We present photometric and spectroscopic observations of the type Ibn supernova (SN) 2019uo, the second ever SN Ibn with flash ionization (He ii, C iii, N iii) features in its early spectra. SN ...2019uo displays a rapid post-peak luminosity decline of 0.1 mag day−1 similar to most of the SNe Ibn, but is fainter ( mag) than a typical SN Ibn and shows a color evolution that places it between SNe Ib and the most extreme SNe Ibn. SN 2019uo shows P-cygni He i features in the early spectra which gradually evolve and become emission dominated post peak. It also shows faster evolution in line velocities as compared to most other members of the type Ibn subclass. The bolometric light curve is fairly well described by a 56Ni + circumstellar interaction model.
We study a sample of 16 Type Ia supernovae (SNe Ia) having both spectroscopic and photometric observations within 2-3 days after the first light. The early B − V colors of such a sample tend to show ...a continuous distribution. For objects with normal ejecta velocity (NV), the C ii λ6580 feature is always visible in the early spectra, while it is absent or very weak in the high-velocity (HV) counterpart. Moreover, the velocities of the detached high-velocity features (HVFs) of the Ca II near-IR triplet (CaIR3) above the photosphere are found to be much higher in HV objects than in NV objects, with typical values exceeding 30,000 km s−1 at 2-3 days. We further analyze the relation between the velocity shift of late-time Fe II lines (vFe II) and host galaxy mass. We find that all HV objects have redshifted vFe II, while NV objects have both blue- and redshifted vFe II. It is interesting to point out that the objects with redshifted vFe II are all located in massive galaxies, implying that HV and a portion of NV objects may have similar progenitor metallicities and explosion mechanisms. We propose that, with a geometric/projected effect, the He-detonation model may account for the similarity in birthplace environment and the differences seen in some SNe Ia, including B − V colors, C II features, CaIR3 HVFs at early times, and vFe II in the nebular phase. Nevertheless, some features predicted by He-detonation simulation, such as the rapidly decreasing light curve, deviate from the observations, and some NV objects with blueshifted nebular vFe II may involve other explosion mechanisms.
Abstract
We have conducted photometric and spectroscopic observations of the peculiar Type Ia supernova (SN Ia) 2016ije that was discovered through the Tsinghua-NAOC Transient Survey. This peculiar ...object exploded in the outskirts of a metal-poor, low-surface brightness galaxy (i.e.,
M
g
= −14.5 mag). Our photometric analysis reveals that SN 2016ije is subluminous (
M
B
,
max
= −17.65 ± 0.06 mag) but exhibits relatively broad light curves (Δ
m
15
(
B
) = 1.35 ± 0.14 mag), similar to the behavior of SN 2002es. Our analysis of the bolometric light curve indicates that only 0.14 ± 0.04
M
⊙
of
56
Ni was synthesized in the explosion of SN 2016ije, which suggests a less energetic thermonuclear explosion when compared to normal SNe Ia, and this left a considerable amount of unburned materials in the ejecta. Spectroscopically, SN 2016ije resembles other SN 2002es-like SNe Ia, except that the ejecta velocity inferred from its carbon absorption line (∼4500 km s
−1
) is much lower than that from silicon lines (∼8300 km s
−1
) at around the maximum light. Additionally, most of the absorption lines are broader than other 02es-like SNe Ia. These peculiarities suggest the presence of significant unburned carbon in the inner region and a wide line-forming region along the line of sight. These characteristics suggest that SN 2016ije might originate from the violent merger of a white dwarf binary system, when viewed near an orientation along the iron-group-element cavity caused by the companion star.
ABSTRACT We present well-sampled optical observations of the bright Type Ia supernova (SN Ia) SN 2011fe in M101. Our data, starting from ∼16 days before maximum light and extending to ∼463 days after ...maximum, provide an unprecedented time series of spectra and photometry for a normal SN Ia. Fitting the early-time rising light curve, we find that the luminosity evolution of SN 2011fe follows a tn law, with the index n being close to 2.0 in the VRI bands but slightly larger in the U and B bands. Combining the published ultraviolet (UV) and near-infrared (NIR) photometry, we derive the contribution of UV/NIR emission relative to the optical. SN 2011fe is found to have stronger UV emission and reaches its UV peak a few days earlier than other SNe Ia with similar Δm15(B), suggestive of less trapping of high-energy photons in the ejecta. Moreover, the U-band light curve shows a notably faster decline at late phases (t 100-300 days), which also suggests that the ejecta may be relatively transparent to UV photons. These results favor the notion that SN 2011fe might have a progenitor system with relatively lower metallicity. On the other hand, the early-phase spectra exhibit prominent high-velocity features (HVFs) of O i λ7773 and the Ca ii NIR triplet, but only barely detectable in Si ii 6355. This difference can be caused by either an ionization/temperature effect or an abundance enhancement scenario for the formation of HVFs; it suggests that the photospheric temperature of SN 2011fe is intrinsically low, perhaps owing to incomplete burning during the explosion of the white dwarf.
We present optical spectroscopic and photometric observations of the nearby type Ic supernova (SN Ic) SN 2014L. This SN was discovered by the Tsinghua-NAOC Transient Survey (TNTS) in the nearby ...type-Sc spiral galaxy M99 (NGC 4254). Fitting to the early-time light curve indicates that SN 2014L was detected at only a few hours after the shock breakout, and it reached a peak brightness of MV = −17.73 0.28 mag (L = 2.06 0.50 ×1042 erg s−1) approximately 13 days later. SN 2014L shows a close resemblance to SN 2007gr in the photometric evolution, while it shows stronger absorption features of intermediate-mass elements (especially Ca ii) in the early-time spectra. Based on simple modeling of the observed light curves, we derived the mass of synthesized 56Ni as MNi = 0.075 0.025 M , and the mass and total energy of the ejecta as Mej = 1.00 0.20M and Eej = 1.45 0.25 foe, respectively. Given these typical explosion parameters, the early detection, and the extensive observations, we suggest that SN 2014L could be a template sample for the investigation of SNe Ic.
ABSTRACT The high-velocity features (HVFs) in optical spectra of type Ia supernovae (SNe Ia) are examined with a large sample including very early-time spectra (e.g., t < −7 days). Multiple Gaussian ...fits are applied to examine the HVFs and their evolutions, using constraints on expansion velocities for the same species (i.e., Si ii 5972 and Si ii 6355). We find that strong HVFs tend to appear in SNe Ia with smaller decline rates (e.g., Δm15(B) ), clarifying that the finding by Childress et al. for the Ca-HVFs in near-maximum-light spectra applies both to the Si-HVFs and Ca-HVFs in the earlier phase. The Si-HVFs seem to be more common in rapidly expanding SNe Ia, which is different from the earlier result that Ca-HVFs are associated with SNe Ia that have slower Si ii 6355 velocities at maximum light (i.e., VSimax). Moreover, SNe Ia with both stronger HVFs at early phases and larger VSimax are found to have noticeably redder colors and to occur preferentially in the inner regions of their host galaxies, while those with stronger HVFs but smaller VSimax show opposite tendencies, suggesting that these two subclasses have different explosion environments and their HVFs may have different origins. We further examine the relationships between the absorption features of Si ii 6355 and Ca ii IR lines, and find that their photospheric components are well correlated in velocity and strength but that the corresponding HVFs show larger scatter. These results cannot be explained with ionization and/or thermal processes alone, and different mechanisms are required for the creation of HVF-forming regions in SNe Ia.
ABSTRACT The absorption feature O i λ7773 is an important spectral indicator for type Ia supernovae (SNe Ia) that can be used to trace the unburned material in outer layers of the exploding white ...dwarf (WD). In this work, we use a large sample of SNe Ia to examine this absorption at early phases (i.e., −13 day t −7 day) and make comparisons with the absorption features of Si ii λ6355 and the Ca ii near-infrared triplet. We show that for a subgroup of spectroscopically normal SNe with normal photospheric velocities (i.e., vsi 12,500 km s−1 at optical maximum), the line strength of the high velocity feature (HVF) of O i is inversely correlated with that of Si ii (or Ca ii), and this feature also shows a negative correlation with the luminosity of SNe Ia. This finding, together with other features we find for the O i HVF, reveal that for this subgroup of SNe Ia, explosive oxygen burning occurs in the outermost layer of the SN. Differences in the oxygen burning could lead to the observed diversity, which is in remarkable agreement with the popular delayed-detonation model of Chandrasekhar mass WDs.
Abstract
In this paper, we present photometric and spectroscopic observations of the subluminous Type Ia supernova (SN Ia) 2012ij, which has an absolute
B
-band peak magnitude
M
B
,
max
=
−
17.95
±
...0.15
mag. The
B
-band light curve exhibits a fast postpeak decline with Δ
m
15
(
B
) = 1.86 ± 0.05 mag. All the
R
- and
I
/
i
-band light curves show a weak secondary peak/shoulder feature at about 3 weeks after the peak, like some transitional subclass of SNe Ia, which could result from an incomplete merger of near-infrared (NIR) double peaks. The spectra are characterized by Ti
ii
and strong Si
ii
λ
5972 absorption features that are usually seen in low-luminosity objects like SN 1999by. The NIR spectrum before maximum light reveals weak carbon absorption features, implying the existence of unburned materials. We compare the observed properties of SN 2012ij with those predicted by the sub-Chandrasekhar-mass and the Chandrasekhar-mass delayed-detonation models and find that both optical and NIR spectral properties can be explained to some extent by these two models. By comparing the secondary maximum features in the
I
and
i
bands, we suggest that SN 2012ij is a transitional object linking normal SNe Ia to typical 91bg-like ones. From the published sample of SNe Ia from the Carnegie Supernova Project II, we estimate that the fraction of SN 2012ij–like SNe Ia is not lower than ∼2%.
Abstract
We present extensive optical photometric and spectroscopic observations of the high-velocity (HV) Type Ia supernova (SN Ia) 2017fgc, covering the phase from ∼12 days before to ∼389 days ...after maximum brightness. SN 2017fgc is similar to normal SNe Ia, with an absolute peak magnitude of
M
max
B
≈
−19.32 ± 0.13 mag and a post-peak decline of Δ
m
15
(
B
) = 1.05 ± 0.07 mag. Its peak bolometric luminosity is derived as (1.32 ± 0.13) × 10
43
erg s
−1
, corresponding to a
56
Ni mass of 0.51 ± 0.03
M
⊙
. The light curves of SN 2017fgc are found to exhibit excess emission in the
UBV
bands in the early nebular phase and pronounced secondary shoulder/maximum features in the
RrIi
bands. Its spectral evolution is similar to that of HV SNe Ia, with a maximum-light Si
ii
velocity of 15,000 ± 150 km s
−1
and a post-peak velocity gradient of ∼120 ± 10 km s
−1
day
−1
. The Fe
ii
and Mg
ii
lines blended near 4300 Å and the Fe
ii
, Si
ii
, and Fe
iii
lines blended near 4800 Å are obviously stronger than those of normal SNe Ia. Inspecting a large sample reveals that the strength of the two blends in the spectra, and the secondary peak in the
i
/
r
-band light curves, are found to be positively correlated with the maximum-light Si
ii
velocity. Such correlations indicate that HV SNe Ia may experience more complete burning in the ejecta and/or that their progenitors have higher metallicity. Examining the birthplace environment of SN 2017fgc suggests that it likely arose from a stellar environment with young and high-metallicity populations.
ABSTRACT
We present extensive observations of SN 2018zd covering the first ∼450 d after the explosion. This SN shows a possible shock-breakout signal ∼3.6 h after the explosion in the unfiltered ...light curve, and prominent flash-ionization spectral features within the first week. The unusual photospheric temperature rise (rapidly from ∼12 000 to above 18 000 K) within the earliest few days suggests that the ejecta were continuously heated. Both the significant temperature rise and the flash spectral features can be explained by the interaction of the SN ejecta with the massive stellar wind ($0.18^{+0.05}_{-0.10}\, \rm M_{\odot }$), which accounts for the luminous peak ($L_{\rm max} = 1.36\pm 0.63 \times 10^{43}\, \rm erg\, s^{-1}$) of SN 2018zd. The luminous peak and low expansion velocity (v ≈ 3300 km s−1) make SN 2018zd like a member of the LLEV (luminous SNe II with low expansion velocities) events originating due to circumstellar interaction. The relatively fast post-peak decline allows a classification of SN 2018zd as a transition event morphologically linking SNe IIP and SNe IIL. In the radioactive-decay phase, SN 2018zd experienced a significant flux drop and behaved more like a low-luminosity SN IIP both spectroscopically and photometrically. This contrast indicates that circumstellar interaction plays a vital role in modifying the observed light curves of SNe II. Comparing nebular-phase spectra with model predictions suggests that SN 2018zd arose from a star of $\sim 12\, \rm M_{\odot }$. Given the relatively small amount of 56Ni ($0.013\!-\!0.035 \rm M_{\odot }$), the massive stellar wind, and the faint X-ray radiation, the progenitor of SN 2018zd could be a massive asymptotic giant branch star that collapsed owing to electron capture.
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