Modelling accreting white dwarf populations in galaxies

This thesis has focused on modelling population of accreting white dwarf binaries in galaxies. We adopt a hybrid binary population synthesis approach to study the formation and evolution of accreting white dwarfs. Based on results of the modelling, we predict the emission from their population in optical, UV and soft X-ray bands and properties of the nova population in galaxies with different star formation histories and compare our predictions with observations. Accreting white dwarf binaries are binaries in which white dwarfs accrete material from non-degenerate donors. Depending on white dwarf mass and accretion rate, the H-rich material on the surface will burn stably or unstably. If the accreted material burns stably, accreting white dwarf will radiate predominantly in the soft X-ray or EUV band. The emission of these accreting white dwarfs is able to ionize the interstellar medium producing a number of characteristic recombination lines, such as He II $4686{\rm \AA}$ and forbidden lines of metals. Previous efforts have been made to use the observational results of the emission of accreting white dwarfs to constrain the progenitor model of type Ia supernovae in single degenerate scenario. These observations should be also good to constrain the accreting white dwarf population in general. On the other hand, if the material burns unstably, it will lead to nova explosions. Several previous observational studies have focused on the properties of nova population in galaxies of different Hubble types. Comparison of results of observations with the predictions of population synthesis models is a powerful tool to benchmark and verify the fundamental assumptions underlying the population synthesis calculations. In this thesis, we model the formation and evolution of accreting white dwarfs population with a hybrid binary population synthesis approach using \textsc{bse} (Binary Star Evolution) and \textsc{mesa} (Modules for Experiments in Stellar Astrophysics) codes. First, we use the rapid binary population synthesis code \textsc{bse} to generate a population of WDs with non-degenerate companions on the verge of filling their Roche lobes. Then we follow their evolution with the detailed stellar evolution code \textsc{mesa}. We investigate the evolution of the number of different types of accreting white dwarfs and type Ia supernova rate. We show that the accurate treatment of mass transfer is critical for binary population synthesis calculations. Using our advanced code, we confirm that the delay time distribution of type Ia supernovae in single degenerate scenario is inconsistent with observations and the type Ia supernova rate of Galaxy in our calculation is much smaller than observation, in agreement with previous studies. Based the above binary population synthesis results and using simple assumptions regarding the emission of accreting white dwarfs, we investigate the soft X-ray ( 0.3-0.7\;keV) luminosity of accreting white dwarfs and their H and He II ionizing luminosity. In addition, using results of detailed photoionization calculations, we study the influence of the radiation of accreting white dwarfs on the line emission from interstellar medium in elliptical galaxies, in particular, He II $4686{\rm \AA}$ and H$\beta$. Then we compare these results with \textit{Chandra} and Sloan Digital Sky Survey observations of elliptical galaxies. We demonstrate that the results of binary population synthesis with commonly adopted assumptions are inconsistent with the observation of galaxies with stellar ages of $\lesssim 4-8\;$Gyr. We discuss various possibilities to resolve this discrepancy and find that the possible solution to this discrepancy is to improve the typically used criteria of dynamically unstable mass-loss for giant stars. With the improved binary population synthesis model, we make predictions about the number of observable supersoft X-ray sources in galaxies of different types. We then model the formation and evolution of nova populations for galaxies with different star formation histories using the improved binary population synthesis model. We study the properties of nova populations in galaxies with different star formation histories. In particular, we present the evolution of nova rate, the distribution of white dwarf mass, mass loss time, recurrence period and maximum magnitude of novae. We found that the mass-specific nova rate in spiral galaxies is about $10-20$ times larger than in elliptical galaxies. The novae tend to be hosted by low mass white dwarfs in elliptical galaxies and massive white dwarfs in spiral galaxies. The majority of current novae in elliptical galaxies are relatively faint and have long mass-loss times and long recurrence periods. In spiral galaxies, on the contrary, the majority of the current nova are relatively bright and have short mass-loss times and short recurrence periods. The results of our composite model using star formation history of M31 galaxy are in agreement with observational data of this galaxy.Diese Arbeit beschäftigt sich mit der Modellierung von Binärsystemen akkretierender weißer Zwerge in Galaxien. Wir verwenden einen hybriden Binärpopulation-Synthese Ansatz um die Entstehung und Entwicklung von akkretierenden weißen Zwergen zu untersuchen. Mit den Ergebnissen des Models machen wir Vorhersagen über die Emission der Populationen im optischen, UV und weichen Röntgenband und die Eigenschaften der Nova Populationen in Galaxien mit unterschiedlicher Sternentstehungsgeschichte und vergleichen unsere Vorhersagen mit Beobachtungen. In akkretierenden Binärsystemen weißer Zwerge akkretieren die weißen Zwerge Material von nicht entarteten Spendern. Abhängig von der Masse und der Akkretionsrate des weißen Zwergs brennt das wasserstoffreiche Material auf der Oberfläche stabil oder instabil. Wenn das akkretierte Material stabil brennt, strahlt der akkretierende weiße Zwerg hauptsächlich im weichen Röntgenstrahlen- oder EUV-Band. Die Emission dieser weißen Zwerge ist imstande, das interstellare Medium zu ionisieren, was eine Reihe von charakteristischen Rekombinations Linien erzeugt, wie die von He II $4686{\rm \AA}$ und verbotene Linien von Metallen. Frühere Arbeiten haben die Beobachtungsresultate der Emission von akkreditierenden weißen Zwergen benutzt um das Vorgänger Modell von Supernovae vom Typ Ia im einfach entarteten Szenario einzugrenzen. Diese Beobachtungen sollten auch gut geeignet sein, um die Population von akkretierenden weißen Zwergen insgesamt einzugrenzen. Wenn das akkreditierende Material instabil brennt, führt das zu Nova Explosionen. Mehrere frühere beobachtende Studien haben sich auf die Eigenschaften von Nova Populationen in Galaxien von verschiedenen Hubble Typen spezialisiert. Der Vergleich von Beobachtungen mit den Vorhersagen von Populations- Synthese -Modellen ist ein mächtiges Werkzeug um die grundlegenden Annahmen der Populations - Synthese Rechnungen zu testen und zu verifizieren. In dieser Arbeit modellieren wir die Entstehung und Entwicklung der Population akkreditierender weißer Zwerge mit einem hybriden Binärpopulations-Synthese Ansatz mit Hilfe der Codes \textsc{bse} (Binary Star Evolution) und \textsc{mesa} (Modules for Experiments in Stellar Astrophysics). Zunächst benutzen wir den schnellen Binärpopulations-Synthese Code um eine Population von weißen Zwergen mit nicht entarteten Begleitern zu berechnen, welche sich an der Grenze zur Füllung ihres Roche Lobe befinden. Dann folgen wir ihrer Entwicklung mit dem detaillierten Sternentwicklungscode \textsc{mesa}. Wir untersuchen die Entwicklung der Anzahl von verschiedenen Typen akkretierender weißer Zwerge und der Raten von Typ Ia Supernova. Wir zeigen, dass eine genaue Behandlung des Massentransfers essentiell für Populations-Synthese-Berechnungen ist. Wir benutzen unseren erweiterten Code, um zu zeigen, dass die Verteilung der Zeitverzögerung von Typ Ia Supernovae im einfach entarteten Szenario inkonsistent mit Beobachtungen ist. Aus unseren Berechnungen ergibt sich die Typ Ia Supernova Rate in unserer Galaxie wesentlich geringer als in Beobachtungen - in Übereinstimmung mit früheren Studien. Basierend auf den obigen Resultaten und einfachen Annahmen bezüglich der Emission von akkrederenden weißen Zwergen untersuchen wir die weiche Röntgen-Leuchtkraft ( 0.3-0.7\;keV) und die H und He II ionisierende Leuchtkraft von akkretierenden weißen Zwergen. Zusätzlich benutzen wir detaillierte Photoionisierungs-Berechnungen um den Einfluss der Strahlung der akkretierenden weißen Zwerge auf die Linien-Emission des interstellaren Mediums in elliptischen Galaxien zu untersuchen - insbesondere He II $4686{\rm \AA}$ und H$\beta$. Danach vergleichen wir diese Ergebnisse mit \textit{Chandra} und Sloan Digital Sky Survey Beobachtungen elliptischer Galaxien. Wir zeigen, dass die Resultate von Binärpopulations-Synthese-Rechnungen mit üblichen Annahmen inkonsistent mit Beobachtungen von Galaxien mit Sternaltern von $\lesssim 4-8\;$ Milliarden Jahren sind. Wir diskutieren verschiedene Möglichkeiten diese Diskrepanz zu beheben. Eine mögliche Lösung ist es, die Kriterien des dynamisch instabilen Masseverlustes von Riesen zu verbessern. Mit einem verbesserten Binärpopulation-Synthese Modell machen wir Vorhersagen zur Anzahl beobachtbarer überweicher Röntgenquellen in Galaxien verschiedener Arten. Danach modellieren wir die Entstehung und Entwicklung von Nova Populationen von Galaxien mit unterschiedlichen Sternentstehungs-Geschichten mit dem verbesserten Binärpopulations-Synthese-Modell. Wir untersuchen die Eigenschaften von Nova Populationen in Galaxien mit unterschiedlichen Sternenstehungs-Geschichten. Insbesondere berechnen wir die Entwicklung der Nova Rate, die Massenverteilung von weißen Zwergen, die Massenverlust-Dauer, die Wiederaufrtritts-Periode und die maximale Magnitude von Novae. Wir finden, dass die massenspezifische Nova Rate in Spiralgalaxien etwa $10-20$ mal höher ist als in elliptischen Galaxien. Novae scheinen in elliptischen Galaxien von weißen Zwergen niedriger Masse und in Spiralgalaxien von weißen Zwergen hoher Masse zu stammen. Die Mehrzahl derzeitiger Novae in elliptischen Galaxien ist relativ lichtschwach und hat eine lange Massenverlust-Dauer und lange Wiederauftritts-Perioden. Die Ergebnisse unseres zusammengesetzten Modells, welches die Sternentstehungs-Geschichte der M31 Galaxie verwendet, sind in Übereinstimmung mit Beobachtungsdaten dieser Galaxie

The Impact of ChatGPT on the Demand for Human Content Generating and Editing Services: Evidence from an Online Labor Market

The rise of generative AI has been a subject of debate among researchers and practitioners regarding its effect on the labor market. While some argue that it may displace jobs, others suggest it could create new opportunities and improve productivity. This study examines the impact of the ChatGPT launch on 18,130 services with 199,430 observations using a difference-in-differences approach and data from the online labor marketplace Fiverr. The findings suggest that ChatGPT had a negative effect on the demand for human content generating and editing services, with a concentration on writing services. However, there was no significant effect on the demand for editing services. The study also found that the demand for services with higher prices was more negatively affected. These results contribute to the ongoing debate on the impact of generative AI on the labor market and offer practical recommendations for service providers to navigate this new AI-driven landscape

Female-exclusive Support and Gender Gap on Digital Platforms

This study investigates the gender gap issue in the mobile platform context by analyzing performance disparities between male and female-led app companies. First, we identify the existence of the performance gap between the apps from two gender groups, utilizing a representative sample of mobile apps on Apple App Store. By employing the occurrence of Apple Entrepreneur Camp 2019 as an exogenous event, we assess the effectiveness of the platform’s female-exclusive support in mitigating the gap. The results demonstrate that following the event, apps from the supported female-led companies exhibit improved performance compared to their male-led counterparts, whereas there is no significant increase in the performance of apps from unsupported female-led companies. Our empirical analysis highlights that as the age of apps from the unsupported female group increases, the event shows a positively significant impact on app performance. This finding suggests the presence of a conditional spillover effect of the platform’s support. This study contributes to understanding the gender gap and emphasizes the importance and limitations of platform support initiatives for promoting gender equality within the platform. Managerial implications suggest implementing the supporting strategies to foster diversity and inclusion in the platform ecosystem

Platform Commission Reduction And Mobile App Performance

Commission is a common platform pricing strategy for charging a portion of transaction revenues. However, digital platform firms face long-standing disputes with app developers and even lawsuits regarding their commission rules. This study investigates how reducing platform commission affects mobile app performance and developers’ behaviors. We leverage a natural experiment based on a commission policy change implemented by Apple and conduct a difference-in-differences (DID) analysis. Surprisingly, we find a negative impact of commission reduction on app performance measured by daily active users and downloads. The impact of commission reduction on app performance is heterogeneous across apps with different ranks and across apps in the game and non-game app categories. Further analysis of the mechanism reveals that the apps eligible to enjoy the benefits of commission reduction are updated less frequently, indicating that developers devote less effort to improving and advancing these apps. Our findings provide important theoretical and managerial implications

Interplay between Social Media and Traditional Media: An Empirical Study in the Motion Picture Industry

Marketers leverage multiple media outlets to promote products. There are three media types: paid (e.g., advertising on TV), owned (e.g., company website), and earned (e.g., consumers’ word-of-mouth) media. The effects of individual media channels and the interrelationships within paid media have been examined by prior literature. However, little is known about interplay across different types of media channels. We investigate how social media marketing (both owned and earned media) and the interplay between traditional paid media and social media affect product sales. We analyze how promotional activities across different media channels influence the box office revenues of 200 movies in their opening weekends and subsequent weeks, respectively. We find empirical evidence that social media marketing is positively associated with product sales, especially in the initial product launch period. Additionally, traditional media and social media are substitutes. We discuss the implications of these results and outline our on-going research plan

The Relationship between Mobile Web and Mobile App Channels for Retailers

As smartphones and tablets are becoming ubiquitous, mobile ecommerce is also evolving rapidly. Consumers can shop on mobile devices in two ways; they either open a mobile browser and visit a retailer\u27s website, or download the retailer\u27s mobile app and shop within the app. However, it is unclear how retailers should manage these two emerging channels together. This proposed study aims to investigate the relationship between mobile web and mobile app channels by analyzing how a change to one channel affects the outcome in the other. To infer causality, we utilize an exogenous event in the mobile web channel to assess how it influences the demand of retailers\u27 mobile apps. The results could reveal whether these two mobile channels complement or substitute each other. This study contributes to the literature of multi-channel management in mobile commerce and provides important managerial implications for retailers to better leverage the growing mobile channels

Examining the Causal Relationship between Screen Size and Cellular Data Consumption

This study utilizes a terabyte dataset from a telecommunications company to examine the relationship between screen size and cellular data consumption for a large number of phone and tablet users. We find the relationship exhibits a different pattern within the two device categories of phones and tablets. For phone users, there is an overall positive and significant relationship over the range of screen size from 1 inch to below 7 inches, which is, however, mainly driven by the dramatic decrease in usage on traditional phones with screens less than 3 inches. Particularly for smartphones with screens 3.5 inches or higher, we do not find a significant relationship between screen size and cellular data consumption measured by either the time spent on the mobile network or the amount of data transmitted. For tablet users, we find evidence that suggests that people spend less time on tablets with bigger screens, which could potentially be due to the reduced portability of large tablets. Our findings can provide important implications for mobile network operators in promoting data plans to users with different devices

ISTopic: Understanding Information Systems Research through Topic Models

What are the fundamental research questions in Information Systems? How do various research topics relate with one another to form the IS research landscape and how do they evolve over time? This study is an initial attempt to answer these questions using topic models to investigate the topics examined by the premier IS journals in 1977-2014. We present an IS Topic Graph that contains 33 research areas, 31 of which are closely connected with one another. Further analyses of this graph reveal how different IS research areas are intertwined to the extent that they are almost inseparable. Looking into IS research at a finer level, we identify 300 research topics, and a chronological analysis reveals a trend of topic diversification and externalization. To guide future research, an intelligent literature search tool called ISTopic is built and is available at http://www.istopic.org for public access

TiO2 Nanocrystals Grown on Graphene as Advanced Photocatalytic Hybrid Materials

Graphene/TiO2 nanocrystals hybrid is successfully prepared by directly growing TiO2 nanocrystals on graphene oxide (GO) sheets. The direct growth of nanocrystals on GO sheets was achieved by a two-step method, in which TiO2 was coated on GO sheets by hydrolysis first and crystallized into anatase nanocrystals by hydrothermal treatment in second step. Slow hydrolysis reaction through the use of EtOH/H2O mixed solvents and addition of H2SO4 allows the selectively growing TiO2 on GO and suppressing free growth in solution. The method offers easy access to the GO/TiO2 nanocrystals hybrid with well controlled coating and strong interactions between TiO2 and the underlying GO sheets. The strong coupling could lead to advanced hybrid materials for various applications including photocatalysis. The prepared graphene/TiO2 nanocrystals hybrid has demonstrated superior photocatalytic activity in degradation of rhodamine B over other TiO2 materials, showing an impressive 3-fold photocatalytic enhancement over P25. It is expected that the hybrid material could also be promising for various other applications including lithium ion battery where strong electrical coupling to TiO2 nanoparticles is essential.Comment: Nano Research, in pres