5,692 research outputs found
Massive star evolution: Luminous Blue Variables as unexpected Supernova progenitors
Stars more massive than about 8 Msun end their lives as a Supernova (SN), an
event of fundamental importance Universe-wide. Theoretically, these stars have
been expected to be either at the red supergiant, blue supergiant, or
Wolf-Rayet stage before the explosion. We performed coupled stellar evolution
and atmospheric modeling of stars with initial masses between 20 Msun and 120
Msun. We found that the 20 Msun and 25 Msun rotating models, before exploding
as SN, have spectra that do not resemble any of the aforementioned classes of
massive stars. Rather, they have remarkable similarities with rare, unstable
massive stars known as Luminous Blue Variables (LBV). While observations show
that some SNe seem to have had LBVs as progenitors, no theoretical model had
yet predicted that a star could explode at this stage. Our models provide
theoretical support for relatively low-luminosity LBVs exploding as SN in the
framework of single stellar evolution. This is a significant shift in paradigm,
meaning that a fraction of LBVs could be the end stage of massive star
evolution, rather than a transitory evolutionary phase. We suggest that type
IIb SN could have LBV as progenitors, and a prime example could be SN 2008ax.Comment: 4 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
Estimating parameters of fluctuations in the cosmic microwave background
We address questions that arise in statistical analyses of recently detected fluctuations in the Cosmic Microwave Background (CMB). Estimators of the quadrupole amplitude, Q, and spectral index, n, of the CMB angular fluctuation power spectrum are considered. Families of unbiased estimators of Q2 and existence conditions for minimum variance estimators of n are given. We find that the common practice of excluding the quadrupole is not recommended if one is interested in unbiased estimators. We explain previousiy reported correlations of the estimators and show how they depend on the multiple used to normalize the spectrum. We show that a finite beam resolution does not justify the use of truncated least-squares to estimate harmonic coefficients of CMB data
Maximum Likelihood Estimation and Lagrange Multiplier Tests for Panel Seemingly Unrelated Regressions with Spatial Lag and Spatial Errors: An Application to Hedonic Housing Prices in Paris
This paper proposes maximum likelihood estimators for panel seemingly unrelated regressions with both spatial lag and spatial error components. We study the general case where spatial effects are incorporated via spatial errors terms and via a spatial lag dependent variable and where the heterogeneity in the panel is incorporated via an error component specification. We generalize the approach of Wang and Kockelman (2007) and propose joint and conditional Lagrange Multiplier tests for spatial autocorrelation and random effects for this spatial SUR panel model. The small sample performance of the proposed estimators and tests are examined using Monte Carlo experiments. An empirical application to hedonic housing prices in Paris illustrates these methods. The proposed specification uses a system of three SUR equations corresponding to three types of flats within 80 districts of Paris over the period 1990-2003. We test for spatial effects and heterogeneity and find reasonable estimates of the shadow prices for housing characteristics.spatial lag, panel spatial dependence, maximum likelihood, Lagrange multiplier tests, hedonic housing prices, spatial error, SUR
Episodic modulations in supernova radio light curves from luminous blue variable supernova progenitor models
Ideally, one would like to know which type of core-collapse SNe is
produced by different progenitors and the channels of stellar evolution leading
to these progenitors. These links have to be very well known to use the
observed frequency of different types of SN events for probing the star
formation rate and massive star evolution in different types of galaxies.
We investigate the link between LBV as SN progenitors and the appearance
of episodic radio light curve modulations of the SN event. We use the
20Msun and 25Msun models with rotation at solar metallicity, part of an
extended grid of stellar models computed by the Geneva team. At their pre-SN
stage, these two models have recently been shown to have spectra similar to
those of LBV stars and possibly explode as Type IIb SNe. Based on the wind
properties before the explosion, we derive the density structure of their
circumstellar medium. This structure is used as input for computing the SN
radio light curve. We find that the 20Msun model shows radio light
curves with episodic luminosity modulations, similar to those observed in some
Type IIb SNe. This occurs because the evolution of the 20Msun model terminates
in a region of the HR diagram where radiative stellar winds present strong
density variations, caused by the bistability limit. The 25Msun model, ending
its evolution in a zone of the HR diagram where no change of the mass-loss
rates is expected, presents no such modulations in its radio SN light curve.
Our results reinforce the link between SN progenitors and LBV
stars. We also confirm the existence of a physical mechanism for a single star
to have episodic radio light curve modulations. In the case of the 25Msun
progenitors, we do not obtain modulations in the radio light curve, but our
models may miss some outbursting behavior in the late stages of massive stars.Comment: 5 pages, 3 figures, accepted by Astronomy & Astrophysics Letter
Coexistence of solutions in dynamical mean-field theory of the Mott transition
In this paper, I discuss the finite-temperature metal-insulator transition of
the paramagnetic Hubbard model within dynamical mean-field theory. I show that
coexisting solutions, the hallmark of such a transition, can be obtained in a
consistent way both from Quantum Monte Carlo (QMC) simulations and from the
Exact Diagonalization method. I pay special attention to discretization errors
within QMC. These errors explain why it is difficult to obtain the solutions by
QMC close to the boundaries of the coexistence region.Comment: 3 pages, 2 figures, RevTe
Spectral properties in the charge density wave phase of the half-filled Falicov-Kimball Model
We study the spectral properties of charge density wave (CDW) phase of the
half-filled spinless Falicov-Kimball model within the framework of the
Dynamical Mean Field Theory. We present detailed results for the spectral
function in the CDW phase as function of temperature and . We show how the
proximity of the non-fermi liquid phase affects the CDW phase, and show that
there is a region in the phase diagram where we get a CDW phase without a gap
in the spectral function. This is a radical deviation from the mean-field
prediction where the gap is proportional to the order parameter
Forecasting with Spatial Panel Data
This paper compares various forecasts using panel data with spatial error correlation. The true data generating process is assumed to be a simple error component regression model with spatial remainder disturbances of the autoregressive or moving average type. The best linear unbiased predictor is compared with other forecasts ignoring spatial correlation, or ignoring heterogeneity due to the individual effects, using Monte Carlo experiments. In addition, we check the performance of these forecasts under misspecification of the spatial error process, various spatial weight matrices, and heterogeneous rather than homogeneous panel data models.forecasting, BLUP, panel data, spatial dependence, heterogeneity
Heavy-fermion and spin-liquid behavior in a Kondo lattice with magnetic frustration
We study the competition between the Kondo effect and frustrating exchange
interactions in a Kondo-lattice model within a large- dynamical
mean-field theory. We find a T=0 phase transition between a heavy Fermi-liquid
and a spin-liquid for a critical value of the exchange , the
single-impurity Kondo temperature. Close to the critical point, the Fermi
liquid coherence scale is strongly reduced and the effective mass
strongly enhanced. The regime is characterized by spin-liquid
magnetic correlations and non-Fermi-liquid properties. It is suggested that
magnetic frustration is a general mechanism which is essential to explain the
large effective mass of some metallic compounds such as LiVO.Comment: 7 pages, 1 figure. Late
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