Weak magnetic fields in white dwarfs and their direct progenitors?

We have carried out a re-analysis of polarimetric data of central stars of planetary nebulae, hot subdwarfs, and white dwarfs taken with FORS1 (FOcal Reducer and low dispersion Spectrograph) on the VLT (Very Large Telescope), and added a large number of new observations in order to increase the sample. A careful analysis of the observations using only one wavelength calibration for the polarimetrically analysed spectra and for all positions of the retarder plate of the spectrograph is crucial in order to avoid spurious signals. We find that the previous detections of magnetic fields in subdwarfs and central stars could not be confirmed while about 10% of the observed white dwarfs have magnetic fields at the kilogauss level.Comment: 6 pages, Proceedings of the 18th European White Dwarf Workshop, ASP Conference Serie

The spectroscopic Hertzsprung-Russell diagram of Galactic massive stars

The distribution of stars in the Hertzsprung-Russell diagram narrates their evolutionary history and directly assesses their properties. Placing stars in this diagram however requires the knowledge of their distances and interstellar extinctions, which are often poorly known for Galactic stars. The spectroscopic Hertzsprung-Russell diagram (sHRD) tells similar evolutionary tales, but is independent of distance and extinction measurements. Based on spectroscopically derived effective temperatures and gravities of almost 600 stars, we derive for the first time the observational distribution of Galactic massive stars in the sHRD. While biases and statistical limitations in the data prevent detailed quantitative conclusions at this time, we see several clear qualitative trends. By comparing the observational sHRD with different state-of-the-art stellar evolutionary predictions, we conclude that convective core overshooting may be mass-dependent and, at high mass ($\geq 15\,M_\odot$), stronger than previously thought. Furthermore, we find evidence for an empirical upper limit in the sHRD for stars with $T_{\rm{eff}}$ between 10000 and 32000 K and, a strikingly large number of objects below this line. This over-density may be due to inflation expanding envelopes in massive main-sequence stars near the Eddington limit.Comment: 5 pages, 2 figures, 1 table; accepted for publication in A&A Letter

gamma Doradus pulsation in two pre-main sequence stars discovered by CoRoT

Pulsations in pre-main sequence stars have been discovered several times within the last years. But nearly all of these pulsators are of delta Scuti-type. gamma Doradus-type pulsation in young stars has been predicted by theory, but lack observational evidence. We present the investigation of variability caused by rotation and (gammaDoradus-type) pulsation in two pre-main sequence members of the young open cluster NGC2264 using high-precision time series photometry from the CoRoT satellite and dedicated high-resolution spectroscopy. Time series photometry of NGC2264VAS20 and NGC 2264VAS87 was obtained by the CoRoT satellite during the dedicated short run SRa01 in March 2008. NGC2264VAS87 was re-observed by CoRoT during the short run SRa05 in December 2011 and January 2012. Frequency analysis was conducted using Period04 and SigSpec. The spectral analysis was performed using equivalent widths and spectral synthesis. The frequency analysis yielded 10 and 14 intrinsic frequencies for NGC2264VAS20 and NGC2264VAS 87, respectively, in the range from 0 to 1.5c/d which are attributed to be caused by a combination of rotation and pulsation. The effective temperatures were derived to be 6380$\pm$150K for NGC2264VAS20 and 6220$\pm$150K for NGC2264VAS87. Membership of the two stars to the cluster is confirmed independently using X-ray fluxes, radial velocity measurements and proper motions available in the literature. The derived Li abundances of log n(Li)=3.34 and 3.54 for NGC2264VAS20 and NGC2264VAS87, respectively, are in agreement with the Li abundance for other stars in NGC2264 of similar Teff reported in the literature. We conclude that the two objects are members of NGC2264 and therefore are in their pre-main sequence evolutionary stage. Assuming that part of their variability is caused by pulsation, these two stars might be the first pre-main sequence gamma Doradus candidates.Comment: 11 pages, 10 figures, A&A accepte

The Contribution of Blazars to the Extragalactic Diffuse Gamma-ray Background and Their Future Spatial Resolution

We examine the constraints on the luminosity-dependent density evolution model for the evolution of blazars given the observed spectrum of the diffuse gamma-ray background (DGRB), blazar source-count distribution, and the blazar spectral energy distribution sequence model, which relates the observed the blazar spectrum to its luminosity. We show that the DGRB observed by the Large Area Telescope (LAT) aboard the Fermi Gamma Ray Space Telescope can be produced entirely by gamma-ray emission from blazars and nonblazar active galactic nuclei, and that our blazar evolution model is consistent with and constrained by the spectrum of the DGRB and flux source-count distribution function of blazars observed by Fermi-LAT. Our results are consistent with previous work that used EGRET spectral data to forecast the Fermi-LAT DGRB. The model includes only three free parameters, and forecasts that >~ 95% of the flux from blazars will be resolved into point sources by Fermi-LAT with 5 years of observation, with a corresponding reduction of the flux in the DGRB by a factor of ~2 to 3 (95% confidence level), which has implications for the Fermi-LAT's sensitivity to dark matter annihilation photons.Comment: 13 pages, 7 figures; v3: minor changes, matches version to appear in Phys. Rev.

The Blazar Sequence: Validity and Predictions

The "blazar sequence" posits that the most powerful BL Lacertae objects and flat-spectrum radio quasars should have relatively small synchrotron peak frequencies, nu_peak, and that the least powerful such objects should have the highest nu_peak values. This would have strong implications for our understanding of jet formation and physics and the possible detection of powerful, moderately high-redshift TeV blazars. I review the validity of the blazar sequence by using the results of very recent surveys and compare its detailed predictions against observational data. I find that the blazar sequence in its simplest form is ruled out. However, powerful flat-spectrum radio quasars appear not to reach the nu_peak typical of BL Lacs. This could indeed be related to some sort of sequence, although it cannot be excluded that it is instead due to a selection effect.Comment: 9 pages, 4 figures, invited talk at the Workshop "The Multi-messenger approach to high energy gamma-ray sources", Barcelona, Spain, July 4-7, 2006, to appear in the proceeding

Abdominal cyst after early failure of polyethylene liner in total hip arthroplasty

We report a case in which the early failure of a polyethylene liner, coupled with a 32-mm CrCo ball head, caused pelvic cyst formation simulating an abdominal mass. The presence of the mass with inguinal swelling lead us to diagnosis liner failure, as shown by radiography. An extraperitoneal cyst surrounding the iliopsoas muscle from the lesser trochanter up to the lumbosacral junction was demonstrated with pre-operative computed tomography and sonography. The cyst contained fluid and many large particles of polyethylene debris. The liner and the head were substituted and the cyst was removed through a different abdominal approach. We hypothesize that debris falls out and concentrates along the iliopsoas muscle from the very beginning of wear, and then the muscle concentration forces pumped it along the muscle belly. From the histologic point of view, large polyethylene particles were observed in the removed tissue, and no major osteoclastic activation was found

Impact of MgII interstellar medium absorption on near-ultraviolet exoplanet transit measurements

Ultraviolet (UV) transmission spectroscopy probes atmospheric escape, which has a significant impact on planetary atmospheric evolution. If unaccounted for, interstellar medium absorption (ISM) at the position of specific UV lines might bias transit depth measurements, and thus potentially affect the (non-)detection of features in transmission spectra. Ultimately, this is connected to the so called ``resolution-linked bias'' (RLB) effect. We present a parametric study quantifying the impact of unresolved or unconsidered ISM absorption in transit depth measurements at the position of the MgII h&k resonance lines (i.e. 2802.705 {\AA} and 2795.528 {\AA} respectively) in the near-ultraviolet spectral range. We consider main-sequence stars of different spectral types and vary the shape and amount of chromospheric emission, ISM absorption, and planetary absorption, as well as their relative velocities. We also evaluate the role played by integration bin and spectral resolution. We present an open-source tool enabling one to quantify the impact of unresolved or unconsidered MgII ISM absorption in transit depth measurements. We further apply this tool to a few already or soon to be observed systems. On average, we find that ignoring ISM absorption leads to biases in the MgII transit depth measurements comparable to the uncertainties obtained from the observations published to date. However, considering the bias induced by ISM absorption might become necessary when analysing observations obtained with the next generation space telescopes with UV coverage (e.g. LUVOIR, HABEX), which will provide transmission spectra with significantly smaller uncertainties compared to what obtained with current facilities (e.g. HST).Comment: Accepted for publication in MNRA

Finite element modeling of nonequilibrium fluid-wall interaction at high-Mach regime

The numerical modeling of the aerodynamic interactions at high-Altitudes and high-Mach numbers is considered in view of its importance when studying problems where the continuum hypothesis at the foundation of the Navier- Stokes equations becomes invalid. One of the difficulties associated with these flight conditions is that both the velocity and the temperature of the fluid do not abide by the no-slip conditions at the wall. A weak Galerkin finite element formulation of the Maxwell-Smoluchowki model is introduced to discretize the velocity slip and temperature jump conditions with better accuracy than the standard finite element approximation. The methodology is assessed on configurations such as cylinders and spheres for flow conditions ranging from quasi-equilibrium to nonequilibrium. Improvements are observed in the slip regime compared with available data. Nonetheless, the results in the transition regime highlight the need for more sophisticated physical modeling to address nonequilibrium at the wall