5 research outputs found
Dating young open clusters using delta Scuti stars. Results for Trumpler 10 and Praesepe
Aims. The main goal of this work is to date young open clusters using
Sct stars. Seismic indices such as the large separation and the
frequency at maximum power can help to constrain the models to better
characterise the stars. We propose a reliable method to identify some radial
modes, which gives us greater confidence in the constrained models. Methods. We
extract the frequency content of a sample of Sct stars belonging to
the same open cluster. We estimate the low-order large separation by means of
different techniques and the frequency at maximum power for each member of the
sample. We use a grid of models built with the typical parameters of
Sct stars, including mass, metallicity and rotation as independent variables,
and determine the oscillation modes. We select the observed frequencies whose
ratios match those of the models. Once we find a range of radial modes matching
the observed frequencies, mainly the fundamental mode, we add it to the other
seismic parameters to derive the stellar age. Assuming star groups have similar
chemistry and age, we estimate their mean age by computing a weighted
probability density function fit to the age distribution of the seismically
constrained models. Results. We estimate the age of Trumpler 10 to be
Myr, and that of Praesepe to be Myr. In this
latter case, we find two apparent populations of Sct stars in the same
cluster, one at Myr and another at Myr. This may be
due to two different formation events, different rotational velocities of the
members in our sample of stars (as rapid rotation may modify the observed large
separation), or to membership of unresolved binary systems.Comment: 20 pages, 25 figure, uses the open-source code MultiModes (see
https://github.com/davidpamos/MultiModes). It will be published in A&
Choledocholithiasis: repetitive thick-slab single-shot projection magnetic resonance cholangiopancreaticography versus endoscopic ultrasonography.
This prospective study compares repetitive thick-slab single-shot projection magnetic resonance cholangiopancreatography (MRCP) with endoscopic ultrasonography (EUS) for the detection of choledocholithiasis. Fifty-seven consecutive patients (36 women, mean age 61) referred for suspected choledocholithiasis underwent MRCP, followed by EUS. Each procedure was performed by different operators blinded to the results of the other investigation. MR technique included a turbo spin-echo T2-weighted axial sequence with selective fat saturation (SPIR/TSE, TE=70 ms, TR=1,600 ms), followed by coronal dynamic MRCP. The same thick-slab slice was sequentially acquired 12 times as breath-hold single-shot projection imaging (SSh, TE=900 ms, TE=8,000 ms) centred on the common bile duct (CBD). Two experienced radiologists independently and blindly evaluated MR images for the detection of CBD stones. Their inter-observer agreement kappa was determined. Secondly, the two observers read MR images in consensus again. CBD stones were demonstrated in 18 out of 57 patients (31.6 %) and confirmed by endoscopic retrograde cholangiography (ERCP, n=17) or intraoperative cholangiography (n=1). Clinical follow-up served as the "gold standard" in patients with negative results without following invasive procedure (n=28). Sensitivity, specificity, accuracy, positive and negative predictive value for MRCP resulting from consensus reading were 94.9%, 94.4%, 94.7%, 97.4% and 89.5%, respectively. Corresponding values of EUS were 97.4%, 94.4%, 96.5%, 97.4% and 94.4%. Inter-observer agreement kappa was 0.81. Repetitive thick-slab single-shot projection MRCP is an accurate non-invasive imaging modality for suspected choledocholithiasis and should be increasingly used to select those patients who require a subsequent therapeutic procedure, namely ERCP