320 research outputs found
The Expansion Center and Dynamical Age of the Galactic Supernova Remnant Cassiopeia A
We present proper motions for 21 bright main shell and 17 faint,
higher-velocity, outer ejecta knots in the Cas A supernova remnant and use them
to derive new estimates for the remnant's expansion center and age. Our study
included 1951 - 1976 Palomar 5 m prime focus plates, 1988 - 1999 CCD images
from the KPNO 4 m and MDM 2.4 m telescopes, and 1999 HST WFPC2 images.
Measurable positions covered a 23 to 41 yr time span for most knots, with a few
outer knots followed for almost 48 yr. We derive an expansion center of alpha =
23h 23m 27s.77 +- 0s.05, delta = 58d 48' 49".4 +- 0".4 (ICRS), with little
difference between centers derived using outer or main shell knots. This
position is 3.0 arcsec due north of that estimated by van den Bergh and Kamper
(1983). It also lies 6.6 +- 1.5 arcsec almost due north (PA = 354 deg) of the
remnant's recently-detected central X-ray point source, implying a transverse
velocity for the X-ray point source of about 330 km/s at a distance of 3.4 kpc.
Using the knots which lie out ahead of the remnant's forward blast wave, we
estimate a knot convergent date of A.D. 1671.3 +- 0.9, assuming no
deceleration. However, a deceleration of just approximately 1.6 km/(s yr) over
a 300 yr time span would produce an explosion date A.D. 1680, consistent with
the suspected sighting of the Cas A supernova by J. Flamsteed.Comment: Astron. Journal in press, sched. July 2001. AASTex5, 17 pages, 2 jpeg
greyscale figures, 3 postscript figure
Interpretation of UV Absorption Lines in SN1006
We present a theoretical interpretation of the broad silicon and iron UV
absorption features observed with the Hubble Space Telescope in the spectrum of
the Schweizer-Middleditch star behind the remnant of Supernova 1006. These
features are caused by supernova ejecta in SN1006. We propose that the
redshifted SiII2 1260 A feature consists of both unshocked and shocked SiII.
The sharp red edge of the line at 7070 km/s indicates the position of the
reverse shock, while its Gaussian blue edge reveals shocked Si with a mean
velocity of 5050 km/s and a dispersion of 1240 km/s, implying a reverse shock
velocity of 2860 km/s. The measured velocities satisfy the energy jump
condition for a strong shock, provided that all the shock energy goes into
ions, with little or no collisionless heating of electrons. The line profiles
of the SiIII and SiIV absorption features indicate that they arise mostly from
shocked Si. The total mass of shocked and unshocked Si inferred from the SiII,
SiIII and SiIV profiles is M_Si = 0.25 \pm 0.01 Msun on the assumption of
spherical symmetry. Unshocked Si extends upwards from 5600 km/s. Although there
appears to be some Fe mixed with the Si at lower velocities < 7070 km/s, the
absence of FeII absorption with the same profile as the shocked SiII suggests
little Fe mixed with Si at higher (before being shocked) velocities. The column
density of shocked SiII is close to that expected for SiII undergoing steady
state collisional ionization behind the reverse shock, provided that the
electron to SiII ratio is low, from which we infer that most of the shocked Si
is likely to be of a fairly high degree of purity, unmixed with other elements.
We propose that the ambient interstellar density on the far side of SN1006 is
anomalously low compared to the density around the rest of the remnant. ThisComment: 24 pages, with 8 figures included. Accepted for publication in the
Astrophysical Journa
Late-Time Optical and UV Spectra of SN 1979C and SN 1980K
A low-dispersion Keck I spectrum of SN 1980K taken in August 1995 (t = 14.8
yr after explosion) and a November 1997 MDM spectrum (t = 17.0 yr) show broad
5500 km s^{-1} emission lines of H\alpha, [O I] 6300,6364 A, and [O II]
7319,7330 A. Weaker but similarly broad lines detected include [Fe II] 7155 A,
[S II] 4068,4072 A, and a blend of [Fe II] lines at 5050--5400 A. The presence
of strong [S II] 4068,4072 A emission but a lack of [S II] 6716,6731 A emission
suggests electron densities of 10^{5-6} cm^{-3}. From the 1997 spectra, we
estimate an H\alpha flux of 1.3 \pm 0.2 \times 10^{-15} erg cm^{-2} s^{-1}
indicating a 25% decline from 1987--1992 levels during the period 1994 to 1997,
possibly related to a reported decrease in its nonthermal radio emission.Comment: 21 pages, 8 figures, submitted to the Astronomical Journa
A Catalog of Outer Ejecta Knots in the Cassiopeia A Supernova Remnant
Hubble Space Telescope images of the core-collapse supernova remnant Cassiopeia A are used to identify high-velocity knots of ejecta located outside the remnant's main emission shell of expanding debris. These ejecta fragments are found near or ahead of the remnant's forward shock front and mostly lie from 12000 to 30000 in radial distance from the remnant's center of expansion. Filter flux ratios when correlated with published spectra show that these knots can be divided into three emission classes: (1) knots dominated by [N II] lambda lambda 6548, 6583 emissions, ( 2) knots dominated by [O II] kk7319, 7330 emissions, and (3) knots displaying filter flux ratios suggestive of [ S II], [ O II], and [Ar III] lambda 7135 emission line strengths similar to the "fast-moving knots'' (FMKs) found in the remnant's bright main shell. Of 1825 knots identified, 444 are strong [N II] emission knots, 192 are strong [O II] emission knots, and 1189 are FMK-like knots. In terms of location around the remnant, 972, 207, and 646 knots are found in the remnant's northeast jet, southwest jet, and non-jet regions, respectively. Assuming a distance of 3.4 kpc, derived knot transverse velocities based on proper motion measurements spanning a 9 month interval indicate maximum transverse expansion velocities for these three knot classes of 14,500, 13,500, and 11,500 km s(-1), respectively. We present a catalog of these outlying ejecta clumps comprising finding charts, epoch 2004.2 knot positions, proper motions, photometric filter fluxes, and estimated knot emission type, along with cross-references to previous knot identifications and data. This compilation represents a nearly tenfold increase in the number of outlying, high-velocity ejecta knots identified around the Cassiopeia A remnant
Near-Infrared Spectroscopy of the Cassiopeia A and Kepler Supernova Remnants
Near-infrared spectra (0.95 - 2.4 micron) of the Cassiopeia A and Kepler supernova remnants (SNRs) are presented. Low-dispersion (R = 700) spectra were obtained for five bright fast-moving ejecta knots (FMKs) at two locations on the main shell and for three bright circumstellar knots (QSFs) near the southwest rim of Cas A. The main shell FMKs in Cas A exhibit a sparse near-infrared spectrum dominated by [S II] 1.03 micron emission with a handful of other, fainter emission lines. Among these are two high-ionization silicon lines, [Si VI] 1.96 micron and [Si X] 1.43 micron, which have been detected in AGNs and novae but never before in a supernova remnant. The near-infrared spectra of circumstellar QSFs in Cas A show a much richer spectrum, with strong He I 1.083 micron emission and over a dozen bright [Fe II] lines. Observed [Fe II] line ratios indicate electron densities of 5 - 9 * 10^4 cm^-3 in the QSFs. The Cas A QSF data are quite similar to the observed spectrum of a bright circumstellar knot along the northwest rim of the Kepler SNR, which also shows strong He I and [Fe II] emission with a measured electron density of 2.5 - 3 * 10^4 cm^-3. Finally, we present J- and K-band images of Cas A. The K-band image shows faint diffuse emission which has no optical or mid-infrared counterpart but is morphologically similar to radio continuum maps and may be infrared synchrotron radiation
Chandra Observations of the Luminous, O-Rich SNR in the Irregular Galaxy NGC 4449
An analysis of a 29 ksec Chandra ACIS-S observation of the young, Cassiopeia-A like supernova remnant in the irregular galaxy NGC 4449 is presented. The observed 0.5-2.1 keV spectrum reveals the likely presence of several emission lines including O VIII at 0.65 keV and 0.77 keV, Ne X at 1.05 keV, Mg XI at 1.5 keV, and Si XIII at 1.85 keV. From the observed spectrum, we derive an N_H = 10^21 cm^-2 and an X-ray temperature of T = 9 * 10^6 K. A non-equilibrium ionization fit to the spectrum suggests an overabundance of oxygen around 20 times solar, consistent with the remnant's UV and optical emission-line properties. We discuss tht remnant's approximate X-ray derived elemental abundances and compare its X-ray spectrum and luminosity to other oxygen-rich remnants
The Evolution of Late-time Optical Emission from SN 1986J
We present late-time optical images and spectra of the Type IIn supernova SN
1986J. HST ACS/WFC images obtained in February 2003 show it to be still
relatively bright with m(F606W) = 21.4 and m(F814W) = 20.0 mag. Compared
against December 1994 HST WFPC2 images, SN 1986J shows a decline of only <1 mag
in brightness over eight years. Ground-based spectra taken in 1989, 1991 and
2007 show a 50% decline in Halpha emission between 1989-1991 and an order of
magnitude drop between 1991-2007, along with the disappearance of He I line
emissions during the period 1991-2007. The object's [O I] 6300, 6364, [O II]
7319, 7330 and [O III] 4959, 5007 emission lines show two prominent peaks near
-1000 km/s and -3500 km/s, with the more blueshifted component declining
significantly in strength between 1991 and 2007. The observed spectral
evolution suggests two different origins for SN 1986J's late-time optical
emission: dense, shock-heated circumstellar material which gave rise to the
initially bright Halpha, He I, and [N II] 5755 lines, and reverse-shock heated
O-rich ejecta on the facing expanding hemisphere dominated by two large clumps
generating two blueshifted emission peaks of [O I], [O II], and [O III] lines.Comment: 4 pages, 3 figures. Submitted to the Astrophysical Journal (Letters
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