38 research outputs found
Bayesian analysis of cosmic structures
We revise the Bayesian inference steps required to analyse the cosmological
large-scale structure. Here we make special emphasis in the complications which
arise due to the non-Gaussian character of the galaxy and matter distribution.
In particular we investigate the advantages and limitations of the
Poisson-lognormal model and discuss how to extend this work. With the lognormal
prior using the Hamiltonian sampling technique and on scales of about 4 h^{-1}
Mpc we find that the over-dense regions are excellent reconstructed, however,
under-dense regions (void statistics) are quantitatively poorly recovered.
Contrary to the maximum a posteriori (MAP) solution which was shown to
over-estimate the density in the under-dense regions we obtain lower densities
than in N-body simulations. This is due to the fact that the MAP solution is
conservative whereas the full posterior yields samples which are consistent
with the prior statistics. The lognormal prior is not able to capture the full
non-linear regime at scales below ~ 10 h^{-1} Mpc for which higher order
correlations would be required to describe the matter statistics. However, we
confirm as it was recently shown in the context of Ly-alpha forest tomography
that the Poisson-lognormal model provides the correct two-point statistics (or
power-spectrum).Comment: 11 pages, 1 figure, report for the Astrostatistics and Data Mining
workshop, La Palma, Spain, 30 May - 3 June 2011, to appear in Springer Series
on Astrostatistic
A very faint core-collapse supernova in M85
An anomalous transient in the early Hubble-type (S0) galaxy Messier 85 (M85)
in the Virgo cluster was discovered by Kulkarni et al. (2007) on 7 January 2006
that had very low luminosity (peak absolute R-band magnitude MR of about -12)
that was constant over more than 80 days, red colour and narrow spectral lines,
which seem inconsistent with those observed in any known class of transient
events. Kulkarni et al. (2007) suggest an exotic stellar merger as the possible
origin. An alternative explanation is that the transient in M85 was a type
II-plateau supernova of extremely low luminosity, exploding in a lenticular
galaxy with residual star-forming activity. This intriguing transient might be
the faintest supernova that has ever been discovered.Comment: 7 pages, 2 figures. Submitted to Nature "Brief Communication Arising"
on 18 July 2007, Accepted on 17 August 2007. Arising from: Kulkarni et al.
2007, Nature, 447, 458-46
Neutrino-driven Explosions
The question why and how core-collapse supernovae (SNe) explode is one of the
central and most long-standing riddles of stellar astrophysics. A solution is
crucial for deciphering the SN phenomenon, for predicting observable signals
such as light curves and spectra, nucleosynthesis, neutrinos, and gravitational
waves, for defining the role of SNe in the evolution of galaxies, and for
explaining the birth conditions and properties of neutron stars (NSs) and
stellar-mass black holes. Since the formation of such compact remnants releases
over hundred times more energy in neutrinos than the SN in the explosion,
neutrinos can be the decisive agents for powering the SN outburst. According to
the standard paradigm of the neutrino-driven mechanism, the energy transfer by
the intense neutrino flux to the medium behind the stagnating core-bounce
shock, assisted by violent hydrodynamic mass motions (sometimes subsumed by the
term "turbulence"), revives the outward shock motion and thus initiates the SN
blast. Because of the weak coupling of neutrinos in the region of this energy
deposition, detailed, multidimensional hydrodynamic models including neutrino
transport and a wide variety of physics are needed to assess the viability of
the mechanism. Owing to advanced numerical codes and increasing supercomputer
power, considerable progress has been achieved in our understanding of the
physical processes that have to act in concert for the success of
neutrino-driven explosions. First studies begin to reveal observational
implications and avenues to test the theoretical picture by data from
individual SNe and SN remnants but also from population-integrated observables.
While models will be further refined, a real breakthrough is expected through
the next Galactic core-collapse SN, when neutrinos and gravitational waves can
be used to probe the conditions deep inside the dying star. (abridged)Comment: Author version of chapter for 'Handbook of Supernovae,' edited by A.
Alsabti and P. Murdin, Springer. 54 pages, 13 figure
Bayesian astrostatistics: a backward look to the future
This perspective chapter briefly surveys: (1) past growth in the use of
Bayesian methods in astrophysics; (2) current misconceptions about both
frequentist and Bayesian statistical inference that hinder wider adoption of
Bayesian methods by astronomers; and (3) multilevel (hierarchical) Bayesian
modeling as a major future direction for research in Bayesian astrostatistics,
exemplified in part by presentations at the first ISI invited session on
astrostatistics, commemorated in this volume. It closes with an intentionally
provocative recommendation for astronomical survey data reporting, motivated by
the multilevel Bayesian perspective on modeling cosmic populations: that
astronomers cease producing catalogs of estimated fluxes and other source
properties from surveys. Instead, summaries of likelihood functions (or
marginal likelihood functions) for source properties should be reported (not
posterior probability density functions), including nontrivial summaries (not
simply upper limits) for candidate objects that do not pass traditional
detection thresholds.Comment: 27 pp, 4 figures. A lightly revised version of a chapter in
"Astrostatistical Challenges for the New Astronomy" (Joseph M. Hilbe, ed.,
Springer, New York, forthcoming in 2012), the inaugural volume for the
Springer Series in Astrostatistics. Version 2 has minor clarifications and an
additional referenc
A low energy core-collapse supernova without a hydrogen envelope
The final fate of massive stars depends on many factors, including mass,
rotation rate, magnetic fields and metallicity. Theory suggests that some
massive stars (initially greater than 25-30 solar masses) end up as Wolf-Rayet
stars which are deficient in hydrogen because of mass loss through strong
stellar winds. The most massive of these stars have cores which may form a
black hole and theory predicts that the resulting explosion produces ejecta of
low kinetic energy, a faint optical display and a small mass fraction of
radioactive nickel(1,2,3). An alternative origin for low energy supernovae is
the collapse of the oxygen-neon core of a relatively lowmass star (7-9 solar
masses) through electron capture(4,5). However no weak, hydrogen deficient,
core-collapse supernovae are known. Here we report that such faint, low energy
core-collapse supernovae do exist, and show that SN2008ha is the faintest
hydrogen poor supernova ever observed. We propose that other similar events
have been observed but they have been misclassified as peculiar thermonuclear
supernovae (sometimes labelled SN2002cx-like events(6)). This discovery could
link these faint supernovae to some long duration gamma-ray bursts. Extremely
faint, hydrogen-stripped core-collapse supernovae have been proposed to produce
those long gamma-ray bursts whose afterglows do not show evidence of
association with supernovae (7,8,9).Comment: Submitted 12 January 2009 - Accepted 24 March 200
The Evolution of Compact Binary Star Systems
We review the formation and evolution of compact binary stars consisting of
white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and
BHs are thought to be the primary astrophysical sources of gravitational waves
(GWs) within the frequency band of ground-based detectors, while compact
binaries of WDs are important sources of GWs at lower frequencies to be covered
by space interferometers (LISA). Major uncertainties in the current
understanding of properties of NSs and BHs most relevant to the GW studies are
discussed, including the treatment of the natal kicks which compact stellar
remnants acquire during the core collapse of massive stars and the common
envelope phase of binary evolution. We discuss the coalescence rates of binary
NSs and BHs and prospects for their detections, the formation and evolution of
binary WDs and their observational manifestations. Special attention is given
to AM CVn-stars -- compact binaries in which the Roche lobe is filled by
another WD or a low-mass partially degenerate helium-star, as these stars are
thought to be the best LISA verification binary GW sources.Comment: 105 pages, 18 figure
Production of dust by massive stars at high redshift
The large amounts of dust detected in sub-millimeter galaxies and quasars at
high redshift pose a challenge to galaxy formation models and theories of
cosmic dust formation. At z > 6 only stars of relatively high mass (> 3 Msun)
are sufficiently short-lived to be potential stellar sources of dust. This
review is devoted to identifying and quantifying the most important stellar
channels of rapid dust formation. We ascertain the dust production efficiency
of stars in the mass range 3-40 Msun using both observed and theoretical dust
yields of evolved massive stars and supernovae (SNe) and provide analytical
expressions for the dust production efficiencies in various scenarios. We also
address the strong sensitivity of the total dust productivity to the initial
mass function. From simple considerations, we find that, in the early Universe,
high-mass (> 3 Msun) asymptotic giant branch stars can only be dominant dust
producers if SNe generate <~ 3 x 10^-3 Msun of dust whereas SNe prevail if they
are more efficient. We address the challenges in inferring dust masses and
star-formation rates from observations of high-redshift galaxies. We conclude
that significant SN dust production at high redshift is likely required to
reproduce current dust mass estimates, possibly coupled with rapid dust grain
growth in the interstellar medium.Comment: 72 pages, 9 figures, 5 tables; to be published in The Astronomy and
Astrophysics Revie
The Fourteenth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the Extended Baryon Oscillation Spectroscopic Survey and from the Second Phase of the Apache Point Observatory Galactic Evolution Experiment
The fourth generation of the Sloan Digital Sky Survey (SDSS-IV) has been in operation since 2014 July. This paper describes the second data release from this phase, and the 14th from SDSS overall (making this Data Release Fourteen or DR14). This release makes the data taken by SDSS-IV in its first two years of operation (2014–2016 July) public. Like all previous SDSS releases, DR14 is cumulative, including the most recent reductions and calibrations of all data taken by SDSS since the first phase began operations in 2000. New in DR14 is the first public release of data from the extended Baryon Oscillation Spectroscopic Survey; the first data from the second phase of the Apache Point Observatory (APO) Galactic Evolution Experiment (APOGEE-2), including stellar parameter estimates from an innovative data-driven machine-learning algorithm known as "The Cannon"; and almost twice as many data cubes from the Mapping Nearby Galaxies at APO (MaNGA) survey as were in the previous release (N = 2812 in total). This paper describes the location and format of the publicly available data from the SDSS-IV surveys. We provide references to the important technical papers describing how these data have been taken (both targeting and observation details) and processed for scientific use. The SDSS web site (www.sdss.org) has been updated for this release and provides links to data downloads, as well as tutorials and examples of data use. SDSS-IV is planning to continue to collect astronomical data until 2020 and will be followed by SDSS-V