514 research outputs found
The Role of Opacities in Stellar Pulsation
We examine the role of opacities in stellar pulsation with reference to
Cepheids and RR Lyraes, and examine the effect of augmented opacities on the
theoretical pulsation light curves in key temperature ranges. The temperature
ranges are provided by recent experimental and theoretical work that have
suggested that the iron opacities have been considerably underestimated. For
Cepheids, we find that the augmented opacities have noticeable effects in
certain period ranges (around ) even though there is a
degeneracy with mixing length. We also find significant effects in theoretical
models of B-star pulsators.Comment: 6 pages, 3 Figures, Proceeding for the "Workshop on Astrophysical
Opacities
Classical Cepheid Pulsation Models: IX. New Input Physics
We constructed several sequences of classical Cepheid envelope models at
solar chemical composition () to investigate the dependence of
the pulsation properties predicted by linear and nonlinear hydrodynamical
models on input physics. To study the dependence on the equation of state (EOS)
we performed several numerical experiments by using the simplified analytical
EOS originally developed by Stellingwerf and the recent analytical EOS
developed by Irwin. Current findings suggest that the pulsation amplitudes as
well as the topology of the instability strip marginally depend on the adopted
EOS.
We also investigated the dependence of observables predicted by theoretical
models on the mass-luminosity (ML) relation and on the spatial resolution
across the Hydrogen and the Helium partial ionization regions. We found that
nonlinear models are marginally affected by these physical and numerical
assumptions. In particular, the difference between new and old models in the
location as well as in the temperature width of the instability strip is on
average smaller than 200 K. However, the spatial resolution somehow affects the
pulsation properties. The new fine models predict a period at the center of the
Hertzsprung Progression (9.84 days) that reasonably agree with
empirical data based on light curves ( days;
\citealt{mbm92}) and on radial velocity curves ( days;
\citealt{mall00}), and improve previous predictions by Bono, Castellani, and
Marconi (2000, hereinafter BCM00).Comment: 35 pages, 7 figures. Accepted for publication in the Astrophysical
Journa
Investigations of the Non-Linear LMC Cepheid Period-Luminosity Relation with Testimator and Schwarz Information Criterion Methods
In this paper, we investigate the linearity versus non-linearity of the Large
Magellanic Cloud (LMC) Cepheid period-luminosity (P-L) relation using two
statistical approaches not previously applied to this problem: the testimator
method and the Schwarz Information Criterion (SIC). The testimator method is
extended to multiple stages for the first time, shown to be unbiased and the
variance of the estimated slope can be proved to be smaller than the standard
slope estimated from linear regression theory. The Schwarz Information
Criterion (also known as the Bayesian Information Criterion) is more
conservative than the Akaike Information Criterion and tends to choose lower
order models. By using simulated data sets, we verify that these statistical
techniques can be used to detect intrinsically linear and/or non-linear P-L
relations. These methods are then applied to independent LMC Cepheid data sets
from the OGLE project and the MACHO project, respectively. Our results imply
that there is a change of slope in longer period ranges for all of the data
sets. This strongly supports previous results, obtained from independent
statistical tests, that the observed LMC P-L relation is non-linear with a
break period at/around 10 days.Comment: 9 pages, 5 figures and 3 tables, PASP accepte
A multi-wavelength analysis of BL Her stars: Models versus Observations
We present new theoretical period--luminosity (PL) and period--radius (PR)
relations at multiple wavelengths (Johnson--Cousins--Glass and {\sl Gaia}
passbands) for a fine grid of BL~Herculis models computed using {\sc mesa-rsp}.
The non-linear models were computed for periods typical of BL~Her stars, i.e.
, covering a wide range of input parameters:
metallicity (2.0 dex [Fe/H] 0.0 dex), stellar mass (0.5--0.8
M), luminosity (50--300 L) and effective temperature (full
extent of the instability strip; in steps of 50K). We investigate the impact of
four sets of convection parameters on multi-wavelength properties. Most
empirical relations match well with theoretical relations from the BL~Her
models computed using the four sets of convection parameters. No significant
metallicity effects are seen in the PR relations. Another important result from
our grid of BL~Her models is that it supports combining PL relations of RR
Lyrae and Type~II Cepheids together as an alternative to classical Cepheids for
the extragalactic distance scale calibration.Comment: 8 pages, 3 figures, Proceedings for IAUS376 'At the cross-roads of
astrophysics and cosmology: Period-luminosity relations in the 2020s',
Budapest, April 202
Determination of Cepheid parameters by light-curve template-fitting
We describe techniques to characterise the light-curves of regular variable
stars by applying principal component analysis (PCA) to a training set of high
quality data, and to fit the resulting light-curve templates to sparse and
noisy photometry to obtain parameters such as periods, mean magnitudes etc. The
PCA approach allows us to efficiently represent the multi-band light-curve
shapes of each variable, and hence quantitatively describe the average
behaviour of the sample as a smoothly varying function of period, and also the
range of variation around this average.
In this paper we focus particularly on the utility of such methods for
analysing HST Cepheid photometry, and present simulations which illustrate the
advantages of our PCA template-fitting approach. These are: accurate parameter
determination, including light-curve shape information; simultaneous fitting to
multiple passbands; quantitative error analysis; objective rejection of
variables with non Cepheid-like light-curves or those with potential period
aliases.
We also use PCA to confirm that Cepheid light-curve shapes are systematically
different (at the same period) between the Milky Way (MW) and the Large and
Small Magellanic Clouds (LMC, SMC), and consider whether light-curve shape
might therefore be used to estimate the mean metallicities of Cepheid samples,
thus allowing metallicity corrections to be applied to derived distance
estimates.Comment: MNRAS in press: revised in light of referees comment
Microlensing Candidates in M87 and the Virgo Cluster with the Hubble Space Telescope
The position of the giant elliptical galaxy M87 at the center of the Virgo
Cluster means that the inferred column density of dark matter associated with
both the cluster halo and the galaxy halo is quite large. This system is thus
an important laboratory for studying massive dark objects in elliptical
galaxies and galaxy clusters by gravitational microlensing, strongly
complementing the studies of spiral galaxy halos performed in the Local Group.
We have performed a microlensing survey of M87 with the WFPC2 instrument on the
Hubble Space Telescope. Over a period of thirty days, with images taken once
daily, we discover seven variable sources. Four are variable stars of some
sort, two are consistent with classical novae, and one exhibits an excellent
microlensing lightcurve, though with a very blue color implying the somewhat
disfavored possibility of a horizontal branch source being lensed. Based on
sensitivity calculations from artificial stars and from artificial lightcurves,
we estimate the expected microlensing rate. We find that the detection of one
event is consistent with a dark halo with a 20% contribution of microlensing
objects for both M87 and the Virgo Cluster, similar to the value found from
observations in the Local Group. Further work is required to test the
hypothesized microlensing component to the cluster.Comment: 22 pages, 9 figures, revised version submitted to Ap
Taxing the Informal Economy: The Current State of Knowledge and Agendas for Future Research
This paper reviews the literature on taxation of the informal economy, taking stock of key debates
and drawing attention to recent innovations. Conventionally, the debate on whether to tax has frequently focused
on the limited revenue potential, high cost of collection, and potentially adverse impact on small firms. Recent
arguments have increasingly emphasised the more indirect benefits of informal taxation in relation to economic
growth, broader tax compliance, and governance. More research is needed, we argue, into the relevant costs and
benefits for all, including quasi-voluntary compliance, political and administrative incentives for reform, and
citizen-state bargaining over taxation
The extra-galactic Cepheid distance scale from LMC and Galactic period-luminosity relations
In this paper, we recalibrate the Cepheid distance to some nearby galaxies observed by the HST Key Project and the Sandage-Tammann-Saha group. We use much of the Key Project methodology in our analysis but apply new techniques, based on Fourier methods to estimate the mean of a sparsely sampled Cepheid light curve, to published extra-galactic Cepheid data. We also apply different calibrating PL relations to estimate Cepheid distances, and investigate the sensitivity of the distance moduli to the adopted calibrating PL relation. We re-determine the OGLELMC PL relations using a more conservative approach and also study the effect of using Galactic PL relations on the distance scale. For the Key Project galaxies after accounting for charge transfer effects, we find good agreement with an average discrepancy of -0.002 and 0.075 mag when using the LMC and Galaxy, respectively, as a calibrating PL relation. For NGC 4258 which has a geometric distance of 29.28 mag, we find a distance modulus of 29. 44 ± 0.06(random) mag, after correcting for metallicity. In addition we have calculated the Cepheid distance to 8 galaxies observed by the Sandage-Tammann-Saha group and find shorter distance moduli by -0.178 mag (mainly due to the use of different LMC PL relations) and -0.108 mag on average again when using the LMC and Galaxy, respectively, as a calibrating PL relation. However care must be taken to extrapolate these changed distances to changes in the resulting values of the Hubble constant because STS also use distances to NGC 3368 and 4414 and because STS calibration of SN la is often decoupled from the distance to the host galaxy through their use of differential extinction arguments. We also calculate the distance to all these galaxies using PL relations at maximum light and find very good agreement with mean light PL distances. However, after correcting for metallicity effects, the difference between the distance moduli obtained using the two sets of calibrating PL relations becomes negligible. This suggests that Cepheids in the LMC and Galaxy do follow different PL relations and constrains the sign for the coefficient of the metallicity correction, γ, to be negative, at least at the median period log(P) approximately equals 1.4, of the target galaxies
Poverty and 'Second Economy' in South Africa: An Attempt to Clarify Applicable Concepts and Quantify Extent of Relevant Challenges
In brief, the paper firstly summarises old and new theoretical and technical issues on measuring poverty, secondly analyses poverty from different perspectives and highlights various research findings on poverty trends in South Africa and thirdly clarifies the notion of "second economy" and attempts to "measure" it
Kepler photometry of RRc stars: peculiar double-mode pulsations and period doubling
We present the analysis of four first overtone RR Lyrae stars observed with the Kepler space telescope, based on data obtained over nearly 2.5 yr. All four stars are found to be multiperiodic.
The strongest secondary mode with frequency f2 has an amplitude of a few mmag, 20–45 times lower than the main radial mode with frequency f1. The two oscillations have a period ratio of P2/P1 = 0.612–0.632 that cannot be reproduced by any two radial modes. Thus, the secondary mode is non-radial. Modes yielding similar period ratios have also recently been discovered in other variables of the RRc and RRd types. These objects form a homogenous group and constitute a new class of multimode RR Lyrae pulsators, analogous to a similar class of multimode classical Cepheids in the Magellanic Clouds. Because a secondary mode with P2/P1 ∼ 0.61 is found in almost every RRc and RRd star observed from space, this form of multiperiodicity must be common. In all four Kepler RRc stars studied, we find subharmonics of f2 at ∼1/2f2 and at ∼3/2f2. This is a signature of period doubling of the secondary oscillation, and is the first detection of period doubling in RRc stars. The amplitudes and phases of f2 and its subharmonics are variable on a time-scale of 10–200 d. The dominant radial mode also shows variations on the same time-scale, but with much smaller amplitude. In three Kepler RRc stars we detect additional periodicities, with amplitudes below 1 mmag, that must correspond to non-radial g-modes. Such modes never before have been observed in RR Lyrae variables
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