| Supernovae and Interacting Binaries |
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| Supernovae and Interacting Binaries |
| Supernovae | Interacting Binaries |
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Explosive Activity of the Padova-Asiago
Supernova Group
Supernovae (SNe) are instrumental to many topics of modern Astrophysics.
They are fundamental probes of stellar
evolution theories and allow to link different kinds of
explosion mechanisms to stars of different masses.
The material ejected during the explosion, at
velocities exceeding 10000 km/s, in many cases reach gas
previously expelled at lower velocities thus uncovering
the latest stages of the evolution of the progenitor, and(or) of the companion star.
The SN ejecta are enriched
in heavy chemical elements, e.g. oxygen, calcium, nitrogen and iron, which are
the result of both thermonuclear burnings during the life of the star and of
explosive nucleosynthesis. SNe are, therefore, key actors in the
chemical evolution of galaxies. Core-collapse SNe,
the endpoints of the evolution of massive stars, leave
behind compact remnants, neutron stars or black holes, while thermonuclear
explosions yield type Ia SNe which, thanks to their relatively homogeneous behaviour,
are successfully used as distance indicators up to cosmological distances.
The discovery that our Universe is expanding with a positive acceleration, probably
because of an undefined Dark Energy, is mainly based on SNe
at high-redshift.
Finally, a number of nearby long GRBs have been recently associated to a subclass of
core collapse SNe, the high-energy type Ic, thus establishing a link between
these two classes of cosmic explosions.
The activity of our team develops in this general context with a number of projects, both
observational and theoretical, aimed at
understanding the physics of the phenomena and unveil the nature of the
progenitor systems.
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| Figure 1: The "Zorro" diagram for Type Ia Supernovae (from Mazzali et al 2007 |
We are addressing the issue of the physical origin of the SNIa and their diversity from an observational/theoretical point of view. In the recent past we have studied the distribution of SNIa absolute magnitudes, ejecta expansion velocities and spectral line ratios in comparison with the expectations from different progenitor scenarios and explosion mechanisms (deflagration, delayed detonation, etc.) Benetti et al 2005 (ApJ 623, 1011) and Mazzali et al 2007 (Science 315, 825). We have found that all SNIa burn roughtly the same amount of mass, which implies that their progenitors had the same mass before explosion. A single explosion scenario, possibly a delayed detonation, may thus explain most SNeIa. The drive for the variations in the explosion observables is likely the strength of the central deflagration and the extent of stellar mass burned before the detonation goes off. The remaining diversity among SNIa could be explained by 3-D effects in the outer envelopes like ejection of blobs of burned material along the line of sight and interaction with the CSM. Our current goal is thus to increase the sample of nearby SNIa with excellent monitoring from UV to near IR domains, and from photospheric up to nebular phases, to put the results mentioned above on a firm statistical ground. In these effort, we take part in a major international collaborations and we exploit several observing programs running on some of the major ground-based observatories (ESO-VLT, ESO-NTT, TNG) and space missions (Galex, Swift).
Other indications on the nature of the progenitors of SNIa can be derived from the study of CircumStellar Matter. Thanks to high quality and very high resolution multi-epochs spectrograms taken with VLT+UVES we have been able to reveal for the first time signatures of matter unequivocally lost by the progenitor system in the form of NaI shells expanding with velocities of about 50 km/s. These observations favour the Single Degenerate scenario for SNIa explosion ( Patat et al 2007, Science 317, 924) in which the WD companion is a giant star. We are working to enlarge the sample of Type Ia Supernovae observed with this innovative approach. Recent studies have shown that the extinction curves toward a few, highly-reddened SNIa have values of RV significantly lower than the canonical 3.1 (Elias-Rosa et al. 2006, 2008). These findings rose new questions. Is the peculiar extinction law derived in the direction to these SNIa common to all the highly-reddened objects? Do low-reddening SNIa (used as distance indicators) follow similar extinction laws? What is the effect on the calibration of nearby SNIa? Are the extinction laws the same at low and high redshift? Is this effect "local", i.e. somehow related to the nature of the progenitor system, or due to the overall dust properties of the host galaxy? To answer these questions observational campaigns of highly and moderately reddened SNe of all types over extended wavelength intervals from the UV (SWIFT) to the near IR (TNG, NTT and VLT) are in progress.
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| Figure 2: Selection of distribution functions of the delay times from Greggio (2005). For better readability the scale on the x-axis is expanded at delay times shorter than 1 Gyr. The insert shows the correlation between the SNIa rate per unit K band luminosity and the color of the parent galaxy, as observed (Mannucci et al. 2005 ,black dots) and predicted by the 4 models (same color coding as the distribution functions). |
A complementary approach to investigate on the progenitors of type Ia SNe
consists in studying the rate at which they occur in different contexts.
Indeed, different evolutionary paths may lead to the thermonuclear
explosion of a CO WD in a close binary system, each corresponding to a
different distribution for the delay times (DTD).
Evolutionary channels which imply DTDs more skewed at short delay
times predict a stronger increase of the SNIa rate per unit mass,
when going from old to young stellar populations. Thus, the analysis of the
SNIa rate per unit mass (or any other mass tracer) in systems with different
age distributions constrains the shape of the DTD, and, in turn, the SNIa
progenitor model.
Greggio (2005) developped
an analytic descriptions of the DTD rooted on stellar evolution, with a
built-in parametrization of those astrophysical variables which play an
important role in shaping it, e.g. the clock of the explosion
and the mass distribution of the binary components in successful systems.
These functions are a flexible tool to investigate on the predictions from
different progenitor models on various observables.
An example for four possible SNIa evolutionary channels is shown in Figure 2,
together with the correlation between the specific SNIa rate and the
parent galaxy colour which they imply. The constraints on the DTD derived from
this kind of correlation appear to depend on the photometric bands used to
trace the average age of the stellar populations. We are investigating on a
more effective way to derive the star formation history in galaxies,in order
to improve on the diagnostic on the DTD from the trend of the SNIa rate with
the parent galaxies' properties.
These DTDs are also a handy tool to
investigate on the impact of the different SNIa models on the redshift
dipendence of the cosmic SNIa rate, on the Fe enrichment timescales
in galaxies and in galaxy clusters, on the global Fe budget of galaxy
clusters, and of the universe (
Blanc and Greggio, 2008), as well as on the evolution of the cosmic mix of SNIa events
if both the DD and SD evolutionary channels are at work (
Greggio, Renzini and Daddi 2008)
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| Figure 3: The transients UGC4904-V1 and SN2006jc (from Pastorello et al 2007) |
Core collapse SNe span a huge range in luminosities, explosion energies,
ejecta masses. Using detailed observations and modelling we are collecting
individual case studies that are used to sort out a coherent general
scenario for massive star evolution and explosion.
To achieve this aim we have developed a semi-analytic tool
(Zampieri et al 2003, MNRAS 338, 711)
in order to study
the explosion mechanism(s) and the progenitors characteristics (in particular its
main sequence mass) from a statistical point of view.
The main input parameters are the supernova luminosity, the photospheric
expansion velocities, the effective temperature while in output
we obtain the explosion energy, the ejected
56Ni mass, and the mass of the ejecta.
The application of the code has shown an overall continuity in the physical parameters
of type II SNe
from the lowest energy explosions (e.g. SN 1997D) to the brightest objects (e.g.
SN 1983K), a clear indication of the existence of a single explosion mechanism.
Several gaps in the distributions of energies and masses still exist which we are planning to fill
by means of dedicated observational programs.
We are also concentrating our efforts on the studies of the most interesting
objects in the core collapse zoo. A recent example is SN 2006jc,
possibly the first
example of the rare explosion of extremely massive stars and
the first SN ever for which a pre-SN burst has been recorded (Pastorello et al. 2007, Nature 447, 829).
We are also studying the type Ic SNe, contact point between
Supernovae and GRBs.
Energetic SNeIc seem also to be the central engines that sustain a few very luminous
SN IIn which have been discovered recently. The huge luminosity has been related to the
shock of the SN ejecta with dense massive shells of material ejected by the SN progenitor
shortly before explosion (Agnoletto et al. 2009, ApJ 691,1348)
SN rates provide unique information on the initial mass function
of stars over an extended mass range and are therefore fundamental
tools for understanding the formation and chemical evolution of
galaxies. The advent of Wide Field Imagers on large telescopes
has allowed measurement of SN rates at high redshifts
and of the evolution of the star
formation history over a large fraction of the age of the Universe.
The rate of type II+Ib/c SNe, in particular, measures the death rate
of young, massive stars via core-collapse and, because
of the short time-scale of the progenitor evolution,
directly reflects the on-going star formation rate (SFR) in a given environment.
On the contrary, the rate of type Ia SNe, whose precursors span a wide range of
lifetimes, reflects the whole star formation history.
Most of the available determinations of SN rates at high-z are based on the material
of SN searches focused on the search of SNIa for determining the geometry of the
Universe. At the end of a pilot program, especially designed to
determine the rates of all SN types, we have obtained estimates
of both core collapse and SNIa rates. In particular, we have been able to
demonstrated that "only"
3 billions years ago the star formation rate was 3 times higher than in the
present Universe (Cappellaro et al. 2005
, Botticella et al 2008).
On the other hand, the evolution of the rate of SNIa with redshift seems to
suggest that after a single episode of star formation SNIa may occur with a wide range of ages, starting from 100 millions up to 10 billions
years afterwards.
In this field, we have now started two new observational projects namely a deep SN search with LBC at the LBT, which is
intended to measure the rate evolution at very high redshift (0.5<z<1.3), and a infrared SN search in Starburst galaxies
using HAWK-I at the VLT, which is aimed to verify the link between SFR and SN rate and get a better handle on the extinction bias.