3,434 research outputs found
Spitzer White Dwarf Planet Limits
We present preliminary limits on the presence of planets around white dwarf
stars using the IRAC photometer on the Spitzer space telescope. Planets emit
strongly in the mid-infrared which allows their presence to be detected as an
excess at these wavelengths. We place limits of for 8 stars assuming
ages of , and for 23 stars.We describe our survey, present our
results and comment on approaches to improve our methodology.Comment: 4 pages, 3 figures, to appear in Proceedings of 15th European White
Dwarf Worksho
Magnetospherically-trapped dust and a possible model for the unusual transits at WD 1145+017
The rapidly evolving dust and gas extinction observed towards WD 1145+017 has
opened a real-time window onto the mechanisms for destruction-accretion of
planetary bodies onto white dwarf stars, and has served to underline the
importance of considering the dynamics of dust particles around such objects.
Here it is argued that the interaction between (charged) dust grains and the
stellar magnetic field is an important ingredient in understanding the physical
distribution of infrared emitting particles in the vicinity of such white
dwarfs. These ideas are used to suggest a possible model for WD 1145+017 in
which the unusual transit shapes are caused by opaque clouds of dust trapped in
the stellar magnetosphere. The model can account for the observed transit
periodicities if the stellar rotation is near 4.5 h, as the clouds of trapped
dust are then located near or within the co-rotation radius. The model requires
the surface magnetic field to be at least around some tens of kG. In contrast
to the eccentric orbits expected for large planetesimals undergoing tidal
disintegration, the orbits of magnetospherically-trapped dust clouds are
essentially circular, consistent with the observations.Comment: 5 pages, accepted to MNRAS Letter
Constraining the Surface Inhomogeneity and Settling Times of Metals on Accreting White Dwarfs
Due to the short settling times of metals in DA white dwarf atmospheres, any
white dwarfs with photospheric metals must be actively accreting. It is
therefore natural to expect that the metals may not be deposited uniformly on
the surface of the star. We present calculations showing how the temperature
variations associated with white dwarf pulsations lead to an observable
diagnostic of the surface metal distribution, and we show what constraints
current data sets are able to provide. We also investigate the effect that
time-variable accretion has on the metal abundances of different species, and
we show how this can lead to constraints on the gravitational settling times.Comment: 4 pages, 5 figures, accepted for publication in the Astrophysical
Journal Letters, updated reference
Learning from openness : the dynamics of breadth in external innovation linkages
We explore how openness in terms of external linkages generates learning effects, which enable firms to generate more innovation outputs from any given breadth of external linkages. Openness to external knowledge sources, whether through search activity or linkages to external partners in new product development, involves a process of interaction and information processing. Such activities are likely to be subject to a learning process, as firms learn which knowledge sources and collaborative linkages are most useful to their particular needs, and which partnerships are most effective in delivering innovation performance. Using panel data from Irish manufacturing plants, we find evidence of such learning effects: establishments with substantial experience of external collaborations in previous periods derive more innovation output from openness in the current period
Outlaw Community Innovations
Recent studies of outlaw communities provide qualitative evidence of their existence and the organisation of the underlying innovation processes. We provide descriptive results from a large scale survey of two online outlaw communities focussing on Microsoft's XBox. In line with previous findings, we identify two types of participants in outlaw communities - user innovators and adopters. Based on 2,256 responses, we find that users modify their XBox mainly to be able to increase the set of available functions of their XBox. Users are also motivated to modify their XBox for the sake of having fun and to conduct pirate behaviour. Finally, the results from our survey suggest that user innovators are largely intrinsically motivated by fun and the intellectual stimulation of writing code for homebrew software
Discovery of Photospheric Calcium Line Strength Variations in the DAZd White Dwarf G29-38
Metals in the photospheres of white dwarfs with Teff between 12,000 and
25,000 K should gravitationally settle out of these atmospheres in 1-2 weeks.
Temporal variations in the line strengths of these metals could provide a
direct measurement of episodic metal accretion. Using archival VLT and Keck
spectroscopy, we find evidence that the DAZd white dwarf G29-38 shows
significant changes in its Ca II K line strength. At the two best-observed
epochs, we find that the Ca line equivalent width (EW) = 165 +- 4 mA (in
1996.885) and 280 +- 8 mA (in 1999.653), which is an increase of 70%. We
consider the effect that pulsation has on the Ca EWs for this known variable
star, and find that it adds an error of < 1 mA to these measurements. Calcium
line strengths at other observational epochs support variations with timescales
as short as two weeks. These Ca EW variations indicate that the metal accretion
process in G29-38, presumably from its debris disk, is episodic on timescales
of a few weeks or less, and thus the accretion is not dominated by
Poynting-Robertson drag from an optically thin, continuous disk, which has a
timescale of ~1 year.Comment: 15 pages, 4 figures, ApJ accepte
A BAYESIAN ANALYSIS OF THE AGES OF FOUR OPEN CLUSTERS
In this paper we apply a Bayesian technique to determine the best fit of stellar evolution models to find the main sequence turn off age and other cluster parameters of four intermediate-age open clusters: NGC 2360, NGC 2477, NGC 2660, and NGC 3960. Our algorithm utilizes a Markov chain Monte Carlo technique to fit these various parameters, objectively finding the best fit isochrone for each cluster. The result is a high precision isochrone fit. We compare these results with the those of traditional âby eyeâ isochrone fitting methods. By applying this Bayesian technique to NGC 2360, NGC 2477, NGC 2660, and NGC 3960 we determine the ages of these clusters to be 1.35 ± 0.05, 1.02 ± 0.02, 1.64 ± 0.04, and 0.860 ± 0.04 Gyr, respectively. The results of this paper continue our effort to determine cluster ages to higher precision than that offered by these traditional methods of isochrone fitting
BAYESIAN ANALYSIS OF TWO STELLAR POPULATIONS IN GALACTIC GLOBULAR CLUSTERS I: STATISTICAL AND COMPUTATIONAL METHODS
We develop a Bayesian model for globular clusters composed of multiple stellar populations, extending earlier statistical models for open clusters composed of simple (single) stellar populations (e.g., van Dyk et al. 2009; Stein et al. 2013). Specifically, we model globular clusters with two populations that differ in helium abundance. Our model assumes a hierarchical structuring of the parameters in which physical propertiesâage, metallicity, helium abundance, distance, absorption, and initial massâare common to (i) the cluster as a whole or to (ii) individual populations within a cluster, or are unique to (iii) individual stars. An adaptive Markov chain Monte Carlo (MCMC) algorithm is devised for model fitting that greatly improves convergence relative to its precursor non-adaptive MCMC algorithm. Our model and computational tools are incorporated into an open-source software suite known as BASE-9. We use numerical studies to demonstrate that our method can recover parameters of two-population clusters, and also show model misspecification can potentially be identified. As a proof of concept, we analyze the two stellar populations of globular cluster NGC 5272 using our model and methods. (BASE-9 is available from GitHub: https://github.com/argiopetech/base/releases)
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