222 research outputs found
VEWS: A Wikipedia Vandal Early Warning System
We study the problem of detecting vandals on Wikipedia before any human or
known vandalism detection system reports flagging potential vandals so that
such users can be presented early to Wikipedia administrators. We leverage
multiple classical ML approaches, but develop 3 novel sets of features. Our
Wikipedia Vandal Behavior (WVB) approach uses a novel set of user editing
patterns as features to classify some users as vandals. Our Wikipedia
Transition Probability Matrix (WTPM) approach uses a set of features derived
from a transition probability matrix and then reduces it via a neural net
auto-encoder to classify some users as vandals. The VEWS approach merges the
previous two approaches. Without using any information (e.g. reverts) provided
by other users, these algorithms each have over 85% classification accuracy.
Moreover, when temporal recency is considered, accuracy goes to almost 90%. We
carry out detailed experiments on a new data set we have created consisting of
about 33K Wikipedia users (including both a black list and a white list of
editors) and containing 770K edits. We describe specific behaviors that
distinguish between vandals and non-vandals. We show that VEWS beats ClueBot NG
and STiki, the best known algorithms today for vandalism detection. Moreover,
VEWS detects far more vandals than ClueBot NG and on average, detects them 2.39
edits before ClueBot NG when both detect the vandal. However, we show that the
combination of VEWS and ClueBot NG can give a fully automated vandal early
warning system with even higher accuracy.Comment: To appear in Proceedings of the 21st ACM SIGKDD Conference of
Knowledge Discovery and Data Mining (KDD 2015
The observed chemical structure of L1544
Prior to star formation, pre-stellar cores accumulate matter towards the
centre. As a consequence, their central density increases while the temperature
decreases. Understanding the evolution of the chemistry and physics in this
early phase is crucial to study the processes governing the formation of a
star. We aim at studying the chemical differentiation of a prototypical
pre-stellar core, L1544, by detailed molecular maps. In contrast with single
pointing observations, we performed a deep study on the dependencies of
chemistry on physical and external conditions. We present the emission maps of
39 different molecular transitions belonging to 22 different molecules in the
central 6.25 arcmin of L1544. We classified our sample in five families,
depending on the location of their emission peaks within the core. Furthermore,
to systematically study the correlations among different molecules, we have
performed the principal component analysis (PCA) on the integrated emission
maps. The PCA allows us to reduce the amount of variables in our dataset.
Finally, we compare the maps of the first three principal components with the
H column density map, and the T map of the core. The results of
our qualitative analysis is the classification of the molecules in our dataset
in the following groups: (i) the -CH family (carbon chain
molecules), (ii) the dust peak family (nitrogen-bearing species), (iii) the
methanol peak family (oxygen-bearing molecules), (iv) the HNCO peak family
(HNCO, propyne and its deuterated isotopologues). Only HCO and
CS do not belong to any of the above mentioned groups. The principal
component maps allow us to confirm the (anti-)correlations among different
families that were described in a first qualitative analysis, but also points
out the correlation that could not be inferred before.Comment: 29 pages, 19 figures, 2 appendices, accepted for publication in A&A,
arXiv abstract has been slightly modifie
A 1.3 cm line survey toward IRC +10216
IRC +10216 is the prototypical carbon star exhibiting an extended molecular
circumstellar envelope. Its spectral properties are therefore the template for
an entire class of objects. The main goal is to systematically study the
1.3 cm spectral line characteristics of IRC +10216. We carried
out a spectral line survey with the Effelsberg-100 m telescope toward IRC
+10216. It covers the frequency range between 17.8 GHz and 26.3 GHz (K-band).
In the circumstellar shell of IRC +10216, we find 78 spectral lines, among
which 12 remain unidentified. The identified lines are assigned to 18 different
molecules and radicals. A total of 23 lines from species known to exist in this
envelope are detected for the first time outside the Solar System and there are
additional 20 lines first detected in IRC +10216. The potential orgin of "U"
lines is also discussed. Assuming local thermodynamic equilibrium (LTE), we
then determine rotational temperatures and column densities of 17 detected
molecules. Molecular abundances relative to H are also estimated. A
non-LTE analysis of NH shows that the bulk of its emission arises from
the inner envelope with a kinetic temperature of 7020 K. Evidence for
NH emitting gas with higher kinetic temperature is also obtained, and
potential abundance differences between various C-bearing isotopologues
of HCN are evaluated. Overall, the isotopic C/C ratio is
estimated to be 499. Finally, a comparison of detected molecules in the
1.3 cm range with the dark cloud TMC-1 indicates that
silicate-bearing molecules are more predominant in IRC +10216.Comment: 32 pages, 9 figures, Accepted by A&
The chemical structure of the very young starless core L1521E
L1521E is a dense starless core in Taurus that was found to have relatively
low molecular depletion by earlier studies, thus suggesting a recent formation.
We aim to characterize the chemical structure of L1521E and compare it to the
more evolved L1544 pre-stellar core. We have obtained 2.52.5
arcminute maps toward L1521E using the IRAM-30m telescope in transitions of
various species. We derived abundances for the species and compared them to
those obtained toward L1544. We estimated CO depletion factors. Similarly to
L1544, -CH and CHOH peak at different positions. Most species
peak toward the -CH peak. The CO depletion factor derived toward the
dust peak is 4.31.6, which is about a factor of three lower
than that toward L1544. The abundances of sulfur-bearing molecules are higher
toward L1521E than toward L1544 by factors of 2-20. The abundance of
methanol is similar toward the two cores. The higher abundances of
sulfur-bearing species toward L1521E than toward L1544 suggest that significant
sulfur depletion takes place during the dynamical evolution of dense cores,
from the starless to pre-stellar stage. The CO depletion factor measured toward
L1521E suggests that CO is more depleted than previously found. Similar
CHOH abundances between L1521E and L1544 hint that methanol is forming at
specific physical conditions in Taurus, characterized by densities of a few
10 cm and (H)10 cm, when CO
starts to catastrophically freeze-out, while water can still be significantly
photodissociated, so that the surfaces of dust grains become rich in solid CO
and CHOH, as already found toward L1544. Methanol can thus provide
selective crucial information about the transition region between dense cores
and the surrounding parent cloud.Comment: Accepted for publication in A&A, abstract abridge
Rotational spectroscopy of the HCCO and DCCO radicals in the millimeter and submillimeter range
The ketenyl radical, HCCO, has recently been detected in the ISM for the
first time. Further astronomical detections of HCCO will help us understand its
gas-grain chemistry, and subsequently revise the oxygen-bearing chemistry
towards dark clouds. Moreover, its deuterated counterpart, DCCO, has never been
observed in the ISM. HCCO and DCCO still lack a broad spectroscopic
investigation, although they exhibit a significant astrophysical relevance. In
this work we aim to measure the pure rotational spectra of the ground state of
HCCO and DCCO in the millimeter and submillimeter region, considerably
extending the frequency range covered by previous studies. The spectral
acquisition was performed using a frequency-modulation absorption spectrometer
between 170 and 650 GHz. The radicals were produced in a low-density plasma
generated from a select mixture of gaseous precursors. For each isotopologue we
were able to detect and assign more than 100 rotational lines. The new lines
have significantly enhanced the previous data set allowing the determination of
highly precise rotational and centrifugal distortion parameters. In our
analysis we have taken into account the interaction between the ground
electronic state and a low-lying excited state (Renner-Teller pair) which
enables the prediction and assignment of rotational transitions with up
to 4. The present set of spectroscopic parameters provides highly accurate,
millimeter and submillimeter rest-frequencies of HCCO and DCCO for future
astronomical observations. We also show that towards the pre-stellar core
L1544, ketenyl peaks in the region where - peaks,
suggesting that HCCO follows a predominant hydrocarbon chemistry, as already
proposed by recent gas-grain chemical models
A study of the -/- ratio in low-mass star forming regions
We use the deuteration of - to probe the physical
parameters of starless and protostellar cores, related to their evolutionary
states, and compare it to the -deuteration in order to
study possible differences between the deuteration of C- and N-bearing species.
We observed the main species -, the singly and doubly
deuterated species - and -, as
well as the isotopologue - toward 10 starless
cores and 5 protostars in the Taurus and Perseus Complexes. We examined the
correlation between the
(-)/(-) ratio and the dust
temperature along with the column density and the CO depletion
factor. The resulting
(-)/(-) ratio is within the
error bars consistent with in all starless cores with detected
-. This also accounts for the protostars except for the
source HH211, where we measure a high deuteration level of . The
deuteration of follows the same trend but is considerably
higher in the dynamically evolved core L1544. Toward the protostellar cores the
coolest objects show the largest deuterium fraction in
-. We show that the deuteration of
- can trace the early phases of star formation and is
comparable to that of . However, the largest
- deuteration level is found toward protostellar cores,
suggesting that while - is mainly frozen onto dust
grains in the central regions of starless cores, active deuteration is taking
place on ice
Nitrogen fractionation towards a pre-stellar core traces isotope-selective photodissociation
Context. Isotopologue abundance ratios are important to understand the evolution of astrophysical objects and ultimately the origins of a planetary system such as our own. With nitrogen being a fundamental ingredient of pre-biotic material, understanding its chemistry and inheritance is of fundamental importance to understand the formation of the building blocks of life.
Aims. We aim to study the 14N/15N ratio in HCN, HNC, and CN across the prototypical pre-stellar core L1544. This study allows us to test the proposed fractionation mechanisms for nitrogen.
Methods. We present here single-dish observations of the ground state rotational transitions of the 13C and 15N isotopologues of HCN, HNC, and CN with the IRAM 30 m telescope. We analyse their column densities and compute the 14N/15N ratio map across the core for HCN. The 15N fractionation of CN and HNC is computed towards different offsets across L1544.
Results. The 15 N-fractionation map of HCN towards a pre-stellar core is presented here for the first time. Our map shows a very clear decrease in the 14N/15N ratio towards the southern edge of L1544, where carbon chain molecules present a peak, strongly suggesting that isotope-selective photodissociation has a strong effect on the fractionation of nitrogen across pre-stellar cores. The 14N/15N ratio in CN measured towards four positions across the core also shows a decrease towards the south-east of the core, while HNC shows the opposite behaviour. We also measured the 12CN/13CN ratio towards four positions across the core.
Conclusions. The uneven illumination of the pre-stellar core L1544 provides clear evidence that 15 N fractionation of HCN and CN is enhanced towards the region more exposed to the interstellar radiation field. Isotope-selective photodissociation of N2 is then a crucial process to understand 15N fractionation, as already found in protoplanetary disks. Therefore, the 15N fractionation in prestellar material is expected to change depending on the environment within which pre-stellar cores are embedded. The 12CN/13CN ratio also varies across the core, but its variation does not affect our conclusions as to the effect of the environment on the fractionation of nitrogen. Nevertheless, the interplay between the carbon and nitrogen fractionation across the core warrants follow-up studies
A 1.3 cm Line Survey toward Orion KL
Orion KL has served as a benchmark for spectral line searches throughout the
(sub)millimeter regime. The main goal is to systematically study spectral
characteristics of Orion KL in the 1.3 cm band. We carried out a spectral line
survey (17.9 GHz to 26.2 GHz) with the Effelsberg-100 m telescope towards Orion
KL. We find 261 spectral lines, yielding an average line density of about 32
spectral features per GHz above 3. The identified lines include 164
radio recombination lines (RRLs) and 97 molecular lines. A total of 23
molecular transitions from species known to exist in Orion KL are detected for
the first time in the interstellar medium. Non-metastable 15NH3 transitions are
detected in Orion KL for the first time. Based on the velocity information of
detected lines and the ALMA images, the spatial origins of molecular emission
are constrained and discussed. A narrow feature is found in SO2
(), possibly suggesting the presence of a maser line. Column
densities and fractional abundances relative to H2 are estimated for 12
molecules with LTE methods. Rotational diagrams of non-metastable 14NH3
transitions with J=K+1 to J=K+4 yield different results; metastable 15NH3 is
found to have a higher excitation temperature than non-metastable 15NH3,
indicating that they may trace different regions. Elemental and isotopic
abundance ratios are estimated: 12C/13C=63+-17, 14N/15N=100+-51,
D/H=0.0083+-0.0045. The dispersion of the He/H ratios derived from
H/He pairs to H/He pairs is very small, which
is consistent with theoretical predictions that the departure coefficients bn
factors for hydrogen and helium are nearly identical. Based on a non-LTE code
neglecting excitation by the infrared radiation field and a likelihood
analysis, we find that the denser regions have lower kinetic temperature, which
favors an external heating of the Hot Core.Comment: 70 pages, 26 figures, 12 tables, accepted for publication in A&A.
Figs. 1, 2, 8, 9 have been downsize
Mapping deuterated methanol toward L1544: I. Deuterium fraction and comparison with modeling
The study of deuteration in pre-stellar cores is important to understand the
physical and chemical initial conditions in the process of star formation. In
particular, observations toward pre-stellar cores of methanol and deuterated
methanol, solely formed on the surface of dust grains, may provide useful
insights on surface processes at low temperatures. Here we analyze maps of CO,
methanol, formaldehyde and their deuterated isotopologues toward a well-known
pre-stellar core. This study allows us to test current gas-dust chemical
models. Single-dish observations of CHOH, CHDOH, HCO,
H_2\,^{13}CO, HDCO, DCO and CO toward the prototypical pre-stellar
core L1544 were performed at the IRAM 30 m telescope. We analyze their column
densities, distributions, and compare these observations with gas-grain
chemical models. The maximum deuterium fraction derived for methanol is
[CHDOH]/[CHOH] 0.080.02, while the measured deuterium
fractions of formaldehyde at the dust peak are [HDCO]/[HCO]
0.030.02, [DCO]/[HCO] 0.040.03 and [DCO]/[HDCO]
1.20.3. Observations differ significantly from the predictions of
models, finding discrepancies between a factor of 10 and a factor of 100 in
most cases. It is clear though that to efficiently produce methanol on the
surface of dust grains, quantum tunneling diffusion of H atoms must be switched
on. It also appears that the currently adopted reactive desorption efficiency
of methanol is overestimated and/or that abstraction reactions play an
important role. More laboratory work is needed to shed light on the chemistry
of methanol, an important precursor of complex organic molecules in space.Comment: Accepted for publication in A&
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