Most important are 1)
the fraction of mass that is accreted by the companion star during mass
transfer, 2) the amount of specific angular momentum which is carried away with
the mass that leaves the binary system.Comment: 7 pages, 0 figures to appear in the proceeding of the IAU Symposium
200, "The Formation of Binary Stars" eds. H. Zinnecker and R. Mathie
Comparison of LISA and Atom Interferometry for Gravitational Wave Astronomy in Space
One of the atom interferometer gravitational wave missions proposed by
Dimopoulos et al.1 in 2008 was called AGIS-Sat. 2. It had a suggested
gravitational wave sensitivity set by the atom state detection shot noise level
that started at 1 mHz, was comparable to LISA sensitivity from 1 to about 20
mHz, and had better sensitivity from 20 to 500 mHz. The separation between the
spacecraft was 1,000 km, with atom interferometers 200 m long and shades from
sunlight used at each end. A careful analysis of many error sources was
included, but requirements on the time-stability of both the laser wavefront
aberrations and the atom temperatures in the atom clouds were not investigated.
After including these considerations, the laser wavefront aberration stability
requirement to meet the quoted sensitivity level is about 1\times10-8
wavelengths, and is far tighter than for LISA. Also, the temperature
fluctuations between atom clouds have to be less than 1 pK. An alternate atom
interferometer GW mission in Earth orbit called AGIS-LEO with 30 km satellite
separation has been suggested recently. The reduction of wavefront aberration
noise by sending the laser beam through a high-finesse mode-scrubbing optical
cavity is discussed briefly, but the requirements on such a cavity are not
given. Unfortunately, such an Earth-orbiting mission seems to be considerably
more difficult to design than a non-geocentric mission and does not appear to
have comparably attractive scientific goals.Comment: Submitted to Proc. 46th Rencontres de Moriond: Gravitational Waves
and Experimental Gravity, March 20 - 27, 2011, La Thuile, Ital
Progenitors of Supernovae Type Ia
Despite the significance of Type Ia supernovae (SNeIa) in many fields in
astrophysics, SNeIa lack a theoretical explanation. The standard scenarios
involve thermonuclear explosions of carbon/oxygen white dwarfs approaching the
Chandrasekhar mass; either by accretion from a companion or by a merger of two
white dwarfs. We investigate the contribution from both channels to the SNIa
rate with the binary population synthesis (BPS) code SeBa in order to constrain
binary processes such as the mass retention efficiency of WD accretion and
common envelope evolution. We determine the theoretical rates and delay time
distribution of SNIa progenitors and in particular study how assumptions affect
the predicted rates.Comment: 6 pages, 6 figures, appeared in proceedings for "The 18th European
White Dwarf Workshop
Prospects for observing ultra-compact binaries with space-based gravitational wave interferometers and optical telescopes
Space-based gravitational wave interferometers are sensitive to the galactic
population of ultra-compact binaries. An important subset of the ultra-compact
binary population are those stars that can be individually resolved by both
gravitational wave interferometers and electromagnetic telescopes. The aim of
this paper is to quantify the multi-messenger potential of space-based
interferometers with arm-lengths between 1 and 5 Gm. The Fisher Information
Matrix is used to estimate the number of binaries from a model of the Milky Way
which are localized on the sky by the gravitational wave detector to within 1
and 10 square degrees and bright enough to be detected by a magnitude limited
survey. We find, depending on the choice of GW detector characteristics,
limiting magnitude, and observing strategy, that up to several hundred
gravitational wave sources could be detected in electromagnetic follow-up
observations.Comment: 6 pages, 3 figures Updated to include new results. Submitted to MNRA
Population synthesis of classical low-mass X-ray binaries in the Galactic Bulge
Aims. We model the present-day population of 'classical' low-mass X-ray
binaries (LMXBs) with neutron star accretors, which have hydrogen-rich donor
stars. Their population is compared with that of hydrogen-deficient LMXBs,
known as ultracompact X-ray binaries (UCXBs). We model the observable LMXB
population and compare it to observations. Methods. We combine the binary
population synthesis code SeBa with detailed LMXB evolutionary tracks to model
the size and properties of the present-day LMXB population in the Galactic
Bulge. Whether sources are persistent or transient, and what their
instantaneous X-ray luminosities are, is predicted using the thermal-viscous
disk instability model. Results. We find a population of ~2.1 x 10^3 LMXBs with
neutron star accretors. Of these about 15 - 40 are expected to be persistent
(depending on model assumptions), with luminosities higher than 10^35 erg s^-1.
About 7 - 20 transient sources are expected to be in outburst at any given
time. Within a factor of two these numbers are consistent with the observed
population of bright LMXBs in the Bulge. This gives credence to our prediction
of the existence of a population of ~1.6 x 10^3 LMXBs with low donor masses
that have gone through the period minimum, and have present-day mass transfer
rates below 10^-11 Msun yr^-1. Conclusions. Even though the observed population
of hydrogen-rich LMXBs in the Bulge is larger than the observed population of
(hydrogen-deficient) UCXBs, the latter have a higher formation rate. While
UCXBs may dominate the total LMXB population at the present, the majority would
be very faint, or may have become detached and produced millisecond radio
pulsars. In that case UCXBs would contribute significantly more to the
formation of millisecond radio pulsars than hydrogen-rich LMXBs. [abridged]Comment: 8 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysics. v2: minor language correction
The gravitational wave signal from the Galactic disk population of binaries containing two compact objects
We review the properties of Galactic binaries containing two compact objects,
as derived by means of population synthesis. Using this information we
calculate the gravitational wave signal of these binaries. At frequencies below
f ~< 2 mHz the double white dwarf population forms an unresolved background for
the low-frequency gravitational wave detector LISA. Above this limit some few
thousand double white dwarfs and few tens of binaries containing neutron stars
will be resolved. Of the resolved double white dwarfs ~500 have a total mass
above the Chandrasekhar limit. About ~95 of these have a measurable frequency
change allowing a determination of their chirp mass. We discuss the properties
of the resolved systems.Comment: Accepted for publication in A&
VLT spectroscopy and non-LTE modeling of the C/O-dominated accretion disks in two ultracompact X-ray binaries
We present new medium-resolution high-S/N optical spectra of the ultracompact
low-mass X-ray binaries 4U0614+091 and 4U1626-67, taken with the ESO Very Large
Telescope. They are pure emission line spectra and the lines are identified as
due to C II-IV and O II-III Line identification is corroborated by first
results from modeling the disk spectra with detailed non-LTE radiation transfer
calculations. Hydrogen and helium lines are lacking in the observed spectra.
Our models confirm the deficiency of H and He in the disks. The lack of neon
lines suggests an Ne abundance of less than about 10 percent (by mass),
however, this result is uncertain due to possible shortcomings in the model
atom. These findings suggest that the donor stars are eroded cores of C/O white
dwarfs with no excessive neon overabundance. This would contradict earlier
claims of Ne enrichment concluded from X-ray observations of circumbinary
material, which was explained by crystallization and fractionation of the white
dwarf core.Comment: Accepted for publication in A&A. Alternative download from
http://astro.uni-tuebingen.de/publications/author_title.shtm
The Formation of Cataclysmic Variables: The Influence of Nova Eruptions
The theoretical and observed populations of pre-cataclysmic variables are dominated by systems with low-mass white dwarfs (WDs), while the WD masses in cataclysmic variables (CVs) are typically high. In addition, the space density of CVs is found to be significantly lower than in the theoretical models. We investigate the influence of nova outbursts on the formation and initial evolution of CVs. In particular, we calculate the stability of the mass transfer in the case where all of the material accreted on the WD is lost in classical novae and part of the energy to eject the material comes from a common-envelope-like interaction with the companion. In addition, we study the effect of an asymmetry in the mass ejection that may lead to small eccentricities in the orbit. We find that a common-envelope-like ejection significantly decreases the stability of the mass transfer, particularly for low-mass WDs. Similarly, the influence of asymmetric mass loss can be important for short-period systems and even more so for low-mass WDs; however, this influence likely disappears long before the next nova outburst due to orbital circularization. In both cases the mass-transfer rates increase, which may lead to observable (and perhaps already observed) consequences for systems that do survive to become CVs. However, a more detailed investigation of the interaction between nova ejecta and the companion and the evolution of slightly eccentric CVs is needed before definite conclusions can be drawn
Short-period AM CVn systems as optical, X-ray and gravitational wave sources
We model the population of AM CVn systems in the Galaxy and discuss the
detectability of these systems with optical, X-ray and gravitational wave
detectors. We concentrate on the short period (P < 1500 s) systems, some of
which are expected to be in a phase of direct impact accretion. Using a
self-consistent model for the star formation history and radial distribution of
stars in the Galaxy plus simple models for the emission of optical and X-ray
radiation from the AM CVn systems and interstellar absorption, we derive the
sample of short-period AM CVn systems that can be detected in the optical
and/or X-ray bands. At the shortest periods the detectable systems are all
X-ray sources, some with periods as short as three minutes. At periods above 10
minutes most detectable systems are optical sources. About one third of the
X-ray sources are also detectable in the optical band. We also calculate the
gravitational wave signal of the short-period AM CVn systems. We find that
potentially several thousand AM CVn systems can be resolved by the
gravitational wave detector LISA, comparable to the expected number of detached
double white dwarfs that can be resolved. We estimate that several hundreds of
the AM CVn systems resolvable by LISA, are also detectable in the optical
and/or X-ray bands.Comment: Accepted for publication in MNRA
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