On the Rotation of Supermassive Stars

Supermassive stars born from pristine gas in atomically-cooled haloes are thought to be the progenitors of supermassive black holes at high redshifts. However, the way they accrete their mass is still an unsolved problem. In particular, for accretion to proceed, a large amount of angular momentum has to be extracted from the collapsing gas. Here, we investigate the constraints stellar evolution imposes on this angular momentum problem. We present an evolution model of a supermassive Population III star including simultaneously accretion and rotation. We find that, for supermassive stars to form by accretion, the accreted angular momentum has to be about 1% of the Keplerian angular momentum. This tight constraint comes from the $\Omega\Gamma$-limit, at which the combination of radiation pressure and centrifugal force cancels gravity. It implies that supermassive stars are slow rotators, with a surface velocity less than 10-20% of their first critical velocity, at which the centrifugal force alone cancels gravity. At such low velocities, the deformation of the star due to rotation is negligible

The Evolution of Supermassive Population III Stars

Supermassive primordial stars forming in atomically-cooled halos at $z \sim15-20$ are currently thought to be the progenitors of the earliest quasars in the Universe. In this picture, the star evolves under accretion rates of $0.1 - 1$ $M_\odot$ yr$^{-1}$ until the general relativistic instability triggers its collapse to a black hole at masses of $\sim10^5$ $M_\odot$. However, the ability of the accretion flow to sustain such high rates depends crucially on the photospheric properties of the accreting star, because its ionising radiation could reduce or even halt accretion. Here we present new models of supermassive Population III protostars accreting at rates $0.001 - 10$ $M_\odot$ yr$^{-1}$, computed with the GENEVA stellar evolution code including general relativistic corrections to the internal structure. We use the polytropic stability criterion to estimate the mass at which the collapse occurs, which has been shown to give a lower limit of the actual mass at collapse in recent hydrodynamic simulations. We find that at accretion rates higher than $0.001$ $M_\odot$ yr$^{-1}$ the stars evolve as red, cool supergiants with surface temperatures below $10^4$ K towards masses $>10^5$ $M_\odot$, and become blue and hot, with surface temperatures above $10^5$ K, only for rates $\lesssim0.001$ $M_\odot$ yr$^{-1}$. Compared to previous studies, our results extend the range of masses and accretion rates at which the ionising feedback remains weak, reinforcing the case for direct collapse as the origin of the first quasars

On the Detection of Supermassive Primordial Stars. II. Blue Supergiants

Supermassive primordial stars in hot, atomically-cooling haloes at $z \sim$ 15 - 20 may have given birth to the first quasars in the universe. Most simulations of these rapidly accreting stars suggest that they are red, cool hypergiants, but more recent models indicate that some may have been bluer and hotter, with surface temperatures of 20,000 - 40,000 K. These stars have spectral features that are quite distinct from those of cooler stars and may have different detection limits in the near infrared (NIR) today. Here, we present spectra and AB magnitudes for hot, blue supermassive primordial stars calculated with the TLUSTY and CLOUDY codes. We find that photometric detections of these stars by the James Webb Space Telescope (JWST) will be limited to $z \lesssim$ 10 - 12, lower redshifts than those at which red stars can be found, because of quenching by their accretion envelopes. With moderate gravitational lensing, Euclid and the Wide-Field Infrared Space Telescope (WFIRST) could detect blue supermassive stars out to similar redshifts in wide-field surveys.Comment: 9 pages, 5 figures, accepted by MNRA

Diffuse Gas in Retired Galaxies: Nebular Emission Templates and Constraints on the Sources of Ionization

We present emission line templates for passively evolving ("retired") galaxies, useful for investigation of the evolution of the ISM in these galaxies, and characterization of their high-temperature source populations. The templates are based on high signal-to-noise ($>800$) co-added spectra ($3700-6800$\AA) of $\sim11500$ gas-rich Sloan Digital Sky Survey galaxies devoid of star-formation and active galactic nuclei. Stacked spectra are provided for the entire sample and sub-samples binned by mean stellar age. In Johansson~et al (2014), these spectra provided the first measurements of the He II 4686\AA\ line in passively-evolving galaxies, and the observed He II/H$\beta$ ratio constrained the contribution of accreting white dwarfs (the "single-degenerate" scenario) to the type Ia supernova rate. In this paper, the full range of unambiguously detected emission lines are presented. Comparison of the observed [O I] 6300\AA/H$\alpha$ ratio with photoionization models further constrains any high-temperature single-degenerate scenario for type Ia supernovae (with 1.5 $\lesssim$ T/$10^{5}K$ $\lesssim$ 10) to $\lesssim$3-6\% of the observed rate in the youngest age bin (i.e. highest SN Ia rate). Hence, for the same temperatures, in the presence of an ambient population of post-AGB stars, we exclude additional high-temperature sources with a combined ionizing luminosity of $\approx 1.35\times 10^{30} L_{\odot}/M_{\odot,*}$ for stellar populations with mean ages of 1 - 4 Gyrs. Furthermore, we investigate the extinction affecting both the stellar and nebular continuum. The latter shows about five times higher values. This contradicts isotropically distributed dust and gas that renders similar extinction values for both cases.Comment: Accepted for publication in MNRAS, 16 pages, 12 figure

Diffuse gas in galaxies sheds new light on the origin of Type Ia supernovae

Date of Acceptance: 05/05/2014We measure the strength of He II λ4686 nebular emission in passively evolving ('retired') galaxies, aiming to constrain their populations of hot accreting white dwarfs (WDs) in the context of the single-degenerate (SD) scenario of Type Ia supernovae (SNe Ia). In the SD scenario, as a WD burns hydrogen-rich material accreted from a companion star, it becomes a powerful source of ionizing ultraviolet emission. If significant populations of such sources exist in galaxies, strong emission in the recombination lines of He II should be expected from the interstellar medium. To explore this conjecture, we select from the Sloan Digital Sky Survey ~11 500 emission-line galaxies with stellar ages >1 Gyr showing no signs of active galactic nuclei activity and co-add their spectra in bins of stellar population age. For the first time, we detect He II λ4686 nebular emission in retired galaxies and find it to be significantly weaker than that expected in the SD scenario, especially in the youngest age bin (1-4 Gyr) where the SN Ia rate is the highest. Instead, the strength of the observed He II λ4686 nebular emission is consistent with post-asymptotic giant branch stars being the sole ionizing source in all age bins. These results limit populations of accretingWDs with photospheric temperatures (Teff) in the range ~(1.5-6) × 105 K to the level at which they can account for no more than ~5-10 per cent of the observed SN Ia rate. Conversely, should all WD progenitors of SN Ia go through the phase of steady nuclear burning with Teff ~ (1.5-6) × 105 K, they do not increase their mass by more than ~0.03M⊙ in this regime.Peer reviewedFinal Accepted Versio

Emission line diagnostics of the progenitors of type Ia supernova

In this dissertation, we consider the origin of thermonuclear supernovae, known by their observational classification as type Ia (hereafter SNe Ia). In particular, we develop an entirely new means to test the ``single-degenerate'' hypothesis, in which the progenitors of these tremendous explosions are suggested to be hot and luminous accreting white dwarfs. We then strongly constrain the role of any such ``hot-mode'' SN Ia progenitor channel using both a population-based argument and an individual case study, before concluding with some more general considerations of nebulae ionized by accreting white dwarfs. Type Ia supernovae have now been the subject of intensive study for decades, particularly in light of their role as standard(-izable) candles in measuring cosmological distances. However, there remains no consensus model for the evolutionary channel(s) by which they originate. In the so-called ``double-degenerate'' scenario, a binary pair of white dwarfs shed angular momentum through gravitational-wave radiation, until they inspiral and merge, triggering an explosion. Alternatively, in the classic picture of the single-degenerate scenario, a white dwarf accretes hydrogen-rich material from some main sequence or red giant companion, and grows through nuclear burning of this material at its surface until reaching sufficient mass to trigger an explosion. This suggests that single-degenerate progenitors should be extremely luminous sources in the EUV and soft X-ray bands during the accretion phase (lasting $\sim 10^{5}$--$10^{6}$ years). For this reason, such objects are generally associated with observed ``supersoft X-ray sources'' (SSSs). Previous efforts to detect or constrain the role of any such channel have focused on detecting these objects directly in the soft X-ray band (photon energies in the range 0.3 -- 0.7 keV), either on an individual basis or as the combined emission of a diffuse population. Such an approach has yielded important constraints, but only if white dwarfs accrete principally at very high temperatures (T $\sim$ $5\times 10^{5}$K). However, observed SSSs are understood to lie in a broad range of temperatures, with a possible range of at least $2 \times 10^{5}$--$10^{6}$K, and some theoretical models suggest even lower temperatures are possible. This necessitates the development of an alternative, complimentary test which can constrain the luminosity of accreting white dwarfs across a wider range of photospheric temperatures. In this work, we demonstrate that if the single-degenerate model is correct, then accreting, nuclear-burning white dwarfs should provide the dominant source of ionizing radiation in passively-evolving galaxies, roughly 40\% of which are known to host extended low-ionization emission-line regions (so-called ``retired'' galaxies, i.e. emission-line galaxies without either a central AGN or significant ongoing star formation). Therefore, one can search for the presence of any high-temperature single-degenerate progenitor population in these galaxies by looking for emission lines characteristic of ionization by very high-temperature ($10^{5}$ K -- $10^{6}$ K) sources. In particular, we find that recombination lines of He II, and forbidden lines of [N I] and [O I], provide the most sensitive diagnostics in retired galaxies to assess the role of accreting white dwarfs as SN Ia progenitors in any ``{\bf hot}-mode'' (T $\gtrsim 1.2\times 10^{5}$K) accretion regime. Following this, we limit the contribution of any high-temperature single-degenerate channel to the SN Ia rate at relatively early delay-times (1 Gyr $\leq$ t $\leq$ 4 Gyr) to $<$~5--10\% (for T$\gtrsim 1.2\times 10^{5}$K) using He II 4686\AA\ and [O I] 6300\AA\ measurements from a stacked sample (provided by Dr Jonas Johansson) of several thousand retired galaxies in the Sloan Digital Sky Survey. We also discuss how these constraints, as well as the observed soft X-ray emission of several nearby galaxies, reveal fundamental problems in our present understanding of the population synthesis of SSSs and other accreting white dwarf binaries. We then revise the standard picture for the observational appearance of nebulae ionized by individual accreting white dwarfs, accounting for a more realistic assessment of the typical ISM densities in which such objects are likely embedded. We then provide the first formal justification for why so few SSS nebulae have been detected thus far, and demonstrate that a complete survey is within the means of modern large-aperture telescopes (such as ESO's Very Large Telescope). We then show how this approach can be extended to individual SNe Ia, by searching for fossil nebulae in the vicinity of nearby events. In particular, we use an archival pre-explosion narrow-band H$\alpha$ + [N II] image of the vicinity of SN2014J to place constraints on the luminosity of any putative high-temperature progenitor for SN2014J (such as an accreting white dwarf).Diese Dissertation beschäftigt sich mit der Frage der Entstehung thermonuklearer Supernovae, welche die beobachtende Astronomie als Typ Ia Supernovae (im folgenden ”SNe Ia“) klassifiziert. Insbesondere wird in dieser Arbeit eine völlig neue Methode beschrieben, mit deren Hilfe sich die sogenannte ”einfach entartete“ Hypothese ¨uberpr¨ufen lässt. Nach dieser Hypothese entsteht eine SN Ia aus einem heißen und sehr leuchtkräftig, akkretierenden Weißen Zwerg. Anschließend kann die Bedeutung dieser Art der ”hot-mode“-SN Ia, sowohl durch ein Populationsargument als auch anhand eines Einzelbeispiels, stark eingeschränkt werden. Abschließend werden generelle Effekte der ionisierenden Wirkung akkretierender Weißer Zwerge auf Nebel erörtert. Typ Ia Supernovae sind seit Jahrzehnten Gegenstand intensiver Forschung, besonders aufgrund ihrer Eignung als ”Standardkerzen“ zur Messung kosmologischer Distanzen. Nichtsdestotrotz gibt es keinen Konsens bez¨uglich der Entwicklungspfade, die zu solchen Sternexplosionen f¨uhren. Das sogenannte ”zweifach entartete Szenario“ beschreibt den Drehimpulsverlust zweier Weißer Zwerge in einem Binärsystem durch die Abstrahlung von Gravitationswellen. Infolgedessen kommt es zur spiralförmigen Annäherung beider Weißer Zwerge, bis diese verschmelzen und damit die eigentliche Explosion auslösen. Im Gegensatz hierzu wird im klassischen ”einfach entartete Szenario“ einem Weißen Zwerg, der wasserstoffreiches Gas von einem Hauptreihenstern oder einem Roten Riesen akkretiert, durch nukleares Brennen dieses Materials an der Oberfläche des Weißen Zwerges, Masse zugef¨uhrt, bis dessen Masse einen kritischen Wert ¨ubersteigt, um die Explosion zu z¨unden. Als Konsequenz sollten diese Objekte während der Akkretionsphase (10^5−10^6 Jahre) im extremen Ultraviolett- (EUV) und weichen Röngtenspektralbereich äußerst leuchtkräftig sein. Solche Objekte werden im Allgemeinen mit beobachteten ”superweichen Röntgenquellen“ (SSSs) assoziiert. Bisherige Anstrengungen die Relevanz dieses Entwicklungspfades zu messen oder einzuschränken, konzentrierten sich immer auf die direkte Beobachtung dieser Objekte im weichen Röngtenspektralbereich (Photonenergien im Bereich von 0.3 - 0.7 keV), entweder durch Beobachtung einzelner Objekte oder der kombinierten Emission einer diffusen Population von Quellen. Dieser Ansatz liefert Einschränkungen, aber nur unter der Annahme, dass Weiße Zwerge Materie bei sehr hohen Temperaturen (T ∼ 5 × 105K) akkretieren. Jedoch fullen beobachtete SSSs einen weiten Temperaturbereich von 2×10^5K bis 10^6K aus, und zudem lassen einige theoretische Modelle noch niedrigere Temperaturen zu. Deshalb ist die Entwicklung eines alternativen, komplementären Tests, mit dem man die Leuchtkraft akkretierender Weißer Zwerge in einem breiteren Spektralbereich photosphärischer Temperaturen eingrenzen kann, notwendig. Diese Arbeit demonstriert, unter Annahme des ”einfach entarteten“ Modells, dass akkretierende, thermonuklear-brennende Weiße Zwerge den dominanten Anteil ionisierender Strahlungsquellen in ”passively-evolving“ Galaxien darstellen. Von etwa 40% dieser Galaxien ist bekannt, dass sie ”low-ionization emission-line regions“ beherbergen (auch als ”retired“ bezeichnet, d. h. ”emission-line galaxies“ entweder ohne aktiven Galaxienkern (AGN) oder ohne signifikante Sternentstehungsrate). Folglich können solche Galaxien auf eine heiße ”einfach entartete“ Sternpopulation untersucht werden, indem nach Emissionslinien gesucht wird, die charakteristisch für die Ionisation durch Objekte mit hohen Temperturen (10^5K - 10^6K) sind. Insbesondere zeigt sich, dass die Rekombinationslinien von He II und die verbotene ¨Ubergänge von [N I] und [O I], die genauesten diagnostischen Größen in diesen ”retired“ Galaxien darstellen, um die Rolle der, im ”hot-mode“ (T > 1.2×105K) akkretierenden Weißen Zwerge, als Vorläufer von Typ Ia SNe zu beurteilen. Diesem Ansatz folgend, kann der Anteil des hochtemperatur-”einfach entarteten“-Entwicklungspfades an der Supernovarate vom Typ Ia zu relativ frühen ”delay-times“ (1Gyr ≤ t ≤ 4Gyr) auf 1.2×105K) beschränkt werden, unter Verwendung von He II 4686°A- und [OI] 6300°A-Messungen in einem kombinierten Satz von einigen tausend ”retired“ Galaxien aus dem Sloan Digital Sky Survey (zur Verf¨ugung gestellt von Dr. Jonas Johansson). Außerdem diskutiert diese Arbeit, wie die gefundenen Obergrenzen und die beobachtete weiche Röntgenemission einiger naher Galaxien fundamentale Probleme im Verständnis der Populationssynthese von SSSs und anderen akkretierenden Binärsystemen mit Weißen Zwergen aufzeigen. Im Weiteren wird die gängige Theorie eines durch einen akkretierenden Weißen Zwerg ionisierten Nebels verfeinert, wobei eine realistischere Abschätzung typischer ISM-Dichten, in denen das Objekt mit großer Wahrscheinlichkeit eingebettet ist, verwendet wird. Damit kann zum ersten Mal rigoros gezeigt werden, weshalb so wenige SSS-Nebel beobachtet wurden und dass eine allumfassende Studie mit heutigen, modernen Teleskopen (beispielsweise das ”Very Large Telescope“ der ESO) möglich ist. Es zeigt sich außerdem, dass dieser Ansatz auf einzelne SN Ia erweitert werden kann, indem nach fossilen Nebeln in der Umgebung eines nahen Ereignisses gesucht wird. Insbesondere wurden schmalbandige Hα+[N II]-Archivaufnahmen der Umgebung von SN2014J verwendet, um die Leuchtkraft eines potentielle Hochtemperatur-Vorläufersterns einzuschränken

On Monolithic Supermassive Stars

Supermassive stars have been proposed as the progenitors of the massive ($\sim 10^{9}\,\rm{M}_{\odot}$) quasars observed at $z\sim7$. Prospects for directly detecting supermassive stars with next-generation facilities depend critically on their intrinsic lifetimes, as well as their formation rates. We use the 1D stellar evolution code Kepler to explore the theoretical limiting case of zero-metallicity, non-rotating stars, formed monolithically with initial masses between $10\,\rm{kM}_{\odot}$ and $190\,\rm{kM}_{\odot}$. We find that stars born with masses between $\sim60\,\rm{kM}_{\odot}$ and $\sim150\,\rm{kM}_{\odot}$ collapse at the end of the main sequence, burning stably for $\sim1.5\,\rm{Myr}$. More massive stars collapse directly through the general relativistic instability after only a thermal timescale of $\sim3\,\rm{kyr}$--$4\,\rm{kyr}$. The expected difficulty in producing such massive, thermally-relaxed objects, together with recent results for currently preferred rapidly-accreting formation models, suggests that such ``truly direct'' or ``dark'' collapses may not be typical for supermassive objects in the early Universe. We close by discussing the evolution of supermassive stars in the broader context of massive primordial stellar evolution and the possibility of supermassive stellar explosions.Comment: 9 pages, 6 figures, 1 table, MNRAS accepte

Probing the Diversity of Type Ia Supernova Light Curves in the Open Supernova Catalog

The ever-growing sample of observed supernovae enhances our capacity for comprehensive supernova population studies, providing a richer dataset for understanding the diverse characteristics of Type Ia supernovae and possibly that of their progenitors. Here, we present a data-driven analysis of observed Type Ia supernova photometric light curves collected in the Open Supernova Catalog. Where available, we add the environmental information from the host galaxy. We focus on identifying sub-classes of Type Ia supernovae without imposing the pre-defined sub-classes found in the literature to date. To do so, we employ an implicit-rank minimizing autoencoder neural network for developing low-dimensional data representations, providing a compact representation of the supernova light curve diversity. When we analyze light curves alone, we find that one of our resulting latent variables is strongly correlated with redshift, allowing us to approximately ``de-redshift'' the other latent variables describing each event. After doing so, we find that three of our latent variables account for $\sim$95\% of the variance in our sample, and provide a natural separation between 91T and 91bg thermonuclear supernovae. Of note, the 02cx subclass is not unambiguously delineated from the 91bg sample in our results, nor do either the over-luminous 91T or the under-luminous 91bg/02cx samples form a clearly distinct population from the broader sample of ``other'' SN Ia events. We identify the physical characteristics of supernova light curves which best distinguish SNe 91T from SNe 91bg \& 02cx, and discuss prospects for future refinements and applications to other classes of supernovae as well as other transients.Comment: Astrophysical Journal, accepet