Type Ia supernovae (SNe Ia) are powerful standardizable candles for constraining cosmological models and provided the first evidence of the accelerated expansion of the universe. Their precision ...derives from empirical correlations, now measured from >1000 SNe Ia, between their luminosities, light-curve shapes, colors, and most recently with the stellar mass of their host galaxy. As mass correlates with other galaxy properties, alternative parameters have been investigated to improve SN Ia standardization though none have been shown to significantly alter the determination of cosmological parameters. We re-examine a recent claim, based on 34 SN Ia in nearby passive host galaxies, of a 0.05 mag Gyr−1 dependence of standardized SN Ia luminosity on host age, which, if extrapolated to higher redshifts, would be a bias up to 0.25 mag, challenging the inference of dark energy. We reanalyze this sample of hosts using both the original method and a Bayesian hierarchical model and find after a fuller accounting of the uncertainties the significance of a dependence on age to be ≤2 and ∼1 after the removal of a single poorly sampled SN Ia. To test the claim that a trend seen in old stellar populations can be applied to younger ages, we extend our analysis to a larger sample that includes young hosts. We find the residual dependence of host age (after all standardization typically employed for cosmological measurements) to be consistent with zero for 254 SNe Ia from the Pantheon sample, ruling out the large but low significance trend seen in passive hosts.
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
Magnetic fields in Wolf–Rayet (WR) stars are not well explored, although there is indirect evidence, e.g. from spectral variability and X-ray emission, that magnetic fields should be present ...in these stars. Being in an advanced stage of their evolution, WR stars have lost their hydrogen envelope, but their dense winds make the stellar core almost unobservable. To substantiate the expectations on the presence of magnetic fields in the most-evolved massive stars, we selected two WR stars, WR 46 and WR 55, for the search of the presence of magnetic fields using FORS 2 spectropolarimetric observations. We achieve a formally definite detection of a variable mean longitudinal magnetic field of the order of a few hundred gauss in WR 55. The field detection in this star, which is associated with the ring nebula RCW 78 and the molecular environment, is of exceptional importance for our understanding of star formation. No field detection at a significance level of 3σ was achieved for WR 46, but the variability of the measured field strengths can be rather well phased with the rotation period of 15.5 h previously suggested by FUSE(Far Ultraviolet Spectroscopic Explorer) observations.
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.
We present the first uniform treatment of long duration gamma-ray burst (GRB) host galaxy detections and upper limits over the redshift range 3 < z < 5, a key epoch for observational and theoretical ...efforts to understand the processes, environments, and consequences of early cosmic star formation (SF). We contribute deep imaging observations of 13 GRB positions yielding the discovery of 8 new host galaxies. We use this data set in tandem with previously published observations of 31 further GRB positions to estimate or constrain the host galaxy rest-frame ultraviolet (UV; lambda = 1600 Angstrom) absolute magnitudes M sub(UV). We then use the combined set of 44 M sub(UV) estimates and limits to construct the M sub(UV) luminosity function (LF) for GRB host galaxies over 3 < z < 5 and compare it to expectations from Lyman break galaxy (LBG) photometric surveys with the Hubble Space Telescope. Adopting standard prescriptions for the luminosity dependence of galaxy dust obscuration (and hence, total SF rate), we find that our LF is compatible with LBG observations over a factor of 600 x in host luminosity, from M sub(UV) = -22.5 mag to >-15.6 mag, and with extrapolations of the assumed Schechter-type LF well beyond this range. We review proposed astrophysical and observational biases for our sample, and find that they are for the most part minimal. We therefore conclude, as the simplest interpretation of our results, that GRBs successfully trace UV metrics of cosmic SF over the range 3 < z < 5. Our findings suggest that GRBs provide an accurate picture of star formation processes from z approx = 3 out to the highest redshifts.
Context. Studies of the magnetic characteristics of massive stars have recently received significant attention because they are progenitors of highly magnetised compact objects. Stars initially more ...massive than about 8 M ⊙ leave behind neutron stars and black holes by the end of their evolution. The merging of binary compact remnant systems produces astrophysical transients detectable by gravitational wave observatories. Studies of magnetic fields in massive stars with low metallicities are of particular interest because they provide important information on the role of magnetic fields in the star formation of the early Universe. Aims. While several detections of massive Galactic magnetic stars have been reported in the last few decades, the impact of a low-metallicity environment on the occurrence and strength of stellar magnetic fields has not yet been explored. Because of the similarity between Of?p stars in the Magellanic Clouds (MCs) and Galactic magnetic Of?p stars, which possess globally organised magnetic fields, we searched for magnetic fields in Of?p stars in the MCs. Additionally, we observed the massive contact binary Cl NGC 346 SSN7 in the Small Magellanic Cloud to test the theoretical scenario that the origin of magnetic fields involves a merger event or a common envelope evolution. Methods. We obtained and analysed measurements of the magnetic field in four massive Of?p stars in the MCs and the binary Cl NGC 346 SSN7 using the ESO/VLT FORS2 spectrograph in spectropolarimetric mode. Results. We detected kilogauss-scale magnetic fields in two Of?p-type stars and in the contact binary Cl NGC 346 SSN7. These results suggest that the impact of low metallicity on the occurrence and strength of magnetic fields in massive stars is low. However, because the explored stellar sample is very small, additional observations of massive stars in the MCs are necessary.
ABSTRACT Studies of magnetic fields in the most evolved massive stars, the Wolf–Rayet stars, are of special importance because they are progenitors of certain types of supernovae. The first detection ...of a magnetic field of the order of a few hundred gauss in the WN7 star WR 55, based on a few FORS2 low-resolution spectropolarimetric observations, was reported in 2020. In this work, we present new FORS2 observations allowing us to detect magnetic and spectroscopic variability with a period of 11.90 h. No significant frequencies were detected in TESS and ASAS-SN photometric observations. Importantly, magnetic field detections are achieved currently only in two Wolf–Rayet stars, WR 6 and WR 55, both showing the presence of corotating interacting regions.
ABSTRACT
The O9.7 V star HD 54879 is currently the only massive magnetic star whose magnetic field geometry and rotation period are not constrained. Over the last 3 yr, we gathered additional ...observations of this star, obtained using various instruments at several astronomical facilities, with the aim to constrain the rotation period and the magnetic field geometry. The new data include the first full Stokes vector observations with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI), installed at the Large Binocular Telescope. The acquired spectropolarimetric observations show a very slow magnetic field variability related to the extremely slow rotation of HD 54879, which is also indicated in a dynamical spectrum, displaying variability of the Hα line. The most intriguing result of our study is the discovery of differences in longitudinal magnetic field strengths measured using different least-squares deconvolution (LSD) masks containing lines belonging to different elements. It is the first time that such a differential analysis of the field strength in dependence of the used lines is carried out for a magnetic O-type star. Since the LSD Stokes I profiles of the studied O, Si, and He line masks remain stable over all observing epochs, we conclude that the detection of different field strengths using lines belonging to these elements is related to the different formation depths, with the He lines formed much higher in the stellar atmosphere compared to the silicon and the oxygen lines, and non-local thermodynamic equilibrium (NLTE) effects. Our numerical magnetospherical model suggests the presence of enhanced gas density that fills the volume inside the field lines close to the star.
We present a spectroscopic analysis of the most nearby Type I superluminous supernova (SLSN-I), SN 2018bsz. The photometric evolution of SN 2018bsz has several surprising features, including an ...unusual pre-peak plateau and evidence for rapid formation of dust ≳200 d post-peak. We show here that the spectroscopic and polarimetric properties of SN 2018bsz are also unique. While its spectroscopic evolution closely resembles SLSNe-I, with early O
II
absorption and C
II
P Cygni profiles followed by Ca, Mg, Fe, and other O features, a multi-component H
α
profile appearing at ∼30 d post-maximum is the most atypical. The H
α
is at first characterised by two emission components, one at ∼+3000 km s
−1
and a second at ∼ − 7500 km s
−1
, with a third, near-zero-velocity component appearing after a delay. The blue and central components can be described by Gaussian profiles of intermediate width (
FWHM
∼ 2000–6000 km s
−1
), but the red component is significantly broader (
FWHM
≳ 10 000 km s
−1
) and Lorentzian. The blue H
α
component evolves towards a lower-velocity offset before abruptly fading at ∼ + 100 d post-maximum brightness, concurrently with a light curve break. Multi-component profiles are observed in other hydrogen lines, including Pa
β
, and in lines of Ca
II
and He
I
. Spectropolarimetry obtained before (10.2 d) and after (38.4 d) the appearance of the H lines shows a large shift on the Stokes
Q
–
U
plane consistent with SN 2018bsz undergoing radical changes in its projected geometry. Assuming the supernova is almost unpolarised at 10.2 d, the continuum polarisation at 38.4 d reaches
P
∼ 1.8%, implying an aspherical configuration. We propose that the observed evolution of SN 2018bsz can be explained by highly aspherical, possibly disk-like, circumstellar material (CSM) with several emitting regions. After the supernova explosion, the CSM is quickly overtaken by the ejecta, but as the photosphere starts to recede, the different CSM regions re-emerge, producing the peculiar line profiles. Based on the first appearance of H
α
, we can constrain the distance of the CSM to be less than ∼6.5 × 10
15
cm (430 AU), or even lower (≲87 AU) if the pre-peak plateau is related to an eruption that created the CSM. The presence of CSM has been inferred previously for other SLSNe-I, both directly and indirectly. However, it is not clear whether the rare properties of SN 2018bsz can be generalised for SLSNe-I, for example in the context of pulsational pair instability, or whether they are the result of an uncommon evolutionary path, possibly involving a binary companion.
Context.
Polarimetry is a very powerful tool for uncovering various properties of astronomical objects that otherwise remain hidden in standard imaging or spectroscopic observations. While common ...observations only measure the intensity of light, polarimetric measurements allow us to distinguish and measure the two perpendicular components of the electric field associated with the incoming light. By using polarimetry it is possible to unveil asymmetries in supernova explosions, properties of intervening dust, characteristics of atmosphere of planets, among others. However, the reliable measurement of the low polarization signal from astronomical sources requires a good control of spurious instrumental polarization induced by the various components of the optical system and the detector.
Aims.
We perform a detailed multi-wavelength calibration study of the FORS2 instrument at the VLT operating in imaging polarimetric mode to characterize the spatial instrumental polarization that may affect the study of extended sources.
Methods.
We used imaging polarimetry of high signal-to-noise ratio blank field
B
V
R
I
observations during the full moon, when the polarization is expected to be constant across the field of view and deviations originate from the instrument, and a crowded star cluster in broad-band
R
I
and narrow-band
H
α
filters, where the individual polarization values of each star across the field can be measured.
Results.
We find an instrumental polarization pattern that increases radially outwards from the optical axis of the instrument reaching up to 1.4% at the edges, depending on the filter. Our results are closely approximated by an elliptical paraboloid down to less than ∼0.05% accuracy, and ∼0.02% when using non-analytic fits. We present 2D maps to correct for this spurious instrumental polarization. We also give several tips and tricks for analyzing polarimetric measurements of extended sources.
Conclusions.
FORS2 is a powerful instrument that allows the linear polarimetry of extended sources to be mapped. We present and discuss a methodology that can be used to measure the polarization of such sources, and to correct for the spatial polarization induced in the optical system. This methodology could be applied to polarimetric measurements using other dual-beam polarimeters.
PDF
