233 research outputs found
FORECASTING URBAN EXPANSION BASED ON NIGHT LIGHTS
Forecasting urban expansion models are a very powerful tool in the hands of urban planners in order to anticipate and mitigate future
urbanization pressures. In this paper, a linear regression forecasting urban expansion model is implemented based on the annual
composite night lights time series available from National Oceanic and Atmospheric Administration (NOAA). The product known as
'stable lights' is used in particular, after it has been corrected with a standard intercalibration process to reduce artificial year-to-year
fluctuations as much as possible. Forecasting is done for ten years after the end of the time series. Because the method is spatially
explicit the predicted expansion trends are relatively accurately mapped. Two metrics are used to validate the process. The first one is
the year-to-year Sum of Lights (SoL) variation. The second is the year-to-year image correlation coefficient. Overall it is evident that
the method is able to provide an insight on future urbanization pressures in order to be taken into account in planning. The trends are
quantified in a clear spatial manner
A postmortem investigation of the Type IIb supernova 2001ig
We present images taken with the GMOS instrument on Gemini-South, in
excellent (<0.5 arcsec) seeing, of SN 2001ig in NGC 7424, ~1000 days after
explosion. A point source seen at the site of the SN is shown to have colours
inconsistent with being an H II region or a SN 1993J-like remnant, but can be
matched to a late-B through late-F supergiant with A_V<1. We believe this
object is the massive binary companion responsible for periodic modulation in
mass loss material around the Wolf-Rayet progenitor which gave rise to
significant structure in the SN radio light curve.Comment: 5 pages, 3 figures. Accepted for publication in MNRAS Letters. Fig. 1
resolution degraded to meet size limitations; full resolution version
available from http://www.aao.gov.au/local/www/sdr/pubs/sn2001ig_gmos.ps.g
Broad Line Emission in Low-Metallicity Blue Compact Dwarf Galaxies: Evidence for Stellar Wind, Supernova and Possible AGN Activity
We present spectra of a large sample of low-metallicity blue compact dwarf
galaxies which exhibit broad components in their strong emission lines, mainly
in Hbeta, [O III]4959, 5007 and Halpha. Twenty-three spectra have been obtained
with the MMT, 14 of which show broad emission. The remaining 21 spectra with
broad emission have been selected from the Data Release 5 of the Sloan Digital
Sky Survey. The most plausible origin of broad line emission is the evolution
of massive stars and their interaction with the circumstellar and interstellar
medium. The broad emission with the lowest H luminosities (10^36 -
10^39 erg/s) is likely produced in circumstellar envelopes around hot Ofp/WN9
and/or LBV stars. The broad emission with the highest Halpha luminosities
(10^40 - 10^42 erg/s) probably arises from type IIp or type IIn supernovae
(SNe). It can also come from active galactic nuclei (AGN) containing
intermediate-mass black holes, although we find no strong evidence for hard
non-thermal radiation in our sample galaxies. The oxygen abundance in the host
galaxies with SN candidates is low and varies in the range 12 + log O/H = 7.36
- 8.31. However, type IIn SN / AGN candidates are found only in galaxies with
12 + log O/H < 7.99. Spectroscopic monitoring of these type IIn SN / AGN
candidates over a time scale of several years is necessary to distinguish
between the two possibilities.Comment: 50 pages, 6 figures. Accepted for publication in the Astrophysical
Journa
Helium Emission in the Type Ic SN 1999cq
We present the first unambiguous detection of helium emission lines in
spectra of Type Ic supernovae (SNe Ic). The presence of He I lines, with full
width at half maximum ~ 2000 km/s, and the distinct absence of any other
intermediate-width emission (e.g., Halpha), implies that the ejecta of SN Ic
1999cq are interacting with dense circumstellar material composed of almost
pure helium. This strengthens the argument that the progenitors of SNe Ic are
core-collapse events in stars that have lost both their hydrogen and helium
envelopes, either through a dense wind or mass-transfer to a companion. In this
way, SN 1999cq is similar to supernovae such as SN 1987K and SN 1993J that
helped firmly establish a physical connection between Type Ib and Type II
supernovae. The light curve of SN 1999cq is very fast, with an extremely rapid
rise followed by a quick decline. SN 1999cq is also found to exhibit a high
level of emission at blue wavelengths (< 5500 A), likely resulting from either
an unusually large amount of iron and iron-group element emission or
uncharacteristically low reddening compared with other SNe Ic.Comment: 17 pages (AASTeX V5.0), 4 figures, accepted for publication in the
Astronomical Journa
Narrow Lines in Type II Supernovae -- Probing the Circumstellar Nebulae of the Progenitors
We have carried out a high-dispersion (R~30,000) echelle spectroscopic survey
of 16 Type II supernovae (SNe) to search for narrow emission lines from
circumstellar nebulae ejected by their massive progenitors. Circumstellar
nebulae, if detected, provide invaluable opportunities to probe SN progenitors.
Of the 16 SNe observed, SN ejecta are clearly detected in 4 SNe and possibly in
another 2 SNe, interstellar gas is detected in 12 SNe, and circumstellar
material is detected only in SN 1978K and SN 1998S. In the case of SN 1978K we
are able to place an upper limit of ~2.2 pc for the size of the circumstellar
ejecta nebula and note that this is more consistent with the typical sizes
observed for ejecta nebulae around luminous blue variables rather than
Wolf-Rayet stars. In the case of SN 1998S, our observations of the narrow lines
\~1 year after the SN explosion show variations compared to early epochs. The
nebular lines we observe from SN 1998S originate from either the low--density,
outer region of a circumstellar nebula or have become dominated by an
interstellar component.Comment: (20 pages, 7 figures, revised from previous version, resubmitted to
AJ, 28 Jan 2002
A Smart Approach to Measuring the Performance - Efficiency, Effectiveness and Productivity - of the General University Hospital of Alexandroupolis - G.U.H.A. using Special Indicators
Evidence for Asphericity in the Type IIn Supernova 1998S
We present optical spectropolarimetry obtained at the Keck-II 10-m telescope
on 1998 March 7 UT along with total flux spectra spanning the first 494 days
after discovery (1998 March 2 UT) of the peculiar type IIn supernova (SN)
1998S. The SN is found to exhibit a high degree of linear polarization,
implying significant asphericity for its continuum-scattering environment.
Prior to removal of the interstellar polarization, the polarization spectrum is
characterized by a flat continuum (at p ~ 2%) with distinct changes in
polarization associated with both the broad (FWZI >= 20,000 km/s) and narrow
(unresolved, FWHM < 300 km/s) line emission seen in the total flux spectrum.
When analyzed in terms of a polarized continuum with unpolarized broad-line
recombination emission, an intrinsic continuum polarization of p ~ 3% results
(the highest yet found for a SN), suggesting a global asphericity of >= 45%
from the oblate, electron-scattering dominated models of Hoflich (1991). The
smooth, blue continuum evident at early times is shown to be inconsistent with
a reddened, single-temperature blackbody, instead having a color temperature
that increases with decreasing wavelength. Broad emission-line profiles with
distinct blue and red peaks are seen in the total flux spectra at later times,
perhaps suggesting a disk-like or ring-like morphology for the dense (n_e ~
10^7 cm^{-3}) circumstellar medium. Implications of the circumstellar
scattering environment for the spectropolarimetry are discussed, as are the
effects of uncertain removal of interstellar polarization.Comment: 25 pages + 2 tables + 14 figures, Submitted to The Astrophysical
Journa
Identification of the Red Supergiant Progenitor of Supernova 2005cs: Do the Progenitors of Type II-P Supernovae Have Low Mass?
The stars that end their lives as supernovae (SNe) have been directly
observed in only a handful of cases, due mainly to the extreme difficulty in
identifying them in images obtained prior to the SN explosions. Here we report
the identification of the progenitor for the recent Type II-plateau
(core-collapse) SN 2005cs in pre-explosion archival images of the Whirlpool
Galaxy (M51) obtained with the Hubble Space Telescope (HST) Advanced Camera for
Surveys (ACS). From high-quality ground-based images of the SN from the
Canada-France-Hawaii Telescope, we precisely determine the position of the SN
and are able to isolate the SN progenitor to within 0".04 in the HST/ACS
optical images. We further pinpoint the SN location to within 0".005 from
HST/ACS ultraviolet images of the SN, confirming our progenitor identification.
From photometry of the SN progenitor obtained with the pre-SN ACS images, and
also limits to its brightness in pre-SN HST/NICMOS images, we infer that the
progenitor is a red supergiant star of spectral type K0--M3, with initial mass
7--9 Msun. We also discuss the implications of the SN 2005cs progenitor
identification and its mass estimate. There is an emerging trend that the most
common Type II-plateau SNe originate from low-mass supergiants 8--15 Msun.Comment: Submitted to ApJ. A high resolution version can be found at
http://astron.berkeley.edu/~weidong/sn05cs.p
The MACHO Project: Microlensing Results from 5.7 Years of LMC Observations
We report on our search for microlensing towards the Large Magellanic Cloud
(LMC). Analysis of 5.7 years of photometry on 11.9 million stars in the LMC
reveals 13 - 17 microlensing events. This is significantly more than the
2 to 4 events expected from lensing by known stellar populations. The
timescales (\that) of the events range from 34 to 230 days. We estimate the
microlensing optical depth towards the LMC from events with 2 < \that < 400
days to be 1.2 ^{+0.4}_ {-0.3} \ten{-7} 0.15 \msun 0.9 \msun$, depending on the halo model, and the total mass in MACHOs out
to 50 kpc is found to be 9+4-3 10^{10} msun, independent of the halo model.
These results are marginally consistent with our previous results, but are
lower by about a factor of two. Besides a larger data set, this work also
includes an improved efficiency determination, improved likelihood analysis,
and more thorough testing of systematic errors, especially with respect to the
treatment of potential backgrounds to microlensing, such as supernovae in
galaxies behind the LMC. [Abridged]Comment: 53 pages, Latex with 12 postscript figures, submitted to Ap
Numerical simulations of super-luminous supernovae of type IIn
We present numerical simulations that include 1-D Eulerian multi-group
radiation-hydrodynamics, 1-D non-LTE radiative transfer, and 2-D polarised
radiative transfer for super-luminous interacting supernovae (SNe). Our
reference model is a ~10Msun inner shell with 10^51erg ramming into a ~3Msun
cold outer shell (the circumstellar-medium, or CSM) that extends from 10^15cm
to 2x10^16cm and moves at 100km/s. We discuss the light curve evolution, which
cannot be captured adequately with a grey approach. In these interactions, the
shock-crossing time through the optically-thick CSM is much longer than the
photon diffusion time. Radiation is thus continuously leaking from the shock
through the CSM, in disagreement with the shell-shocked model that is often
invoked. Our spectra redden with time, with a peak distribution in the near-UV
during the first month gradually shifting to the optical range over the
following year. Initially Balmer lines exhibit a narrow line core and the broad
line wings that are characteristic of electron scattering in the SNe IIn
atmospheres (CSM). At later times they also exhibit a broad blue shifted
component which arises from the cold dense shell. Our model results are broadly
consistent with the bolometric light curve and spectral evolution observed for
SN2010jl. Invoking a prolate pole-to-equator density ratio in the CSM, we can
also reproduce the ~2% continuum polarisation, and line depolarisation,
observed in SN2010jl. By varying the inner shell kinetic energy and the mass
and extent of the outer shell, a large range of peak luminosities and
durations, broadly compatible with super-luminous SNe IIn like 2010jl or
2006gy, can be produced.Comment: paper accepted to MNRA
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