143 research outputs found
Spectroscopic and redox properties of amine-unctionalized K_2[Os-^(II)(bpy)(CN)_4] complexes
We report the first examples of amine-functionalized K_2[Os^(II)(bpy)(CN)_4] (bpy = 2,2'-bipyridine) complexes. The tetracyanoosmate complexes were prepared by UV irradiation (λ = 254 nm) of K_4[Os^(II)(CN)_6] and primary amine-functionalized bpy ligands in acidic aqueous media. The aqueous solution pH dependences of the spectroscopic and redox properties of 4,4'- and 5,5'-substituted complexes have been investigated. The pendant amine functional groups and coordinated cyanide ligands are basic sites that can be sequentially protonated, thereby allowing systematic tuning of electrochemical and optical spectroscopic properties
Towards a Continuous Record of the Sky
It is currently feasible to start a continuous digital record of the entire
sky sensitive to any visual magnitude brighter than 15 each night. Such a
record could be created with a modest array of small telescopes, which
collectively generate no more than a few Gigabytes of data daily.
Alternatively, a few small telescopes could continually re-point to scan and
reco rd the entire sky down to any visual magnitude brighter than 15 with a
recurrence epoch of at most a few weeks, again always generating less than one
Gigabyte of data each night. These estimates derive from CCD ability and
budgets typical of university research projects. As a prototype, we have
developed and are utilizing an inexpensive single-telescope system that obtains
optical data from about 1500 square degrees. We discuss the general case of
creating and storing data from a both an epochal survey, where a small number
of telescopes continually scan the sky, and a continuous survey, composed of a
constellation of telescopes dedicated each continually inspect a designated
section of the sky. We compute specific limitations of canonical surveys in
visible light, and estimate that all-sky continuous visual light surveys could
be sensitive to magnitude 20 in a single night by about 2010. Possible
scientific returns of continuous and epochal sky surveys include continued
monitoring of most known variable stars, establishing case histories for
variables of future interest, uncovering new forms of stellar variability,
discovering the brightest cases of microlensing, discovering new novae and
supernovae, discovering new counterparts to gamma-ray bursts, monitoring known
Solar System objects, discovering new Solar System objects, and discovering
objects that might strike the Earth.Comment: 38 pages, 9 postscript figures, 2 gif images. Revised and new section
added. Accepted to PASP. Source code submitted to ASCL.ne
An architecture for consolidating multidimensional time-series data onto a common coordinate grid
Modelling the Galactic distribution of free electrons
In this paper we test 8 models of the free electron distribution in the Milky
Way that have been published previously, and we introduce 4 additional models
that explore the parameter space of possible models further. These new models
consist of a simple exponential thick disk model, and updated versions of the
models by Taylor & Cordes and Cordes & Lazio with more extended thick disks.
The final model we introduce uses the observed H-alpha intensity as a proxy for
the total electron column density, also known as the dispersion measure (DM).
We use the latest available data sets of pulsars with accurate distances
(through parallax measurements or association with globular clusters) to
optimise the parameters in these models. In the process of fitting a new scale
height for the thick disk in the model by Cordes & Lazio we discuss why this
thick disk cannot be replaced by the thick disk that Gaensler et al. advocated
in a recent paper. In the second part of our paper we test how well the
different models can predict the DMs of these pulsars at known distances.
Almost all models perform well, in that they predict DMs within a factor of
1.5-2 of the observed DMs for about 75% of the lines of sight. This is somewhat
surprising since the models we tested range from very simple models that only
contain a single exponential thick disk to very complex models like the model
by Cordes & Lazio. We show that the model by Taylor & Cordes that we updated
with a more extended thick disk consistently performs better than the other
models we tested. Finally, we analyse which sightlines have DMs that prove
difficult to predict by most models, which indicates the presence of local
features in the ISM between us and the pulsar. (abridged)Comment: 16 pages, 10 figures, 5 tables. Accepted for publication in the
Monthly Notices of the RAS by the Royal Astronomical Society and Blackwell
Publishin
On the discovery of an enormous ionized halo around the hot DO white dwarf PG1034+001
Very recently, the discovery of the largest known planetary nebula on the sky
surrounding the DO white dwarf PG1034+001 with an apparent diameter of about 2
degrees, corresponding to a linear diameter of 3.5 - 7.0 pc at the likely
distance of 100 - 200 pc, has been reported by Hewett et al. (2003). A careful
inspection of available sky survey data has now shown that this planetary
nebula, Hewett 1, is surrounded by an elliptical emission shell with an
apparent diameter of 6 times 9 degrees (16.2 +6.1/-4.5 times 24.3 +9.1/-6.8 pc
at d = 155 +58/-43 pc. A further emission structure, detected northeast of the
central star may indicate another shell with a size of 10 times 16 degr. From
presently available observational data we do not have indications whether the
emission arises from material which was ejected from PG1034+001 or from ionized
ambient ISM.
Improved proper motion data combined with radial velocity and distance from
the literature have enabled us to derive a Galactic orbit for the central star
PG1034+001. Its thin disk orbit and the morphology of the first halo suggest
that the nebula is in an advanced stage of interaction with the interstellar
medium.Comment: 4 pages, 3 figure
Hierarchical Star Formation: Stars and Stellar Clusters in the Gould Belt
We perform a study of the spatial and kinematical distribution of young open
clusters in the solar neighborhood, discerning between bound clusters and
transient stellar condensations within our sample. Then, we discriminate
between Gould Belt (GB) and local Galactic disk (LGD) members, using a previous
estimate of the structural parameters of both systems obtained from a sample of
O-B6 Hipparcos stars. Using this classified sample we analyze the spatial
structure and the kinematic behavior of the cluster system in the GB. The two
star formation regions that dominate and give the GB its characteristic
inclined shape show a striking difference in their content of star clusters:
while Ori OB1 is richly populated by open clusters, not a single one can be
found within the boundaries of Sco OB2. This is mirrored in the velocity space,
translating again into an abundance of clusters in the region of the kinematic
space populated by the members of Ori OB1, and a marginal number of them
associated to Sco OB2. In the light of these results we study the nature of the
GB with respect to the optical segment of the Orion Arm, and we propose that
the different content of star clusters, the different heights over the Galactic
plane and the different residual velocities of Ori OB1 and Sco OB2 can be
explained in terms of their relative position to the density maximum of the
Local Arm in the solar neighborhood. Although morphologically intriguing, the
GB appears to be the result of our local and biased view of a larger star
cluster complex in the Local Arm, that could be explained by the internal
dynamics of the Galactic disk.Comment: 23 pages, including 12 figures. Accepted for publication in MNRA
The influence of binarity on dust obscuration events in the planetary nebula M 2-29 and its analogues
The central star of the planetary nebula (CSPN) M 2-29 shows an extraordinary
R Coronae Borealis-like fading event in its optical lightcurve. The only other
CSPN to show these events are CPD-568032 (Hen 3-1333) and V651 Mon (NGC 2346).
Dust cloud formation in the line of sight appears responsible but the exact
triggering mechanism is not well understood. Understanding how planetary
nebulae (PNe) trigger dust obscuration events may help understand the same
process in a wide range of objects including Population-I WC9 stars, symbiotic
stars and perhaps Asymptotic Giant Branch (AGB) stars with long secondary
periods (LSPs). A binary scenario involving an eccentric, wide companion that
triggers dust formation via interaction at periastron is a potential
explanation that has been suggested for LSP variables. Model fits to the
lightcurves of CPD-568032 and M 2-29 show the dust forms in excess of 70 AU at
the inner edge of a dust disk. In the case of CPD-568032 this radius is far too
large to coincide with a binary companion trigger, although a binary may have
been responsible for the formation of the dust disk. We find no direct evidence
to support previous claims of binarity in M 2-29 either from the OGLE
lightcurve or deep medium-resolution VLT FLAMES spectroscopy of the CSPN. We
classify the CSPN as Of(H) with T_eff=50+-10 kK and log g=4.0+-0.3. We find a
mean distance of 7.4+-1.8 kpc to M 2-29 at which the M_V=-0.9 mag CSPN could
potentially hide a subgiant luminosity or fainter companion. A companion would
help explain the multiple similarities with D'-type symbiotic stars whose outer
nebulae are thought to be bona-fide PNe. The 7.4 kpc distance, oxygen abundance
of 8.3 dex and Galactic coordinates (l=4.0, b=-3.0) prove that M 2-29 is a
Galactic Bulge PN and not a Halo PN as commonly misconceived.Comment: 15 pages, 14 figures, 7 tables. Accepted for publication in A\&
Planck Intermediate Results. IX. Detection of the Galactic haze with Planck
Using precise full-sky observations from Planck, and applying several methods
of component separation, we identify and characterize the emission from the
Galactic "haze" at microwave wavelengths. The haze is a distinct component of
diffuse Galactic emission, roughly centered on the Galactic centre, and extends
to |b| ~35 deg in Galactic latitude and |l| ~15 deg in longitude. By combining
the Planck data with observations from the WMAP we are able to determine the
spectrum of this emission to high accuracy, unhindered by the large systematic
biases present in previous analyses. The derived spectrum is consistent with
power-law emission with a spectral index of -2.55 +/- 0.05, thus excluding
free-free emission as the source and instead favouring hard-spectrum
synchrotron radiation from an electron population with a spectrum (number
density per energy) dN/dE ~ E^-2.1. At Galactic latitudes |b|<30 deg, the
microwave haze morphology is consistent with that of the Fermi gamma-ray "haze"
or "bubbles," indicating that we have a multi-wavelength view of a distinct
component of our Galaxy. Given both the very hard spectrum and the extended
nature of the emission, it is highly unlikely that the haze electrons result
from supernova shocks in the Galactic disk. Instead, a new mechanism for
cosmic-ray acceleration in the centre of our Galaxy is implied.Comment: 15 pages, 9 figures, submitted to Astronomy and Astrophysic
The WHAM Northern Sky Survey and the Nature of the Warm Ionized Medium in the Galaxy
The Wisconsin H-Alpha Mapper (WHAM) has completed a velocity-resolved map of
diffuse H-alpha emission of the entire northern sky, providing the first
comprehensive picture of both the distribution and kinematics of diffuse
ionized gas in the Galaxy. WHAM continues to advance our understanding of the
physical conditions of the warm ionized medium through observations of other
optical emission lines throughout the Galactic disk and halo. We discuss some
highlights from the survey, including an optical window into the inner Galaxy
and the relationship between HI and HII in the diffuse ISM.Comment: 9 pages, 3 figures. To be published in "How does the Galaxy work?",
eds. E.J. Alfaro, E. Perez & J. Franco, Kluwer, held 23-27 June 2003 in
Granada, Spain. Higher resolution version available at
http://www.astro.wisc.edu/~madsen/prof/pubs.htm
Comparison of filters for the detection of point sources in Planck simulations
We study the detection of extragalactic point sources in two-dimensional flat
simulations for all the frequencies of the forthcoming ESA's Planck mission. In
this work we have used the most recent available templates of the microwave
sky: as for the diffuse Galactic components and the Sunyaev-Zel'dovich clusters
we have used the ``Plank Reference Sky Model''; as for the extragalactic point
sources, our simulations - which comprise all the source populations relevant
in this frequency interval - are based on up-to-date cosmological evolution
models for sources. To consistently compare the capabilities of different
filters for the compilation of the - hopefully - most complete blind catalogue
of point sources, we have obtained three catalogues by filtering the simulated
sky maps with: the Matched Filter (MF), the Mexican Hat Wavelet (MHW1) and the
Mexican Hat Wavelet 2 (MHW2), the first two members of the MHW Family. For the
nine Planck frequencies we show the number of real and spurious detections and
the percentage of spurious detections at different flux detection limits as
well as the completeness level of the catalogues and the average errors in the
estimation of the flux density of detected sources. Allowing a 5 % of spurious
detections, we obtain the following number of detections by filtering with the
MHW2 an area equivalent to half of the sky: 580 (30 GHz), 342 (44 GHz), 341 (70
GHz), 730 (100 GHz), 1130 (143 GHz), 1233 (217 GHz), 990 (353 GHz), 1025 (545
GHz) and 3183 (857 GHz). Our current results indicate that the MF and the MHW2
yield similar results, whereas the MHW1 performs worse in some cases and
especially at very low fluxes. This is a relevant result, because we are able
to obtain comparable results with the well known Matched Filter and with this
specific wavelet, the MHW2, which is much easier to implement and use.Comment: 20 pages, 14 figures. Accepted for publication in MNRA
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