| 1. |
- Ahlgren, Björn, 1990-
(författare)
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Subphotospheric dissipation in gamma-ray bursts observed by the Fermi Gamma-ray Space Telescope
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gamma-ray bursts (GRBs) are the brightest events in the Universe, for a short time outshining the rest of the Universe combined, as they explode with isotropic equivalent luminosities up to $10^{54}$ erg s$^{-1}$. These events are believed to be connected to supernovae and to binary compact object mergers, such as binary neutron stars or neutron star -- black hole systems. The origin of the so-called prompt emission in GRBs remains an unsolved problem, although some progress is being made. Spectral analysis of prompt emission has traditionally been performed with the Band function, an empirical model with no physical interpretation, and it is just recently that physical models have started to be fitted to data. This thesis presents spectral analysis of GRB data from the Fermi Gamma-ray Space Telescope using a physical model for subphotospheric dissipation. The model is developed using a numerical code and implemented as a table model in {\scriptsize XSPEC}. Paper \rom{1} presents the model and provides a proof-of-concept of fitting GRB data with such a model. Specifically, two GRBs are fitted and compared with the corresponding Band function fits. In paper \rom{2}, a sample of 37 bursts are fitted with an extended version of the model and improved analysis tools. Overall, about a third of the fitted spectra can be described by the model. From these fits it is concluded that the scenario of subphotospheric dissipation can describe all spectral shapes present in the sample. The key characteristic of the spectra that are not fitted by the model is that they are very luminous. Within the context of the model, this suggests that the assumption of internal shocks as a dissipation mechanism cannot explain the full population of GRBs. Alternatively, additional emission components may required. The thesis concludes that subphotospheric dissipation is viable as a possible origin of GRB prompt emission. Furthermore, it shows the importance of using physically motivated models when analysing GRBs.
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| 2. |
- Amaro-Seoane, Pau, et al.
(författare)
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Astrophysics with the Laser Interferometer Space Antenna
- 2023
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Ingår i: Living Reviews in Relativity. - : Springer Science and Business Media LLC. - 1433-8351. ; 26
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Forskningsöversikt (refereegranskat)abstract
- The Laser Interferometer Space Antenna (LISA) will be a transformative experiment for gravitational wave astronomy, and, as such, it will offer unique opportunities to address many key astrophysical questions in a completely novel way. The synergy with ground-based and space-born instruments in the electromagnetic domain, by enabling multi-messenger observations, will add further to the discovery potential of LISA. The next decade is crucial to prepare the astrophysical community for LISA’s first observations. This review outlines the extensive landscape of astrophysical theory, numerical simulations, and astronomical observations that are instrumental for modeling and interpreting the upcoming LISA datastream. To this aim, the current knowledge in three main source classes for LISA is reviewed; ultra-compact stellar-mass binaries, massive black hole binaries, and extreme or interme-diate mass ratio inspirals. The relevant astrophysical processes and the established modeling techniques are summarized. Likewise, open issues and gaps in our understanding of these sources are highlighted, along with an indication of how LISA could help making progress in the different areas. New research avenues that LISA itself, or its joint exploitation with upcoming studies in the electromagnetic domain, will enable, are also illustrated. Improvements in modeling and analysis approaches, such as the combination of numerical simulations and modern data science techniques, are discussed. This review is intended to be a starting point for using LISA as a new discovery tool for understanding our Universe.
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| 3. |
- Angelou, George C., et al.
(författare)
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Diagnostics of stellar modelling from spectroscopy and photometry of globular clusters
- 2015
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 450:3, s. 2423-2440
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Tidskriftsartikel (refereegranskat)abstract
- We conduct a series of comparisons between spectroscopic and photometric observations of globular clusters and stellar models to examine their predictive power. Data from medium-to-high resolution spectroscopic surveys of lithium allow us to investigate first dredge-up and extra mixing in two clusters well separated in metallicity. Abundances at first dredge-up are satisfactorily reproduced but there is preliminary evidence to suggest that the models overestimate the luminosity at which the surface composition first changes in the lowest metallicity system. Our models also begin extra mixing at luminosities that are too high, demonstrating a significant discrepancy with observations at low metallicity. We model the abundance changes during extra mixing as a thermohaline process and determine that the usual diffusive form of this mechanism cannot simultaneously reproduce both the carbon and lithium observations. Hubble Space Telescope photometry provides turn-off and bump magnitudes in a large number of globular clusters and offers the opportunity to better test stellar modelling as function of metallicity. We directly compare the predicted main-sequence turn-off and bump magnitudes as well as the distance-independent parameter $\Delta M_V \,<^>{\rm {MSTO}}_{\rm {bump}}$. We require 15 Gyr isochrones to match the main-sequence turn-off magnitude in some clusters and cannot match the bump in low-metallicity systems. Changes to the distance modulus, metallicity scale and bolometric corrections may impact on the direct comparisons but $\Delta M_V \,<^>{\rm {MSTO}}_{\rm {bump}}$, which is also underestimated from the models, can only be improved through changes to the input physics. Overshooting at the base of the convective envelope with an efficiency that is metallicity dependent is required to reproduce the empirically determined value of $\Delta M_V \,<^>{\rm {MSTO}}_{\rm {bump}}$.
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| 4. |
- Angelou, George C., et al.
(författare)
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The Role of Thermohaline Mixing in Intermediate- and Low-Metallicity Globular Clusters
- 2012
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Ingår i: Astrophysical Journal. - 0004-637X. ; 749:2
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Tidskriftsartikel (refereegranskat)abstract
- It is now widely accepted that globular cluster red giant branch (RGB) stars owe their strange abundance patterns to a combination of pollution from progenitor stars and in situ extra mixing. In this hybrid theory a first generation of stars imprints abundance patterns into the gas from which a second generation forms. The hybrid theory suggests that extra mixing is operating in both populations and we use the variation of [C/Fe] with luminosity to examine how efficient this mixing is. We investigate the observed RGBs ofM3, M13, M92, M15, and NGC 5466 as a means to test a theory of thermohaline mixing. The second parameter pair M3 and M13 are of intermediate metallicity and our models are able to account for the evolution of carbon along the RGB in both clusters, although in order to fit the most carbon-depleted main-sequence stars in M13 we require a model whose initial [C/Fe] abundance leads to a carbon abundance lower than is observed. Furthermore, our results suggest that stars in M13 formed with some primary nitrogen (higher C+N+O than stars in M3). In the metal-poor regime only NGC 5466 can be tentatively explained by thermohaline mixing operating in multiple populations. We find thermohaline mixing unable to model the depletion of [C/Fe] with magnitude in M92 and M15. It appears as if extra mixing is occurring before the luminosity function bump in these clusters. To reconcile the data with the models would require first dredge-up to be deeper than found in extant models.
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| 5. |
- Angelou, George C., et al.
(författare)
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Thermohaline Mixing And Its Role In The Evolution Of Carbon And Nitrogen Abundances In Globular Cluster Red Giants: The Test Case Of Messier 3
- 2011
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Ingår i: Astrophysical Journal. - 0004-637X. ; 728:2
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Tidskriftsartikel (refereegranskat)abstract
- We review the observational evidence for extra mixing in stars on the red giant branch (RGB) and discuss why thermohaline mixing is a strong candidate mechanism. We recall the simple phenomenological description of thermohaline mixing and aspects of mixing in stars in general. We use observations of M3 to constrain the form of the thermohaline diffusion coefficient and any associated free parameters. This is done by matching [C/Fe] and [N/Fe] along the RGB of M3. After taking into account a presumed initial primordial bimodality of [C/Fe] in the CN-weak and CN-strong stars, our thermohaline mixing models can explain the full spread of [C/Fe]. Thermohaline mixing can produce a significant change in [N/Fe] as a function of absolute magnitude on the RGB for initially CN-weak stars, but not for initially CN-strong stars, which have so much nitrogen to begin with that any extra mixing does not significantly affect the surface nitrogen composition.
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| 6. |
- Angelou, George C., et al.
(författare)
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Thermohaline mixing and the variation of [C/Fe] with magnitude in M3
- 2010
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Ingår i: 11th Symposium on Nuclei in the Cosmos, NIC 2010; Heidelberg; Germany; 19 July 2010 through 23 July 2010.
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Konferensbidrag (refereegranskat)abstract
- We use observations of M3 to constrain thermohaline mixing. This is done by matching [C/Fe] and [N/Fe] along the RGB of M3. We find our models can explain observations if it is assumed there is a spread of ∼ 0.3 - 0.4 dex in [C/Fe] in the stars in M3 from their birth. We reproduce the full spread in [C/Fe] at the tip of the RGB. Thermohaline mixing can produce a significant change in [N/Fe] as a function of absolute magnitude on the RGB for initially CN-weak stars, but not for initially CN-strong stars, which have so much nitrogen to begin with that any deep mixing does not significantly affect the surface nitrogen composition.
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| 7. |
- Arnadottir, Anna, et al.
(författare)
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The Meridian S02E00 : The Winter Solstice special episode
- 2021
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Konstnärligt arbete (övrigt vetenskapligt/konstnärligt)abstract
- In this very special solstice episode of The Meridian Nic and Rebecca look back at some of the highlights of 2021. With the days growing ever longer here in the Northern Hemisphere they have rallied a troupe of PhD students to compete in the 2021 Astronomy Games! Nic is joined by Alvaro and Johan on his team and Rebecca has convinced Daniel and Madeleine to join her team. The Games were recorded in front of a live studio audience.
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| 8. |
- Arnadottir, Anna, et al.
(författare)
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The Meridian S03E01 : Human Exploration of Space
- 2023
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Konstnärligt arbete (övrigt vetenskapligt/konstnärligt)abstract
- Joining Nic and Rebecca in this first episode of the third season of The Meridian is none other than NASA astronaut Jim Pawelczyk, professor at Penn State University in Kinesiology and Physiology, who looks into how the human body adapts when in space.In this third season we are also including some Cosmic Curiosities. These are some ideas, events or trivia from astronomic history that Ross, Victor and Lucian feel are worth bringing out of obscurity for one more look.
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| 9. |
- Arnadottir, Anna, et al.
(författare)
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The Meridian S03E02 : The Hunt for Dark Matter
- 2023
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Konstnärligt arbete (övrigt vetenskapligt/konstnärligt)abstract
- We are on a hunt for the invisible in this second episode of the third season of The Meridian. Ruth Pöttgen is a senior lecturer at the Department of Physics where she is studying fundamental particles. She is especially interested in finding that mysterious and elusive Dark Matter particle.In this third season we are also including some Cosmic Curiosities. These are some ideas, events or trivia from astronomic history that Ross, Victor and Lucian feel are worth bringing out of obscurity for one more look.
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| 10. |
- Arnadottir, Anna, et al.
(författare)
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The Meridian S03E03 : Planetary systems beyond our own
- 2023
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Konstnärligt arbete (övrigt vetenskapligt/konstnärligt)abstract
- Judith Korth joins the podcast this week and tell us about how she found her way into astronomy research, how she ended up here at Lund University and how she now studies distant worlds far beyond our own Solar System.In this third season we are also including some Cosmic Curiosities. These are some ideas, events or trivia from astronomic history that Ross, Victor and Lucian feel are worth bringing out of obscurity for one more look.
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