27,877 research outputs found
An independent Cepheid distance scale: Current status
An independent distance scale for Cepheid variables is discussed. The apparent magnitude and the visual surface brightness, inferred from an appropriate color index, are used to determine the angular diameter variation of the Cepheid. When combined with the linear displacement curve obtained from the integrated radial velocity curve, the distance and linear radius are determined. The attractiveness of the method is its complete independence of all other stellar distance scales, even though a number of practical difficulties currently exist in implementing the technique
The High Chromospheres of the Late A Stars
We report the detection of N V 1239 A transition region emission in HST/GHRS
spectra of the A7 V stars, Alpha Aql and Alpha Cep. Our observations provide
the first direct evidence of 1-3 x 10^5 K material in the atmospheres of normal
A-type stars. For both stars, and for the mid-A--type star Tau3 Eri, we also
report the detection of chromospheric emission in the Si III 1206 A line. At a
B-V color of 0.16 and an effective temperature of 8200 K, Tau3 Eri becomes the
hottest main sequence star known to have a chromosphere and thus an outer
convection zone. We see no firm evidence that the Si III line surface fluxes of
the A stars are any lower than those of moderately active, solar-type, G and K
stars. This contrasts sharply with their coronal X-ray emission, which is >100
times weaker than that of the later-type stars. Given the strength of the N V
emission observed here, it now appears unlikely that the X-ray faintness of the
A stars is due to their forming very cool, <= 1 MK coronae. An alternative
explanation in terms of mass loss in coronal winds remains a possibility,
though we conclude from moderate resolution spectra of the Si III lines that
such winds, if they exist, do not penetrate into the chromospheric Si
III--forming layers of the star, since the profiles of these lines are *not*
blueshifted, and may well be redshifted with respect to the star.Comment: LaTex, 12 pages, 3 Postscript figures, uses aaspp4, accepted by Ap
Field support, data analysis and associated research for the acoustic grenade sounding program
Temperature and horizontal winds in the 30 to 90 km altitude range of the upper atmosphere, were determined by acoustic grenade soundings conducted at Wallops Island, Virginia and Kourou, French Guiana. Field support provided at these locations included deployment of the large area microphone system, supervision, maintenance and operation of sound ranging stations; and coordination of activities. Data analysis efforts included the analysis of field data to determine upper atmospheric meteorological parameters. Profiles for upper atmospheric temperature, wind and density are provided in plots and tables for each of the acoustic grenade soundings conducted during the contract period. Research efforts were directed toward a systematic comparison of temperature data from acoustic grenade with other meteorological sensor probes in the upper atmosphere
Freely decaying turbulence in two-dimensional electrostatic gyrokinetics
In magnetized plasmas, a turbulent cascade occurs in phase space at scales
smaller than the thermal Larmor radius ("sub-Larmor scales") [Phys. Rev. Lett.
103, 015003 (2009)]. When the turbulence is restricted to two spatial
dimensions perpendicular to the background magnetic field, two independent
cascades may take place simultaneously because of the presence of two
collisionless invariants. In the present work, freely decaying turbulence of
two-dimensional electrostatic gyrokinetics is investigated by means of
phenomenological theory and direct numerical simulations. A dual cascade
(forward and inverse cascades) is observed in velocity space as well as in
position space, which we diagnose by means of nonlinear transfer functions for
the collisionless invariants. We find that the turbulence tends to a
time-asymptotic state, dominated by a single scale that grows in time. A theory
of this asymptotic state is derived in the form of decay laws. Each case that
we study falls into one of three regimes (weakly collisional, marginal, and
strongly collisional), determined by a dimensionless number D*, a quantity
analogous to the Reynolds number. The marginal state is marked by a critical
number D* = D0 that is preserved in time. Turbulence initialized above this
value become increasingly inertial in time, evolving toward larger and larger
D*; turbulence initialized below D0 become more and more collisional, decaying
to progressively smaller D*.Comment: 12 pages, 12 figures; replaced to match published versio
Simulating `Complex' Problems with Quantum Monte Carlo
We present a new quantum Monte Carlo algorithm suitable for generically
complex problems, such as systems coupled to external magnetic fields or anyons
in two spatial dimensions. We find that the choice of gauge plays a nontrivial
role, and can be used to reduce statistical noise in the simulation.
Furthermore, it is found that noise can be greatly reduced by approximate
cancellations between the phases of the (gauge dependent) statistical flux and
the external magnetic flux.Comment: Revtex, 11 pages. 3 postscript files for figures attache
Hadronic Spectral Function and Charm Meson Production
At the chiral restoration/deconfinement transition, most hadrons undergo a
Mott transition from being bound states in the confined phase to resonances in
the deconfined phase. We investigate the consequences of this qualitative
change in the hadron spectrum on final state interactions of charmonium in hot
and dense matter, and show that the Mott effect for D-mesons leads to a
critical enhancement of the J/Psi dissociation rate. Anomalous J/Psi
suppression in the NA50 experiment is discussed as well as the role of the Mott
effect for the heavy flavor kinetics in future experiments at the LHC. The
status of our calculations of hadron-hadron cross sections using the quark
interchange and chiral Lagrangian approaches is reviewed, and an Ansatz for a
unification of these schemes is given.Comment: 12 pages, 6 figures, Proceedings of the Budapest'02 Workshop on Quark
& Hadron Dynamics, Budapest, Hungary, March 3-7, 200
BB Intermeson Potentials in the Quark Model
In this paper we derive quark model results for scattering amplitudes and
equivalent low energy potentials for heavy meson pairs, in which each meson
contains a heavy quark. This "BB" system is an attractive theoretical
laboratory for the study of the nuclear force between color singlets; the
hadronic system is relatively simple, and there are lattice gauge theory (LGT)
results for V_BB(r) which may be compared to phenomenological models. We find
that the quark model potential (after lattice smearing) has qualitative
similarities to the LGT potential in the two B*B* channels in which direct
comparison is possible, although there is evidence of a difference in length
scales. The quark model prediction of equal magnitude but opposite sign for I=0
and I=1 potentials also appears similar to LGT results at intermediate r. There
may however be a discrepancy between the LGT and quark model I=1 BB potentials.
A numerical study of the two-meson Schrodinger equations in the (bqbar)(bqbar)
and (cqbar)(cqbar) sectors with the quark model potentials finds a single
"molecule", in the I=0 BB* sector. Binding in other channels might occur if the
quark model forces are augmented by pion exchange.Comment: 30 pages, 5 figures, revtex and epsfig. Submitted to Phys. Rev.
Direct Distances to Cepheids in the Large Magellanic Cloud: Evidence for a Universal Slope of the Period-Luminosity Relation up to Solar Abundance
We have applied the infrared surface brightness (ISB) technique to derive
distances to 13 Cepheids in the LMC which span a period range from 3 to 42
days. From the absolute magnitudes of the variables calculated from these
distances, we find that the LMC Cepheids define tight period-luminosity
relations in the V, I, W,
J and K bands which agree exceedingly well with the corresponding Galactic PL
relations derived from the same technique, and are significantly steeper than
the LMC PL relations in these bands observed by the OGLE-II Project in V, I and
W, and by Persson et al. in J and K. We find that the tilt-corrected true
distance moduli of the LMC Cepheids show a significant dependence on period,
which hints at a systematic error in the ISB technique related to the period of
the stars. We identify as the most likely culprit the p-factor which converts
the radial into pulsational velocities; our data imply a much steeper period
dependence of the p-factor than previously thought, and we derive p=1.58
(+/-0.02) -0.15 (+/-0.05) logP as the best fit from our data, with a zero point
tied to the Milky Way open cluster Cepheids. Using this revised p-factor law,
the period dependence of the LMC Cepheid distance moduli disappears, and at the
same time the Milky Way and LMC PL relations agree among themselves, and with
the directly observed LMC PL relations, within the 1 sigma uncertainties. Our
main conclusion is that the previous, steeper Galactic PL relations were caused
by an erroneous calibration of the p-factor law, and that there is now evidence
that the slope of the Cepheid PL relation is independent of metallicity up to
solar metallicity, in both optical, and near-infrared bands.Comment: ApJ accepte
Distances to six Cepheids in the LMC cluster NGC1866 from the near-IR surface-brightness method
We derive individual distances to six Cepheids in the young populous star
cluster NGC1866 in the Large Magellanic Cloud employing the near-IR surface
brightness technique. With six stars available at the exact same distance we
can directly measure the intrinsic uncertainty of the method. We find a
standard deviation of 0.11 mag, two to three times larger than the error
estimates and more in line with the estimates from Bayesian statistical
analysis by Barnes et al. (2005). Using all six distance estimates we determine
an unweighted mean cluster distance of 18.30+-0.05. The observations indicate
that NGC1866 is close to be at the same distance as the main body of the LMC.
If we use the stronger dependence of the p-factor on the period as suggested by
Gieren et al. (2005) we find a distance of 18.50+-0.05 (internal error) and the
PL relations for Galactic and MC Cepheids are in very good agreement.Comment: Presented at the conference "Stellar Pulsation and Evolution" in
Monte Porzio Catone, June 2005. To appear in Mem. Soc. Ast. It. 76/
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