A Broadband Scalar Vortex Coronagraph

Broadband coronagraphy with deep nulling and small inner working angle has the potential of delivering images and spectra of exoplanets and other faint objects. In recent years, many coronagraphic schemes have been proposed, the most promising being the optical vortex phase mask coronagraphs. In this paper, a new scheme of broadband optical scalar vortex coronagraph is proposed and characterized experimentally in the laboratory. Our setup employs a pair of computer generated phase gratings (one of them containing a singularity) to control the chromatic dispersion of phase plates and achieves a constant peak-to-peak attenuation below $1\cdot 10^{-3}$ over a bandwidth of 120 nm centered at 700 nm. An inner working angle of ~\lambda/D is demonstrated along with a raw contrast of 11.5 magnitudes at 2\lambda/D.Comment: Accepted, 6 pages, 6 image

Investigation of a transiting planet candidate in Trumpler 37: An astro-physical false positive eclipsing spectroscopic binary star

We report our investigation of the first transiting planet candidate from the YETI project in the young (∼4 Myr old) open cluster Trumpler 37. The transit-like signal detected in the lightcurve of F8V star 2M21385603+5711345 repeats every 1.364894 + 0.000015 days, and has a depth of 54.5 + 0.8 mmag in R. Membership in the cluster is supported by its mean radial velocity and location in the color-magnitude diagram, while the Li diagnostic and proper motion are inconclusive in this regard. Follow-up photometric monitoring and adaptive optics imaging allow us to rule out many possible blend scenarios, but our radial-velocity measurements show it to be an eclipsing single-lined spectroscopic binary with a late-type (mid-M) stellar companion, rather than one of planetary nature. The estimated mass of the companion is 0.15–0.44 M⊙. The search for planets around very young stars such as those targeted by the YETI survey remains of critical importance to understand the early stages of planet formation and evolution

EXOhSPEC folded design optimization and performance estimation

The EXOplanet high resolution SPECtrograph (EXOhSPEC) instrument is an echelle spectrograph dedicated to the detection of exoplanets by using the radial velocity method using 2m class telescopes. This spectrograph is specified to provide spectra with a spectral resolution R < 70, 000 over the spectral range from 400 to 700 nm and to reach a shortterm radial velocity precision of 3 m/s. To achieve this the separation between two adjacent spectral orders is specified to be greater than 30 pixels and to enable a wide range of targets the throughput of the instrument is specified to be higher than 4%. We present the results of the optimization of the spectrograph collimator performed and initial tests of its optical performance. First, we consider the spectrograph design and we estimate its theoretical performance. We show that the theoretical image quality is close to the diffraction limit. Second, we describe the method used to perform the tolerancing analyzes using ZEMAX software to estimate the optical performance of the instrument after manufacturing, assembly and alignment. We present the results of the performance budget and we show that the estimated image quality performance of EXOhSPEC are in line with the specifications. Third, we present the results of the stray light analysis and we show that the minimum ratio between the scientific signal and the stray light halo signal is higher than 1,000. Finally, we provide a status on the progress of the EXOhSPEC project and we show the first results obtained with a preliminary version of the prototype.Final Accepted Versio

Doing Big Things in a Small Way: A Social Media Analytics Approach to Information Diffusion During Crisis Events in Digital Influencer Networks

Digital influencers play an essential role in determining information diffusion during crisis events. This paper demonstrates that information diffusion (retweets) on the social media platform Twitter (now X) highly depends on digital influencers’ number of followers and influencers’ location within communication networks. We show (study 1) that there is significantly more information diffusion in regional (vs. national or international) crisis events when tweeted by micro-influencers (vs. meso- and macro-influencers). Further, study 2 demonstrates that this pattern holds when micro-influencers operate in a local location (are located local to the crisis). However, effects become attenuated when micro-influencers are situated in a global location (outside of the locality of the event). We term this effect ‘influencer network compression’ – the smaller in scope a crisis event geography (regional, national, or international) and influencer location (local or global) becomes, the more effective micro-influencers are at diffusing information. This shows that those who possess the most followers (meso- and macro-influencers) are less effective at attracting retweets than micro-influencers situated local to a crisis. As online information diffusion plays a critical role during public crisis events, this paper contributes to both practice and theory by exploring the role of digital influencers and their network geographies in different types of crisis events

Nachbeobachtungen von Transitkandidaten und jungen bedeckungsveränderlichen Doppelsternen in der scheinbaren Umgebung des offenen Sternhaufens Trumpler 37

Seit der Entdeckung des ersten Exoplaneten 1995 wurden bisher über 1000 Planeten gefunden. Dadurch hat sich das Verständnis von Planeten- und Sternentstehung stark verbessert. Es ist nötig möglichst viele Parameter der Planeten und Sterne (Masse, Radius, Alter) direkt zu bestimmen um diese Werte mit Modellen vergleichen zu können. Bisher wurden vorwiegend Planeten älter als 100 Mio. Jahre untersucht. In dieser Arbeit wurde mit Hilfe der Transitmethode nach jungen Exoplaneten im 4 Mio. Jahre jungen Sternhaufen Trumpler 37 gesucht. Die Region des Sternhaufens wurde dabei mit dem Teleskop des "Astrophysikalischen Instituts und Universitäts-Sternwarte" Jena, sowie anderen Teleskopen des YETI (Young Exoplanet Transit Initiative) Netzwerkes in 188 Nächten über einen Zeitraum von drei Jahren beobachtet. Aus den korrigierten Aufnahmen wurden für über 17000 Sterne (Mitgliedssterne, sowie Vorder- und Hintergrundsterne) die Lichtkurven extrahiert. Dabei kam differentielle Photometrie zum Einsatz. In den Lichtkurven wurde nach Periodizität sowie typischen Transitsignalen gesucht. Es wurden über 400 variable Sterne gefunden, darunter zwei transitartige. Für diese Sterne wurden Nachbeobachtungen durchgeführt um andere, falsch-positive Ursachen auszuschließen. Es zeigte sich, dass beide Signale von Doppelsternen verursacht wurden. Das eine System besteht aus einem sonnenartigen und einem sehr massearmen Stern, während es sich bei dem anderen System um zwei Sternen ähnlicher Größe und Leuchtkraft handelt, bei welchen sich jedoch ein weiterer Stern innerhalb der optischen Punktbidfunktion des Doppelsterns befindet. Im letzten Teil der Arbeit wurden die in Trumpler 37 gefundenen Doppelsterne spektroskopisch untersucht und die Lichtkurven und Radialgeschwindigkeitsdaten gemeinsam modelliert. Dadurch konnten unter anderem Massen und Radien der beiden Komponenten eines Systems bestimmt werden, welche im Anschluss mit verschiedenen Modellen zur Sternentstehung verglichen wurden

The pirate and the navy: Challenger brands and their utilization of counter-hegemonic ideology in identity communication

Challenger branding is a phenomenon that is gaining significant exposure in academia. However, most research on this branding approach relates to a practical or technical knowledge interest, ignoring the counter-hegemonic critique that exemplifies challenger branding. The critique that these brands base their identity on often shifts after they have been acquired into a hegemonic context, by joining a large corporation or becoming hegemonic brands themselves. This thesis aims to extend critical knowledge by connecting theories of ideology, hegemony, and brand communication to the identity construction of two challenger brands, The Body Shop and Innocent Drinks, and further explores the identity shift of the brands after their acquisition by L’Oréal and Coca-Cola. Counter-hegemony was found to be a pervasive tool activated through a number of signs for challenger identity construction. Further, challenger brand identity differed substantially before and after the challenger brands joined L’Oréal and Coca-Cola