103 research outputs found
Improving the extragalactic distance scale using Cepheids in M33
Cepheids have been used as distance indicators for almost a century. Through their well defined period-luminosity relation, their distance can be found from their period of pulsation alone. However, in recent years, an important uncertainty has arisen in its calibration, namely the effect of metallicity on the period-luminosity relation. It is important that we quantify this effect as Cepheids are used to tie down the base of the extragalactic distance scale. Without an accurate calibration for the nearest distances, the distances we measure for all other objects will be subject to systematic uncertainties. This thesis studies how the chemical composition of a Cepheid affects the zero-point of the period-luminosity relation, and quantifies the resulting change in measured distance modulus. The first study uses Cepheids in two areas of M33; a sample from the metal-rich central region is compared with the more metal-poor sample from the southern spiral arm. Period-luminosity relations in the reddening-free Wesenheit index Wvi are constructed for the two samples and the measured distance moduli are compared. A significant offset is found between the two samples. Effects such as reddening and blending are ruled out, leaving the change in metallicity as the only possible explanation for the discrepancy. The second study presented covers the whole of M33. Here, a sample of around 600 Cepheids is used, and the effect of metallicity on Wgi and Wri is measured. The same conclusion is found; the change in composition significantly affects the measured distance modulus of the Cepheid and the period-luminosity relation must be changed to take this into account. In addition, the Sloan band period-luminosity relations are derived empirically for the first time. The metallicity gradient of M33 is also assessed. It is found that the slope must be steep, at least in the central few kpc of the galaxy, but may flatten off at larger radial distances
Physical parameters and the projection factor of the classical Cepheid in the binary system OGLE-LMC-CEP-0227
A novel method of analysis of double-lined eclipsing binaries containing a
radially pulsating star is presented. The combined pulsating-eclipsing light
curve is built up from a purely eclipsing light curve grid created using an
existing modeling tool. For every pulsation phase the instantaneous radius and
surface brightness are taken into account, being calculated from the
disentangled radial velocity curve of the pulsating star and from its
out-of-eclipse pulsational light curve and the light ratio of the components,
respectively. The best model is found using the Markov Chain Monte Carlo
method.
The method is applied to the eclipsing binary Cepheid OGLE-LMC-CEP-0227
(P_puls = 3.80 d, P_orb = 309 d). We analyze a set of new spectroscopic and
photometric observations for this binary, simultaneously fitting OGLE V-band,
I-band and Spitzer 3.6 {\mu}m photometry. We derive a set of fundamental
parameters of the system significantly improving the precision comparing to the
previous results obtained by our group. The Cepheid mass and radius are M_1 =
4.165 +/- 0.032 M_solar and R_1 = 34.92 +/- 0.34 R_solar, respectively.
For the first time a direct, geometrical and distance-independent
determination of the Cepheid projection factor is presented. The value p = 1.21
+/- 0.03(stat.) +/- 0.04(syst.) is consistent with theoretical expectations for
a short period Cepheid and interferometric measurements for {\delta} Cep. We
also find a very high value of the optical limb darkening coefficients for the
Cepheid component, in strong disagreement with theoretical predictions for
static atmospheres at a given surface temperature and gravity.Comment: 16 pages, 17 figures, accepted for publication in MNRA
A large-scale survey for variable stars in M33
We have started a survey of M 33 in order to find variable stars and Cepheids
in particular. We have obtained more than 30 epochs of g'r'i' data with the
CFHT and the one-square-degree camera MegaCam. We present first results from
this survey, including the search for variable objects and a basic
characterization of the various groups of variable stars.Comment: To appear in the proceedings of the "Nonlinear stellar
hydrodynamics", conference in honor of Robert Buchler's 65th birthday, July
2007, Pari
On the distance of the globular cluster M4 (NGC 6121) using RR Lyrae stars: I. optical and near-infrared Period-Luminosity and Period-Wesenheit relations
We present new distance determinations to the nearby globular M4 (NGC~6121)
based on accurate optical and Near Infrared (NIR) mean magnitudes for
fundamental (FU) and first overtone (FO) RR Lyrae variables (RRLs), and new
empirical optical and NIR Period-Luminosity (PL) and Period-Wesenheit (PW)
relations. We have found that optical-NIR and NIR PL and PW relations are
affected by smaller standard deviations than optical relations. The difference
is the consequence of a steady decrease in the intrinsic spread of cluster RRL
apparent magnitudes at fixed period as longer wavelengths are considered. The
weighted mean visual apparent magnitude of 44 cluster RRLs is
\left=13.329\pm0.001 (standard error of the mean) 0.177
(weighted standard deviation) mag. Distances were estimated using RR Lyr itself
to fix the zero-point of the empirical PL and PW relations. Using the entire
sample (FUFO) we found weighted mean true distance moduli of
11.350.030.05 mag and 11.320.020.07 mag. Distances were
also evaluated using predicted metallicity dependent PLZ and PWZ relations. We
found weighted mean true distance moduli of 11.2830.0100.018 mag (NIR
PLZ) and 11.2720.0050.019 mag (optical--NIR and NIR PWZ). The above
weighted mean true distance moduli agree within 1. The same result is
found from distances based on PWZ relations in which the color index is
independent of the adopted magnitude (11.2720.0040.013 mag). These
distances agree quite well with the geometric distance provided by
\citep{kaluzny2013} based on three eclipsing binaries. The available evidence
indicates that this approach can provide distances to globulars hosting RRLs
with a precision better than 2--3\%.Comment: Accepted for publication on Ap
Young and Intermediate-age Distance Indicators
Distance measurements beyond geometrical and semi-geometrical methods, rely
mainly on standard candles. As the name suggests, these objects have known
luminosities by virtue of their intrinsic proprieties and play a major role in
our understanding of modern cosmology. The main caveats associated with
standard candles are their absolute calibration, contamination of the sample
from other sources and systematic uncertainties. The absolute calibration
mainly depends on their chemical composition and age. To understand the impact
of these effects on the distance scale, it is essential to develop methods
based on different sample of standard candles. Here we review the fundamental
properties of young and intermediate-age distance indicators such as Cepheids,
Mira variables and Red Clump stars and the recent developments in their
application as distance indicators.Comment: Review article, 63 pages (28 figures), Accepted for publication in
Space Science Reviews (Chapter 3 of a special collection resulting from the
May 2016 ISSI-BJ workshop on Astronomical Distance Determination in the Space
Age
The Large Magellanic Cloud and the Distance Scale
The Magellanic Clouds, especially the Large Magellanic Cloud, are places
where multiple distance indicators can be compared with each other in a
straight-forward manner at considerable precision. We here review the distances
derived from Cepheids, Red Variables, RR Lyraes, Red Clump Stars and Eclipsing
Binaries, and show that the results from these distance indicators generally
agree to within their errors, and the distance modulus to the Large Magellanic
Cloud appears to be defined to 3% with a mean value of 18.48 mag, corresponding
to 49.7 Kpc. The utility of the Magellanic Clouds in constructing and testing
the distance scale will remain as we move into the era of Gaia.Comment: 23 pages, accepted for publication in Astrophysics and Space Science.
From a presentation at the conference The Fundamental Cosmic Distance Scale:
State of the Art and the Gaia Perspective, Naples, May 201
Toward an internally consistent astronomical distance scale
Accurate astronomical distance determination is crucial for all fields in
astrophysics, from Galactic to cosmological scales. Despite, or perhaps because
of, significant efforts to determine accurate distances, using a wide range of
methods, tracers, and techniques, an internally consistent astronomical
distance framework has not yet been established. We review current efforts to
homogenize the Local Group's distance framework, with particular emphasis on
the potential of RR Lyrae stars as distance indicators, and attempt to extend
this in an internally consistent manner to cosmological distances. Calibration
based on Type Ia supernovae and distance determinations based on gravitational
lensing represent particularly promising approaches. We provide a positive
outlook to improvements to the status quo expected from future surveys,
missions, and facilities. Astronomical distance determination has clearly
reached maturity and near-consistency.Comment: Review article, 59 pages (4 figures); Space Science Reviews, in press
(chapter 8 of a special collection resulting from the May 2016 ISSI-BJ
workshop on Astronomical Distance Determination in the Space Age
The Scottish Bladder Cancer Quality Performance Indicators Influencing Outcomes, Prognosis, and Surveillance (Scot BC Quality OPS) Clinical Project
The aim of the Scot BC Quality OPS clinical project is to create a reliable prospective data set for evaluating real-world effectiveness and efficiency consequent to standardisation and monitoring of bladder cancer treatment (through the national Quality Performance Indicator programme) and streamlined surveillance in Scotland. Several work packages have been created, reflecting wide clinical and research collaboration
SMHASH: Anatomy of the Orphan Stream using RR Lyrae stars
Stellar tidal streams provide an opportunity to study the motion and structure of the disrupting galaxy as well as the gravitational potential of its host. Streams around the Milky Way are especially promising as phase space positions of individual stars will be measured by ongoing or upcoming surveys. Nevertheless, it remains a challenge to accurately assess distances to stars farther than 10 kpc from the Sun, where we have the poorest knowledge of the Galaxy's mass distribution. To address this we present observations of 32 candidate RR Lyrae stars in the Orphan tidal stream taken as part of the Spitzer Merger History and Shape of the Galactic Halo (SMHASH) program. The extremely tight correlation between the periods, luminosities, and metallicities of RR Lyrae variable stars in the Spitzer IRAC band allows the determination of precise distances to individual stars; the median statistical distance uncertainty to each RR Lyrae star is . By fitting orbits in an example potential we obtain an upper limit on the mass of the Milky Way interior to 60 kpc of , bringing estimates based on the Orphan Stream in line with those using other tracers. The SMHASH data also resolve the stream in line--of--sight depth, allowing a new perspective on the internal structure of the disrupted dwarf galaxy. Comparing with N--body models we find that the progenitor had an initial dark halo mass of approximately , placing the Orphan Stream's progenitor amongst the classical dwarf spheroidals
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