Infrared Continuum and Line Evolution of the Equatorial Ring around SN 1987A

1.4 MB·

by  Richard G. Arendt

Draftversion January12,2016
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
INFRARED CONTINUUM AND LINE EVOLUTION OF THE EQUATORIAL RING AROUND SN 1987A
Richard G. Arendt1,2, Eli Dwek2, Patrice Bouchet3, I. John Danziger4, Kari A. Frank5,
Robert D. Gehrz6, Sangwook Park7, Charles E. Woodward6
1CRESST/UMBC;[email protected],
2Observational CosmologyLab,Code665,NASA/GSFC,Greenbelt,MD20771, USA
3LaboratoireAIMParis-Saclay,CEA-IRFU/SAp,CNRS,Universit´eParisDiderot,F-91191Gif-sur-Yvette,France
4INAF-OsservatorioAstronomicodiTrieste,viaG.B.Tiepolo11,34143Trieste,Italy
6 5DepartmentofAstronomyandAstrophysics,PennsylvaniaStateUniversity,UniversityPark,PA16802,USA
1 6MinnesotaInstituteforAstrophysics,School ofPhysicsandAstronomy,UniversityofMinnesota,
0 116ChurchStreet, SE,Minneapolis,MN55455, USA
2 7DepartmentofPhysics,UniversityofTexasatArlington,Arlington,TX76019,USA

n Draft version January 12, 2016
a
J ABSTRACT

1 Spitzer observations of SN 1987A have now spanned more than a decade. Since day ∼4,000, mid-
1 infrared(mid-IR) emissionhasbeen dominatedby thatfromshock-heateddustin the equatorialring
(ER).From6,000to8,000daysafterthe explosion,Spitzerobservationsincludedbroadbandphotom-

] etry at 3.6 – 24 µm, and low and moderate resolution spectroscopy at 5 – 35 µm. Here we present
R
later Spitzer observations, through day 10,377, which include only the broadband measurements at
S
3.6 and 4.5 µm. These data show that the 3.6 and 4.5 µm brightness has clearly begun to fade after
h. day ∼8,500, and no longer tracks the X-ray emission as well as it did at earlier epochs. This can be
p explainedbythedestructionofthedustintheERontimescalesshorterthanthecoolingtimeforthe
- shockedgas. We find that the evolution of the late time IR emission is also similar to the now fading
o optical emission. We provide the complete record of the IR emission lines, as seen by Spitzer prior
r
to day 8,000. The past evolution of the gas as seen by the IR emission lines seems largely consistent
t
s with the optical emission, although the IR [Fe II] and [Si II] lines show different, peculiar velocity
a
structures.
[
Subject headings: dust, extinction — infrared: general — supernovae: individual (SN 1987A)

1
v
6 1. INTRODUCTION ejecta. Photometric and spectroscopic evidence of
0 dust formation in the ejecta began to appear after
SN 1987Apresents, thus far, a unique opportunity for
5 day ∼ 450 (Danziger et al. 1989; Suntzeff & Bouchet
the detailed study of the evolutionof a supernova into a
2 1990). From days 615 – 1,144, mid-IR spectrophotom-
supernovaremnant(see reviewsby McCray1993, 2007).
0 etry by Moseley et al. (1989); Dwek et al. (1992) and
Its proximity allows the imaging of small scale structure
1. ofboththeSNejectaandthecircumstellarenvironment. Wooden et al.(1993)monitoredthe emissionofthe dust
0 Italsoissufficientlybrightthatatmostwavelengthshigh directly. However at much later times, the ejecta has
6 S/N observations can be obtained very quickly, even at cooled and faded, and has been too faint to be detected
1 by Spitzer at mid- or far-IR wavelengths. At present
high spectral resolution. SN 1987A was not a prototyp-
: times, the ejecta dust has been detected in the far-IR
v ical Type II supernova, exhibiting a subluminous light
with larger,more sensitive instruments such as Herschel
i curve with a late maximum, and having a known blue
X and ALMA (Matsuura et al. 2011; Laki´cevi´cet al.
supergiantprogenitor(e.g.Arnett et al.1989). However,
2012; Kamenetzky et al. 2013; Indebetouw et al.
r anumber ofother blue supergiantstarshavebeenfound
a 2014; Zanardo et al. 2014; Matsuura et al. 2015;
to have similar circumstellar structures (Brandner et al.
1997a,b; Smith 2007; Smith et al. 2013; Muratore et al. Dwek & Arendt 2015). The ∼ 0.4 − 0.7 M⊙ of cold
ejecta dust seen by Herschel (Matsuura et al. 2011)
2015; Gvaramadze et al. 2015).
Shortly after its launch, the Spitzer Space Telescope is greatly in excess of the ∼ 2 × 10−6 M⊙ of warmer
(Werner et al. 2004; Gehrz et al. 2007) began a long- circumstellar dust located in the ER (Bouchet et al.
termcampaignofimagingandspectroscopyofSN1987A 2006).
atwavelengthsfrom3.6–160µm. Atthenear-andmid- The SN ejecta were first beginning to impact the ER
IR wavelengths Spitzer has been able to monitor ther- creating hotspots visible in the high resolution HST
mal emission from warm dust in the SN. Despite insuf- images at days ∼3,000 – 4,400 (Lawrence et al. 2000,
ficient angular resolution to resolve detailed structure, andreferencestherein). Spitzerobservationscommenced
this emission has been identified (Bouchet et al. 2006) near day 6,100 (Bouchet et al. 2006) while the number
asarisingfromdustinthe equatorialring(ER)thatwas andbrightnessofhotspotsaroundtheERwereincreasing
formedbywindsduringmasslossepisodesoftheSNpro- (Gr¨oningsson et al. 2008a; Fransson et al. 2015). This
genitorstar(orbinarysystem,e.g.Podsiadlowski& Joss epoch is also when the soft X-ray light curve turned
1989). up (Park et al. 2005, 2006), indicating that the shock
Dust is also known to be present in SN 1987A’s started interacting with dense protrusions from the ER.
Thus Spitzer data are well suited for assessing what
2
kind of dust exists in the circumstellar environment and SN1987A,andcharacterizetheIRevolutionwithrespect
whether this dust can survive in the hot shocked envi- to the X-ray emission, and the optical emission. In Sec-
ronment downstream of the SN blast wave. tion 4, we present the evolution of the IR emission lines
Bouchet et al. (2004), using the T-ReCS instrument during Spitzer’s cryogenic mission (days 6,000 - 8,000).
(Telesco et al. 1998; De Buizer & Fisher 2005) on the The results are discussed in Section 5 and summarized
Gemini South 8m telescope, detected 10 µm emission in Section 6.
from SN 1987A at Day 6,067 and resolved it as being
dominated by the ER (i.e. pre-existing circumstellar 2. SPITZER INSTRUMENTS AND DATA
dust), with only a small contribution from the ejecta
The Spitzer Space Telescope carriesthree instruments,
(i.e. newly-formed SN dust). Bouchet et al. (2006) pre-
all of which were used for observation of SN 1987A.
sented later T-ReCS observations (Day 6,526) showing
The Multiband Imaging Photometer for Spitzer
the ejecta to be fading and the ER brightening. They
(MIPS; Rieke et al. 2004) provides broadband imaging
also analyzed Spitzer IRS spectroscopy at day 6,190, re-
at 24, 70, and 160 µm. It also has an SED mode to pro-
vealingthatthe dustpossessesstrongsilicatefeaturesat
vide very low resolution spectra over 52 - 97 µm. Over
10 and 20 µm, which are typical of silicate grains in the
the course of the cryogenicmission, the SN was only de-
ISM. The derived dust temperature of ∼ 180 K (with a
tected in the 24 µm band. No lines or continuum were
surprisinglackofevidence forvariationsindusttemper-
detected in the SED mode.
ature)isfarwarmerthantypicalISMdusttemperatures
The Infrared Spectrograph (IRS; Houck et al. 2004)
(∼20K)andprovidesaclearindicationofstrongheating provideslowresolution(R∼100)intwomodules(short-
of the dust. The observed dust temperature was deter-
low = SL and long-low = LL) covering 5.2-38 µm. it
minedtobe consistentwithcollisionalheatingofdustin also provides high resolution (R ∼ 600) spectra in two
the shocked X-ray emitting gas. Radiative heating was
modules (short-high = SH and long-high = LH) cover-
estimatedtoproducedusttemperaturesofonly∼125K.
ing 9.9-37.2 µm. The low resolution spectra are best for
Whileradiativeheatingwasnotruledout,thisdoessug-
study of the continuum, and the high resolution spectra
gest it is not the dominant dust heating mechanism.
are best for the line emission.
About 1,000 days later, Dwek et al. (2008) reported
The Infrared Array Camera (IRAC; Fazio et al. 2004)
thatfurtherSpitzerobservationshadshownthedustwas
provides broadband imaging at 3.6, 4.5, 5.8, and 8 µm.
unchanged in temperature and increasing in brightness,
The IRS low resolution spectra provide overlapwith the
butnotincreasingasrapidlyasthe X-rayemission. The
twolongerbands,butdonotcovertheshortertwobands.
decreasing ratio of IR to X-ray emission (IRX) was in-
These shorter two bands are the only channels that re-
terpreted as destruction of dust behind the blast wave
main operational on Spitzer since the telescope warmed
on timescales shorter than that for the cooling of the
up after the depletion of cryogen on day 8,117.
shocked, X-ray emitting gas. However, this conclusion
Figure1showscompositeIRACimagesofSN1987Aat
was based on only two epochs of data.
3.6and4.5 µm. Combining allepochs oftargetedobser-
Dwek et al. (2010) presented the full set of Spitzer’s
vations after day 7,000 provides sufficient depth, dither-
cryogenic imaging and spectroscopy (continuum results ing, and rotation of the PSF, such that mapping at 0′.′4
only), through day 7,983. Over this longer baseline it
(here) reveals more detail than mapping at the detector
became apparent that, although there appeared to be pixel scale of 1′.′2 (Bouchet et al. 2006). In Figure 1, we
a marginally significant decrease in the IRX after the
can see the SN marginally resolved from Stars 2 and 3.
first Spitzer observations, the IRX ratio was constant
for all further observations from days 6,500 to 8,000. 3. EVOLUTION OF THE IR CONTINUUM
The favored interpretation was that the grain destruc-
In sections 3.1 and 3.2 we consider the IR evolution
tion timescale was sufficiently long that no major de-
in comparison to the X-ray emission. The IR and X-
struction of the dust had yet taken place. They pre-
ray emission should be directly related for collisionally
dicted that grain destruction may become appreciable
after day ∼9,200. This work also drew attention to the heated dust. In Section 3.3 we look at the IR evolution
in terms of the dust mass and temperature, without re-
shortwavelengthcontinuumemission,whichwaspresent
at . 8 µm, in excess of the ∼ 180 K silicate dust. This gard for the heating mechanism. Section 3.4 examines
the relation between the IR and the optical emission, as
indicatesahotterdustcomponent,butlackinganyspec-
potentially related if radiative heating of the dust is im-
tral features, the composition and temperature of this
portant. The“models”inthissectionareempiricalpara-
hot component were indeterminate.
metric descriptions of the evolution of the IR emission.
In this paper, we examine continued Spitzer obser-
They can be used as simplified descriptions of the data
vations of SN 1987A. This paper is an observational
(e.g. for extrapolation or interpolation) and to identify
update to, and relies upon, previous analysis of the
the behavior of correlations between the IR, and X-ray
Spitzer photometry and spectroscopy presented in the
and optical emission. In some cases the model parame-
works of Bouchet et al. (2006), Dwek et al. (2008), and
tersarerelatedtooneormoreoftheunderlyingphysical
Dwek et al. (2010). In the Spitzer post-cryo era, only
parameters of the system.
the 3.6 and4.5 µm imaging capability remains, but con-
tinued observations reveal the evolution of the emission
3.1. Infrared and X-ray Brightness Evolution
of the hot dust component (the component emitting at
λ < 8 µm), which may provide clues to the nature of Examination of the cryogenic data had indicated that
this dust. Section 2 summarizes the characteristics of the ratio of 24 µm IR to X-ray brightness (IRX) was
the Spitzer instruments and data used in this research. approximately constant between days 6,000 and 8,000.
In Section3, we presentthe latest Spitzer lightcurvesof However,continuedobservationsat 3.6 and4.5 µm have
3
Figure 1. (top)SN1987AasseenbyHubble/ACS(F814W,left),Spitzer/IRAC(3.6and4.5µm,middle),andChandra/HRC(right). In
theIRACimagestheangularresolutionis&12timesworsethantheACSimage. Stars2and3totheNWandSEareonlybarleyresolved
from the SN. The ER is elongated, but not resolved. (bottom) The full IRAC 3.6 and 4.5 µm 2-color image, with SN 1987A centered in
thebluebox,reveals that theERisredderthanmoststarsinthefieldandthat itdoes notshow thesymmetricPSFthat stellarsources
exhibit. TheopticalandIRimagesarelogarithmicallyscaled;theX-rayimageisscaledlinearly.
4
shownatrendthatclearlydivergesfromtheX-rayemis- where T is the gas temperature, Λ (T ) and
gas dust gas
sion. Figure2depictstheobservedlightcurves,aslisted Λ (T ) are the in-band cooling functions of the dust
gas gas
in Table 1. and gas, n is the number density of dust grains, and
dust
Thesoft0.5-2keVin-bandX-rayflux isusedthrough- n is the number density of hydrogen. This is for col-
H
out for comparison with the IR emission because it lisional heating of the dust. For radiative heating, the
is more representative than the harder X-ray emission dust cooling function will depend on the radiation field
of the forward shock that propagates through the ER rather than the gas temperature. The relative impor-
(Dwek et al. 1987). The X-ray data used here are those tance of radiative vs. collisional heating of dust in su-
measured with the Chandra ACIS and published by pernova remnants has been discussed by Arendt et al.
Helder et al.(2013),towhichweadd3additionalepochs (1999) and Andersen et al. (2011), and in more general
ofsimilarlyobservedandreducedChandra observations. circumstances by Bocchio et al. (2013). In all cases we
A more physically motivated comparison might use the only use data from days 6,000 to 8,000 to determine the
integrated X-ray emission of the softer component of 2- parametera. The fit is generally not very good, indicat-
componentfitstothetotalX-rayspectrum. Thetemper- ingthatthesimplestmodel,givenbyEquation(1),fails.
atureofthe softcomponentvariesfrom0.2-0.4keV,and However, the fit is significantly improved if the model
issufficientlylowthatthefractionofemissionatenergies allows a constant flux density offset, b:
> 2 keV is negligible. The soft component accounts for
∼ 33% of the flux in the 0.5-2 keV band at day 5,000, S(λ,t)=a(λ)S(X,t)+b(λ) (3)
risingto∼41%atday10,000. However,givenuncertain-
asshowninFigure3. Anon-zerovaluefortheparameter
ties in the corrections of systematic effects in the X-ray
b could indicate a systematic error in the measurement
data, and the relatively low signal-to-noise of the data
of the flux densities of SN 1987A, e.g. error in the sub-
atsome epochs, both of whichaffect the spectralfitting,
tractionofthecontributionofnearbyStars2and3. The
the lightcurveofthe integratedsoftX-raycomponentis
parameter b would also account for a component of the
much noisier than that of the simple 0.5-2 keV in-band
SN’s IR or X-rayemission that doesn’t exhibit the same
flux.
correlatedtemporalvariationasthebulkoftheemission.
A strong correlation between the evolution of the X-
Using the 24 µm emission as the reference instead of
ray and IR luminosity would be an indication that the
the X-ray emission provides a check on whether the IR
mid-IRemissionofSN1987Aisfromcollisionallyheated
emission is evolving in a consistent manner across all
dustintheER,asboththeIRandX-rayemissionshould
wavelengths. This comparisonis also shown in Figure 3.
be proportional to the mass of shocked material. Fig-
As with the X-ray emission, the shorter wavelengths are
ure 3 shows the evolution of flux density, S(λ), at each
consistent with the 24 µm emission only if a constant
IRwavelength,λ,comparedtoscaledversionsofthesoft
emission component is present at each IR wavelength.
X-ray emission, S(X):
The fit parameters (a, b) required here are consistent
S(λ,t)=a(λ)S(X,t). (1) with those derived from the X-ray comparison given by
Equation (3).
The empirical fitting parameter a corresponds to a At 3.6 and 4.5 µm, the comparison to X-ray emission
constant IRX ratio (Dwek et al. 1987; Dwek & Arendt can be extended over for the longer time intervals from
1992): day6,000today11,000. Inthesecases,therequiredcon-
stanttermisincreasedsuchthatitaccountsfornearlyall
IRX =n Λ (T )/n Λ (T ), (2)
dust dust gas H gas gas
oftheemissionatday6,000,andtheratiooftheevolving
IR to X-ray emission drops correspondingly. Consistent
results are obtained when the 3.6 µm flux density evolu-
tion is compared directly to that at 4.5 µm.
3.2. IRX Evolution
The empirical models described in the preceding sub-
section assume that the ratio of IR to X-ray emission
should remain constant as the brightness evolves. How-
ever, a changing value of IRX could eliminate the need
for a strong non-evolving IR emission component in the
comparison of IR and X-ray emission. The IRX would
beexpectedtochangeifthephysicalconditions(temper-
ature, density, or dust-to-gas mass ratio) in the shocked
gas change over time. The ratio of hard to soft X-ray
emissionhasremainednearlyconstantsinceday∼6,000
(seeTable1ofHelder et al.2013),thereforeadecreasing
IRX may indicate ongoing grain destruction and evolu-
Figure 2. EvolutionofSN1987Aasseeninthe3.6–24µmflux tionofthedusttogasmassrationdust/ngas. Apartfrom
densitiesobservedwithSpitzer,andthesoftX-rayfluxesdetected the explicit dependence of the IRX on the dust to gas
withChandra(Helderetal.2013). Until2009,theIRemissionhad mass ratio and the gas temperature, see Equation (2),
beennearlyproportionaltothesoftX-rayemission. Spitzer/IRAC
its numerical value will also depend on the dust grain
warmmissiondatasince2009,haveshownthatthe3.6and4.5µm
fluxdensitieshavedeviatedfromtheX-raytrend,andhavebegun size distribution, and on the abundances and ionization
todeclineslowly. state of the gas.
5
Figure 3. Theevolutionofthefluxdensityat3.6–8µm(solidblackcircles)iscomparedtothatat24µm(opencircles)andinthesoft
X-rays(×). Inthe bottom panel, the 24µm evolution (solidblack circles)issimilarlycompared tothe softX-rayevolution. Thescaling
of the reference emission is determined both directly: S(λ,t) = a(λ) S(X,t) (red symbols), and with the presence of a constant offset:
S(λ,t) = a(λ) S(X,t)+b(λ) (blue symbols). In all cases, only the data from the cryogenic mission (earlier than day 8,000) are used to
determine the scale parameters. Priorto day 8,000, the IR emissioncan followthe X-rayemissionclosely if an additional constant term
ispresent. However,atshortwavelengths (3.6and4.5µm)theIRemissiondeviates afterday8,000.
Asimplemodelthatwouldallowdivergentbutrelated For the choice of t = 4,600, we derive β = −1.0 for
0
evolution of the IR and X-ray emission can be written both 3.6 and 4.5 µm data. However, at this same t , β
0
as: increases with wavelength, reaching −0.6 for the 24 µm
S(λ,t)=α S(X,t)(t−t )β (4) data. Because there is a strong correlation between t
0 0
and β in this model, we can force β = −1.0 for each
where t represents the time at which the forwardshock
0 wavelength by making t (λ) a function of wavelength,
0
began interaction with the ER, α is the nominal IRX at
e.g. t (24 µm)=3,270.
0
that time, and β characterizesthe evolutionof IRX over The value of β = −1 is of particular interest. If the
time. If β = 0 we recover the constant IRX assumption
volume of the X-ray emitting gas is accumulating as a
ofthe model in Equation(1). Assuming t0, we cansolve power law function of t −t , integrated from t to t ,
0 0 1
for the free parameters α and β by performing a linear
but the volume of the IR emitting region is restricted to
fit to:
a layer representing a finite duration behind the shock
log[S(λ,t)/S(X,t)]=βlog(t−t )+α. (5) front, i.e an integral from t1 −∆t to t1, then the IRX
0
6
should evolve as (t−t )−1 for ∆t ≪(t−t ) (see Fig. 7 whereκ (λ)isthemassabsorptioncoefficientofthedust,
0 0 i
of Dwek et al. 2010). The limited thickness of the IR- disthedistancetotheSN,B (T)=2hν3/c21/(ehν/kT−
ν
emitting region would arise if the dust destruction time 1) is the Planck function. The mass absorption coeffi-
scale is short compared to the cooling time of the gas. cient for silicate is taken from Draine & Lee (1984), and
Therefore if we make the assumption that β = −1, foramorphouscarbonisfromRouleau & Martin(1991).
then we can derive t0 and α via a linear fit to: Wechoosepowerlawfunctionsoftimeasasimplemeans
S(X,t)/S(λ,t)=α−1(t−t ). (6) of characterizing the evolution of the mass and temper-
0 ature of separate warm (silicate) and hot (e.g. carbon)
Such fits are shown in Figure 4. If we use this prescrip- components:
tion to transform the observed X-ray fluxes to IR flux
densities, then the agreement is very good throughout M (t)=M (t−t )µi (8)
i i,0 1
the span of the IR observations (Figure 5). Extrap-
olation back to earlier times will fail if t − t0 is less and
than the dust destruction timescale. The extrapolation
of the 24 µm emission to later times may be verified Ti(t)=Ti,0 (t−t1)θi. (9)
in the future with SOFIA (Young et al. 2012) or JWST
(Gardner et al. 2006). Dwek et al.(2010)illustratehowthe masspowerlawin-
dex, µ, changes for idealized shocks expanding into 1-,
3.3. Dust Mass and Temperature Evolution 2-,or3-dimensionalmedia,underthe casesoffreeorSe-
dov expansion, and with or without rapid destruction of
Comparison of the IR and X-ray emission in the pre-
dust.
vious subsections indicates that the IRX does not re-
If the temporal evolution is measured with respect to
mainconstant. Thechangesmayindicatethatthedecay
the SN explosion (t = 0) then t−t only varies by a
timescaleofthe IRemissionismuchshorterthanforthe 1 1
factor of 1.33 from day 6,000 to 8,000, which forces a
X-ray emission. Decreasing IR emission can occur if the
large value of µ ∼ 4−5 to match the observed change
dust cools or is destroyed. i
in brightness. It is more realistic to choose t as the
Thefollowingmodelsspecificallyexaminewhether the 1
start of the interaction between the blast wave and the
IRAC and MIPS data indicate evolution of dust mass,
ER. Assuming that t = 5,500 (Dwek et al. 2010), we
dust temperature, or both. These models use two dust 1
find that µ = 0.87 (silicate) and µ = 0.98 (carbon).
compositions with independent temperatures to fit the 1 2
The value of µ .1 could indicate free expansion into a
IRAC and MIPS SED of SN 1987Aat 9 epochs between i
1-dimensional medium without grain destruction or 2-
days 6,000 and 8,000, when observations at all wave-
dimensional medium with grain destruction, or Sedov
lengths were obtained. Guided by the spectral models
expansion into a 2 dimensional medium (see Figure 7
presented by Dwek et al. (2010), the warm dust com-
of Dwek et al. 2010). For both dust components θ ∼0,
ponent giving rise to the 8-35 µm emission is assumed i
indicating very little evolution of the temperature.
to be silicate, with a temperature of ∼ 180 K, and the
These results are shown in Figure 6, where the fit-
hot dust component giving rise to the 3 – 8 µm emis-
ted dust temperatures are T = 190 K for the warm
sion is assumed to be amorphous carbon, with a tem- 1,0
silicate component, and T = 525 K for the hot amor-
perature of ∼ 460 K. The hot dust component could 2,0
phous carbon component. These dust temperatures are
alternately be any other composition with a generally
warmer, though not significantly, than those derived by
featureless spectrum (e.g. iron), and with temperatures
Dwek et al. (2010). Figure 7 compares the changes in
that could be as much as ∼ 100 K lower (Dwek et al.
the λ / 24 µm colors of the data and the model. These
2010). These components are derived from fits to the
changes are mostly driven by variation of the relative
Spitzer IRS low resolution spectra (5 – 30 µm) obtained
massesofthewarmandhotcomponents,notbychanges
between days 6,000and 8,000. The warmdust is consis-
in the dust temperatures. Figure 8 (top) shows the ob-
tent with collisional heating by the X-ray emitting gas,
servedandmodeled evolutionof the flux densities. Over
and with even small dust grains residing at their equi-
the fitted interval of days 6,000 – 8,000, the model is in
librium temperatures (Bouchet et al. 2006; Dwek et al.
fair agreement with the data. However, extrapolating
2008). The hotter dust can also be collisionally heated,
the model to later times at3.6and 4.5µm showsa clear
if the grainsizes are sufficiently small, dependent on the
deviation between the data and the model.
unknown dust composition (Dwek et al. 2010). Small
Comparison between the model and observations is
carbon grains can reach these high temperatures under
shown in Figure 8 (bottom). The ratio between the
thermalequilibrium,butothercompositionsmayrequire
model and data is constant until about day 7,500, after
stochasticheatingtoreachsufficientlyhightemperatures
which the data fall linearly with respect to the model.
(Dwek et al.2010). However,iftransientlyheatedgrains
Thus, we have constructed an altered model for the 3.6
areproducingthehotcomponent,thentheyshouldradi-
and4.5µmdata. Onlythehotcarbondustcomponentis
atemorestronglyatlowertemperatures,producingmore
used, because the warm silicate dust provides negligible
emission at λ > 10 µm which would dilute the relative
emission at these wavelengths. The dust temperature is
strength of the 10 and 20 µm silicate features.
stillafree parameter,but itis heldconstantbecause the
The fitting is constrained at different epochs by as-
previous models suggested very little evolution in tem-
suming that the temperature and dust mass evolve as
perature. Starting at day 7,500, a linear decrease in the
functions of time:
mass is applied. The slope of this decrease indicates a
2 4κ (λ) destruction timescale for the dust, t−1 , which is a free
i destr
S(λ,t)=X 4πd2 Mi(t)πBν[Ti(t)] (7) parameter to be derived from the data. The form of the
i=1
7
Figure 4. CorrelationbetweentandS(X,t)/S(λ,t). Thex-axisinterceptsoftheselinearfitscorrespondtot0 inEquation(6). Averaged
overall5wavelengths, ht0i=4,376±78d.
model is thus: 2015). The correlations are moderately good in both
cases,but arebetter for the B band, whichis dominated
4κ(λ)
S(λ,t)= M (t−t )µ πB (T) (10) by the emission lines of Hγ, Hδ, and [S II] 0.4069 µm
4πd2 0 0 ν
(Fransson et al. 2015). Figure 10 (upper panels) illus-
for t<7,500
tratesthattheapplicationofEquation(1),butusingthe
4κ(λ) opticalinsteadofthe X-rayemission,providesasurpris-
= M (t−t )µ[1−(t−7,500)/t ] πB (T)
4πd2 0 0 destr ν ingly effective model of the IR emission, especially when
for t>7,500 using the B band optical emission. However, the lower
panels of the figure show that the IR emission is more
The fits for this models are shown in Figure 8. The closely matched by the previously discussed functions of
relative accuracy of this model is ∼ 3% at 3.6 µm and the X-ray emission.
∼2% at 4.5 µm (shown later in Figure 10). These correlations between the optical and the IR
emission may indicate the IR and optical emission origi-
3.4. IR – Optical Evolution
nateinthesame,orcloselyrelated,regionsoftheshocked
Figure 9 shows the evolution of the IR emission mea- ER. However, examination of the radiative luminosity
sured by Spitzer compared to the optical emission in of the SN at various wavelengths indicates that it is
the B and R bands measured by HST (Fransson et al.
8
Figure 5. ObservedIRfluxdensities(•)canbewellfitbymodels
of the IR emission in the form of S(λ,t) = αS(X,t)/[t−t0(λ)]
(coloredlines,codedasinFigure2). TheobservedX-rayemission
isindicatedbytheblack×symbols.
Figure 8. The top panel shows the evolution of the flux densi-
tiesforthedata(•)andthemodelofEquation(7)(dashedlines).
Resultsatλ=3.6,4.5,5.8,8,and24µm arecoloredviolet, blue,
green,orange, andred,respectively. Atlatertimestheextrapola-
tion of the models is not a good fit to the 3.6 and 4.5 µm data.
Thebottompanelshowsthattheratiobetweenthismodelandthe
data is constant at early times, but appears to fall linearly after
day∼7,500. Modifyingthemodel withanother factor represent-
Figure 6. Fits of a two-temperature, two-composition model to ingalineardecayafterday7,500[Equation(10)],providesagood
the SED of SN 1987A at 9 different epochs show little apparent fittothe3.6and4.5µmdata(solidlines)inboththefluxdensities
evolutionofthespectrum. Thewarm(190K)componentissilicate; (top)andtheratios(bottom).
the hot (525 K) component is assumed to be amorphous carbon.
Theevolutionofthemassanddusttemperatureofeachcomponent unlikely the dust heating can be predominantly radia-
areconstrainedtobeapowerlawfunctionoft−t1,witht1=5,500. tive. Radiation from the ER is dominated by UV -
X-ray emission with a total 0.01 - 8 keV luminosity of
∼ 1,460 L⊙ (France et al. 2015). The ER dust has an
integratedIRluminosityof∼1,450L⊙. Radiativeheat-
ing would require that half of the ER emission is ab-
sorbed by dust within the ring. The required optical
depthisinconsistentwithgasphaseabundancesindicat-
inglittledepletionintodust(e.g.Mattila et al.2010)and
spectral analysis which requires only Galactic + LMC
line of sight extinction corrections (e.g. Pun et al. 2002;
Gr¨oningssonet al. 2008b).
4. EVOLUTION OF THE IR EMISSION LINES
4.1. Line Identification
The atomic spectral lines seen in the mid-IR are usu-
allythegroundstatefine-structuretransitionsofvarious
Figure 7. Evolution of the λ/24 µm colors for the data (•) and ionized species. One or more higher-lying H recombina-
the model (dashed lines). The colors are normalized with respect tion lines are seen, and one excited state transition of
to the color at the firstepoch. Results at λ=3.6, 4.5, 5.8, and 8 [Fe II] is detected. Cutouts of the lines in the SH and
µm are colored violet, blue, green, and orange, respectively. The
LHdata areshownin Figure 11,andlines inthe SL and
dashed lines indicate that the model of Equation (7) provides a
reasonable representation of the color evolution for days 6,000 - LL data are shown in Figure 12. Here we only discuss
8,000. thelines,asthecontinuumintheSLandLLspectrahas
beenanalyzedbyBouchet et al.(2006)and(Dwek et al.
9
Figure 9. IRfluxdensityevolution(•)inthe3.6,4.5,5.8,8,and24µmbands(violet–red,scaledasinFigure2)comparedtothescaled
opticalevolution(⋆)intheR(left)andB(right)bands(Franssonetal.2015). Theevolutionofthe3.6and4.5µmemissionissimilarto
thatoftheopticalemission,especiallytheBband.
Figure 10. Ratios of the IR to R and B band emission is shown in the top two panels. The ratios are relatively constant with time,
especiallyat3.6and4.5µmandwithrespecttotheBband. ThebottomleftpanelshowsthattheIRemissionismoreaccuratelymodeled
as proportional to S(X)/(t−t0). The bottom rightpanel shows the ratiobetween the IR emissionof the model of Equation (10) witha
rolloffafterday7,500. ThismodelprovidesthemostaccuratefittotheIRdata.
2008,2010). ThecontinuumintheSHandLHdataadds 300kms−1)widthcomponentsinhighresolutionoptical
no new information and is less reliably measured as the spectra (e.g. Gr¨oningsson et al. 2008a).
concatenation of many spectral orders. One apparent exception is the [SiII] line at 34.81 µm.
Nearly all of the observed emission lines can be at- Atallepochs (barringthe lowsignalto noisedata ofthe
tributedtothe emissionring,asthe widths ofmostlines initialobservationsatday6,109),thisemissionlineseems
seem to be unresolved at the R ∼ 600 resolution of the to be flanked by a pair of lines at velocities of ± 1,700
IRS SH and LH modules. However this spectral reso- kms−1withrespecttothecentralcomponent. Theback-
lution is insufficient to distinguish between the ionized groundspectradonotshowanyindicationofthe shifted
but unshocked component of the ring and the shocked lines. The velocities of the flanking lines are large com-
component of the ring, which are clearly seen as nar- pared to the velocities associated with shocked emission
row(FWHM∼10kms−1)andintermediate(FWHM ∼ ring, but fairly small compared to the velocity of the
SN ejecta. The distinctness of the flanking lines implies
10
Figure 11. Spectral lines of SN 1987A as seen in the high spectral resolution SH+LH Spitzer IRS data. The rising continuum levels
correlatewiththedateoftheobservations. Thedottedlinesatconstantwavelengthindicatetherestwavelengthofeachline. Thespectra
are not adjusted for the systemic velocity of the ER, ∼ 287 km s−1 (Gr¨oningssonetal. 2008a). For each line, the wavelength range
displayedcorrespondstoavelocityrangeof[-7,500,+7,500]kms−1.