4,078 research outputs found
Can Light Echoes Account for the Slow Decay of Type IIn Supernovae?
The spectra of type IIn supernovae indicate the presence of apre-existing
slow, dense circumstellar wind (CSW). If the CSW extends sufficiently far from
the progenitor star, then dust formation should occur in the wind. The light
from the supernova explosion will scatter off this dust and produce a light
echo. Continuum emission seen after the peak will have contributions from both
this echo as well as from the shock of the ejecta colliding with the CSW, with
a fundamental question of which source dominates the continuum. We calculate
the brightness of the light echo as a function of time for a range of dust
shell geometries, and use our calculations to fit to the light curves of SN
1988Z and SN 1997ab, the two slowest declining IIn supernovae on record. We
find that the light curves of both objects can be reproduced by the echo model.
However, their rate of decay from peak, color at peak and their observed peak
absolute magnitudes when considered together are inconsistent with the echo
model. Furthermore, when the observed values of M are corrected for the
effects of dust scattering, the values obtained imply that these supernovae
have unrealistically high luminosities. We conclude that light echoes cannot
properly account for the slow decline seen in some IIn's, and that the shock
interaction is likely to dominate the continuum emission.Comment: 15 pages, 9 figure
Detecting z > 2 Type IIn Supernovae
Type IIn supernovae (SNe IIn) dominate the brightest supernova events in
observed FUV flux (~1200-2000A). We show that multi-band, multi-epoch optical
surveys complete to m_r = 27 can detect the FUV emission of ~25 z > 2 SNe IIn
deg^-2 yr^-1 rest-frame (~10 SNe IIn deg^-2 yr^-1 observed-frame) to 4 sigma
using a technique monitoring color-selected galaxies. Moreover, the strength
and evolution of the bright emission lines observed in low redshift SNe IIn
imply that the Ly-a emission features in ~70% of z > 2 SNe IIn are above
8m-class telescope spectroscopic thresholds for ~2 yr rest-frame. As a result,
existing facilities have the capability to both photometrically detect and
spectroscopically confirm z > 2 SNe IIn and pave the way for efficient searches
by future 8m-class survey and 30m-class telescopes. The method presented here
uses the sensitivities and wide-field capabilities of current optical
instruments and exploits (1) the efficiency of z > 2 galaxy color-selection
techniques, (2) the intrinsic brightness distribution ( = -19.0 +/-0.9)
and blue profile of SNe IIn continua, (3) the presence of extremely bright,
long-lived emission features, and (4) the potential to detect blueshifted SNe
Ly-a emission shortward of host galaxy Ly-a features.Comment: 26 pages (pre-print), 6 figures, accepted Ap
Far Ultraviolet Spectroscopic Explorer Spectroscopy of the Nova-like BB Doradus
We present an analysis of the Far Ultraviolet Spectroscopic Explorer
({\it{FUSE}}) spectra of the little-known southern nova-like BB Doradus. The
spectrum was obtained as part of our Cycle 8 {\it FUSE} survey of high
declination nova-like stars.
The FUSE spectrum of BB Dor, observed in a high state, is modeled with an
accretion disk with a very low inclination (possibly lower than 10deg).
Assuming an average WD mass of 0.8 solar leads to a mass accretion rate of
1.E-9 Solar mass/year and a distance of the order of 650 pc, consistent with
the extremely low galactic reddening in its direction. The spectrum presents
some broad and deep silicon and sulfur absorption lines, indicating that these
elements are over-abundant by 3 and 20 times solar, respectively
Data mining techniques for the assessment of factors contributing to the damage of residential houses in Australia
This paper reports on the preparation and management processes of inconsistent data on damage on residential houses in Victoria, Australia. There are no existing specific and fully relevant databases readily available except for the incomplete paper-based and electronic-based reports. Therefore, the extracting of information from the reports is complicated and time consuming in order to extract and include all the necessary information needed for analysis of damage on residential houses founded on expansive soils. Data mining is adopted to develop a database. Statistical methods and Artificial Intelligence methods are used to quantify the quality of data. The paper concludes that the development of such database could enable BHC to evaluate the usefulness of the reports prepared on the reported damage properties for further analysis
Image Processing Based Control for Scaled Automated Vehicles
This paper presents a way to design a lateral controller for an automated vehicle using information gained through image processing with the control objective being to stay on a desired path. Two possible ways to obtain the information necessary for lateral control by image processing are presented, one based on pixel intensity summation and the other on vanishing point calculations. The paper also describes two algorithms for the actual lateral control, one based on classical control theory and the other on modern H∞ control. The resulting control algorithms were implemented on a scaled autonomous vehicle system
Complete Treatment of Galaxy Two-Point Statistics: Gravitational Lensing Effects and Redshift-Space Distortions
We present a coherent theoretical framework for computing gravitational
lensing effects and redshift-space distortions in an inhomogeneous universe and
investigate their impacts on galaxy two-point statistics. Adopting the
linearized FRW metric, we derive the gravitational lensing and the generalized
Sachs-Wolfe effects that include the weak lensing distortion, magnification,
and time delay effects, and the redshift-space distortion, Sachs-Wolfe, and
integrated Sachs-Wolfe effects, respectively. Based on this framework, we first
compute their effects on observed source fluctuations, separating them as two
physically distinct origins: the volume effect that involves the change of
volume and is always present in galaxy two-point statistics, and the source
effect that depends on the intrinsic properties of source populations. Then we
identify several terms that are ignored in the standard method, and we compute
the observed galaxy two-point statistics, an ensemble average of all the
combinations of the intrinsic source fluctuations and the additional
contributions from the gravitational lensing and the generalized Sachs-Wolfe
effects. This unified treatment of galaxy two-point statistics clarifies the
relation of the gravitational lensing and the generalized Sachs-Wolfe effects
to the metric perturbations and the underlying matter fluctuations. For near
future dark energy surveys, we compute additional contributions to the observed
galaxy two-point statistics and analyze their impact on the anisotropic
structure. Thorough theoretical modeling of galaxy two-point statistics would
be not only necessary to analyze precision measurements from upcoming dark
energy surveys, but also provide further discriminatory power in understanding
the underlying physical mechanisms.Comment: 20 pages, 5 figures, Fig.4 corrected, appendix added, accepted for
publication in Physical Review
Unveiling the nature of three INTEGRAL sources through optical spectroscopy
The results of an optical spectroscopy campaign performed at the Astronomical
Observatory of Bologna in Loiano (Italy) on three hard X-ray sources detected
by INTEGRAL (IGR J17303-0601, IGR J18027-1455 and IGR J21247+5058) are
presented. These data have allowed a determination of the nature for two of
them, with IGR J17303-0601 being a low mass X-ray binary in the Galaxy and IGR
J18027-1455 a background Type 1 Seyfert galaxy at redshift z = 0.035. IGR
J21247+5058, instead, has a quite puzzling spectroscopic appearance, with a
broad, redshifted H_alpha complex superimposed onto a `normal' F/G-type
Galactic star continuum: these features, together with the spatially coincident
extended radio emission, might suggest a chance alignment between a relatively
nearby star and a background radio galaxy. These results underline the still
non-negligible importance of smaller telescopes in modern astrophysics.Comment: 4 pages, 2 figures. Accepted for publication in A&A Letter
Hard X-ray flares in IGR J08408-4503 unveil clumpy stellar winds
Context : A 1000-s flare from a new hard X-ray transient, IGR J08408-4503,
was observed by INTEGRAL on May 15, 2006 during the real-time routine
monitoring of IBIS/ISGRI images performed at the INTEGRAL Science Data Centre.
The flare, detected during a single one-hour long pointing, peaked at 250 mCrab
in the 20-40 keV energy range.
Aims : Multi-wavelength observations, combining high-energy and optical data,
were used to unveil the nature of IGR J08408-4503.
Methods : A search in all INTEGRAL public data for other bursts from IGR
J08408-4503 was performed, and the detailed analysis of another major flare is
presented. The results of two Swift Target of Opportunity observations are also
described. Finally, a study of the likely optical counterpart, HD 74194, is
provided.
Results : IGR J08408-4503 is very likely a supergiant fast X-ray transient
(SFXT) system. The system parameters indicate that the X-ray flares are
probably related to the accretion of wind clumps on a compact object orbiting
about 1E13 cm from the supergiant HD 74194. The clump mass loss rate is of the
order of 1E-6 solar mass/yr.
Conclusions : Hard X-ray flares from SFXTs allow to probe the stellar winds
of massive stars, and could possibly be associated with wind perturbations due
to line-driven instabilities.Comment: 5 pages with 5 figures. Published as a Letter in Astronomy &
Astrophysic
Spectrophotometrically Identified stars in the PEARS-N and PEARS-S fields
Deep ACS slitless grism observations and identification of stellar sources
are presented within the Great Observatories Origins Deep Survey (GOODS) North
and South fields which were obtained in the Probing Evolution And Reionization
Spectroscopically (PEARS) program. It is demonstrated that even low resolution
spectra can be a very powerful means to identify stars in the field, especially
low mass stars with stellar types M0 and later. The PEARS fields lay within the
larger GOODS fields, and we used new, deeper images to further refine the
selection of stars in the PEARS field, down to a magnitude of mz = 25 using a
newly developed stellarity parameter. The total number of stars with reliable
spectroscopic and morphological identification was 95 and 108 in the north and
south fields respectively. The sample of spectroscopically identified stars
allows constraints to be set on the thickness of the Galactic thin disk as well
as contributions from a thick disk and a halo component. We derive a thin disk
scale height, as traced by the population of M4 to M9 dwarfs along two
independent lines of sight, of h_thin = 370 +60/-65 pc. When including the more
massive M0 to M4 dwarf population, we derive h_thin = 300 +/- 70pc. In both
cases, we observe that we must include a combination of thick and halo
components in our models in order to account for the observed numbers of faint
dwarfs. The required thick disk scale height is typically h_thick=1000 pc and
the acceptable relative stellar densities of the thin disk to thick disk and
the thin disk to halo components are in the range of 0.00025<f_halo<0.0005 and
0.05<f_thick<0.08 and are somewhat dependent on whether the more massive M0 to
M4 dwarfs are included in our sample
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