149 research outputs found
Maternal and neonatal outcomes of pregnancy in women with chronic hypertension
No full textPoster abstract 782Kim Broekhuijsen, Josje Langenveld, Paul van den Berg, Anita Ravelli, Ben Willem Mol, Maureen Fransse
Relevance of individual participant data meta-analysis for studies in obstetrics: delivery versus expectant monitoring for hypertensive disorders of pregnancy
No full textAbstract not availableKim Broekhuijsen, Thomas Bernardes, Gert-Jan van Baaren, Parvin Tajik, Natalia Novikova, Shakila Thangaratinam, Kim Boers, Corine M. Koopmans, Kedra Wallace, Andrew H. Shennan, Josje Langenveld, Henk Groen, Paul P. van den Berg, Ben Willem J. Mol, Maureen T.M. Fransse
Multi-wavelength studies of compact binaries
No full textBinary stars with small orbits in which at least one of the components is a white dwarf, neutron star or a black hole are usually referred to as compact binaries. The presence of very large gravitational potentials in these systems offer the unique opportunity to study astrophysical phenomena under conditions that can never be achieved on Earth. Thanks to their excellent angular resolution and sensitivity, telescopes like Hubble and Chandra opened a new window in the study of compact binaries in crowded stellar environments such as globular clusters. Due to the high stellar interaction rates in clusters, it is expected that they harbour large populations of such compact binaries. However, their identification and study in globular clusters is very challenging. Only with multi-wavelength studies we can get new insights about their physics, formation and evolution. This thesis focuses on the study of compact systems which harbour white dwarfs and neutron stars. The combination of observations in optical, ultraviolet and X-rays in one cluster resulted in the discovery of several millisecond radio pulsar companions, tens of new cataclysmic variables and the second double degenerate candidate to date in any globular cluster. A comparison with the cataclysmic variables in other clusters was also carried out. An extensive X-ray study of a transient neutron star low-mass X-ray binary in another cluster revealed a different behavior to other neutron star transients. Additionally, the two confirmed transitional millisecond pulsars in the Galactic field were studied using mid-ultraviolet observations for the first time in this thesis
X-ray diversity in old star clusters
No full textThe X-ray sky is host to a variety of objects with very distinct properties and underlying physics. In particular, groups of stars known as star clusters are excellent environments to study an array of X-ray sources. In this thesis, I discuss the properties of low-luminosity X-ray sources in old star clusters from two distinct standpoints – the properties of faint X-ray sources within old open clusters, and the cooling of accretion-heated neutron-star crusts in low-mass X-ray binary (LMXB) systems in globular clusters. The first part of the thesis focuses on the properties of close, interacting binary star populations in old open clusters. These properties are a powerful diagnostic of the stellar dynamics taking place inside the cluster. I used X-ray and optical telescope data to unravel the nature of such sources. I determined the total X-ray luminosity per unit mass of the cluster and found that the results support earlier findings that old open clusters are more luminous in X-rays than other old stellar populations. In the second part of the thesis, I studied the cooling of accretion-heated neutron-star crusts in two LMXBs. Using X-ray monitoring observations of the neutron stars during quiescence, I followed the cooling of the crusts over time. Such studies of the thermal evolution of an accretion-heated neutron-star crust help in improving our understanding of the structure and composition of the crust, and how the thermal conductivities of different neutron stars vary under different physical conditions
Timing of delivery for women with non-severe hypertensive disorders of pregnancy
No full textThis thesis consists of two parts. Part I focusses on the HYPITAT-II study, which evaluated whether women with non-severe hypertensive disorders of pregnancy between 34 and 37 weeks of pregnancy should be delivered in an attempt to prevent complications, or monitored until 37 weeks to allow the unborn child to mature. The results show that any beneficial effect of immediate delivery on maternal outcomes is probably small. However, expectant monitoring clearly benefits the child. As quality of life did not differ between the two strategies and economic analyses favoured expectant monitoring as well, we concluded that expectant monitoring should be offered to women with non-severe hypertensive disorders of pregnancy between 34 and 37 weeks of gestation. Part II discusses other aspects of timing of delivery for women with non-severe hypertensive disorders of pregnancy. First, we studied maternal and neonatal outcomes of women with chronic hypertension in pregnancy. The results show that these women have an increased risk of complications, which might be reduced by planned delivery between 39 and 40 weeks of pregnancy. Second, we explored the potential of so-called “individual patient data meta-analysis” to provide definitive and in-depth answers on timing of delivery for individual women with non-severe hypertensive disorders of pregnancy. In conclusion, the work in this thesis indicates that immediate delivery does not benefit all women with non-severe hypertensive disorders of pregnancy. Additional research, using individual data from existing trials, is necessary to develop optimal management for individual women with these disorders
Optical studies of X-ray sources in the old open cluster M67
No full textM67 is an old open star cluster with an estimated age of ~4 billion years. It is
one of the best studied open clusters. This thesis contains an optical study of X-
ray sources in M67; in addition several sources in the old clusters NGC752 and
NGC6940 are studied. X-ray observations of old open clusters have detected
many magnetically active binaries. This is not unexpected: at ages higher than
~1 billion years single late-type stars are believed to rotate too slowly to emit
detectable X-rays. Tidal interaction in a close binary is therefore required to
induce rotation at a higher rate than is typical for single stars. However, in this
work I mainly studied stars whose X-rays are not as easily explained: binaries
with orbits too wide for tidal interaction, and stars whose evolutionary statuses
are badly understood; in these cases the X-rays might provide a clue to the
nature of the stars.
Many of the peculiar X-ray sources that I investigated are binaries. It is
found that the properties of some systems are difficult to understand in the
context of binary evolution alone. Therefore, one of the conclusions is that
interactions between cluster stars have produced these peculiar systems and
that the properties of the stars that are the outcome of such interactions are
still poorly understood.
One of the stars that is studied in detail is the blue straggler S 1082 in
M67 which is probably a good example of a system that is the product of one
or multiple encounters. The present study solved its apparently contradictory
properties: eclipses with a period of ~1 day had been observed but the radial-
velocity variations of the narrow lines in the spectrum indicate that the star
moves in an orbit of ~1000 days. We concluded that S 1082 must be triple,
and indeed found the signature of three stars. As we find that two stars in the
system are blue stragglers on their own account and that the total mass of the
inner binary is about three times the turnoff mass of M67, probably five stars
were involved in the formation of S 1082.
Two other stars in M67 that are studied are the sub-subgiants S 1063 and
S 1113. Even though the peculiar positions of these binaries in the colour-
magnitude diagram are so similar, we found that their different orbital properties
make it difficult to find one explanation for their origin. In fact, we have great
difficulty to explain the properties of these stars at all, and are left to conclude
that they were involved in a recent interaction with other cluster stars
ON THE ORIGIN OF THE NEAR-INFRARED EMISSION FROM THE NEUTRON-STAR LOW-MASS X-RAY BINARY GX 9+1
No full textWe have determined an improved position for the luminous persistent neutron-star low-mass X-ray binary and atoll source GX 9+1 from archival Chandra X-ray Observatory data. The new position significantly differs from a previously published Chandra position for this source. Based on the revised X-ray position we have identified a new near-infrared (NIR) counterpart to GX 9+1 in K[subscript s]-band images obtained with the PANIC and FourStar cameras on the Magellan Baade Telescope. NIR spectra of this K[subscript s]=16.5 ± 0.1 mag star, taken with the FIRE spectrograph on the Baade Telescope, show a strong Br γ emission line, which is a clear signature that we discovered the true NIR counterpart to GX 9+1. The mass donor in GX 9+1 cannot be a late-type giant, as such a star would be brighter than the estimated absolute Ks magnitude of the NIR counterpart. The slope of the dereddened NIR spectrum is poorly constrained due to uncertainties in the column density NH and NIR extinction. Considering the source's distance and X-ray luminosity, we argue that NH likely lies near the high end of the previously suggested range. If this is indeed the case, the NIR spectrum is consistent with thermal emission from a heated accretion disk, possibly with a contribution from the secondary. In this respect, GX 9+1 is similar to other bright atolls and the Z sources, whose NIR spectra do not show the slope that is expected for a dominant contribution from optically thin synchrotron emission from the inner regions of a jet
Complex anisotropy beneath the Peruvian flat slab from frequency-dependent, multiple-phase shear wave splitting analysis
No full textFlat or shallow subduction is a relatively widespread global occurrence, but the dynamics remain poorly understood. In particular, the interaction between flat slabs and the surrounding mantle flow has yet to be studied in detail. Here we present measurements of seismic anisotropy to investigate mantle flow beneath the Peruvian flat-slab segment, the largest present-day region of flat subduction. We conduct a detailed shear wave splitting analysis at a long-running seismic station (NNA) located near Lima, Peru. We present measurements of apparent splitting parameters (fast direction ? and delay time ?t) for SKS, ScS, and local S phases from 80 events. We observe well-defined frequency dependence and backazimuthal variability, indicating the likely presence of complex anisotropy. Forward modeling the observations with two or three layers of anisotropy reveals a likely layer with a trench-normal fast direction underlying a layer with a more trench-oblique (to trench-subparallel) fast direction. In order to further constrain the anisotropic geometry, we analyzed the source-side splitting from events originating within the slab measured at distant stations. Beneath the flat-slab segment, we found trench-normal fast splitting directions in the subslab mantle, while within the dipping portion of the slab further to the east, likely trench-subparallel anisotropy within the slab itself. This subslab pattern contradicts observations from elsewhere in South America for “normal” (i.e., more steeply dipping) slab conditions. It is similar, however, to inferences from other shallowly dipping subduction zones around the world. While there is an apparent link between slab dip and the surrounding mantle flow, at least beneath Peru, the precise nature of the relationship remains to be clarified
A Chandra X-Ray Census of the Interacting Binaries in Old Open Clusters—Collinder 261
No full textWe present the first X-ray study of Collinder 261 (Cr 261), which at an age of 7 Gyr is one of the oldest open clusters known in the Galaxy. Our observation with the Chandra X-Ray Observatory is aimed at uncovering the close interacting binaries in Cr 261, and reaches a limiting X-ray luminosity of (0.3–7 keV) for stars in the cluster. We detect 107 sources within the cluster half-mass radius r h , and we estimate that among the sources with , ~26 are associated with the cluster. We identify a mix of active binaries and candidate active binaries, candidate cataclysmic variables, and stars that have "straggled" from the main locus of Cr 261 in the color–magnitude diagram. Based on a deep optical source catalog of the field, we estimate that Cr 261 has an approximate mass of 6500 M ⊙, roughly the same as the old open cluster NGC 6791. The X-ray emissivity of Cr 261 is similar to that of other old open clusters, supporting the trend that they are more luminous in X-rays per unit mass than old populations of higher (globular clusters) and lower (the local neighborhood) stellar density. This implies that the dynamical destruction of binaries in the densest environments is not solely responsible for the observed differences in X-ray emissivity
Neutron star atmosphere composition: the quiescent, low-mass X-ray binary in the globular cluster M28
No full textUsing deep Chandra observations of the globular cluster M28, we study the quiescent X-ray emission of a neutron star in a low-mass X-ray binary in order to constrain the chemical composition of the neutron star atmosphere and the equation of state of dense matter. We fit the spectrum with different neutron star atmosphere models composed of hydrogen, helium or carbon. The parameter values obtained with the carbon model are unphysical and such a model can be ruled out. Hydrogen and helium models give realistic parameter values for a neutron star, and the derived mass and radius are clearly distinct depending on the composition of the atmosphere. The hydrogen model gives masses/radii consistent with the canonical values of 1.4 M_Sun and 10 km, and would allow for the presence of exotic matter inside neutron stars. On the other hand, the helium model provides solutions with higher masses/radii, consistent with the stiffest equations of state. Measurements of neutron star masses/radii by spectral fitting should consider the possibility of heavier element atmospheres, which produce larger masses/radii for the same data, unless the composition of the accretor is known independently
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