Chi-Chao Wu

Interpretation of Ultraviolet Absorption Lines in SN 1006

We present a theoretical interpretation of the broad silicon and iron ultraviolet absorption features observed with the Hubble Space Telescope (HST) in the spectrum of the Schweizer-Middleditch star behind the remnant of SN 1006. These features are caused by supernova ejecta in SN 1006. We propose that the redshifted Si II 1260 A feature consists of both unshocked and shocked Si II. The sharp red edge of the line at 7070 km s-1 indicates the position of the reverse shock, while its Gaussian blue edge reveals shocked Si with a mean velocity of 5050 km s-1 and a dispersion of 1240 km s-1, which implies a reverse shock velocity of 2860 km s-1. 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 Si III and Si IV absorption features indicate that they arise mostly from shocked Si. The total mass of shocked and unshocked Si inferred from the Si II, Si III, and Si IV profiles is MSi = 0.25 ± 0.01 M☉ on the assumption of spherical symmetry. Unshocked Si extends upward from 5600 km s-1. Although there appears to be some Fe mixed with the Si at lower velocities 7070 km s-1, the absence of Fe II absorption with the same profile as the shocked Si II suggests little Fe mixed with Si at higher (before being shocked) velocities. The column density of shocked Si II is close to that expected for Si II undergoing steady state collisional ionization behind the reverse shock, provided that the electron to Si II 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 SN 1006 is anomalously low compared to the density around the rest of the remnant. This would simultaneously explain the high velocity of the redshifted Si absorption, the absence of blueshifted Si absorption, and the low density of the absorbing Si compared to the high Si density required to produce the observed Si X-ray line emission. We have reanalyzed the Fe II absorption lines and have concluded that the earlier evidence for high-velocity blueshifted Fe II extending to ~-8000 km s-1 is not compelling. We interpret the blue edge on the Fe II profiles at -4200 km s-1 as the position of the reverse shock on the near side of SN 1006. The mass of Fe II inferred from the red edge of the Fe II profile is MFe II = 0.029 ± 0.004 M☉ up to 7070 km s-1, if spherical symmetry is assumed. The low ionization state of unshocked Si inferred from our analysis of the silicon features, Si II/Si = 0.92 ± 0.07, suggests a correspondingly low ionization state of unshocked iron, with Fe II/Fe = 0.66 -->+ 0.29−0.22. If this is correct, then the total mass of Fe up to 7070 km s-1 is MFe = 0.044 -->−0.013+0.022 M☉ with a 3 σ upper limit of MFe < 0.16 M☉. Such a low ionization state and mass of iron is consistent with the recent observation of Fe III 1123 A with the Hopkins Ultraviolet Telescope (HUT), which indicates Fe III/Fe II = 1.1 ± 0.9 but conflicts with the expected presence of several tenths of a solar mass of iron in this suspected Type Ia supernova remnant. However, the inference from the present HST data is too indirect, and the HUT data are too noisy, to rule out a large mass of iron. Reobservation of the Fe III 1123 A line at higher signal-to-noise ratio with Far Ultraviolet Space Explorer will be important in determining the degree of ionization and hence mass of iron in SN 1006.

Prominent ultraviolet emission lines from Type 1 Seyfert galaxies

IUE Explorer data with a 6-A resolution are reported for Ly-alpha, C IV 1550 A, C III semiforbidden line 1909 A, and Mg II 2800 A, as well as the continuum at 1450 A, for 20 Seyfert galaxies and one quasar. Good correlation is found between the continuum and the line fluxes, indicating that photoionization is the dominant heating mechanism for these active galactic nuclei. A combination of the data for Seyferts and high redshift quasars covering five orders of magnitude in continuum luminosity shows the lower luminosity objects to have higher equivalent widths for their emission lines, suggesting that the covering factor increases with decreasing luminosity.

IUE Spectra and photoionization models of the Seyfert 2 glaxies NGC 7674 and I Zw 92

The physical conditions in the narrow-line regions of two Seyfert 2 galaxies, NGC 7674 and I Zw 92, are examined using IUE spectra, published optical spectra and multifrequency observations, and photoionization models. For each Seyfert galaxy, the emission-line fluxes were dereddened using the He II lambda(1640)/lambda(4686) ratio. Photoionization models were calculated using a power-law index determined from the He II lambda(4686)/H-beta ratio; the index is very similar to that obtained from a fit to the observed multifrequency continuum from the infrared to the X-rays. The models were calculated in a way that minimized the number of assumptions, and given the uncertainties in the reddening corrections, the calculated ratios match nearly all of the dereddened ratios successfully. a multicomponent model (three components with different densities and ionization parameters) was required to fit the spectrum of I Zw 92, whereas a single component was sufficient for NGC 7674. The CNO abundances are close to solar, although a reduced abundance of up to one-third solar for one or more of the heavy elements is possible. In contrast to a previous study of Mrk 3, dust inside the narrow-line region (NLR) louds was not required to fit the spectra of these two Seyfert galaxies, although the emission lines experience considerable reddening from external dust. Higher signal-to-noise spectra in the UV are essential for placing further restrictions on the reddening and physical conditions in the narrow-line regions of Seyfert galaxies.

High-velocity iron absorption lines in supernova remnant 1006

The very strong, broad absorption lines of a sdOB star continuum shown in the International Ultraviolet Explorer spectrum of the Schweizer-Middleditch star projected near the center of supernova remnant SNR 1006 are investigated. It is found that strong Fe(+) resonance absorption lines are present whose centers show zero radial velocity while their profiles are broadened by approximately 5000-6000 km/s. Also identified are redshifted Si(+), Si(2+), and Si(3+) lines at velocities of approximately 5000 km/s. Results show that the absorptions must occur in the ejecta of the supernova. The strength and symmetric width of the Fe(+) line indicates that the bulk of the ejecta is iron, in agreement with the current theory for the origin of Type I supernova. It is suggested that the previous failure to detect strong Fe emission lines in the X-ray spectra of this and other young Type I SNRs may be a result of the ejecta not having had time to interact significantly with the ambient medium. In addition, the presence of redshifted adsorption lines due to supernova ejecta in its spectrum shows that this star is located behind the SNR and is not physically associated with it.

Faint Object Spectrograph Spectra of the Broad Fe II Absorption Lines in the Remnant of SN 1006

Ultraviolet spectra of a hot subdwarf star located behind the remnant of SN 1006 have been obtained with the Faint Object Spectrograph on the Hubble Space Telescope. The FOS UV spectra show strong and extremely broad Fe II λ2343, λ2374, λ2382, λ2586, and λ2599 lines in absorption from the supernova remnant, confirming an earlier detection with the International Ultraviolet Explorer. The better resolution and signal-to-noise of the FOS spectra allow for the removal of narrow interstellar and stellar lines and the deblending of the broad Fe II lines, resulting in an intrinsic Fe II profile suitable for detailed comparison with model predictions

High-velocity ultraviolet iron, silicon, oxygen, and sulfur absorption features associated with the remnant of SN 1006

New low-dispersion IUE spectra of a faint sdOb star located in a direction near the center of the SN 1006 remnant are presented. The UV spectrum of the star exhibits several strong absorption features which are uncharacteristic of its optical sdOB star classification. The identification by Wu et al. (1983) of very broad absorption features at 1610, 2370, 2600 A as Fe II gas associated with the SN 1006 remnant is supported. The observed Fe II line profiles indicate a concentration of Fe(+) toward the remnants center with a radial velocity range on the order of + or - 5000 km/s. Strong absorption lines at 1281, 1331, and 1420 A are interpreted as originating from clumps of O-, Si- and S-rich ejecta with central radial velocities in the range 5000-6500 km/s. The presence in the SN 1006 remnant of an expanding sphere of iron-rich ejecta interior to O-, Si-, and S-rich clumps of ejecta having velocities over the maximum seen for the Fe II absorbing gas is consistent with type Ia Sn observations and carbon deflagration models. 64 references.

Far-Ultraviolet Absorption Lines in the Remnant of SN 1006*

We have obtained a far-ultraviolet spectrum (1150-1600 A) of a hot subdwarf star behind the remnant of supernova 1006 with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope. The high-quality spectrum is used to test previous identifications of the strong absorption features discovered with the International Ultraviolet Explorer. These features have FWHM = 4000 (±300) km s-1 and are not at the rest wavelengths of known interstellar lines, as opposed to the broader (~8000 km s-1 FWHM) Fe II lines from the remnant centered at 0 km s-1 in near-UV FOS spectra. We confirm that the broad absorption features are principally due to redshifted Si II, Si III, and Si IV lines, which are centered at a radial velocity of 5100 (±200) km s-1. The Si II λ1260.4 profile is asymmetric, with a nearly flat core and sharp red wing, unlike the Si II λ1526.7 and Si IV λλ1393.8, 1402.8 profiles. One possible explanation is additional absorption from another species. Previous work has suggested that S II λλ1250.6, 1253.8, 1259.5 at a radial velocity of ~6000 km s-1 is responsible, but this would require a sulfur-to-silicon abundance ratio that is at least a factor of 10 higher than expected. Another possible explanation is that the Si II and Si IV profiles are intrinsically different, but this does not explain the symmetric (albeit weaker) Si II λ1526.7 profile.

Faint Object Spectrograph Spectra of the UV Emission Lines in NGC 5558: Detection of Strong Narrow Components

Ultraviolet spectra of the Seyfert 1 galaxy NGC 5548 were obtained with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope on 1992 July 5, when the UV continuum and broad emission lines were at their lowest ever observed level. The high resolution of the spectra, relative to previous UV observations, and the low state of NGC 5548 allow the detection and accurate measurement of strong narrow components of the emission lines of Ly alpha, C IV 1549, and C III 1909. Isolation of the UV narrow components enables a detailed comparison of narrow-line region (NLR) properties in Seyfert 1 and 2 galaxies, and removal of their contribution is important for studies of the broad-line region (BLR). Relative to the other narrow lines, C IV 1549 is much stronger in NGC 5548 than in Seyfert 2 galaxies, and Mg II 2798 is very weak or absent.

The C IV 1550 profile in type 1 Seyfert galaxies

The paper presents C IV 1550 A line profiles for the type 1 Seyfert galaxies NGC 5548, Mrk 509, NGC 7469, and MCG-2-58-22. Several line broadening mechanisms and theoretical line profiles are considered, and random motion of discrete clouds is ruled out. A spherical ensemble of discrete clouds with steady outflow or inflow produces a logarithmic profile, but does not account for the highly extended wings. A spherical ensemble with ballistic outflow produces a profile of the first exponential integral function, and fits the observed profile to the continuum level. Although C IV profiles favor the ballistic model, both Mrk 509 and NGC 7469 have significant asymmetry, and Balmer lines with a higher optical depth show higher asymmetry and redshift than all four galaxies.