175 research outputs found
Evidence for an oscillation of the magnetic axis of the white dwarf in the polar DP Leonis
From 1979 to 2001, the magnetic axis of the white dwarf in the polar DP Leo
slowly rotated by 50 deg in azimuth, possibly indicating a small asynchronism
between the rotational and orbital periods of the magnetic white dwarf. We have
obtained phase-resolved orbital light curves between 2009 and 2013, which show
that this trend has not continued in recent years. Our data are consistent with
the theoretically predicted oscillation of the magnetic axis of the white dwarf
about an equilibrium orientation, which is defined by the competition between
the accretion torque and the magnetostatic interaction of the primary and
secondary star. Our data indicate an oscillation period of ~60 yr, an amplitude
of about 25 deg, and an equilibrium orientation leading the connecting line of
the two stars by about 7 deg.Comment: Accepted by A&
A long-term optical and X-ray ephemeris of the polar EK Ursae Majoris
We searched for long-term period changes in the polar EK UMa using new
optical data and archival X-ray/EUV data. An optical ephemeris was derived from
data taken remotely with the MONET/N telescope and compared with the X-ray
ephemeris based on Einstein, Rosat, and EUVE data. A three-parameter fit to the
combined data sets yields the epoch, the period, and the phase offset between
the optical minima and the X-ray absorption dips. An added quadratic term is
insignificant and sets a limit to the period change. The derived linear
ephemeris is valid over 30 years and the common optical and X-ray period is
P=0.0795440225(24) days. There is no evidence of long-term O-C variations or a
period change over the past 17 years Delta P = -0.14+-0.50 ms. We suggest that
the observed period is the orbital period and that the system is tightly
synchronized. The limit on Delta P and the phase constancy of the bright part
of the light curve indicate that O-C variations of the type seen in the polars
DP Leo and HU Aqr or the pre-CV NN Ser do not seem to occur in EK UMa. The
X-ray dips lag the optical minima by 9.5+-0.7 deg in azimuth, providing some
insight into the accretion geometry.Comment: 4 pages, 2 Postscript figures, accepted for publication in Astronomy
& Astrophysic
Nonlinear Outcome of Gravitational Instability in Disks with Realistic Cooling
We consider the nonlinear outcome of gravitational instability in optically
thick disks with a realistic cooling function. We use a numerical model that is
local, razor-thin, and unmagnetized. External illumination is ignored. Cooling
is calculated from a one-zone model using analytic fits to low temperature
Rosseland mean opacities. The model has two parameters: the initial surface
density Sigma_0 and the rotation frequency Omega. We survey the parameter space
and find: (1) The disk fragments when t_c,eff Omega = 1, where t_c,eff is an
effective cooling time defined as the average internal energy of the model
divided by the average cooling rate. This is consistent with earlier results
that used a simplified cooling function. (2) The initial cooling time t_c0 or a
uniform disk with Q = 1 can differ by orders of magnitude from t_c,eff in the
nonlinear outcome. The difference is caused by sharp variations in the opacity
with temperature. The condition t_c0 Omega = 1 therefore does not necessarily
indicate where fragmentation will occur. (3) The largest difference between
t_c,eff and t_c0 is near the opacity gap, where dust is absent and hydrogen is
largely molecular. (4) In the limit of strong illumination the disk is
isothermal; we find that an isothermal version of our model fragments for Q <
1.4. Finally, we discuss some physical processes not included in our model, and
find that most are likely to make disks more susceptible to fragmentation. We
conclude that disks with t_c,eff Omega < 1 do not exist.Comment: 30 pages, 12 figure
HI ``Tails'' from Cometary Globules in IC1396
IC 1396 is a relatively nearby (750 pc), large (>2 deg), HII region ionized
by a single O6.5V star and containing bright-rimmed cometary globules. We have
made the first arcmin resolution images of atomic hydrogen toward IC 1396, and
have found remarkable ``tail''-like structures associated with some of the
globules and extending up to 6.5 pc radially away from the central ionizing
star. These HI ``tails'' may be material which has been ablated from the
globule through ionization and/or photodissociation and then accelerated away
from the globule by the stellar wind, but which has since drifted into the
``shadow'' of the globules.
This report presents the first results of the Galactic Plane Survey Project
recently begun by the Dominion Radio Astrophysical Observatory.Comment: 11 pages, 5 postscript figures, uses aaspp4.sty macros, submitted in
uuencoded gzipped tar format, accepted for publication in Astrophysical
Journal Letters, colour figures available at
http://www.drao.nrc.ca/~schieven/news_sep95/ic1396.htm
Electronic structure of strained InP/GaInP quantum dots
We calculate the electronic structure of nm scale InP islands embedded in
. The calculations are done in the envelope approximation
and include the effects of strain, piezoelectric polarization, and mixing among
6 valence bands. The electrons are confined within the entire island, while the
holes are confined to strain induced pockets. One pocket forms a ring at the
bottom of the island near the substrate interface, while the other is above the
island in the GaInP. The two sets of hole states are decoupled. Polarization
dependent dipole matrix elements are calculated for both types of hole states.Comment: Typographical error corrected in strain Hamiltonia
Very Low Mass Stars and Brown Dwarfs in Taurus-Auriga
We present high resolution optical spectra obtained with the HIRES
spectrograph on the Keck I telescope of low mass T Tauri stars and brown dwarfs
(LMTTs) in Taurus-Auriga. Of particular interest is the previously classified
"continuum T Tauri star" GM Tau, which has a spectral type of M6.5 and a mass
just below the stellar/substellar boundary. None of the LMTTs in Taurus are
rapidly rotating (vsini < 30 km/s), unlike low mass objects in Orion. Many of
the slowly rotating, non-accreting stars and brown dwarfs exhibit prominent
H-alpha emission (EWs of 3 - 36 A), indicative of active chromospheres. We
demonstrate empirically that the full-width at 10% of the H-alpha emission
profile peak is a more practical and possibly more accurate indicator of
accretion than either the equivalent width of H-alpha or optical veiling:
10%-widths > 270 km/s are classical T Tauri stars (i.e. accreting), independent
of stellar spectral type. Although LMTTs can have accretion rates comparable to
that of more typical, higher-mass T Tauri stars (e.g. K7-M0), the average mass
accretion rate appears to decrease with decreasing mass. The diminished
frequency of accretion disks for LMTTs, in conjunction with their lower, on
average, mass accretion rates, implies that they are formed with less massive
disks than higher-mass T Tauri stars. The radial velocities, circumstellar
properties and known binaries do not support the suggestion that many of the
lowest mass members of Taurus have been ejected from higher stellar density
regions within the cloud. Instead, LMTTs appear to have formed and are evolving
in the same way as higher-mass T Tauri stars, but with smaller disks and
shorter disk lifetimes.Comment: 27 pages, plus 8 figures, accepted for publication in Ap
A 150MG magnetic white dwarf in the cataclysmic variable RX J1554.2+2721
We report the detection of Zeeman-split Lalpha absorption pi and sigma+ lines
in the far-ultraviolet Hubble Space Telescope/Space Telescope Imaging
Spectrograph spectrum of the magnetic cataclysmic variable RX J1554.2+2721.
Fitting the STIS data with magnetic white dwarf model spectra, we derive a
field strength of B~144MG and an effective temperature of 17000K<Teff<23000K.
This measurement makes RX J1554.2+2721 only the third cataclysmic variable
containing a white dwarf with a field exceeding 100MG. Similar to the other
high-field polar AR UMa, RX J1554.2+2721 is often found in a state of feeble
mass transfer, which suggests that a considerable number of high-field polars
may still remain undiscovered.Comment: 4 pages, accepted for ApJ Letter
Rapid variability of accretion in AM Herculis
We present the last pointed observation of AM Her carried out during the life
of the BeppoSAX satellite. It was bright at the beginning of the observation,
but dropped to the lowest X-ray level ever observed so far. The X-ray emission
during the bright period is consistent with accretion occurring onto the main
pole of the magnetized white dwarf. The rapid change from the active state to
the low deep state indicates a drop by a factor of 17 in the accretion rate and
hence that accretion switched-off. The short timescale (less than one hour) of
this variation still remains a puzzle. Optical photometry acquired
simultaneousy during the low state shows that the white dwarf remains heated,
although a weak emission from the accretion stream could be still present.
Cyclotron radiation, usually dominating the V and R bands, is negligible thus
corroborating the possibility that AM Her was in an off-accretion state. The
X-ray emission during the inactive state is consistent with coronal emission
from the secondary late type star.Comment: 6 pages A&A-Latex, 6 Figures, accepted for publication in A&
High angular resolution imaging and infrared spectroscopy of CoRoT candidates
Studies of transiting extrasolar planets are of key importance for
understanding the nature of planets outside our solar system because their
masses, diameters, and bulk densities can be measured. An important part of
transit-search programmes is the removal of false-positives. The critical
question is how many of the candidates that passed all previous tests are false
positives. For our study we selected 25 CoRoT candidates that have already been
screened against false-positives using detailed analysis of the light curves
and seeing-limited imaging, which has transits that are between 0.7 and 0.05%
deep. We observed 20 candidates with the adaptive optics imager NaCo and 18
with the high-resolution infrared spectrograph CRIRES. We found previously
unknown stars within 2 arcsec of the targets in seven of the candidates. All of
these are too faint and too close to the targets to have been previously
detected with seeing-limited telescopes in the optical. Our study thus leads to
the surprising results that if we remove all candidates excluded by the
sophisticated analysis of the light-curve, as well as carrying out deep imaging
with seeing-limited telescopes, still 28-35% of the remaining candidates are
found to possess companions that are bright enough to be false-positives. Given
that the companion-candidates cluster around the targets and that the J-K
colours are consistent with physical companions, we conclude that the
companion-candidates are more likely to be physical companions rather than
unrelated field stars.Comment: 12 pages, 12 figures, A&A in pres
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