856 research outputs found
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/
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
Are the HI deficient galaxies on the outskirts of Virgo recent arrivals?
The presence on the Virgo cluster outskirts of spiral galaxies with gas
deficiencies as strong as those of the inner galaxies stripped by the
intracluster medium has led us to explore the possibility that some of these
peripheral objects are not newcomers. A dynamical model for the collapse and
rebound of spherical shells under the point mass and radial flow approximations
has been developed to account for the amplitude of the motions in the Virgo I
cluster (VIC) region. According to our analysis, it is not unfeasible that
galaxies far from the cluster, including those in a gas-deficient group well to
its background, went through its core a few Gyr ago. The implications would be:
(1) that the majority of the HI-deficient spirals in the VIC region might have
been deprived of their neutral hydrogen by interactions with the hot
intracluster medium; and (2) that objects spending a long time outside the
cluster cores might keep the gas deficient status without altering their
morphology.Comment: Accepted for publication in ApJ. 4 pages, 3 figures. Uses emulateapj
Recommended from our members
A New Calibration Of Galactic Cepheid Period-Luminosity Relations From B To K Bands, And A Comparison To LMC Relations
Context. The universality of the Cepheid period-luminosity (PL) relations has been under discussion since metallicity effects were assumed to play a role in the value of the intercept and, more recently, of the slope of these relations. Aims. The goal of the present study is to calibrate the Galactic PL relations in various photometric bands (from B to K) and to compare the results to the well-established PL relations in the LMC. Methods. We use a set of 59 calibrating stars, the distances of which are measured using five different distance indicators: Hubble Space Telescope and revised Hipparcos parallaxes, infrared surface brightness and interferometric Baade-Wesselink parallaxes, and classical Zero-Age-Main-Sequence-fitting parallaxes for Cepheids belonging to open clusters or OB stars associations. A detailed discussion of absorption corrections and projection factor to be used is given. Results. We find no significant difference in the slopes of the PL relations between LMC and our Galaxy. Conclusions. We conclude that the Cepheid PL relations have universal slopes in all photometric bands, not depending on the galaxy under study (at least for LMC and Milky Way). The possible zero-point variation with metal content is not discussed in the present work, but an upper limit of 18.50 for the LMC distance modulus can be deduced from our data.McDonald Observator
Measurement of Source Star Colors with the K2C9-CFHT Multi-color Microlensing Survey
K2 Campaign 9 (K2C9) was the first space-based microlensing parallax survey
capable of measuring microlensing parallaxes of free-floating planet candidate
microlensing events. Simultaneous to K2C9 observations we conducted the K2C9
Canada-France-Hawaii Telescope Multi-Color Microlensing Survey (K2C9-CFHT MCMS)
in order to measure the colors of microlensing source stars to improve the
accuracy of K2C9's parallax measurements. We describe the difference imaging
photometry analysis of the K2C9-CFHT MCMS observations, and present the
project's first data release. This includes instrumental difference flux
lightcurves of 217 microlensing events identified by other microlensing
surveys, reference image photometry calibrated to PanSTARRS data release 1
photometry, and tools to convert between instrumental and calibrated flux
scales. We derive accurate analytic transformations between the PanSTARRS
bandpasses and the Kepler bandpass, as well as angular diameter-color relations
in the PanSTARRS bandpasses. To demonstrate the use of our data set, we analyze
ground-based and K2 data of a short timescale microlensing event,
OGLE-2016-BLG-0795. We find the event has a timescale ~days and microlens parallax or , subject to the standard satellite parallax degeneracy. We argue that the
smaller value of the parallax is more likely, which implies that the lens is
likely a stellar-mass object in the Galactic bulge as opposed to a
super-Jupiter mass object in the Galactic disk.Comment: Submitted to PAS
Additive-multiplicative stochastic models of financial mean-reverting processes
We investigate a generalized stochastic model with the property known as mean
reversion, that is, the tendency to relax towards a historical reference level.
Besides this property, the dynamics is driven by multiplicative and additive
Wiener processes. While the former is modulated by the internal behavior of the
system, the latter is purely exogenous. We focus on the stochastic dynamics of
volatilities, but our model may also be suitable for other financial random
variables exhibiting the mean reversion property. The generalized model
contains, as particular cases, many early approaches in the literature of
volatilities or, more generally, of mean-reverting financial processes. We
analyze the long-time probability density function associated to the model
defined through a It\^o-Langevin equation. We obtain a rich spectrum of shapes
for the probability function according to the model parameters. We show that
additive-multiplicative processes provide realistic models to describe
empirical distributions, for the whole range of data.Comment: 8 pages, 3 figure
Pricing Options in Incomplete Equity Markets via the Instantaneous Sharpe Ratio
We use a continuous version of the standard deviation premium principle for
pricing in incomplete equity markets by assuming that the investor issuing an
unhedgeable derivative security requires compensation for this risk in the form
of a pre-specified instantaneous Sharpe ratio. First, we apply our method to
price options on non-traded assets for which there is a traded asset that is
correlated to the non-traded asset. Our main contribution to this particular
problem is to show that our seller/buyer prices are the upper/lower good deal
bounds of Cochrane and Sa\'{a}-Requejo (2000) and of Bj\"{o}rk and Slinko
(2006) and to determine the analytical properties of these prices. Second, we
apply our method to price options in the presence of stochastic volatility. Our
main contribution to this problem is to show that the instantaneous Sharpe
ratio, an integral ingredient in our methodology, is the negative of the market
price of volatility risk, as defined in Fouque, Papanicolaou, and Sircar
(2000).Comment: Keywords: Pricing derivative securities, incomplete markets, Sharpe
ratio, correlated assets, stochastic volatility, non-linear partial
differential equations, good deal bound
Extreme times for volatility processes
We present a detailed study on the mean first-passage time of volatility
processes. We analyze the theoretical expressions based on the most common
stochastic volatility models along with empirical results extracted from daily
data of major financial indices. We find in all these data sets a very similar
behavior that is far from being that of a simple Wiener process. It seems
necessary to include a framework like the one provided by stochastic volatility
models with a reverting force driving volatility toward its normal level to
take into account memory and clustering effects in volatility dynamics. We also
detect in data a very different behavior in the mean first-passage time
depending whether the level is higher or lower than the normal level of
volatility. For this reason, we discuss asymptotic approximations and confront
them to empirical results with a good agreement, specially with the ExpOU
model.Comment: 10, 6 colored figure
Self-Averaging Scaling Limits of Two-Frequency Wigner Distribution for Random Paraxial Waves
Two-frequency Wigner distribution is introduced to capture the asymptotic
behavior of the space-frequency correlation of paraxial waves in the radiative
transfer limits. The scaling limits give rises to deterministic transport-like
equations. Depending on the ratio of the wavelength to the correlation length
the limiting equation is either a Boltzmann-like integral equation or a
Fokker-Planck-like differential equation in the phase space. The solutions to
these equations have a probabilistic representation which can be simulated by
Monte Carlo method. When the medium fluctuates more rapidly in the longitudinal
direction, the corresponding Fokker-Planck-like equation can be solved exactly.Comment: typos correcte
- âŠ