Aims. This work provides the results of the first six years of operation by the systematic night-sky monitoring at ESO-Paranal (Chile). Methods. The UBVRI night-sky brightness was estimated on about ...10 000 VLT-FORS1 archival images, obtained on more than 650 separate nights, distributed over 6 years, and covering the descent from maximum to minimum of sunspot cycle n. 23. Additionally, a set of about 1000 low-resolution, optical, night-sky spectra were extracted and analysed. Results. The unprecedented database discussed in this paper has led to detecting a clear seasonal variation of the broad-band night-sky brightness in the VRI passbands, similar to the well-known semi-annual oscillation of the Na I D doublet. The spectroscopic data demonstrate that this seasonality is common to all spectral features, with the remarkable exception of the OH rotational-vibrational bands. A clear dependency on the solar activity is detected in all passbands and is particularly pronounced in the U band, where the sky brightness decreased by ~0.6 mag arcsec-2 from maximum to minimum of solar cycle n. 23. No correlation is found between solar activity and the intensity of the Na I D doublet and the OH bands. A strong correlation between the intensity of N I 5200 Å and OI6300, 6364 Å is reported here for the first time. The paper also addresses the determination of the correlation time-scales with solar activity and the possible connection with the flux of charged particles emitted by the Sun.
Gravitational waves were discovered with the detection of binary black-hole mergers and they should also be detectable from lower-mass neutron-star mergers. These are predicted to eject material rich ...in heavy radioactive isotopes that can power an electromagnetic signal. This signal is luminous at optical and infrared wavelengths and is called a kilonova. The gravitational-wave source GW170817 arose from a binary neutron-star merger in the nearby Universe with a relatively well confined sky position and distance estimate. Here we report observations and physical modelling of a rapidly fading electromagnetic transient in the galaxy NGC 4993, which is spatially coincident with GW170817 and with a weak, short γ-ray burst. The transient has physical parameters that broadly match the theoretical predictions of blue kilonovae from neutron-star mergers. The emitted electromagnetic radiation can be explained with an ejected mass of 0.04 ± 0.01 solar masses, with an opacity of less than 0.5 square centimetres per gram, at a velocity of 0.2 ± 0.1 times light speed. The power source is constrained to have a power-law slope of -1.2 ± 0.3, consistent with radioactive powering from r-process nuclides. (The r-process is a series of neutron capture reactions that synthesise many of the elements heavier than iron.) We identify line features in the spectra that are consistent with light r-process elements (atomic masses of 90-140). As it fades, the transient rapidly becomes red, and a higher-opacity, lanthanide-rich ejecta component may contribute to the emission. This indicates that neutron-star mergers produce gravitational waves and radioactively powered kilonovae, and are a nucleosynthetic source of the r-process elements.
ABSTRACT
The origin of the diverse light-curve shapes of Type II supernovae (SNe), and whether they come from similar or distinct progenitors, has been actively discussed for decades. Here, we report ...spectropolarimetry of two fast declining Type II (Type IIL) SNe: SN 2013ej and SN 2017ahn. SN 2013ej exhibited high continuum polarization from very soon after the explosion to the radioactive tail phase with time-variable polarization angles. The origin of this polarimetric behaviour can be interpreted as the combination of two different aspherical structures, namely an aspherical interaction of the SN ejecta with circumstellar matter (CSM) and an inherently aspherical explosion. Aspherical explosions are a common feature of slowly declining Type II (Type IIP) SNe. By contrast, SN 2017ahn showed low polarization not only in the photospheric phase but also in the radioactive tail phase. This low polarization in the tail phase, which has never before been observed in other Type IIP/L SNe, suggests that the explosion of SN 2017ahn was nearly spherical. These observations imply that Type IIL SNe have, at least, two different origins: they result from stars that have different explosion properties and/or different mass-loss processes. This fact might indicate that 13ej-like Type IIL SNe originate from a similar progenitor to those of Type IIP SNe accompanied by an aspherical CSM interaction, while 17ahn-like Type IIL SNe come from a more massive progenitor with less hydrogen in its envelope.
ABSTRACT Type IIP supernovae (SNe IIP), which represent the most common class of core-collapse (CC) SNe, show a rapid increase in continuum polarization just after entering the tail phase. This ...feature can be explained by a highly asymmetric helium core, which is exposed when the hydrogen envelope becomes transparent. Here we report the case of an SN IIP (SN 2017gmr) that shows an unusually early rise of the polarization, ≳30 d before the start of the tail phase. This implies that SN 2017gmr is an SN IIP that has very extended asphericity. The asymmetries are not confined to the helium core, but reach out to a significant part of the outer hydrogen envelope, hence clearly indicating a marked intrinsic diversity in the aspherical structure of CC explosions. These observations provide new constraints on the explosion mechanism, where viable models must be able to produce such extended deviations from spherical symmetry, and account for the observed geometrical diversity.
Abstract
We present optical spectropolarimetry and spectroscopy of the broad-lined Type Ic (Ic-bl) SN 2014ad. Our spectropolarimetric observations cover seven epochs, from −2 to 66 d after V-band ...maximum, and the spectroscopic data were acquired from −2 to +107 d. The photospheric velocity estimates showed ejecta speeds similar to those of SN 1998bw and other SNe associated with gamma-ray bursts (GRBs). The spectropolarimetric data revealed aspherical outer ejecta and a nearly spherical interior. The polarization associated with O i λ7774 and the Ca ii infrared triplet suggests a clumpy and highly asymmetrical distribution of these two species within the ejecta. Furthermore, it was shown that the two line forming regions must have been spatially distinct and oxygen was found to have higher velocities than calcium. Another oxygen line-forming region was also identified much closer to the core of the explosion and distributed in a spherical shell. It is difficult to reconcile the geometry of the deeper ejecta with a jet driven explosion, but the high ejecta velocities of SN 2014ad are typical of those observed in SNe Ic-bl with GRBs and the behaviour of the oxygen and calcium line-forming regions is consistent with fully jet-driven models. The metallicity of the host galaxy of SN 2014ad was also calculated and compared to that of the hosts of other SNe Ic-bl with and without GRBs, but due to the overlap in the two populations no conclusion could be drawn.
Aims. The aim of this paper is twofold: 1) to investigate the properties of extragalactic dust and compare them to what is seen in the Galaxy; 2) to address in an independent way the problem of the ...anomalous extinction curves reported for reddened Type Ia Supernovae (SN) in connection to the environments in which they explode. Methods. The properties of the dust are derived from the wavelength dependence of the continuum polarization observed in four reddened Type Ia SN: 1986G, 2006X, 2008fp, and 2014J. The method is based on the observed fact that Type Ia SN have a negligible intrinsic continuum polarization. This and their large luminosity makes them ideal tools to probe the dust properties in extragalactic environments. Results. All four objects are characterized by exceptionally low total-to-selective absorption ratios (RV) and display an anomalous interstellar polarization law, characterized by very blue polarization peaks. In all cases the polarization position angle is well aligned with the local spiral structure. While SN 1986G is compatible with the most extreme cases of interstellar polarization known in the Galaxy, SN 2006X, 2008fp, and 2014J show unprecedented behaviours. The observed deviations do not appear to be connected to selection effects related to the relatively large amounts of reddening characterizing the objects in the sample. Conclusions. The dust responsible for the polarization of these four SN is most likely of interstellar nature. The polarization properties can be interpreted in terms of a significantly enhanced abundance of small grains. The anomalous behaviour is apparently associated with the properties of the galactic environment in which the SN explode, rather than with the progenitor system from which they originate. For the extreme case of SN 2014J, we cannot exclude the contribution of light scattered by local material; however, the observed polarization properties require an ad hoc geometrical dust distribution.
Type Ia supernovae are key tools for measuring distances on a cosmic scale. They are generally thought to be the thermonuclear explosion of an accreting white dwarf in a close binary system. The ...nature of the mass donor is still uncertain. In the single-degenerate model it is a main-sequence star or an evolved star, whereas in the double-degenerate model it is another white dwarf. We show that the velocity structure of absorbing material along the line of sight to 35 type Ia supernovae tends to be blueshifted. These structures are likely signatures of gas outflows from the supernova progenitor systems. Thus, many type Ia supernovae in nearby spiral galaxies may originate in single-degenerate systems.
High-dispersion observations of the Na I D lambdalambda5890, 5896 and K I lambdalambda7665, 7699 interstellar lines, and the diffuse interstellar band at 5780 A in the spectra of 32 Type Ia ...supernovae are used as an independent means of probing dust extinction. We show that the dust extinction of the objects where the diffuse interstellar band at 5780 A is detected is consistent with the visual extinction derived from the supernova colors. This strongly suggests that the dust producing the extinction is predominantly located in the interstellar medium of the host galaxies and not in circumstellar material associated with the progenitor system. One quarter of the supernovae display anomalously large Nai column densities in comparison to the amount of dust extinction derived from their colors. Remarkably, all of the cases of unusually strong Na I D absorption correspond to "Blueshifted" profiles in the classification scheme of Sternberg et al. This coincidence suggests that outflowing circumstellar gas is responsible for at least some of the cases of anomalously large Na i column densities. Two supernovae with unusually strong Nai D absorption showed essentially normal K I column densities for the dust extinction implied by their colors, but this does not appear to be a universal characteristic. Overall, we find the most accurate predictor of individual supernova extinction to be the equivalent width of the diffuse interstellar band at 5780 A, and provide an empirical relation for its use. Finally, we identify ways of producing significant enhancements of the Na abundance of circumstellar material in both the single-degenerate and double-degenerate scenarios for the progenitor system.
Abstract
We present a JWST/MIRI low-resolution mid-infrared (MIR) spectroscopic observation of the normal Type Ia supernova (SN Ia) SN 2021aefx at +323 days past rest-frame
B
-band maximum light. The ...spectrum ranges from 4 to 14
μ
m and shows many unique qualities, including a flat-topped Ar
iii
8.991
μ
m profile, a strongly tilted Co
iii
11.888
μ
m feature, and multiple stable Ni lines. These features provide critical information about the physics of the explosion. The observations are compared to synthetic spectra from detailed non–local thermodynamic equilibrium multidimensional models. The results of the best-fitting model are used to identify the components of the spectral blends and provide a quantitative comparison to the explosion physics. Emission line profiles and the presence of electron capture elements are used to constrain the mass of the exploding white dwarf (WD) and the chemical asymmetries in the ejecta. We show that the observations of SN 2021aefx are consistent with an off-center delayed detonation explosion of a near–Chandrasekhar mass (
M
Ch
) WD at a viewing angle of −30° relative to the point of the deflagration to detonation transition. From the strengths of the stable Ni lines, we determine that there is little to no mixing in the central regions of the ejecta. Based on both the presence of stable Ni and the Ar velocity distributions, we obtain a strict lower limit of 1.2
M
⊙
for the initial WD, implying that most sub-
M
Ch
explosions models are not viable models for SN 2021aefx. The analysis here shows the crucial importance of MIR spectra in distinguishing between explosion scenarios for SNe Ia.
A key tracer of the elusive progenitor systems of Type Ia supernovae (SNe Ia) is the detection of narrow blueshifted time-varying Na i D absorption lines, interpreted as evidence of circumstellar ...material surrounding the progenitor system. The origin of this material is controversial, but the simplest explanation is that it results from previous mass-loss in a system containing a white dwarf and a non-degenerate companion star. We present new single-epoch intermediate-resolution spectra of 17 low-redshift SNe Ia taken with XShooter on the European Southern Observatory Very Large Telescope. Combining this sample with events from the literature, we confirm an excess (∼20 per cent) of SNe Ia displaying blueshifted narrow Na i D absorption features compared to redshifted Na i D features. The host galaxies of SNe Ia displaying blueshifted absorption profiles are skewed towards later-type galaxies, compared to SNe Ia that show no Na i D absorption and SNe Ia displaying blueshifted narrow Na i D absorption features have broader light curves. The strength of the Na i D absorption is stronger in SNe Ia displaying blueshifted Na i D absorption features than those without blueshifted features, and the strength of the blueshifted Na i D is correlated with the B − V colour of the SN at maximum light. This strongly suggests the absorbing material is local to the SN. In the context of the progenitor systems of SNe Ia, we discuss the significance of these findings and other recent observational evidence on the nature of SN Ia progenitors. We present a summary that suggests that there are at least two distinct populations of normal, cosmologically useful SNe Ia.
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