1,144 research outputs found
The Quiescent Spectrum of the AM CVn star CP Eri
We used the 6.5m MMT to obtain a spectrum of the AM CVn star CP Eri in
quiescence. The spectrum is dominated by He I emission lines, which are clearly
double peaked with a peak-to-peak separation of ~1900 km/s. The spectrum is
similar to that of the longer period AM CVn systems GP Com and CE 315, linking
the short and the long period AM CVn systems. In contrast with GP Com and CE
315, the spectrum of CP Eri does not show a central 'spike' in the line
profiles, but it does show lines of SiII in emission. The presence of these
lines indicates that the material being transferred is of higher metallicity
than in GP Com and CE 315, which, combined with the low proper motion of the
system, probably excludes a halo origin of the progenitor of CP Eri. We
constrain the primary mass to M_1>0.27 M_sun and the orbital inclination to 33
degr < i < 80 degr. The presence of the He I lines in emission opens up the
possibility for phase resolved spectroscopic studies which allows a
determination of the system parameters and a detailed study of helium accretion
disks under highly varying circumstances.Comment: 12 pages, 2 figures, accepted for publication in ApJ Letter
The Helium-Rich Cataclysmic Variable ES Ceti
We report photometry of the helium-rich cataclysmic variable ES Ceti during
2001-2004. The star is roughly stable at V ~ 17.0 and has a light curve
dominated by a single period of 620 s, which remains measurably constant over
the 3 year baseline. The weight of evidence suggests that this is the true
orbital period of the underlying binary, not a "superhump" as initially
assumed. We report GALEX ultraviolet magnitudes, which establish a very blue
flux distribution (F_nu ~ nu^1.3), and therefore a large bolometric correction.
Other evidence (the very strong He II 4686 emission, and a ROSAT detection in
soft X-rays) also indicates a strong EUV source, and comparison to
helium-atmosphere models suggests a temperature of 130+-10 kK. For a distance
of 350 pc, we estimate a luminosity of (0.8-1.7)x10^34 erg/s, yielding a mass
accretion rate of (2-4)x10^-9 M_sol/yr onto an assumed 0.7 M_sol white dwarf.
This appears to be about as expected for white dwarfs orbiting each other in a
10 minute binary, assuming that mass transfer is powered by gravitational
radiation losses. We estimate mean accretion rates for other helium-rich
cataclysmic variables, and find that they also follow the expected M-dot ~
P_o^-5 relation. There is some evidence (the lack of superhumps, and the small
apparent size of the luminous region) that the mass transfer stream in ES Cet
directly strikes the white dwarf, rather than circularizing to form an
accretion disk.Comment: PDF, 26 pages, 3 tables, 9 figures; accepted, in press, to appear
February 2005, PASP; more info at http://cba.phys.columbia.edu
On Echo Outbursts and ER UMa Supercycles in SU UMa-type Cataclysmic Variables
I present a variation on Osaki's tidal-thermal-instability model for SU UMa
behavior. I suggest that in systems with the lowest mass ratios, the
angular-momentum dissipation in an eccentric disk is unable to sustain the disk
on the hot side of the thermal instability. This decoupling of the tidal and
thermal instabilities in systems with q < 0.07 allows a better explanation of
the `echo' outbursts of EG Cnc and the short supercycles of RZ LMi and DI UMa.
The idea might also apply to the soft X-ray transients.Comment: To appear in PASP, April 2001 (6 pages, 4 figs
Coaxial Jets and Sheaths in Wide-Angle-Tail Radio Galaxies
We add 20, 6 and 3.6 cm wavelength VLA observations of two WATs, 1231+674 and
1433+553, to existing VLA data at 6 and 20 cm, in order to study the variations
of spectral index as a function of position. We apply the spectral tomography
process that we introduced in our analysis of 3C67, 3C190 and 3C449. Both
spectral tomography and polarization maps indicate that there are two distinct
extended components in each source. As in the case of 3C449, we find that each
source has a flat spectrum jet surrounded by a steeper spectrum sheath. The
steep components tend to be more highly polarized than the flat components. We
discuss a number of possibilities for the dynamics of the jet/sheath systems,
and the evolution of their relativistic electron populations. While the exact
nature of these two coaxial components is still uncertain, their existence
requires new models of jets in FR I sources and may also have implications for
the dichotomy between FR Is and FR IIs.Comment: 29 text pages plus 13 figures. Scheduled for publication in May 10,
1999 Ap
Persistence in Cluster--Cluster Aggregation
Persistence is considered in diffusion--limited cluster--cluster aggregation,
in one dimension and when the diffusion coefficient of a cluster depends on its
size as . The empty and filled site persistences are
defined as the probabilities, that a site has been either empty or covered by a
cluster all the time whereas the cluster persistence gives the probability of a
cluster to remain intact. The filled site one is nonuniversal. The empty site
and cluster persistences are found to be universal, as supported by analytical
arguments and simulations. The empty site case decays algebraically with the
exponent . The cluster persistence is related to the
small behavior of the cluster size distribution and behaves also
algebraically for while for the behavior is
stretched exponential. In the scaling limit and with fixed the distribution of intervals of size between
persistent regions scales as , where is the average interval size and . For finite the
scaling is poor for , due to the insufficient separation of the two
length scales: the distances between clusters, , and that between
persistent regions, . For the size distribution of persistent regions
the time and size dependences separate, the latter being independent of the
diffusion exponent but depending on the initial cluster size
distribution.Comment: 14 pages, 12 figures, RevTeX, submitted to Phys. Rev.
MMT Observations of the Black Hole Candidate XTE J1118+480 near and in Quiescence
We report on the analysis of new and previously published MMT optical spectra
of the black hole binary XTE J1118+480 during the decline from the 2000
outburst to true quiescence. From cross-correlation with template stars, we
measure the radial velocity of the secondary to derive a new spectroscopic
ephemeris. The observations acquired during approach to quiescence confirm the
earlier reported modulation in the centroid of the double-peaked Halpha
emission line. Additionally, our data combined with the results presented by
Zurita et al. (2002) provide support for a modulation with a periodicity in
agreement with the expected precession period of the accretion disk of ~52 day.
Doppler images during the decline phase of the Halpha emission line show
evidence for a hotspot and emission from the gas stream: the hotspot is
observed to vary its position, which may be due to the precession of the disk.
The data available during quiescence show that the centroid of the Halpha
emission line is offset by about -100 km/s from the systemic velocity which
suggests that the disk continues to precess. A Halpha tomogram reveals emission
from near the donor star after subtraction of the ring-like contribution from
the accretion disk which we attribute to chromospheric emission. No hotspot is
present suggesting that accretion from the secondary has stopped (or decreased
significantly) during quiescence. Finally, a comparison is made with the black
hole XRN GRO J0422+32: we show that the Halpha profile of this system also
exhibits a behaviour consistent with a precessing disk.Comment: 11 pages, 5 figures, accepted by Ap
Mode Identification from Combination Frequency Amplitudes in ZZ Ceti Stars
The lightcurves of variable DA stars are usually multi-periodic and
non-sinusoidal, so that their Fourier transforms show peaks at eigenfrequencies
of the pulsation modes and at sums and differences of these frequencies. These
combination frequencies provide extra information about the pulsations, both
physical and geometrical, that is lost unless they are analyzed. Several
theories provide a context for this analysis by predicting combination
frequency amplitudes. In these theories, the combination frequencies arise from
nonlinear mixing of oscillation modes in the outer layers of the white dwarf,
so their analysis cannot yield direct information on the global structure of
the star as eigenmodes provide. However, their sensitivity to mode geometry
does make them a useful tool for identifying the spherical degree of the modes
that mix to produce them. In this paper, we analyze data from eight hot,
low-amplitude DAV white dwarfs and measure the amplitudes of combination
frequencies present. By comparing these amplitudes to the predictions of the
theory of Goldreich & Wu, we have verified that the theory is crudely
consistent with the measurements. We have also investigated to what extent the
combination frequencies can be used to measure the spherical degree (ell) of
the modes that produce them. We find that modes with ell > 2 are easily
identifiable as high ell based on their combination frequencies alone.
Distinguishing between ell=1 and 2 is also possible using harmonics. These
results will be useful for conducting seismological analysis of large ensembles
of ZZ Ceti stars, such as those being discovered using the Sloan Digital Sky
Survey. Because this method relies only on photometry at optical wavelengths,
it can be applied to faint stars using 4 m class telescopes.Comment: 73 pages, 22 figures, accepted in the Ap
High Resolution Spectroscopy of the Pulsating White Dwarf G29-38
We present the analysis of time-resolved, high resolution spectra of the cool
white dwarf pulsator, G29-38. From measuring the Doppler shifts of the H-alpha
core, we detect velocity changes as large as 16.5 km/s and conclude that they
are due to the horizontal motions associated with the g-mode pulsations on the
star. We detect seven pulsation modes from the velocity time-series and
identify the same modes in the flux variations. We discuss the properties of
these modes and use the advantage of having both velocity and flux measurements
of the pulsations to test the convective driving theory proposed for DAV stars.
Our data show limited agreement with the expected relationships between the
amplitude and phases of the velocity and flux modes. Unexpectedly, the velocity
curve shows evidence for harmonic distortion, in the form of a peak in the
Fourier transform whose frequency is the exact sum of the two largest
frequencies. Combination frequencies are a characteristic feature of the
Fourier transforms of light curves of G29-38, but before now have not been
detected in the velocities, nor does published theory predict that they should
exist. We compare our velocity combination frequency to combination frequencies
found in the analysis of light curves of G29-38, and discuss what might account
for the existence of velocity combinations with the properties we observe.
We also use our high-resolution spectra to determine if either rotation or
pulsation can explain the truncated shape observed for the DAV star's line
core. We are able to eliminate both mechanisms: the average spectrum does not
fit the rotationally broadened model and the time-series of spectra provides
proof that the pulsations do not significantly truncate the line.Comment: 24 pages, 9 figures, Accepted for publication in ApJ (June
First Attempt at Spectroscopic Detection of Gravity Modes in a Long-Period Pulsating Subdwarf B Star -- PG 1627+017
In the first spectroscopic campaign for a PG 1716 variable (or long-period
pulsating subdwarf B star), we succeeded in detecting velocity variations due
to g-mode pulsations at a level of 1.0-1.5 km/s.The observations were obtained
during 40 nights on 2-m class telescopes in Arizona, South Africa,and
Australia. The target,PG1627+017, is one of the brightest and largest amplitude
stars in its class.It is also the visible component of a post-common envelope
binary.Our final radial velocity data set includes 84 hours of time-series
spectroscopy over a time baseline of 53 days. Our derived radial velocity
amplitude spectrum, after subtracting the orbital motion, shows three potential
pulsational modes 3-4 sigma above the mean noise level, at 7201.0s,7014.6s and
7037.3s.Only one of the features is statistically likely to be real,but all
three are tantalizingly close to, or a one day alias of, the three strongest
periodicities found in the concurrent photometric campaign. We further
attempted to detect pulsational variations in the Balmer line amplitudes. The
single detected periodicity of 7209 s, although weak, is consistent with
theoretical expectations as a function of wavelength.Furthermore, it allows us
to rule out a degree index of l= 3 or l= 5 for that mode. Given the extreme
weakness of g-mode pulsations in these stars,we conclude that anything beyond
simply detecting their presence will require larger telescopes,higher
efficiency spectral monitoring over longer time baselines,improved longitude
coverage, and increased radial velocity precision.Comment: 39 pages, 9 figures, 4 tables, ApJ accepted. See postscript for full
abtrac
Position-specific Performance Indicators that Discriminate between Successful and Unsuccessful Teams in Elite Womenâs Indoor Field Hockey: Implications for Coaching
The aim of this investigation was to establish median performance profiles for the six playing positions in elite womenâs indoor hockey and then identify whether these position-specific profiles could discriminate between qualifying (top four), mid-table and relegated teams in the 2011-12 England Hockey premier league. Successful passing in relegated teams was significantly lower (p<0.008) than in mid-table and qualifying teams in four of the five outfield positions. Furthermore, the right backs of qualifying teams demonstrated significantly fewer (p<0.008) unsuccessful passes (xÌ=15.5 ±CLs 15.0 and 10.0 respectively) and interceptions (xÌ=4.0 ±CLs 4.0 and 3.0 respectively) than relegated teams (xÌ=19.5 ±CLs 21.0 and 17.0; xÌ=7.5 ±CLs 8.0 and 6.0 respectively). Finally, the right forwards of relegated teams demonstrated significantly fewer (p<0.008) successful interceptions (xÌ=4.0 ±CLs 5.0 and 4.0 respectively) than qualifying teams (xÌ=5.0 ±CLs 6.0 and 3.0 respectively) and significantly more (p<0.008) unsuccessful interceptions (xÌ=5.5 ±CLs 6.0 and 4.0 respectively) than mid-table teams (xÌ=3.0 ±CLs 3.0 and 2.0 respectively). Based on these findings, coaches should adapt tactical strategies and personnel deployment accordingly to enhance the likelihood of preparing a qualifying team. Research should build from these data to examine dribbling, pressing and patterns of play when outletting
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