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Measuring Ejecta Velocity Improves Type Ia Supernova Distances
Cosmology and Nongalactic Astrophysics · Submitted 19 Nov 2010
abstract compiled · 2026-09-23 · no language-model review
The authors' main claim 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.
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The abstract, sentence by sentence
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.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | 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. | result 0.64 | 0.96 | 0.36 | asserted 1.00 |
| s1 | 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"). | background 0.99 | 0.06 | 0.04 | asserted 1.00 |
| s2 | 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. | result 0.99 | 0.96 | 0.08 | asserted 1.00 |
| s3 | 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. | result 1.00 | 0.94 | 0.08 | asserted 1.00 |
| s4 | Together, this evidence indicates a difference in intrinsic color for the subsamples. | interpretation 0.91 | 0.87 | 0.30 | hedged possibility 0.97 |
| s5 | 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. | result 0.99 | 0.94 | 0.53 | asserted 1.00 |
| s6 | The scatter can be further reduced to 0.109 mag by exclusively using SNe Ia from the Normal subsample. | result 1.00 | 0.93 | 0.20 | asserted 0.99 |
| s7 | Additionally, this result can at least partially explain the anomalously low values of R_V found in large SN Ia samples. | interpretation 0.97 | 0.66 | 0.06 | hedged possibility 0.96 |
| s8 | We explain the correlation between ejecta velocity and color as increased line blanketing in the High-Velocity SNe Ia, causing them to become redder. | interpretation 1.00 | 0.69 | 0.10 | asserted 0.97 |
| s9 | 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. | future work 0.99 | 0.41 | 0.05 | asserted 0.55 |
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The paper
Original abstract
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.
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References
How the literature uses it
5 arXiv papers cite this in 9 sentences
Papers citing it · 6 sentences without a confident label
Compares with it 1 paper
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The ejecta velocity distribution has a mean velocity $v_ej = 11,060$ and standard deviation $1,596$ km s$^-1$, consistent with spectroscopic Si II measurements [ref] .
Lightcurve Modelling of 2,205 ZTF DR2 Type~Ia Supernovae: Implications for SN Ia Physics and Cosmology · Physical Parameter Distributions · 0.83
Cites it as background 1 paper
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Ejecta velocity is correlated with intrinsic SN Ia color [ref] , and potentially impacts cosmological distance measurements [ref] .
SN 2025ogs: A Spectroscopically-Normal Type Ia Supernova at z = 2 as a Benchmark for Redshift Evolution · Introduction · 0.87
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