101 research outputs found
Evidence for a Spectroscopic Sequence Among SNe Ia
In this Letter we present evidence for a spectral sequence among Type Ia
supernovae (SNe Ia). The sequence is based on the systematic variation of
several features seen in the near-maximum light spectrum. This sequence is
analogous to the recently noted photometric sequence among SNe Ia which shows a
relationship between the peak brightness of a SN Ia and the shape of its light
curve. In addition to the observational evidence we present a partial
theoretical explanation for the sequence. This has been achieved by producing a
series of non-LTE synthetic spectra in which only the effective temperature is
varied. The synthetic sequence nicely reproduces most of the differences seen
in the observed one and presumably corresponds to the amount of 56Ni produced
in the explosion.Comment: To appear in Astrophysical Journal Letters, uuencoded, gzipped
postscript file, also available from http://www.nhn.uoknor.edu/~baron
Multi-Color Light Curves of Type Ia Supernovae on the Color-Magnitude Diagram: a Novel Step Toward More Precise Distance and Extinction Estimates
We show empirically that fits to the color-magnitude relation of Type Ia
supernovae after optical maximum can provide accurate relative extragalactic
distances. We report the discovery of an empirical color relation for Type Ia
light curves: During much of the first month past maximum, the magnitudes of
Type Ia supernovae defined at a given value of color index have a very small
magnitude dispersion; moreover, during this period the relation between
magnitude and color (or or color) is strikingly linear, to
the accuracy of existing well-measured data. These linear relations can provide
robust distance estimates, in particular, by using the magnitudes when the
supernova reaches a given color. After correction for light curve strech factor
or decline rate, the dispersion of the magnitudes taken at the intercept of the
linear color-magnitude relation are found to be around 0.08 for the
sub-sample of supernovae with \BVm , and around 0.11 for the
sub-sample with \BVm . This small dispersion is consistent with
being mostly due to observational errors. The method presented here and the
conventional light curve fitting methods can be combined to further improve
statistical dispersions of distance estimates. It can be combined with the
magnitude at maximum to deduce dust extinction. The slopes of the
color-magnitude relation may also be used to identify intrinsically different
SN Ia systems. The method provides a tool that is fundamental to using SN Ia to
estimate cosmological parameters such as the Hubble constant and the mass and
dark energy content of the universe.Comment: ApJ, in pres
Cepheid Calibration of the Peak Brightness of SNe Ia. X. SN 1991T in NGC 4527
Repeated imaging observations have been made of NGC 4527 with the Hubble
Space Telescope between April and June 1999, over an interval of 69 days.
Images were obtained on 12 epochs in the F555W band and on five epochs in the
F814W band. The galaxy hosted the type Ia supernova SN1991T, which showed
relatively unusual behavior by having both an abnormal spectrum near light
maximum, and a slower declining light curve than the proto-typical Branch
normal SNe Ia.
A total of 86 variables that are putative Cepheids have been found, with
periods ranging from 7.4 days to over 70 days. From photometry with the DoPHOT
program, the de-reddened distance modulus is determined to be (m-M)_0 = 30.67
+/- 0.12 (internal uncertainty) using a subset of the Cepheid data whose
reddening and error parameters are secure. A parallel analysis of the Cepheids
using photometry with ROMAFOT yields (m -M)_0 =30.82 +/- 0.11. The final
adopted modulus is (m -M)_0 =30.74 +/- 0.12 +/- 0.12 (d=14.1 +/- 0.8 +/- 0.8
Mpc).
The photometric data for SN1991T are used in combination with the Cepheid
distance to NGC 4527 to obtain the absolute magnitude for this supernova of
M_V^0(max) = -19.85 +/- 0.29. The relatively large uncertainty is a result of
the range in estimates of the reddening to the supernova. Thus SN1991T is seen
to be only moderately brighter (by ~ 0.3 mag) than the mean for
spectroscopically normal supernovae, although magnitude differences of up to
0.6 mag cannot be ruled out.Comment: 46 pages, LATEX using aaspp4.sty, including 9 embedded tables, 19
figures (gif and jpg files), a full-resolution version (ps files) is
available at http://www.astro.unibas.ch/forschung/ll/cepheid.shtml, accepted
for publication in the Astrophysical Journa
The Distribution of High and Low Redshift Type Ia Supernovae
The distribution of high redshift Type Ia supernovae (SNe Ia) with respect to
projected distance from the center of the host galaxy is studied and compared
to the distribution of local SNe. The distribution of high-z SNe Ia is found to
be similar to the local sample of SNe Ia discovered with CCDs, but different
than the sample discovered photographically. This is shown to be due to the
Shaw effect. These results have implications for the use of SNe Ia to determine
cosmological parameters if the local sample of supernovae used to calibrate the
light curve decline relationships is drawn from a sample discovered
photographically. A K-S test shows that the probability that the high redshift
SNe of the Supernova Cosmology Project are drawn from the same distribution as
the low redshift calibrators of Riess et al. is 0.1%. This is a potential
problem because photographically discovered SNe are preferentially discovered
farther away from the galaxy nucleus, where SNe show a lower scatter in
absolute magnitude, and are on average 0.3 magnitudes fainter than SNe located
closer to the center of their host galaxy. This raises questions about whether
or not the calibration SNe sample the full range of parameters potentially
present in high redshift SNe Ia. The limited data available suggest that the
calibration process is adequate; however, it would be preferable if high
redshift SNe and the low redshift SNe used to calibrate them were drawn from
the same sample, as subtle differences may be important. Data are also
presented which suggest that the seeming anti-Malmquist trend noticed by
Tammann et al.(1996, 1998) for SNe Ia in galaxies with Cepheid distances may be
due to the location of the SNe in their host galaxies.Comment: 16 pages, 2 figures Accepted for publication in the Astrophysical
Journa
The Hubble constant from Co-powered Nebular Candles
Type Ia supernovae (SNe Ia), produced by the thermonuclear explosion of white
dwarf (WD) stars, are used here to derive extragalactic distances and an
estimate of the Hubble constant from their emission signatures at late phases
({\it Nebular SNe Ia Method}, NSM). The method, first developed in
Ruiz--Lapuente \& Lucy (1992), makes use here of an improved modeling of the
forbidden line emission at late phases. Hydrodynamic models of the explosion of
WDs of different masses, both sub--Chandrasekhar and Chandrasekhar, provide the
basis for comparison with the observations. It is shown that it is possible to
probe the overall density structure of the ejecta and the mass of the exploding
WD by the effect that the electron density profile has in shaping the forbidden
line emission of the iron ions, and that a robust diagnostic of the mass of the
exploding WD can be obtained. Cosmic distance scale can thus be related to
basic diagnostics of excitation of iron lines. Once the most adequate model is
selected, comparison of the predicted line emission at these phases with the
observed spectra gives an internal estimate of both the reddening and the
distance to the SNe Ia. The results presented here favor denser models than
those corresponding to sub--Chandrasekhar explosions. From a sample of seven
SNe Ia in Leo, Virgo, Fornax and beyond, a value of the Hubble constant is derived. The depth
of the Virgo cluster is found to be large, ranging from 13 to 23 Mpc at least.
If NGC 4526 traces well the core of the Virgo Cluster, then the latter is
located at . The galaxy NGC 3267 in Leo appears to be located at
9.8 1.5 Mpc.Comment: 17 pages, including 2 figures. uuencoded, gzipped ps file. Submitted
to the ApJ (Letters
Diversity of Decline-Rate-Corrected Type Ia Supernova Rise Times: One Mode or Two?
B-band light-curve rise times for eight unusually well-observed nearby Type
Ia supernovae (SNe) are fitted by a newly developed template-building
algorithm, using light-curve functions that are smooth, flexible, and free of
potential bias from externally derived templates and other prior assumptions.
From the available literature, photometric BVRI data collected over many
months, including the earliest points, are reconciled, combined, and fitted to
a unique time of explosion for each SN. On average, after they are corrected
for light-curve decline rate, three SNe rise in 18.81 +- 0.36 days, while five
SNe rise in 16.64 +- 0.21 days. If all eight SNe are sampled from a single
parent population (a hypothesis not favored by statistical tests), the rms
intrinsic scatter of the decline-rate-corrected SN rise time is 0.96 +0.52
-0.25 days -- a first measurement of this dispersion. The corresponding global
mean rise time is 17.44 +- 0.39 days, where the uncertainty is dominated by
intrinsic variance. This value is ~2 days shorter than two published averages
that nominally are twice as precise, though also based on small samples. When
comparing high-z to low-z SN luminosities for determining cosmological
parameters, bias can be introduced by use of a light-curve template with an
unrealistic rise time. If the period over which light curves are sampled
depends on z in a manner typical of current search and measurement strategies,
a two-day discrepancy in template rise time can bias the luminosity comparison
by ~0.03 magnitudes.Comment: As accepted by The Astrophysical Journal; 15 pages, 6 figures, 2
tables. Explanatory material rearranged and enhanced; Fig. 4 reformatte
Tycho Brahe's supernova: light from centuries past
The light curve of SN 1572 is described in the terms used nowadays to
characterize SNeIa. By assembling the records of the observations done in
1572--74 and evaluating their uncertainties, it is possible to recover the
light curve and the color evolution of this supernova. It is found that, within
the SNe Ia family, the event should have been a SNIa with a normal rate of
decline, its stretch factor being {\it s} 0.9. Visual light curve near
maximum, late--time decline and the color evolution sustain this conclusion.
After correcting for extinction, the luminosity of this supernova is found to
be M --19.58 --5 log (D/3.5 kpc) 0.42.Comment: 28 pages, 3 figures, 3 tables. submitted to ApJ (Main Journal
Cepheid Calibration of the Peak Brightness of SNe Ia -- IX. SN 1989B in NGC 3627
(Abridged) Repeated imaging observations have been made of NGC 3627 with the
HST in 1997/98, over an interval of 58 days. Images were obtained on 12 epochs
in the F555W band and on five epochs in the F814W band. The galaxy hosted the
prototypical, `Branch normal', type Ia supernova SN 1989B. A total of 83
variables have been found, of which 68 are definite Cepheid variables with
periods ranging from 75 days to 3.85 days. The de-reddened distance modulus is
determined to be (m-M)_0= 30.22+/-0.12 (internal uncertainty) using a subset of
the Cepheid data whose reddening and error parameters are secure.
The photometric data of Wells et al. (1994), combined with the Cepheid data
for NGC 3627 give M_B(max)= -19.36+/-0.18 and M_V(max)= -19.34+/-0.16 for SN
1989B. Combined with the previous six calibrations in this program, plus two
additional calibrations determined by others gives the mean absolute magnitudes
at maximum of = -19.48+/-0.07 and = -19.48 +/-0.07 for `Branch
normal' SNe Ia at this interim stage in the calibration program.
The second parameter correlations of M(max) of blue SNe Ia with decay rate,
color at maximum, and Hubble type are re-investigated. The dependence of
on decay rate is non-linear, showing a minimum for decay rates between
1.0< Delta m_15 <1.6. Magnitudes corrected for decay rate show no dependence on
Hubble type, but a dependence on color remains. Correcting both the fiducial
sample of 34 SNe Ia with decay-rate data and the current 8 calibrating SNe Ia
for the correlation with decay rate as well as color gives H_0= 60+/-2
(internal) km/s/Mpc, in both B and V. The same value to within 4% is obtained
if only the SNe Ia in spirals (without second parameter corrections) are
considered.Comment: 32 pages (with 7 tables and 14 figures) LaTeX, uses emulateapj.sty; a
full-resolution version with complete figs. 4 and 5 is available at
http://www.astro.unibas.ch/cosmology/papers.html ; accepted for publication
in Ap
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