1,718 research outputs found
Precise Atmospheric Parameters for the Shortest Period Binary White Dwarfs: Gravitational Waves, Metals, and Pulsations
We present a detailed spectroscopic analysis of 61 low mass white dwarfs and
provide precise atmospheric parameters, masses, and updated binary system
parameters based on our new model atmosphere grids and the most recent
evolutionary model calculations. For the first time, we measure systematic
abundances of He, Ca and Mg for metal-rich extremely low mass white dwarfs and
examine the distribution of these abundances as a function of effective
temperature and mass. Based on our preliminary results, we discuss the
possibility that shell flashes may be responsible for the presence of the
observed He and metals. We compare stellar radii derived from our spectroscopic
analysis to model-independent measurements and find good agreement except for
those white dwarfs with Teff < 10,000 K. We also calculate the expected
gravitational wave strain for each system and discuss their significance to the
eLISA space-borne gravitational wave observatory. Finally, we provide an update
on the instability strip of extremely low mass white dwarf pulsators.Comment: 18 pages, 13 figures, 3 tables, accepted for publication in Ap
3D Model Atmospheres for Extremely Low-Mass White Dwarfs
We present an extended grid of mean three-dimensional (3D) spectra for
low-mass, pure-hydrogen atmosphere DA white dwarfs (WDs). We use CO5BOLD
radiation-hydrodynamics 3D simulations covering Teff = 6000-11,500 K and logg =
5-6.5 (cgs units) to derive analytical functions to convert spectroscopically
determined 1D temperatures and surface gravities to 3D atmospheric parameters.
Along with the previously published 3D models, the 1D to 3D corrections are now
available for essentially all known convective DA WDs (i.e., logg = 5-9). For
low-mass WDs, the correction in temperature is relatively small (a few per cent
at the most), but the surface gravities measured from the 3D models are lower
by as much as 0.35 dex. We revisit the spectroscopic analysis of the extremely
low-mass (ELM) WDs, and demonstrate that the 3D models largely resolve the
discrepancies seen in the radius and mass measurements for relatively cool ELM
WDs in eclipsing double WD and WD + milli-second pulsar binary systems. We also
use the 3D corrections to revise the boundaries of the ZZ Ceti instability
strip, including the recently found ELM pulsators.Comment: 11 pages, 8 figures, accepted for publication in the Astrophysical
Journa
On Gauge-Invariant Decomposition of Nucleon Spin
We investigate the relation between the known decompositions of the nucleon
spin into its constituents, thereby clarifying in what respect they are common
and in what respect they are different essentially. The decomposition recently
proposed by Chen et al. can be thought of as a nontrivial generalization of the
gauge-variant Jaffe-Manohar decomposition so as to meet the gauge-invariance
requirement of each term of the decomposition. We however point out that there
is another gauge-invariant decomposition of the nucleon spin, which is closer
to the Ji decomposition, while allowing the decomposition of the gluon total
angular momentum into the spin and orbital parts. After clarifying the reason
why the gauge-invariant decomposition of the nucleon spin is not unique, we
discuss which decomposition is more preferable from the experimental viewpoint.Comment: The version to appear in Phys. Rev.
The Rapid Rotation of the Strongly Magnetic Ultramassive White Dwarf EGGR 156
The distribution of white dwarf rotation periods provides a means for
constraining angular momentum evolution during the late stages of stellar
evolution, as well as insight into the physics and remnants of double
degenerate mergers. Although the rotational distribution of low mass white
dwarfs is relatively well constrained via asteroseismology, that of high mass
white dwarfs, which can arise from either intermediate mass stellar evolution
or white dwarf mergers, is not. Photometric variability in white dwarfs due to
rotation of a spotted star is rapidly increasing the sample size of high mass
white dwarfs with measured rotation periods. We present the discovery of 22.4
minute photometric variability in the lightcurve of EGGR 156, a strongly
magnetic, ultramassive white dwarf. We interpret this variability as rapid
rotation, and our data suggest that EGGR 156 is the remnant of a double
degenerate merger. Finally, we calculate the rate of period change in rapidly
rotating, massive, magnetic WDs due to magnetic dipole radiation. In many
cases, including EGGR 156, the period change is not currently detectable over
reasonable timescales, indicating that these WDs could be very precise clocks.
For the most highly magnetic, rapidly rotating massive WDs, such as ZTF
J1901+1450 and RE J0317853, the period change should be detectable and may
help constrain the structure and evolution of these exotic white dwarfs.Comment: Replaced to correct two typos in equations on page 12. No
calculations or conclusions affected. 15 pages, 5 figures, accepted for
publication in the Astronomical Journa
Destroying Aliases from the Ground and Space: Super-Nyquist ZZ Cetis in K2 Long Cadence Data
With typical periods of order 10 minutes, the pulsation signatures of ZZ Ceti
variables (pulsating hydrogen-atmosphere white dwarf stars) are severely
undersampled by long-cadence (29.42 minutes per exposure) K2 observations.
Nyquist aliasing renders the intrinsic frequencies ambiguous, stifling
precision asteroseismology. We report the discovery of two new ZZ Cetis in
long-cadence K2 data: EPIC 210377280 and EPIC 220274129. Guided by 3-4 nights
of follow-up, high-speed (<=30 s) photometry from McDonald Observatory, we
recover accurate pulsation frequencies for K2 signals that reflected 4-5 times
off the Nyquist with the full precision of over 70 days of monitoring (~0.01
muHz). In turn, the K2 observations enable us to select the correct peaks from
the alias structure of the ground-based signals caused by gaps in the
observations. We identify at least seven independent pulsation modes in the
light curves of each of these stars. For EPIC 220274129, we detect three
complete sets of rotationally split ell=1 (dipole mode) triplets, which we use
to asteroseismically infer the stellar rotation period of 12.7+/-1.3 hr. We
also detect two sub-Nyquist K2 signals that are likely combination (difference)
frequencies. We attribute our inability to match some of the K2 signals to the
ground-based data to changes in pulsation amplitudes between epochs of
observation. Model fits to SOAR spectroscopy place both EPIC 210377280 and EPIC
220274129 near the middle of the ZZ Ceti instability strip, with Teff =
11590+/-200 K and 11810+/-210 K, and masses 0.57+/-0.03 Msun and 0.62+/-0.03
Msun, respectively.Comment: 13 pages, 9 figures, 7 tables; accepted for publication in Ap
Avaliação de impacto ambiental em estabelecimentos de pecuária de leite com manejo orgânico e convencional.
Este trabalho teve por objetivo avaliar o impacto ambiental da pecuária de leite convencional e orgânica em dois estabelecimentos representativos do município de Serra Negra, SP, utilizando o sistema APOIA ? Novo Rural. A pecuária orgânica apresentou um índice geral de impacto ambiental (0,77)superior ao da pecuária convencional (0,67), sendo que as dimensões Ecologia da Paisagem, Qualidade da Água, Valores Econômicos e Gestão e Administração foram as que melhor qualificaram essa atividade em termos de sua contribuição para o desenvolvimento local sustentável
A Dark Spot on a Massive White Dwarf
We present the serendipitous discovery of eclipse-like events around the
massive white dwarf SDSS J152934.98+292801.9 (hereafter J1529+2928). We
selected J1529+2928 for time-series photometry based on its spectroscopic
temperature and surface gravity, which place it near the ZZ Ceti instability
strip. Instead of pulsations, we detect photometric dips from this white dwarf
every 38 minutes. Follow-up optical spectroscopy observations with Gemini
reveal no significant radial velocity variations, ruling out stellar and brown
dwarf companions. A disintegrating planet around this white dwarf cannot
explain the observed light curves in different filters. Given the short period,
the source of the photometric dips must be a dark spot that comes into view
every 38 min due to the rotation of the white dwarf. Our optical spectroscopy
does not show any evidence of Zeeman splitting of the Balmer lines, limiting
the magnetic field strength to B<70 kG. Since up to 15% of white dwarfs display
kG magnetic fields, such eclipse-like events should be common around white
dwarfs. We discuss the potential implications of this discovery on transient
surveys targeting white dwarfs, like the K2 mission and the Large Synoptic
Survey Telescope.Comment: ApJ Letters, in pres
Research in the general area of non-linear dynamical systems Final report, 8 Jun. 1965 - 8 Jun. 1967
Nonlinear dynamical systems research on systems stability, invariance principles, Liapunov functions, and Volterra and functional integral equation
- …