17,503 research outputs found
Incorporating a Tracking Signal into State Space Models for Exponential Smoothing
It is a common practice to complement a forecasting method such as simple exponential smoothing with a monitoring scheme to detect those situations where forecasts have failed to adapt to structural change. It will be suggested in this paper that the equations for simple exponential smoothing can be augmented by a common monitoring statistic to provide a method that automatically adapts to structural change without human intervention. It is shown that the resulting equations conform to those of damped trend corrected exponential smoothing. In a similar manner, exponential smoothing with drift, when augmented by the same monitoring statistic, produces equations that split the trend into long term and short term components.Forecasting, exponential smoothing, tracking signals.
The UCSD HIRES/KeckI Damped Lya Abundance Database: II. The Implications
We present a comprehensive analysis of the damped Lya abundance database
presented in the first paper of this series. This database provides a
homogeneous set of abundance measurements for many elements including Si, Cr,
Ni, Zn, Fe, Al, S, Co, O, and Ar from 38 damped Lya systems with z > 1.5. With
little exception, these damped Llya systems exhibit very similar relative
abundances. There is no significant correlation in X/Fe with [Fe/H] metallicity
and the dispersion in X/Fe is small at all metallicity.
We search the database for trends indicative of dust depletion and in a few
cases find strong evidence. Specifically, we identify a correlation between
[Si/Ti] and [Zn/Fe] which is unambiguous evidence for depletion.
We present a discussion on the nucleosynthetic history of the damped Lya
systems by focusing on abundance patterns which are minimally affected by dust
depletion. We find [Si/Fe] -> +0.25 dex as [Zn/Fe] -> 0 and that the [Si/Fe]
values exhibit a plateau of ~+0.3 dex at [Si/H] < -1.5 dex. Together these
trends indicate significant alpha-enrichment in the damped Lya systems at low
metallicity, an interpretation further supported by the observed O/Fe, S/Fe and
Ar/Fe ratios. We also discuss Fe-peak nucleosynthesis and the odd-even effect.
To assess the impact of dust obscuration, we present estimates of the
dust-to-gas ratios for the damped Lya sightlines and crudely calculate dust
extinction corrections. The distribution of extinction corrections suggests the
effects of dust obscuration are minimal and that the population of 'missing'
damped systems has physical characteristics similar to the observed sample.
We update our investigation on the chemical evolution of the early universe
in neutral gas. [significantly abridged]Comment: 29 pages, 26 figures. Uses emulateapj.sty. Accepted to ApJ: Oct 15,
200
Sulphur and zinc abundances in Galactic stars and damped Lyman-alpha systems
High resolution spectra of 34 halo population dwarf and subgiant stars have
been obtained with VLT/UVES and used to derive sulphur abundances from the
8694.0, 8694.6 A and 9212.9, 9237.5 A SI lines. In addition, iron abundances
have been determined from 19 FeII lines and zinc abundances from the 4722.2,
4810.5 ZnI lines. The abundances are based on a classical 1D, LTE model
atmosphere analysis, but effects of 3D hydrodynamical modelling on the [S/Fe],
[Zn/Fe] and [S/Zn] ratios are shown to be small. We find that most halo stars
with metallicities in the range -3.2 < [Fe/H] < -0.8 have a near-constant
[S/Fe] = +0.3; a least square fit to [S/Fe] vs. {Fe/H] shows a slope of only
-0.04 +/- 0.01. Among halo stars with -1.2 < [Fe/H] < -0.8 the majority have
[S/Fe] ~ +0.3, but two stars (previously shown to have low [alpha/Fe] ratios)
have [S/Fe] ~ 0. For disk stars with [Fe/H] > -1, [S/Fe] decreases with
increasing [Fe/H]. Hence, sulphur behaves like other typical alpha-capture
elements, Mg, Si and Ca. Zinc, on the other hand, traces iron over three orders
of magnitude in [Fe/H], although there is some evidence for a small systematic
Zn overabundance ([Zn/Fe] ~ +0.1) among metal-poor disk stars and for halo
stars with [Fe/H] < -2.0. Recent measurements of S and Zn in ten damped
Ly-alpha systems (DLAs) with redshifts between 1.9 and 3.4 and zinc abundances
in the range -2.1 < [Zn/H] < -0.15 show an offset relative to the [S/Zn] -
[Zn/H] relation in Galactic stars. Possible reasons for this offset are
discussed, including low and intermittent star formation rates in DLAs.Comment: Accepted for publication in A&A. 16pages; 9 figure
Do Damped and Sub-damped Lyman-alpha Absorbers Arise in Galaxies of Different Masses?
We consider the questions of whether the damped Lyman-alpha (DLA) and sub-DLA
absorbers in quasar spectra differ intrinsically in metallicity, and whether
they could arise in galaxies of different masses. Using the recent measurements
of the robust metallicity indicators Zn and S in DLAs and sub-DLAs, we confirm
that sub-DLAs have higher mean metallicities than DLAs, especially at . We find that the intercept of the metallicity-redshift relation
derived from Zn and S is higher than that derived from Fe by 0.5-0.6 dex. We
also show that, while there is a correlation between the metallicity and the
rest equivalent width of Mg II or Fe II for DLAs,
no correlation is seen for sub-DLAs. Given this, and the similar Mg II or Fe II
selection criteria employed in the discovery of both types of systems at lower
redshifts, the difference between metallicities of DLAs and sub-DLAs appears to
be real and not an artefact of selection. This conclusion is supported by our
simulations of Mg II and Fe II lines for a wide
range of physical conditions. On examining the velocity spreads of the
absorbers, we find that sub-DLAs show somewhat higher mean and median velocity
spreads (), and an excess of systems with km
s, than DLAs. Compared to DLAs, the [Mn/Fe] vs. [Zn/H] trend for
sub-DLAs appears to be steeper and closer to the trend for Galactic bulge and
thick disk stars, possibly suggesting different stellar populations. The
absorber data appear to be consistent with galaxy down-sizing. The data are
also consistent with the relative number densities of low-mass and high-mass
galaxies. It is thus plausible that sub-DLAs arise in more massive galaxies on
average than DLAs.Comment: 27 pages, 5 figures, 4 tables. Accepted for publication in New
Astronom
Assessment of stochastic and deterministic models of 6304 quasar lightcurves from SDSS Stripe 82
The optical light curves of many quasars show variations of tenths of a
magnitude or more on time scales of months to years. This variation often
cannot be described well by a simple deterministic model. We perform a Bayesian
comparison of over 20 deterministic and stochastic models on 6304 QSO light
curves in SDSS Stripe 82. We include the damped random walk (or
Ornstein-Uhlenbeck [OU] process), a particular type of stochastic model which
recent studies have focused on. Further models we consider are single and
double sinusoids, multiple OU processes, higher order continuous autoregressive
processes, and composite models. We find that only 29 out of 6304 QSO
lightcurves are described significantly better by a deterministic model than a
stochastic one. The OU process is an adequate description of the vast majority
of cases (6023). Indeed, the OU process is the best single model for 3462 light
curves, with the composite OU process/sinusoid model being the best in 1706
cases. The latter model is the dominant one for brighter/bluer QSOs.
Furthermore, a non-negligible fraction of QSO lightcurves show evidence that
not only the mean is stochastic but the variance is stochastic, too. Our
results confirm earlier work that QSO light curves can be described with a
stochastic model, but place this on a firmer footing, and further show that the
OU process is preferred over several other stochastic and deterministic models.
Of course, there may well exist yet better (deterministic or stochastic) models
which have not been considered here.Comment: accepted by AA, 12 pages, 11 figures, 4 table
Automatic Time Series Forecasting: The forecast Package for R
Automatic forecasts of large numbers of univariate time series are often needed in business and other contexts. We describe two automatic forecasting algorithms that have been implemented in the forecast package for R. The first is based on innovations state space models that underly exponential smoothing methods. The second is a step-wise algorithm for forecasting with ARIMA models. The algorithms are applicable to both seasonal and non-seasonal data, and are compared and illustrated using four real time series. We also briefly describe some of the other functionality available in the forecast package.
Automatic time series forecasting: the forecast package for R.
Automatic forecasts of large numbers of univariate time series are often needed in business and other contexts. We describe two automatic forecasting algorithms that have been implemented in the forecast package for R. The first is based on innovations state space models that underly exponential smoothing methods. The second is a step-wise algorithm for forecasting with ARIMA models. The algorithms are applicable to both seasonal and non-seasonal data, and are compared and illustrated using four real time series. We also briefly describe some of the other functionality available in the forecast package.ARIMA models; automatic forecasting; exponential smoothing; prediction intervals; state space models; time series, R.
Oscillations in the dark energy EoS: new MCMC lessons
We study the possibility of detecting oscillating patterns in the equation of
state (EoS) of the dark energy using different cosmological datasets. We follow
a phenomenological approach and study three different oscillating models for
the EoS, one of them periodic and the other two damped (proposed here for the
first time). All the models are characterised by the amplitude value, the
centre and the frequency of oscillations. In contrast to previous works in the
literature, we do not fix the value of the frequency to a fiducial value
related to the time extension of chosen datasets, but consider a discrete set
of values, so to avoid arbitrariness and try and detect any possible time
period in the EoS. We test the models using a recent collection of SNeIa,
direct Hubble data and Gamma Ray Bursts data. Main results are: I. even if
constraints on the amplitude are not too strong, we detect a trend of it versus
the frequency, i.e. decreasing (and even negatives) amplitudes for higher
frequencies; II. the centre of oscillation (which corresponds to the present
value of the EoS parameter) is very well constrained, phantom behaviour is
excluded at level and trend which is in agreement with the one for
the amplitude appears; III. the frequency is hard to constrain, showing similar
statistical validity for all the values of the discrete set chosen, but the
best fit of all the scenarios considered is associated with a period which is
in the redshift range depicted by our cosmological data. The "best" oscillating
models are compared with CDM using dimensionally consistent a Bayesian
approach based information criterion and the conclusion reached is the non
existence of significant evidence against dark energy oscillations.Comment: 12 papers, mn2e, 8 figure
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