Disentangling the Hercules stream

Using high-resolution spectra of nearby F and G dwarf stars, we have investigated the detailed abundance and age structure of the Hercules stream. We find that the stars in the stream have a wide range of stellar ages, metallicities, and element abundances. By comparing to existing samples of stars in the solar neighbourhood with kinematics typical of the Galactic thin and thick disks we find that the properties of the Hercules stream distinctly separate into the abundance and age trends of the two disks. Hence, we find it unlikely that the Hercules stream is a unique Galactic stellar population, but rather a mixture of thin and thick disk stars. This points toward a dynamical origin for the Hercules stream, probably caused by the Galactic bar.Comment: Accepted for publication in ApJ Letter

Spectroscopic Identification of Faint White Dwarf Candidates in the Praesepe Open Star Cluster

We present spectroscopic observations of the remaining four candidate white dwarfs in Praesepe. All four candidates are quasars with redshifts between 0.8 and 2.8. One quasar, LB 6072, is observed to have a strong metal-line absorption system blueward of the quasar redshift. The lack of additional white dwarfs in Praesepe leaves the total known white-dwarf population of the cluster at five, well below the number expected from commonly-assumed initial mass functions, though several undiscovered cluster WDs may lie in the outer regions of the cluster. All known Praesepe member white dwarfs are concentrated within 0.6 deg of the cluster center, and the radial profile of cluster white dwarfs is quite similar to the profile of massive cluster stars. This profile is mildly inconsistent with that of ~1Mo cluster stars and suggests that the white dwarfs did not receive a velocity kick during the progenitor star's mass loss phases. If complete, the observed Praesepe white dwarf population is consistent with a steeper high-end initial-mass function than commonly assumed, though the calculated slopes are inconsistent with the present-day mass function of Praesepe. Searches for white dwarfs outside the core of Praesepe and further study of the white dwarf populations of additional open clusters is necessary to constrain further the underlying cause of the white dwarf deficit.Comment: 6 pages, 5 figures, uses emulateapj.cls; accepted for publication in the Astronomical Journa

Gamma knife radiosurgery for arteriovenous malformations: general principles and preliminary results in a Swiss cohort.

Arteriovenous malformations (AVMs) are a type of vascular malformation characterised by an abnormal connection between arteries and veins, bypassing the capillary system. This absence of capillaries generates an elevated pressure (hyperdebit), in both the AVM and the venous drainage, increasing the risk of rupture. Management modalities are: observation, microsurgical clipping, endovascular treatment and radiosurgery. The former can be used alone or in the frame of a multidisciplinary approach. We review our single-institution experience with gamma knife radiosurgery (GKR) over a period of 5 years. The study was open-label, prospective and nonrandomised. Fifty-seven consecutive patients, benefitting from 64 GKR treatments, were included. All were treated with Leksell Gamma Knife Perfexion (Elekta Instruments, AB, Sweden) between July 2010 and August 2015. All underwent stereotactic multimodal imaging: standard digital subtraction angiography, magnetic resonance imaging and computed tomography angiography. We report obliteration rates, radiation-induced complications and haemorrhages during follow-up course. The mean age was 46 years (range 13-79 years). The mean follow-up period was 36.4 months (median 38, range 12-75 months). Most common pretherapeutic clinical presentation was haemorrhage (50%). The most common Pollock-Flickinger score was between 1.01 and 1.5 (46%) and Spetzler-Martin grade III (46%). In 39 (60.1%) of cases, GKR was performed as upfront therapeutic option. The mean gross target volume (GTV) was 2.3 ml (median 1.2, range 0.03-11.3 ml). Mean marginal dose was 22.4 Gy (median 24, range 18-24 Gy). The mean prescription isodose volume (PIV) was 2.9 ml (median 1.8, range 0.065-14.6 ml). The overall obliteration rates (all treatments combined) at 12, 24, 36, 48 and 60 months were 4.8, 16.9%, 37.4, 63.6 and 78.4%, respectively. The main predictive factors for complete obliteration were: higher mean marginal dose (23.3 vs 21.0 Gy), lower GTV (mean 1.5 vs 3.5 ml) and absence of previous embolisation (at 60 months 61.8% prior embolisation compared with 82.4% without prior embolisation) (for all p <0.05). Eight (14%) patients experienced complications after GKR. Overall definitive morbidity rate was 3.1%. No patient died from causes related to GKR. However, during the obliteration period, one case of extremely rare fatal haemorrhage occurred. Radiosurgery is a safe and effective treatment modality for intracranial AVMs in selected cases. It can be used as upfront therapy or in the frame of a combined management. Obliteration rates are high, with minimal morbidity. The treatment effect is progressive and subsequent and regular clinical and radiological follow-up is needed to evaluate this effect

Transient central hypoxemia due to intermittent high-degree atrioventricular block in a heart-transplanted patient diagnosed during routine electroencephalography: a case report.

BACKGROUND Bradycardia frequently occurs in heart-transplanted patients, mainly as a temporally restricted manifestation early after transplantation and often without symptoms. A high-degree atrioventricular block is mostly symptomatic through cerebral hypoxia induced through cerebral hypoperfusion. Only a few published cases show this specific electroencephalography result in this context. The purpose of this case is to bring attention to atypical manifestations of typical cardiac complications after heart transplantation and the importance of perseverance in the diagnostic. CASE PRESENTATION A Central European man in his 50s with history of heart transplantation 31 years previously was admitted to the internal medicine ward for short-lived recurrent episodes of generalized weakness with multiple falls but without loss of consciousness. During routine electroencephalography, the patient perceived this recurrent sensation. This episode coincided with a transient third-degree atrioventricular block followed 8-10 seconds later by a generalized slowing of the electroencephalography, reflecting cerebral hypoxia due to cerebral hypoperfusion. Holter monitoring confirmed the diagnosis. A pacemaker was implanted, consequently resolving the episodes. CONCLUSION This case report illustrates the pathophysiological central hypoxemic origin of episodes of generalized weakness caused by a high-degree atrioventricular block in a patient surviving 29 years after heart transplant. It highlights the benefit of electroencephalography as a diagnostic tool in well-selected patients

Clinical impact of double protease inhibitor boosting with Lopinavir/Ritonavir and Amprenavir as part of salvage antiretroviral therapy

Purpose: Double protease inhibitor (PI) boosting is being explored as a new strategy in salvage antiretroviral (ARV) therapy. However, if a negative drug interaction leads to decreased drug levels of either or both PIs, double PI boosting could lead to decreased virologic response. A negative drug interaction has been described between amprenavir (APV) and lopinavir/ritonavir (LPV/r). This observational cohort study assessed the virologic impact of the addition of APV to a salvage ARV regimen, which also contains LPV/r, compared to a regimen containing LPV/r alone. Method: Patients initiated on a salvage ARV regimen that included LPV/r obtained from the expanded access program in Toronto, Canada, were evaluated. APV (600-1,200 mg bid) was added at the discretion of the treating physician. Results: Using multivariate Cox proportional hazards models, we found that the addition of APV to a LPV/r-containing salvage regimen was not significantly associated with time to virologic suppression (< 50 copies/mL; adjusted hazard ratio [HR] = 0.75, p = .12) or with time to virologic rebound (adjusted HR = 1.46, p = .34). Those patients who received higher doses of APV had an increased chance of virologic suppression (p = .03). In a subset of 27 patients, the median LPV Ctrough was significantly lower in patients receiving APV (p = .04), and the median APV Ctrough was reduced compared to reported controls. Conclusion: Our data do not support an additional benefit in virologic reduction of double boosting with APV and LPV/r relative to LPV/r alone in salvage ARV therapy. Our study's limitations include its retrospective nature and the imbalance between the two groups potentially confounding the results. Although these factors were adjusted for in the multivariate analysis, a prospective randomized controlled trial is warranted to confirm our findings

An HST/WFPC2 Survey for Brown Dwarf Binaries in the alpha Per and the Pleiades Open Clusters

We present the results of a high-resolution imaging survey for brown dwarf (BD) binaries in two open clusters. The observations were carried out with WFPC2 onboard HST. Our sample consists of 8 BD candidates in the alpha Per cluster and 25 BD candidates in the Pleiades. We have resolved 4 binaries in the Pleiades with separations in the range 0".094--0".058, corresponding to projected separations between 11.7~AU and 7.2~AU. No binaries were found among the alpha Per targets. Three of the binaries have proper motions consistent with cluster membership in the Pleiades cluster, and for one of them we report the detection of Halpha in emission and LiI absorption obtained from Keck~II/ESI spectroscopy. One of the binaries does not have a proper motion consistent with Pleiades membership. We estimate that BD binaries wider than 12~AU are less frequent than 9% in the alphaPer and Pleiades clusters. This is consistent with an extension to substellar masses of a trend observed among stellar binaries: the maximum semimajor axis of binary systems decreases with decreasing primary mass. We find a binary frequency of 2 binaries over 13 BDs with confirmed proper motion membership in the Pleiades, corresponding to a binary fraction of 15%(1 sigma error bar +15%/-5%). These binaries are limited to the separation range 7-12~AU and their mass ratios are larger than 0.7. The relatively high binary frequency (>10%), the bias to separations smaller than about 15 AU and the trend to high mass ratios (q>0.7) are fundamental properties of BDs. Current theories of BD formation do not appear to provide a good description of all these properties.Comment: Accepted by ApJ (scheduled publication in volume 594, September 1, 2003

The Pattern Speed of the Galactic Bar

Most late-type stars in the solar neighborhood have velocities similar to the local standard of rest (LSR), but there is a clearly separated secondary component corresponding to a slower rotation and a mean outward motion. Detailed simulations of the response of a stellar disk to a central bar show that such a bi-modality is expected from outer-Lindblad resonant scattering. When constraining the run of the rotation curve by the proper motion of Sgr A* and the terminal gas velocities, the value observed for the rotation velocity separating the two components results in a value of (53+/-3)km/s/kpc for the pattern speed of the bar, only weakly dependent on the precise values for Ro and bar angle phi.Comment: 5 pages LaTeX, 2 Figs, accepted for publication in ApJ Letter

Shape parameters of Galactic open clusters

(abridged) In this paper we derive observed and modelled shape parameters (apparent ellipticity and orientation of the ellipse) of 650 Galactic open clusters identified in the ASCC-2.5 catalogue. We provide the observed shape parameters of Galactic open clusters, computed with the help of a multi-component analysis. For the vast majority of clusters these parameters are determined for the first time. High resolution ("star by star") N-body simulations are carried out with the specially developed $\phi$GRAPE code providing models of clusters of different initial masses, Galactocentric distances and rotation velocities. The comparison of models and observations of about 150 clusters reveals ellipticities of observed clusters which are too low (0.2 vs. 0.3), and offers the basis to find the main reason for this discrepancy. The models predict that after $\approx 50$ Myr clusters reach an oblate shape with an axes ratio of $1.65:1.35:1$, and with the major axis tilted by an angle of $q_{XY} \approx 30^\circ$ with respect to the Galactocentric radius due to differential rotation of the Galaxy. Unbiased estimates of cluster shape parameters require reliable membership determination in large cluster areas up to 2-3 tidal radii where the density of cluster stars is considerably lower than the background. Although dynamically bound stars outside the tidal radius contribute insignificantly to the cluster mass, their distribution is essential for a correct determination of cluster shape parameters. In contrast, a restricted mass range of cluster stars does not play such a dramatic role, though deep surveys allow to identify more cluster members and, therefore, to increase the accuracy of the observed shape parameters.Comment: 13 pages, 12 figures, accepted for publication in Astronomy and Astrophysic

Variability of Be Stars in Southern Open Clusters

We recently discovered a large number of highly active Be stars in the open cluster NGC 3766, making it an excellent location to study the formation mechanism of Be star disks. To explore whether similar disk appearances and/or disappearances are common among the Be stars in other open clusters, we present here multiple epochs of H-alpha spectroscopy for 296 stars in eight open clusters. We identify 12 new transient Be stars and confirm 17 additional Be stars with relatively stable disks. By comparing the H-alpha equivalent widths to the photometric y - H-alpha colors, we present a method to estimate the strength of the H-alpha emission when spectroscopy is not available. For a subset of 128 stars in four open clusters, we also use blue optical spectroscopy and available Stromgren photometry to measure their projected rotational velocities, effective temperatures, and polar surface gravities. We combine our Be star detections from these four clusters to investigate physical differences between the transient Be stars, stable Be stars, and normal B-type stars with no line emission. Both types of Be stars are faster rotating populations than normal B-type stars, and we find no significant physical differences between the transient and stable Be stars in our sample.Comment: Accepted to ApJ; small corrections to Table 5 and associated figure