# The Absolute Magnitudes of 1991T-like Supernovae111This paper includes data
gathered with the 6.5 meter Magellan telescopes at Las Campanas Observatory,
Chile.

M. M. Phillips Carnegie Observatories, Las Campanas Observatory, Casilla 601,
La Serena, Chile  C. Ashall Department of Physics, Virginia Polytechnic
Institute and State University, 850 West Campus Drive, Blacksburg, VA 24061,
USA  Christopher R. Burns Observatories of the Carnegie Institution for
Science, 813 Santa Barbara St., Pasadena, CA 91101, USA  Carlos Contreras
Carnegie Observatories, Las Campanas Observatory, Casilla 601, La Serena,
Chile  L. Galbany Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer
de Can Magrans, s/n, E-08193 Barcelona, Spain.  Institut d’Estudis Espacials
de Catalunya (IEEC), E-08034 Barcelona, Spain.  P. Hoeflich Department of
Physics, Florida State University, 77 Chieftan Way, Tallahassee, FL 32306, USA
E. Y. Hsiao Department of Physics, Florida State University, 77 Chieftan Way,
Tallahassee, FL 32306, USA  Nidia Morrell Carnegie Observatories, Las Campanas
Observatory, Casilla 601, La Serena, Chile  Peter Nugent Department of
Astronomy, University of California, Berkeley, USA  Lawrence Berkeley National
Laboratory, USA  Syed A. Uddin George P. and Cynthia Woods Mitchell Institute
for Fundamental Physics and Astronomy, Texas A&M University, Department of
Physics and Astronomy, College Station, TX 77843, USA  E. Baron University of
Oklahoma 440 W. Brooks, Rm 100, Norman, Oklahoma, 73019, USA  Wendy L.
Freedman Department of Astronomy and Astrophysics, University of Chicago, 5640
S. Ellis Ave, Chicago, IL 60637, USA  Chelsea E. Harris Center for Data
Intensive and Time Domain Astronomy, Department of Physics and Astronomy,
Michigan State University, East Lansing, MI 48824, USA  Kevin Krisciunas
George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and
Astronomy, Texas A&M University, Department of Physics and Astronomy, College
Station, TX 77843, USA  S. Kumar Department of Physics, Florida State
University, 77 Chieftan Way, Tallahassee, FL 32306, USA  J. Lu Department of
Physics, Florida State University, 77 Chieftan Way, Tallahassee, FL 32306, USA
S. E. Persson Observatories of the Carnegie Institution for Science, 813 Santa
Barbara St., Pasadena, CA 91101, USA  Anthony L. Piro Observatories of the
Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101,
USA  Abigail Polin Observatories of the Carnegie Institution for Science, 813
Santa Barbara St., Pasadena, CA 91101, USA  TAPIR, Walter Burke Institute for
Theoretical Physics, Caltech, 1200 East California Boulevard, Pasadena, CA
91125, USA  Shahbandeh, M. Department of Physics, Florida State University, 77
Chieftan Way, Tallahassee, FL 32306, USA  Maximilian Stritzinger Department of
Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C,
Denmark  Nicholas B. Suntzeff George P. and Cynthia Woods Mitchell Institute
for Fundamental Physics and Astronomy, Texas A&M University, Department of
Physics and Astronomy, College Station, TX 77843, USA

###### Abstract

1991T-like supernovae are the luminous, slow-declining extreme of the Branch
shallow-silicon (SS) subclass of Type Ia supernovae. They are distinguished by
extremely weak Ca II H & K and Si II $\lambda 6355$ and strong Fe III
absorption features in their optical spectra at pre-maximum phases, and have
long been suspected to be over-luminous compared to normal Type Ia supernovae.
In this paper, the pseudo equivalent width of the Si II $\lambda$6355
absorption obtained at light curve phases from $\leq+10$ days is combined with
the morphology of the $i$-band light curve to identify a sample of 1991T-like
supernovae in the Carnegie Supernova Project II. Hubble diagram residuals show
that, at optical as well as near-infrared wavelengths, these events are over-
luminous by $\sim$0.1–0.5 mag with respect to the less extreme Branch SS
(1999aa-like) and Branch core-normal supernovae with similar $B$-band light
curve decline rates.

Type Ia supernovae (1728), Supernovae (1668), Observational cosmology (1146)

††journal: ApJ††facilities: Magellan:Baade (IMACS imaging spectrograph,
FourStar wide-field near-infrared camera, Magellan:Clay (LDSS3 imaging
spectrograph), Swope (SITe3 CCD imager, e2v 4K x 4K CCD imager), du Pont
(SITe2 CCD imager, Tek5 CCD imager, WFCCD imaging spectrograph, RetroCam near-
infrared imager), NOT (ALFOSC), La Silla-QUEST, CRTS, PTF, iPTF, OGLE, ASAS-
SN, PS1, KISS, ISSP, MASTER, SMT)††software: IRAF(Tody, 1986, 1993), SNooPy
(Burns et al., 2011)

##  1 Introduction

Type Ia supernovae (SNe Ia) provide a powerful tool for modern cosmology.
Although they are not perfect standard candles, the luminosities and colors of
SNe Ia vary smoothly with light curve width (Phillips, 1993; Hamuy et al.,
1996; Riess et al., 1996; Phillips et al., 1999; Burns et al., 2014, 2018)
making it possible to determine reliable distances to a precision of 5–10%,
especially when near-infrared (NIR) photometry is available (e.g., Krisciunas
et al., 2004; Phillips, 2012; Burns et al., 2018; Avelino et al., 2019).
Observations of SNe Ia have provided the most precise local measurements of
the Hubble constant to date (e.g., Riess et al., 2016; Dhawan et al., 2018;
Burns et al., 2018; Freedman et al., 2019; Riess et al., 2021; Uddin et al.,
2022) and led to the discovery of the accelerated expansion of the Universe
(Riess et al., 1998; Perlmutter et al., 1999).

At the luminous extreme of the absolute magnitude versus light-curve width
relation for SNe Ia are the slow-declining 1991T-like (henceforth “91T-like”)
events, named after the well-observed SN 1991T (Filippenko et al., 1992;
Phillips et al., 1992; Ruiz-Lapuente et al., 1992; Jeffery et al., 1992;
Mazzali et al., 1995; Gómez et al., 1996; Lira et al., 1998; Blondin et al.,
2012; Silverman et al., 2012a). At pre-maximum phases, this supernova (SN)
displayed strong absorption features of Fe III at optical wavelengths, whereas
the Si II, Ca II, and S II features that typify normal SNe Ia222In this paper,
a “normal SN Ia” refers to the “core-normal” definition of Branch et al.
(2006). spectra at these epochs were extremely weak or missing altogether. By
two weeks after maximum light, however, the spectrum of SN 1991T was
essentially that of a normal Type Ia event. The decline rate of the $B$-band
light curve was measured to be $\Delta{\rm m}_{15}(B)$ $=0.95\pm 0.05$
mag333$\Delta{\rm m}_{15}(B)$ is defined as the amount in magnitudes that the
SN fades in the first 15 days following the epoch of $B$-band maximum
(Phillips, 1993). (Lira et al., 1998), and the absolute magnitude, $M(B)$, at
maximum may have been as much as 0.5 mag brighter than those of normal SNe Ia
with similar decline rates (Mazzali et al., 1995; Saha et al., 2001). Like
other slow-declining SNe Ia, the 91T-like SNe are associated with star-forming
galaxies (e.g, see Hamuy et al., 2000).

A few years after the discovery of SN 1991T, Nugent et al. (1995) demonstrated
that the luminosity versus decline rate relationship for SNe Ia was
accompanied by a parallel sequence of smoothly varying spectral features at
maximum light. These spectral variations were ascribed to differences in
temperature corresponding to the amount of 56Ni produced in the explosion,
with 91T-like events representing the highest temperatures (largest 56Ni
masses). The discovery of SN 1999aa (Filippenko et al., 1999), which showed a
pre-maximum spectral evolution intermediate between that of SN 1991T and
normal SNe Ia (Li et al., 2001b; Garavini et al., 2004), lent credence to the
idea that 91T-like events are simply extreme examples of normal SNe Ia
(Branch, 2001; Garavini et al., 2004).

Figure 1 displays spectra at three different phases of SN 1991T, SN 1999aa,
and the normal SNe 1999ee and 2011fe (Hamuy et al., 2002b; Stritzinger et al.,
2002). At pre-maximum epochs, the spectrum of SN 1991T is remarkable for the
extreme weakness of the Ca II H & K and Si II $\lambda 6355$ absorption
features, which are clearly visible in the spectra of SN 1999ee and SN 2011fe,
although in the case of the day $-9$ spectrum of SN 1999ee, the Ca II and Si
II absorptions are dominated by a high-velocity component (Mazzali et al.,
2005). At these pre-maximum phases, the Ca II and Si II absorptions in the
pre-maxima spectra of SN 1999aa are seen to be intermediate in strength
between SN 1991T and SN 1999ee. At two weeks past maximum, however, the
spectra of all four SNe show only minor differences. Note that the $\Delta{\rm
m}_{15}(B)$ values reported in the literature for SN 1999aa ($0.85\pm 0.08$;
Jha et al., 2006) and SN 1999ee ($0.94\pm 0.06$; Stritzinger et al., 2002) are
similar to within the errors to that of SN 1991T, while SN 2011fe had a
slightly faster decline rate of $1.10\pm 0.04$ (Pereira et al., 2013).

Figure 1: Optical spectra of SN 1991T obtained at $-8$ days, $-3$ days, and
$+14$ days with respect to the time of maximum light in the $B$-band. Spectra
of SN 1999aa, SN 1999ee, and SN 2011fe at similar phases are plotted for
comparison. The spectra for SN 1991T are taken from Phillips et al. (1992),
those of SN 1999aa from Blondin et al. (2012), those of SN 1999ee from Hamuy
et al. (2002b), and those of SN 2011fe from Pereira et al. (2013). Each
spectrum was corrected for Galactic reddening using the the Schlafly &
Finkbeiner (2011) recalibration of the Schlegel, Finkbeiner & Davis (1998)
infrared dust maps. In addition, based on SNooPy (Burns et al., 2011) fits to
the photometry using the color versus decline rate calibration of Burns et al.
(2014), corrections for host galaxy reddenings of $E(B-V)_{host}=0.15$ and
$0.31$ mag were applied to the spectra of SN 1991T and SN 1999ee,
respectively, while $E(B-V)_{host}$ was assumed to be zero for both SN 1999aa
and SN 2011fe.

Because they are among the most luminous SNe Ia, 91T-like events will
inevitably be included in distant samples used to study cosmology. A better
understanding of these objects is therefore crucial not only to deciphering
the progenitors and explosion mechanism, but also to avoid possible systematic
errors in cosmological samples of SNe Ia. In this paper, we examine the
absolute magnitudes of a sample of 91T-like SNe observed during the course of
the second phase of the Carnegie Supernova Project (CSP-II; Phillips et al.,
2019). In §2, a new method for classifying them based on photometry and one or
more optical spectra taken between $-10$ and $+10$ days with respect to the
epoch of $B$ maximum is presented. In §3, this technique is employed to
identify ten 91T-like SNe observed by CSP-II. A brief description of the
optical spectral evolution of 91T-like SNe is also included in this section.
Next, in §4, the optical and near-infrared (NIR) Hubble diagram residuals of
the 91T-like events in the CSP-II sample are compared to 1999aa-like
(henceforth “99aa-like”) and normal SNe Ia with similar decline rates.
Finally, in §5, our conclusions are summarized.

##  2 Identifying 91T-like Supernovae

Branch et al. (2006) devised a system for classifying SNe Ia into four
subtypes based on the pseudo-equivalent widths (pEW) of the Si II
$\lambda\lambda 5972,6355$ absorption features near maximum light. Figure 2a
shows this “Branch diagram” for SNe Ia observed spectroscopically by the CfA
Supernova Group (Blondin et al., 2012) and the first phase of the Carnegie
Supernova Project (CSP-I; Folatelli et al., 2013). Branch et al. (2009)
divided the diagram into four areas delineated approximately by the dashed
lines drawn in Figure 2a: CN for “core normal”, SS for “shallow silicon”, BL
for “broad line”, and CL for “cool”. Blondin et al. (2012) and Folatelli et
al. (2013) divided up the four areas in a slightly different manner; in this
paper, we adopt the boundaries of Folatelli et al. which are indicated by the
colors of the symbols in Figure 2a. A recent cluster analysis of the Branch
diagram by Burrow et al. (2020) using Gaussian mixture models concluded that
the general division into the CN, SS, BL, and CL groups was robust and that,
when Si II velocity information is included in the analysis, the BL group
appears to be distinct with respect to the other three groups.

The positions of SN 1991T and SN 1999aa are shown in Figure 2a. Both fall
clearly in the SS portion of the Branch diagram, with SN 1991T having one of
the smallest values of pEW(Si II $\lambda 6355$) of any of the SNe plotted.
The positions of the CN SNe 1999ee and 2011fe in the diagram are also shown
for reference.

Figure 2: (a) Branch diagram for SNe Ia observed by the CfA Supernova Group
and the CSP-I. The dashed lines approximate the boundaries between the
different subtypes adopted by Branch et al. (2009) (See Figure 10 of Silverman
et al., 2012b)), while the different colors correspond to the boundaries
assumed by Folatelli et al. (2013), which are similar to those used by Blondin
et al. (2012); (b) The light curve decline rate parameter, $\Delta{\rm
m}_{15}(B)$, plotted versus the pseudo-equivalent width of the Si II $\lambda
6355$ absorption for the Branch SS (green triangles) and CN (black circles)
subtypes in the CfA Supernova Group and the CSP-I samples. In this paper, we
refer to CN events lying below the red horizontal line ($\Delta{\rm
m}_{15}(B)$ $<1.1$) as the “slow-declining Branch CN” SNe (see §4).

In Figure 2b, the decline rate parameter, $\Delta{\rm m}_{15}(B)$, is plotted
versus pEW(Si II $\lambda 6355$) for the SS and CN SNe in the CfA and CSP I
samples. The measurements for the CfA sample are taken from Blondin et al.
(2012), while those for the CSP-I are from Folatelli et al. (2013). Points
corresponding to SN 1991T, SN 1999aa, SN 1999ee, and SN 2011fe are indicated.
Not surprisingly, this diagram resembles the region of the Branch diagram
occupied by the SS and CN SNe since pEW(Si II $\lambda 5972$) is known to
correlate strongly with $\Delta{\rm m}_{15}(B)$ (Hachinger et al., 2008;
Folatelli et al., 2013). This plot clearly demonstrates that 91T-like SNe, and
SS events in general, cannot be distinguished from many CN events on the basis
of their decline rates alone.

The Branch diagram is an excellent tool for identifying 91T-like SNe, but it
requires that an optical spectrum be obtained within a few days of maximum
light. Often this is not the case, and so we have developed an alternative
method based on the evolution of pEW(Si II $\lambda 6355$). Figure 3a shows
our measurements of pEW(Si II $\lambda 6355$) plotted as a function of phase
with respect to the epoch of $B$ maximum, $t-t(B_{max})$, for the CN and SS
SNe in the CfA and CSP-I spectroscopic samples. Again, the measurements for SN
1991T, SN 1999aa, SN 1999ee, and SN 2011fe are highlighted for reference. For
the SS SNe, pEW(Si II $\lambda 6355$) is observed to increase approximately
monotonically between $-15$ d $\lesssim t-t(B_{max})\lesssim+3$ d, and then
stays roughly constant to $+10$ d. The trajectory of SN 1991T lies at the
extreme lower edge of the distribution of SS events, while that of SN 1999aa
sits more or less in the middle of the SS SNe. The evolution of pEW(Si II
$\lambda 6355$) for the CN objects contrasts with that of the SS subtype in
that events caught early enough display a rapid initial decline in equivalent
width until $t-t(B_{max})\sim-8$ d, followed by a period of slowly increasing
values that extends to $t-t(B_{max})\sim+3$ d, and then a final slow decline
to $t-t(B_{max})=+10$ d. Inspection of the CN spectra obtained during the
initial sharp decline in pEW(Si II $\lambda 6355$) indicates that this
phenomenon is due to the presence of high-velocity Si II absorption at the
earliest epochs, which rapidly decays in strength. Note that this behavior is
not observed for the SS SNe, implying either an absence of high-velocity Si
and Ca or higher ionization conditions in the outermost ejecta.

Figure 3: (a) Evolution of the pEW(Si II $\lambda 6355$) for SS (green dotted
lines) and CN (black dotted lines) SNe from the Blondin et al. (2012) and
Folatelli et al. (2013) spectroscopic samples. Measurements for SN 1991T, SN
1999aa, SN 1999ee, and SN 2011fe are highlighted. The spectra for SN 1991T are
from Phillips et al. (1992) and Mazzali et al. (1995), those of SN 1999aa are
from Garavini et al. (2004) and Blondin et al. (2012), those of SN 1999ee are
from Hamuy et al. (2002b) and Blondin et al. (2012), and those of SN 2011fe
are from Pereira et al. (2013). Also included are measurements for the 02cx-
like SN2005hk, and for the 03fg-like event SN2007if. The spectra for SN2005hk
are from Phillips et al. (2007) and Blondin et al. (2012), and those for
SN2007if are from Scalzo et al. (2010) and Blondin et al. (2012).; (b)
Evolution of the pseudo equivalent widths of the Ca II H & K and Si II
$\lambda 6355$ absorption features as a function of light curve phase with
respect to $t(B_{max})$ for SN 1991T, SN 1999aa, SN 1999ee, and SN 2011fe.
Each point in the diagram represents a spectrum, with a selection of the
points labelled with the light curve phase.

In principle, the pseudo equivalent width of the Ca II H & K absorption
feature can also be used to identify 91T-like SNe. This is illustrated in
Figure 3b where pEW(Ca II H & K) is plotted versus pEW(Si II $\lambda 6355$)
for light curve epochs spanning the range from $-10$ d $\leq
t-t(B_{max})\leq+10$ d for SN 1991T, SN 1999aa, SN 1999ee, and SN 2011fe. The
phases of selected spectra are labelled to indicate how the pseudo equivalent
width measurements evolve as a function of time. The trajectories of the
points for SN 1991T and SN 1999aa are seen to be similar but offset, with SN
1991T always displaying weaker Si II $\lambda 6355$ and Ca II H & K absorption
when compared with spectra at the same phases as SN 1999aa. This effect is
clearly seen in Figure 1 where the spectrum of SN 1999aa at $-8$ days is seen
to be virtually identical to the spectrum of SN 1991T at $-3$ days. The
trajectories of the CN SNe 1999ee and 2011fe are considerably more complex and
different – not just with respect to SN 1991T and SN 1999aa, but also one from
the other. Some of these differences are attributable to the presence of high-
velocity Ca II at pre-maximum phases, which is particularly strong in SN
1999ee (see Figure 1 and Mazzali et al., 2005).

The pseudo equivalent widths for the Si II $\lambda 6355$ and Ca II H & K
features were measured by defining a straight “continuum” level between the
two flux peaks to either side of the absorption as illustrated in Figure 4 of
Folatelli et al. (2013). Errors estimated using the IRAF444IRAF is distributed
by the National Optical Astronomy Observatory, which is operated by the
Association of Universities for Research in Astronomy (AURA) under cooperative
agreement with the National Science Foundation. splot task were typical 20–30%
for the weakest features and 5% or less for stronger lines. However,
equivalent width measurements are susceptible to additional unaccounted errors
in the subtraction of host galaxy light from the SN spectrum. This effect
likely explains some of the “jaggedness” in the temporal evolution of the
pEW(Si II $\lambda 6355$) measurements of the CN and SS SNe in Figure 3a.
These errors can make it difficult in some cases to correctly separate
91T-like SNe from 99aa-like events.

In this paper, we use Figure 3a to identify true 91T-like SNe. However,
certain peculiar SNe Ia can masquerade as 91T-like events if photometric
information is not also available. In particular, the pre-maximum spectra of
02cx-like SNe (also known as “Type Iax” events; Foley et al., 2013) are
likewise dominated by Fe III absorption lines, but at much lower expansion
velocities than normal SNe Ia (Li et al., 2003). Figure 3 shows that the time
evolution of the pEW(Si II $\lambda 6355$) measurements for the well-observed
02cx-like event, SN2005hk, closely mimic those of SN1991T. However, the 02cx-
like events are sub-luminous compared to 91T-like SNe and, as opposed to
91T-like and normal SNe Ia, their $i$/$I$-band and NIR light curves peak after
$t(B_{max})$ and do not show a secondary maximum (Li et al., 2003; Phillips et
al., 2007; Ashall et al., 2020). Certain 03fg-like SNe555These SNe are
commonly referred to in the literature as “Super-Chandrasekhar” events.
However, since the latter name presumes knowledge of the progenitors which, to
date, has not been definitively proved, we will refer to them as “03fg-like”
SNe, after the first recognized example (Howell et al., 2006). can also
display 91T-like spectra at pre-maximum phases, the best example being the
well-studied SN2007if (Scalzo et al., 2010; Yuan et al., 2010). The evolution
of pEW(Si II $\lambda 6355$) for SN2007if is also plotted in Figure 3a. Again,
such objects can be discriminated from 91T-like SNe from their exceptionally
broad $i$/$I$-band and NIR light curves that peak after $t(B_{max})$ and lack
of a clear secondary maximum (Ashall et al., 2020, 2021).

To summarize, we employ both spectroscopic and photometric requirements for
classifying a SN as a 91T-like event, which we define as follows:

  * •

Spectroscopic: At least one pEW(Si II $\lambda 6355$) measurement before 10
days after $B$ maximum that is consistent with the trajectory of SN 1991T in
Figure 3a. In practical terms, this translates to pEW(Si II $\lambda 6355$)
$<$ $\sim$20 Å at $t-t(B_{max})=-10$ d, $<$ than $\sim$40 Å at maximum, and
$<$ $\sim$50 Å at $t-t(B_{max})=+10$ d.

  * •

Photometric: An $i$/$I$-band light curve that reaches maximum before the epoch
of $B$ maximum and displays a clear secondary maximum.

Before ending this section, it is important to emphasize that programs
providing automated spectral classification such as SNID (Blondin & Tonry,
2007) make little distinction between 91T-like and 99aa-like SNe. Indeed, the
standard SNID template subgroups are “Ia-norm”, “Ia-91T”, “Ia-91bg”, “Ia-csm”,
and “Ia-pec”. Hence, many SNe that SNID classifies as “Ia-91T” are actually
“99aa-like” events.

##  3 91T-like Supernovae in the CSP-II Sample

Using the above-described criteria, we have identified ten 91T-like events
observed by the CSP-II. The CSP-II was carried out between 2011 October and
2015 May, during which high-quality optical and NIR light curves were obtained
for 214 SNe Ia, 125 of which were located in the smooth Hubble flow at
redshifts $0.027<z<0.137$ (Phillips et al., 2019). These data are being
published separately in the final data release of CSP-II photometry (Suntzeff
et al., 2022). Details concerning these ten 91T-like SNe are given in Appendix
A, including plots of the temporal evolution of pEW(Si II $\lambda 6355$) and
the individual light curves.

Figure 4: Optical spectra of the ten 91T-like SNe in the CSP-II sample
obtained $-8$ to $+7$ days with respect to the time of maximum light in the
$B$-band. Spectra of SN1991T at three phases covering this range are plotted
in black for comparison. Where necessary, the spectra have been adjusted via a
low-order polynomial to match the photometry. They have also been corrected
for Galactic reddening using the Schlafly & Finkbeiner (2011) recalibration of
the Schlegel, Finkbeiner & Davis (1998) infrared-based dust map and assuming
$R_{V}=3.1$. Unremoved telluric absorption features are marked with an
$\oplus$ symbol.

Selected optical spectra of the 91T-like SNe identified in the CSP-II sample
are found in Figure 4. This figure illustrates that the spectra of events
obtained at similar phases are quite similar overall. The largest differences
are in the slopes (i.e., the colors) of the spectra, which are consistent with
differences in host galaxy reddening. At pre-maximum epochs, the spectra are
dominated by the Fe III $\lambda$4404 and $\lambda$5129 absorption features.
Note, however, that the overall strengths of the Fe III lines can vary
considerably from object to object. A prime example of this is illustrated in
Figure 4, where the spectrum of SN2014dl is observed to have very strong Fe
III absorption at $-7.8$ days, while these same lines are nearly washed out in
the spectra of CSS130303:105206-133424 at a phase of $-7.9$ days and
OGLE-2014-SN-141 at $-7.7$ days. In between these two extremes is the spectrum
of ASASSN-14kd obtained at $-7.1$ days. As noted by Sasdelli et al. (2014),
the Fe III absorption observed at pre-maximum epochs in 91T-like SNe must be
produced mostly by stable 54Fe since 56Co would not have had enough time since
explosion to decay to 56Fe. These authors concluded that the amount of stable
Fe required to produce the observed Fe III absorption in SN 1991T is
consistent with solar metalicity material in the progenitor. It is tempting to
speculate that the observed variations in the strength of the Fe III
absorption at pre-maximum epochs could reflect differences in the
metallicities of the progenitors. It seems unlikely that this is an ionization
effect since the strength of the Si III $\lambda$4564 absorption (see below)
shows less variation than Fe III $\lambda$4404 in spite of the very similar
ionization potentials of Fe++ and Si++. And while differing amounts of host
galaxy light contamination in the spectra might produce such variations, there
is no evidence to support this, particularly in the case of
CSS130303:105206-133424 which exploded in the outer region of its faint host.

Other weaker absorption features visible between and redward of the Fe III
$\lambda\lambda$4404,5129 lines at these early phases (see top panel of Figure
1) have been ascribed to transitions of Ni III (Ruiz-Lapuente et al., 1992;
Mazzali et al., 1995; Fisher et al., 1999; Hatano et al., 2002). The $-8.9$
days spectrum of SN 1991T plotted in Figure 4 shows a strong absorption
feature at 3200 Å which, according to Sasdelli et al. (2014), is a blend of Co
III and Fe III lines, with the Co arising from the decay of 56Ni. These
authors concluded that Ni, Co, and Fe must be present to velocities as high as
$\sim$17,000 ${\rm km~{}s}^{-1}$ to match the shape of the spectral features
produced by these elements.

Absorption due to multiplet 2 of Si III ($\lambda_{0}\simeq 4564$ Å) is
present in the pre-maximum spectra of 91T-like events beginning at the
earliest epochs (Jeffery et al., 1992; Mazzali et al., 1995). However, Si II
$\lambda$6355 absorption does not become clearly visible until approximately a
week before maximum light, and Ca II H & K and the iconic “W” absorption
feature of S II at $\sim$5400 Å are undetectable until only a few days before
maximum light. The presence of Si III absorption early on demonstrates that
intermediate-mass elements are present in the outermost layers of 91T-like
SNe, but are largely invisible due to the unusually high radiation
temperatures at these phases (Mazzali et al., 1995; Nugent et al., 1995;
Sasdelli et al., 2014). This may also account for the lack of observable high-
velocity Ca II and Si II in the outer ejecta of 99aa-like and 91T-like SNe.

As mentioned in §1 and illustrated in the bottom panel of Figure 1, by two
weeks after maximum, the optical spectra of 91T-like SNe begin to closely
resemble those of normal SNe Ia. P-Cygni lines of Ca II H & K and the
$\lambda\lambda$8498,8542,8662 triplet are clearly visible, and Fe II features
begin to dominate much of the spectrum (e.g., see Figure 10 of Jeffery et al.,
1992). Broad absorption is present at $\sim$7500 Å (see Figure 5 of Filippenko
et al., 1992) which several authors have identified with O I $\lambda$7774
(Filippenko et al., 1992; Jeffery et al., 1992; Sasdelli et al., 2014).
Unfortunately, at these phases, the strength of the Si II absorption is no
longer a useful tool for separating 91T-like events from 99aa-like and slow-
declining Branch CN SNe.

##  4 Absolute Magnitudes

As highlighted in the introduction to this paper, and also summarized by
Taubenberger (2017) in his review article, 91T-like SNe have long been
suspected of being intrinsically brighter than “normal” SNe Ia at the luminous
end of the absolute magnitude versus decline rate relation. In the case of SN
1991T itself, if we assume the Cepheid-based distance modulus of $30.74\pm
0.12$ (statistical) $\pm 0.12$ (systematic) measured by Saha et al. (2001) and
use the SNooPy EBV_model2 (Burns et al., 2011) to fit the $UBVRI$ photometry
published by Lira et al. (1998), a host reddening of $E(B-V)=0.15\pm 0.01$ mag
and an absolute magnitude of $M_{V}=-19.62\pm 0.22$ are implied. This is
$0.23\pm 0.23$ mag brighter than a SN with $s_{BV}$ $=1.1$666The color stretch
parameter, $s_{BV}$, is a dimensionless parameter defined as the time
difference between $B$-band maximum and the reddest point in the $(B-V)$ color
curve divided by 30 days, where typical SNe Ia have $s_{BV}$$\sim$1 (Burns et
al., 2014). according to the $M_{V}$ versus $s_{BV}$ relation for SNe Ia given
by Burns et al. (2018) for events with $s_{BV}$$>0.5$ and $E(B-V)_{host}<0.5$
mag. While interesting, the large error in this measurement does not allow a
definitive conclusion. A better approach would be to compare a sample of
91T-like SNe in the smooth Hubble flow where the redshift can be used as a
precise relative measurement of distance. The CSP-II sample of SNe Ia provides
such an opportunity.

Figure 5: Hubble diagram distance modulus residuals, $\Delta(m-M)$, for
91T-like, 99aa-like, and slow-declining Branch CN SNe observed by the CSP-II.
The sample of slow-declining Branch CN SNe is defined as those events with
$\Delta{\rm m}_{15}(B)$ $<1.10$. The larger symbols are SNe whose values of
$\Delta{\rm m}_{15}(B)$ were measured directly; the smaller symbols correspond
to those where SNooPy template fits were used to measure the decline rate. The
samples were limited to those with redshifts $z\geq 0.02$ to minimize
uncertainties due to peculiar velocities. Note that negative values of
$\Delta(m-M)$ correspond to overluminous events.

In Figure 5, we plot Hubble diagram residuals, $\Delta(m-M)$, versus
$\Delta{\rm m}_{15}(B)$ for the 91T-like and 99aa-like SNe observed by the
CSP-II. The Hubble diagram residual is defined as the difference between the
distance modulus predicted from the host galaxy redshift using a best-fit
cosmological model777Assumes standard $\Lambda$CDM cosmology and a fixed
Hubble constant $H_{0}=72~{}{\rm km~{}s^{-1}~{}Mpc^{-1}}$, density parameter
$\Omega_{m}=0.27$, and cosmological constant parameter
$\Omega_{\Lambda}=0.73$. to the full set of SNe Ia observed by the CSP-I and
CSP-II, and the distance modulus derived from the intrinsic color analysis
described in detail by Burns et al. (2018). For comparison, the Hubble diagram
residuals for the slow-declining Branch CN SNe in the CSP-II sample, which we
define as those events with $\Delta{\rm m}_{15}(B)$ $<1.10$, are also plotted
in Figure 5. Negative values therefore indicate more luminous SNe Ia. This
figure clearly shows that all of the 91T-like SNe have negative Hubble diagram
residuals, implying that they are over-luminous by $\sim 0.1$–$0.5$ mag. It
also confirms that 91T-like (and 99aa-like) SNe cannot be distinguished from
slow-declining Branch CN events with similar decline rates on the basis of
their $\Delta{\rm m}_{15}(B)$ measurements alone.

To separate the 91T-like events from the 99aa-like and slow-declining Branch
CN SNe, Hubble diagram residuals are plotted versus pEW(Si II $\lambda 6355$)
at maximum light in Figure 6. The pEW(Si II $\lambda 6355$) values for the
99aa-like and slow-declining Branch CN SNe are taken from Morrell et al.
(2022), while for the 91T-like SNe, a low-order polynomial fit to the shape of
the evolution of the equivalent width values for SN 1991T was used to
extrapolate the pEW measurements to maximum light. The error bars reflect the
average error obtained in applying the same extrapolation method to the
individual measurements of pEW(Si II $\lambda 6355$) for SN 1991T. Again, we
see that the distance modulus residuals of the 99aa-like and Branch CN SNe
scatter about zero, whereas the values for the 91T-like SNe are systematically
negative.

Figure 6: (above) Hubble diagram residuals in magnitudes plotted versus the
pseudo equivalent width of the Si II $\lambda 6355$ absorption at maximum
light for 91T-like, 99aa-like, and CN SNe observed by the CSP-II. (below)
$E(B-V)_{host}$ values derived using SNooPy for the same SNe.

As will be presented in a future paper, the $(B-v)$ and $(r-i)$ colors of the
91T-like SNe at maximum light appear to be similar to those of the 99aa-like
and slow-declining Branch CN SNe. Nevertheless, the possibility must be
considered that the apparent overluminous nature of the 91T-like events is the
result of overestimates of the effect of host galaxy reddening due to
incorrect assumptions of the colors of these extreme SNe. In the lower half of
Figure 6 we plot the $E(B-V)_{host}$ values returned by SNooPy for the SNe
plotted in the upper half of this figure. The mean host reddening for the
91T-like SNe is $\sim$0.06 mag higher than the average values for the 99aa-
like and slow-declining Branch CN events (see Table 1). Two of the four most
luminous 91T-like objects, SN 2013U and SN 2013bz, have relatively large
values of $E(B-V)_{host}$ ($0.20\pm 0.02$ and $0.23\pm 0.02$ mag,
respectively). However, the two others, OGLE-2014-SN-107 and SN 2014dl, have
more modest values of $E(B-V)_{host}=0.13\pm 0.03$ mag and $0.05\pm 0.01$ mag.

A way of checking whether errant host galaxy dust corrections are responsible
for the systematically-higher luminosities of the 91T-like SNe compared to the
99aa-like and slow-declining Branch CN events is to look at the Hubble diagram
residuals as a function of wavelength. In particular, we would expect dust
absorption to have a much smaller effect on the Hubble diagram residuals for
the NIR filters compared to the optical. The values of $\Delta(m-M)$ plotted
in the upper half of Figure 6 are derived from combining the SNooPy fits to
the $uBgVriYJH$ light curves. However, we can also look at the residuals as a
function of the individual filters. Table 1 indicates that the weighted mean
Hubble diagram residuals for the 91T-like, 99aa-like, and CN samples are
remarkably constant from the optical to the NIR. This is displayed graphically
in Figure 7 where the residuals in $V$, $r$, $Y$, and $H$ for each SN are
plotted versus the residuals in $B$. Excellent correlations are observed in
all four bands, with the 91T-like SNe consistently occupying the most-luminous
part.

Figure 7: $B$-band Hubble diagram residuals in magnitudes plotted versus the
residuals in $V$, $r$, $Y$, and $H$ for 91T-like, 99aa-like, and and CN SNe
observed by the CSP-II. The solid black line in each plot indicates the
expected correlation in the residuals for $R_{V}=3$ and a systematic error
between $\pm 0.5$ mag in $E(B-V)$; the dashed black line shows the same for
$R_{V}=2$. Table 1: Mean values of $\Delta{\rm m}_{15}(B)$, $E(B-V)_{host}$,
and $\Delta(m-M)$ for CSP-II SNe Ia

Parameter | 91T-like | n | SS | n | CN | n
---|---|---|---|---|---|---
$\Delta{\rm m}_{15}(B)$ | 0.87 (0.17) | 10 | 0.92 (0.11) | 13 | 0.92 (0.10) | 27
$E(B-V)_{host}$ | 0.12 (0.11) | 10 | 0.07 (0.19) | 13 | 0.06 (0.13) | 27
$\Delta(m-M)_{B}$ |  $-0.31$ (0.18) | 10 |  $-0.01$ (0.19) | 13 |  $+0.04$ (0.22) | 27
$\Delta(m-M)_{V}$ |  $-0.33$ (0.16) | 10 |  $-0.03$ (0.19) | 13 |  $+0.05$ (0.22) | 27
$\Delta(m-M)_{r}$ |  $-0.32$ (0.16) | 10 |  $-0.03$ (0.18) | 13 |  $+0.05$ (0.21) | 27
$\Delta(m-M)_{Y}$ |  $-0.27$ (0.21) | 10 |  $+0.02$ (0.21) | 11 |  $-0.01$ (0.22) | 20
$\Delta(m-M)_{H}$ |  $-0.25$ (0.19) | 7 |  $+0.02$ (0.17) | 8 |  $+0.03$ (0.21) | 15
$\Delta(m-M)_{uBgVriYJH}$aaAverage cross-validated distance modulus calculated from the $u$, $B$, $g$, $V$, $r$, $i$, $Y$, $J$, and $H$ filters (see Burns et al., 2018) |  $-0.33$ (0.18) | 10 |  $-0.01$ (0.15) | 13 |  $+0.05$ (0.22) | 27

Note. — All values shown are weighted means. The root mean square dispersion
is given in parentheses, followed by the number of SNe, “n”, used in
calculating the mean.

The SNooPy color_model method used to calculate the Hubble diagram residuals
in Table 1 and Figures 5–7 employs intrinsic colors from Burns et al. (2018)
to constrain $E(B-V)$ and $R_{V}$ for each SN. These values, in turn, are used
to compute reddening-corrected magnitudes. Errors in $E(B-V)$ and $R_{V}$ will
therefore propagate a systematic error in the Hubble diagram residual in all
filters. However, such errors should grow smaller with wavelength (see Figure
7) and cannot account for the strong, slope $=$ 1, correlations observed in
Figure 7 which are dominated by the peculiar velocities of the host galaxies.
Moreover, except for OGLE-2014-SN-141, the color_model method gives acceptable
fits in the $BVriYJH$ bands (see Appendix A). Still, it is important to
confirm that the high luminosities of 91T-like SNe in all filters are not an
artifact of the color_model method. The Hubble diagram residuals for CSP-II
SNe Ia published by Uddin et al. (2022) provide an alternative check since
these were derived by applying the empirical Tripp (1998) luminosity–color
relation to maximum light magnitudes derived from the SNooPy max_model method.
This analysis yields average residuals in all filters of approximately
$-0.22\pm 0.07$ mag, $-0.04\pm 0.05$ mag, and $+0.08\pm 0.04$ mag for the
91T-like, 99aa-like, and slow-declining Branch CN SNe, respectively,
confirming that 91T-like SNe are systematically more luminous than their 99aa-
like and slow-declining Branch CN counterparts. Figures 5–7, as well as Table
1, also suggest that 99aa-like SNe may be intermediate in luminosity between
91T-like and slow-declining Branch CN events, but this will require future
confirmation from larger samples.

##  5 Conclusions

91T-like SNe are the extreme members of the Branch SS group in displaying the
weakest Si II$~{}\lambda 6355$ and Ca II H & K absorption absorption at
maximum light of any SNe Ia. We show that these objects can be identified from
one or more optical spectra by plotting pEW(Si II $\lambda 6355$) versus light
curve phase for $t-t(B_{max})\leq+10$ days. They are differentiated from 02cx-
like and 03fg-like SNe in possessing $i/I$-band light curves that reach
maximum before the of epoch $B$ maximum, and that also display a clear
secondary maximum. Lacking accurate knowledge of light curve phase, 91T-like
SNe may be easily confused with the less extreme 99aa-like events.

From a sample of ten 91T-like SNe observed during the course of the CSP-II, we
find clear evidence that 91T-like SNe are over-luminous by $\sim$0.1–0.5 mag
compared to 99aa-like and slow-declining Branch CN SNe. This difference in
luminosity is remarkably constant from optical to NIR wavelengths, arguing
that overestimates of the host galaxy dust corrections cannot explain this
finding. The data further suggest that 99aa-like events may be intermediate in
luminosity between 91T-like SNe and slow-declining Branch CN SNe, although
this requires further confirmation.

Based on an analysis by Leaman et al. (2011) of 726 SNe discovered by the Lick
Observatory Supernova Search (LOSS), Li et al. (2011) concluded that 9% of all
SNe Ia in the local Universe are 91T-like events. However, Li et al. employed
a broad definition for what constituted a “Ia 91T” that included 99aa-like
events. In any event, they are likely to be sufficiently rare in the local
Universe that their effect on measurements of the Hubble constant should be
small. This is borne out by an analysis of the CSP-II sample using the
methodology of Uddin et al. (2022). Restricting the sample to the 178 SNe Ia
with $s_{BV}$ $>0.8$ so as to exclude lower-luminosity events, and then
repeating the calculation after removing the ten 91T-like events, the value of
the Hubble constant changes by only 0.1%. However, there is no assurance that
the frequency of 91T-like remains constant at greater and greater look-back
times. And even if their relative numbers have not changed as the Universe has
evolved, we can expect to observe proportionately more of them at the
magnitude limits of discovery surveys due to Malmquist bias. Unfortunately,
the light curve width alone cannot be used to identify 91T-like SNe, although
this may not be true at ultraviolet wavelengths (Jiang et al., 2018). At least
for now, spectroscopy combined with rest frame $i$/$I$-band photometry remains
the most reliable tool for discriminating between 91T-like, 99aa-like, and
slow-declining Branch CN SNe.

The work of the CSP-II has been generously supported by the National Science
Foundation under grants AST-1613426, AST-1613455, and AST-1613472. The CSP-II
was also supported in part by the Danish Agency for Science and Technology and
Innovation through a Sapere Aude Level 2 grant. M. Stritzinger acknowledges
funding by a research grant (13261) from VILLUM FONDEN. E.B. is supported in
part by NASA grant 80NSSC20K0538. L.G. acknowledges financial support from the
Spanish Ministerio de Ciencia e Innovación (MCIN), the Agencia Estatal de
Investigación (AEI) 10.13039/501100011033, and the European Social Fund (ESF)
”Investing in your future” under the 2019 Ramón y Cajal program
RYC2019-027683-I and the PID2020-115253GA-I00 HOSTFLOWS project, from Centro
Superior de Investigaciones Científicas (CSIC) under the PIE project
20215AT016, and the program Unidad de Excelencia María de Maeztu
CEX2020-001058-M. We gratefully acknowledge the use of WISeREP –
https://wiserep.weizmann.ac.il. M.M.P. thanks Richard Scalzo for interesting
discussions of extreme SNe Ia. This research has made use of the NASA/IPAC
Extragalactic Database (NED), which is funded by the National Aeronautics and
Space Administration and operated by the California Institute of Technology.
Finally, we thank the referee for suggestions on improving the text and
figures.

##  Appendix A 91T-like SNe from the CSP-II

A total of ten 91T-like events in the CSP-II sample were identified using the
spectroscopic and photometric criteria defined in §2. These objects are listed
in Table A. Figure 8 shows the temporal evolution of pEW(Si II $\lambda 6355$)
for the CSP-II sample, and, in Figures 9–11, SNooPY color_model fits are
plotted. These fits employ the intrinsic colors from Burns et al. (2018) to
constrain $E(B-V)$ and $R_{V}$ for each SN.

Comments on individual objects are as follows:

  * •

LSQ12gdj: Extensive observations of this SN have been previously published by
Scalzo et al. (2014), who concluded that it was “spectroscopically 91T-like”.
Figure 8 shows that the evolution of the pseudo equivalent width of Si II
$\lambda 6355$ was more similar to SN 1991T than SN 1999aa. The light curves
plotted in Figure 10 also clearly show that LSQ12gdj had a strong secondary
maximum in the $i$ band, and that primary maximum in $i$ was reached before
$B$ maximum.

  * •

SN 2013U: This SN was discovered by the Puckett Observatory Supernova
Search888http://www.cometwatch.com/supernovasearch.html and was correctly
classified as a 91T-like event from a spectrum obtained with the Asiago 1.82-m
Copernico Telescope (Gagliano et al., 2013; Tomasella et al., 2013). The
SNooPy color_model templates displayed in Figure 10 provide excellent fits at
maximum light in the $BVgri$ filters, but underpredict the observed brightness
in $u$. Comparing with the other SNe with $u$-band photometry plotted in
Figure 10, this appears to be a general characteristic of 91T-like SNe (see
also Scalzo et al., 2014).

  * •

CSS130303:105206-133424: This object was discovered by the Catalina Real-Time
Transient Survey (CRTS; Djorgovski et al., 2011). A spectrum obtained by the
Public ESO Spectroscopic Survey of Transient Objects (PESSTO; Smartt et al.,
2015) with the ESO La Silla 3.6 m NTT was classified by Blagorodnova et al.
(2013) as corresponding to a SN Ia caught $\sim$7 days before maximum light.
As discussed in §3, this spectrum is peculiar even for a 91T-like event in
showing only very weak absorption features, including those of Fe III.

  * •

MASTER OT J093953.18+165516.4: The discovery of this SN by the MASTER network
of robotic telescopes (Gorbovskoy et al., 2013) was announced by Shumkov et
al. (2013). It was independently discovered by the CRTS with the designation
MLS130215:093953+165516. A PESSTO spectrum obtained by Benitez-Herrera et al.
(2013) was stated to show “similarity with that of the peculiar, 1991T-like SN
1998es” near maximum light. SN 1998es was actually a 99aa-like event according
to the classification method presented in §2. However, as seen in Figure 8,
measurement of pEW(Si II $\lambda 6355$) from this spectrum of MASTER OT
J093953.18+165516.4 is consistent with a 91T-like event a few days after
maximum light. Technically speaking, the CSP-II $i$-band photometry plotted in
Figure 10 does not begin early enough to confirm that maximum was reached
before maximum light in $B$, but the excellent fits to the SNooPy templates
leave little doubt that this was a bonafide 91T-like SN.

  * •

SN 2013bz: This is another SN discovered by the CRTS (Howerton et al., 2013)
which designated it as SNhunt188. A spectrum taken with the Asiago 1.82-m
Copernico Telescope by Ochner et al. (2013) was said to be “a good match …
with the so-called ‘super-Chandrasekhar’ SNe Ia 2006gz and 2009dc.” However, a
spectrum obtained 4 days earlier with the 4.2-m William Herschel Telescope by
Chen et al. (Howerton et al., 2013) was stated to be consistent with either a
normal SN Ia at maximum light, or a 91T-like event a week after maximum. The
CSP-II light curves indicate that this spectrum, which is not publicly
available, was obtained at $+3$ days. The value of pEW(Si II $\lambda 6355$)
measured from the Asiago spectrum is fully consistent with a 91T-like
classification (see Figure 8). This SN has a striking excess at $u$-band
maximum (see Figure 10).

  * •

SN 2014dl: Yet another SN discovered by the CRTS (Drake et al., 2014). Spectra
obtained at the Asiago and Las Campanas Observatories were found to be similar
to that of a 91T-like event $\sim$1 week before maximum (Drake et al., 2014).

  * •

OGLE-2014-SN-107: This object was discovered in the course of the OGLE-IV
Real-Time Transient Search (Wyrzykowski et al., 2014). From a PESSTO spectrum,
Takats et al. (2014) classified it as a 91T-like SN observed approximately
$\sim$5 days before maximum light. This is confirmed in Figure 8. As
illustrated in Figure 10, the minimum between the primary and secondary maxima
of the $i$-band light curve of this SN is particularly shallow.

  * •

ASASSN-14kd: The discovery of this SN in the course of the All-Sky Automated
Survey for Supernovae (ASASSN) (Shappee et al., 2014; Kochanek et al., 2017)
was announced by (Nicholls et al., 2014). From a PESSTO spectrum obtained
within hours of discovery, Firth et al. (2014) identified it as a SN Ia that
was best matched by SNID 91T-like templates at a phase of $-7$ days, which we
confirm in Figure 8.

  * •

OGLE-2014-SN-141: This is another SN discovered by the OGLE-IV Real-Time
Transient Search. A single PESSTO spectrum was obtained by Dimitriadis et al.
(2014), who classified it as a SN Ia at a phase of approximately $-7$ days and
noted that it was an “excellent match to a SN1991T-like SN Ia.” This is
confirmed in Figure 8. Interestingly, although the SNooPy color_model gives a
good fit to the $BVRi$ light curves with a host galaxy color excess of
$0.08\pm 0.02$ mag and $R_{V}=3.4\pm 0.9$, the $Y$ and $J$ templates over
predict the brightness in the NIR by $\sim$0.4–0.6 mag.

  * •

SN2014eg: This SN was serendipitously discovered in ESO 154-G010 by PESSTO
observers who were obtaining follow-up observations of the Type II-n SN 2013fc
in the same host galaxy. A spectrum was obtained six nights later by Smith et
al. (2014) who classified SN2014eg as a 91T-like caught approximately one week
before maximum. Extensive spectroscopic and photometry observations of this SN
may be found in Dimitriadis (2017).

Table 2: 91T-like Supernovae Observed by the CSP-II

SN Name | Host Galaxy |  $z_{\rm{helio}}$aaHeliocentric redshift are from the NASA/IPAC Extragalactic Database (NED) unless otherwise indicated. |  $\Delta{\rm m}_{15}(B)$bb$\Delta{\rm m}_{15}(B)$ decline rate in magnitudes (Phillips, 1993) as measured with SNooPy. The 1$\sigma$ error is given in between parentheses. The * symbol indicates values derived from template fits only. |  $s_{BV}$cc$s_{BV}$ color stretch (Burns et al., 2014) as measured with SNooPy templates. The 1$\sigma$ error is given in between parentheses. | SpectroscopyddReferences to spectroscopic observations.
---|---|---|---|---|---
LSQ12gdj | ESO 472-G 007 | $0.0303$ | 0.73 (0.03) | 1.14 (0.05) | 1
SN2013U | CGCG 008-023 | $0.0345$ | 1.02 (0.13) | 1.25 (0.05) | 2,3,4
CSS130303:105206-133424 | GALEXASC J105206.27-133420.2 | $0.0789$ | 1.08 (0.16) | 1.19 (0.05) | 5
MASTER OT J093953.18+165516.4 | CGCG 092-024 | $0.0478$ | 0.84 (0.04) | 1.12 (0.05) | 6
SN2013bz (SNhunt188) | 2MASX J13265081-1001263 | $0.0192$ | 0.76 (0.02) | 1.12 (0.05) | 4,7
SN2014dl (CSS140925:162946+083831) | UGC 10414 | $0.0330$ | 1.05 (0.07)∗ | 1.22 (0.05) | 3,4,8,9
OGLE-2014-SN-107 | APMUKS(BJ) B004021.02-650219.5 |  $0.0664$eeRedshift measured by CSP-II. | 1.12 (0.05) | 1.19 (0.05) | 10
ASASSN-14kd | 2MASX J22532475+0447583 |  $0.0243$eeRedshift measured by CSP-II. | 0.79 (0.01) | 1.13 (0.05) | 3,11
OGLE-2014-SN-141 | 2MASX J05371898-7543157 |  $0.0625$eeRedshift measured by CSP-II. | 0.62 (0.09) | 1.24 (0.05) | 12
SN2014eg | ESO 154-G 010 | $0.0186$ | 0.92 (0.01) | 1.17 (0.05) | 3,4

References. — (1) Scalzo et al. (2014); (2) Tomasella et al. (2013); (3) This
paper; (4) PESSTO; (5) Blagorodnova et al. (2013); (6) Benitez-Herrera et al.
(2013); (7) Ochner et al. (2013); (8) Drake et al. (2014); (9) Stahl et al.
(2020) (10) Takats et al. (2014); (11) Firth et al. (2014); (12) Dimitriadis
et al. (2014)

Figure 8: Evolution of pEW(Si II $\lambda 6355$) for 91T-like SNe identified
in the CSP-II sample. The trajectories of SN 1991T and SN 1999aa in this
diagram are plotted for reference.

Figure 9: SNooPy fits to the 91T-like SNe LSQ12gdj, SN2013U,
CSS130303:105206-133424, and SN 2014eg derived using the intrinsic color
analysis described by Burns et al. (2018). Magnitudes plotted on the y-axis
for each filter are as observed. The x-axis gives the time in days in the
observer frame after the epoch of $B$ maximum.

Figure 10: Same as Figure 9, except for the 91T-like SNe 2013bz, 2014dl,
OGLE-2014-SN-107, and ASASSN-14kd.

Figure 11: Same as Figure 9, except for the 91T-like SNe OGLE-2014-SN-141 and
SN 2014eg.

## References

  * Aldering et al. (2006) Aldering, G., Antilogus, P., Bailey, S., et al. 2006, ApJ, 650, 510
  * Ashall et al. (2019b) Ashall, C., Hsiao, E. Y., Hoeflich, P., et al. 2019, ApJ, 875, L14
  * Ashall et al. (2020) Ashall, C., Lu, J., Burns, C., et al. 2020, ApJ, 895, L3
  * Ashall et al. (2021) Ashall, C., Lu, J., Hsiao, E. Y., et al. 2021, ApJ, 922, 205
  * Avelino et al. (2019) Avelino, A., Friedman, A. S., Mandel, K. S., et al. 2019, ApJ, 887, 106
  * Benitez-Herrera et al. (2013) Benitez-Herrera, S., Taubenberger, S., Garoffolo, A. M., et al. 2013, The Astronomer’s Telegram, 4801
  * Blagorodnova et al. (2013) Blagorodnova, N., Nicholl, M., Walton, N., et al. 2013, The Astronomer’s Telegram, 4852
  * Blondin et al. (2012) Blondin, S., Matheson, T., Kirshner, R. P., et al. 2012, AJ, 143, 126
  * Blondin & Tonry (2007) Blondin, S., & Tonry, J. L. 2007, ApJ, 666, 1024
  * Branch (2001) Branch, D. 2001, PASP, 113, 169
  * Branch et al. (2009) Branch, D., Chau Dang, L., & Baron, E. 2009, PASP, 121, 238
  * Branch et al. (2006) Branch, D., Dang, L. C., Hall, N., et al. 2006, PASP, 118, 560
  * Burns et al. (2018) Burns, C. R., Parent, E., Phillips, M. M., et al. 2018, ApJ, 869, 56
  * Burns et al. (2011) Burns, C. R., Stritzinger, M., Phillips, M. M., et al. 2011, AJ, 141, 19
  * Burns et al. (2014) Burns, C. R., Stritzinger, M., Phillips, M. M., et al. 2014, ApJ, 789, 32
  * Burrow et al. (2020) Burrow, A., Baron, E., Ashall, C., et al. 2020, ApJ, 901, 154
  * Dhawan et al. (2018) Dhawan, S., Jha, S. W., & Leibundgut, B. 2018, A&A, 609, A72
  * Dilday et al. (2012) Dilday, B., Howell, D. A., Cenko, S. B., et al. 2012, Science, 337, 942
  * Dimitriadis (2017) Dimitriadis, G. 2017, Ph.D. thesis, University of Southampton
  * Dimitriadis et al. (2014) Dimitriadis, G., Smith, M., Firth, R., et al. 2014, The Astronomer’s Telegram, 6706
  * Djorgovski et al. (2011) Djorgovski, S. G., Drake, A. J., Mahabal, A. A., et al. 2011, arXiv:1102.5004
  * Drake et al. (2014) Drake, A. J., Djorgovski, S. G., Graham, M. J., et al. 2014, Central Bureau Electronic Telegrams 3995, 1
  * Filippenko et al. (1999) Filippenko, A. V., Li, W. D., & Leonard, D. C. 1999, IAU Circ., 7108, 2
  * Filippenko et al. (1992) Filippenko, A. V., Richmond, M. W., Matheson, T., et al. 1992, ApJ, 384, L15
  * Firth et al. (2014) Firth, R., Dimitriadis, G., Smith, M., et al. 2014, The Astronomer’s Telegram, 6699
  * Fisher et al. (1999) Fisher, A., Branch, D., Hatano, K., & Baron, E. 1999, MNRAS, 304, 67
  * Folatelli et al. (2013) Folatelli, G., Morrell, N., Phillips, M. M., et al. 2013, ApJ, 773, 53
  * Foley et al. (2013) Foley, R. J., Challis, P. J., Chornock, R., et al. 2013, ApJ, 767, 57
  * Freedman et al. (2019) Freedman, W. L., Madore, B. F., Hatt, D., et al. 2019, ApJ, 882, 34
  * Gagliano et al. (2013) Gagliano, R., Newton, J., Puckett, T., et al. 2013, Central Bureau Electronic Telegrams, 3410
  * Garavini et al. (2005) Garavini, G., Aldering, G., Amadon, A., et al. 2005, AJ, 130, 2278
  * Garavini et al. (2004) Garavini, G., Folatelli, G., Goobar, A., et al. 2004, AJ, 128, 387
  * Gómez et al. (1996) Gómez, G., López, R., & Sánchez, F. 1996, AJ, 112, 2094
  * Gorbovskoy et al. (2013) Gorbovskoy, E. S., Lipunov, V. M., Kornilov, V. G., et al. 2013, Astronomy Reports, 57, 233
  * Graham et al. (2019) Graham, M. L., Harris, C. E., Nugent, P. E., et al. 2019, ApJ, 871, 62
  * Hachinger et al. (2008) Hachinger, S., Mazzali, P. A., Tanaka, M., et al. 2008, MNRAS, 389, 1087
  * Hamuy et al. (2002b) Hamuy, M., Maza, J., Pinto, P. A., et al. 2002, AJ, 124, 417 [Erratum: 2002, AJ, 124, 2339]
  * Hamuy et al. (1996) Hamuy, M., Phillips, M. M., Suntzeff, N. B., et al. 1996, AJ, 112, 2391
  * Hamuy et al. (2003b) Hamuy, M., Phillips, M. M., Suntzeff, N. B., et al. 2003, Nature, 424, 651
  * Hamuy et al. (2000) Hamuy, M., Trager, S. C., Pinto, P. A., et al. 2000, AJ, 120, 1479
  * Hatano et al. (2002) Hatano, K., Branch, D., Qiu, Y. L., et al. 2002, New A, 7, 441
  * Howell et al. (2006) Howell, D. A., Sullivan, M., Nugent, P. E., et al. 2006, Nature, 443, 308
  * Howerton et al. (2013) Howerton, S., Drake, A. J., Djorgovski, S. G., et al. 2013, Central Bureau Electronic Telegrams, 3507, 1
  * Hsiao et al. (2019) Hsiao, E. Y., Phillips, M. M., Marion, G. H., et al. 2019, PASP, 131, 014002
  * Jeffery et al. (1992) Jeffery, D. J., Leibundgut, B., Kirshner, R. P., et al. 1992, ApJ, 397, 304
  * Jha et al. (2006) Jha, S., Kirshner, R. P., Challis, P., et al. 2006, AJ, 131, 527
  * Jiang et al. (2018) Jiang, J.-. an ., Doi, M., Maeda, K., et al. 2018, ApJ, 865, 149
  * Kochanek et al. (2017) Kochanek, C. S., Shappee, B. J., Stanek, K. Z., et al. 2017, PASP, 129, 104502
  * Krisciunas et al. (2004) Krisciunas, K., Suntzeff, N. B., Phillips, M. M., et al. 2004, AJ, 128, 3034
  * Leaman et al. (2011) Leaman, J., Li, W., Chornock, R., et al. 2011, MNRAS, 412, 1419
  * Li et al. (2003) Li, W., Filippenko, A. V., Chornock, R., et al. 2003, PASP, 115, 453
  * Li et al. (2001b) Li, W., Filippenko, A. V., Treffers, R. R., et al. 2001b, ApJ, 546, 734
  * Li et al. (2011) Li, W., Leaman, J., Chornock, R., et al. 2011, MNRAS, 412, 1441
  * Lira et al. (1998) Lira, P., Suntzeff, N. B., Phillips, M. M., et al. 1998, AJ, 115, 234
  * Mazzali et al. (2005) Mazzali, P. A., Benetti, S., Stehle, M., et al. 2005, MNRAS, 357, 200
  * Mazzali et al. (1998) Mazzali, P. A., Cappellaro, E., Danziger, I. J., Turatto, M., & Benetti, S. 1998, ApJ, 499, L49
  * Mazzali et al. (1995) Mazzali, P. A., Danziger, I. J., & Turatto, M. 1995, A&A, 297, 509
  * Morrell et al. (2022) Morrell, N, Phillips, M. M., Folatelli, G., et al. 2022, in preparation
  * Nicholls et al. (2014) Nicholls, B., Kiyota, S., Brimacombe, J., et al. 2014, The Astronomer’s Telegram, 6694
  * Nugent et al. (1995) Nugent, P., Phillips, M., Baron, E., Branch, D., & Hauschildt, P. 1995, ApJ, 455, L147
  * Ochner et al. (2013) Ochner, P., Tomasella, L., Pastorello, A., et al. 2013, The Astronomer’s Telegram, 5048
  * Pereira et al. (2013) Pereira, R., Thomas, R. C., Aldering, G., et al. 2013, A&A, 554, A27
  * Perlmutter et al. (1999) Perlmutter, S., Aldering, G., Goldhaber, G., et al. 1999, ApJ, 517, 565
  * Phillips (1993) Phillips, M. M. 1993, ApJ, 413, L105
  * Phillips (2012) Phillips, M. M. 2012, PASA, 29, 434
  * Phillips et al. (2019) Phillips, M. M., Contreras, C., Hsiao, E. Y., et al. 2019, PASP, 131, 014001
  * Phillips et al. (2006) Phillips, M. M., Krisciunas, K., Suntzeff, N. B., et al. 2006, AJ, 131, 2615
  * Phillips et al. (2007) Phillips, M. M., Li, W., Frieman, J. A., et al. 2007, PASP, 119, 360
  * Phillips et al. (1999) Phillips, M. M., Lira, P., Suntzeff, N. B., et al. 1999, AJ, 118, 1766
  * Phillips et al. (1992) Phillips, M. M., Wells, L. A., Suntzeff, N. B., et al. 1992, AJ, 103, 1632
  * Prieto et al. (2007) Prieto, J. L., Garnavich, P. M., Phillips, M. M., et al. 2007, arXiv e-prints, arXiv:0706.4088
  * Riess et al. (2021) Riess, A. G., Casertano, S., Yuan, W., et al. 2021, ApJ, 908, L6
  * Riess et al. (1998) Riess, A. G., Filippenko, A. V., Challis, P., et al. 1998, AJ, 116, 1009
  * Riess et al. (2016) Riess, A. G., Macri, L. M., Hoffmann, S. L., et al. 2016, ApJ, 826, 56
  * Riess et al. (1996) Riess, A. G., Press, W. H., & Kirshner, R. P. 1996, ApJ, 473, 88
  * Ruiz-Lapuente et al. (1992) Ruiz-Lapuente, P., Cappellaro, E., Turatto, M., et al. 1992, ApJ, 387, L33
  * Saha et al. (2001) Saha, A., Sandage, A., Thim, F., et al. 2001, ApJ, 551, 973
  * Sasdelli et al. (2014) Sasdelli, M., Mazzali, P. A., Pian, E., et al. 2014, MNRAS, 445, 711
  * Scalzo et al. (2010) Scalzo, R. A., Aldering, G., Antilogus, P., et al. 2010, ApJ, 713, 1073
  * Scalzo et al. (2014) Scalzo, R. A., Childress, M., Tucker, B., et al. 2014, MNRAS, 445, 30
  * Schlafly & Finkbeiner (2011) Schlafly, E. F., & Finkbeiner, D. P. 2011, ApJ, 737, 103
  * Schlegel, Finkbeiner & Davis (1998) Schlegel, D. J., Finkbeiner, D. P., & Davis. M. 1998, ApJ, 500, 525
  * Shappee et al. (2014) Shappee, B. J., Prieto, J. L., Grupe, D., et al. 2014, ApJ, 788, 48
  * Shumkov et al. (2013) Shumkov, V., Denisenko, D., Balanutsa, P., et al. 2013, The Astronomer’s Telegram, 4799
  * Silverman et al. (2012a) Silverman, J. M., Foley, R. J., Filippenko, A. V., et al. 2012a, MNRAS, 425, 1789
  * Silverman et al. (2012b) Silverman, J. M., Kong, J. J., & Filippenko, A. V. 2012b, MNRAS, 425, 1819
  * Smartt et al. (2015) Smartt, S. J., Valenti, S., Fraser, M., et al. 2015, A&A, 579, A40
  * Smith et al. (2014) Smith, M., Firth, R., Dimitriadis, G., et al. 2014, The Astronomer’s Telegram, 6739
  * Stahl et al. (2020) Stahl, B. E., Zheng, W., de Jaeger, T., et al. 2020, MNRAS, 492, 4325
  * Stritzinger et al. (2002) Stritzinger, M., Hamuy, M., Suntzeff, N. B., et al. 2002, AJ, 124, 2100
  * Suntzeff et al. (2022) Suntzeff, N. B., et al. 2022, in preparation
  * Takats et al. (2014) Takats, K., Romero-Canizales, C., Galbany, L., et al. 2014, The Astronomer’s Telegram, 6612
  * Taubenberger (2017) Taubenberger, S. 2017, Handbook of Supernovae, 317
  * Tody (1986) Tody, D. 1986, Proc. SPIE, 627, 733. doi:10.1117/12.968154
  * Tody (1993) Tody, D. 1993, Astronomical Data Analysis Software and Systems II, 52, 173
  * Tomasella et al. (2013) Tomasella, L., Benetti, S., Cappellaro, E., et al. 2013, The Astronomer’s Telegram, 4796
  * Tripp (1998) Tripp, R. 1998, A&A, 331, 815
  * Uddin et al. (2022) Uddin, S. A., et al. 2022, submitted
  * Wyrzykowski et al. (2014) Wyrzykowski, Ł., Kostrzewa-Rutkowska, Z., Kozłowski, S., et al. 2014, Acta Astron., 64, 197
  * Yuan et al. (2010) Yuan, F., Quimby, R. M., Wheeler, J. C., et al. 2010, ApJ, 715, 1338