High-velocity SNe Ia Research Articles

Measuring the ejecta velocities of type Ia supernovae from the pan-STARRS1 medium deep survey

ABSTRACT There is growing evidence that Type Ia supernovae (SNe Ia) may originate from multiple explosion channels. Previous studies have indicated that the ejecta velocity of SNe Ia is one powerful tool to discriminate between different channels. In this work, we study ∼400 confirmed SNe Ia discovered by the Pan-STARRS1 Medium Deep Survey (PS1-MDS), and obtain a sample of ∼50 SNe Ia that have near-peak $\mathrm{Si}\, {\small II}\, \lambda 6355$ velocity ($v_{\mathrm{Si}\, {\small II}}$) measurements. We investigate the relationships between $v_{\mathrm{Si}\, {\small II}}$ and various parameters, including SN light-curve width, colour, host galaxy properties, and redshift. No significant trends are identified between $v_{\mathrm{Si}\, {\small II}}$ and light-curve parameters. Regarding the host-galaxy properties, we see a significant trend that high-velocity (HV) SNe Ia ($v_{\mathrm{Si}\, {\small II}}\gtrsim 12000$ km s$^{-1}$) tend to reside in more massive galaxies compared to normal velocity (NV) SNe Ia ($v_{\mathrm{Si}\, {\small II}}\lt 12000$ km s$^{-1}$) when combining both the PS1-MDS data set and those from previous low-z studies. While we do not see a significant trend between $v_{\mathrm{Si}\, {\small II}}$ and redshift, HV SNe Ia appear to be more prevalent in low-z samples than in high-z samples. We discuss several possibilities that could potentially contribute to this trend. Furthermore, we investigate the potential bias on SN Ia distances and find no significant difference in Hubble residuals between HV and NV subgroups.

The Blue Excess of High-velocity Type Ia Supernovae: Dust Scattering of Circumstellar Material

Type Ia supernovae (SNe Ia) originate from thermonuclear explosions of carbon-oxygen white dwarfs at masses approaching the Chandrasekhar limit, and are widely used as standard candles for cosmological distances. However, the progenitor system and explosion mechanism of SNe Ia are still unclear. The circumstellar environment of SNe Ia has received increasing attention in recent decades. The distance and other geometric properties of the circumstellar material provide essential clues for exploring the physical origin of SNe Ia. At the same time, the scattering of circumstellar dust can produce observable effects on the light curve, spectrum, and polarization during the late phase of SNe Ia. The spectroscopically normal SNe Ia can be classified into two categories: normal-velocity and high-velocity ones. The high-velocity SNe Ia have an apparent blue excess within a few months after the maximum brightness. This blue excess can be fitted by circumstellar dust scattering. Meanwhile, fitting the late-phase spectrum or imaging polarization of SNe Ia can constrain the grain size or the geometric distribution of circumstellar dust, respectively. The result indicates that multi-epoch image polarimetry during the late phase of SNe Ia is a crucial probe to reveal the characteristics of circumstellar dust.

SN 2019ein: a Type Ia supernova likely originated from a sub-Chandrasekhar-mass explosion

ABSTRACT We present extensive optical photometric and spectroscopic observations for the nearby Type Ia supernova (SN Ia) 2019ein, spanning the phases from ∼3 to ∼330 d after the explosion. This SN Ia is characterized by extremely fast expansion at early times, with initial velocities of Si ii and Ca ii being above ∼25 000–30 000 km s−1. After experiencing an unusually rapid velocity decay, the ejecta velocity dropped to ∼13 000 km s−1 around maximum light. Photometrically, SN 2019ein has a moderate post-peak decline rate (Δm15(B) = 1.35 ± 0.01 mag), while being fainter than normal SNe Ia by about 40 per cent (with $M^{\rm max}_{B} \approx -18.71 \pm 0.15$ mag). The nickel mass synthesized in the explosion is estimated to be 0.27–0.31 M⊙ from the bolometric light curve. Given such a low nickel mass and a relatively high photospheric velocity, we propose that SN 2019ein likely had a sub-Chandrasekhar-mass white dwarf (WD) progenitor, MWD ≲ 1.22 M⊙. In this case, the explosion could have been triggered by a double-detonation mechanism, for which 1D and 2D models with WD mass MWD ≈ 1 M⊙ and a helium shell of 0.01 M⊙ can reasonably produce the observed bolometric light curve and spectra. The predicted asymmetry as a result of double detonation is also favoured by the redshifted Fe ii and Ni ii lines observed in the nebular-phase spectrum. Possible diversity in origin of high velocity SNe Ia is also discussed.

The Foundation Supernova Survey: Photospheric Velocity Correlations in Type Ia Supernovae

Abstract The ejecta velocities of Type Ia supernovae (SNe Ia), as measured by the Si ii λ6355 line, have been shown to correlate with other supernova properties, including color and standardized luminosity. We investigate these results using the Foundation Supernova Survey, with a spectroscopic data release presented here, and photometry analyzed with the SALT2 light-curve fitter. We find that the Foundation data do not show significant evidence for an offset in color between SNe Ia with high and normal photospheric velocities, with Δc = 0.004 ± 0.015. Our SALT2 analysis does show evidence for redder high-velocity SNe Ia in other samples, including objects from the Carnegie Supernova Project, with a combined sample yielding Δc = 0.018 ± 0.008. When split on velocity, the Foundation SNe Ia also do not show a significant difference in Hubble diagram residual, ΔHR = 0.015 ± 0.049 mag. Intriguingly, we find that SN Ia ejecta velocity information may be gleaned from photometry, particularly in redder optical bands. For high-redshift SNe Ia, these rest-frame red wavelengths will be observed by the Nancy Grace Roman Space Telescope. Our results are in line with previous work that suggests SN Ia host-galaxy stellar mass is correlated with ejecta velocity: high-velocity SNe Ia are found nearly exclusively in high-stellar-mass hosts. However, host-galaxy properties alone do not explain velocity-dependent differences in supernova colors and luminosities across samples. Measuring and understanding the connection between intrinsic explosion properties and supernova environments, across cosmic time, will be important for precision cosmology with SNe Ia.

SN 2017fgc: A Fast-expanding Type Ia Supernova Exploded in Massive Shell Galaxy NGC 474

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 Δm 15(B) = 1.05 ± 0.07 mag. Its peak bolometric luminosity is derived as (1.32 ± 0.13) × 1043 erg s−1, corresponding to a 56Ni 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.

Light-curve properties of SN 2017fgc and HV SNe Ia

ABSTRACT Photometric and spectroscopic observations of Type Ia supernova (SN) 2017fgc, which cover the period from −12 to + 137 d since the B-band maximum are presented. SN 2017fgc is a photometrically normal SN Ia with the luminosity decline rate, Δm15(B)true = 1.10 ± 0.10 mag. Spectroscopically, it belongs to the high-velocity (HV) SNe Ia group, with the Si ii λ6355 velocity near the B-band maximum estimated to be 15 200 ± 480 km s−1. At the epochs around the near-infrared secondary peak, the R and I bands show an excess of ∼0.2-mag level compared to the light curves of the normal velocity (NV) SNe Ia. Further inspection of the samples of HV and NV SNe Ia indicates that the excess is a generic feature among HV SNe Ia, different from NV SNe Ia. There is also a hint that the excess is seen in the V band, both in SN 2017fgc and other HV SNe Ia, which behaves like a less prominent shoulder in the light curve. The excess is not obvious in the B band (and unknown in the U band), and the colour is consistent with the fiducial SN colour. This might indicate that the excess is attributed to the bolometric luminosity, not in the colour. This excess is less likely caused by external effects, like an echo or change in reddening but could be due to an ionization effect, which reflects an intrinsic, either distinct or continuous, difference in the ejecta properties between HV and NV SNe Ia.

Constraining the Source of the High-velocity Ejecta in Type Ia SN 2019ein

Abstract We present multiwavelength photometric and spectroscopic observations of SN 2019ein, a high-velocity Type Ia supernova (SN Ia) discovered in the nearby galaxy NGC 5353 with a two-day nondetection limit. SN 2019ein exhibited some of the highest measured expansion velocities of any SN Ia, with a Si ii absorption minimum blueshifted by 24,000 km s−1 at 14 days before peak brightness. More unusually, we observed the emission components of the P Cygni profiles to be blueshifted upward of 10,000 km s−1 before B-band maximum light. This blueshift, among the highest in a sample of 28 other SNe Ia, is greatest at our earliest spectroscopic epoch and subsequently decreases toward maximum light. We discuss possible progenitor systems and explosion mechanisms that could explain these extreme absorption and emission velocities. Radio observations beginning 14 days before B-band maximum light yield nondetections at the position of SN 2019ein, which rules out symbiotic progenitor systems, most models of fast optically thick accretion winds, and optically thin shells of mass at radii . Comparing our spectra to models and observations of other high-velocity SNe Ia, we find that SN 2019ein is well fit by a delayed-detonation explosion. We propose that the high emission velocities may be the result of abundance enhancements due to ejecta mixing in an asymmetric explosion, or optical depth effects in the photosphere of the ejecta at early times. These findings may provide evidence for common explosion mechanisms and ejecta geometries among high-velocity SNe Ia.

The formation of type Ia supernovae from carbon–oxygen–silicon white dwarfs

ABSTRACT The carbon–oxygen white dwarf (CO WD)+He star channel is thought to be one of the promising scenarios that produce young type Ia supernovae (SNe Ia). Previous studies found that if the mass-accretion rate is greater than a critical value, the He-accreting CO WD will undergo inwardly propagating (off-centre) carbon ignition when it increases its mass close to the Chandrasekhar limit. Previous works supposed that the inwardly propagating carbon flame would reach the centre, leading to the production of an oxygen–neon (ONe) WD that may collapse into a neutron star but not an SN Ia. However, it is still uncertain how the carbon flame propagates under the effect of mixing mechanisms. In the present work, we aim to investigate the off-centre carbon burning of He-accreting CO WDs by considering the effect of convective mixing. We found that the temperature of the flame is high enough to burn the carbon into silicon-group elements in the outer part of the CO core even if convective overshooting is considered, but the flame would quench somewhere inside the WD, resulting in the formation of a C–O–Si WD. Owing to the inefficiency of thermohaline mixing, the C–O–Si WD may explode as an SN Ia if it continues to grow in mass. Our radiation transfer simulations show that SN ejecta with silicon-rich outer layers will form high-velocity absorption lines in Si ii, leading to some similarities to a class of high-velocity SNe Ia in spectral evolution. We estimate that the birthrate of SNe Ia with Si-rich envelopes is ∼$1\times 10^{-4}\, \mbox{yr}^{-1}$ in our Galaxy.

SN 2019ein: New Insights into the Similarities and Diversity among High-velocity Type Ia Supernovae

Abstract We present optical observations of the Type Ia supernova (SN) 2019ein, starting two days after the estimated explosion date. The spectra and light curves show that SN 2019ein belongs to a high-velocity (HV) and broad-line group with a relatively rapid decline in the light curves (Δm 15(B) = 1.36 ± 0.02 mag) and a short rise time (15.37 ± 0.55 days). The Si ii λ6355 velocity, associated with a photospheric component but not with a detached high-velocity feature, reached ∼20,000 km s−1 12 days before the B-band maximum. The line velocity, however, decreased very rapidly and smoothly toward maximum light, to ∼13,000 km s−1, which is relatively low among HV SNe. This indicates that the speed of the spectral evolution of HV SNe Ia is correlated with not only the velocity at maximum light, but also the light-curve decline rate, as is the case for normal-velocity (NV) SNe Ia. Spectral synthesis modeling shows that the outermost layer at >17,000 km s−1 is well described by an O–Ne–C burning layer extending to at least 25,000 km s−1, and there is no unburnt carbon below 30,000 km s−1; these properties are largely consistent with the delayed detonation scenario and are shared with the prototypical HV SN 2002bo despite the large difference in Δm 15(B). This structure is strikingly different from that derived for the well-studied NV SN 2011fe. We suggest that the relation between the mass of 56Ni (or Δm 15) and the extent of the O–Ne–C burning layer provides an important constraint on the explosion mechanism(s) of HV and NV SNe.

The Ultraviolet Colors of Type Ia Supernovae and Their Photospheric Velocities

Abstract We compare ultraviolet (UV) and optical colors of a sample of 29 type Ia supernovae (SNe Ia) observed with the Swift satellite’s UltraViolet Optical Telescope with theoretical models of an asymmetric explosion viewed from different angles from Kasen & Plewa. This includes mid-UV (1600–2700 Å; uvw2 and uvm2) and near-UV (2700–4000 Å; uvw1 and u) filters. We find the observed colors to be redder than the model predictions, and that these offsets are unlikely to be caused by dust reddening. We confirm that high-velocity SNe Ia have red UV-optical observed colors. After correcting the colors for dust reddening by assuming a constant b − v color, we find no correlation between the uvw1 − v or u − v colors and the ejecta velocities for 25 SNe Ia with published velocities and/or spectra. When assuming an optical color–velocity relation, weak correlations of 2 and 3.6σ are found for uvw1 − v and u − v. However, we find that weak correlations can be reproduced with shuffled velocities and colors that are corrected for reddening. The slope and significance of a correlation between the UV colors and the velocity is thus dependent on the slope of the optical color–velocity relation. Even with a correction, a significant scatter still remains in the uvw1 − v colors including a large spread at low velocities, demonstrating that the NUV-blue/red spread is not caused by the photospheric velocity. The uvm2 − uvw1 colors also show a large dispersion uncorrelated with the velocity.

SN 2014J in M82: new insights on the spectral diversity of Type Ia supernovae

ABSTRACT We present extensive spectroscopic observations for one of the closest Type Ia supernovae (SNe Ia), SN 2014J discovered in M82, ranging from 10.4 d before to 473.2 d after B-band maximum light. The diffuse interstellar band features detected in a high-resolution spectrum allow an estimate of line-of-sight extinction as Av ∼ 1.9 ± 0.6 mag. Spectroscopically, SN 2014J can be put into the high-velocity (HV) subgroup in Wang’s classification with a velocity of Si ii λ 6355 at maximum light of $v$0 = 1.22 ± 0.01 × 104 km s−1 but has a low velocity gradient (LVG, following Benetti’s classification) of $\dot{v}=41\pm 2$ km s−1 d−1, which is inconsistent with the trend that HV SNe Ia generally have larger velocity gradients. We find that the HV SNe Ia with LVGs tend to have relatively stronger Si iii (at ∼4400 Å) absorptions in early spectra, larger ratios of S ii λ 5468 to S ii λ 5640, and weaker Si ii 5972 absorptions compared to their counterparts with similar velocities but high velocity gradients. This shows that the HV+LVG subgroup of SNe Ia may have intrinsically higher photospheric temperature, which indicates that their progenitors may experience more complete burning in the explosions relative to the typical HV SNe Ia.

MEASURING EJECTA VELOCITY IMPROVES TYPE Ia SUPERNOVA DISTANCES

We use a sample of 121 spectroscopically normal Type Ia supernovae (SNe Ia) to show that their intrinsic color is correlated with their ejecta velocity, as measured from the blueshift of the Si II 6355 feature near maximum brightness, v_Si. The SN Ia sample was originally used by Wang et al. (2009) to show that the relationship between color excess and peak magnitude, which in the absence of intrinsic color differences describes a reddening law, was different for two subsamples split by v_Si (defined as "Normal" and "High-Velocity"). We verify this result, but find that the two subsamples have the same reddening law when extremely reddened events (E(B-V) > 0.35 mag) are excluded. We also show that (1) the High-Velocity subsample is offset by ~0.06 mag to the red from the Normal subsample in the (B_max - V_max) - M_V plane, (2) the B_max - V_max cumulative distribution functions of the two subsamples have nearly identical shapes, but the High-Velocity subsample is offset by ~0.07 mag to the red in B_max - V_max, and (3) the bluest High-Velocity SNe Ia are ~0.10 mag redder than the bluest Normal SNe Ia. Together, this evidence indicates a difference in intrinsic color for the subsamples. Accounting for this intrinsic color difference reduces the scatter in Hubble residuals from 0.190 mag to 0.130 mag for SNe Ia with A_V < 0.7 mag. The scatter can be further reduced to 0.109 mag by exclusively using SNe Ia from the Normal subsample. Additionally, this result can at least partially explain the anomalously low values of R_V found in large SN Ia samples. We explain the correlation between ejecta velocity and color as increased line blanketing in the High-Velocity SNe Ia, causing them to become redder. We discuss some implications of this result, and stress the importance of spectroscopy for future SN Ia cosmology surveys, with particular focus on the design of WFIRST.

Optical Observations of the Rapidly Expanding Type Ia Supernova 2007gi

ABSTRACTWe present optical photometry and spectra for the Type Ia supernova (SN Ia) 2007gi in the nearby galaxy NGC 4036. SN 2007gi is characterized by extremely high-velocity (HV) features of the intermediate-mass elements (Si, Ca, and S), with expansion velocities (vexp) approaching ∼15,500 km s-1 near-maximum brightness (compared to ∼10,600 km s-1 for SNe Ia with normal vexp). SN 2007gi reached a B-band peak magnitude of 13.25 ± 0.04 mag with a decline rate of Δm15(B)(true) = 1.33 ± 0.09 mag. The B-band light curve of SN 2007gi demonstrated an interesting two-stage evolution during the nebular phase, with a decay rate of 1.16 ± 0.05 mag (100 days)-1 during t = 60–90 days and 1.61 ± 0.04 mag (100 days)-1 thereafter. Such behavior was also observed in the HV SN Ia 2006X, and might be caused by the interaction between supernova ejecta and circumstellar material (CSM) around HV SNe Ia. Based on a sample of a dozen well-observed R band (or unfiltered) light curves of SNe Ia, we confirm that the HV events may have a faster rise time to maximum than those with normal vexp.

Evidence for Spectropolarimetric Diversity in Type Ia Supernovae

We present single-epoch, postmaximum spectropolarimetry of four Type Ia supernovae (SNe Ia) that span a range of spectral and photometric properties: SN 2002bf and SN 2004dt exhibit unusually high-velocity (HV) absorption lines. SN 1997dt is probably somewhat subluminous, and SN 2003du is slightly overluminous. We detect polarization modulations across strong lines in all four objects, demonstrating that all are intrinsically polarized. However, the nature and degree of the polarization varies considerably. Including all SNe Ia studied thus far, the following order emerges in terms of increasing strength of line-polarization features: ordinary/overluminous < subluminous < HV SNe Ia, with the strength of the line-polarization features increasing from 0.2% in the slightly overluminous SN 2003du to 2% in both of the HV SNe Ia in our study. The most convincing explanation for the line polarization of all objects is partial obscuration of the photosphere by clumps of intermediate-mass elements forged in the explosion; the polarization characteristics of the HV SNe Ia in particular effectively rule out a simple ellipsoidal asphericity as the root cause of their line polarization. That SNe Ia are separable into different groups based on their spectropolarimetric characteristics may help narrow down progenitor possibilities and/or explosion physics. In particular, the recently proposed gravitationally confined detonation model may provide an attractive explanation for many of the observed polarization characteristics of HV SNe Ia.