76 research outputs found
Molecular Gas in Candidate Double-Barred Galaxies II. Cooler, Less Dense Gas Associated with Stronger Central Concentrations
We have performed a multi-transition CO study of the centers of seven
double-barred galaxies that exhibit a variety of molecular gas morphologies to
determine if the molecular gas properties are correlated with the nuclear
morphology and star forming activity. Near infrared galaxy surveys have
revealed the existence of nuclear stellar bars in a large number of barred or
lenticular galaxies. High resolution CO maps of these galaxies exhibit a wide
range of morphologies. Recent simulations of double-barred galaxies suggest
that variations in the gas properties may allow it to respond differently to
similar gravitational potentials. We find that the 12CO J=3-2/J=2-1 line ratio
is lower in galaxies with centrally concentrated gas distributions and higher
in galaxies with CO emission dispersed around the galactic center in rings and
peaks. The 13CO/12CO J=2-1 line ratios are similar for all galaxies, which
indicates that the J=3-2/J=2-1 line ratio is tracing variations in gas
temperature and density, rather than variations in optical depth. There is
evidence that the galaxies which contain more centralized CO distributions are
comprised of molecular gas that is cooler and less dense. Observations suggest
that the star formation rates are higher in the galaxies containing the warmer,
denser, less centrally concentrated gas. It is possible that either the bar
dynamics are responsible for the variety of gas distributions and densities
(and hence the star formation rates) or that the star formation alone is
responsible for modifying the gas properties.Comment: 27 pages + 6 figures; to appear in the April 20, 2003 issue of Ap
On the Feasibility of Malware Authorship Attribution
There are many occasions in which the security community is interested to
discover the authorship of malware binaries, either for digital forensics
analysis of malware corpora or for thwarting live threats of malware invasion.
Such a discovery of authorship might be possible due to stylistic features
inherent to software codes written by human programmers. Existing studies of
authorship attribution of general purpose software mainly focus on source code,
which is typically based on the style of programs and environment. However,
those features critically depend on the availability of the program source
code, which is usually not the case when dealing with malware binaries. Such
program binaries often do not retain many semantic or stylistic features due to
the compilation process. Therefore, authorship attribution in the domain of
malware binaries based on features and styles that will survive the compilation
process is challenging. This paper provides the state of the art in this
literature. Further, we analyze the features involved in those techniques. By
using a case study, we identify features that can survive the compilation
process. Finally, we analyze existing works on binary authorship attribution
and study their applicability to real malware binaries.Comment: FPS 201
The Different Structures of the Two Classes of Starless Cores
We describe a model for the thermal and dynamical equilibrium of starless
cores that includes the radiative transfer of the gas and dust and simple CO
chemistry. The model shows that the structure and behavior of the cores is
significantly different depending on whether the central density is either
above or below about 10^5 cm-3. This density is significant as the critical
density for gas cooling by gas-dust collisions and also as the critical density
for dynamical stability, given the typical properties of the starless cores.
The starless cores thus divide into two classes that we refer to as thermally
super-critical and thermally sub-critical.This two-class distinction allows an
improved interpretation of the different observational data of starless cores
within a single model.Comment: ApJ in pres
The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Young Clusters
We have obtained images of the Trapezium Cluster (140" x 140"; 0.3 pc x 0.3
pc) with the Hubble Space Telescope Near-Infrared Camera and Multi-Object
Spectrometer (NICMOS). Combining these data with new ground-based K-band
spectra (R=800) and existing spectral types and photometry and the models of
D'Antona & Mazzitelli, we find that the distributions of ages of comparable
samples of stars in the Trapezium, rho Oph, and IC 348 indicate median ages of
\~0.4 Myr for the first two regions and ~1-2 Myr for the latter. The low-mass
IMFs in these sites of clustered star formation are similar over a wide range
of stellar densities and other environmental conditions. With current data, we
cannot rule out modest variations in the substellar mass functions among these
clusters. We then make the best estimate of the true form of the IMF in the
Trapezium by using the evolutionary models of Baraffe et al. and an empirically
adjusted temperature scale and compare this mass function to recent results for
the Pleiades and the field. All of these data are consistent with an IMF that
is flat or rises slowly from the substellar regime to about 0.6 Msun, and then
rolls over into a power law that continues from about 1 Msun to higher masses
with a slope similar to or somewhat larger than the Salpeter value of 1.35. For
the Trapezium, this behavior holds from our completeness limit of ~0.02 Msun
and probably, after a modest completeness correction, even from 0.01-0.02 Msun.
These data include ~50 likely brown dwarfs. We test the predictions of theories
of the IMF against various properties of the observed IMF.Comment: 34 pages, 13 figures, for color image see
http://cfa-www.harvard.edu/~kluhman/trap/colorimage.jp
High-excitation OH and H_2O lines in Markarian 231: the molecular signatures of compact far-infrared continuum sources
The ISO/LWS far-infrared spectrum of the ultraluminous galaxy Mkn 231 shows
OH and H_2O lines in absorption from energy levels up to 300 K above the ground
state, and emission in the [O I] 63 micron and [C II] 158 micron lines. Our
analysis shows that OH and H_2O are radiatively pumped by the far-infrared
continuum emission of the galaxy. The absorptions in the high-excitation lines
require high far-infrared radiation densities, allowing us to constrain the
properties of the underlying continuum source. The bulk of the far-infrared
continuum arises from a warm (T_dust=70-100 K), optically thick
(tau_100micron=1-2) medium of effective diameter 200-400 pc. In our best-fit
model of total luminosity L_IR, the observed OH and H2O high-lying lines arise
from a luminous (L/L_IR~0.56) region with radius ~100 pc. The high surface
brightness of this component suggests that its infrared emission is dominated
by the AGN. The derived column densities N(OH)>~10^{17} cm^{-2} and
N(H_2O)>~6x10^{16} cm^{-2} may indicate XDR chemistry, although significant
starburst chemistry cannot be ruled out. The lower-lying OH, [C II] 158 micron,
and [O I] 63 micron lines arise from a more extended (~350 pc) starburst
region. We show that the [C II] deficit in Mkn 231 is compatible with a high
average abundance of C+ because of an extreme overall luminosity to gas mass
ratio. Therefore, a [C II] deficit may indicate a significant contribution to
the luminosity by an AGN, and/or by extremely efficient star formation.Comment: 16 pages, 6 figures, accepted for publication in The Astrophysical
Journa
A spectral line survey of the starless and proto-stellar cores detected by BLAST toward the Vela-D molecular cloud
We present a 3-mm and 1.3-cm spectral line survey conducted with the Mopra
22-m and Parkes 64-m radio telescopes of a sample of 40 cold dust cores,
previously observed with BLAST, including both starless and proto-stellar
sources. 20 objects were also mapped using molecular tracers of dense gas. To
trace the dense gas we used the molecular species NH3, N2H+, HNC, HCO+, H13CO+,
HCN and H13CN, where some of them trace the more quiescent gas, while others
are sensitive to more dynamical processes. The selected cores have a wide
variety of morphological types and also show physical and chemical variations,
which may be associated to different evolutionary phases. We find evidence of
systematic motions in both starless and proto-stellar cores and we detect line
wings in many of the proto-stellar cores. Our observations probe linear
distances in the sources >~0.1pc, and are thus sensitive mainly to molecular
gas in the envelope of the cores. In this region we do find that, for example,
the radial profile of the N2H+(1-0) emission falls off more quickly than that
of C-bearing molecules such as HNC(1-0), HCO+(1-0) and HCN(1-0). We also
analyze the correlation between several physical and chemical parameters and
the dynamics of the cores. Depending on the assumptions made to estimate the
virial mass, we find that many starless cores have masses below the
self-gravitating threshold, whereas most of the proto-stellar cores have masses
which are near or above the self-gravitating critical value. An analysis of the
median properties of the starless and proto-stellar cores suggests that the
transition from the pre- to the proto-stellar phase is relatively fast, leaving
the core envelopes with almost unchanged physical parameters.Comment: Submitted for publication to Astronomy & Astrophysics on January
18th, 201
PACS and SPIRE photometer maps of M33: First results of the Herschel M33 extended survey (HERM33ES)
Within the framework of the HERM33ES key project, we are studying the star
forming interstellar medium in the nearby, metal-poor spiral galaxy M33,
exploiting the high resolution and sensitivity of Herschel. We use PACS and
SPIRE maps at 100, 160, 250, 350, and 500 micron wavelength, to study the
variation of the spectral energy distributions (SEDs) with galacto-centric
distance. Detailed SED modeling is performed using azimuthally averaged fluxes
in elliptical rings of 2 kpc width, out to 8 kpc galacto-centric distance.
Simple isothermal and two-component grey body models, with fixed dust
emissivity index, are fitted to the SEDs between 24 and 500 micron using also
MIPS/Spitzer data, to derive first estimates of the dust physical conditions.
The far-infrared and submillimeter maps reveal the branched, knotted spiral
structure of M33. An underlying diffuse disk is seen in all SPIRE maps (250-500
micron). Two component fits to the SEDs agree better than isothermal models
with the observed, total and radially averaged flux densities. The two
component model, with beta fixed at 1.5, best fits the global and the radial
SEDs. The cold dust component clearly dominates; the relative mass of the warm
component is less than 0.3% for all the fits. The temperature of the warm
component is not well constrained and is found to be about 60K plus/minus 10K.
The temperature of the cold component drops significantly from about 24K in the
inner 2 kpc radius to 13K beyond 6 kpc radial distance, for the best fitting
model. The gas-to-dust ratio for beta=1.5, averaged over the galaxy, is higher
than the solar value by a factor of 1.5 and is roughly in agreement with the
subsolar metallicity of M33.Comment: 5 pages, 3 figures, accepted for publication in the A&A Herschel
Special Issu
On Detection of Erratic Arguments
Abstract. Due to the erratic nature, the value of a function argument in one normal program execution could become illegal in another normal execution context. Attacks utilizing such erratic arguments are able to evade detections as fine-grained context information is unavailable in many existing detection schemes. In order to obtain such fine-grained context information, a precise model on the internal program states has to be built, which is impractical especially monitoring a closed source program alone. In this paper, we propose an intrusion detection scheme which builds on two diverse programs providing semantically-close functionality. Our model learns underlying semantic correlation of the argument values in these programs, and consequently gains more accurate context information compared to existing schemes. Through experiments, we show that such context information is effective in detecting attacks which manipulate erratic arguments with comparable false positive rates. Key words: Intrusion detection, system call argument, diversity
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