Phillips, MM, Simon, JD, Morrell, N, Burns, CR, Cox, NLJ, Foley, RJ, Karakas,
AI, Patat, F, Sternberg, A, Williams, RE, Gal-Yam, A, Hsiao, EY, Leonard, DC,
Persson, SE, Stritzinger, M, Thompson, IB, Campillay, A, Contreras, C,
Folatelli, G, Freedman, WL, Hamuy, M, Roth, M, Shields, GA, Suntzeff, NB,
Chomiuk, L, Ivans, II, Madore, BF, Penprase, BE, Perley, DA, Pignata, G,
Preston, G and Soderberg, AM
On the source of the dust extinction in type Ia supernovae and the discovery
of anomalously strong Na i absorption
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Citation (please note it is advisable to refer to the publisher’s version if you
intend to cite from this work)
Phillips, MM, Simon, JD, Morrell, N, Burns, CR, Cox, NLJ, Foley, RJ,
Karakas, AI, Patat, F, Sternberg, A, Williams, RE, Gal-Yam, A, Hsiao, EY,
Leonard, DC, Persson, SE, Stritzinger, M, Thompson, IB, Campillay, A,
Contreras, C, Folatelli, G, Freedman, WL, Hamuy, M, Roth, M, Shields, GA,
LJMU Research Online
The Astrophysical Journal, 779:38 (21pp), 2013 December 10 doi:10.1088/0004-637X/779/1/38
C
2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
ON THE SOURCE OF THE DUST EXTINCTION IN TYPE Ia SUPERNOVAE AND
THE DISCOVERY OF ANOMALOUSLY STRONG Na i ABSORPTION
∗
M. M. Phillips
1
, Joshua D. Simon
2
, Nidia Morrell
1
, Christopher R. Burns
2
,NickL.J.Cox
3
, Ryan J. Foley
4
,
Amanda I. Karakas
5
, F. Patat
6
, A. Sternberg
7,21
, R. E. Williams
8
, A. Gal-Yam
9
,E.Y.Hsiao
1
, D. C. Leonard
10
,
Sven E. Persson
2
, Maximilian Stritzinger
11
, I. B. Thompson
2
, Abdo Campillay
1
, Carlos Contreras
1
,
Gast
´
on Folatelli
12
, Wendy L. Freedman
2
, Mario Hamuy
13
, Miguel Roth
1
, Gregory A. Shields
14
,
Nicholas B. Suntzeff
15
, Laura Chomiuk
4
, Inese I. Ivans
16
, Barry F. Madore
2,17
, B. E. Penprase
18
,
Daniel Perley
19
, G. Pignata
20
, G. Preston
2
, and Alicia M. Soderberg
4
1
Carnegie Observatories, Las Campanas Observatory, Casilla 601, La Serena, Chile;
mmp@lco.cl
2
Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA
3
Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D bus 2401, 3001 Leuven, Belgium
4
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
5
Research School of Astronomy and Astrophysics, The Australian National University, Weston, ACT 2611, Australia
6
European Southern Observatory (ESO), Karl Schwarschild Strasse 2, D-85748, Garching bei M
¨
unchen, Germany
7
Max Planck Institute for Astrophysics, Karl Schwarzschild Strasse 1, D-85741 Garching bei M
¨
unchen, Germany
8
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
9
Benoziyo Center for Astrophysics, Faculty of Physics, Weizmann Institute of Science, Rehovot 76100, Israel
10
Department of Astronomy, San Diego State University, San Diego, CA 92182, USA
11
Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark
12
Kavli Institute for the Physics and Mathematics of the Universe, Todai Institutes for Advanced Study,
the University of Tokyo, Kashiwa 277-8583, Japan
13
Universidad de Chile, Departamento de Astronom
´
ıa, Casilla 36-D, Santiago, Chile
14
Department of Astronomy, University of Texas, Austin, TX 78712, USA
15
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
16
Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
17
Infrared Processing and Analysis Center, Caltech/Jet Propulsion Laboratory, Pasadena, CA 91125, USA
18
Department of Physics and Astronomy, Pomona College, 610 N. College Ave., Claremont, CA 91711, USA
19
Cahill Center for Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
20
Departamento de Ciencias Fisicas, Universidad Andres Bello, Avda. Republica 252, Santiago, Chile
Received 2013 July 11; accepted 2013 November 1; published 2013 November 22
ABSTRACT
High-dispersion observations of the Na i D λλ5890, 5896 and K i λλ7665, 7699 interstellar lines, and the diffuse
interstellar band at 5780 Å in the spectra of 32 Type Ia supernovae are used as an independent means of probing
dust extinction. We show that the dust extinction of the objects where the diffuse interstellar band at 5780 Å is
detected is consistent with the visual extinction derived from the supernova colors. This strongly suggests that the
dust producing the extinction is predominantly located in the interstellar medium of the host galaxies and not in
circumstellar material associated with the progenitor system. One quarter of the supernovae display anomalously
large Na i column densities in comparison to the amount of dust extinction derived from their colors. Remarkably, all
of the cases of unusually strong Na i D absorption correspond to “Blueshifted” profiles in the classification scheme
of Sternberg et al. This coincidence suggests that outflowing circumstellar gas is responsible for at least some of the
cases of anomalously large Na i column densities. Two supernovae with unusually strong Na i D absorption showed
essentially normal K i column densities for the dust extinction implied by their colors, but this does not appear to
be a universal characteristic. Overall, we find the most accurate predictor of individual supernova extinction to be
the equivalent width of the diffuse interstellar band at 5780 Å, and provide an empirical relation for its use. Finally,
we identify ways of producing significant enhancements of the Na abundance of circumstellar material in both the
single-degenerate and double-degenerate scenarios for the progenitor system.
Key words: circumstellar matter – dust, extinction – galaxies: ISM – supernovae: general
Online-only material: color figures
1. INTRODUCTION
Type Ia supernovae (SNe Ia) are one of the most effective
observational tools for measuring the expansion history of
the universe. Their successful use in cosmology is due to the
discovery of empirical relations that dramatically decrease the
dispersion in peak luminosities at optical wavelengths. The first
∗
This paper includes data gathered with the 6.5 m Magellan telescopes at Las
Campanas Observatory, Chile.
21
Minerva Fellow.
of these is the well-known correlation with light curve shape:
intrinsically brighter SNe Ia have broader light curves that
decline more slowly from maximum than do the light curves
of less luminous SNe Ia (Phillips
1993). The second is a strong
dependence of peak luminosity on color that is in the same
sense as dust reddening, but with an average value of the ratio
of total-to-selective extinction, R
V
, that is significantly less than
would be produced by normal interstellar dust in the Milky Way
(Tripp
1998). The latter result has variously been interpreted
as possible evidence that the extinction arises in circumstellar
1
The Astrophysical Journal, 779:38 (21pp), 2013 December 10 Phillips et al.
dust (Wang
2005; Goobar 2008), as the consequence of intrinsic
differences in color between SNe Ia with “normal” and “high”
Si ii expansion velocities (Foley & Kasen
2011), or as a bias due
to a misidentification of the dispersion in the luminosity/color-
corrected Hubble diagram with an intrinsic scatter in luminosity
rather than color (Scolnic et al.
2013).
Our understanding of the progenitors and explosion mech-
anism(s) that produce SNe Ia is still quite limited. Although
there is widespread agreement that these objects correspond to
the thermonuclear disruption of a white dwarf in a binary sys-
tem, it is not yet clear if the companion to the white dwarf
is a main sequence or giant star (“single-degenerate” or “SD”
model) or another white dwarf (“double-degenerate” or “DD”
model). In recent years, observational evidence favoring both
scenarios has been put forward (e.g., see Howell
2011; Maoz &
Mannucci
2012; Patat 2013). In both the SD and DD scenarios,
material ejected from the system prior to the explosion may re-
main as circumstellar material (CSM; Moore & Bildsten
2012;
Raskin & Kasen
2013; Shen et al. 2013). Sternberg et al. (2011)
found a strong statistical preference for blueshifted structures
in the narrow Na i D absorption observed in the line-of-sight to
many SNe Ia, suggestive of gas outflows from the progenitor
systems. In a few such cases, temporal variations of blueshifted
components of the Na i D lines apparently due to changing ion-
ization conditions in the CSM have also been observed (e.g.,
Patatetal.
2007; Simon et al. 2009; Dilday et al. 2012). On
the other hand, radio and X-ray observations of the prototypical
Type Ia SN 2011fe place tight upper limits on the amount of
CSM in the progenitor system before explosion (e.g., Horesh
et al.
2012), and early-time photometry of this event apparently
rules out either a red giant or main sequence companion (Bloom
et al.
2012).
In the Milky Way, the strengths of certain interstellar absorp-
tion features such as the Na i D lines and the diffuse interstellar
bands (DIBs) have been known for many years to correlate with
dust extinction (e.g., Merrill & Wilson
1938; Hobbs 1974). In
this paper, we employ high-dispersion spectroscopy to use these
features as an independent probe of the dust affecting the col-
ors of SNe Ia. As we will show, the data indicate that the dust
extinction for the objects where DIBs are observed is gener-
ally consistent with the extinction derived from the SN colors,
and therefore most likely arises in the interstellar medium of
the host galaxy. However, one-fourth of the SNe Ia, all with
blueshifted structures as per Sternberg et al. (
2011), display
anomalously large Na i column densities that, in the interstellar
medium (ISM) of the Milky Way, would correspond to an order
of magnitude or more greater dust extinction than that implied
by the SN colors.
2. OBSERVATIONS AND ANALYSIS
2.1. Column Densities and Equivalent Widths
Our approach is to first examine the relationship between dust
extinction and interstellar absorption lines in the Milky Way.
These results will then be contrasted with a similar comparison
between the dust extinction in the line-of-sight to the SN Ia as
derived from their optical and near-infrared (NIR) light curves,
and the narrow absorption lines producedby the host galaxy ISM
and/or a pre-existing CSM (hereafter referred to collectively as
“host absorption”).
In the first case, we employ a sample of 46 SNe and
active galactic nuclei (AGNs) as external beacons to study the
absorption lines produced by the ISM of the Milky Way. Echelle
spectra of 22 of these objects are drawn from the observations
of thermonuclear and core-collapse SNe published by Sternberg
et al. (
2011) and available through WISeREP (Yaron & Gal-Yam
2012
22
). The remaining 24 spectra in our data set correspond
to unpublished observations of SNe and AGNs obtained with
the Magellan Inamori Kyocera Echelle (MIKE; Bernstein et al.
2003) on the 6.5 m Clay telescope. Table 1 lists the objects in
this sample. Henceforth, we refer to these objects as the “Milky
Way” sample.
To study the relationship between the SN dust extinction
and the narrow host absorption lines, we have put together a
sample of 32 SNe Ia with both high-dispersion spectra and
well-observed light curves. Spectra for 21 of these SNe Ia were
drawn from the Sternberg et al. (
2011) study (also available
through WISeREP), and an additional 6 are taken from Foley
et al. (
2012b). Results for the remaining 5 SNe are taken from
the literature. Table
2 lists the full sample of 32 SNe Ia along
with host galaxy names, morphologies, and references to the
SN photometry. Table
3 gives the sources and wavelength
resolutions of the high-dispersion spectral observations. These
SNe are referred to as the “host absorption” sample in the
remainder of this paper.
Column densities of neutral sodium and potassium were mea-
sured for both the Milky Way and host absorption components
of the Na i D λλ5890, 5896 and K i λλ7665, 7699 doublets
using the Voigt profile fitting program, VPFIT,
23
developed by
R. F. Carswell, J. K. Webb, and others, in combination with
the VPGUESS
24
interface of J. Liske. Upper limits for non-
detections of both Na i and K i were calculated by first estimating
an upper limit to the equivalent width, and then converting this
to a column density using empirical relations between equiva-
lent width and column density derived from weak, unsaturated
lines in other objects. In cases where the Na i D lines were sig-
nificantly saturated (log N
Na i
13 cm
−2
), the much weaker K i
lines were used to determine the velocity and Doppler parame-
ter, b, of each visible component, and this information was em-
ployed in fitting the saturated portion of the Na i D profiles. This
procedure was possible for most of the objects observed from
2006 onward. Four of the SNe in the host absorption sample had
extremely strong D lines (log N
Na i
> 13.5cm
−2
). For one of
these—SN 2002bo—the spectral coverage did not include the
K i lines. A model with two absorption components provided a
significantly better fit to the severely saturated profiles of the D
lines in this SN than did one with a single component, and so
we have adopted the results of the two-component model in this
paper. However, without the additional information provided
by the K i lines, the error associated with the measurement of
log N
Na i
is large.
Kemp et al. (
2002) found that Na i column densities measured
from fitting profiles to the D lines for values log N
Na i
>
12.5cm
−2
were systematically underestimated by 0.40–0.70 dex
compared to column densities measured from the much weaker
Na i UV λλ3302, 3303 doublet. The UV lines are not covered
by our echelle spectra so we cannot confirm this, although
as mentioned in Section
4.5, a comparison of the N
Na i
/N
K i
ratio derived for our Milky Way sample with the measurements
of Kemp et al. (
2002) suggests that our N
Na i
values may be
similarly affected. This should be borne in mind when using
the relations involving the Na i column density developed in
22
http://www.weizmann.ac.il/astrophysics/wiserep/
23
http://www.ast.cam.ac.uk/∼rfc/vpfit.html
24
http://www.eso.org/∼jliske/vpguess/
2
The Astrophysical Journal, 779:38 (21pp), 2013 December 10 Phillips et al.
Tab le 1
Milky Way Na i and K i Column Density Measurements
A
V
log N
Na i
log N
K i
Object (mag) (cm
−2
)(cm
−2
) Reference
(1) (2) (3) (4) (5)
2003gd 0.19 ± 0.03 12.775 ± 0.034 ··· 1
2006be 0.08 ± 0.01 12.068 ± 0.038 ··· 2
2006ca 0.64 ± 0.10 13.181 ± 0.052 ··· 2
2006eu 0.52 ± 0.08 12.914 ± 0.039 ··· 2
2007af 0.11 ± 0.02 11.751 ± 0.106 ··· 2
2007hj 0.26 ± 0.04 12.859 ± 0.102 ··· 2
2007kk 0.64 ± 0.10 12.801 ± 0
.122 ··· 2
2007le 0.09 ± 0.02 11.924 ± 0.055 ··· 2
2007on 0.03 ± 0.01 11.178 ± 0.077 ··· 2
2007sr 0.13 ± 0.02 11.734 ± 0.018 ··· 2
2008C 0.23 ± 0.04 12.777 ± 0.467 ··· 2
2008fp 0.54 ± 0.09 13.141 ± 0.061 11.417 ± 0.070 2
2008ge 0.04 ± 0.01 11.307 ± 0.045 ··· 2
2008hv 0.09
± 0.01 12.276 ± 0.016 ··· 2
2008ia 0.62 ± 0.10 13.149 ± 0.010 11.545 ± 0.070 2
2009ds 0.11 ± 0.02 12.489 ± 0.020 ··· 2
2009ev 0.28 ± 0.05 12.737 ± 0.040 ··· 2
2009iw 0.24 ± 0.04 12.543 ± 0.021 ··· 2
2009le 0.05 ± 0.01 11.793 ± 0.011 ··· 2
2009mz 0.08 ± 0.01 11.972 ±
0.030 ··· 2
2009nr 0.07 ± 0.01 11.926 ± 0.031 ··· 2
2010A 0.08 ± 0.01 11.431 ± 0.032 ··· 2
2010ev 0.29 ± 0.05 12.564 ± 0.028 ··· 2
2010jl 0.07 ± 0.01 11.791 ± 0.119 ··· 1
2010ko 0.39 ± 0.06 12.767 ± 0.103 11.210 ± 0.087 1
2011K 0.27 ± 0.04 12.500 ± 0.016 ··· 1
2011di 0.
29 ± 0.05 12.472 ± 0.025 ··· 1
2011dn 0.49 ± 0.08 12.948 ± 0.011 11.614 ± 0.052 1
2011dq 0.31 ± 0.05 12.808 ± 0.022 11.619 ± 0.024 1
2011dy 0.19 ± 0.03 12.960 ± 0.056 11.103 ± 0.041 1
2011ek 0.97 ± 0.15 12.999 ± 0.024 11.867 ± 0.038 1
2011fj 0.47 ± 0.07 13.043 ± 0.
047 ··· 1
2012cg 0.07 ± 0.01 11.218 ± 0.057 ··· 1
PTF11iqb 0.09 ± 0.02 12.004 ± 0.007 ··· 1
3C273 0.06 ± 0.01 12.073 ± 0.014 ··· 1
IC4329A 0.16 ± 0.02 11.999 ± 0.137 ··· 1
Mk509 0.16 ± 0.02 12.142 ± 0.041 ··· 1
NGC 1068 0.09 ± 0.02 12.618 ± 0.156 ··· 1
NGC 2110 1.03 ± 0.16 13.
149 ± 0.071 11.559 ± 0.049 1
NGC 3783 0.33 ± 0.05 12.656 ± 0.013 11.169 ± 0.055 1
PDS456 1.42 ± 0.23 13.309 ± 0.037 11.757 ± 0.022 1
Fairall51 0.30 ± 0.05 12.388 ± 0.036 ··· 1
IRAS06213+0020 1.77 ± 0.28 13.538 ± 0.049 12.167 ± 0.018 1
IRAS08311-2459 0.29 ± 0.05 12.668 ± 0.040 ··· 1
IRAS09149-6206 0.50
± 0.08 12.643 ± 0.100 11.023 ± 0.075 1
IRAS11353-4854 0.53 ± 0.08 12.771 ± 0.075 11.522 ± 0.027 1
Notes. Columns: (1) Object name; (2) Milky Way dust extinction (Schlafly & Finkbeiner 2011);
(3) Logarithm of the total neutral sodium column density; (4) Logarithm of the total neutral potassium
column density; (5) High-dispersion spectroscopy reference [1 = unpublished MIKE spectrum; 2 =
Sternberg et al. (
2011)].
Section
3.1. Nevertheless, since our approach is to compare
the SN host absorption measurements relative to our same
measurements for the Milky Way, this problem should not affect
our conclusions.
Although the Ca ii H & K lines were also present in many of
the SNe spectra, except for a few specific objects, they are not
included in this study since the column density of Ca ii is poorly
correlated with dust extinction in the Milky Way, presumably
due to variations in the large depletion factor of calcium
(Hobbs
1974).
Equivalent widths of the DIB at 5780 Å were calculated
using the IRAF
25
task fitprofs assuming a Gaussian profile
of 2.1 Å FWHM, a typical value in the Milky Way (Tuairisg et al.
2000; Welty et al. 2006; Hobbs et al. 2008).
26
This feature was
25
IRAF 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.
26
Although very high dispersion spectroscopy has shown that the profile of
the 5780 Å feature is not Gaussian (Galazutdinov et al.
2008), the wavelength
resolution and signal-to-noise ratio of our observations do not warrant a more
sophisticated method of determining the equivalent width.
3
