1,195 research outputs found
The population of AM CVn stars from the Sloan Digital Sky Survey
The AM Canum Venaticorum stars are rare interacting white dwarf binaries,
whose formation and evolution are still poorly known. The Sloan Digital Sky
Survey provides, for the first time, a sample of 6 AM CVn stars (out of a total
population of 18) that is sufficiently homogeneous that we can start to study
the population in some detail.
We use the Sloan sample to `calibrate' theoretical population synthesis
models for the space density of AM CVn stars. We consider optimistic and
pessimistic models for different theoretical formation channels, which yield
predictions for the local space density that are more than two orders of
magnitude apart. When calibrated with the observations, all models give a local
space density of 1-3x10^{-6} pc^{-3}, which is lower than expected.
We discuss the implications for the formation of AM CVn stars, and conclude
that at least one of the dominant formation channels (the double-degenerate
channel) has to be suppressed relative to the optimistic models. In the
framework of the current models this suggests that the mass transfer between
white dwarfs usually cannot be stabilized. We furthermore discuss evolutionary
effects that have so far not been considered in population synthesis models,
but which could be of influence for the observed population. We finish by
remarking that, with our lower space density, the expected number of Galactic
AM CVn stars resolvable by gravitational-wave detectors like LISA should be
lowered from current estimates, to about 1,000 for a mission duration of one
year.Comment: Accepted to MNRA
On the orbital periods of the AM CVn stars HP Librae and V803 Centauri
We analyse high-time-resolution spectroscopy of the AM CVn stars HP Librae
and V803 Centauri, taken with the New Technology Telescope (NTT) and the Very
Large Telescope (VLT) of the European Southern Observatory, Chile.
We present evidence that the literature value for V803 Cen's orbital period
is incorrect, based on an observed `S-wave' in the binary's spectrogram. We
measure a spectroscopic period P=1596.4+/-1.2s of the S-wave feature, which is
significantly shorter than the 1611-second periods found in previous
photometric studies. We conclude that the latter period likely represents a
`superhump'. If one assumes that our S-wave period is the orbital period, V803
Cen's mass ratio can be expected to be much less extreme than previously
thought, at q~0.07 rather than q~0.016. This relaxes the constraints on the
masses of the components considerably: the donor star does then not need to be
fully degenerate, and the mass of the accreting white dwarf no longer has to be
very close to the Chandrasekhar limit.
For HP Lib, we similarly measure a spectroscopic period P=1102.8+/-0.2s. This
supports the identification of HP Lib's photometric periods found in the
literature, and the constraints upon the masses derived from them.Comment: Accepted for publication in MNRA
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
UVES and X-Shooter spectroscopy of the emission line AM CVn systems GP Com and V396 Hya
We present time-resolved spectroscopy of the AM CVn-type binaries GP Com and
V396 Hya obtained with VLT/X-Shooter and VLT/UVES. We fully resolve the narrow
central components of the dominant helium lines and determine radial velocity
semi-amplitudes of km s for GP Com and
km s for V396 Hya. The mean velocities of
the narrow central components show variations from line to line. Compared to
calculated line profiles that include Stark broadening we are able to explain
the displacements, and the appearance of forbidden helium lines, by additional
Stark broadening of emission in a helium plasma with an electron density
cm. More than nitrogen and more than
neon lines emission lines were detected in both systems. Additionally,
nitrogen absorption lines are only seen in GP Com. The radial velocity
variations of these lines show the same phase and velocity amplitude as the
central helium emission components. The small semi-amplitude of the central
helium emission component, the consistency of phase and amplitude with the
absorption components in GP Com as well as the measured Stark effect shows that
the central helium emission component, the so-called central-spike, is
consistent with an origin on the accreting white dwarf. We use the dynamics of
the bright spot and the central spike to constrain the binary parameters for
both systems and find a donor mass of - M for GP
Com and - M for V396 Hya. We find an upper limit
for the rotational velocity of the accretor of km s for
GP Com and km s for V396 Hya which excludes a fast
rotating accretor in both systems.Comment: Accepted for publication in MNRAS, 15 pages, 14 figures, 5 table
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 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
Identification of 13 DB + dM and 2 DC + dM binaries from the Sloan Digital Sky Survey
We present the identification of 13 DB + dM binaries and 2 DC + dM binaries
from the Sloan Digital Sky Survey (SDSS). Before the SDSS only 2 DB + dM
binaries and 1 DC + dM binary were known. At least three, possibly 8, of the
new DB + dM binaries seem to have white dwarf temperatures well above 30000 K
which would place them in the so called DB-gap. Finding these DB white dwarfs
in binaries may suggest that they have formed through a different evolutionary
channel than the ones in which DA white dwarfs transform into DB white dwarfs
due to convection in the upper layers.Comment: 4 pages, 2 figures, accepted for publication in A&A Letter
SDSS J124058.03-015919.2: A new AM CVn star with a 37-minute orbital period
We present high time resolution VLT spectroscopy of SDSS J124058.03-015919.2,
a new helium-transferring binary star identified in the Sloan Digital Sky
Survey. We measure an orbital period of 37.355+/-0.002 minutes, confirming the
AM CVn nature of the system. From the velocity amplitudes of the accretor and
the accretion stream--disc impact, we derive a mass ratio q=0.039+/-0.010. Our
spectral coverage extends from 3700A--9500A and shows the presence of helium,
nitrogen, silicon and iron in the accretion disc, plus the redshifted,
low-velocity "central spikes" in the helium lines, known from the low-state AM
CVn stars GP Com and CE 315. Doppler tomography of the helium and silicon
emission lines reveals an unusual pattern of two bright emission sites in the
tomograms, instead of the usual one emission site identified with the impact of
the mass stream into the accretion disc. One of the two is preferred as the
conventional stream--disc impact point in velocity space, at the 3-sigma
confidence level. We speculate briefly on the origin of the second.Comment: Accepted for publication in MNRA
Estimation of aerosol water and chemical composition from AERONET Sunâsky radiometer measurements at Cabauw, the Netherlands
Remote sensing of aerosols provides important information on atmospheric
aerosol abundance. However, due to the hygroscopic nature of aerosol
particles observed aerosol optical properties are influenced by atmospheric
humidity, and the measurements do not unambiguously characterize the aerosol
dry mass and composition, which complicates the comparison with aerosol
models. In this study we derive aerosol water and chemical composition by a
modeling approach that combines individual measurements of remotely sensed
aerosol properties (e.g., optical thickness, single-scattering albedo,
refractive index and size distribution) from an AERONET (Aerosol Robotic
Network) Sunâsky radiometer with radiosonde measurements of relative
humidity. The model simulates water uptake by aerosols based on the chemical
composition (e.g., sulfates, ammonium, nitrate, organic matter and black
carbon) and size distribution. A minimization method is used to calculate
aerosol composition and concentration, which are then compared to in situ
measurements from the Intensive Measurement Campaign At the Cabauw Tower
(IMPACT, May 2008, the Netherlands). Computed concentrations show good
agreement with campaign-average (i.e., 1â14 May) surface observations (mean
bias is 3% for PM<sub>10</sub> and 4â25% for the individual compounds). They
follow the day-to-day (synoptic) variability in the observations and are in
reasonable agreement for daily average concentrations (i.e., mean bias is
5% for PM<sub>10</sub> and black carbon, 10% for the inorganic salts and
18% for organic matter; root-mean-squared deviations are 26% for
PM<sub>10</sub> and 35â45% for the individual compounds). The modeled water
volume fraction is highly variable and strongly dependent on composition.
During this campaign we find that it is >0.5 at approximately 80% relative humidity
(RH) when the aerosol composition is dominated by hygroscopic inorganic salts, and
<0.1 when RH is below 40%, especially when the composition is
dominated by less hygroscopic compounds such as organic matter. The
scattering enhancement factor (f(RH), the ratio of the scattering coefficient
at 85% RH and its dry value at 676 nm) during 1â14 May is
2.6 ± 0.5. The uncertainty in AERONET (real) refractive index
(0.025â0.05) is the largest source of uncertainty in the modeled aerosol
composition and leads to an uncertainty of 0.1â0.25 (50â100%) in aerosol
water volume fraction. Our methodology performs relatively well at Cabauw,
but a better performance may be expected for regions with higher aerosol
loading where the uncertainties in the AERONET inversions are smaller
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