| 1. |
- Ergon, Mattias, et al.
(author)
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Monte-Carlo methods for NLTE spectral synthesis of supernovae
- 2018
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 620
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Journal article (peer-reviewed)abstract
- We present JEKYLL, a new code for modelling of supernova (SN) spectra and lightcurves based on Monte-Carlo (MC) techniques for the radiative transfer. The code assumes spherical symmetry, homologous expansion and steady state for the matter, but is otherwise capable of solving the time-dependent radiative transfer problem in non-local-thermodynamic-equilibrium (NLTE). The method used was introduced in a series of papers by Lucy, but the full time-dependent NLTE capabilities of it have never been tested. Here, we have extended the method to include non-thermal excitation and ionization as well as charge-transfer and two-photon processes. Based on earlier work, the non-thermal rates are calculated by solving the Spencer-Fano equation. Using a method previously developed for the SUMO code, macroscopic mixing of the material is taken into account in a statistical sense. To save computational power a diffusion solver is used in the inner region, where the radiation field may be assumed to be thermalized. In addition, a statistical Markov-chain model is used to sample the emission frequency more efficiently, and we introduce a method to control the sampling of the radiation field, which is used to reduce the noise in the radiation field estimators. Except for a description of JEKYLL, we provide comparisons with the ARTIS, SUMO and CMFGEN codes, which show good agreement in the calculated spectra as well as the state of the gas. In particular, the comparison with CMFGEN, which is similar in terms of physics but uses a different technique, shows that the Lucy method does indeed converge in the time-dependent NLTE case. Finally, as an example of the time-dependent NLTE capabilities of JEKYLL, we present a model of a Type IIb SN, taken from a set of models presented and discussed in detail in an accompanying paper. Based on this model we investigate the effects of NLTE, in particular those arising from non-thermal excitation and ionization, and find strong effects even on the bolometric lightcurve. This highlights the need for full NLTE calculations when simulating the spectra and lightcurves of SNe.
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| 2. |
- Gumaelius, Lena, et al.
(author)
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Outreach initiatives operated by universities for increasing interest in science and technology
- 2016
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In: European Journal of Engineering Education. - : Taylor & Francis Group. - 0304-3797 .- 1469-5898. ; , s. 1-34
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Journal article (peer-reviewed)abstract
- Since the 1990s, the low number of students choosing to study science and technology in higher education has been on the societal agenda and many initiatives have been launched to promote awareness regarding career options. The initiatives particularly focus on increasing enrolment in the engineering programmes. This article describes and compares eight European initiatives that have been established and operated by universities (and in some cases through collaboration with other actors in society). Each initiative is summarised in a short essay that discusses motivation, organisation, pedagogical approach, and activities. The initiatives are characterised by comparing the driving forces behind their creation, how the initiative activities relate to the activities at the university, size based on the number of participants and cost per participant and pedagogical framework. There seem to be two main tracks for building outreach activities, one where outreach activities are based on the university's normal activities, and one where outreach activities are designed specifically for the visiting students.
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| 3. |
- Lundqvist, Peter, et al.
(author)
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No trace of a single-degenerate companion in late spectra of supernovae 2011fe and 2014J
- 2015
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 577
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Journal article (peer-reviewed)abstract
- Aims. This study aims at constraining the origin of the nearby Type Ia supernovae (SNe), 2011fe and 2014J. The two most favoured scenarios for triggering the explosion of the white dwarf supernova progenitor is either mass loss from a non-degenerate companion or merger with another white dwarf. In the former, there could be a significant amount of leftover material from the companion at the centre of the supernova. Detecting such material would therefore favour the single-degenerate scenario. Methods. The left-over material from a possible non-degenerate companion can reveal itself after about one year, and in this study such material was searched for in the spectra of SN 2011fe (at 294 days after the explosion) using the Large Binocular Telescope and for SN 2014J using the Nordic Optical Telescope (315 days past explosion). The observations were interpreted using numerical models simulating the expected line emission from ablated material from the companion star. The spectral lines sought for are H alpha, [O I] lambda 6300, and [Ca II] lambda lambda 7291,7324, and the expected width of these lines is similar to 1000 km s(-1), which in the case of the [Ca II] lines blend to a broader feature. Results. No signs of H alpha, [O I] lambda 6300, or [Ca II] lambda lambda 7291, 7324 could be traced for in any of the two supernovae. When systematic uncertainties are included, the limits on hydrogen-rich ablated gas are 0 : 003 M-circle dot in SN 2011fe and 0 : 0085 M-circle dot in SN 2014J, 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, and here limits from [O I] lambda 6300 provide the upper mass limits 0 : 002 M-circle dot and 0 : 005 M-circle dot for SNe 2011fe and 2014J, respectively. These numbers 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 very likely be ruled out, whereas a main-sequence donor system with large intrinsic separation is still possible for SN 2014J. 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. Published data for SNe 1998bu, 2000cx, 2001el, 2005am, and 2005cf are used to constrain their origin. We emphasise that the results of this study depend on the sought-after lines emerging unattenuated from the central regions of the nebula. Detailed radiative transfer calculations with longer line lists than are presently used are needed to confirm that this is, in fact, true. Finally, the broad lines of SNe 2011fe and 2014J are discussed, and it is found that the [Ni II] lambda 7378 emission is redshifted by similar to+ 1300 km s(-1), as opposed to the known blueshift of similar to-1100 km s(-1) for SN 2011fe. [Fe II] lambda 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] lambda 7155 and [Ni II] lambda 7378, and a slow decline of the Si II lambda 6355 absorption trough just after B-band maximum.
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| 4. |
- Verma, Geeta, et al.
(author)
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Creating inclusive learning opportunities for newly arrived students in Sweden : Examining an afterschool program
- 2017
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Conference paper (peer-reviewed)abstract
- We examined an afterschool STEM program (robotics club) designed specifically for newly arrived students in Stockholm, Sweden. The newly arrived students came from various countries (Afghanistan, Iran [Afghans living as guest workers in Iran], Somalia, and/or Ethiopia] and attended either middle or high school in Sweden. The robotics club was established in collaboration with teachers who primarily work with the newly arrived students, Vetenskapens Hus (Stockholm House of Science), a near-peer mentor, and university researchers. The House of Science as well as the near-peer mentor acted as resources (materials and instruction) for the robotics club. The newly arrived students spoke various languages (Pasto, Dari, Hindi, English, Swedish, and an African dialect). Language fluency in Swedish was on a continuum based on their duration of stay in Sweden. The students learnt to do basic coding using Blue-Bots and Lego robotics to accomplish various tasks. In this qualitative study, we focused on the larger question, “do larger social structures of formal schooling limit equitable access to STEM learning for newly arrived students”. Weworked with 12 teens during the 4-week robotics club activities. Data sources included pre-surveys, participant observations, and focus group interviews. Findings suggested that various task structures in the robotics club (i.e., coding robots to do various tasks), participant structures (i.e., community of practice), and discourse structures (i.e., student talk) served as the mediating processes and were engaging for students.
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