169 research outputs found
Type II Supernovae as Standardized Candles
We present evidence for a correlation between expansion velocities of the
ejecta of Type II plateau supernovae and their bolometric luminosities during
the plateau phase. This correlation permits one to standardize the candles and
decrease the scatter in the Hubble diagram from ~1 mag to a level of 0.4 and
0.3 mag in the V and I bands, respectively. When we restrict the sample to the
eight objects which are well in the Hubble flow (cz > 3,000 km/s) the scatter
drops even further to only 0.2 mag (or 9% in distance), which is comparable to
the precision yielded by Type Ia supernovae and far better than the ``expanding
photosphere method'' applied to Type II supernovae. Using SN 1987A to calibrate
the Hubble diagrams we get Ho=55+/-12.Comment: 9 pages, 3 figures, 1 table, accepted by ApJ
Selection Effects, Biases, and Constraints in the Calan/Tololo Supernova Survey
We use Monte Carlo simulations of the Calan/Tololo photographic supernova
survey to show that a simple model of the survey's selection effects accounts
for the observed distributions of recession velocity, apparent magnitude,
angular offset, and projected radial distance between the supernova and the
host galaxy nucleus for this sample of Type Ia supernovae (SNe Ia). The model
includes biases due to the flux-limited nature of the survey, the different
light curve morphologies displayed by different SNe Ia, and the difficulty of
finding events projected near the central regions of the host galaxies. From
these simulations we estimate the bias in the zero-point and slope of the
absolute magnitude-decline rate relation used in SNe Ia distance measurements.
For an assumed intrinsic scatter of 0.15 mag about this relation, these
selection effects decrease the zero-point by 0.04 mag. The slope of the
relation is not significantly biased. We conclude that despite selection
effects in the survey, the shape and zero-point of the relation determined from
the Calan/Tololo sample are quite reliable. We estimate the degree of
incompleteness of the survey as a function of decline rate and estimate a
corrected luminosity function for SNe Ia in which the frequency of SNe appears
to increase with decline rate (the fainter SNe are more common). Finally, we
compute the integrated detection efficiency of the survey in order to infer the
rate of SNe Ia from the 31 events found. For a value of Ho=65 km/sec/Mpc we
obtain a SN Ia rate of 0.21(+0.30)(-0.13) SNu. This is in good agreement with
the value 0.16+/-0.05 SNu recently determined by Capellaro et al. (1997).Comment: 36 pages, 19 figures as extra files, to appear in the A
Hydrodynamical models of type II plateau supernovae
We present bolometric light curves of Type II plateau supernovae obtained using a newly developed, one-dimensional Lagrangian hydrodynamic code with flux-limited radiation diffusion. Using our code we calculate the bolometric light curve and photospheric velocities of SN 1999em, obtaining a remarkably good agreement with observations despite the simplifications used in our calculation. The physical parameters used in our calculation are E = 1.25foe, M = 19 M ⊙, R = 800 R ⊙, and M Ni = 0.056 M ⊙. We find that an extensive mixing of 56Ni is needed in order to reproduce a plateau as flat as that shown by the observations. We also study the possibility to fit the observations with lower values of the initial mass consistently with upper limits that have been inferred from pre-supernova imaging of SN 1999em in connection with stellar evolution models. We cannot find a set of physical parameters that reproduce well the observations for models with pre-supernova mass of ≤12 M ⊙, although models with 14 M ⊙ cannot be fully discarded.Facultad de Ciencias AstronĂłmicas y GeofĂsica
The Hubble Diagram of the Calan/Tololo Type Ia Supernovae and the value of Ho
The Calan/Tololo supernova survey has discovered ~30 Type Ia supernovae out
to z~0.1. Using BVI data for these objects and nearby SNe Ia, we have shown
that there exists a significant dispersion in the intrinsic luminosities of
these objects. We have devised a robust chisquare minimization technique
simultaneously fitting the BVI light curves to parametrize the SN event as a
function of (tb,m, m15(B)) where tb is the time of B maximum, m is the peak BVI
magnitude corrected for luminosity variations, and m15(B) is a single parameter
describing the whole light curve morphology. When properly corrected for
m15(B), SNe Ia prove to be high precision distance indicators,yielding relative
distances with errors 7-10%. The corrected peak magnitudes are used to
construct BVI Hubble diagrams (HD), and with Cepheid distances recently
measured with the HST to four nearby SNe Ia (37C, 72E, 81B, 90N) we derive a
value of the Hubble constant of 63.1+/-3.4 (internal) km/s/Mpc. This value is
~10-15% larger than the value obtained by assuming that SNe Ia are perfect
standard candles. As we have shown in Paper V, there is now strong evidence
that galaxies with younger stellar population appear to host the
slowest-declining, and therefore most luminous SNe Ia. Hence, the use of Pop I
objects such as Cepheids to calibrate the zero point of the SNe Ia HD can
easily bias the results toward luminous SNe Ia, unless the absolute
magnitude-decline relation is taken into account.Comment: 32 pages, figures attached, all tables available, to appear in the
Astronomical Journa
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