The He-rich core-collapse supernova 2007Y: Observations from X-ray to Radio Wavelengths

Abstract

A detailed study spanning approximately a year has been conducted on the Type
Ib supernova 2007Y. Imaging was obtained from X-ray to radio wavelengths, and a
comprehensive set of multi-band (w2m2w1u'g'r'i'UBVYJHKs) light curves and
optical spectroscopy is presented. A virtually complete bolometric light curve
is derived, from which we infer a (56)Ni-mass of 0.06 M_sun. The early spectrum
strongly resembles SN 2005bf and exhibits high-velocity features of CaII and
H_alpha; during late epochs the spectrum shows evidence of a ejecta-wind
interaction. Nebular emission lines have similar widths and exhibit profiles
that indicate a lack of major asymmetry in the ejecta. Late phase spectra are
modeled with a non-LTE code, from which we find (56)Ni, O and total-ejecta
masses (excluding He) to be 0.06, 0.2 and 0.42 M_sun, respectively, below 4,500
km/s. The (56)Ni mass confirms results obtained from the bolometric light
curve. The oxygen abundance suggests the progenitor was most likely a ~3.3
M_sun He core star that evolved from a zero-age-main-sequence mass of 10-13
M_sun. The explosion energy is determined to be ~10^50 erg, and the mass-loss
rate of the progenitor is constrained from X-ray and radio observations to be
<~10^-6 M_sun/yr. SN 2007Y is among the least energetic normal Type Ib
supernovae ever studied.

Full text

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arXiv:0902.0609v2 [astro-ph.HE] 5 Mar 2009
Draft version March 5, 2009
Preprint typeset using LATEX style emulateapj v. 10/09/06
THE HE-RICH CORE-COLLAPSE SUPERNOVA 2007Y: OBSERVATIONS FROM X-RAY TO RADIO
WAVELENGTHS1,2
Maximilian Stritzinger,3,4Paolo Mazzali,5,6Mark M. Phillips,3Stefan Immler,7,8Alicia Soderberg,9,10Jesper
Sollerman,4,11Luis Boldt,3Jonathan Braithwaite,12Peter Brown,13Christopher R. Burns,14Carlos
Contreras,3Ricardo Covarrubias,3Gast´ on Folatelli,15Wendy L. Freedman,14Sergio Gonz´ alez,3Mario
Hamuy,15Wojtek Krzeminski,3Barry F. Madore,14,16Peter Milne,17Nidia Morrell,3S. E. Persson,14Miguel
Roth,3Mathew Smith,18and Nicholas B. Suntzeff19
(Received 2009 Jan. 19; Accepted 2009 Feb. 3)
Draft version March 5, 2009
ABSTRACT
A detailed study spanning approximately a year has been conducted on the Type Ib supernova
2007Y. Imaging was obtained from X-ray to radio wavelengths, and a comprehensive set of multi-
band (w2m2w1u′g′r′i′UBV Y JHKs) light curves and optical spectroscopy is presented. A virtually
complete bolometric light curve is derived, from which we infer a56Ni-mass of 0.06 M⊙. The early
spectrum strongly resembles SN 2005bf and exhibits high-velocity features of Ca II and Hα; during late
epochs the spectrum shows evidence of a ejecta-wind interaction. Nebular emission lines have similar
widths and exhibit profiles that indicate a lack of major asymmetry in the ejecta. Late phase spectra
are modeled with a non-LTE code, from which we find56Ni, O and total-ejecta masses (excluding
He) to be 0.06, 0.2 and 0.42 M⊙, respectively, below 4,500 km s−1. The56Ni mass confirms results
obtained from the bolometric light curve. The oxygen abundance suggests the progenitor was most
likely a ≈3.3 M⊙He core star that evolved from a zero-age-main-sequence mass of 10–13 M⊙. The
explosion energy is determined to be ≈1050erg, and the mass-loss rate of the progenitor is constrained
from X-ray and radio observations to be ? 10−6M⊙ yr−1. SN 2007Y is among the least energetic
normal Type Ib supernovae ever studied.
Subject headings: galaxies: individual (NGC 1187) — supernovae: general — supernovae: individual
(SN 2007Y)
1
This paper includes data gathered with the 6.5 meter
Magellan telescope at Las Campanas Observatory, Chile.
2Partly based on observations collected at the European
Southern Observatory, La Silla and Paranal Observatories, Chile
(ESO Programme 078.D-0048 and 380.D-0272.)
3Las Campanas Observatory, Carnegie Observatories, Casilla
601,LaSerena,Chile;mstritzinger@lco.cl,
lboldt@lco.cl, ccontreras@lco.cl, ricardo@lco.cl, sgonzalez@lco.cl,
wojtek@lco.cl, nmorrell@lco.cl, miguel@lco.cl.
4Dark Cosmology Centre, Niels Bohr Institute, University
of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø,
Denmark; max@dark-cosmology.dk, jesper@dark-cosmology.dk.
5Max-Planck-Institut f¨ ur Astrophysik, Karl-Schwarzschild-Str.
1,85741 Garching bei M¨ unchen,
garching.mpg.de.
6INAF-OsservatorioAstronomico
dell’Osservatorio 5, 35122 Padova, Italy.
7Astrophysics Science Division, X-RayAstrophysical Labora-
tory, Code662, NASAGoddardSpaceFlight Center, Greenbelt, MD
20771, USA; stefan.m.immler@nasa.gov.
8Universities Space Research Association, 10211 Wincopin
Circle, Columbia, MD 21044, USA
9DepartmentofAstrophysical
versity, Ivy Lane,Princeton, New Jersey 08544,
cia@astro.princeton.edu.
10Harvard-Smithsonian Center for Astrophysics, 60 Garden
Street, Cambridge, MA 02138, USA.
11Department of Astronomy, Stockholm University, AlbaNova,
SE-10691 Stockholm, Sweden.
12Canadian Institute for Theoretical Astrophysics, 60 St.George
St., Toronto M5S 3H8, Canada; jon@cita.utoronto.ca.
13Pennsylvania StateUniversity,
omy&Astrophysics,University
pbrown@astro.psu.edu.
14Observatories of the Carnegie Institution of Washington,
813 Santa Barbara St., Pasadena, CA 91101; cburns@ociw.edu,
wendy@ociw.edu, barry@ociw.edu, persson@ociw.edu.
15UniversidaddeChile,Departamento
mmp@lco.cl,
Germany;mazzali@mpa-
diPadova,vicolo
Sciences,PrincetonUni-
USA; ali-
Department
Park,
of Astron-
USA;PA16802,
deAstronom´ ıa,
1. INTRODUCTION
A decade has passed since it was first recognized that
some extragalactic gamma-ray transients could be linked
to the death of massive stars (Galama et al. 1998).
Soon afterwards the connection between long gamma-
ray bursts (GRBs) and energetic Type Ic supernovae
(SNe Ic) was firmly established (Hjorth et al. 2003;
Matheson et al. 2003; Stanek et al. 2003; Malesani et al.
2004). Over the last several years it has also become
clear that some X-ray flashes, a less energetic version
of GRBs, are produced by a similar type of SN Ic
(Fynbo et al. 2004; Modjaz et al. 2006; Pian et al. 2006;
Sollerman et al. 2006).
Recently, X-ray emission was detected from the He-rich
Type Ib SN 2008D (Soderberg et al. 2008; Mazzali et al.
2008; Malesani et al. 2009; Modjaz et al. 2009).
finding has given fresh impetus to the study of nor-
mal SNe Ib, which has been somewhat neglected, only
a handful of events having been well-observed. In this
article we report detailed observations, obtained through
the course of the Carnegie Supernova Project (here after
This
Casilla
mhamuy@das.uchile.cl.
16Infrared Processing and Analysis Center, Caltech/Jet Propul-
sion Laboratory, Pasadena, CA 91125.
17Department of Astronomy and Steward Observatory, Univer-
sity of Arizona, Tucson, AZ 85721, USA; pmilne@as.asrizona.edu.
18Cosmology and Gravity Group, Department of Mathematics
and Applied Mathematics, University of Cape Town, South Africa;
mathew.smith@uct.ac.za.
19Texas A&M University, Physics Department, College Station,
TX 77843; nsuntzeff@tamu.edu.
36-D,Santiago,Chile;gaston@das.uchile.cl,

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2Stritzinger et al.
CSP; Hamuy et al. 2006), of the Type Ib SN 2007Y. The
earliest spectra of this event bear a striking resemblance
to the unusual Type Ib SN 2005bf (Anupama et al. 2005;
Folatelli et al. 2006), however its light curves exhibit a
more ordinary evolution.
SN 2007Y (see Fig. 1) was discovered in the nearby spi-
ral galaxy NGC 1187 on Feb. 15.77 UT20(Monard 2007),
and was initially classified as a young peculiar SN Ib/c
(Folatelli et al. 2007). As this was a young and nearby
supernova it made an excellent target for our follow-up
program.
In this article a comprehensive set of X-ray, ultra-
violet, optical, near-infrared and radio observations of
SN 2007Y are presented to gain insight into the explosion
physics, and determine the stellar evolutionary path of
the progenitor. X-ray and ultraviolet observations were
carried out with the X-Ray Telescope (XRT) and the
UltraViolet Optical Telescope (UVOT) aboard the Swift
satellite (Roming et al. 2005). Early-phase spectroscopy
and imaging was collected at Las Campanas Observa-
tory (LCO). Late-time spectroscopy and imaging came
from facilities at LCO and at the European Organisation
for Astronomical Research in the Southern Hemisphere’s
(ESO) Paranal and La Silla Observatories. Finally, our
radio observations were obtained with the Very Large
Array.21
In § 2 a concise description of the observations is
given. More complete details regarding the data reduc-
tion methods applied to the optical and near-infrared
observations can be found in the Appendix. Section 3
contains the analysis of the data, while the discussion is
presented in § 4. We conclude with a summary in § 5.
2. OBSERVATIONS
Six weeks of imaging was obtained at LCO covering
the flux evolution from −14 to +41 days past maximum
light.22
Optical imaging was performed with a set of
Sloan u′g′r′i′and Johnson B and V filters, while in the
near-infrared a set of JHKsfilters identical to those used
by Persson et al. (1998) were employed. The Y band
is defined in Hillenbrand et al. (2002), and details con-
cerning its calibration are given in Hamuy et al. (2006,
hereafter Paper I) and Contreras et al. (2009). In ad-
dition, two epochs of optical and near-infrared imaging
were conducted at late epochs with the VLT.
Nineteen epochs of early phase photometry obtained
with UVOT and originally published in Brown et al.
(2008) are also included in our analysis. These data were
obtained with w2m2w1UBV passbands and are critical
to calculating a nearly complete bolometric light curve.
SN 2007Y was also observed with XRT. No X-ray emis-
sion was detected, but these observations allow us to
place an upper limit on the mass-loss rate of the pro-
genitor prior to explosion.
Early and late phase radio monitoring spanning from
−8 to 653 days past maximum were obtained with the
VLA. With these observations constraints are placed on
the mass-loss rate of the progenitor star and the interac-
20Universal Time (UT) is used through out this article.
21The Very Large Array is a facility of the National Science
Foundation operated under cooperative agreement by Associated
Universities, Inc.
22Maximum light refers to the time of peak bolometric bright-
ness (Lmax), i.e. March 3.5 or JD-2454163.12 (see § 3.4).
tion of circumstellar material (CSM) with the supernova
shock wave. Our radio observations are complementary
to the XRT observations and provide a more stringent
limit on the mass-loss rate.
Table 1 contains the positions and average magnitudes
of the optical and near-infrared local sequence stars used
to compute final photometry from the CSP images. The
definitive optical and near-infrared photometry in the
standard Landolt (1992) (BV ) and Smith et al. (2002)
(u′g′r′i′) systems are given in Table 2 and Table 3, re-
spectively.
Early phase ultraviolet, optical, and near-infrared light
curves of SN 2007Y are shown in Fig. 2. Late phase pho-
tometry is combined with the corresponding early epoch
light curves in Fig. 3.Note, the VLT optical images
were taken with a set of facility Johnson Kron-Cousins
BV RI and the near-infrared images with JsHKsfilters
(Persson et al. 1998; Hillenbrand et al. 2002).
A total of twelve epochs of long slit optical spec-
troscopy were collected over the course of ∼300 days. Ta-
ble 4 contains the journal of spectroscopic observations
and gives details concerning the telescope, instrument
setup, and final data product. The spectra corrected to
the rest frame of the host galaxy are presented in Fig. 4.
For clarity each spectrum has been shifted below the top
one by an arbitrary constant. The number in parenthe-
ses gives the epoch with respect to Lmax. Four of these
spectra were taken when the ejecta was fully nebular. A
subset of these are modeled in order to derive physical
parameters of SN 2007Y (see § 3.7).
3. ANALYSIS
3.1. Distance and reddening
With coordinates of α = 03h02m35.s92 and δ =
−22◦53′50.1′′, SN 2007Y was located 24′′west and 123′′
south of the nucleus of NGC 1187; well outside of any
spiral arm. The NASA/IPAC Extragalactic Database
(NED) lists a heliocentric recessional velocity for this
galaxy of 1390 km s−1. With a value of H◦ = 72±8
km s−1Mpc−1(Freedman et al. 2001) and adopting a
400 km s−1uncertainty in the redshift distance, the dis-
tance to NGC 1187 is 19.31±5.56 Mpc (µ = 31.43 ±
0.55). If this distance is corrected for Virgo infall it be-
comes 10% smaller. In this paper we will use the more
traditional heliocentric distance, but note that a Virgo
infall corrected distance would make our estimated values
of the peak absolute magnitude, luminosity, and mass of
56Ni about 10% less.
Accordingtotheinfrared
Schlegel, Finkbeiner, & Davis
due to the Milky Way in the direction of NGC 1187 is
E(B−V )gal= 0.022 mag. The optical colors of SN 2007Y
near peak brightness (see below) are relatively blue,
suggesting minimal host galaxy extinction. A spectrum
obtained near maximum light shows the presence of a
weak Na I D absorption feature at the redshift of the
host galaxy with an equivalent width of 0.56˚ A. Using
the relationship between the equivalent width of the
Na I D and E(B-V) (see Turatto, Benetti, & Cappellaro
2003; Taubenberger et al. 2006) suggests a host galaxy
reddening of E(B − V )host= 0.09 mag. Combining this
with the Galactic reddening gives E(B − V )tot= 0.112
mag. This value is adopted in the subsequent analysis.
dustmaps
reddening
of
(1998)the

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The Type Ib supernova 2007Y3
3.2. X-Ray Observations and Limiting Luminosity
Swift X-Ray Telescope (XRT; Burrows et al. 2005)
observations were obtained simultaneously with the
UVOT observations. To search for X-ray emission from
SN 2007Y, we extracted X-ray counts from a circular re-
gion with a 10 pixel radius (23.′′7, corresponding to the
XRT on-axis 90% encircled energy radius) centered on
the optical position of the SN. The background was ex-
tracted locally from a source-free region of radius of 40′′
to account for detector and sky background, and for dif-
fuse emission from the host galaxy.
No X-ray source is detected in the merged 66.5 ks XRT
data obtained in photon-counting mode. The 3σ upper
limit to the XRT net count rate is 2.63 × 10−4cts s−1,
corresponding to an unabsorbed (0.2–10 keV band) X-
ray flux of f0.2−10 < 1.24 × 10−14erg cm−2s−1and a
luminosity L0.2−10< 5.5 × 1038erg s−1for an adopted
thermal plasma spectrum with a temperature of kT =
10 keV (see Fransson, Lundqvist & Chevalier 1996, and
references therein), a Galactic foreground column density
of NH= 1.91×1020cm−2(Dickey & Lockman 1990) and
a distance of 19.31 Mpc.
In order to search for a possible late X-ray turn-on
phase, we binned the XRT data into two time intervals
ranging from day −14 to +23 and day +290 to +387 with
exposure times of 56.5 ks and 10.1 ks, respectively. No X-
ray source is found at the position of SN 2007Y in either
of the two epochs, down to 3σ limiting luminosities of <
6.5×1038erg s−1and < 1.9×1039erg s−1, respectively.
The lack of X-ray emission can be used to put loose
limits on the mass-loss rate of the progenitor system.
Following the discussion in Immler et al. (2007, and ref-
erences therein), an upper limit to the mass-loss rate of
˙M ≈ 2×10−5M⊙yr−1(vw/10 km s−1) with an uncer-
tainty of a factor of 2–3 is obtained. In this calculation
the model assumes thermal emission, which is more ap-
propriate for Type II SNe. In addition the wind velocity
of Wolf-Rayet stars is known to be of the order of 1000
km s−1. Taking this into account then implies a limit on
the mass-loss of ∼ 2× 10−3M⊙yr−1. To obtain a more
robust estimate on the mass-loss rate we turn to con-
straints provided by the radio observations (see § 3.8).
3.3. Ultraviolet, Optical and Near-infrared Light Curves
The nicely sampled ultraviolet, optical, near-infrared
light curves presented in Fig. 2 constitute one of the more
complete collections obtained for a He-rich SN Ib.
Under close inspection the CSP and UVOT B- and V -
band light curves exhibit excellent agreement with one
another. However, the U-band light curve of UVOT is
systematically ∼0.7 mag brighter than CSP’s u′-band
light curve. This is not surprising considering the ap-
preciable difference between the transmission functions
of these two filters. The u′-band has an effective wave-
length λeff = 3681˚ A and a FWHM of 487˚ A, while the
U-band has λeff= 3465˚ A and a FWHM of 785˚ A.
To see if this discrepancy could be attributed to the
differences in filter transmissivities we performed an ex-
periment using spectrophotometry of SN 2007Y. First,
appropriate zero-points were computed for each pass-
band using a library of spectrophotometric flux stan-
dards (Stritzinger et al. 2005). Armed with these zero-
points and the early phase spectra that had their wave-
length coverageextended to the atmospheric cutoff by as-
suming zero flux, synthetic magnitudes of SN 2007Y were
determined for the two passbands. We find the U-band
filter consistently gives synthetic magnitudes brighter
than u′-band, and the disparity is in agreement with the
average difference between the broad-band magnitudes.
Figure 5 shows the (u′− B) and (B − V ) color curves
of SN 2007Y, both of which track photospheric tem-
perature variations.Included for comparison are the
color curves of two well-observed SNe IIb: SN 2008ax
(Pastorello et al. 2008) and SN 2008aq (CSP paper in
preparation). Also included is the (B-V) color curve of
the Type Ib SN 1999ex (Stritzinger et al. 2002). The
color curves of each SNe have been corrected for extinc-
tion using reddening values of E(B − V ) of 0.30, 0.11,
0.05 and 0.30 for SN 1999ex, SN 2007Y, SN 2008aq and
SN 2008ax, respectively. As the brightness of the su-
pernova increases the photospheric temperature also in-
creases, which in turn, causes the color curves to evolve
to the blue. Specifically, the color of SN 2007Y decreases
from a value of (B −V ) = 0.47 on day −12.8 to (B −V )
= 0.06 on day −4.3. The colors then evolve monotoni-
cally back to the red where (B −V ) = 1.0 on day +31.2.
Then the colors moved marginally back towards the blue.
Interestingly, around 3 weeks past maximum the UVOT
ultraviolet light curves appear to stop declining in bright-
ness. Although UVOT observations were discontinued
near this epoch the u′-band light curve indicates that the
flux in this passband not only stopped declining but actu-
ally increased(!). Specifically, the u′band decreases from
19.800 mag on day +22 to 19.408±0.110 nineteen days
later. This phenomenon coincides with a (u′− B) color
evolution to the blue that is more prevalent than in other
He-rich core-collapse SN like SN 2008ax (see bottom
panel of Fig. 5). A blue excess is also seen in SN 2008aq
where the u′-band light curve becomes brighter by 0.63
mag from day +30 to +96. This is followed by a dimming
of the u′light curve by 0.55 mag from day +96 to +123.
Notice in the bottom panel of Fig. 5 the (u′− B) col-
ors curves of SN 2007Y and SN 2008aq are very similar
between days +25 and +35.
The excess in ultraviolet emission could be connected
to a change in opacity of the ejecta. Unfortunately the
spectra do not extend far enough in the blue to deter-
mine if the spectral energy distribution of the supernova
actually increases in the region covered by the u′-band fil-
ter. Alternatively, this could be the consequence of shock
heating produced from the interaction of high-velocity
supernova ejecta with CSM (e.g. Immler et al. 2006). As
we will see below there is evidence in the late epoch op-
tical spectrum of such an interaction.
We now turn to the broad-band absolute magnitudes
of SN 2007Y. First the time and observed magnitude at
maximum light of each filter light curve was estimated
with the use of moderate order (5–8) polynomial fits;
these values are given in Table 5.
SNe Ib/c, the ultraviolet/blue light curves of SN 2007Y
peak ∼4 days prior to the optical light curves. This is fol-
lowed by the near-infrared bands peaking 3–5 days later.
Peak absolute magnitudes were computed using the red-
dening value of E(B−V )tot= 0.112 mag, an RV value of
3.1, and a distance of 19.31±5.56 Mpc (see § 3.1). These
values are also listed in Table 5 with the quoted errors
accounting for uncertainty in the estimated value of peak
Similarly to other

Page 4

4 Stritzinger et al.
magnitude and the distance to the host galaxy. With an
absolute magnitude of MV =−16.5±0.6 mag, SN 2007Y
is similar in brightness to SN 2008D, which peaked at
MV ≈−16.7 mag (Soderberg et al. 2008). This turns out
to be on the faint end of the absolute magnitude distri-
bution of SNe Ib/c. For example SN 2006aj peaked at
MV =−18.7±0.1 (Modjaz et al. 2006), while other pre-
viously observed GRB/X-ray transient related SNe Ib/c
have peaked at MV < −20.0 mag (see Richardson et al.
2006, and references therein).
In Fig. 6 the B- and V -band light curves of SN 2007Y
are compared with those of four other well-observed
SNe Ib/c: SN 1994I (Richmond et al. 1996), SN 1998bw
(Galama et al. 1998), SN 2005bf (Folatelli et al. 2006),
and SN 2006aj (Mirabal et al. 2006; Sollerman et al.
2006).The observed magnitudes of the comparison
SNe Ib/c have been normalized to the peak magnitudes
of SN 2007Y and shifted in time to their respective epoch
of peak brightness. The light curves of SN 1998bw and
SN 2006aj have been corrected for time dilation, and
SN 2006aj has also had its underlying host galaxy con-
tribution removed.SN 2007Y rises to maximum like
SN 1998bw and SN 2005bf, however, after maximum it
declines rapidly like SN 2006aj. Then about three weeks
past maximum the rate of decline begins to slow, and
again, the V -band light curve of SN 2007Y is similar
to that of SN 1998bw. We note that SN 1999ex and
SN 2008D were not included in Fig. 6, but if they were,
one would see that the width of their light curves is sig-
nificantly broader than SN 2007Y. Overall the shape of
the V -band light curve, which contains the majority of
flux around maximum, is most similar to SN 2006aj.
Looking at Arnett (1982)’s analytical description of the
shape of the light curve around peak brightness, we can
infer from the similar shaped light curves that the effec-
tive diffusion time τm, which can expressed as
τm∝ κ1/2
is similar in SNe 2006aj and 2007Y. In this expression
κoptrefers to the mean optical opacity, Mejcis the ejecta
mass, and Ekin is the kinetic energy of the expansion.
This then implies the ratio of M3
between these two events.
Now we turn our attention to the late phase VLT pho-
tometry shown in Fig. 3. The optical light curves indi-
cate magnitude differences from peak to the last observed
epoch of ∆m(B) = 7.9 and ∆m(R) = 6.9.
more a weighted least square analysis gives decline rates
per hundred days of B = 1.30±0.31, V = 1.73 ± 0.20,
R = 1.65 ± 0.25 and I = 1.82 ± 0.25. Clearly these de-
cline rates are faster than the decline of 0.98 magnitude
per hundred days that is expected in the case of complete
trapping of the γ rays produced from the decay of56Co
to56Fe. Although the measured magnitude decline rates
are rather uncertain given that only two epochs were
observed, they are nonetheless in agreement with other
SNe Ib/c (e.g. Barbon et al. 1994; Sollerman et al. 1998;
Patat et al. 2001; Elmhamdi et al. 2004; Tomita et al.
2006; Clocchiatti et al. 2008), and hints towards a low
ejecta mass.
optM3/4
ejcE−1/4
kin
,(1)
ejcto Ekinis also similar
Further-
3.4. Bolometric Light Curve
The comprehensive wavelength coverage provided by
the early-time observations affords an excellent oppor-
tunity to construct a nearly complete bolometric light
curve of SN 2007Y. To this end it was first necessary to
account for gaps in any individual light curve when a
particular passband was not observed on a given night.
In these cases it proved necessary to interpolate between
the existing light curve using low order polynomial fits.
As the m2-band light curve covers only a limited time
around maximum its temporal coverage was extended
by extrapolation assuming a color correction (based on
the available photometry) of (w2 − m2) = −0.929. Sim-
ilarly as the Ks light curve sampling was rather sparse
a (V − Ks) = 1.0 color was adopted to extend the light
curve during the pre-maximum phase, and a (V − Ks)
= 0.2 color was adopted to extrapolate the light curve
from day +12 to +42. Although these two assumptions
are rather simple they do provide a reasonably accurate
extrapolation of the light curves.
Next, the photometry was corrected for extinction us-
ing our adopted value of reddening and a RV = 3.1
extinction curve for the optical and near-infrared data,
while the extinction curve of Pei (1992) was used to cor-
rect the ultraviolet photometry. The magnitudes were
then converted to flux at the effective wavelength of each
filter. In the case of the UVOT photometry, zero-points
and count-rate-to-flux conversion factors given in Table 6
and Table 9 of Poole et al. (2008) were used.
To compute the quasi-bolometric (UV to NIR, here-
after UVOIR) light curve the observed magnitudes of
each light curve were converted to flux and then in-
tegrated over frequency.The summed flux was then
converted to luminosity using our adopted distance to
NGC 1187.Note the last four epochs of the early
phase UVOIR light curve were computed without ul-
traviolet observations, and the late phase bolometric
fluxes were derived with BV RIJsH photometry.
Fig. 7 the UVOIR light curve of SN 2007Y is compared
to the UVOIR light curves of several low luminosity
SNe Ib.These include: SN 1999ex (Stritzinger et al.
2002), SN 2005bf (Folatelli et al. 2006), SN 2008D
(Malesani et al. 2009) and the Type IIb SN 1993J
(Richmond et al. 1994). Extinction values of AV = 0.6,
0.93 and 2.5 mag, and distances of 3.6, 44.1 and 30.0 Mpc
were adopted for SN 1993J, SN 1999ex and SN 2008D, re-
spectively. In the case of SN 2005bf, values of ‘Lbol’ were
taken directly from Table 2 of Folatelli et al. (2006).
The UVOIR light curve of SN 2007Y indicates a peak
luminosity of ∼1.30×1042erg s−1. The main uncertainty
in this estimate is the error in the distance which is of
the order of 30%. One way to estimate the abundance
of56Ni, the main radioactive isotope that powers the
light curves of supernovae, is through the use of Arnett’s
Rule (Arnett 1982). Although based on a number of un-
derlying assumptions, Richardson et al. (2006) have used
this method to obtain reasonable estimates of the56Ni
yields for a sample of two dozen SNe Ib/c. In the case
of SN 2007Y, if we assume a rise time of 18 days, appli-
cation of Arnett’s Rule tells us ≈0.06±0.02 M⊙of56Ni
was synthesized in SN 2007Y.23
An alternative method of estimating the56Ni yield is
In
23We note in passing that the rise times of SNe Ib vary sig-
nificantly. The rise times of SNe 1999ex and 2008D were 18 days
(Stritzinger et al. 2002; Modjaz et al. 2009), while SN 2006aj ex-
hibited a rise time of 11 days (Sollerman et al. 2006).

Page 5

The Type Ib supernova 2007Y5
to fit a radioactive decay energy deposition function to
the late phase UVOIR light curve. Under the reasonable
assumptions that the majority of energy deposited in the
ejecta at late phases is from the56Co →56Fe decay,
and that the optical depth, τ, is much less than one, a
simple model that estimates the56Ni-mass can be fitted
to the UVOIR light curve from 50 to 350 days. The
functional form of the model used (see Sollerman et al.
1998) is L=1.3 × 1043MNi e−t/111.3(1 − 0.966e−τ)
erg s−1, where τ is given by (t1/t)2. In this form t1sets
the time when the optical depth to γ rays is unity.
Before the energy deposition function could be used
to estimate the56Ni mass it first deemed necessary to
extend the UVOIR light curve from day +41 to +60.
This was achieved by applying a linear least squares fit
to the UVOIR light curve from day +28 to +41; this
best fit was then used to extrapolate the light curve to
day +60. Note the assumption that the decline rate be-
tween days +28 to +41 can be used to extend all but
the u′-band light curves out to day +60 is reasonable as
other SNe Ib/c show no changes in decline rate during
the epochs in question (see Clocchiatti et al. 2008). Fi-
nally the best fit of the toy model to the UVOIR light
curve was obtained with a t1 of 35.8 days and a56Ni
mass of 0.06 M⊙.
The broad wavelength coverage obtained for SN 2007Y
also allows for an estimation of the distribution of flux
(as a function of time) in the different wavelength do-
mains. These estimates are shown in Fig. 8. Similar to
Type Ia supernovae at maximum the majority of flux
is emitted in the optical, however the ultraviolet light
curves contained a significant portion of flux (?20%);
while the near-infrared contribution was small, ∼5%. By
two weeks past Lmaxthe flux in the near-infrared pass-
bands increased to roughly 20% while in the ultraviolet
it dropped to ≤10%. During the last two epochs (day
+270 and +344) ∼40% of the observed flux was con-
tained in the R and I bands because these passbands
coincide with the strong emission lines of O I, Hα, Fe II,
and Ca II that dominate the spectrum (see below). By
day +270 ∼5% of the flux was contained in the H band
while the near-infrared as a whole contributes no more
than 15%. Evidently at this phase near-infrared emission
lines were not a major coolant.
3.5. Early Phase Spectroscopic Evolution
The early spectra of SN 2007Y (Fig. 4) consist of a se-
ries of P-Cygni features superimposed on a blue pseudo
continuum. Multiple absorption and emission features
of Fe II dominate the blue-end of the spectra while an
exceptionally strong Ca II λλ8498, 8542, 8662 triplet
rounds out the red-end. Distinctive features include: (i)
an absorption trough between 4000 and 4200˚ A caused
by the blending of Fe-group emission lines (i.e. Sc II,
Ti II or alternatively Fe II), (ii) absorption at ∼5900
˚ A caused by a blend of the Na I λλ5890, 5896 doublet
with He I λ5876, (iii) a distinctive absorption feature at
∼6200˚ A that is possibly formed from a blending of Si II
with high-velocity Hα, and (iv) a relatively weak O I
λ7774 absorption feature.
As SN 2007Y evolved through maximum, the strength
of the absorption at 6200˚ A steadily declined until it
disappeared around 10 days past Lmax. Meanwhile the
depth of the Ca II triplet also decreased until day +6
when it transformed back to a deep absorption. This
phenomenon reflects the transition of the Ca II triplet
being formed from a high-velocity component to a photo-
spheric component (see Fig. 10, and Folatelli et al. 2006
and Parrent et al. 2007). After maximum, conspicuous
lines of He I λλ4471, 4921, 5016, 6678, 7065, 7281 emerge
and begin to dominate the spectrum.
Figure 9 displays the time evolution of the blue-shifts
(vexp) for the ions that produced the dominant fea-
tures in the optical spectrum of SN 2007Y. The expan-
sion velocity of Fe II λ6159 is often used as an indi-
cator of the photospheric velocity (Branch et al. 2002;
Richardson et al. 2006). In the earliest spectra the ab-
sorption minimum of this line indicates that vexp∼10,000
km s−1, while He I is measured as vexp∼7000 km s−1.
This is reminiscent of SN 2005bf, which to date has been
the only other SN Ib to show Fe II absorption blue-shifted
more than its He I absorption. As SN 2007Y evolved in
time, Fe II λ6159 monotonically declined, reaching a min-
imum value of ∼4500 km s−1three weeks past maximum.
On the other hand, the blue-shift of He I λ5876 shows an
increase of ∼1500 km s−1from day −14 to Lmax. There-
after the blue-shift of He I λ5876 evolved from 8500 to
6500 km s−1over the next month.
Contrarily to all other ions, the velocity at maximum
absorption of the 6200˚ A feature (assuming it is related
to Hα, see § 4) and the Ca II triplet, suggest these
features were produced from a component of gas that
was detached from the photosphere with vexp ∼15,500
km s−1. By a week past maximum Hα had decreased
to vexp ∼10,000 km s−1, while the Ca II triplet began
to follow the velocity evolution of He I with vexp∼ 6500
km s−1. The abnormally low blue-shifts of Si II argues
against the case that it is the dominate ion responsible
for the formation of the 6200˚ A feature.
In Fig. 10 spectra of SN 2007Y at days −14, −8, −1
and +39 are compared to spectra of SN 2005bf obtained
on days −8, −3, +4 and +42.24The similarity between
the earliest spectra of these two SNe Ib is remarkable.
Two subtle differences exist, namely a somewhat stronger
Ca II triplet in SN 2007Y, while the Fe II multiplet 42
λλ4924, 5018, 5169 features in SN 2005bf are more nar-
row. In this figure one can see the position of maxi-
mum absorption of the Ca II triplet clearly evolves in
time. The narrow features in SN 2005bf, on the other
hand, have been interpreted to be from a high-velocity
component of Fe gas in the ejecta (Anupama et al. 2005;
Folatelli et al. 2006; Parrent et al. 2007), that maybe re-
lated to a polar outflow or a failed jet (Folatelli et al.
2006; Maeda et al. 2007b; Maund et al. 2007). The lack
of these narrow features in SN 2007Y suggests its ejecta
was more spherical than in the case of SN 2005bf.
Finally, at the bottom of Fig. 10, spectra of SNe 2007Y
and 2005bf taken over a month past maximum are com-
pared. By this time the spectra of both events resemble
normal SNe Ib.
3.6. Nebular Phase Spectroscopy
Months later, as SN 2007Y continued to expand, the
photosphere receded deep into the ejecta leading to the
24Epochs of SN 2005bf spectra are with respect to the time of
its first maximum.