IR Shell Surrounding the Pulsar
Wind Nebula G54.1+0.3
Tea Temim
(CfA, Univ. of MN)
Collaborators: P. Slane, S. Reynolds, J. Raymond, K. Borkowski
SNRs and PWNe in the Chandra Era
Boston, July 8, 2009
Outline
1. Structure of PWNe Evolving Inside SNRs
1. IR Observations of G54.1+0.3: Evidence for
Interaction with SN Ejecta
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IRAC and MIPS Imaging
IRS Spectroscopy
Dust emission – evidence for freshly formed dust
New interpretation for the origin of IR emission
2. Summary & Conclusions
Evolution of PWNe inside SNRs
• Pulsar wind is shocked at the
termination shock
• PWN drives a shock into the freely
expanding, cold SN ejecta
• SNR blast wave sweeps up ISM material
and reverse shock heats the inner ejecta
• Reverse shock encounters the PWN
surface and crushes the nebula
• G54.1+0.3  PWN sweeping up
inner ejecta
Gaensler & Slane 2006
Examples of PWNe interacting with SN
ejecta include: Crab (Hester et al. 2008 for
review), 3C58 (Bocchino et al. 2001, Slane et
al. 2004), 0540-69.3 (e.g., Reynolds 1985)
G54.1+0.3
Pulsar: J1930+1852
P = 138 ms
(Camilo et al. 2002)
Edot = 1.2 x 1037 ergs/s
Characteristic age = 2900 yr
Actual age = 1500 – 6000 yr
Distance = 6 kpc (5 – 8 kpc)
(Lu et al. 2002, Camilo et al. 2002,
Leahy et al. 2008)
PWN radius = 1 arcmin (1.8 pc)
NH = 1.95 (0.04) x 1022 cm-2
Chandra
• X-ray spectra well described by a power-law model with the spectral
index steepening with distance from the pulsar (Lu et al. 2002)
• There has been no evidence for emission from a thermal component
in X-rays - new Spitzer IR observations provide evidence for an
interaction of the PWN with SN ejecta
Spitzer Imaging
5.8 mm
8.0 mm
24 mm
70 mm
Radio, MIPS 70 mm, MIPS 24 mm, X-ray
Infrared images reveal a shell with a
radius of 1.5 arcmin - X-ray nebula fills
the cavity of the shell
Total IR Fluxes:
5.8 mm ~ 0.3 Jy
8.0 mm ~ 1 Jy
24 mm
40 (4) Jy
70 mm
76 (15) Jy
MIPS 24 mm image shows a dozen point sources
embedded in the IR shell – suggested to be young
stellar objects (Koo et al. 2008)
Spitzer Spectroscopy
IRS slits overlaid on the MIPS 24 mm image
IR spectrum shows various emission
lines (strongest from Si, S, Ne, and Ar)
and a rising continuum with broad dust
features around 13 and 21 mm
Spitzer Spectroscopy
Some emission lines are significantly
broadened, up to a FWHM = 1000 km/s
(expected resolution of IRS = 500 km/s)
Shock Diagnostics
• Models with cosmic abundances
and depleted refractory elements run
for several shock speeds and preshock densities (Hartigan et al. 1987)
• A pre-shock density of 10 cm-3
matches the SIII line ratio.
• A shock speed of at least 100 km/s
needed to produce SIV, but a shock
faster than 110 km/s would produce
too much OIV.
• A factor of 3 depletion in refractory
elements needed at position 1, and a
larger factor at position 2
Spectral Line Profiles
Chevalier 2005 – Models for young PWNe
expanding into SN ejecta:
Vsh = 0.25Rp/t
 t = 4500 yr
Vexp = 400 km/s
Vobs = 500 km/s  observed line broadening
Msw = EdotRp-2t3  Msw = 0.5 M
Dust Emission
Cas A
• Continuum emission in
G54.1+0.3 closely resembles
the IR spectrum of Cas A –
same broad 21 mm feature
G54.1+0.3
Figures: Rho et al. 2008, 2009
Combined
Silicon
• Distribution of 21 mm dust
in Cas A similar to that of SN
ejecta -> freshly formed SN
dust (Rho et al. 2008)
Dust Mass Estimate
Rough dust temperature and mass estimates made by
fitting the IR spectrum and the total 24 and 70 mm fluxes
with forsterite (Mg2SiO4) grain compositions:
Mdust = 0.015 – 0.05 M (T = 60 – 70 K)
•Spitzer imaging and spectral maps of SNRs have allowed
estimates of masses of freshly formed - dust emission
coincides with lines from SN ejecta dust (Rho et al. 2009)
Argon
21 mm Dust Feature
Cas A
E0102
N132D
0.02 - 0.05 M
0.007 - 0.015 M
0.008 M (lower limit)
Dust Emission: Heating by Stellar Sources
Koo et al. 2008
• Colors of the point
sources in the IR shell
resemble colors of young
stellar objects
• In this case the IR shell
would have to be a preexisting shell, but we see
no evidence for outer
blast wave
MIPS 24 mm
Alternative Explanation for IR Point Sources
---- PSF Profiles
SURFACE BRIGHTNESS
• Dust model (Borkowski 1994) for forsterite with
power law distribution of grain sizes, and a grain
mass density of 0.007 M/pc3, heated by a main
sequence B0 star with T=30,000 K  dozen stars and
0.1 M of dust needed to reproduce 24/70 mm ratio
• Model shows that ejecta dust heated by main
sequence stars can produce IR emission resembling
point sources at 24 mm  SN exploding in a stellar
cluster could explain IR observations
70 mm
24 mm
RADIUS (arcsec)
Conclusions
IR observations of the shell surrounding G54.1+0.3 provide
first evidence of the PWN interacting with SN ejecta:
• Morphological association between the shell and the PWN
• Spectral lines broadened to 1000 km/s (FWHM)
• Shock velocities on the order of 100 km/s  leads to an age of 4500 yr
and a shell velocity of 500 km/s, consistent with line broadening
• Dust emission features resemble freshly formed dust in Cas A
• Estimated dust mass is in the same range as for ejecta dust in other SNRs
• IR point sources at 24 mm may be explained by radiative heating of ejecta
dust by main sequence stars in a cluster
• We may be probing SN dust that is usually destroyed by shocks!
Unidentified 21 mm Feature
• Similar feature observed in carbon-rich protoplanetary
nebulae (Posch et al. 2004 for review) – most likely
candidates are FeO (Zhang et al. 2009) and SiC grains
(Speck & Hofmeister 2004)
• SiO2 used to fit the 21 mm feature in Cas A
(Rho et al. 2009)
• More detailed spectral fitting required to determine dust
composition in the shell of G54.1+0.3
Spatial Variation in Line Intensities
• 21 mm feature most pronounced at the bright IR knot
• [SIII] 18.7 mm enhanced at the IR knot -> higher density in this region
• Silicon enhanced at the interface between the PWNe and the IR shell
• [ArII] also peaks at the position of the shell cavity
Dust Emission
Rough dust temperature and mass estimates made by fitting the IR
spectrum and the total 24 and 70 micron fluxes with astronomical silicates
and forsterite (Mg2SiO4) grain compositions
Total Dust Mass in the IR Shell:
Mdust = 0.015 – 0.05 M (T = 60 – 70 K)
• Spitzer imaging and spectral maps of SNRs have allowed estimates of
masses of freshly formed dust
• Dust emission coincides with lines from SN ejecta (Rho et al. 2009)
Cas A
0.02 - 0.05 M
E0102
0.007 - 0.015 M
N132D
0.008 M (lower limit)
G11.2-0.3same order of magnitude