2,154 research outputs found
HST Snapshot Survey of Post-AGB Objects
The results from a Hubble Space Telescope (HST) snapshot survey of post-AGB
objects are shown. The aim of the survey is to complement existing HST images
of PPN and to connect various types of nebulosities with physical and chemical
properties of their central stars. Nebulosities are detected in 15 of 33
sources. Images and photometric and geometric measurements are presented. For
sources with nebulosities we see a morphological bifurcation into two groups,
DUPLEX and SOLE, as previous studies have found. We find further support to the
previous results suggesting that this dichotomy is caused by a difference in
optical thickness of the dust shell. The remaining 18 sources are classified as
stellar post-AGB objects, because our observations indicate a lack of
nebulosity. We show that some stellar sources may in fact be DUPLEX or SOLE
based on their infrared colors. The cause of the differences among the groups
are investigated. We discuss some evidence suggesting that high progenitor-mass
AGB stars tend to become DUPLEX post-AGB objects. Intermediate progenitor-mass
AGB stars tend to be SOLE post-AGB objects. Most of the stellar sources
probably have low mass progenitors and do not seem to develop nebulosities
during the post-AGB phase and therefore do not become planetary nebulae.Comment: 21 pages, 11 figure
H2O Maser Observations of Candidate Post-AGB Stars and Discovery of Three High-velocity Water Sources
We present the results of 22 GHz H_2O maser observations of a sample of 85
post-Asymptotic Giant Branch (post-AGB) candidate stars, selected on the basis
of their OH 1612 MHz maser and far-infrared properties. All sources were
observed with the Tidbinbilla 70-m radio telescope and 21 detections were made.
86 GHz SiO Mopra observations of a subset of the sample are also presented. Of
the 21 H_2O detections, 15 are from sources that are likely to be massive AGB
stars and most of these show typical, regular H_2O maser profiles. In contrast,
nearly all the detections of more evolved stars exhibited high-velocity H_2O
maser emission. Of the five sources seen, v223 (W43A, IRAS 18450-0148) is a
well known `water-fountain' source which belongs to a small group of post-AGB
stars with highly collimated, high-velocity H_2O maser emission. A second
source in our sample, v270 (IRAS 18596+0315), is also known to have
high-velocity emission. We report the discovery of similar emission from a
further three sources, d46 (IRAS 15445-5449), d62 (IRAS 15544-5332) and b292
(IRAS 18043-2116). The source d46 is an evolved post-AGB star with highly
unusual maser properties. The H_2O maser emission from d62 is probably
associated with a massive star. The source b292 is a young post-AGB star that
is highly likely to be a water-fountain source, with masers detected over a
velocity range of 210 km s^{-1}.Comment: 47 pages, 9 figures, 4 tables, accepted by Ap
LOFAR early-time search for coherent radio emission from GRB 180706A
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.The nature of the central engines of gamma-ray bursts (GRBs) and the composition of their relativistic jets are still under debate. If the jets are Poynting flux dominated rather than baryon dominated, a coherent radio flare from magnetic re-connection events might be expected with the prompt gamma-ray emission. There are two competing models for the central engines of GRBs; a black hole or a newly formed milli-second magnetar. If the central engine is a magnetar it is predicted to produce coherent radio emission as persistent or flaring activity. In this paper, we present the deepest limits to date for this emission following LOFAR rapid response observations of GRB 180706A. No emission is detected to a 3 limit of 1.7 mJy beam at 144 MHz in a two-hour LOFAR observation starting 4.5 minutes after the gamma-ray trigger. A forced source extraction at the position of GRB 180706A provides a marginally positive (1 sigma) peak flux density of mJy. The data were time-sliced into different sets of snapshot durations to search for FRB like emission. No short duration emission was detected at the location of the GRB. We compare these results to theoretical models and discuss the implications of a non-detection.Peer reviewedFinal Accepted Versio
Multichannel interference mitigation methods in radio astronomy
Radio-astronomical observations are increasingly corrupted by RF
interference, and online detection and filtering algorithms are becoming
essential. To facilitate the introduction of such techniques into radio
astronomy, we formulate the astronomical problem in an array signal processing
language, and give an introduction to some elementary algorithms from that
field. We consider two topics in detail: interference detection by rank
estimation of short-term covariance matrices, and spatial filtering by subspace
estimation and projection. We discuss experimental data collected at the
Westerbork radio telescope, and illustrate the effectiveness of the space-time
detection and blanking process on the recovery of a 3C48 absorption line in the
presence of GSM mobile telephony interference.Comment: 39 pages, 18 figures.Enhanced figures can be downloaded from
http://cas.et.tudelft.nl/~leshem/postscripts/leshem/figs34567.ps.gz To appear
in Astrophysical Journal Supplements serie
Webteaching: sequencing of subject matter in relation to prior knowledge of pupils
Two experiments are discussed in which the sequencing procedure of webteaching is compared with a linear sequence for the presentation of text material.\ud
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In the first experiment variations in the level of prior knowledge of pupils were studied for their influence on the sequencing mode of text presentation. Prior knowledge greatly reduced the effect of the size of sequencing procedures.\ud
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In the second experiment pupils with a low level of prior knowledge studied a text, following either a websequence or a linear sequence. Webteaching was superior to linear teaching on a number of dependent variables. It is concluded that webteaching is an effective sequencing procedure in those cases where substantial new learning is required
Mid-Infrared interferometry of dust around massive evolved stars
We report long-baseline interferometric measurements of circumstellar dust
around massive evolved stars with the MIDI instrument on the Very Large
Telescope Interferometer and provide spectrally dispersed visibilities in the
8-13 micron wavelength band. We also present diffraction-limited observations
at 10.7 micron on the Keck Telescope with baselines up to 8.7 m which explore
larger scale structure. We have resolved the dust shells around the late type
WC stars WR 106 and WR 95, and the enigmatic NaSt1 (formerly WR 122), suspected
to have recently evolved from a Luminous Blue Variable (LBV) stage. For AG Car,
the protoypical LBV in our sample, we marginally resolve structure close to the
star, distinct from the well-studied detached nebula. The dust shells around
the two WC stars show fairly constant size in the 8-13 micron MIDI band, with
gaussian half-widths of ~ 25 to 40 mas. The compact dust we detect around NaSt1
and AG Car favors recent or ongoing dust formation.
Using the measured visibilities, we build spherically symmetric radiative
transfer models of the WC dust shells which enable detailed comparison with
existing SED-based models. Our results indicate that the inner radii of the
shells are within a few tens of AU from the stars. In addition, our models
favor grain size distributions with large (~ 1 micron) dust grains. This
proximity of the inner dust to the hot central star emphasizes the difficulty
faced by current theories in forming dust in the hostile environment around WR
stars. Although we detect no direct evidence for binarity for these objects,
dust production in a colliding-wind interface in a binary system is a feasible
mechanism in WR systems under these conditions.Comment: 21 pages, 4 tables, 13 figures. Accepted for publication in the
Astrophysical Journa
Echelle long-slit optical spectroscopy of evolved stars
We present echelle long-slit optical spectra of a sample of objects evolving
off the AGB, most of them in the pre-planetary nebula (pPN) phase, obtained
with the ESI and MIKE spectrographs at Keck-II and Magellan-I, respectively.
The total wavelength range covered with ESI (MIKE) is ~3900 to 10900 A (~3600
to 7200A). In this paper, we focus our analysis mainly on the Halpha profiles.
Prominent Halpha emission is detected in half of the objects, most of which
show broad Halpha wings (up to ~4000 km/s). In the majority of the
Halpha-emission sources, fast, post-AGB winds are revealed by P-Cygni profiles.
In ~37% of the objects Halpha is observed in absorption. In almost all cases,
the absorption profile is partially filled with emission, leading to complex,
structured profiles that are interpreted as an indication of incipient post-AGB
mass-loss. All sources in which Halpha is seen mainly in absorption have F-G
type central stars, whereas sources with intense Halpha emission span a larger
range of spectral types from O to G. Shocks may be an important excitation
agent of the close stellar surroundings for objects with late type central
stars. Sources with pure emission or P Cygni Halpha profiles have larger J-K
color excess than objects with Halpha mainly in absorption, which suggests the
presence of warm dust near the star in the former. The two classes of profile
sources also segregate in the IRAS color-color diagram in a way that intense
Halpha-emitters have dust grains with a larger range of temperatures.
(abridged)Comment: 68 pages, 14 figures, accepted for publication in ApJS (abstract
abridged
Near-Infrared Photometric Survey of Proto-Planetary Nebula Candidates
We present JHK' photometric measurements of 78 objects mostly consisting of
proto-planetary nebula candidates. Photometric magnitudes are determined by
means of imaging and aperture photometry. Unlike the observations with a
photometer with a fixed-sized beam, the method of imaging photometry permits
accurate derivation of photometric values because the target sources can be
correctly identified and confusion with neighboring sources can be easily
avoided. Of the 78 sources observed, we report 10 cases in which the source
seems to have been misidentified or confused by nearby bright sources. We also
present nearly two dozen cases in which the source seems to have indicated a
variability which prompts a follow-up monitoring. There are also a few sources
that show previously unreported extendedness. In addition, we present H band
finding charts of the target sources.Comment: 3 tables, 1 figur
The HUSH complex is a gatekeeper of type I interferon through epigenetic regulation of LINE-1s
The Human Silencing Hub (HUSH) complex is necessary for epigenetic repression of LINE-1 elements. We show that HUSH-depletion in human cell lines and primary fibroblasts leads to induction of interferon-stimulated genes (ISGs) through JAK/STAT signaling. This effect is mainly attributed to MDA5 and RIG-I sensing of double-stranded RNAs (dsRNAs). This coincides with upregulation of primate-conserved LINE-1s, as well as increased expression of full-length hominid-specific LINE-1s that produce bidirectional RNAs, which may form dsRNA. Notably, LTRs nearby ISGs are derepressed likely rendering these genes more responsive to interferon. LINE-1 shRNAs can abrogate the HUSH-dependent response, while overexpression of an engineered LINE-1 construct activates interferon signaling. Finally, we show that the HUSH component, MPP8 is frequently downregulated in diverse cancers and that its depletion leads to DNA damage. These results suggest that LINE-1s may drive physiological or autoinflammatory responses through dsRNA sensing and gene-regulatory roles and are controlled by the HUSH complex
Risk stratification and subclinical phenotyping of dilated and/or arrhythmogenic cardiomyopathy mutation-positive relatives:CVON eDETECT consortium
In relatives of index patients with dilated cardiomyopathy and arrhythmogenic cardiomyopathy, early detection of disease onset is essential to prevent sudden cardiac death and facilitate early treatment of heart failure. However, the optimal screening interval and combination of diagnostic techniques are unknown. The clinical course of disease in index patients and their relatives is variable due to incomplete and age-dependent penetrance. Several biomarkers, electrocardiographic and imaging (echocardiographic deformation imaging and cardiac magnetic resonance imaging) techniques are promising non-invasive methods for detection of subclinical cardiomyopathy. However, these techniques need optimisation and integration into clinical practice. Furthermore, determining the optimal interval and intensity of cascade screening may require a personalised approach. To address this, the CVON-eDETECT (early detection of disease in cardiomyopathy mutation carriers) consortium aims to integrate electronic health record data from long-term follow-up, diagnostic data sets, tissue and plasma samples in a multidisciplinary biobank environment to provide personalised risk stratification for heart failure and sudden cardiac death. Adequate risk stratification may lead to personalised screening, treatment and optimal timing of implantable cardioverter defibrillator implantation. In this article, we describe non-invasive diagnostic techniques used for detection of subclinical disease in relatives of index patients with dilated cardiomyopathy and arrhythmogenic cardiomyopathy
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