The origin of the high-velocity circumstellar gas around SN 1998S

Modelling of high-resolution Balmer line profiles in the early-time spectra of SN 1998S shows that the inferred fast (≈400 km s⁻¹) circumstellar (CS) gas on days 23 and 42 post-explosion is confined to a narrow, negative velocity gradient shell just above the photosphere. This gas may be identified with a slow  (v<40 km s⁻¹)  progenitor wind accelerated at the ejecta–wind interface. In this scenario, the photosphere coincides with a cool dense shell formed in the reverse shock. Acceleration by radiation from the supernova or by a shock-accelerated relativistic particle precursor are both possible explanations for the observed fast CS gas. An alternative, equally plausible scenario is that the fast CS gas is accelerated within shocked clouds engulfed by the outer shock, as it propagates through the intercloud wind.     

HST/STIS observations of the high-velocity interstellar cloud HVC 291.2−41.2+80: a warm, mainly ionized high-velocity cloud

We present intermediate-resolution HST /STIS spectra of a high-velocity interstellar cloud ( v _LSR=+80 km s⁻¹) towards DI 1388, a young star in the Magellanic Bridge located between the Small and Large Magellanic Clouds. The STIS data have a signal-to-noise ratio (S/N) of 20–45 and a spectral resolution of about 6.5 km s⁻¹ (FWHM). The high-velocity cloud absorption is observed in the lines of C  ii , O  i , Si  ii , Si  iii , Si  iv and S  iii . Limits can be placed on the amount of S  ii and Fe  ii absorption that is present. An analysis of the relative abundances derived from the observed species, particularly C  ii and O  i , suggests that this high-velocity gas is warm ( T _k∼10³–10⁴ K) and predominantly ionized. This hypothesis is supported by the presence of absorption produced by highly ionized species, such as Si  iv . This sightline also intercepts two other high-velocity clouds that produce weak absorption features at v _LSR=+113 and +130 km s⁻¹ in the STIS spectra.     

Strong helium 10 830-Å absorption in Sakurai's object (V4334 Sgr)

We report the appearance and evolution during 1998 of strong neutral helium ³S–³P^o absorption at ∼10 830 Å in Sakurai's Object (V4334 Sgr), which is believed to be a planetary nebula nucleus (PNN) undergoing a final helium shell-flash. First detected on 1998 March 18, the profile of the He  i feature is P Cygni-like. The absorption depth has increased in three subsequent spectra in 1998. If this is owing to a wind, the profile indicates a wind velocity of ∼670±50 km s⁻¹. The strong C  i 10 690-Å line seen prior to the appearance of the helium feature has disappeared; however Sr  ii and CN absorption features remain present. We tentatively identify several new features as Si  i . Taken together with other observations we suggest that the data are consistent with Sakurai's Object entering a phase in which it seems to have become a member of the R Coronae Borealis-type class of stars.     

Spectral analysis of the 91bg-like Type Ia SN 2005bl: low luminosity, low velocities, incomplete burning

The properties of underluminous Type Ia supernovae (SNe Ia) of the 91bg subclass have yet to be theoretically understood. Here, we take a closer look at the structure of the dim SN Ia 2005bl. We infer the abundance and density profiles needed to reproduce the observed spectral evolution between −6 d and  +12.9 d  with respect to B maximum. Initially, we assume the density structure of the standard explosion model W7; then we test whether better fits to the observed spectra can be obtained using modified density profiles with different total masses and kinetic energies. Compared to normal SNe Ia, we find a lack of burning products especially in the rapidly expanding outer layers  ( v ≳ 15 000 km s⁻¹)  . The zone between ∼8500 and 15 000 km s⁻¹ is dominated by oxygen and includes some amount of intermediate-mass elements. At lower velocities, intermediate-mass elements dominate. This holds down to the lowest zones investigated in this work. This fact, together with negligible-to-moderate abundances of Fe-group elements, indicates large-scale incomplete Si burning or explosive O burning, possibly in a detonation at low densities. Consistently with the reduced nucleosynthesis, we find hints of a kinetic energy lower than that of a canonical SN Ia: the spectra strongly favour reduced densities at  ≳13 000 km s⁻¹  compared to W7, and are very well fitted using a rescaled W7 model with original mass  (1.38 M_⊙)  , but a kinetic energy reduced by ∼30 per cent (i.e. from  1.33 × 10⁵¹  to  0.93 × 10⁵¹ erg  ).     

The quasar-frame velocity distribution of narrow C iv absorbers

We report on a survey for narrow (full widths at half-minimum <600 km s⁻¹) C  iv absorption lines in a sample of bright quasars at redshifts  1.8 ≤ z < 2.25  in the Sloan Digital Sky Survey. Our main goal is to understand the relationship of narrow C  iv absorbers to quasar outflows and, more generally, to quasar environments. We determine velocity zero-points using the broad Mg  ii emission line, and then measure the absorbers' quasar-frame velocity distribution. We examine the distribution of lines arising in quasar outflows by subtracting model fits to the contributions from cosmologically intervening absorbers and absorption due to the quasar host galaxy or cluster environment. We find that a substantial number (  ≥43 ± 6  per cent) of absorbers with   W ^λ1548₀ > 0.3  Å in the velocity range  +750 ≲ v ≲+ 12 000  km s⁻¹ are intrinsic to the active galactic nucleus outflow. This 'outflow fraction' peaks near   v =+2000  km s⁻¹ with a value of   f _outflow≃ 0.81 ± 0.13  . At velocities below   v ≈+ 2000  km s⁻¹, the incidence of outflowing systems drops, possibly due to geometric effects or to the over-ionization of gas that is nearer the accretion disc. Furthermore, we find that outflow absorbers are on average broader and stronger than cosmologically intervening systems. Finally, we find that ∼14 per cent of the quasars in our sample exhibit narrow, outflowing C  iv absorption with   W ^λ1548₀ > 0.3  Å, slightly larger than that for broad absorption line systems.     

[Al ii] in the ultraviolet spectrum of the symbiotic star RR Telescopii

An inspection of a GHRS/ HST spectrum of the symbiotic star RR Telescopii reveals the presence of the [Al  ii ] 3s²¹S – 3s3p ³P₂ line at a vacuum wavelength of 2661.06±0.08 Å, 8.89±0.08 Å away from the Al  ii ] 3s²¹S – 3s3p ³P₁ intercombination transition at 2669.95 Å, in good agreement with the theoretical prediction of Δ λ =8.80 Å. We also find that the Al  ii ] line profile is asymmetric, showing a strong low-density component with a weak high-density wing, redshifted by 30 km s⁻¹, in agreement with the findings of Schild & Schmid, which were based on optical observations. Our measurement of the emission-line ratio R I (2661.06 Å)/ I (2669.95 Å)=0.027±0.003 implies log  N _e=5.8±0.2, in good agreement with the densities found from other ions, such as Si  iii . These results provide strong evidence that we have detected the [Al  ii ] line, the first time (to our knowledge) that this feature has been reliably identified in an astrophysical or laboratory spectrum.     

Optical photometry and spectroscopy of the Type Ibn supernova SN 2006jc until the onset of dust formation

We present optical UBVRI photometric and spectroscopic data of the Type Ibn supernova SN 2006jc, until the onset of the dust-forming phase. The optical spectrum shows a blue continuum and is dominated by the presence of moderately narrow (velocity ∼2500 km s⁻¹) He  i emission lines superimposed over a relatively weak supernova spectrum. The helium lines are produced in a pre-existing He-rich circumstellar shell. The observed helium line fluxes indicate the circumstellar shell is dense, with a density of  ∼10⁹–10¹⁰ cm⁻³  . The helium mass in this shell is estimated to be  ≲0.07 M_⊙  . The optical light curves show a clear signature of dust formation, indicated by a sharp decrease in the magnitudes around day 50, accompanied by a reddening of the colours. The evolution of the optical light curves during the early phase and that of the uvoir bolometric light curve at all phases is reasonably similar to normal Ib/c supernovae.     

Spectroscopy of SN 1987A at 0.9–2.4 μm: days 1348–3158

We present near-infrared spectroscopic observations of SN 1987A covering the period 1358 to 3158 d post explosion. This is the first time that IR spectra of a supernova have been obtained to such late epochs. The spectra comprise emission from both the ejecta and the bright, ring-shaped circumstellar medium (CSM). The most prominent CSM emission lines are recombination lines of H  i and He  i , and forbidden lines of [S  iii ] and [Fe  ii ]. The ejecta spectra include allowed lines of H  i , He  i and Na  i and forbidden lines of [Si  i ], [Fe  i ], [Fe  ii ] and possibly [S  i ]. The intensity ratios and widths of the H  i ejecta lines are consistent with a low-temperature Case B recombination spectrum arising from non-thermal ionization/excitation in an extended, adiabatically-cooled H envelope, as predicted by several authors. The slow decline of the ejecta forbidden lines, especially those of [Si  i ], indicates that pure non-thermal excitation was taking place, driven increasingly by the decay of ⁴⁴Ti. The ejecta iron exhibits particularly high velocities  (4000–4500 km s^-1)  , supporting scenarios where fast radioactive nickel is created and ejected just after the core bounce. In addition, the ejecta lines continue to exhibit blueshifts with values ∼−200 to −800 km s⁻¹ to at least day 2000. These blueshifts, which first appeared around day 600, probably indicate that very dense concentrations of dust persist in the ejecta, although an alternative explanation of asymmetry in the excitation conditions is not ruled out.     

The 1.05-μm feature in the spectrum of the Type Ia supernova 1994D: He in SNe Ia?

A P Cygni profile with absorption at 1.05 μm was observed in three pre-maximum J -band spectra of the Type Ia supernova (SN) 1994D. The feature was not present in two post-maximum spectra. The line was attributed to He I 10830 ... or Mg II 10926 ..., based on a local thermodynamic equilibrium (LTE) treatment. The detection of He in the ejecta of a SN Ia would be useful for determining the pre-SN evolution and the explosion mechanism of SNe Ia.
In this paper, synthetic spectra are presented for both the He and Mg models. The population of the He levels has been computed in non-local thermodynamic equilibrium (NLTE), including non-thermal excitation and ionization effects resulting from the deposition of γ-rays from the decay of ⁵⁶Ni and ⁵⁶Co.
The J -band feature in the pre-maximum spectra can be reproduced either assuming the presence of a narrow shell, between 10000 and 12500 km s⁻¹, containing about 0.01 M⊙ of He, or increasing the abundance of Mg by about a factor of 5 with respect to the W7 value, implying a Mg mass of about 0.08 M⊙ above 10000 km s⁻¹. Both models are in good agreement with the optical spectrum. In particular, a strong He I 10830-... line does not imply a strong 5876-... line, because the departure coefficients of the 2p and 2s levels of He I differ by about an order of magnitude.
Unfortunately, neither model is able to reproduce the sudden disappearance of the J -band feature in the post-maximum spectra. Possible explanations are discussed.     

Si x emission lines in spectra obtained with the Solar EUV Rocket Telescope and Spectrograph (SERTS)

New R -matrix calculations of electron impact excitation rates for transitions among the 2s²2p, 2s2p² and 2p³ levels of Si  x are presented. These data are subsequently used, in conjunction with recent estimates for proton excitation rates, to derive theoretical electron density sensitive emission-line ratios involving transitions in the ∼253–356 Å wavelength range. A comparision of these with observations of a solar active region and subflare, obtained during the 1989 flight of the Solar EUV Rocket Telescope and Spectrograph ( SERTS ), reveals that the electron densities determined from most of the Si  x line ratios are consistent with one another for both solar features. In addition, the derived densities are also in good agreement with the values of N _e estimated from diagnostic lines in other species formed at similar electron temperatures to Si  x , such as Fe  xii and Fe  xiii . These results provide observational support for the general accuracy of the adopted atomic data, and hence line ratio calculations, employed in the present analysis. However, we find that the Si  x 256.32-Å line is blended with the He  ii transition at the same wavelength, while the feature at 292.25 Å is not due to Si  x , but currently remains unidentified. The intensity of the 253.81-Å line in the SERTS active region spectrum is about a factor of 3 larger than expected from theory, but the reason for this is unclear, and requires additional observations to explain the discrepancy.     

The Cepheid distance to M96 and the Hubble constant

HST WFPC2 observations of Cepheids in M96 (NGC 3368) are used to find a distance to that galaxy of 11.2±1.0  Mpc. This estimate is based on a calibration of the Cepheid period–luminosity relation in the Large Magellanic Cloud, and includes a correction for the difference in metallicity between the two systems. There are good reasons for believing M96 is at the same distance as four E/S0 galaxies in the Leo-I group, and hence we calibrate secondary distance indicators based on the early-type galaxies, namely the fundamental plane and surface-brightness-fluctuation method. Also the Type Ia supernova 1998bu occurred in M96 itself and is used to calibrate the SN Ia distance scale. These methods reach to recession velocities of greater than 5000 km s⁻¹ and can therefore allow us to evaluate the Hubble constant without reference to the peculiar velocity of M96 itself. In fact, these indicators agree well between themselves and hence we find H ₀=67±7 km s⁻¹ Mpc⁻¹ where the quoted error includes estimates of potential systematic effects.     

The coronal line regions of planetary nebulae NGC 6302 and 6537: 3–13 μm grating and echelle spectroscopy

We report on advances in the study of the cores of NGC 6302 and 6537 using infrared grating and echelle spectroscopy. In NGC 6302, emission lines from species spanning a large range of ionization potential, and in particular [Si  ix ] 3.934 μm, are interpreted using photoionization models (including cloudy ), which allow us to re-estimate the temperature of the central star to be about 250 000 K. All of the detected lines are consistent with this value, except for [Al  v ] and [Al  vi ]. Aluminium is found to be depleted to one hundredth of the solar abundance, which provides further evidence for some dust being mixed with the highly ionized gas (with photons harder than 154 eV). A similar depletion pattern is observed in NGC 6537. Echelle spectroscopy of IR coronal ions in NGC 6302 reveals a stratified structure in ionization potential, which confirms photoionization to be the dominant ionization mechanism. The lines are narrow (<22 km s⁻¹ FWHM), with no evidence of the broad wings found in optical lines from species with similar ionization potentials, such as [Ne  v ] 3426 Å. We note the absence of a hot bubble, or a wind-blown bipolar cavity filled with a hot plasma, at least on 1 arcsec and 10 km s⁻¹ scales. The systemic heliocentric velocities for NGC 6302 and 6537, measured from the echelle spectra of IR recombination lines, are found to be −34.8±1 km s⁻¹ and −17.8±3 km s⁻¹. We also provide accurate new wavelengths for several of the infrared coronal lines observed with the echelle.     

Circinus X-1: survivor of a highly asymmetric supernova

We have analysed the kinematical parameters of Cir X-1 to constrain the nature of its companion star, the eccentricity of the binary and the pre-supernova parameter space. We argue that the companion is most likely to be a low-mass (≲2.0 M_⊙) unevolved star and that the eccentricity of the orbit is 0.94±0.04. We have evaluated the dynamical effects of the supernova explosion and we find it must have been asymmetric. On average , we find that a kick of ∼740 km s⁻¹ is needed to account for the recently measured radial velocity of +430 km s⁻¹ (Johnston, Fender & Wu) for this extreme system. The corresponding minimum kick velocity is ∼500 km s⁻¹. This is the largest kick needed to explain the motion of any observed binary system. If Cir X-1 is associated with the supernova remnant G321.9-0.3 then we find a limiting minimum age of this remnant of ∼60 000 yr. Furthermore, we predict that the companion star has lost ∼10 per cent of its mass as a result of stripping and ablation from the impact of the supernova shell shortly after the explosion.     

Globular cluster calibration of the peak brightness of the Type Ia supernova 1992A and the value of H0

We have determined the absolute magnitude at maximum light of SN 1992A by using the turnover magnitude of the globular cluster luminosity function of its parent galaxy, NGC 1380. A recalibration of the peak of the turnover magnitude of the Milky Way clusters using the latest Hipparcos results has been made with an assessment of the complete random and systematic error budget. The following results emerge: a distance to NGC 1380 of 18.6 ± 1.4 Mpc, corresponding to ( m  −  M )  31.35 ± 0.16, and an absolute magnitude of SN 1992A at maximum of M _B (max)  −18.79 ± 0.16. Taken at face value, SN 1992A seems to be more than half a magnitude fainter than the other SNe Ia for which accurate distances exist. We discuss the implications of this result and present possible explanations. We also discuss the Phillips relationship between rate of decline and the absolute magnitude at maximum, on the basis of nine SNe Ia, the individual distances of which have been obtained with Cepheids and the globular cluster luminosity function. The new calibration of this relationship, applied to the most distant SNe of the Calan–Tololo survey, yields H ₀ = 62 ± 6 km s⁻¹ Mpc⁻¹.     

The type IIn supernova 1995G: interaction with the circumstellar medium

We present the photometric and spectroscopic evolution of the type IIn SN 1995G in NGC 1643, on the basis of 4 years of optical and infrared observations. This supernova shows very flat optical light curves similar to SN 1988Z, with a slow decline rate at all times. The spectra are characterized by strong Balmer lines with multiple components in emission and with a P Cygni absorption component blueshifted by only 700 km s⁻¹. This feature indicates the presence of a slowly expanding shell above the SN ejecta as in the case of SNe 1994aj and 1996L. As in other SNe IIn, the slow luminosity decline cannot be explained only with a radioactive energy input, and an additional source of energy is required, most likely that produced by the interaction between supernova ejecta and a pre-existent circumstellar medium (CSM). It was estimated that the shell material has a density   n _H≫10⁸ cm^-3  , consistent with the absence of forbidden lines in the spectra. About 2 years after the burst the low-velocity shell is largely overtaken by the SN ejecta and the luminosity drops at a faster rate.     

The circumstellar material around SN IIn 1997eg: another detection of a very narrow P Cygni profile

We report the detection of a very narrow P Cygni profile on top of the broad emission H α and H β lines of the Type IIn Supernova 1997eg. A similar feature has been detected in SN 1997ab, SN 1998S and SN 1995G . The detection of the narrow P Cygni profile indicates the existence of a dense circumstellar material (CSM), into which the ejecta of the supernova is expanding. From the analysis of the spectra of SN 1997eg we deduce (i) that such CSM is very dense  ( n ≳5×10⁷ cm^-3)  , (ii) that it has a low expanding velocity of about 160 km s⁻¹. The origin of such dense CSM can be either a very dense progenitor wind  ( M˙ ∼10^-2 M_⊙ yr^-1)  or a circumstellar shell product of the progenitor wind expanding into a high-pressure environment.     

The mass ratio and the orbital parameters of the sdOB binary AA Doradus

The time sequence of 105 spectra covering one full orbital period of AA Dor has been analysed. Direct determination of   V  sin  i   for the sdOB component from 97 spectra outside of the eclipse for the lines Mg  ii 4481 Å and Si  iv 4089 Å clearly indicated a substantially smaller value than estimated before. Detailed modelling of line-profile variations for eight spectra during the eclipse for the Mg  ii 4481 Å line, combined with the out-of-eclipse fits, gave   V  sin  i = 31.8 ± 1.8 km s⁻¹  . The previous determinations of   V  sin  i   , based on the He  ii 4686 Å line, appear to be invalid because of the large natural broadening of the line. With the assumption of the solid-body, synchronous rotation of the sdOB primary, the measured values of the semi-amplitude K ₁ and   V  sin  i   lead to the mass ratio   q = 0.213 ± 0.013  which in turn gives K ₂ and thus the masses and radii of both components. The sdOB component appears to be less massive than assumed before,   M ₁= 0.25 ± 0.05 M_⊙  , but the secondary has its mass–radius parameters close to theoretically predicted for a brown dwarf,   M ₂= 0.054 ± 0.010 M_⊙  and   R ₂= 0.089 ± 0.005 R_⊙  . Our results do not agree with the recent determination of Vŭcković et al. based on a K ₂ estimate from line-profile asymmetries.     

Time-series Paschen-β spectroscopy of SU Aurigae

We present time-series echelle spectra of the Paβ line of the T Tauri star SU Aur, observed over three consecutive nights. The line shows strong variability (∼10 per cent) over the velocity range 100–420 km s⁻¹ in the red broad absorption component, and weaker variability (∼2 per cent) over the velocity range  −200–0 km s⁻¹  in the blue wing. The variability in the velocity range  −200–0 km s⁻¹  is correlated with that in  200–400 km s⁻¹  , and the variability in these velocity ranges anticorrelates with that in  0–100 km s⁻¹  . The mean spectrum from the second night shows the suggestion of a blueshifted absorption component at about  −150 km s⁻¹  , similar to that found in the Hα and Hβ lines. We find the position of the subpeak in the red absorption component changes steadily with time, and its motion modulates at half the rotational period. We also find that the modulation of the line equivalent width is possibly associated with a half and a third of the rotational period, which is consistent with the surface Doppler images of SU Aur. Radiative transfer models of a rotationally modulated Paβ line, produced in the shock-heated magnetospheric accretion flow, are also presented. Models with a magnetic dipole offset reproduce the overall characteristics of the observed line variability, including the line equivalent width and the motion of the subpeak in the red absorption trough.     

The high-velocity molecular stream in W51B: a ring structure with compact H ii regions

We present ¹³ CO J  = 1 − 0 line observations of the H  ii region complex W51B located in the high-velocity (HV) stream. These observations reveal a filamentary and clumpy structure in the molecular gas. The mean local standard of rest (LSR) velocity ∼ + 65 km s⁻¹ of the molecular gas in this region is greater than the maximum velocities allowed by kinematic Galactic rotation curves. The size and mass of the molecular clouds are ∼ 48 × 17 pc² and ∼ 2.4 × 10⁵ M⊙ respectively. In a position–velocity diagram, molecular gas in the southern part comprises a redshifted ring structure with v _LSR=+ 60 to +73 km s⁻¹. The velocity gradient of this ring is ∼ 0.5 km s⁻¹ pc⁻¹, and the mass is ∼ 6.2 × 10⁴ M⊙. If we assume that the ring is expanding with a uniform velocity, the expansion velocity, radius and kinetic energy are ∼ 7 km s⁻¹, ∼ 13 pc and ∼ 3.0 × 10 ⁴⁹ erg respectively. The kinetic energy and mass spectrum of the ring could be explained by an expanding cylindrical cloud with a centrally condensed mass distribution. The locations of two compact H  ii regions, G49.0−0.3 and G48.9−0.3, coincide with the two molecular clumps in this ring. We discuss star formation, and the mechanism that produced the ring structure.