5,853 research outputs found
Hipparcos period-luminosity relations for Miras and semiregular variables
We present period-luminosity diagrams for nearby Miras and semiregulars,
selecting stars with parallaxes better than 20 per cent and well-determined
periods. Using K-band magnitudes, we find two well-defined P-L sequences, one
corresponding to the standard Mira P-L relation and the second shifted to
shorter periods by a factor of about 1.9. The second sequence only contains
semiregular variables, while the Mira sequence contains both Miras and
semiregulars. Several semiregular stars show double periods in agreement with
both relations. The Whitelock evolutionary track is shown to fit the data,
indicating that the semiregulars are Mira progenitors. The transition between
the two sequences may correspond to a change in pulsation mode or to a change
in the stellar structure. Large amplitude pulsations leading to classical Mira
classification occur mainly near the tip of the local AGB luminosity function.Comment: 10 pages with figures, accepted by ApJ Letter
Atomic quasi-Bragg diffraction in a magnetic field
We report on a new technique to split an atomic beam coherently with an
easily adjustable splitting angle. In our experiment metastable helium atoms in
the |{1s2s}^3S_1 M=1> state diffract from a polarization gradient light field
formed by counterpropagating \sigma^+ and \sigma^- polarized laser beams in the
presence of a homogeneous magnetic field. In the near-adiabatic regime, energy
conservation allows the resonant exchange between magnetic energy and kinetic
energy. As a consequence, symmetric diffraction of |M=0> or |M=-1> atoms in a
single order is achieved, where the order can be chosen freely by tuning the
magnetic field. We present experimental results up to 6th order diffraction (24
\hbar k momentum splitting, i.e., 2.21 m/s in transverse velocity) and present
a simple theoretical model that stresses the similarity with conventional Bragg
scattering. The resulting device constitutes a flexible, adjustable,
large-angle, three-way coherent atomic beam splitter with many potential
applications in atom optics and atom interferometry.Comment: 4 pages, 5 figure
Gaia: Organisation and challenges for the data processing
Gaia is an ambitious space astrometry mission of ESA with a main objective to
map the sky in astrometry and photometry down to a magnitude 20 by the end of
the next decade. While the mission is built and operated by ESA and an
industrial consortium, the data processing is entrusted to a consortium formed
by the scientific community, which was formed in 2006 and formally selected by
ESA one year later. The satellite will downlink around 100 TB of raw telemetry
data over a mission duration of 5 years from which a very complex iterative
processing will lead to the final science output: astrometry with a final
accuracy of a few tens of microarcseconds, epoch photometry in wide and narrow
bands, radial velocity and spectra for the stars brighter than 17 mag. We
discuss the general principles and main difficulties of this very large data
processing and present the organisation of the European Consortium responsible
for its design and implementation.Comment: 7 pages, 2 figures, Proceedings of IAU Symp. 24
Limits on I-band microvariability of the Galactic Bulge Miras
We search for microvariability in a sample of 485 Mira variables with high
quality I-band light curves from the second generation Optical Gravitational
Lensing Experiment (OGLE-II). Rapid variations with amplitudes in the ~0.2-1.1
mag range lasting hours to days were discovered in Hipparcos data by de Laverny
et al. (1998). Our search is primarily sensitive to events with time-scales of
about 1 day, but retains a few percent efficiency (per object) for detecting
unresolved microvariability events as short as 2 hours. We do not detect any
candidate events. Assuming that the distribution of the event time profiles is
identical to that from the Hipparcos light curves we derive the 95% confidence
level upper limit of 0.038 per year per star for the rate of such events (1 per
26 years per average object of the ensemble). The high event rates of the order
of 1 per year per star implied by the Hipparcos study in the H_P band are
excluded with high confidence by the OGLE-II data in the I band. Our
non-detection could still be explained by much redder spectral response of the
I filter compared to the H_P band or by population differences between the
bulge and the solar neighborhood. In any case, the OGLE-II I-band data provide
the first limit on the rate of the postulated microvariability events in Mira
stars and offer new quantitative constraints on their properties. Similar
limits are obtained for other pulse shapes and a range of the assumed
time-scales and size-frequency distributions.Comment: Accepted for publication in Ap
Why are the K dwarfs in the Pleiades so Blue?
The K dwarfs in the Pleiades fall nearly one half magnitude below a main
sequence isochrone when plotted in a color-magnitude diagram utilizing V
magnitude as the luminosity index and B-V as the color index. This peculiarity
has been known for forty years but has gone unexplained and mostly ignored.
When compared to Praesepe members, the Pleiades K dwarfs again are subluminous
(or blue) in a color-magnitude diagram using B-V as the color index. However,
using V-I as the color index, stars in the two clusters are coincident to M_V ~
10; using V-K as the color index, Pleiades late K and M stars fall above the
main sequence locus defined by Praesepe members. We believe that the anomalous
spectral energy distributions for the Pleiades K dwarfs, as compared to older
clusters, are a consequence of rapid stellar rotation and may be primarily due
to spottedness. If so, the required areal filling factor for the cool component
has to be very large (=> 50%). Weak-lined T Tauri stars have similar color
anomalies, and we suspect this is a common feature of all very young K dwarfs
(sp. type > K3). The peculiar spectral energy distribution needs to be
considered in deriving accurate pre-main sequence isochrone-fitting ages for
clusters like the Pleiades since the age derived will depend on the temperature
index used.Comment: 41 pages, 15 figures, AASTeX5.0. Accepted 05 May 2003; Scheduled for
publication in the Astronomical Journal (August 2003
Adiabatic Formation of Rydberg Crystals with Chirped Laser Pulses
Ultracold atomic gases have been used extensively in recent years to realize
textbook examples of condensed matter phenomena. Recently, phase transitions to
ordered structures have been predicted for gases of highly excited, 'frozen'
Rydberg atoms. Such Rydberg crystals are a model for dilute metallic solids
with tunable lattice parameters, and provide access to a wide variety of
fundamental phenomena. We investigate theoretically how such structures can be
created in four distinct cold atomic systems, by using tailored
laser-excitation in the presence of strong Rydberg-Rydberg interactions. We
study in detail the experimental requirements and limitations for these
systems, and characterize the basic properties of small crystalline Rydberg
structures in one, two and three dimensions.Comment: 23 pages, 10 figures, MPIPKS-ITAMP Tandem Workshop, Cold Rydberg
Gases and Ultracold Plasmas (CRYP10), Sept. 6-17, 201
Building the cosmic distance scale: from Hipparcos to Gaia
Hipparcos, the first ever experiment of global astrometry, was launched by
ESA in 1989 and its results published in 1997 (Perryman et al., Astron.
Astrophys. 323, L49, 1997; Perryman & ESA (eds), The Hipparcos and Tycho
catalogues, ESA SP-1200, 1997). A new reduction was later performed using an
improved satellite attitude reconstruction leading to an improved accuracy for
stars brighter than 9th magnitude (van Leeuwen & Fantino, Astron. Astrophys.
439, 791, 2005; van Leeuwen, Astron. Astrophys. 474, 653, 2007).
The Hipparcos Catalogue provided an extended dataset of very accurate
astrometric data (positions, trigonometric parallaxes and proper motions),
enlarging by two orders of magnitude the quantity and quality of distance
determinations and luminosity calibrations. The availability of more than 20000
stars with a trigonometric parallax known to better than 10% opened the way to
a drastic revision of our 3-D knowledge of the solar neighbourhood and to a
renewal of the calibration of many distance indicators and age estimations. The
prospects opened by Gaia, the next ESA cornerstone, planned for launch in June
2013 (Perryman et al., Astron. Astrophys. 369, 339, 2001), are still much more
dramatic: a billion objects with systematic and quasi simultaneous astrometric,
spectrophotometric and spectroscopic observations, about 150 million stars with
expected distances to better than 10%, all over the Galaxy. All stellar
distance indicators, in very large numbers, will be directly measured,
providing a direct calibration of their luminosity and making possible detailed
studies of the impacts of various effects linked to chemical element
abundances, age or cluster membership. With the help of simulations of the data
expected from Gaia, obtained from the mission simulator developed by DPAC, we
will illustrate what Gaia can provide with some selected examples.Comment: 16 pages, 16 figures, Conference "The Fundamental Cosmic Distance
scale: State of the Art and the Gaia perspective, 3-6 May 2011, INAF,
Osservatorio Astronomico di Capodimonte, Naples. Accepted for publication in
Astrophysics & Space Scienc
Ages for illustrative field stars using gyrochronology: viability, limitations and errors
We here develop an improved way of using a rotating star as a clock, set it
using the Sun, and demonstrate that it keeps time well. This technique, called
gyrochronology, permits the derivation of ages for solar- and late-type main
sequence stars using only their rotation periods and colors. The technique is
clarified and developed here, and used to derive ages for illustrative groups
of nearby, late-type field stars with measured rotation periods. We first
demonstrate the reality of the interface sequence, the unifying feature of the
rotational observations of cluster and field stars that makes the technique
possible, and extends it beyond the proposal of Skumanich by specifying the
mass dependence of rotation for these stars. We delineate which stars it cannot
currently be used on. We then calibrate the age dependence using the Sun. The
errors are propagated to understand their dependence on color and period.
Representative age errors associated with the technique are estimated at ~15%
(plus possible systematic errors) for late-F, G, K, & early-M stars. Ages
derived via gyrochronology for the Mt. Wilson stars are shown to be in good
agreement with chromospheric ages for all but the bluest stars, and probably
superior. Gyro ages are then calculated for each of the active main sequence
field stars studied by Strassmeier and collaborators where other ages are not
available. These are shown to be mostly younger than 1Gyr, with a median age of
365Myr. The sample of single, late-type main sequence field stars assembled by
Pizzolato and collaborators is then assessed, and shown to have gyro ages
ranging from under 100Myr to several Gyr, and a median age of 1.2Gyr. Finally,
we demonstrate that the individual components of the three wide binaries
XiBooAB, 61CygAB, & AlphaCenAB yield substantially the same gyro ages.Comment: 58 pages, 18 color figures, accepted for publication in The
Astrophysical Journal; Age uncertainties slightly modified upon correcting an
algebraic error in Section
Applying a cost-based pricing model for innovative cancer treatments subject to indication expansion:A case study for pembrolizumab and daratumumab
BACKGROUND: Expanding the indication of already approved immuno-oncology drugs presents treatment opportunities for patients but also strains healthcare systems. Cost-based pricing models are discussed as a possibility for cost containment. This study focuses on two drugs, pembrolizumab (Keytruda) and daratumumab (Darzalex), to explore the potential effect of indication broadening on the estimated price when using the cost-based pricing (CBP) model proposed by Uyl-de Groot and Löwenberg (2018). METHODS: The model was used to calculate cumulative yearly prices, cumulative prices per indication, and non-cumulative indication-based prices using inputs such as research and development (R&D) costs, manufacturing costs, eligible patient population, and a profit margin. A deterministic stepwise analysis and scenario analysis were conducted to examine how sensitive the estimated price is to the different input assumptions. RESULTS: The yearly cumulative cost-based prices (CBPs) ranged from €52 to €885 for pembrolizumab per vial and €823 to €31,941 for daratumumab per vial. Prices were higher in initial years or indications due to smaller patient populations, decreased over time or after additional indications. Sensitivity analysis showed that the number of eligible patients had the most significant impact on the estimated price. In the scenario analysis the profit margin contributed most to a higher CBPs for both drugs. Lower estimates resulted from assumed lower R&D costs. DISCUSSION: The estimated CBPs are consistently lower than Dutch list prices for pembrolizumab (€2,861), mainly resulting from larger patient populations in registered indications. However, daratumumab's list prices fall within the range of modeled CBPs depending on the year or indication (€4,766). Both CBPs decrease over time or with additional indications. The number of eligible patients and initial R&D costs have the most significant influence on the CBPs. These findings contribute to the ongoing discussions on pharmaceutical pricing, especially concerning cancer drugs with expanding indications.</p
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