171 research outputs found
Analysis of refill curve shape in ultrasound contrast agent studies
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135021/1/mp9534.pd
Extended envelopes around Galactic Cepheids III. Y Oph and alpha Per from near-infrared interferometry with CHARA/FLUOR
Unbiased angular diameter measurements are required for accurate distances to
Cepheids using the interferometric Baade Wesselink method (IBWM). The precision
of this technique is currently limited by interferometric measurements at the
1.5% level. At this level, the center-to-limb darkening (CLD) and the presence
of circumstellar envelopes (CSE) seem to be the two main sources of bias. The
observations we performed aim at improving our knowledge of the interferometric
visibility profile of Cepheids. In particular, we assess the systematic
presence of CSE around Cepheids in order determine accurate distances with the
IBWM free from CSE biased angular diameters. We observed a Cepheid (Y Oph) for
which the pulsation is well resolved and a non-pulsating yellow supergiant
(alpha Per) using long-baseline near-infrared interferometry. We interpreted
these data using a simple CSE model we previously developed. We found that our
observations of alpha Per do not provide evidence for a CSE. The measured CLD
is explained by an hydrostatic photospheric model. Our observations of Y Oph,
when compared to smaller baseline measurements, suggest that it is surrounded
by a CSE with similar characteristics to CSE found previously around other
Cepheids. We have determined the distance to Y Oph to be d=491+/-18 pc.
Additional evidence points toward the conclusion that most Cepheids are
surrounded by faint CSE, detected by near infrared interferometry: after
observing four Cepheids, all show evidence for a CSE. Our CSE non-detection
around a non-pulsating supergiant in the instability strip, alpha Per, provides
confidence in the detection technique and suggests a pulsation driven mass-loss
mechanism for the Cepheids.Comment: accepted for publication in Ap
UVSat: a concept of an ultraviolet/optical photometric satellite
Time-series photometry from space in the ultraviolet can be presently done
with only a few platforms, none of which is able to provide wide-field
long-term high-cadence photometry. We present a concept of UVSat, a twin space
telescope which will be capable to perform this kind of photometry, filling an
observational niche. The satellite will host two telescopes, one for
observations in the ultraviolet, the other for observations in the optical
band. We also briefly show what science can be done with UVSat.Comment: 6 pages, 2 figures, accepted for publication in the Proceedings of
the PAS (Proc. of the 2nd BRITE Science conference, Innsbruck
Period and light curve fluctuations of the Kepler Cepheid V1154 Cyg
We present a detailed period analysis of the bright Cepheid-type variable
star V1154 Cygni (V =9.1 mag, P~4.9 d) based on almost 600 days of continuous
observations by the Kepler space telescope. The data reveal significant
cycle-to-cycle fluctuations in the pulsation period, indicating that classical
Cepheids may not be as accurate astrophysical clocks as commonly believed:
regardless of the specific points used to determine the O-C values, the cycle
lengths show a scatter of 0.015-0.02 days over the 120 cycles covered by the
observations. A very slight correlation between the individual Fourier
parameters and the O-C values was found, suggesting that the O - C variations
might be due to the instability of the light curve shape. Random fluctuation
tests revealed a linear trend up to a cycle difference 15, but for long term,
the period remains around the mean value. We compare the measurements with
simulated light curves that were constructed to mimic V1154 Cyg as a perfect
pulsator modulated only by the light travel time effect caused by low-mass
companions. We show that the observed period jitter in V1154 Cyg represents a
serious limitation in the search for binary companions. While the Kepler data
are accurate enough to allow the detection of planetary bodies in close orbits
around a Cepheid, the astrophysical noise can easily hide the signal of the
light-time effect.Comment: published in MNRAS: 8 pages, 7 figure
An RR Lyrae family portrait: 33 stars observed in Pisces with K2-E2
A detailed analysis is presented of 33 RR Lyrae stars in Pisces observed with the Kepler space telescope over the 8.9-day long K2 Two-Wheel Concept Engineering Test. The sample includes not only fundamental-mode and first overtone (RRab and RRc) stars but the first two double-mode (RRd) stars that Kepler detected and the only modulated first-overtone star ever observed from space so far. The precision of the extracted K2 light curves made it possible to detect low-amplitude additional modes in all subtypes. All RRd and non-modulated RRc stars show the additional mode at PX /P1 ∼ 0.61 that was detected in previous space-based photometric measurements. A periodicity longer than the fundamental mode was tentatively identified in one RRab star that might belong to a gravity mode. We determined the photometric [Fe/H] values for all fundamental-mode stars and provide the preliminary results of our efforts to fit the double-mode stars with non-linear hydrodynamic pulsation models. The results from this short test run indicate that the K2 mission will be, and has started to be, an ideal tool to expand our knowledge about RR Lyrae stars. As a by-product of the target search and analysis, we identified 165 bona-fide double-mode RR Lyrae stars from the Catalina Sky Survey observations throughout the sky, 130 of which are new discoveries
Mean angular diameters, distances and pulsation modes of the classical Cepheids FF Aql and T Vul - CHARA/FLUOR near-infrared interferometric observations
We report the first angular diameter measurements of two classical Cepheids,
FF Aql and T Vul, that we have obtained with the FLUOR instrument installed at
the CHARA interferometric array. We obtain average limb-darkened angular
diameters of \theta_LD = 0.878 +/- 0.013 mas and \theta_LD = 0.629 +/- 0.013
mas, respectively for FF Aql and T Vul. Combining these angular diameters with
the HST-FGS trigonometric parallaxes leads to linear radii R = 33.6 +/- 2.2
Rsol and R = 35.6 +/- 4.4 Rsol, respectively. The comparison with empirical and
theoretical Period-Radius relations leads to the conclusion that these Cepheids
are pulsating in their fundamental mode. The knowledge of the pulsation mode is
of prime importance to calibrate the Period-Luminosity relation with a uniform
sample of fundamental mode Cepheids
The pulsating hot subdwarf Balloon 090100001: results of the 2005 multisite campaign
We present the results of a multisite photometric campaign on the pulsating
sdB star Balloon 090100001. The star is one of the two known hybrid hot
subdwarfs with both long- and short-period oscillations. The campaign involved
eight telescopes with three obtaining UBVR data, four B-band data, and one
Stromgren uvby photometry. The campaign covered 48 nights, providing a temporal
resolution of 0.36microHz with a detection threshold of about 0.2mmag in
B-filter data.
Balloon 090100001 has the richest pulsation spectrum of any known pulsating
subdwarf B star and our analysis detected 114 frequencies including 97
independent and 17 combination ones. The strongest mode (f_1) in the 2.8mHz
region is most likely radial while the remaining ones in this region form two
nearly symmetric multiplets: a triplet and quintuplet, attributed to
rotationally split \ell=1 and 2 modes, respectively. We find clear increases of
splitting in both multiplets between the 2004 and 2005 observing campaigns,
amounting to 15% on average. The observed splittings imply that the rotational
rate in Bal09 depends on stellar latitude and is the fastest on the equator. We
use a small grid of models to constrain the main mode (f_1), which most likely
represents the radial fundamental pulsation. The groups of p-mode frequencies
appear to lie in the vicinity of consecutive radial overtones, up to the third
one. Despite the large number of g-mode frequencies observed, we failed to
identify them, most likely because of the disruption of asymptotic behaviour by
mode trapping. The observed frequencies were not, however, fully exploited in
terms of seismic analysis which should be done in the future with a larger grid
of reliable evolutionary models of hot subdwarfs.Comment: accepted for publication in MNRA
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