Applying the expanding photosphere and standardized candle methods to Type II-Plateau supernovae at cosmologically significant redshifts: the distance to SN 2013eq

Based on optical imaging and spectroscopy of the Type II-Plateau SN 2013eq, we present a comparative study of commonly used distance determination methods based on Type II supernovae. The occurrence of SN 2013eq in the Hubble flow (z = 0.041 +/- 0.001) prompted us to investigate the implications of the difference between "angular" and "luminosity" distances within the framework of the expanding photosphere method (EPM) that relies upon a relation between flux and angular size to yield a distance. Following a re-derivation of the basic equations of the EPM for SNe at non-negligible redshifts, we conclude that the EPM results in an angular distance. The observed flux should be converted into the SN rest frame and the angular size, theta, has to be corrected by a factor of (1+z)^2. Alternatively, the EPM angular distance can be converted to a luminosity distance by implementing a modification of the angular size. For SN 2013eq, we find EPM luminosity distances of D_L = 151 +/- 18 Mpc and D_L = 164 +/- 20 Mpc by making use of different sets of dilution factors taken from the literature. Application of the standardized candle method for Type II-P SNe results in an independent luminosity distance estimate (D_L = 168 +/- 16 Mpc) that is consistent with the EPM estimate.Comment: 12 pages, 4 figures, accepted by A&

Three-dimensional simulations of the interaction between Type Ia supernova ejecta and their main sequence companions

The identity of the progenitor systems of SNe Ia is still uncertain. In the single-degenerate (SD) scenario, the interaction between the SN blast wave and the outer layers of a main sequence (MS) companion star strips off H-rich material which is then mixed into the ejecta. Strong contamination of the SN ejecta with stripped material could lead to a conflict with observations of SNe Ia. This constrains the SD progenitor model. In this work, our previous simulations based on simplified progenitor donor stars have been updated by adopting more realistic progenitor-system models that result from fully detailed, state-of-the-art binary evolution calculations. We use Eggleton's stellar evolution code including the optically thick accretion wind model and the possibility of the effects of accretion disk instabilities to obtain realistic models of companions for different progenitor systems. The impact of the SN blast wave on these companion stars is followed in three-dimensional hydrodynamic simulations employing the SPH code GADGET3. We find that the stripped masses range from 0.11 to 0.18 M_sun. The kick velocity is between 51 and 105 km/s. We find that the stripped mass and kick velocity depend on the ratio of the orbital separation to the radius of a companion. They can be fitted by a power law for a given companion model. However, the structure of the companion star is also important for the amount of stripped material. With more realistic companion star models than in previous studies, our simulations show that the H masses stripped from companions are inconsistent with the best observational limits (< 0.01 M_sun) derived from nebular spectra. However, a rigorous forward modeling based on impact simulations with radiation transfer is required to reliably predict observable signatures of the stripped H and to conclusively assess the viability of the considered SN Ia progenitor scenario.Comment: 14 pages, 13 figures, accepted for publication by A&

Exploring different methods of cellulose extraction for 14C dating

In this study we aim to identify the optimal cellulose extraction protocol for 14C dating of wood, with a focus on glacial trees. To achieve this, we compare three cellulose extraction methods on the basis of cellulose yield and 14C age. The study is conducted on 12 wood samples of different species, in varying states of preservation with ages covering the full 14C age range. Cellulose is extracted from each sample following three different protocols selected from the literature: ABA-B, BABAB and 2Chlorox. The extracted cellulose was graphitised and dated with the MICADAS (Mini Carbon Dating System) at the ETH AMS laboratory. Although all three methods are considered efficient, the BABAB protocol, despite being a more aggressive procedure, allows the extraction of a sufficient amount of cellulose to be 14C dated and leads to the most reliable results, particularly for very old and background samples (samples with 14C content of zero)

Novice teachers\u27 experiences of community service-learning

This study focuses on beginning teachers\u27 experiences with a currently popular curriculum strategy in the US: community service-learning. To determine the personal and contextual factors influencing novice teachers\u27 experiences, we surveyed over 300 early career teachers and interviewed 30 of the larger sample. The study provides evidence that some beginning teachers are willing to implement strategies they learned in their teacher education programs, and can do so successfully, in spite of being busy and unsupported. Results indicate that specific preparation features and school characteristics may play a large role in whether novice teachers implement service-learning activities in their classrooms

No trace of a single-degenerate companion in late spectra of SNe 2011fe and 2014J

Left-over, ablated material from a possible non-degenerate companion can reveal itself after about one year in spectra of Type Ia SNe (SNe Ia). We have searched for such material in spectra of SN 2011fe (at 294 days after the explosion) and for SN 2014J (315 days past explosion). The observations are compared with numerical models simulating the expected line emission. The spectral lines sought for are H-alpha, [O I] 6300 and [Ca II] 7291,7324, and the expected width of these lines is about 1000 km/s. No signs of these lines can be traced in any of the two supernovae. When systematic uncertainties are included, the limits on hydrogen-rich ablated gas in SNe 2011fe and 2014J are 0.003 M_sun and 0.0085 M_sun, respectively, where the limit for SN 2014J is the second lowest ever, and the limit for SN 2011fe is a revision of a previous limit. Limits are also put on helium-rich ablated gas. These limits are used, in conjunction with other data, to argue that these supernovae can stem from double-degenerate systems, or from single-degenerate systems with a spun up/spun down super-Chandrasekhar white dwarf. For SN 2011fe, other types of hydrogen-rich donors can likely be ruled out, whereas for SN 2014J a main-sequence donor system with large intrinsic separation is still possible. Helium-rich donor systems cannot be ruled out for any of the two supernovae, but the expected short delay time for such progenitors makes this possibility less likely, especially for SN 2011fe. The broad [Ni II] 7378 emission in SN 2014J is redshifted by about +1300 km/s, as opposed to the known blueshift of roughly -1100 km/s for SN 2011fe. [Fe II] 7155 is also redshifted in SN 2014J. SN 2014J belongs to a minority of SNe Ia that both have a nebular redshift of [Fe II] 7155 and [Ni II] 7378, and a slow decline of the Si II 6355 absorption trough just after B-band maximum.Comment: 13 pages, submitted to A&

The peculiar Type Ia supernova iPTF14atg: Chandrasekhar-mass explosion or violent merger?

iPTF14atg, a subluminous peculiar Type Ia supernova (SN Ia) similar to SN 2002es, is the first SN Ia for which a strong UV flash was observed in the early-time light curves. This has been interpreted as evidence for a single-degenerate (SD) progenitor system where such a signal is expected from interactions between the SN ejecta and the non-degenerate companion star. Here, we compare synthetic observables of multi-dimensional state-of-the-art explosion models for different progenitor scenarios to the light curves and spectra of iPTF14atg. From our models, we have difficulties explaining the spectral evolution of iPTF14atg within the SD progenitor channel. In contrast, we find that a violent merger of two carbon-oxygen white dwarfs with 0.9 and 0.76 solar masses, respectively, provides an excellent match to the spectral evolution of iPTF14atg from 10d before to several weeks after maximum light. Our merger model does not naturally explain the initial UV flash of iPTF14atg. We discuss several possibilities like interactions of the SN ejecta with the circum-stellar medium and surface radioactivity from a He ignited merger that may be able to account for the early UV emission in violent merger models.Comment: 12 pages, 7 figures, accepted for publication in MNRA

Sub-luminous type Ia supernovae from the mergers of equal-mass white dwarfs with M~0.9 M_sun

Type Ia supernovae (SNe Ia) are thought to result from thermonuclear explosions of carbon-oxygen white dwarf stars. Existing models generally explain the observed properties, with the exception of the sub-luminous 1991-bg-like supernovae. It has long been suspected that the merger of two white dwarfs could give rise to a type Ia event, but hitherto simulations have failed to produce an explosion. Here we report a simulation of the merger of two equal-mass white dwarfs that leads to an underluminous explosion, though at the expense of requiring a single common-envelope phase, and component masses of ~0.9 M_sun. The light curve is too broad, but the synthesized spectra, red colour and low expansion velocities are all close to what is observed for sub-luminous 1991bg-like events. While mass ratios can be slightly less than one and still produce an underluminous event, the masses have to be in the range 0.83-0.9 M_sun.Comment: Accepted to Natur