KICP Seminars & Colloquia, Fall 2009

Seminar schedule for Fall 2009
September 30, 2009
Astronomy Colloquium
Michael Gladders
University of Chicago
Strong Lensing by Optically-Selected Galaxy Clusters   [Abstract]
October 2, 2009
Friday noon seminar
Tom Crawford
The University of Chicago
A preview of upcoming results from the South Pole Telescope   [Abstract]
October 7, 2009
Wednesday colloquium
Steven Allen
KIPAC (Stanford/SLAC)
X-ray Cluster Cosmology   [Abstract]
October 9, 2009
Friday noon seminar
Dawn Erb
UCSB
Galaxies in the Young Universe: Metallicity, Kinematics, and Gas Flows   [Abstract]
October 13, 2009
Astronomy Special Seminar
Aldo Dall'aglio
Astrophysikalisches Institut Potsdam
The evolution of the cosmic UV background photoionisation rate   [Abstract]
October 14, 2009
Wednesday colloquium
Miguel A Mostafa
Colorado State University
The latest results from the Pierre Auger Observatory   [Abstract]
October 16, 2009
Friday noon seminar
Ali Vanderveld
Caltech/JPL
Testing General Relativity on Cosmological Scales with Weak Gravitational Lensing   [Abstract]
October 20, 2009
Astronomy Special Seminar
Priscilla Frisch
The University of Chicago
The Interstellar Boundary Explorer (IBEX): Mapping the boundary between the Sun and the cosmos with energetic neutral atoms   [Abstract]
October 21, 2009
Astronomy Colloquium
Kip Thorne
California Institute of Technology
Gravitational Waves: A New Window onto the Universe   [Abstract]
October 23, 2009
Friday noon seminar
Jens Chluba
CITA
Signals from the Cosmological Recombination Epoch   [Abstract]
October 28, 2009
Wednesday colloquium
Andrey Kravtsov
The University of Chicago
"Living on the Edges: Modeling Formation of the Smallest and Largest Luminous Systems in the Universe"   [Abstract]
October 30, 2009
Friday noon seminar
Andrew Wetzel
UC Berkeley
Satellite Galaxies in LambdaCDM: Orbits, Merging & Disruption   [Abstract]
November 2, 2009
Astronomy Special Seminar
Abigail Crites
The University of Chicago
Summary and Recommendations from Women in Astronomy 2009
November 4, 2009
Astronomy Colloquium
Josh Winn
MIT
Exoplanets and their Odd Orbital Orientations   [Abstract]
November 6, 2009
Friday noon seminar
Daniel Grin
California Institute of Technology
Cosmological hydrogen recombination: the effect of extremely high-n states and forbidden transitions   [Abstract]
November 13, 2009
Friday noon seminar
Amit Yadav
Institute for Advanced Study, Princeton
Probing the Physics of the Beginning: Primordial Non-Gaussianity and Gravitational Waves   [Abstract]
November 18, 2009
Astronomy Colloquium
Justin Khoury
University of Pennsylvania
Before the Big Bang   [Abstract]
November 20, 2009
Friday noon seminar
Simona Murgia
SLAC/KIPAC
Recent Results on Dark Matter Searches with Fermi   [Abstract]
December 1, 2009
Special seminar
Guido D'Amico
SISSA
The effective theory of quintessence and its observational signatures   [Abstract]
December 2, 2009
Astronomy Colloquium
Miguel Morales
University of Washington
Reionization and Cosmology with 21 cm Fluctuations   [Abstract]
December 4, 2009
Friday noon seminar
Assaf Horesh
Tel-Aviv University
New Results for Giant Arc Statistics in ~100 Clusters   [Abstract]
December 8, 2009
Astronomy Special Seminar
Vikram Dwarkadas
The University of Chicago
High Resolution X-Ray spectroscopy and Theoretical Interpretation of SN 1996cr   [Abstract]
December 9, 2009
Wednesday colloquium
Claude Canizares
Massachusetts Institute of Technology
Searching for 'Missing Baryons' in the Warm-Hot Intergalactic Medium with High Resolution X-ray Spectroscopy   [Abstract]
December 10, 2009
Special seminar
Claude Canizares
Massachusetts Institute of Technology
NRC Report on STEM Faculty: Gender Differences at Critical Transitions in the Careers of Science, Engineering and Mathematics Faculty   [Abstract]
 
WEDNESDAY COLLOQUIA

  • October 7, 2009 | 3:30 PM | RI 480
    X-ray Cluster Cosmology
    Steven Allen, KIPAC (Stanford/SLAC)

    X-ray observations of galaxy clusters provide powerful cosmological constraints via two independent methods. The first uses measurements of the baryonic mass fraction in the largest, dynamically relaxed clusters. This method, like type Ia supernovae studies, measures distance as a function of redshift and traces the acceleration of the Universe directly. It also provides a tight constraint on the mean matter density. The second method uses the observed evolution of the cluster mass function. It leads to tight constraints on the amplitude of mass fluctuations and powerful, complementary constraints on dark energy. I will present the latest results from our team's work using both methods, employing a rigorous, self-consistent approach that accounts for survey biases, captures fully the important degeneracies between parameters and includes conservative allowances for systematic uncertainties. I will place the results in context with other current experiments and highlight the prospects for improvements in the near-to-mid term with the incorporation of new SZ, optical and X-ray data and improved hydrodynamical simulations.
  • October 14, 2009 | 3:30 PM | RI 480
    The latest results from the Pierre Auger Observatory
    Miguel A Mostafa, Colorado State University

    Since the first detection of a cosmic ray event with energy above 1020 eV in 1962, their nature and origin remain unknown. Due to the extreme rarity of these ultra high energy cosmic rays, they must be observed indirectly through the observation of extensive air showers, and the lack of knowledge of hadronic interactions at these energies leads to inherent difficulties in characterizing the properties of the primary particle. A new generation cosmic ray detector, the Pierre Auger Observatory, has been designed to study cosmic rays with energy above 1018 eV and answer the crucial questions of ultra high energy cosmic ray physics. The Southern Observatory in Argentina has been collecting data since January 2004, and its exposure is larger than that of any other cosmic ray experiment. Among the first results from the Pierre Auger Collaboration are the most precise measurement of the suppression of the cosmic ray flux at the highest energies, the first anisotropy result above 6 x 1019 eV, the first photon limit with a fluorescence detector, and the best neutrino limit at EeV energies. Then, after five years of operation, a good question is, "What is left to be done?" In this colloquium, I will describe the Pierre Auger Observatory in its astrophysical context, our most recent results, and the exciting prospects for the near future.
  • October 28, 2009 | 3:30 PM | RI 480
    "Living on the Edges: Modeling Formation of the Smallest and Largest Luminous Systems in the Universe"
    Andrey Kravtsov, The University of Chicago

    The current Cold Dark Matter paradigm of structure formation in the universe has proven its mettle in numerous stringent tests against observations over the last three decades. Nevertheless, many aspects of the theory related to the physics of baryonic component of galaxies and galaxy clusters remain relatively poorly understood and are therefore subject to continuous rigorous testing. In this talk, I will focus on formation of the smallest luminous galaxies (some of which contain only a few hundred stars) and the largest virialized systems - galaxy clusters. These systems occupy extremes of the mass range of collapsed objects and are interesting from both astrophysical and cosmological standpoints. The faintest dwarf galaxies give us a window into the process of star formation and stellar feedback in extremely low density and low metallicity environments and, at the same time, provide constraints on the properties of dark matter particles. Clusters of galaxies are excellent laboratories for studying details of galaxy formation and interaction of galaxies with surrounding gas and, at the same time, can be used as sensitive probes of cosmological parameters. I will review some of the recent research developments in modeling these systems and will discuss implications both for our understanding of galaxy formation and for cosmology.
  • December 9, 2009 | 3:30 PM | RI 480
    Searching for 'Missing Baryons' in the Warm-Hot Intergalactic Medium with High Resolution X-ray Spectroscopy
    Claude Canizares, Massachusetts Institute of Technology

    Baryons make up 4.6% of matter and energy content of the universe, but roughly half the baryons in the current epoch have not yet been observed. Simulations suggest that the missing matter is primarily in a "cosmic web" of hot, low-density plasma that traces the large-scale structure of the universe. At temperatures above 106 K, the most promising signature of this so-called WHIM is the resonance line of He-like oxygen, O VII He α, around 20 Å. This line can be observed in absorption against background X-ray point sources with the high resolution spectrometers on the Chandra and XMM-Newton observatories (an X-ray analog to the Ly α forrest). The observations, however, are challenging and results have been controversial. This talk will review evidence for (and against) the detection of the WHIM, including a recently confirmed observation of an X-ray absorption line associated with the large-scale galaxy enhancement in the direction of Sculptor.

 
FRIDAY NOON SEMINARS

  • October 2, 2009 | 12:00 PM | LASR Conference Room
    A preview of upcoming results from the South Pole Telescope
    Tom Crawford, The University of Chicago

    The South Pole Telescope (SPT) is a 10-meter, millimeter-wave telescope operating at the Amundsen-Scott South Pole Station. The first camera on the SPT, a ~1000-element, three-color, bolometric receiver, has been operational since 2007 and is currently surveying hundreds of square degrees of the southern sky per year in observing bands centered on 90, 150, and 220 GHz. Analysis of the 2008 SPT data has resulted in several interesting discoveries and new results, which will be made public soon, and which I will give an informal survey of here. These results include the apparent discovery of a new population of star-forming galaxies, measurements of the SZ effect from known clusters out to unprecedented clustercentric radii, the first cosmologically interesting catalog of SZ-selected galaxy clusters, and measurements of high-l CMB power spectrum.
  • October 9, 2009 | 12:00 PM | LASR Conference Room
    Galaxies in the Young Universe: Metallicity, Kinematics, and Gas Flows
    Dawn Erb, UCSB

    A large fraction of the stars in the universe today formed during the redshift interval 1.5<z<3, when the universe was only about 25% of its current age. However, the quantitative study of galaxies in this redshift range from large spectroscopic samples has only recently become feasible. Such spectra offer a unique opportunity to quantify the physical conditions in these distant galaxies and their interactions with the surrounding intergalactic gas. I will discuss the results of a large near-IR spectroscopic survey of star-forming galaxies at z~2, highlighting the galaxies' kinematics, elemental abundances, and large-scale outflows and inflows of gas.
  • October 16, 2009 | 12:00 PM | LASR Conference Room
    Testing General Relativity on Cosmological Scales with Weak Gravitational Lensing
    Ali Vanderveld, Caltech/JPL

    Weak gravitational lensing is a powerful probe of modifications of General Relativity on cosmological scales, since such modifications can affect both how matter produces gravitational potential wells and how photons move within these wells. I will discuss alternative theories of gravitation and how we may constrain such theories using weak lensing observables, including those that could be obtained with the balloon-borne High Altitude Lensing Observatory (HALO). I will also discuss the ''parametrized-post-Friedmannian'' approach for obtaining model-independent constraints, in which new parameters are introduced to characterize the departure from General Relativity on large scales.
  • October 23, 2009 | 12:00 PM | LASR Conference Room
    Signals from the Cosmological Recombination Epoch
    Jens Chluba, CITA

    Very soon the Planck Surveyor will start observing the CMB temperature and polarization anisotropies with unprecedented precision. For the analysis of these data sets it will be very important to understand the ionization history of the Universe at redshift z~1100 with very high accuracy, since otherwise uncertainties in the modelling of the recombination process may lead to significant biases in the deduced values of some cosmological parameters. In addition to the simple fact that electrons are captured by protons and helium ions also some photons are released during the cosmological recombination process, leading to small distortions in the CMB blackbody spectrum which should still be present today. This recombination radiation carries valuable information about the dynamics of recombination and the underlying cosmological parameters, which until now has not been accessed. In my talk I will review some of the recent computations in connection with the ionization history of the Universe and the CMB power spectra, showing that neglecting details in the physics of recombination will lead to important biases in the values of n_s and Omega_b. Furthermore, I will try to show that one could learn a lot about cosmological parameter, details in the recombination dynamics, energy release at high redshift and possible dark matter annihilations during recombination by directly measuring the cosmological recombination radiation.
  • October 30, 2009 | 12:00 PM | LASR Conference Room
    Satellite Galaxies in LambdaCDM: Orbits, Merging & Disruption
    Andrew Wetzel, UC Berkeley

    Dark matter halos that merge with larger halos persist as subhalos, which host satellite galaxies. While subhalos are rapidly stripped of their dark mass, the compact luminous material remains intact longer, making the correspondence of galaxies with severely stripped suhalos unclear. I use a high-resolution, cosmological N-body simulation to explore satellite galaxy merging and tidal disruption. Satellite subhalos must be well-resolved down to ~1% of their mass at infall, and many satellites experience tidal disruption prior to merging with the central galaxy. Using abundance matching to assign stellar mass to subhalos, I compare with observed galaxy clustering, satellite fractions, and cluster luminosity functions. I also explore a simple analytic model based on dynamical friction for satellite infall. Finally, I examine the orbital distribution of infalling satellites and its evolution with halo mass and redshift.
  • November 6, 2009 | 12:00 PM | LASR Conference Room
    Cosmological hydrogen recombination: the effect of extremely high-n states and forbidden transitions
    Daniel Grin, California Institute of Technology

    Thanks to the ongoing Planck mission, a new window will be opened on the properties of the primordial density field, the cosmological parameters, and the physics of reionization. Much of Planck's new leverage on these quantities will come from temperature measurements at small angular scales and from polarization measurements. These both depend on the details of cosmological hydrogen recombination; use of the CMB as a probe of energies greater than 10^16 GeV compels us to get the ~eV scale atomic physics right.

    One question that remains is how high in hydrogen principle quantum number we have to go to make sufficiently accurate predictions for Planck. Using sparse matrix methods to beat computational difficulties, I have modeled the influence of very high (up to and including n=200) excitation states of atomic hydrogen on the recombination history of the primordial plasma, resolving all angular momentum sub-states separately and including, for the first time, the effect of hydrogen quadrupole transitions. I will review the basic physics, explain the resulting plasma properties, discuss recombination histories, and close by discussing the effects on CMB observables.
  • November 13, 2009 | 12:00 PM | LASR Conference Room
    Probing the Physics of the Beginning: Primordial Non-Gaussianity and Gravitational Waves
    Amit Yadav, Institute for Advanced Study, Princeton

    In the last few decades, advances in observational cosmology have given us a standard model of cosmology. We know the content of the universe to within a few percent. With more ambitious experiments on the way, we hope to move beyond the knowledge of what the universe is made of, to why the universe is the way it is. In my talk, I will focus on what we can expect to learn about the dynamics of the universe at the very earliest moments. I will discuss theoretical predictions from inflationary models and their observational consequences in the cosmic microwave background anisotropies. In particular, I will focus on two observational signatures, primordial non-Gaussianity and gravitational waves, as probes for the physics of the beginning.
  • November 20, 2009 | 12:00 PM | LASR Conference Room
    Recent Results on Dark Matter Searches with Fermi
    Simona Murgia, SLAC/KIPAC

    The Fermi Large Area Telescope (LAT) has been successfully launched from Cape Canaveral on 11 June 2008. It is exploring the gamma ray sky in the energy range from 20 MeV to over 300 GeV with unprecedented sensitivity. One of the most exciting science questions that the Fermi LAT will address is the nature of dark matter. Several theoretical models have been proposed that predict the existence of Weakly Interacting Massive Particles (WIMPs) that are excellent dark matter candidates. The Fermi LAT investigates the existence of WIMPs indirectly, primarily through their annihilation or decay into photons and into electrons and positrons. I will present recent results on these searches.
  • December 4, 2009 | 12:00 PM | LASR Conference Room
    New Results for Giant Arc Statistics in ~100 Clusters
    Assaf Horesh, Tel-Aviv University

    Gravitational lensing is frequently used to map the evolution of cluster mass profiles, ellipticities, and substructure. One approach is to perform detailed modeling of individual clusters using strong and weak lensing, but this is generally suited only to deep data for individual clusters that exhibit numerous lensed images, and the results may not be representative of the majority of clusters. A complementary approach is to measure the statistics of lensed arcs in large samples of clusters. Over the past decade, a debate has gone on about the nature of cluster samples selected by different methods, and about whether or not real clusters are highly more efficient producers of arcs than expected from theory. I will present an analysis of the arc statistics in a sample of ~100 galaxy clusters observed with the HST/ACS. X-ray selected clusters are much more efficient lenses than optically-selected clusters of similar optical luminosity, showing that optical selection yields lower-mass, perhaps marginally bound, structures. I will also present our past and in-progress comparison of observed arc statistics with simulations that aim to include all the theoretical and observational aspects of the problem.

 
SPECIAL SEMINARS

  • December 1, 2009 | |
    The effective theory of quintessence and its observational signatures
    Guido D'Amico, SISSA

    I will study generic single-field dark energy models, by a parametrization of the most general theory of their perturbations around a given background, including higher derivative terms. In appropriate limits this approach reproduces standard quintessence, k-essence and ghost condensation. There are no general pathologies associated to an equation of state w_Q < -1 or in crossing the phantom divide w_Q = -1. Stability requires that, when w_Q < -1, dark energy behaves, on cosmological scales, as a fluid with a virtually zero speed of sound. Theoretical and stability constraints are summarized on the quintessential plane (1+w_Q) vs. speed of sound squared.
    Finally, I will discuss the effect of dark energy with a zero speed of sound on non-linear scales, by calculating the halo mass function.
  • December 10, 2009 | 12:00 PM | BSLC 109
    NRC Report on STEM Faculty: Gender Differences at Critical Transitions in the Careers of Science, Engineering and Mathematics Faculty
    Claude Canizares, Massachusetts Institute of Technology

    In June 2009, the National Research Council released a report on gender differences in the careers of science, engineering, and mathematics faculty at research universities. Claude Canizares, Co-Chair of the NRC report committee, will present the results of two surveys commissioned for the report, and discuss the key findings and recommendations of the committee.

 
ASTRONOMY COLLOQUIA

  • September 30, 2009 | 3:30 PM | RI 480
    Strong Lensing by Optically-Selected Galaxy Clusters
    Michael Gladders, University of Chicago

    Gravitational lensing by galaxy clusters was predicted in the 1930s, and finally discovered in 1980s. In the two decades following the initial discovery, several dozen significant cluster lenses were found, though only a handful of these have been studied extensively. Lensing clusters probe the distribution of massive halos in the universe; the expected arc production frequency can be predicted from simulations and compared to existing data. Massive lensing clusters act as 'natural telescopes', providing highly magnified images of background sources which cannot otherwise be studied using the current generation of telescopes. The details of the observed lensing in clusters also probes the internal properties of these massive halos. Most cluster strong lens studies to date have been limited by the small number and heterogeneous nature of the sample of known lenses (most of which are one-off discoveries). I will report on efforts to take the study of strong lensing clusters to a new statistical regime, by identifying and studying two new samples of strong lenses within large catalogs of optically selected galaxy clusters from the RCS-2 and SDSS surveys; in total we have found hundreds of new giant arcs. These efforts are now approximately three-quarters-complete; in this progress report I will describe some of the spectacular successes of these studies, and the remaining challenges.
  • October 21, 2009 | 3:30 PM | KPTC 106
    Gravitational Waves: A New Window onto the Universe
    Kip Thorne, California Institute of Technology

    Over the next decade or so, the gravitational-wave window onto the Universe will be opened in four frequency bands that span 22 orders of magnitude: The high-frequency band, 10 to 10,000 Hz (ground-based interferometers such as LIGO), the low-frequency band, 10^-5 to 0.1 Hz (the space-based interferometer LISA), the very-low-frequency band, 10^-9 to 10^-7 Hz (pulsar timing arrays), and the extremely-low-frequency band, 10^-18 to 10^-16 Hz (polarization of the cosmic microwave background). This lecture will describe these four bands, the detectors that are being developed to explore them, and what we are likely to learn about black holes, neutron stars, white dwarfs and early-universe exotica from these detectors' observations. [I will focus largely on LIGO and LISA but, unless you advise otherwise, I think it useful to include PTAs and CMB polarization as well.]
  • November 4, 2009 | 3:30 PM | RI 480
    Exoplanets and their Odd Orbital Orientations
    Josh Winn, MIT

    In the Solar system, the planets follow orbits that are aligned with the Sun's equatorial plane to within about 7 degrees. What about planets around other stars? Recently we have measured the orbital orientations (relative to their parent stars' equators) of more than a dozen different exoplanets, using a technique first theorized in the 19th century. Many systems have good alignment, as in the Solar system -- but there are a few surprises. I will discuss these results and their implications for theories of planet formation and migration.
  • November 18, 2009 | 3:30 PM | RI 480
    Before the Big Bang
    Justin Khoury, University of Pennsylvania

    This talk will explore the idea that our universe existed before the big bang, as an alternative to the standard big bang/inflationary model. I will show how a phase of slow contraction before the big bang can explain the observed degree of flatness and homogeneity of our universe, as well as generate a nearly scale-invariant spectrum of primordial density perturbations. I will contrast the predictions of this model with those of inflationary cosmology, and discuss observational prospects for distinguishing between the two scenarios.
  • December 2, 2009 | 3:30 PM | RI 480
    Reionization and Cosmology with 21 cm Fluctuations
    Miguel Morales, University of Washington

    TBA

 
ASTRONOMY SPECIAL SEMINARS

  • October 13, 2009 | 12:00 PM | AAC 123
    The evolution of the cosmic UV background photoionisation rate
    Aldo Dall'aglio, Astrophysikalisches Institut Potsdam

    We present a new determination of the hydrogen photoionisation rate due to the metagalactic UV radiation field, between redshifts z~2 and z~5. We have developed a novel and unbiased method to estimate this quantity, exploiting the distribution properties of the strength of the Proximity Effect in samples of high-quality quasar absorption line spectra. Our new method overcomes the systematic bias towards too high values of the UVB, inherent in the standard analysis technique of the proximity effect. We have applied our method to a sample of 40 high S/N, high-resolution quasar spectra taken with the ESO-VLT, finding that there is at best marginal evidence for a decrease of the UVB intensity between z~2 and z~4. With the same method we now have analysed a sample of 2000 quasar spectra from the SDSS, extending our redshift range up almost z~5 and greatly improving the statistical sampling rate. With this dataset we can estimate the UVB photoionisation rate independently for several redshift bins. We find that the UVB remains approximately constant up to the highest redshifts covered. Comparing our results with synthesis models of the UVB from the population properties of possible UV sources, we conclude that quasars do not contribute significantly to the UVB at and above a redshift of z~3. Our measurements are in excellent agreement with UV light from star-forming galaxies dominating the UVB around z~3-3.5, but current surveys appear to fall short of uncovering the required UV emissivity to account for a flat UVB at z>~4.
  • October 20, 2009 | 12:00 PM | AAC 123
    The Interstellar Boundary Explorer (IBEX): Mapping the boundary between the Sun and the cosmos with energetic neutral atoms
    Priscilla Frisch, The University of Chicago

    The Interstellar Boundary Explorer (IBEX) spacecraft has just completed the first skymaps of the heliosphere boundaries. The contact region between the solar wind and interstellar medium is imaged in maps of energetic neutral atoms (ENAs) in the energy range 10 eV to 6 keV, formed when energetic ions charge-exchange with interstellar neutrals. The spectra of the ENA emissions provide data on the energy distribution of the underlying ions from which they are formed. Seen in these maps are the effects of the heliosphere nose-tail morphology, where the nose is directed near the galactic center and the tail is directed towards Taurus and Orion. The flat spectra at high ecliptic latitudes follow the latitudinal dependence of the solar wind energy flux. IBEX has also made the first in situ measurement of interstellar oxygen atoms. Most surprising of all, a remarkable Ribbon of ENA emission arcs 300 degrees across the sky. The Ribbon traces the region where the interstellar magnetic field is perpendicular to the sightline (as viewed from the Sun). Forces resulting from the motion of charged particles through magnetic fields in the outer heliosphere appear to explain the Ribbon structure. makahiki > imprint 101509_ibex_abstract.
  • November 2, 2009 | 4:00 PM | AAC 123
    Summary and Recommendations from Women in Astronomy 2009
    Abigail Crites, The University of Chicago
  • December 8, 2009 | 12:00 PM | AAC 123
    High Resolution X-Ray spectroscopy and Theoretical Interpretation of SN 1996cr
    Vikram Dwarkadas, The University of Chicago

    SN1996cr is the one of the five closest SNe to explode in the past 30 yr, yet lay undiscovered for many years. Due to its fortuitous location in the Circinus Galaxy at 3.7 Mpc, we have a wealth of serendipitous archival data available to piece together its early evolution. In addition to this, our team was awarded a 500,000 sec Chandra HETG observation of the SN, which was completed in late 2008-early 2009. In order to interpret this data, we have carried out hydrodynamic simulations, followed by computations of non-equilibrium ionization simulated spectra that can be directly compared with the observations. These calculations allow us to infer the evolution of the SN shock wave, and the abundances of the ejecta and surrounding medium to reasonable accuracy. The data imply that SN 1996cr exploded in a low-density medium before interacting with a dense shell of material about 0.03pc away from the star. In this talk we will discuss in detail the data, the abundance calculations and its theoretical implications. In particular we will show how these calculations allow us to probe the stellar mass loss in the very last (0.1%) phases of a massive star's life. SN 1996cr seems to be one of several recently discovered SNe whose emission reveals evidence of abrupt mass-loss in the very last stages of a massive star's life.