10,832 research outputs found
Photometry of the Stingray Nebula (V839 Ara) from 1889-2015 Across the Ionization of Its Planetary Nebula
Up until around 1980, the Stingray was an ordinary B1 post-AGB star, but then
it suddenly sprouted bright emission lines like in a planetary nebula (PN), and
soon after this the Hubble Space Telescope (HST) discovered a small PN around
the star, so apparently we have caught a star in the act of ionizing a PN. We
report here on a well-sampled light curve from 1889 to 2015, with unique
coverage of the prior century plus the entire duration of the PN formation plus
three decades of its aftermath. Surprisingly, the star anticipated the 1980's
ionization event by declining from B=10.30 in 1889 to B=10.76 in 1980. Starting
in 1980, the central star faded fast, at a rate of 0.20 mag/year, reaching
B=14.64 in 1996. This fast fading is apparently caused by the central star
shrinking in size. From 1994-2015, the V-band light curve is almost entirely
from the flux of two bright [OIII] emission lines from the unresolved nebula,
and it shows a consistent decline at a rate of 0.090 mag/year. This steady
fading (also seen in the radio and infrared) has a time scale equal to that
expected for ordinary recombination within the nebula, immediately after a
short-duration ionizing event in the 1980s. We are providing the first direct
measure of the rapidly changing luminosity of the central star on both sides of
a presumed thermal pulse in 1980, with this providing a strong and critical set
of constraints, and these are found to sharply disagree with theoretical models
of PN evolution.Comment: ApJ accepted, 54 pages, 4 figures, one long data tabl
The Globular Cluster Systems of Five Nearby Spiral Galaxies: New Insights from Hubble Space Telescope Imaging
We use available multifilter Hubble Space Telescope (HST) WFPC2 imaging of
five (M81, M83, NGC 6946, M101, and M51) low inclination, nearby spiral
galaxies to study ancient star cluster populations. M81 globular clusters (GC)
have an intrinsic color distribution which is very similar to those in the
Milky Way and M31, with ~40% of the clusters having colors expected for a
metal-rich population. On the other hand, the GC system in M51 appears almost
exclusively blue and metal poor. This lack of metal-rich GCs associated with
the M51 bulge indicates that the bulge formation history of this Sbc galaxy may
have differed significantly from that of our own. Ancient clusters in M101, and
possibly in NGC 6946, appear to have luminosity distributions which continue to
rise to our detection limit (M_V ~ -6.0), well beyond the expected turnover
(M_V ~ -7.4) in the luminosity function. This is reminiscent of the situation
in M33, a Local Group galaxy of similar Hubble type. The faint ancient cluster
candidates in M101 and NGC 6946 have colors and radii similar to their more
luminous counterparts, and we suggest that these are either intermediate age
(3-9 Gyr) disk clusters or the low mass end of the original GC population. If
the faint, excess GC candidates are excluded, we find that the specific
frequency (S_N) of ancient clusters formed in later-type spirals is roughly
constant, with S_N=0.5 +- 0.2. By combining the results of this study with
literature values for other systems, we find that the total GC specific
frequencies in spirals appear to correlate best with Hubble type and
bulge/total ratio, rather than with galaxy luminosity or galaxy mass
(abridged).Comment: 31 pages, 11 tables, 10 figure
Non-Ground-State Bose-Einstein Condensates of Trapped Atoms
The possibility of creating a Bose condensate of trapped atoms in a
non-ground state is suggested. Such a nonequilibrium Bose condensate can be
formed if one, first, obtains the conventional Bose condensate in the ground
state and then transfers the condensed atoms to a non-ground state by means of
a resonance pumping. The properties of ground and non-ground states are
compared and plausible applications of such nonequilibrium condensates are
discussed.Comment: 1 file, 16 pages, RevTe
Constraining the Progenitor Companion of the Nearby Type Ia SN 2011fe with a Nebular Spectrum at +981 Days
We present an optical nebular spectrum of the nearby Type Ia supernova
2011fe, obtained 981 days after explosion. SN 2011fe exhibits little evolution
since the +593 day optical spectrum, but there are several curious aspects in
this new extremely late-time regime. We suggest that the persistence of the
~\AA\ feature is due to Na I D, and that a new emission feature at
~\AA\ may be [Ca II]. Also, we discuss whether the new emission
feature at ~\AA\ might be [Fe I] or the high-velocity hydrogen
predicted by Mazzali et al. The nebular feature at 5200~\AA\ exhibits a linear
velocity evolution of per 100 days from at least
+220 to +980 days, but the line's shape also changes in this time, suggesting
that line blending contributes to the evolution. At days after
explosion, flux from the SN has declined to a point where contribution from a
luminous secondary could be detected. In this work we make the first
observational tests for a post-impact remnant star and constrain its
temperature and luminosity to and
. Additionally, we do not see any evidence for narrow H
emission in our spectrum. We conclude that observations continue to strongly
exclude many single-degenerate scenarios for SN 2011fe.Comment: 11 pages, 10 figures, published by MNRA
Variability of Fe II Emission Features in the Seyfert 1 Galaxy NGC 5548
We study the low-contrast Fe II emission blends in the ultraviolet
(1250--2200A) and optical (4000--6000A) spectra of the Seyfert 1 galaxy NGC
5548 and show that these features vary in flux and that these variations are
correlated with those of the optical continuum. The amplitude of variability of
the optical Fe II emission is 50% - 75% that of Hbeta and the ultraviolet Fe II
emission varies with an even larger amplitude than Hbeta. However, accurate
measurement of the flux in these blends proves to be very difficult even using
excellent Fe II templates to fit the spectra. We are able to constrain only
weakly the optical Fe II emission-line response timescale to a value less than
several weeks; this upper limit exceeds all the reliably measured emission-line
lags in this source so it is not particularly meaningful. Nevertheless, the
fact that the optical Fe II and continuum flux variations are correlated
indicates that line fluorescence in a photoionized plasma, rather than
collisional excitation, is responsible for the Fe II emission. The iron
emission templates are available upon request.Comment: 34 pages including 12 figures and 2 tables. Accepted for publication
by ApJ (tentatively in vol. 626 June 10, 2005
Atomic Deuterium Adsorbed on the Surface of Liquid Helium
We investigate deuterium atoms adsorbed on the surface of liquid helium in
equilibrium with a vapor of atoms of the same species. These atoms are studied
by a sensitive optical method based on spectroscopy at a wavelength of 122 nm,
exciting the 1S-2P transition. We present a direct measurement of the
adsorption energy of deuterium atoms on helium and show evidence for the
existence of resonantly enhanced recombination of atoms residing on the surface
to molecules.Comment: 6 pages 4 figure
Crossover from hydrodynamic to acoustic drag on quartz tuning forks in normal and superfluid 4He
We present measurements of the drag forces on quartz tuning forks oscillating at low velocities in normal and superfluid 4He. We have investigated the dissipative drag over a wide range of frequencies, from 6.5 to 600 kHz, by using arrays of forks with varying prong lengths and by exciting the forks in their fundamental and first overtone modes. At low frequencies the behavior is dominated by laminar hydrodynamic drag, governed by the fluid viscosity. At higher frequencies acoustic drag is dominant and is described well by a three-dimensional model of sound emission
Lattice Distortion and Magnetism of 3d- Perovskite Oxides
Several puzzling aspects of interplay of the experimental lattice distortion
and the the magnetic properties of four narrow -band perovskite oxides
(YTiO, LaTiO, YVO, and LaVO) are clarified using results of
first-principles electronic structure calculations. First, we derive parameters
of the effective Hubbard-type Hamiltonian for the isolated bands using
newly developed downfolding method for the kinetic-energy part and a hybrid
approach, based on the combination of the random-phase approximation and the
constraint local-density approximation, for the screened Coulomb interaction
part. Then, we solve the obtained Hamiltonian using a number of techniques,
including the mean-field Hartree-Fock (HF) approximation, the second-order
perturbation theory for the correlation energy, and a variational superexchange
theory. Even though the crystal-field splitting is not particularly large to
quench the orbital degrees of freedom, the crystal distortion imposes a severe
constraint on the form of the possible orbital states, which favor the
formation of the experimentally observed magnetic structures in YTiO,
YVO_, and LaVO even at the HF level. Beyond the HF approximation, the
correlations effects systematically improve the agreement with the experimental
data. Using the same type of approximations we could not reproduce the correct
magnetic ground state of LaTiO. However, we expect that the situation may
change by systematically improving the level of approximations for dealing with
the correlation effects.Comment: 30 pages, 17 figures, 8 tables, high-quality figures are available
via e-mai
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