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Register a new userThe Carnegie Supernova Project: Analysis of the First Sample of Low-Redshift Type-Ia Supernovae
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Gast\\'on Folatelli
Publication date
2009
fields
Physics
and research's language is
English
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We present the analysis of the first set of low-redshift Type Ia supernovae (SNe Ia) by the Carnegie Supernova Project. Well-sampled, high-precision optical (ugriBV) and near-infrared (NIR; YJHKs) light curves obtained in a well-understood photometric system are used to provide light-curve parameters, and ugriBVYJH template light curves. The intrinsic colors at maximum light are calibrated to compute optical--NIR color excesses for the full sample, thus allowing the properties of the reddening law in the host galaxies to be studied. A low value of Rv~1.7, is derived when using the entire sample of SNe. However, when the two highly reddened SNe in the sample are excluded, a value Galactic standard of Rv~3.2 is obtained. The colors of these two events are well matched by a reddening model due to circumstellar dust. The peak luminosities are calibrated using a two-parameter linear fit to the decline rates and the colors, or alternatively, the color excesses. In both cases, dispersions in absolute magnitude of 0.12--0.16 mag are obtained, depending on the filter-color combination. In contrast to the results obtained from color excesses, these fits give Rv~1--2, even when the two highly reddened SNe are excluded. This discrepancy suggests that, beyond the normal interstellar reddening produced in the host galaxies, there is an intrinsic dispersion in the colors of SNe Ia which is correlated with luminosity but independent of the decline rate. Finally, a Hubble diagram is produced by combining the results of the fits for each filter. The resulting scatter of 0.12 mag appears to be limited by peculiar velocities as evidenced by the strong correlation between the distance-modulus residuals among the different filters. The implication is that the actual precision of SN Ia distances is 3--4%.
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The Carnegie Supernova Project (CSP) is a five-year survey being carried out at the Las Campanas Observatory to obtain high-quality light curves of ~100 low-redshift Type Ia supernovae in a well-defined photometric system. Here we present the first release of photometric data that contains the optical light curves of 35 Type Ia supernovae, and near-infrared light curves for a subset of 25 events. The data comprise 5559 optical (ugriBV) and 1043 near-infrared (YJHKs) data points in the natural system of the Swope telescope. Twenty-eight supernovae have pre-maximum data, and for 15 of these, the observations begin at least 5 days before B maximum. This is one of the most accurate datasets of low-redshift Type Ia supernovae published to date. When completed, the CSP dataset will constitute a fundamental reference for precise determinations of cosmological parameters, and serve as a rich resource for comparison with models of Type Ia supernovae.
This is the first release of optical spectroscopic data of low-redshift Type Ia supernovae (SNe Ia) by the Carnegie Supernova Project including 604 previously unpublished spectra of 93 SNe Ia. The observations cover a range of phases from 12 days before to over 150 days after the time of B-band maximum light. With the addition of 228 near-maximum spectra from the literature we study the diversity among SNe Ia in a quantitative manner. For that purpose, spectroscopic parameters are employed such as expansion velocities from spectral line blueshifts, and pseudo-equivalent widths (pW). The values of those parameters at maximum light are obtained for 78 objects, thus providing a characterization of SNe Ia that may help to improve our understanding of the properties of the exploding systems and the thermonuclear flame propagation. Two objects, namely SNe 2005M and 2006is, stand out from the sample by showing peculiar Si II and S II velocities but otherwise standard velocities for the rest of the ions. We further study the correlations between spectroscopic and photometric parameters such as light-curve decline rate and color. In agreement with previous studies, we find that the pW of Si II absorption features are very good indicators of light-curve decline rate. Furthermore, we demonstrate that parameters such as pW2(SiII4130) and pW6(SiII5972) provide precise calibrations of the peak B-band luminosity with dispersions of ~0.15 mag. In the search for a secondary parameter in the calibration of peak luminosity for SNe Ia, we find a ~2--3-sigma correlation between B-band Hubble residuals and the velocity at maximum light of S II and Si II lines.
We use the spectroscopy and homogeneous photometry of 97 Type Ia supernovae obtained by the emph{Carnegie Supernova Project} as well as a subset of 36 Type Ia supernovae presented by Zheng et al. (2018) to examine maximum-light correlations in a four-dimensional (4-D) parameter space: $B$-band absolute magnitude, $M_B$, ion{Si}{2}~$lambda6355$ velocity, vsi, and ion{Si}{2} pseudo-equivalent widths pEW(ion{Si}{2}~$lambda6355$) and pEW(ion{Si}{2}~$lambda5972$). It is shown using Gaussian mixture models (GMMs) that the original four groups in the Branch diagram are well-defined and robust in this parameterization. We find three continuous groups that describe the behavior of our sample in [$M_B$, vsi] space. Extending the GMM into the full 4-D space yields a grouping system that only slightly alters group definitions in the [$M_B$, vsi] projection, showing that most of the clustering information in [$M_B$, vsi] is already contained in the 2-D GMM groupings. However, the full 4-D space does divide group membership for faster objects between core-normal and broad-line objects in the Branch diagram. A significant correlation between $M_B$ and pEW(ion{Si}{2}~$lambda5972$) is found, which implies that Branch group membership can be well-constrained by spectroscopic quantities alone. In general, we find that higher-dimensional GMMs reduce the uncertainty of group membership for objects between the originally defined Branch groups. We also find that the broad-line Branch group becomes nearly distinct with the inclusion of vsi, indicating that this subclass of SNe Ia may be somehow different from the other groups.
We present a multi-wavelength photometric and spectroscopic analysis of thirteen Super-Chandrasekhar Mass/2003fg-like type Ia Supernova (SNe~Ia). Nine of these objects were observed by the Carnegie Supernova Project. 2003fg-like have slowly declining light curves ($Delta m_{15}$(B) $<$1.3 mag), and peak absolute $B$-band magnitudes between $-19<M_{B}<-21$~mag. Many 2003fg-like are located in the same part of the luminosity width relation as normal SNe~Ia. In the optical $B$ and $V$ bands, 2003fg-like look like normal SNe~Ia, but at redder wavelengths they diverge. Unlike other luminous SNe~Ia, 2003fg-like generally have only one $i$-band maximum which peaks after the epoch of $B$-band maximum, while their NIR light curve rise times can be $gtrsim$40 days longer than those of normal SNe~Ia. They are also at least one magnitude brighter in the NIR bands than normal SNe~Ia, peaking above $M_H < -19$~mag, and generally have negative Hubble residuals, which may be the cause of some systematics in dark energy experiments. Spectroscopically, 2003fg-like exhibit peculiarities such as unburnt carbon well past maximum light, a large spread (8000--12000~km/s) in SiII $lambda$6355 velocities at maximum light with no rapid early velocity decline, and no clear $H$-band break at +10~d, e. We find that SNe with a larger pseudo equivalent width of CII at maximum light have lower SiII $lambda$6355 velocities and slower declining light curves. There are also multiple factors that contribute to the peak luminosity of 2003fg-like. The explosion of a C-O degenerate core inside a carbon-rich envelope is consistent with these observations. Such a configuration may come from the core degenerate scenario.
In this first paper in a series we present 1298 low-redshift (zleq0.2) optical spectra of 582 Type Ia supernovae (SNe Ia) observed from 1989 through 2008 as part of the Berkeley SN Ia Program (BSNIP). 584 spectra of 199 SNe Ia have well-calibrated light curves with measured distance moduli, and many of the spectra have been corrected for host-galaxy contamination. Most of the data were obtained using the Kast double spectrograph mounted on the Shane 3 m telescope at Lick Observatory and have a typical wavelength range of 3300-10,400 Ang., roughly twice as wide as spectra from most previously published datasets. We present our observing and reduction procedures, and we describe the resulting SN Database (SNDB), which will be an online, public, searchable database containing all of our fully reduced spectra and companion photometry. In addition, we discuss our spectral classification scheme (using the SuperNova IDentification code, SNID; Blondin & Tonry 2007), utilising our newly constructed set of SNID spectral templates. These templates allow us to accurately classify our entire dataset, and by doing so we are able to reclassify a handful of objects as bona fide SNe Ia and a few other objects as members of some of the peculiar SN Ia subtypes. In fact, our dataset includes spectra of nearly 90 spectroscopically peculiar SNe Ia. We also present spectroscopic host-galaxy redshifts of some SNe Ia where these values were previously unknown. [Abridged]
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