The 1988 IUE SWP observations of a faint sdOB star situated behind the remnant of the supernova of AD 1006 are presented. These spectra along with IUE spectra of the star taken between 1982 and 1986 provide a detailed look at the elemental composition and dynamical properties of the SN 1006 remnant. Over the 6 years there were no significant changes in the absorption features associated with the remnant at 1281, 1330, or 1420 A. While the lack of variability in these absorption lines makes it impossible to decide whether the ejecta is distributed smoothly or in knots, it is clear that the 1281 A feature is a blend, requiring either S II absorption redshifted at 6000 km/sec plus Si II 1260 at 5200 km/sec, or else 2 individual Si absorbing regions

Fesen, Robert A.

Hamilton, Andrew J. S.

English

NASA Technical Reports Server

//17 ''/./,' /1-
 q89- 10700 121
EJECTA IN SN 1()06: THE KNOTTY ISSUE
Rol)ert A. Fesen Andrew J. S. Hamilton
Cent, er for Astrophysics
and Space Astronomy
University of Colorado
Boulder, Colorado 80309
ABSTRACT
We present new 1988 IUE SWP observations of a fidnt
sdOB star situated behind the remnant of the supernova of
AD 1006. These spectra along with previous ]UE specl, ra
of the star taken between 1982 and 1986 l)rovide a detailed
look at the elemental ('omposition and dynamical propertms
of the SN 1006 renmant. Over the six years there have been
no significant changes in the absorption featmes associated
with the iemnant at 1281, 1330, (,r 1420/_. Whih.' the lack
of variability in these absorl)tion lines makes it impossible
to decide whether tile ejecta is distributed smoothly or in
knots, it is now clear that the 1281 /_ featm-e is a bhmd,
requiring either S II absorption redshifted at 6000 km s -l
plus Si II 1260 at 5200 km s-', or else two individual $1
absorbing regions.
Keywords: supernovae - su, pernova remT_ant._ znter-
,stellar medzum star.q: sdOB
I INTRODUCTION
Tile galactic ra(ho source G327.6+14.6 is tile prol)able rClll-
nant of the bright historical SUl)ernova observed in 1006
AD. This remnant appears in the radio and X-rays as a
limb brightened 30' diameter shell. Optically, there are just
a few thin Balmer dominated filaments along the northwest
rim. The smfilarity of the SN 1006 remnant's morphology
and optical emission to Tycho's SNR, its high galactic lat-
itude, the supernova's reported visual brightness and time
of visibility, together with a lack of _ nearby OB associa-
tion have strongly suggested a Type Ia origin for SN 1006.
Current models for Type Ia SN involve the dibruption of a
white dwarf either through inass accretion in a close binary
or the merge of two white dwarfs.
Strong support for both the Type Ia nature of SN 1000
and exploding carbon-deflagration white dwarf nmdels has
come from IUE obserwttions of a faint sdOB star positioned
behind the SN 1006 remnant. Low dispersion IUE spectra
of this star, referred to as the S-M star (Schweizer and Mid-
dlcditch 1980), reveal several strong and broad absorption
features which are unchaiacteristic of a sdOB star (Wu et
Joint. Institute for Labor_tory Astrophysics
and National Bureau of Standards
University of Colorado
Boulder, Colorado 803O9
al. 1983, Fesen et al. 1988). Specifically, there are very
stlong Fe II absorptions at 1610, 2370, and 2600 _ with
a velocity range of 4-5000 kin s -1, plus broad fi-'atures at
1281, 1330, and 1420 ._. The latter features have been in-
terpreted by \¥u et al. (1983) and Fesen et al. (1988) as
1260 Si If, 1255, 1259 S II, 1302 O I, and 1393, 1403 Si
IV redshifted by 5000 - 6500 km s -l. The presence of an
expanding sphere of iron-rich ejecta interior to O, S, and
Si-rich mat¢'_ial having velocities m excess of that seen foi
the iron is consistent both with observations of Type Ia SNe
near maxnnum light, and with carbon deflagration models.
If the Si, S, and O features are caused by fast-moving
knots of ejecta with dimensions on the order of 10 .2 pc,
such as is observed in other young remnants, then notice-
able changes in their absorption profiles and strengths can
be expected on a time scale of about 1(1 years. In order to
investigt_te possible changes in these absori)tion features,
we obtained new 1988 S\VP spectra of the S-M star. Be-
low, we describe the obs¢'rwltions and results and discuss
the issue of knots of ejecta in SN 1006.
II OBSERVATIONS
Low-dispersmn, short wavelength (S\VP) IUE spectra of
the S-M star were obtained on March 25 and 26, 1988.
The exposure times were 400 minutes (SWP 33156) and
415 ininutes (SWP 33164) respectively and were taken dur-
ing very low rad,ation lew-Js in US1 observing shifts. A log
of all IUE SWP of the S-M star is given in Table 1. The
two new low dispersion spectra were extracted fi'om the
si)atially resolved line-by-line file provided by IUESIPS us-
ing either 5 or 7 lines. Standard calibrations and blemish
corrections were applied. Placement of tile S-M star within
Table I
lUg Observing Log - S-M Star
Image Number Date Exposure
SWP 16054 16 Jan 1982 360 rain
SWP 19927 8 May 1983 360 rain
SWI' 27592 25 Jan 1986 400 rain
8WP 27619 27 ,lan 1986 330 rain
SWP 33156 2.5 Mar 1988 q00 rain
SWI' 33161 26 Mar 1988 '115 min
A Decade of UV Astronomy wuh IUE, Proc. Celebratory Symposium, GSFC, Greenbeh USA, 12-15 April 1988, ESA SP-281, Vol. 1 (June 1988)
https://ntrs.nasa.gov/search.jsp?R=19890001329 2020-03-20T05:58:07+00:00Z
122 R A FESEN & A J S HAMILTON
the large aperture was done via a blind offset slew. Slight
relative displacements of the star within the large aper-
ture can result in small wavelength shifts, a not uncommon
problem. Offset displacements of the S-M star of the order
of 2 - 4" apparently occurred for these new observations
since stellar absorption features at 1550 (C IV) and 1640
(He II) in these 1988 spectra showed wavelength differences
of 7 and 6 ._. with respect to rest wavelengths. The spectra
were consequently shifted by these amounts so they could
be compared and added to previous IUE spectra.
The two new spectra smoothed by a 3 point box func-
tion and then added together are shown in Figure 1 along
with 1982/1983 and 1986 summed spectra. The summed
spectrum from the three epochs is shown in Figure 2 with
the major absorption features identified.
X
hi
tad
I I I I
120( 1300 1400 1500 1600 1700
WAVELENGTH
Figure 1 - Plot of summed 1982/83, 1986, and 1988
SWP spectra of the S-M star for the wavelength range 1200
to 1700/_. Summed data as shown have been smoothed by
a three point box-car function and wavelength corrected
relative to the 1982 spectrum (SWP 16054).
X
D
LI
t--
txJ
I I i I
Si II
+ ALL DATA
(_S II
I
.o,,
" " V/Z>`
I I I I
1200 1500 1400 1500 1600 1700
WAVELENGTH
Figure 2 - Sum of all six low-dispersion SWP spectra
of the S-M star for the wavelength region 1200 to 1700/_.
Probable line identifications for the stronger features are
indicated.
III DISCUSSION: KNOTS OR NOT?
From the six IUE spectra taken over a period of six years,
we can now better determine the reality and variability of
absorption features in the S-M star. The absorption lines
consistently present in the wavelength range between 1220
and 1900/_. include the features at 1260, 1281, 1330, 1420,
1527, 1550, and 1640 _. The 1260 and 1527/_. lines appear
to be conventional interstellar Si II 1260 and 1527 absorp-
tions, while the 1550 and 1640 _ features are probably stel-
lar C IV and He II lines, consistent with the star's sdOB
classification (see Fesen et al. 1988). The possible weak stel-
lar Fe V absorption near 1464/_ is only marginally present
in the 1988 spectra. No other features in the spectrum as
strong as the interstellar 1260/_ Si II line appear present.
As shown in Figure 1, the 1281, 1330, and 1420 /_ fea-
tures attributed to absorbing material associated with the
SN 1006 have not changed substantially from 1982 to 1988.
Although all six spectra are underexposed, these three lines
show no equivalent width variations at the 50% level and
probably at the 25% level. Possible minor changes in the
1330 _ feature mentioned by Fesen et al. are not confirmed.
The only possible change observed is the 1420 /_ feature
which may have broadened somewhat during this time pe-
riod (see Figure 1). However, none of the three absorption
features show any change in their central wavelength.
As shown in Figure 3, the line identifications for these
features remain essentially unchanged from those discussed
by Fesen et al. The 1420 /_. feature is probably the Si IV
resonance lines at 1393 and 1403/_ redshifted by 5000 km
s-t; the 1330 * feature can be interpreted as a blend of
Si II 1304 at 5200 km s-1, O I 1302 at 6500 km s -1, and
interstellar C II 1335; and finally, the strongest feature at
1281 _ is most likely caused mainly by the 1260 line of Si
II redshifted at a velocity of 5200 km s -1, i.e. close to the
velocity inferred from the Si IV interpretation of the 1420
feature.
While the strong 1281 /_ feature has not varied, the
new 1988 observations in conjunction with the four previous
spectra indicate that this feature is actually a blend of at
least two lines; one centered near 1282 /_, and a slightly
weaker one near 1277 _. Despite the low signal to noise
near the bottom of such a strong absorption feature, 5 of
the 6 individual spectra suggest an unresolved blend of at
least two components. The other spectrum shows a broad
line profile consistent with the blended profile seen in the
five other spectra. Fesen et al. (1988) concluded that while
most of the 1281 feature was likely due to Si II, about 25%
or so might also bc redshifted S II lines (1251, 1254, 1259)
at a velocity of 6000 km s -I . For this feature to be a blend,
either there must exist two absorbing regions of Si II along
the line-of-sight, or S II absorption must be present at a
more substantial intensity.
The lack of changes in these lines implies a nfinimum di-
mension of 0.06 pc (for d = 1.7 kpc) for the absorbing gases,
but leaves open the question, discussed by Fesen et al., of
whether the Si, S, and O absorbing material is unshocked
ejecta, or shocked ejecta which has cooled and condensed
into knots. This is an important issue for it is not yet clear
whether Type Ia supernovae produce the type of clmnpy
ejecta seen in young sut)ernova remnants such as Cas A
EJECTA IN SN 1006 123
X
3
b_
Ld
_>
LO
[Z
1 I 1
1281 FEATURE
1260 Si f( 5200 km/s
I
S II 6000 km/s 1251 1254 1259
I I I
40 1260 1280 1300 1320
WAVELENGTH
Lt_
LLI
I I I I
1.331 FEATURE
1304 Si II 5200 km/s 1335 C II 0 km/9
I I
... 1302 0 I 6500 km/s
1310
I I I I
1320 1530 1340 1350 1360
WAVELENGTH
X
d
LL
ILl
>
F-
5
LO
i I I
142o FEATURE
Si IV 5000 km/s
._ 1394 14i3
1380 1400 1420 1440 1460
WAVELENGTH
X
J
h
LO
LU
I I I I
1550 FEATURE
Si II 1527 C IV 1548/51
I II
,."", 1527 Si II 5200 km/s
I I I I
1500 1520 1540 1560 1580 1600
WAVELENGTH
Fi,gurc 3
1OSS IUE d_,,,;t(_,(,h,l hn{') c(mq,m,'d _(> 19S2 IOSG d:,t,a ((l()t, lc(1 lira') f(,1 l,hc
5_l()llt_ ,d)_(aptl()nfc,trtu('_]]_ 1]_'_l)c_ 11,t_f' tit(' L3._I'_IHI D;tt;_ _}jmvu have l)c,'u
\'_';t\cIcII_tII ,-,hl[t,-C()llCCtc,l ;IIHI _.lnooth(,d ]ix" ;t thlcc p(mlt ]_o\-('a: fuw:t_o]L 12cd-
'.hfftcd I_()bitl(m ", ()t $1 II. S 11. () I ;Iml SI I\: m(' _.h(,uu v,'at.h zclat, iv(: ()S,_lllat(n
st_cngr, h,. tmhc;m:(11)y the lcngth._ of the vc_ tic;,1 lmc.., Tit(",(: plot,b mc _inular t()
Figm(' S of Fcscn c1, al (19SS) Note the ',]milarlty ()f the ;d).'.ort)tl(,n features seen
m 1h,; 19SS (I,LI,L t(, tlt,_t ()f th,: l)t,'viou ', ,'l)()('h (h,t;L.
124 R A FESEN & A J S HAMILTON
and tile Crab Nebula. Knots in supernova ejecta may form
as a result of thermal instabilities either at the time of the
explosion or in reverse shocked ejecta.
The absence of variability in these lines is at least con-
sistent with smoothly distributed unshocked ejecta. Cal-
culations by Hamilton and Fesen (1988) indicate that any
unshocked silicon should show comparable amounts of Si II
and Si IV as the present time, as is observed. The Fe II
absorption profiles indicate that the reverse shock lies not
umch beyond the 5000 km s-1 free expansion radius. Our
identifications of absorbing material at higher velocities,
namely S II at 6000 km s-' and O I at 6500 km s -1, are
uncertain because of possible blending; for example, the
1330 O I feature might instead be Si II 1304 at 5200 km
s -t and interstellar C II 1335 (see Wu et al. 1983). If the
S II and O I line identifications are incorrect, then it is pos-
sible that no absorbing material has yet reached the reverse
shock.
On the other hand, if the absorbing regions are com-
posed of material shocked by the blast wave's reverse shock,
then the combination of low ionization and high velocity
requires the absorbing material to be condensed knots of
cjccta. Clumpy ejecta would be consistent with the lack
of any blueshifted absorption despite the symmetry of the
Fe II absorption profiles. If our identifications of S II at
6000 km s -1 and O I at 6500 km s-' are correet_i_._._this
absot bing material must lie far at'i;_'_.l'_ dnst_dd_-c\_'i:on,
m the form of shocked and cooled kno_.. _h_ velo_ d,s-
persion of several hundred km s -1 indicated by thg width
of the Si IV 1420 /_ feature is comparable to the velocity
dispersions seen in the reverse shocked metal-rich knots of
Cas A. ..
,g_-
Unshocked ejecta will produce slowly changing lines,
whereas small knots of shocked ejecta should produce more
rapid variations. The most decisive evidence in favor of
knotb would be rapid absorption line variability. However,
until such changes are observed, the question of the struc-
ture and composition of this 1000 yr old Type Ia's ejecta
relnains uncertain.
IV ACKNOWLEDGEMENTS
These data were reduced and processed at the University
of Colorado's IUE-RDAF, which is supported by NASA
contract number NAS5-28731. This work was supported in
part by NASA contract NAS5-31436 to the University of
Colorado.
REFERENCES
F,..s,m; IRA , \Vu, C.-C., Leventhal, M., and Hamilton, A J S.
1988, Ap. J., 327, 164.
" Hamilton, A.J.S., and Fesen, R.A. 1988, Ap. J., 327,178.
Schweizer, P., and Middleditch, J. 1980, Ap J., 241, 1039
\\;u, C-C., Leventhal, M., Sarazin, C L., and Gull, T.R.
1983, Ap. J. (Le_ter,,), 269, L5.


Ejecta in SN 1006: The knotty issue

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server