3,535 research outputs found
An Early Warning System for Asteroid Impact
Earth is bombarded by meteors, occasionally by one large enough to cause a
significant explosion and possible loss of life. Although the odds of a deadly
asteroid strike in the next century are low, the most likely impact is by a
relatively small asteroid, and we suggest that the best mitigation strategy in
the near term is simply to move people out of the way. We describe an "early
warning" system that could provide a week's notice of most sizable asteroids or
comets on track to hit the Earth. This system, dubbed "Asteroid
Terrestrial-impact Last Alert System" (ATLAS), comprises two observatories
separated by about 100km that simultaneously scan the visible sky twice a
night, and can be implemented immediately for relatively low cost. The
sensitivity of ATLAS permits detection of 140m asteroids (100 Mton impact
energy) three weeks before impact, and 50m asteroids a week before arrival. An
ATLAS alarm, augmented by other observations, should result in a determination
of impact location and time that is accurate to a few kilometers and a few
seconds. In addition to detecting and warning of approaching asteroids, ATLAS
will continuously monitor the changing universe around us: most of the variable
stars in our galaxy, many micro-lensing events from stellar alignments,
luminous stars and novae in nearby galaxies, thousands of supernovae, nearly a
million quasars and active galactic nuclei, tens of millions of galaxies, and a
billion stars. With two views per day ATLAS will make the variable universe as
familiar to us as the sunrise and sunset.Comment: 33 pages, 7 figures, accepted for publication in PASP, Jan 201
Determining the Type, Redshift, and Phase of a Supernova Spectrum
We present an algorithm to identify the types of supernova spectra, and
determine their redshift and phase. This algorithm, based on the correlation
techniques of Tonry & Davis, is implemented in the SuperNova IDentification
code (SNID). It is used by members of the ESSENCE project to determine whether
a noisy spectrum of a high-redshift supernova is indeed of type Ia, as opposed
to, e.g., type Ib/c. Furthermore, by comparing the correlation redshifts
obtained using SNID with those determined from narrow lines in the supernova
host galaxy spectrum, we show that accurate redshifts (with a typical error <
0.01) can be determined for SNe Ia for which a spectrum of the host galaxy is
unavailable. Last, the phase of an input spectrum is determined with a typical
accuracy of ~3 days.Comment: 10 pages, 7 figures. To appear in "The Multicoloured Landscape of
Compact Objects and their Explosive Progenitors: Theory vs Observations"
(Cefalu, Sicily, June 2006). Eds. L. Burderi et al. (New York: AIP
Fuzzy Supernova Templates II: Parameter Estimation
Wide field surveys will soon be discovering Type Ia supernovae (SNe) at rates
of several thousand per year. Spectroscopic follow-up can only scratch the
surface for such enormous samples, so these extensive data sets will only be
useful to the extent that they can be characterized by the survey photometry
alone. In a companion paper (Rodney and Tonry, 2009) we introduced the SOFT
method for analyzing SNe using direct comparison to template light curves, and
demonstrated its application for photometric SN classification. In this work we
extend the SOFT method to derive estimates of redshift and luminosity distance
for Type Ia SNe, using light curves from the SDSS and SNLS surveys as a
validation set. Redshifts determined by SOFT using light curves alone are
consistent with spectroscopic redshifts, showing a root-mean-square scatter in
the residuals of RMS_z=0.051. SOFT can also derive simultaneous redshift and
distance estimates, yielding results that are consistent with the currently
favored Lambda-CDM cosmological model. When SOFT is given spectroscopic
information for SN classification and redshift priors, the RMS scatter in
Hubble diagram residuals is 0.18 mags for the SDSS data and 0.28 mags for the
SNLS objects. Without access to any spectroscopic information, and even without
any redshift priors from host galaxy photometry, SOFT can still measure
reliable redshifts and distances, with an increase in the Hubble residuals to
0.37 mags for the combined SDSS and SNLS data set. Using Monte Carlo
simulations we predict that SOFT will be able to improve constraints on
time-variable dark energy models by a factor of 2-3 with each new generation of
large-scale SN surveys.Comment: 20 pages, 7 figures, accepted to ApJ; paper 1 is arXiv:0910.370
K-band versus I-band Surface Brightness Fluctuations as distance indicators
We evaluate the method of optical and infrared Surface Brightness
Fluctuations (SBF) as a distance indicator and its application on 8-m class
telescopes, such as the Very Large Telescope (VLT). The novelty of our approach
resides in the development of Monte Carlo simulations of SBF observations
incorporating realistic elliptical galaxy stellar population models, the
effects induced by globular clusters and background galaxies, instrumental
noise, sky background and PSF blurring. We discuss, for each band and in
different observational conditions, the errors on distance measurements arising
from stellar population effects, data treatment and observational constraints.
With 8-m class telescopes, one can extend I-band SBF measurements out to
6000-10000 km/s. Integration times in the K-band are too expensive from the
ground, due to the high infrared background for large-scale distance
determination projects. Nevertheless ground-based K-band measurements are
necessary to understand stellar population effects on the SBF calibration, and
to prepare future space-based observations, where this band is more efficient.Comment: A&A, in press, 17 pages, 10 figure
Distances from Surface Brightness Fluctuations
The practice of measuring galaxy distances from their spatial fluctuations in
surface brightness is now a decade old. While several past articles have
included some review material, this is the first intended as a comprehensive
review of the surface brightness fluctuation (SBF) method. The method is
conceptually quite simple, the basic idea being that nearby (but unresolved)
star clusters and galaxies appear "bumpy", while more distant ones appear
smooth. This is quantified via a measurement of the amplitude of the Poisson
fluctuations in the number of unresolved stars encompassed by a CCD pixel
(usually in an image of an elliptical galaxy). Here, we describe the technical
details and difficulties involved in making SBF measurements, discuss
theoretical and empirical calibrations of the method, and review the numerous
applications of the method from the ground and space, in the optical and
near-infrared. We include discussions of stellar population effects and the
"universality" of the SBF standard candle. A final section considers the future
of the method.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles',
A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 22
pages, including 3 postscript figures; uses Kluwer's crckapb.sty LaTex macro
file, enclose
The observed infall of galaxies towards the Virgo cluster
We examine the velocity field of galaxies around the Virgo cluster induced by
its overdensity. A sample of 1792 galaxies with distances from the Tip of the
Red Giant Branch, the Cepheid luminosity, the SNIa luminosity, the surface
brightness fluctuation method, and the Tully-Fisher relation has been used to
study the velocity-distance relation in the Virgocentric coordinates. Attention
was paid to some observational biases affected the Hubble flow around Virgo.
We estimate the radius of the zero-velocity surface for the Virgo cluster to
be within (5.0 - 7.5) Mpc corresponding to (17 - 26)^\circ at the mean cluster
distance of 17.0 Mpc. In the case of spherical symmetry with cosmological
parameter \Omega_m=0.24 and the age of the Universe T_0= 13.7 Gyr, it yields
the total mass of the Virgo cluster to be within M_T=(2.7 - 8.9) * 10^{14}
M_\sun in reasonable agreement with the existing virial mass estimates for the
cluster.Comment: 22 pages, 11 figures, 2 tables. Accepted for publication in MNRA
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