1,040 research outputs found
Generators for the hyperelliptic Torelli group and the kernel of the Burau representation at t = -1
We prove that the hyperelliptic Torelli group is generated by Dehn twists about
separating curves that are preserved by the hyperelliptic involution. This verifies a
conjecture of Hain. The hyperelliptic Torelli group can be identified with the kernel
of the Burau representation evaluated at t = −1 and also the fundamental group of
the branch locus of the period mapping, and so we obtain analogous generating sets
for those. One application is that each component in Torelli space of the locus of
hyperelliptic curves becomes simply connected when curves of compact type are added
High molecular weight SOA formation during limonene ozonolysis: insights from ultrahigh-resolution FT-ICR mass spectrometry characterization
The detailed molecular composition of laboratory generated limonene ozonolysis secondary organic aerosol (SOA) was studied using ultrahigh-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Approximately 1200 molecular formulas were identified in the SOA over the mass range of 140 to 850 Da. Four characteristic groups of high relative abundance species were observed; they indicate an array of accretion products that retain a large fraction of the limonene skeleton. The identified molecular formulas of each of the groups are related to one another by CH2, O and CH2O homologous series. The CH2 and O homologous series of the low molecular weight (MW) SOA (m/z \u3c 300) are explained with a combination of functionalization and fragmentation of radical intermediates and reactive uptake of gas-phase carbonyls. They include isomerization and elimination reactions of Criegee radicals, reactions between alkyl peroxy radicals, and scission of alkoxy radicals resulting from the Criegee radicals. The presence of compounds with 10–15 carbon atoms in the first group (e.g. C11H18O6) provides evidence for SOA formation by the reactive uptake of gas-phase carbonyls during limonene ozonolysis. The high MW compounds (m/z \u3e 300) were found to constitute a significant number fraction of the identified SOA components. The formation of high MW compounds was evaluated by molecular formula trends, fragmentation analysis of select high MW compounds and a comprehensive reaction matrix including the identified low MW SOA, hydroperoxides and Criegee radicals as building blocks. Although the formation of high MW SOA may occur via a variety of radical and non-radical reaction channels, the combined approach indicates a greater importance of the non-condensation reactions over aldol and ester condensation reaction channels. Among these hemi-acetal reactions appear to be most dominant followed by hydroperoxide and Criegee reaction channels
An Evaluation of Gravity Waves and Gravity Wave Sources in the Southern Hemisphere in a 7-Km Global Climate Simulation
In this study, gravity waves in the high-resolution GEOS-5 Nature Run are first evaluated with respect to satellite and other model results. Southern Hemisphere winter sources of nonorographic gravity waves in the model are then investigated by linking measures of tropospheric nonorographic gravity wave generation tied to precipitation and frontogenesis with absolute gravity wave momentum flux in the lower stratosphere. Finally, nonorographic gravity wave momentum flux is compared to orographic gravity wave momentum flux and compared to previous estimates. The results show that the global patterns in gravity wave amplitude, horizontal wavelength, and propagation direction are realistic compared to observations. However, like other global models the amplitudes are weaker and horizontal wavelengths longer than observed. The global patterns in absolute gravity wave momentum flux also agree well with previous model and observational estimates. The evaluation of model nonorographic gravity wave sources in the Southern Hemisphere winter shows that strong intermittent precipitation (greater than 10 mm per hr) is associated with gravity wave momentum flux over the South Pacific, whereas frontogenesis and less intermittent, lower precipitation rates (less than 10 mm per hr) are associated with gravity wave momentum flux near 60 degrees South. In the model, orographic gravity waves contribute almost exclusively to a peak in zonal mean momentum flux between 70 and 75 degrees South, while nonorographic waves dominate at 60 degrees South, and nonorographic gravity waves contribute a third to a peak in zonal mean momentum flux between 25 and 30 degrees South
A Very Sensitive 21cm Survey for Galactic High-Velocity HI
Very sensitive HI 21cm observations have been made in 860 directions at dec
>= -43deg in search of weak, Galactic, high-velocity HI emission lines at
moderate and high Galactic latitudes. One-third of the observations were made
toward extragalactic objects. The median 4-sigma detection level is NHI =
8x10^{17} cm^-2 over the 21' telescope beam. High-velocity HI emission is
detected in 37% of the directions; about half of the lines could not have been
seen in previous surveys. The median FWHM of detected lines is 30.3 km/s. High-
velocity HI lines are seen down to the sensitivity limit of the survey implying
that there are likely lines at still lower values of NHI. The weakest lines
have a kinematics and distribution on the sky similar to that of the strong
lines, and thus do not appear to be a new population. Most of the emission
originates from objects which are extended over several degrees; few appear to
be compact sources. At least 75%, and possibly as many as 90%, of the lines are
associated with one of the major high-velocity complexes. The Magellanic Stream
extends at least 10 deg to higher Galactic latitude than previously thought and
is more extended in longitude as well. Although there are many lines with low
column density, their numbers do not increase as rapidly as NHI^-1, so most of
the HI mass in the high-velocity cloud phenomenon likely resides in the more
prominent clouds. The bright HI features may be mere clumps within larger
structures, and not independent objects.Comment: 88 pages includes 22 figures Accepted for Publication in ApJ Suppl.
June 200
Do the Unidentified EGRET Sources Trace Annihilating Dark Matter in the Local Group?
In a cold dark matter (CDM) framework of structure formation, the dark matter
haloes around galaxies assemble through successive mergers with smaller haloes.
This merging process is not completely efficient, and hundreds of surviving
halo cores, or {\it subhaloes}, are expected to remain in orbit within the halo
of a galaxy like the Milky Way. While the dozen visible satellites of the Milky
Way may trace some of these subhaloes, the majority are currently undetected. A
large number of high-velocity clouds (HVCs) of neutral hydrogen {\it are}
observed around the Milky Way, and it is plausible that some of the HVCs may
trace subhaloes undetected in the optical. Confirming the existence of
concentrations of dark matter associated with even a few of the HVCs would
represent a dramatic step forward in our attempts to understand the nature of
dark matter. Supersymmetric (SUSY) extensions of the Standard Model of particle
physics currently suggest neutralinos as a natural well-motivated candidate for
the non-baryonic dark matter of the universe. If this is indeed the case, then
it may be possible to detect dark matter indirectly as it annihilates into
neutrinos, photons or positrons. In particular, the centres of subhaloes might
show up as point sources in gamma-ray observations. In this work we consider
the possibility that some of the unidentified EGRET -ray sources trace
annihilating neutralino dark matter in the dark substructure of the Local
Group. We compare the observed positions and fluxes of both the unidentified
EGRET sources and the HVCs with the positions and fluxes predicted by a model
of halo substructure, to determine to what extent any of these three
populations could be associated.Comment: 12 Pages, 4 figures, to appear in a special issue of ApSS. Presented
at "The Multiwavelength Approach to Unidentified Gamma-Ray Sources" (Hong
Kong, June 1 - 4, 2004; Conference organizers: K.S. Cheng and G.E. Romero
EEG theta/beta ratio as an electrophysiological marker for attentional control and its test-retest reliability
FSW - Self-regulation models for health behavior and psychopathology - ou
Every apple has a voice: using stable isotopes to teach about food sourcing and the water cycle
Agricultural crops such as fruits take up irrigation and meteoric water and
incorporate it into their tissue (fruit water) during growth, and the
geographic origin of a fruit may be traced by comparing the H and O stable
isotope composition (δ2H and δ18O values) of fruit water to the global geospatial distribution of H and O stable isotopes in
precipitation. This connection between common fruits and the global water
cycle provides an access point to connect with a variety of demographic
groups to educate about isotope hydrology and the water cycle. Within the
context of a 1-day outreach activity designed for a wide spectrum of
participants (high school students, undergraduate students, high school
science teachers) we developed introductory lecture materials, in-class
participatory demonstrations of fruit water isotopic measurement in real
time, and a computer lab exercise to couple actual fruit water isotope data
with open-source online geospatial analysis software. We assessed learning
outcomes with pre- and post-tests tied to learning objectives, as well as
participant feedback surveys. Results indicate that this outreach activity
provided effective lessons on the basics of stable isotope hydrology and the
water cycle. However, the computer lab exercise needs to be more specifically
tailored to the abilities of each participant group. This pilot study
provides a foundation for further development of outreach materials that can
effectively engage a range of participant groups in learning about the water
cycle and the ways in which humans modify the water cycle through
agricultural activity
Intergalactic HII Regions Discovered in SINGG
A number of very small isolated HII regions have been discovered at projected
distances up to 30 kpc from their nearest galaxy. These HII regions appear as
tiny emission line objects in narrow band images obtained by the NOAO Survey
for Ionization in Neutral Gas Galaxies (SINGG). We present spectroscopic
confirmation of four isolated HII regions in two systems, both systems have
tidal HI features. The results are consistent with stars forming in interactive
debris due to cloud-cloud collisions. The H-alpha luminosities of the isolated
HII regions are equivalent to the ionizing flux of only a few O stars each.
They are most likely ionized by stars formed in situ, and represent atypical
star formation in the low density environment of the outer parts of galaxies. A
small but finite intergalactic star formation rate will enrich and ionize the
surrounding medium. In one system, NGC 1533, we calculate a star formation rate
of 1.5e-3 msun/yr, resulting in a metal enrichment of ~1e-3 solar for the
continuous formation of stars. Such systems may have been more common in the
past and a similar enrichment level is measured for the `metallicity floor' in
damped Lyman-alpha absorption systems.Comment: accepted for publication in the Astronomical Journal, 19 pages,
including 5 figures, some low resolution. Paper with high resolution images
can be downloaded from
http://astro.ph.unimelb.edu.au/~eryan/publications/eldots.ps.g
INTEGRAL deep observations of the Small Magellanic Cloud
Deep observations of the Small Magellanic Cloud (SMC) and region were carried
out in the hard X-ray band by the INTEGRAL observatory in 2008-2009. The field
of view of the instrument permitted simultaneous coverage of the entire SMC and
the eastern end of the Magellanic Bridge. In total, INTEGRAL detected seven
sources in the SMC and five in the Magellanic Bridge; the majority of the
sources were previously unknown systems. Several of the new sources were
detected undergoing bright X- ray outbursts and all the sources exhibited
transient behaviour except the supergiant system SMC X-1. They are all thought
to be High Mass X-ray Binary (HMXB) systems in which the compact object is a
neutron star.Comment: 7 pages, 10 figures Accepted for publication in MNRA
The many streams of the Magellanic Stream
We present results from neutral hydrogen (HI) observations of the tip of the
Magellanic Stream (MS), obtained with the Arecibo telescope as a part of the
on-going survey by the Consortium for Galactic studies with the Arecibo L-band
Feed Array. We find four large-scale, coherent HI streams, extending
continously over a length of 20 degrees, each stream possessing different
morphology and velocity gradients. The newly discovered streams provide strong
support for the tidal model of the MS formation by Connors et al. (2006), which
suggested a spatial and kinematic bifurcation of the MS. The observed
morphology and kinematics suggest that three of these streams could be
interpreted as a 3-way splitting of the main MS filament, while the fourth
stream appears much younger and may have originated from the Magellanic Bridge.
We find an extensive population of HI clouds at the tip of the MS. Two thirds
of clouds have an angular size in the range 3.5'--10'. We interpret this as
being due to thermal instability, which would affect a warm tail of gas
trailing through the Galactic halo over a characteristic timescale of a few
Myrs to a few hundred Myrs. We show that thermal fragments can survive in the
hot halo for a long time, especially if surrounded by a <10^6 K halo gas. If
the observed clumpy structure is mainly due to thermal instability, then the
tip of the MS is at a distance of ~70 kpc. A significant fraction of HI clouds
at the tip of the MS show multi-phase velocity profiles, indicating the
co-existence of cooler and warmer gas.Comment: Accepted by Ap
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