154 research outputs found
On the pulsation of a star in which there is a prevalent magnetic field
In this paper a simple approximate formula is obtained for the frequency (Ï) of radial pulsation of a gaseous star in which there is a prevalent magnetic field. The formula is Ï2I = - (3Îł - 4) (Ω + B) , where Îł is the ratio of the specific heats, I = â«0Mr2dm (r) , and Ω and B denote the gravitational potential energy and the magnetic energy of the star, respectively. The formula is derived from the virial theorem in the form recently established by Chandrasekhar and Fermi; and it supports their conclusion that the period of pulsation can be made as long as one may desire by letting the magnetic energy approach the upper limit (namely, |Ω|) set by the virial theorem
Hydrodynamical Simulations of the IGM at High Mach Numbers
We present a new approach to doing Eulerian computational fluid dynamics that
is designed to work at high Mach numbers encountered in hydrodynamical
simulations of the IGM. In conventional Eulerian CFD, the thermal energy is
poorly tracked in supersonic bulk flows where local fluid variables cannot be
accurately separated from the much larger bulk flow components. We described a
method in which local fluid quantities can be directly tracked and the Eulerian
fluid equations solved in a local frame moving with the flow. The new algorithm
has been used to run large hydrodynamical simulations on a 1024^3 grid to study
the kinetic SZ effect. The KSZ power spectrum is broadly peaked at l~10^4 with
temperature fluctuations on micro Kelvin levels.Comment: 6 pages, to appear in the Proc. from the IGM/Galaxy Connection
conferenc
Linguistics
Contains reports on four research projects.National Institute of Mental Health (Grant 1 PO1 MH-13390-04
Indications of a positive feedback between coastal development and beach nourishment
Beach nourishment, a method for mitigating coastal storm damage or chronic erosion by deliberately replacing sand on an eroded beach, has been the leading form of coastal protection in the United States for four decades. However, investment in hazard protection can have the unintended consequence of encouraging development in places especially vulnerable to damage. In a comprehensive, parcel-scale analysis of all shorefront single-family homes in the state of Florida, we find that houses in nourishing zones are significantly larger and more numerous than in non-nourishing zones. The predominance of larger homes in nourishing zones suggests a positive feedback between nourishment and development that is compounding coastal risk in zones already characterized by high vulnerability
Linguistics
Contains research objectives and reports on five research projects.National Institutes of Health (Grant 1 P01 MH-13390-02
Galaxy Flow in the Canes Venatici I Cloud
We present an analysis of Hubble Space Telescope/WFPC2 images of eighteen
galaxies in the Canes Venatici I cloud.
We derive their distances from the luminosity of the tip of the red giant
branch stars with a typical accuracy of ~12 %. The resulting distances are 3.9
Mpc (UGC 6541), 4.9 Mpc (NGC 3738), 3.0 Mpc (NGC 3741), 4.5 Mpc (KK 109), >6.3
Mpc (NGC 4150), 4.2 Mpc (UGC 7298), 4.5 Mpc (NGC 4244), 4.6 Mpc (NGC 4395), 4.9
Mpc (UGC 7559), 4.2 Mpc (NGC 4449), 4.4 Mpc (UGC 7605), 4.6 Mpc (IC 3687), 4.7
Mpc (KK 166), 4.7 Mpc (NGC 4736), 4.2 Mpc (UGC 8308), 4.3 Mpc (UGC 8320), 4.6
Mpc (NGC 5204), and 3.2 Mpc (UGC 8833). The CVn I cloud has a mean radial
velocity of 286 +- 9 km/s, a mean distance of 4.1 +- 0.2 Mpc, a radial velocity
dispersion of 50 km/s, a mean projected radius of 760 kpc, and a total blue
luminosity of 2.2 * 10^{10} L_{\sun}. Assuming virial or closed orbital motions
for the galaxies, we estimated their virial and their orbital
mass-to-luminosity ratio to be 176 and 88 M_{\sun}/L_{\sun}, respectively.
However, the CVn I cloud is characterized by a crossing time of 15 Gyr, and is
thus far from a state of dynamical equilibrium. The large crossing time for the
cloud, its low content of dSph galaxies ( %), and the almost
``primordial'' shape of its luminosity function show that the CVn I complex is
in a transient dynamical state, driven rather by the free Hubble expansion than
by galaxy interactions.Comment: 23 pages, 6 figures, A&A in preparation. The version does not include
Figure 2. High resolution figures 1 and 2 (11311k) are available at
http://luna.sao.ru/~sme/figsCVn.tar.g
Indications of a positive feedback between coastal development and beach nourishment: COASTAL DEVELOPMENT BEACH NOURISHMENT
Beach nourishment, a method for mitigating coastal storm damage or chronic erosion by deliberately replacing sand on an eroded beach, has been the leading form of coastal protection in the United States for four decades. However, investment in hazard protection can have the unintended consequence of encouraging development in places especially vulnerable to damage. In a comprehensive, parcelâscale analysis of all shorefront singleâfamily homes in the state of Florida, we find that houses in nourishing zones are significantly larger and more numerous than in nonânourishing zones. The predominance of larger homes in nourishing zones suggests a positive feedback between nourishment and development that is compounding coastal risk in zones already characterized by high vulnerability
Clustering and descendants of MUSYC galaxies at z<1.5
We measure the evolution of galaxy clustering out to a redshift of z~1.5
using data from two MUSYC fields, the Extended Hubble Deep Field South (EHDF-S)
and the Extended Chandra Deep Field South (ECDF-S). We use photometric redshift
information to calculate the projected-angular correlation function,
omega(sigma), from which we infer the projected correlation function Xi(sigma).
We demonstrate that this technique delivers accurate measurements of clustering
even when large redshift measurement errors affect the data. To this aim we use
two mock MUSYC fields extracted from a LambdaCDM simulation populated with
GALFORM semi-analytic galaxies which allow us to assess the degree of accuracy
of our estimates of Xi(sigma) and to identify and correct for systematic
effects in our measurements. We study the evolution of clustering for volume
limited subsamples of galaxies selected using their photometric redshifts and
rest-frame r-band absolute magnitudes. We find that the real-space correlation
length r_0 of bright galaxies, M_r<-21 (rest-frame) can be accurately recovered
out to z~1.5, particularly for ECDF-S given its near-infrared photometric
coverage. There is mild evidence for a luminosity dependent clustering in both
fields at the low redshift samples (up to =0.57), where the correlation
length is higher for brighter galaxies by up to 1Mpc/h between median
rest-frame r-band absolute magnitudes of -18 to -21.5. As a result of the
photometric redshift measurement, each galaxy is assigned a best-fit template;
we restrict to E and E+20%Sbc types to construct subsamples of early type
galaxies (ETGs). Our ETG samples show a strong increase in r_0 as the redshift
increases, making it unlikely (95% level) that ETGs at median redshift
z_med=1.15 are the direct progenitors of ETGs at z_med=0.37 with equivalent
passively evolved luminosities. (ABRIDGED)Comment: 16 pages, 12 figures, 2 tables, accepted for publication in MNRA
Large Scale Pressure Fluctuations and Sunyaev-Zel'dovich Effect
The Sunyaev-Zel'dovich (SZ) effect associated with pressure fluctuations of
the large scale structure gas distribution will be probed with current and
upcoming wide-field small angular scale cosmic microwave background
experiments. We study the generation of pressure fluctuations by baryons which
are present in virialized dark matter halos and by baryons present in small
overdensities. For collapsed halos, assuming the gas distribution is in
hydrostatic equilibrium with matter density distribution, we predict the
pressure power spectrum and bispectrum associated with the large scale
structure gas distribution by extending the dark matter halo approach which
describes the density field in terms of correlations between and within halos.
The projected pressure power spectrum allows a determination of the resulting
SZ power spectrum due to virialized structures. The unshocked photoionized
baryons present in smaller overdensities trace the Jeans-scale smoothed dark
matter distribution. They provide a lower limit to the SZ effect due to large
scale structure in the absence of massive collapsed halos. We extend our
calculations to discuss higher order statistics, such as bispectrum and
skewness in SZ data. The SZ-weak lensing cross-correlation is suggested as a
probe of correlations between dark matter and baryon density fields, while the
probability distribution functions of peak statistics of SZ halos in wide field
CMB data can be used as a probe of cosmology and non-Gaussian evolution of
large scale structure pressure fluctuations.Comment: 16 pages, 9 figures; Revised with expanded discussions. Phys. Rev. D.
(in press
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