247 research outputs found
Stationary structures of irrotational binary systems -- models for close binary systems of compact stars
We propose a new numerical method to calculate irrotational binary systems
composed of compressible gaseous stars in Newtonian gravity. Assuming
irrotationality, i.e. vanishing of the vorticity vector everywhere in the star
in the inertial frame, we can introduce the velocity potential for the flow
field. Using this velocity potential we can derive a set of basic equations for
stationary states which consist of (i) the generalized Bernoulli equation, (ii)
the Poisson equation for the Newtonian gravitational potential and (iii) the
equation for the velocity potential with the Neumann type boundary condition.
We succeeded in developing a new code to compute numerically exact solutions to
these equations for the first time. Such irrotational configurations of binary
systems are appropriate models for realistic neutron star binaries composed of
inviscid gases, just prior to coalescence of two stars caused by emission of
gravitational waves. Accuracies of our numerical solutions are so high that we
can compute reliable models for fully deformed final stationary configurations
and hence determine the inner most stable circular orbit of binary neutron star
systems under the approximations of weak gravity and inviscid limit.Comment: 32 pages, 25 bitmapped ps files, to appear in ApJ supplemen
34) Pieranski, P. Contemp. Phys. 1983, 24, 25. (35) Okubo, T
However, we have no information on the stoichiometric charge number on the surface of tungstic acid colloid. Thus, we estimated the amount of free protons from the solution conductance and from the reference values of equivalent conductance of protons. The evaluated D 1 values for our system were in the range of several thousand angstroms. This correction term then became quite significant for the smaller colloids. The triangles in The significance of the Debye-length has often been pointed out for the solution properties of various colloids and polyelectrolytes in salt-free systems such as the ordered solution structure in monodispersed l a t i c e~,~~-~~ conformation of ionic flexible polymer^,^*-^^ and very low mobility of spherical colloids at ex- tremely low ionic ~t r e n g t h .~* -~~ Next, we examined the viscosity effect of solvent on the relaxation times. In Conclusions A new and convenient way of determining the rotational diffusion coefficient (D,) was successfully achieved by the SP-SF and C S F techniques for the ellipsoidal colloids of tungstic acid. The significant contribution of Debye-screening-length to the D, has been described, particularly in salt-free systems. (32) Ito, K.; Ise, N:; Okubo; T.'J
Nova Sagittarii 1998 (V4633 Sgr) - a permanent superhump system or an asynchronous polar?
We report the results of observations of V4633 Sgr (Nova Sagittarii 1998)
during 1998-2000. Two photometric periodicities were present in the light curve
during the three years of observations: a stable one at P=3.014 h, which is
probably the orbital period of the underlying binary system, and a second one
of lower coherence, approximately 2.5 per cent longer than the former. The
latter periodicity may be a permanent superhump, or alternatively, the spin
period of the white dwarf in a nearly synchronous magnetic system. A third
period, at P=5.06 d, corresponding to the beat between the two periods was
probably present in 1999. Our results suggest that a process of mass transfer
took place in the binary system since no later than two and a half months after
the nova eruption. We derive an interstellar reddening of E(B-V)~0.21 from our
spectroscopic measurements and published photometric data, and estimate a
distance of d~9 kpc to this nova.Comment: 13 pages, latex, 7 figures, accepted for publication in MNRA
Instabilities and Mixing in SN 1993J
Rayleigh-Taylor (R-T) instabilities in the explosion of SN 1993J are
investigated by means of two-dimensional hydrodynamical simulations. It is
found that the extent of mixing is sensitive to the progenitor's core mass and
the envelope mass. Because the helium core mass (3 - 4 \ms) is smaller than
that of SN 1987A, R-T instabilities at the He/C+O interfaces develop to induce
a large scale mixing in the helium core, while the instability is relatively
weak at the H/He interface due to the small envelope mass. The predicted
abundance distribution, in particular the amount of the \ni~ mixing, is
compared with those required in the theoretical light curves and the late time
optical spectra. This enables us to specify the progenitor of SN 1993J in some
detail.Comment: 20 pages, LaTeX (AASTeX), to appear in Ap
Stability of the r-modes in white dwarf stars
Stability of the r-modes in rapidly rotating white dwarf stars is
investigated. Improved estimates of the growth times of the
gravitational-radiation driven instability in the r-modes of the observed DQ
Her objects are found to be longer (probably considerably longer) than 6x10^9y.
This rules out the possibility that the r-modes in these objects are emitting
gravitational radiation at levels that could be detectable by LISA. More
generally it is shown that the r-mode instability can only be excited in a very
small subset of very hot (T>10^6K), rather massive (M>0.9M_sun) and very
rapidly rotating (P_min<P<1.2P_min) white dwarf stars. Further, the growth
times of this instability are so long that these conditions must persist for a
very long time (t>10^9y) to allow the amplitude to grow to a dynamically
significant level. This makes it extremely unlikely that the r-mode instability
plays a significant role in any real white dwarf stars.Comment: 5 Pages, 5 Figures, revte
First-generation black-hole-forming supernovae and the metal abundance pattern of a very iron-poor star
It has been proposed theoretically that the first generation of stars in the
Universe (population III) would be as massive as 100 solar masses (100Mo),
because of inefficient cooling of the precursor gas clouds. Recently, the most
iron-deficient (but still carbon-rich) low-mass star -- HE0107-5240 -- was
discovered. If this is a population III that gained its metals (elements
heavier than helium) after its formation, it would challenge the theoretical
picture of the formation of the first stars. Here we report that the patterns
of elemental abundance in HE0107-5240 (and other extremely metal-poor stars)
are in good accord with the nucleosynthesis that occurs in stars with masses of
20-130Mo when they become supernovae if, during the explosions, the ejecta
undergo substantial mixing and fall-back to form massive black holes. Such
supernovae have been observed. The abundance patterns are not, however,
consistent with enrichment by supernovae from stars in the range 130-300 Mo. We
accordingly infer that the first-generation supernovae came mostly from
explosions of ~ 20-130Mo stars; some of these produced iron-poor but carbon-
and oxygen-rich ejecta. Low-mass second-generation stars, like HE0107-5240,
could form because the carbon and oxygen provided pathways for gas to cool.Comment: To appear in NATURE 422 (2003), 871-873 (issue 24 April 2003); Title
and the first paragraph have been changed and other minor corrections have
been mad
Early phase observations of extremely luminous Type Ia Supernova 2009dc
We present early phase observations in optical and near-infrared wavelengths
for the extremely luminous Type Ia supernova (SN Ia) 2009dc. The decline rate
of the light curve is , which is one of the
slowest among SNe Ia. The peak -band absolute magnitude is mag even if the host extinction is mag. It reaches
mag for the host extinction of mag as
inferred from the observed Na {\sc i} D line absorption in the host. Our
-band photometry shows that the SN is one of the most luminous SNe Ia
also in near-infrared wavelengths. These results indicate that SN 2009dc
belongs to the most luminous class of SNe Ia, like SN 2003fg and SN 2006gz. We
estimate the ejected Ni mass of \Msun for no host
extinction case (or 1.6 0.4 M for the host extinction of
mag). The C {\sc ii} 6580 absorption line keeps visible
until a week after maximum, which diminished in SN 2006gz before its maximum
brightness. The line velocity of Si {\sc ii} 6355 is about 8000 km
s around the maximum, being considerably slower than that of SN 2006gz,
while comparable to that of SN 2003fg. The velocity of the C {\sc ii} line is
almost comparable to that of the Si {\sc ii}. The presence of the carbon line
suggests that thick unburned C+O layers remain after the explosion. SN 2009dc
is a plausible candidate of the super-Chandrasekhar mass SNe Ia
Modeling of the Super-Eddington Phase for Classical Novae: Five IUE Novae
We present a light curve model for the super-Eddington luminosity phase of
five classical novae observed with IUE. Optical and UV light curves are
calculated based on the optically thick wind theory with a reduced effective
opacity for a porous atmosphere. Fitting a model light curve with the UV 1455
\AA light curve, we determine the white dwarf mass and distance to be (1.3
M_sun, 4.4 kpc) for V693 CrA, (1.05 M_sun, 1.8 kpc) for V1974 Cyg, (0.95 M_sun,
4.1 kpc) for V1668 Cyg, (1.0 M_sun, 2.1 kpc) for V351 Pup, and (1.0 M_sun, 4.3
kpc) for OS And.Comment: 9 pages including 8 figures, to appear in the Astrophysical Journa
Serum BDNF levels before and after the development of mood disorders: a case-control study in a population cohort.
Serum levels of brain-derived neurotrophic factor (BDNF) are low in major depressive disorder (MDD), and were recently shown to decrease in chronic depression, but whether this is a trait or state marker of MDD remains unclear. We investigated whether serum BDNF levels decrease before or after the developments of MDD and other mood disorders through a case-control study nested in a cohort of 1276 women aged 75-84 years in 2008. Psychiatrists using the Structured Clinical Interview for DSM-IV identified incident cases of mood disorders at follow-up surveys in 2010 and 2012: 28 of MDDs, 39 of minor depressive disorders (minDDs) and 8 of minor depressive episodes with a history of major depressive episodes (minDEs with MDE history). A total of 106 representative non-depressed controls were also identified in the 2012 follow-up. We assayed BDNF levels in preserved sera of cases and controls at baseline and at follow-up. Serum BDNF levels at baseline in cases of MDD, minDD or minDE with MDE history were no lower than those in controls. The decrease in the serum BDNF level from baseline to follow-up was greater in cases of MDD or minDE with MDE history than in controls or cases of minDD. These results show that serum BDNF levels are not a trait marker of MDD in old women but appeared to be a state marker. The different changes in BDNF levels among diagnostic groups suggest that MDD has a pathophysiologic relation to minDE with MDE history, rather than to minDD.This study was supported in part by JSPS KAKENHI grant number 24590820. The authors thank Drs. Yuichi Amano, Chie Hasegawa, and Hirotaka Iida for their cooperation in psychiatric evaluation and Dr. Masao Okazaki for proofreading and making helpful comments. The authors also thank Mses. Masayo Obata and Jun Komori for their help in participant recruitment and study management. The authors are grateful to Dr. Ben Underwood for his helpful comments on an earlier draft of the manuscript
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