SN 2001el Research Articles

Predicting polarization signatures for double-detonation and delayed-detonation models of Type Ia supernovae

Calculations of synthetic spectropolarimetry are one means to test multi-dimensional explosion models for Type Ia supernovae. In a recent paper, we demonstrated that the violent merger of a 1.1 and 0.9 M$_{\odot}$ white dwarf binary system is too asymmetric to explain the low polarization levels commonly observed in normal Type Ia supernovae. Here, we present polarization simulations for two alternative scenarios: the sub-Chandrasekhar mass double-detonation and the Chandrasekhar mass delayed-detonation model. Specifically, we study a two-dimensional double-detonation model and a three-dimensional delayed-detonation model, and calculate polarization spectra for multiple observer orientations in both cases. We find modest polarization levels ($<$ 1 per cent) for both explosion models. Polarization in the continuum peaks at $\sim$ 0.1$-$0.3 per cent and decreases after maximum light, in excellent agreement with spectropolarimetric data of normal Type Ia supernovae. Higher degrees of polarization are found across individual spectral lines. In particular, the synthetic Si ii {\lambda}6355 profiles are polarized at levels that match remarkably well the values observed in normal Type Ia supernovae, while the low degrees of polarization predicted across the O i {\lambda}7774 region are consistent with the non-detection of this feature in current data. We conclude that our models can reproduce many of the characteristics of both flux and polarization spectra for well-studied Type Ia supernovae, such as SN 2001el and SN 2012fr. However, the two models considered here cannot account for the unusually high level of polarization observed in extreme cases such as SN 2004dt.

DO THE PHOTOMETRIC COLORS OF TYPE II-P SUPERNOVAE ALLOW ACCURATE DETERMINATION OF HOST GALAXY EXTINCTION?

We present infrared photometry of SN 1999em, plus optical photometry, infrared photometry, and optical spectroscopy of SN 2003hn. Both objects were Type II-P supernovae. The V-[RIJHK] color curves of these supernovae evolved in a very similar fashion until the end of plateau phase. This allows us to determine how much more extinction the light of SN 2003hn suffered compared to SN 1999em. Since we have an estimate of the total extinction suffered by SN 1999em from model fits of ground-based and space-based spectra as well as photometry of SN 1999em, we can estimate the total extinction and absolute magnitudes of SN 2003hn with reasonable accuracy. Since the host galaxy of SN 2003hn also produced the Type Ia SN 2001el, we can directly compare the absolute magnitudes of these two SNe of different types.

Late-time emission of type Ia supernovae: optical and near-infrared observations of SN 2001el

Aims. To elucidate the nature of the late-phase emission of the normal type la supernova SN 2001el. Methods. We present optical and near-infrared light curves of SN 2001el from 310 to 445 days past maximum light, obtained with the Very Large Telescope. Results. The late-time optical (UBVRI) light curves decay in a nearly linear fashion with decay time scales of 1.43 ±0.14, 1.43 ±0.06, 1.48 ± 0.06, 1.45 ± 0.07, and 1.03 ± 0.07 magnitudes (per hundred days) in the U, B, V, R, and I bands, respectively. In contrast, in the near-infrared (JHK s ) bands the time evolution of the flux appears to be nearly constant at these epochs. We measure decline rates (per hundred days) of 0.19 ± 0.10 and 0.17 ±0.1 I magnitudes in the J and H bands, respectively. We construct a UVOIR light curve, and find that the late-time luminosity has a decay time scale nearly consistent with complete depostion of positron kinetic energy. Conclusions. The late-time light curves of the normal type la SN 2001 el demonstrate the increased importance of the near-infrared contribution. This was previously observed in the peculiar SN 2000cx, and the results for SN 2001el thus ensure that the conclusions previously based on a single peculiar event are applicable to normal type la supernovae. The measured late-time UVOIR decline rate suggests that a majority of the positrons are trapped within the ejecta. This result does not favor the prediction of a weak and/or radially combed magnetic field configuration.

Comparative Direct Analysis of Type Ia Supernova Spectra. II. Maximum Light

ABSTRACTA comparative study of near–maximum‐light optical spectra of 24 Type Ia supernovae (SNe Ia) is presented. The spectra are assigned to four groups. Seven “core‐normal” SNe Ia have very similar spectra, except for strong, high‐velocity Ca ii absorption in SN 2001el. Seven SNe Ia are assigned to a “broad‐line” group, the most extreme of which is SN 1984A. Five SNe Ia, including SN 1991bg, are assigned to a “cool” group. Five SNe Ia, including SN 1991T, are assigned to a “shallow‐silicon” group. Comparisons with SYNOW synthetic spectra provide a basis for discussion of line identifications and an internally consistent quantification of the maximum‐light spectroscopic diversity among SNe Ia. The extent to which SN Ia maximum‐light spectra appear to have a continuous distribution of properties, rather than consist of discrete subtypes, is discussed.

Early and late time VLT spectroscopy of SN 2001el - progenitor constraints for a type Ia supernova

We present early time high-resolution (VLT/UVES) and late time low-resolution (VLT/FORS) optical spectra of the normal type Ia supernova, SN 2001el. The high-resolution spectra were obtained at -9 and -2 days to allow the detection of narrow hydrogen and/or helium emission lines from the circumstellar medium of the SN. No such lines were detected, and we therefore use photoionisation models to derive upper limits of 9x10^-6 Msun/yr and 5x10^-5 Msun/yr for the mass loss rate from the progenitor system assuming velocities of 10 km/s and 50 km/s, respectively, for a wind extending to outside at least a few x 10^15 cm away from the SN explosion site. These limits exclude a symbiotic star in the upper mass loss rate regime from being the progenitor of SN 2001el. The low resolution spectrum was obtained in the nebular phase of the SN, 400 days after the maximum light, to search for any hydrogen rich gas originating from the SN progenitor system. However, we see no signs of Balmer lines in our spectrum. Therefore, we model the late time spectra to derive an upper limit of ~0.03 Msun for solar abundance material present at velocities lower than 1000 km/s within the SN explosion site. According to simulations of Marietta et al. (2000) this is less than the expected mass lost by a subgiant, red giant or main sequence secondary star at a small binary separation as a result of the SN explosion. Finally, we discuss the origin of high velocity Ca II lines. We see both the CaII IR triplet and the H&K lines in the -9 days spectrum at a very high velocity of up to 34000 km/s. The spectrum also shows a flat-bottomed Si II `6150 A' feature similar to the one previously observed in SN 1990N at -14 days. We compare these spectral features to those observed in SNe 1984A and 1990N at even higher velocities.

Diffuse Interstellar Bands in NGC 1448

We present spectroscopic VLT/UVES observations of two emerging supernovae, the Type Ia SN 2001el and the Type II SN 2003hn, in the spiral galaxy NGC 1448. Our high resolution and high signal-to-noise spectra display atomic lines of Ca  ,N a ,T i and K  in the host galaxy. In the line of sight towards SN 2001el, we also detect over a dozen diffuse interstel- lar bands (DIBs) within NGC 1448. These DIBs have strengths comparable to low reddening galactic lines of sight, albeit with some variations. In particular, a good match is found with the line of sight towards the σ type diffuse cloud (HD 144217). The DIBs towards SN 2003hn are significantly weaker, and this line of sight has also lower sodium column density. The DIB central velocities show that the DIBs towards SN 2001el are closely related to the strongest interstellar Ca  and Na  components, indicating that the DIBs are preferentially produced in the same cloud. The ratio of the λ 5797 and λ 5780 DIB strengths (r ∼ 0.14) suggests a rather strong UV field in the DIB environment towards SN 2001el. We also note that the extinction estimates obtained from the sodium lines using multiple line fitting agree with reddening estimates based on the colors of the Type Ia SN 2001el.

SN 2003du: Signatures of the Circumstellar Environment in a Normal Type Ia Supernova?

We present observations of the Type Ia supernova 2003du obtained with the Hobby-Eberly Telescope and report the detection of a high-velocity component in the Ca II infrared triplet near 8000 A, similar to features previously observed in SN 2000cx and SN 2001el. This feature exhibits a large expansion velocity (≈18,000 km s-1), which is nearly constant between -7 and +2 days relative to maximum light and disappears shortly thereafter. Other than this feature, the spectral evolution and light curve of SN 2003du resemble those of a normal SN Ia. We consider a possible origin for this high-velocity Ca II line in the context of a self-consistent spherical delayed-detonation model for the supernova. We find that the Ca II feature can be caused by a dense shell formed when circumstellar material of solar abundance is overrun by the rapidly expanding outermost layers of the SN ejecta. Model calculations show that the optical and infrared spectra are remarkably unaffected by the circumstellar interaction and the resulting shell. In particular, no hydrogen lines are detectable in either absorption or emission after the phase of dynamic interaction. The only qualitatively different features in the model spectra are the strong, high-velocity feature in the Ca II IR triplet around 8000 A and a somewhat weaker O I feature near 7,300 A. The Doppler shift and time evolution of these features provides an estimate for the amount of accumulated matter (decreasing Doppler shift with increasing shell mass) and also an indication of the mixing within the dense shell. For high shell masses (≈5 × 10-2 M☉), the high-velocity component of the Ca II line merges with the photospheric line forming a broad feature. A cutoff of the blue wings of strong, unblended lines (particularly the Si II feature at about 6,000 A) may also be observable for larger shell masses. The model SN Ia light curves are little effected except at very early times when the shell is partially optically thick because of Thomson scattering, resulting in larger (B-V) colors by up to 0.3 mag. We apply these diagnostic tools to SN 2003du and infer that about 2 × 10-2 M☉ of solar abundance material may have accumulated in a shell prior to the observations. Furthermore, in this interpretation, the early light-curve data imply that the circumstellar material was originally very close to the progenitor system, perhaps from an accretion disk, Roche lobe, or common envelope. Because of the observed confinement of Ca II in velocity space and the lack of ongoing interaction inferred from the light curve, the matter cannot be placed in the outer layers of the exploding white dwarf star or related to a recent period of high mass loss in the progenitor system prior to the explosion. We note that the signatures of circumstellar interaction could be rather common in Type Ia supernovae and may have eluded discovery because optical spectra often do not extend significantly beyond 7500 A.

On the Geometry of the High‐Velocity Ejecta of the Peculiar Type Ia Supernova 2000cx

High-velocity features in Type Ia supernova spectra provide a way to probe the outer layers of these explosions. The maximum-light spectra of the unique Type Ia supernova 2000cx exhibit interesting Ca II features with high-velocity components. The infrared triplet absorption is quadruply notched, while the H and K absorption is wide and flat. Stimulated by a three-dimensional interpretation of similar Ca II features in another Type Ia supernova (SN 2001el; Kasen et al.), we present alternative spherically symmetric and three-dimensional ejecta models to fit the high-velocity (v > 16,000 km s-1) Ca II features of SN 2000cx. We also present simple estimates of the high-velocity ejecta mass for a few trial compositions and discuss their implications for explosion modeling.

Analysis of the Flux and Polarization Spectra of the Type Ia Supernova SN 2001el: Exploring the Geometry of the High‐Velocity Ejecta

SN 2001el is the first normal Type Ia supernova to show a strong, intrinsic polarization signal. In addition, during the epochs prior to maximum light, the Ca II IR triplet absorption is seen distinctly and separately at both normal photospheric velocities and at very high velocities. The high-velocity triplet absorption is highly polarized, with a different polarization angle than the rest of the spectrum. The unique observation allows us to construct a relatively detailed picture of the layered geometrical structure of the supernova ejecta: in our interpretation, the ejecta layers near the photosphere (v ? 10,000 km s-1) obey a nearly axial symmetry, while a detached, high-velocity structure (v ? 18,000-25,000 km s-1) with high Ca II line opacity deviates from the photospheric axisymmetry. By partially obscuring the underlying photosphere, the high-velocity structure causes a more incomplete cancellation of the polarization of the photospheric light and so gives rise to the polarization peak and rotated polarization angle of the high-velocity IR triplet feature. In an effort to constrain the ejecta geometry, we develop a technique for calculating three-dimensional synthetic polarization spectra and use it to generate polarization profiles for several parameterized configurations. In particular, we examine the case in which the inner ejecta layers are ellipsoidal and the outer, high-velocity structure is one of four possibilities: a spherical shell, an ellipsoidal shell, a clumped shell, or a toroid. The synthetic spectra rule out the spherical shell model, disfavor a toroid, and find a best fit with the clumped shell. We show further that different geometries can be more clearly discriminated if observations are obtained from several different lines of sight. Thus, assuming that the high-velocity structure observed for SN 2001el is a consistent feature of at least a known subset of Type Ia supernovae, future observations and analyses such as these may allow one to put strong constraints on the ejecta geometry and hence on supernova progenitors and explosion mechanisms.

Spectropolarimetry of SN 2001el in NGC 1448: Asphericity of a Normal Type Ia Supernova

High-quality spectropolarimetry (range 417-860 nm; spectral resolution 1.27 nm and 0.265 nm pixel-1) of the Type Ia supernova (SN Ia) 2001el was obtained with the ESO Very Large Telescope Melipal (+FORS1) at five epochs. The spectra a week before maximum and around maximum indicate photospheric expansion velocities of about 10,000 km s-1. Prior to optical maximum, the linear polarization of the continuum was ≈0.2%-0.3% with a constant position angle, showing that SN 2001el has a well-defined axis of symmetry. The polarization was nearly undetectable a week after optical maximum. The spectra are similar to those of the normally bright SN 1994D, with the exception of a strong double-troughed absorption feature seen around 800 nm (FWHM about 22 nm). The 800 nm feature is probably due to the Ca II IR triplet at very high velocities (20,000-26,000 km s-1) involving ~0.004 M☉ of calcium and perhaps 0.1 M☉ total mass. The 800 nm feature is distinct in velocity space from the photospheric Ca II IR triplet and has a significantly higher degree of polarization (≈0.7%) and different polarization angle than the continuum. Taken together, these aspects suggest that this high-velocity calcium is a kinematically distinct feature with the matter distributed in a filament, torus, or array of blobs almost edge-on to the line of sight. This feature could thus be an important clue to the binary nature of SNe Ia, perhaps associated with an accretion disk, or to the nature of the thermonuclear burning, perhaps representing a stream of material ballistically ejected from the site of the deflagration to detonation transition. If modeled in terms of an oblate spheroid, the continuum polarization implies a minor to major axis ratio of around 0.9 if seen equator-on; this level of asymmetry would produce an absolute luminosity dispersion of about 0.1 mag when viewed at different viewing angles. If typical for SNe Ia, this would create an rms scatter of several hundredths of a magnitude around the mean brightness-decline relation. We discuss the possible implications of this scatter for the high-precision measurements required to determine the cosmological equation of state.