842 research outputs found
A Survey for Photometric Variability in Isolated Magnetic White DwarfsâMeasuring their Spin Periods
We present the initial findings of a photometric survey of isolated magnetic white dwarfs (MWDs) carried out with the 1.0m Jacobus Kapteyn Telescope. Of our sample of 30 MWDs, we have observed variability in 17 (57%) over our observed timescales (minutes to years), with a further 11 requiring more data, and two that are non-variable at the 1% level. In total we have discovered possible variability in 15 targets that has not been reported before in the literature, and we have measured the spin period of five objects in our sample to within a few percent. We find no correlation between spin period, mass or temperature, but there may be a weak negative correlation between period and field strength for the short-period targets. We have identified 14 MWDs with low field strengths and low temperatures, which are candidates for having star spots on their surfaces and should be followed up with polarimetry. We have also found that three low-field, high temperature MWDs are unexpectedly variable, with no obvious mechanism to cause this
Modulation of Low-Altitude Ionospheric Upflow by Linear and Nonlinear Atmospheric Gravity Waves
This study examines how thermospheric motions due to gravity waves (GWs) drive ion upflow in the F region, modulating the topside ionosphere in a way that can contribute to ion outflow. We present incoherent scatter radar data from Sondrestrom, from 31 May 2003 which showed upflow/downflow motions, having a downward phase progression, in the fieldâaligned velocity, indicating forcing by a thermospheric GW. The GWâupflow coupling dynamics are investigated through the use of a coupled atmosphereâionosphere model to examine potential impacts on topside ionospheric upflow. Specifically, a sequence of simulations with varying wave amplitude is conducted to determine responses to a range of transient forcing reminiscent of the incoherent scatter radar data. Nonlinear wave effects, resulting from increases in amplitude of the modeled GW, are shown to critically impact the ionospheric response. GW breaking deposits energy into smaller scale wave modes, drives periods of large fieldâaligned ion velocities, while also modulating ion densities. Complementary momentum transfer increases the mean flow and, through ionâneutral drag, can increase ion densities above 300 km. Ionospheric collision frequency (cooling) and photoionization effects (heating), both dependent on ionospheric density, modify the electron temperature; these changes conduct quickly up geomagnetic field lines driving ion upflow at altitudes well above initial disturbances. This flow alters ion populations available for highâaltitude acceleration processes that may lead to outflow into the magnetosphere. We have included a representative source of transverse wave heating which, when supplemented by our GWs, illustrates strengthened upward fluxes in the topside ionosphere
Photometric Variability and Rotation in Magnetic White Dwarfs
We present a search for long term (monthsâyears) photometric variability in a sample of ten isolated magnetic white dwarfs using observations taken with the Liverpool Robotic Telescope between March 2005 and January 2007. These stars had previously been found to be photometrically stable on short (hoursâone week) timescales [1]. We construct differential light curves for each target and then use CLEAN and LombâScargle periodograms to determine any periodicity that may be present. Photometric variability is detected in two of the targets during the observed timescaleâG 240â72 and G 227â28. We find no variability in the remaining eight targets above the 1% level. Finally, we search for any correlations between the spin periods and intrinsic physical properties of magnetic white dwarfs, such as the magnetic field strength, temperature, mass and age
Spatial Structure and Collisionless Electron Heating in Balmer-dominated Shocks
Balmer-dominated shocks in supernova remnants (SNRs) produce strong hydrogen
lines with a two-component profile composed of a narrow contribution from cold
upstream hydrogen atoms, and a broad contribution from hydrogen atoms that have
undergone charge transfer reactions with hot protons. Observations of emission
lines from edge-wise shocks in SNRs can constrain the gas velocity and
collisionless electron heating at the shock front. Downstream hydrogen atoms
engage in charge transfer, excitation and ionization reactions, defining an
interaction region called the shock transition zone. The properties of hot
hydrogen atoms produced by charge transfers (called broad neutrals) are
critical for accurately calculating the structure and radiation from the shock
transition zone. This paper is the third in a series describing the kinetic,
fluid and emission properties of Balmer-dominated shocks, and is the first to
properly treat the effect of broad neutral kinetics on shock transition zone
structure. We use our models to extract shock parameters from observations of
Balmer-dominated SNRs. We find that inferred shock velocities and electron
temperatures are lower than those of previous calculations by <10% for v_s<1500
km/s, and by 10-30% for v_s>1500 km/s. This effect is primarily due to the fact
that excitation by proton collisions and charge transfer to excited levels
favor the high speed part of the neutral hydrogen velocity distribution. Our
results have a strong dependence on the ratio of electron to proton
temperatures, \beta=T_e/T_p, which allows us to construct a relation \beta(v_s)
between the temperature ratio and shock velocity. We compare our calculations
to previous results by Ghavamian et al. (2007).Comment: 41 pages, 15 figures, 2 tables. Improved comparison to previous
results, added discussion, and incorporated referee's suggestions. Submitted
to Ap
Born Again Protoplanetary Disk Around Mira B
The Mira AB system is a nearby (~107 pc) example of a wind accreting binary
star system. In this class of system, the wind from a mass-losing red giant
star (Mira A) is accreted onto a companion (Mira B), as indicated by an
accretion shock signature in spectra at ultraviolet and X-ray wavelengths.
Using novel imaging techniques, we report the detection of emission at
mid-infrared wavelengths between 9.7 and 18.3 m from the vicinity of Mira
B but with a peak at a radial position about 10 AU closer to the primary Mira
A. We interpret the mid-infrared emission as the edge of an optically-thick
accretion disk heated by Mira A. The discovery of this new class of accretion
disk fed by M-giant mass loss implies a potential population of young planetary
systems in white-dwarf binaries which has been little explored, despite being
relatively common in the solar neighborhood.Comment: Accepted for Ap
A comparison of DA white dwarf temperatures and gravities from FUSE Lyman line and ground-based Balmer line observations
Observation of the strengths and profiles of the hydrogen Balmer absorption
series is an established technique for determining the effective temperature
and surface gravity of hot H-rich white dwarf stars. In principle, the Lyman
series lines should be equally useful but, lying in the far-UV, are only
accessible from space. Nevertheless, there are situations (for example, where
the optical white dwarf spectrum is highly contaminated by the presence of a
companion) in which use of the Lyman series may be essential. Therefore, it is
important to establish whether or not the Lyman lines provide an equally valid
means of measurement. We have already made a first attempt to study this
problem, comparing Lyman line measurements from a variety of far-UV instruments
with ground-based Balmer line studies. Within the measurement uncertainties we
found the results from each line series to be broadly in agreement. However, we
noted a number of potential systematic effects that could bias either
measurement. With the availability of the FUSE data archive and observations
from our own Guest Observer programmes, we now have an opportunity of examining
the use of the Lyman series in more detail from observations of 16 DA white
dwarfs. The new results partially reproduce the earlier study, showing that
Balmer and Lyman line determined temperatures are in good agreement up to
\~50000K. However, above this value there is an increasing systematic
difference between the Lyman and Balmer line results, the former yielding the
higher temperature.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Society: 15 pages (inc 10 figures
Rotational period of WD1953-011 - a magnetic white dwarf with a star spot
WD1953-011 is an isolated, cool (7920 +/- 200K, Bergeron, Legget & Ruiz,
2001) magnetic white dwarf (MWD) with a low average field strength (~70kG,
Maxted et al. 2000) and a higher than average mass (~0.74 M_sun, Bergeron et
al. 2001). Spectroscopic observations taken by Maxted et al. (2000) showed
variations of equivalent width in the Balmer lines, unusual in a low field
white dwarf. Here we present V band photometry of WD1953-011 taken at 7 epochs
over a total of 22 months. All of the datasets show a sinusoidal variation of
approximately 2% peak-to-peak amplitude. We propose that these variations are
due to a star spot on the MWD, analogous to a sunspot, which is affecting the
temperature at the surface, and therefore its photometric magnitude. The
variations have a best-fit period over the entire 22 months of 1.4418 days,
which we interpret as the rotational period of the WD.Comment: (1) University of Southampton, (2) University of Warwick, (3)
University of Nijmegen, (4) Keele University, (5) University of Leicester. 6
pages, 5 figs, accepted MNRA
Stellar archaeology with Gaia: the Galactic white dwarf population
Gaia will identify several 1e5 white dwarfs, most of which will be in the
solar neighborhood at distances of a few hundred parsecs. Ground-based optical
follow-up spectroscopy of this sample of stellar remnants is essential to
unlock the enormous scientific potential it holds for our understanding of
stellar evolution, and the Galactic formation history of both stars and
planets.Comment: Summary of a talk at the 'Multi-Object Spectroscopy in the Next
Decade' conference in La Palma, March 2015, to be published in ASP Conference
Series (editors Ian Skillen & Scott Trager
Strengthening spatial reasoning: elucidating the attentional and neural mechanisms associated with mental rotation skill development
© 2020, The Author(s). Spatial reasoning is a critical skill in many everyday tasks and in science, technology, engineering, and mathematics disciplines. The current study examined how training on mental rotation (a spatial reasoning task) impacts the completeness of an encoded representation and the ability to rotate the representation. We used a multisession, multimethod design with an active control group to determine how mental rotation ability impacts performance for a trained stimulus category and an untrained stimulus category. Participants in the experimental group (n = 18) showed greater improvement than the active control group (n = 18) on the mental rotation tasks. The number of saccades between objects decreased and saccade amplitude increased after training, suggesting that participants in the experimental group encoded more of the object and possibly had more complete mental representations after training. Functional magnetic resonance imaging data revealed distinct neural activation associated with mental rotation, notably in the right motor cortex and right lateral occipital cortex. These brain areas are often associated with rotation and encoding complete representations, respectively. Furthermore, logistic regression revealed that activation in these brain regions during the post-training scan significantly predicted training group assignment. Overall, the current study suggests that effective mental rotation training protocols should aim to improve the encoding and manipulation of mental representations
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