8,958 research outputs found
Critical fields of liquids of liquid superconducting metallic hydrogen
Liquid metallic hydrogen, in a fully dissociated state, is predicted at certain densities to pass from dirty to clean and from type II to type I superconducting behavior as temperature is lowered
Testing modified gravity with globular cluster velocity dispersions
Globular clusters (GCs) in the Milky Way have characteristic velocity
dispersions that are consistent with the predictions of Newtonian gravity, and
may be at odds with Modified Newtonian Dynamics (MOND). We discuss a modified
gravity (MOG) theory that successfully predicts galaxy rotation curves, galaxy
cluster masses and velocity dispersions, lensing, and cosmological
observations, yet produces predictions consistent with Newtonian theory for
smaller systems, such as GCs. MOG produces velocity dispersion predictions for
GCs that are independent of the distance from the galactic center, which may
not be the case for MOND. New observations of distant GCs may produce strong
criteria that can be used to distinguish between competing gravitational
theories.Comment: 4 pages, 2 figures; accepted for publication in Ap
Resolving the nucleus of Centaurus A at mid-IR wavelengths
We have observed Centaurus A with the MID-infrared Interferometric instrument
(MIDI) at the Very Large Telescope Interferometer (VLTI) at resolutions of 7 -
15 mas (at 12.5 micron) and filled gaps in the (u,v) coverage in comparison to
earlier measurements. We are now able to describe the nuclear emission in terms
of geometric components and derive their parameters by fitting models to the
interferometric data. With simple geometrical models, the best fit is achieved
for an elongated disk with flat intensity profile with diameter 76 +/- 9 mas x
35 +/- 2 mas (1.41 +/- 0.17 pc x 0.65 +/- 0.03 pc) whose major axis is oriented
at a position angle (PA) of 10.1 +/- 2.2 degrees east of north. A point source
contributes 47 +/- 11 % of the nuclear emission at 12.5 micron. There is also
evidence that neither such a uniform nor a Gaussian disk are good fits to the
data. This indicates that we are resolving more complicated small-scale
structure in AGNs with MIDI, as has been seen in Seyfert galaxies previously
observed with MIDI. The PA and inferred inclination i = 62.6 +2.1/-2.6 degrees
of the dust emission are compared with observations of gas and dust at larger
scales.Comment: Accepted for the PASA special issue on Centaurus
On the Internal Absorption of Galaxy Clusters
A study of the cores of galaxy clusters with the Einstein SSS indicated the
presence of absorbing material corresponding to 1E+12 Msun of cold cluster gas,
possibly resulting from cooling flows. Since this amount of cold gas is not
confirmed by observations at other wavelengths, we examined whether this excess
absorption is present in the ROSAT PSPC observations of 20 bright galaxy
clusters. For 3/4 of the clusters, successful spectral fits were obtained with
absorption due only to the Galaxy, and therefore no extra absorption is needed
within the clusters, in disagreement with the results from the Einstein SSS
data for some of the same clusters. For 1/4 of the clusters, none of our
spectral fits was acceptable, suggesting a more complicated cluster medium than
the two-temperature and cooling flow models considered here. However, even for
these clusters, substantial excess absorption is not indicated.Comment: accepted by the Astrophysical Journa
Shock heating in the nearby radio galaxy NGC 3801
Original article can be found at: http://www.journals.uchicago.edu/ApJ/--Copyright American Astronomical SocietyPeer reviewe
A Thin HI Circumnuclear Disk in NGC4261
We report on high sensitivity, spectral line VLBI observations of the HI
absorption feature in the radio galaxy NGC4261. Although absorption is only
detectable on the most sensitive baseline, it can be unambiguously associated
with the counterjet and is interpreted to originate in a thin atomic
circumnuclear disk. This structure is probably a continuation of the dusty
accretion disk inferred from HST imaging, which could be feeding the massive
black hole. HI column densities in front of the counterjet of the order of
10^{21}(T_sp/100 K) cm^{-2} are derived, consistent with X-ray data and VLBI
scale free-free absorption. The data presented here are the result of the first
scientific project processed on the new EVN MkIV data processor.Comment: 4 pages, 3 postscript figures, Astronomy and Astrophysics Letters, in
pres
High spatial resolution mid-infrared observations of the low-mass young star TW Hya
We want to improve knowledge of the structure of the inner few AU of the
circumstellar disk around the nearby T Tauri star TW Hya. Earlier studies have
suggested the existence of a large inner hole, possibly caused by interactions
with a growing protoplanet. We used interferometric observations in the N-band
obtained with the MIDI instrument on the Very Large Telescope Interferometer,
together with 10 micron spectra recorded by the infrared satellite Spitzer. The
fact that we were able to determine N-band correlated fluxes and visibilities
for this comparatively faint source shows that MIR interferometry can be
applied to a large number of low-mass young stellar objects.
The MIR spectra obtained with Spitzer reveal emission lines from HI (6-5), HI
(7-6), and [Ne II] and show that over 90% of the dust we see in this wavelength
regime is amorphous. According to the correlated flux measured with MIDI, most
of the crystalline material is in the inner, unresolved part of the disk, about
1 AU in radius. The visibilities exclude the existence of a very large (3-4 AU
radius) inner hole in the circumstellar disk of TW Hya, which was required in
earlier models. We propose instead a geometry of the inner disk where an inner
hole still exists, but at a much reduced radius, with the transition from zero
to full disk height between 0.5 and 0.8 AU, and with an optically thin
distribution of dust inside. Such a model can comply with SED and MIR
visibilities, as well as with visibility and extended emission observed in the
NIR at 2 micron. If a massive planet was the reason for this inner hole, as has
been speculated, its orbit would have to be closer to the star than 0.3 AU.
Alternatively, we may be witnessing the end of the accretion phase and an early
phase of an inward-out dispersal of the circumstellar disk.Comment: 13 pages, 9 figures, accepted by A&
WQ 2059-247: An unusual high redshift X-ray cluster
X-ray, optical, and radio observations of a high redshift, Bautz-Morgan type I cluster of galaxies are reported. The cD galaxy contains a powerful, flat spectrum radio source coincident with the possibly stellar nucleus. The cluster is an extremely luminous X-ray source; however, unlike nearby luminous X-ray clusters the X-ray spectrum appears to be rather soft. Two possible interpretations of the soruces are suggested: either the intracluster gas is much cooler in high redshift clusters because they are less relaxed, or the X-ray and radio emissions from WQ 2059-247 are the result of a non thermal QSO/BL Lac type object in the nucleus of the cD
Transverse Double-Spin Asymmetries for Muon Pair Production in pp-Collisions
We calculate the rapidity dependence of the transverse double-spin asymmetry
for the Drell-Yan process to next-to-leading order in the strong coupling.
Input transversity distributions are obtained by saturating the Soffer
inequality at a low hadronic mass scale. Results for the polarized BNL-RHIC
proton-proton collider and the proposed HERA-N fixed-target experiment are
presented, and the influence of the limited muon acceptance of the detectors on
measurements of the asymmetry is studied in detail.Comment: 7 pages including 5 figures; significantly shortened, to be published
in Phys. Rev.
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