39 research outputs found
Kinematics of the ultracompact helium accretor AM canum venaticorum
We report on the results from a five-night campaign of high-speed spectroscopy of the 17-min binary AM Canum Venaticorum (AM CVn), obtained with the 4.2-m William Herschel Telescope on La Palma.
We detect a kinematic feature that appears to be entirely analogous to the 'central spike' known from the long-period, emission-line AM CVn stars GP Com, V396 Hya and SDSS J124058.03-015919.2, which has been attributed to the accreting white dwarf. Assuming that the feature indeed represents the projected velocity amplitude and phase of the accreting white dwarf, we derive a mass ratio q = 0.18 +/- 0.01 for AM CVn. This is significantly higher than the value found in previous, less direct measurements. We discuss the implications for AM CVn's evolutionary history and show that a helium star progenitor scenario is strongly favoured. We further discuss the implications for the interpretation of AM CVn's superhump behaviour, and for the detectability of its gravitational-wave signal with the Laser Interferometer Space Antenna (LISA).
In addition, we demonstrate a method for measuring the circularity or eccentricity of AM CVn's accretion disc, using stroboscopic Doppler tomography. We test the predictions of an eccentric, precessing disc that are based on AM CVn's observed superhump behaviour. We limit the effective eccentricity in the outermost part of the disc, where the resonances that drive the eccentricity are thought to occur, to e = 0.04 +/- 0.01, which is smaller than previous models indicated
Phase-resolved spectroscopy of the helium dwarf nova 'SN 2003aw' in quiescence
High time resolution spectroscopic observations of the ultracompact helium dwarf nova 'SN 2003aw' in its quiescent state at V similar to 20.5 reveal its orbital period at 2027.8 +/- 0.5 s or 33.80 min. Together with the photometric 'superhump' period of 2041.5 +/- 0.5 s, this implies a mass ratio q approximate to 0.036. We compare both the average and time-resolved spectra of 'SN 2003aw' and Sloan Digital Sky Survey (SDSS) J124058.03-015919.2. Both show a DB white dwarf spectrum plus an optically thin, helium-dominated accretion disc. 'SN 2003aw' distinguishes itself from the SDSS source by its strong calcium H & K emission lines, suggesting higher abundances of heavy metals than the SDSS source. The silicon and iron emission lines observed in the SDSS source are about twice as strong in 'SN 2003aw'. The peculiar 'double bright spot' accretion disc feature seen in the SDSS source is also present in time-resolved spectra of 'SN 2003aw', albeit much weaker
Massive donors in interacting binaries: effect of metallicity
Metallicity is known to significantly affect the radial expansion of a
massive star: the lower the metallicity, the more compact the star, especially
during its post-MS evolution. We study this effect in the context of binary
evolution. Using the stellar-evolution code MESA, we computed evolutionary
tracks of stars at different metallicities, exploring variations of factors
known to affect the radial expansion (e.g. semiconvection, overshooting,
rotation). We find observational support for an evolution in which already at
metallicity massive stars remain relatively compact during the
Hertzprung-Gap (HG) phase and most of their expansion occurs during core-helium
burning (CHeB). Consequently, we show that metallicity has a strong influence
on the type of mass transfer evolution in binary systems. At solar metallicity,
a case-B mass transfer is initiated shortly after the end of MS, and a giant
donor is almost always a rapidly expanding HG star. At lower metallicity, the
parameter space for mass transfer from a more evolved CHeB star increases
dramatically. This means that envelope stripping and formation of helium stars
in low-metallicity environments occurs later in the evolution of the donor,
implying a much shorter duration of the Wolf-Rayet phase (even by an order of
magnitude) and higher final core masses. This metallicity effect is independent
of the impact of metallicity-dependent stellar winds. At very low
metallicities, a significant fraction of massive stars in binaries engages in
the first episode of mass transfer very late into their evolution, when they
already have a well-developed CO core. The remaining lifetime ( yr) is
unlikely to be enough to strip the entire H-rich envelope. We also briefly
discuss the extremely small parameter space for mass transfer from massive
convective-envelope donors in the context of binary black hole merger
formation.Comment: 15 pages, 8 figures (+ 4 pages, 4 fig. appendix), to appear in A&
Prospects for detection of detached double white dwarf binaries with Gaia, LSST and LISA
Double white dwarf (DWD) binaries are expected to be very common in the Milky
Way, but their intrinsic faintness challenges the detection of these systems.
Currently, only a few tens of detached DWDs are know. Such systems offer the
best chance of extracting the physical properties that would allow us to
address a wealth of outstanding questions ranging from the nature of white
dwarfs, over stellar and binary evolution to mapping the Galaxy. In this paper
we explore the prospects for detections of ultra-compact (with binary
separations of a few solar radii or less) detached DWDs in: 1) optical
radiation with Gaia and the LSST and 2) gravitational wave radiation with LISA.
We show that Gaia, LSST and LISA have the potential to detect respectively
around a few hundreds, a thousand, and 25 thousand DWD systems. Moreover, Gaia
and LSST data will extend by respectively a factor of two and seven the
guaranteed sample of binaries detected in electromagnetic and gravitational
wave radiation, opening the era of multi-messenger astronomy for these sources.Comment: submitted to MNRA
Simulation of the White Dwarf -- White Dwarf galactic background in the LISA data
LISA (Laser Interferometer Space Antenna) is a proposed space mission, which
will use coherent laser beams exchanged between three remote spacecraft to
detect and study low-frequency cosmic gravitational radiation. In the low-part
of its frequency band, the LISA strain sensitivity will be dominated by the
incoherent superposition of hundreds of millions of gravitational wave signals
radiated by inspiraling white-dwarf binaries present in our own galaxy. In
order to estimate the magnitude of the LISA response to this background, we
have simulated a synthesized population that recently appeared in the
literature. We find the amplitude of the galactic white-dwarf binary background
in the LISA data to be modulated in time, reaching a minimum equal to about
twice that of the LISA noise for a period of about two months around the time
when the Sun-LISA direction is roughly oriented towards the Autumn equinox.
Since the galactic white-dwarfs background will be observed by LISA not as a
stationary but rather as a cyclostationary random process with a period of one
year, we summarize the theory of cyclostationary random processes and present
the corresponding generalized spectral method needed to characterize such
process. We find that, by measuring the generalized spectral components of the
white-dwarf background, LISA will be able to infer properties of the
distribution of the white-dwarfs binary systems present in our Galaxy.Comment: 14 pages and 6 figures. Submitted to Classical and Quantum Gravity
(Proceedings of GWDAW9
Binaries discovered by the SPY project. IV, Five single-lined DA double white dwarfs
We present results from our ongoing follow-up observations of double white dwarf binaries detected in the ESO SN Ia Progenitor SurveY (SPY). We discuss our observing strategy and data analysis and present the orbital solutions of five close double white dwarf binaries: HE0320â1917, HE1511â0448, WD0326â273, WD1013â010 and WD1210+140. Their periods range from 0.44 to 3.22 days. In none of these systems we find any spectral lines originating from the companion. This rules out main sequence companions and indicates that the companion white dwarfs are significantly older and cooler than the bright component. Infrared photometry suggests the presence of a cool, helium-rich white dwarf companion in the binary WD 0326â273. We briefly discuss the consequences of our findings for our understanding of the formation and evolution of double white dwarfs
The fast transient sky with Gaia
The ESA Gaia satellite scans the whole sky with a temporal sampling ranging
from seconds and hours to months. Each time a source passes within the Gaia
field of view, it moves over 10 CCDs in 45 s and a lightcurve with 4.5 s
sampling (the crossing time per CCD) is registered. Given that the 4.5 s
sampling represents a virtually unexplored parameter space in optical time
domain astronomy, this data set potentially provides a unique opportunity to
open up the fast transient sky. We present a method to start mining the wealth
of information in the per CCD Gaia data. We perform extensive data filtering to
eliminate known on-board and data processing artefacts, and present a
statistical method to identify sources that show transient brightness
variations on ~2 hours timescales. We illustrate that by using the Gaia
photometric CCD measurements, we can detect transient brightness variations
down to an amplitude of 0.3 mag on timescales ranging from 15 seconds to
several hours. We search an area of ~23.5 square degrees on the sky, and find
four strong candidate fast transients. Two candidates are tentatively
classified as flares on M-dwarf stars, while one is probably a flare on a giant
star and one potentially a flare on a solar type star. These classifications
are based on archival data and the timescales involved. We argue that the
method presented here can be added to the existing Gaia Science Alerts
infrastructure for the near real-time public dissemination of fast transient
events.Comment: 10 pages, 5 figures and 5 tables; MNRAS in pres
The expanding bipolar shell of the helium nova V445 Puppis
From multi-epoch adaptive optics imaging and integral field unit spectroscopy, we report the discovery of an expanding and narrowly confined bipolar shell surrounding the helium nova V445 Puppis (Nova Puppis 2000). An equatorial dust disc obscures the nova remnant, and the outflow is characterized by a large polar outflow velocity of 6720 +/- 650 km s(-1) and knots moving at even larger velocities of 8450 +/- 570 km s(-1). We derive an expansion parallax distance of 8.2 +/- 0.5 kpc and deduce a pre-outburst luminosity of the underlying binary of log L/L-circle dot = 4.34 +/- 0.36. The derived luminosity suggests that V445 Puppis probably contains a massive white dwarf accreting at high rate from a helium star companion making it part of a population of binary stars that potentially lead to supernova Ia explosions due to accumulation of helium-rich material on the surface of a massive white dwarf
An irradiated-Jupiter analogue hotter than the Sun
Planets orbiting close to hot stars experience intense extreme-ultraviolet
radiation, potentially leading to atmosphere evaporation and to thermal
dissociation of molecules. However, this extreme regime remains mainly
unexplored due to observational challenges. Only a single known ultra-hot giant
planet, KELT-9b, receives enough ultraviolet radiation for molecular
dissociation, with a day-side temperature of ~4,600K. An alternative approach
uses irradiated brown dwarfs as hot-Jupiter analogues. With atmospheres and
radii similar to those of giant planets, brown dwarfs orbiting close to hot
Earth-sized white-dwarf stars can be directly detected above the glare of the
star. Here we report observations revealing an extremely irradiated low-mass
companion to the hot white dwarf WD0032-317. Our analysis indicates a day-side
temperature of ~8,000K, and a day-to-night temperature difference of ~6,000K.
The amount of extreme-ultraviolet radiation (with wavelengths 100-912\r{A})
received by WD0032-317B is equivalent to that received by planets orbiting
close to stars as hot as a late B-type stars, and about 5,600 times higher than
that of KELT-9b. With a mass of ~75-88 Jupiter masses, this
near-hydrogen-burning-limit object is potentially one of the most massive brown
dwarfs known.Comment: Authors' version of the article published in Nature Astronomy (DOI
https://doi.org/10.1038/s41550-023-02048-z
Faint Thermonuclear Supernovae from AM Canum Venaticorum Binaries
Helium that accretes onto a Carbon/Oxygen white dwarf in the double white
dwarf AM Canum Venaticorum (AM CVn) binaries undergoes unstable thermonuclear
flashes when the orbital period is in the 3.5-25 minute range. At the shortest
orbital periods (and highest accretion rates, Mdot > 10^-7 Msol/yr), the
flashes are weak and likely lead to the Helium equivalent of classical nova
outbursts. However, as the orbit widens and Mdot drops, the mass required for
the unstable ignition increases, leading to progressively more violent flashes
up to a final flash with Helium shell mass ~ 0.02-0.1 Msol. The high pressures
of these last flashes allow the burning to produce the radioactive elements
48Cr, 52Fe, and 56Ni that power a faint (M_V in the range of -15 to -18) and
rapidly rising (few days) thermonuclear supernova. Current galactic AM CVn
space densities imply one such explosion every 5,000-15,000 years in 10^11 Msol
of old stars (~ 2-6% of the Type Ia rate in E/SO galaxies). These ".Ia"
supernovae (one-tenth as bright for one-tenth the time as a Type Ia supernovae)
are excellent targets for deep (e.g. V=24) searches with nightly cadences,
potentially yielding an all-sky rate of 1,000 per year.Comment: To appear in The Astrophysical Journal Letters; 4 pages, 3 figures.
Expected rates somewhat reduced due to lowered galactic density of AM CVn
binarie