arXiv subjects

Nikolaus Vogt

Publications and source records attributed to Nikolaus Vogt.

14 recordsLinked to original sources

arXiv2025

Was there a (super)nova in 1408?

The 1408 CE "guest star" recorded in Chinese historical texts presents a compelling case for identifying a historical stellar transient. While previous studies debated its nature as a meteor, comet, or nova, we re-evaluate the event using original Ming Dynasty records, including a newly found memorial from the imperial court. The object, described as stationary for over ten days, yellow, and luminous (resembling a "Zhou Bo virtue star"), is inconsistent with cometary behavior. Positional analysis locates it near the Niandao asterism (modern Cygnus-Vulpecula region) within the Milky Way, with a derived brightness of -4 to 0 mag. Light-curve stability over ten days and color descriptions align with a slow nova or a supernova. We cross-correlated the historical coordinates with modern catalogs and found a few possible counterparts. Among them, CK Vul - a luminous red nova remnant from 1670 to 1672 - is the most interesting candidate. Could its progenitor system have experienced a precursor classical nova eruption circa 1408 prior to the merger ~200 years later? We also examine cataclysmic variables and planetary nebulae within the 100 square-degree search field, though most lack sufficient brightness or age characteristics. This study emphasizes the value of integrating detailed historical records with contemporary astrophysical data to resolve long-standing controversies over ancient transients. The 1408 event likely represents a rare, well-documented nova, offering insights into pre-modern stellar phenomena and their modern counterparts.

astro-ph.SR↗

arXiv2024

New orbital periods of high-inclination dwarf novae based on Gaia Alerts photometry

The orbital period of a cataclysmic variable stands as a crucial parameter for investigating the structure and physics of these binary systems, as well as understanding their evolution. We use photometric Gaia data for dwarf novae (DNe) in the quiescent state which are available for a number of years to determine new orbital periods and improve/modify previously suggested values. Two approaches are implemented for selecting high-inclination targets, either eclipsing or with ellipsoidal variations. We determine new orbital periods for 75 DNe and improve ephemerides for 27 more (three of which change significantly), contributing 9.4% of the known DNe periods of 0.05-2.0 days, and doubling the number of known periods exceeding 0.44 days. Their phase-folded light curves are presented and arranged by orbital period, illustrating the transition from short-period systems, dominated by radiation from the accretion disc and the hot spot, to longer-period DNe, where the Roche-lobe-filling secondary star is the primary visual flux source. This transition -- which occurs around the well-known period gap (around 2-3 hours) -- is expected, as DNe with larger orbital periods typically harbour more massive donors, which contribute to the visible flux. However, this transition is not abrupt. Within the same range of periods, we observe systems dominated by ellipsoidal variations, where the companion star is clearly visible, as well as others dominated by the disc and hot spot. The presence of some DNe with ellipsoidal variations near the lower edge of the period gap is striking, as the companions in these systems are expected to be cool low-mass M-dwarfs not visible in the light curve. This could indicate that we are observing systems where the donor star was originally much more massive and underwent significant nuclear evolution before mass-transfer began, as has been suggested previously for QZ Ser.

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arXiv2021

A Search for recurrent novae among Far Eastern guest stars

According to recent theoretical studies, classical novae are expected to erupt every ~$10^5$ years, while the recurrence time scale of modern recurrent novae (N_r) stars ranges from 10 to ~100 years. To bridge this huge gap in our knowledge (three orders of magnitude in time scales), it appears attractive to consider historical data: In Far Eastern sources, we searched for brightening events at different epochs but similar positions that possibly refer to recurrent nova eruptions. Probing a sample of ~185 Asian observations from ~500 BCE to 1700 CE, we present a method to systematically filter possible events. The result are a few search fields with between 2 and 5 flare ups and typical cadences between $10^2$ and $10^3$ years. For most of our recurrence candidates, we found possible counterparts among known cataclysmic variables in the corresponding search areas. This work is based on an interdisciplinary approach, combining methods from digital humanities and computational astrophysics when applying our previously developed methods in searches for classical novae among Far Eastern guest stars. A first and rather preliminary comparison of (possible) historical and (well known) modern recurrent novae reveals first tentative hints on some of their properties, stimulating further studies in this direction.

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arXiv2021

Discovery of a directly imaged planet to the young solar analog YSES 2

Abbreviated. By selecting stars with similar ages and masses, the Young Suns Exoplanet Survey (YSES) aims to detect and characterize planetary-mass companions to solar-type host stars in the Scorpius-Centaurus association. Our survey is carried out with VLT/SPHERE with short exposure sequences on the order of 5 min per star per filter. The subtraction of the stellar point spread function (PSF) is based on reference star differential imaging (RDI) using the other targets in the survey in combination with principal component analysis. We report the discovery of YSES 2b, a planetary-mass companion to the K1 star YSES 2 (TYC 8984-2245-1). The primary has a Gaia EDR3 distance of 110 pc, and we derive a revised mass of $1.1\,M_\odot$ and an age of approximately 14 Myr. We detect the companion in two observing epochs southwest of the star at a position angle of 205$^\circ$ and with a separation of $\sim1.05''$, which translates to a minimum physical separation of 115 au at the distance of the system. We derive a photometric planet mass of $6.3^{+1.6}_{-0.9}\,M_\mathrm{Jup}$ using AMES-COND and AMES-dusty evolutionary models; this mass corresponds to a mass ratio of $q=(0.5\pm0.1)$% with the primary. This is the lowest mass ratio of a direct imaging planet around a solar-type star to date. We discuss potential formation mechanisms and find that the current position of the planet is compatible with formation by disk gravitational instability, but its mass is lower than expected from numerical simulations. Formation via core accretion must have occurred closer to the star, yet we do not find evidence that supports the required outward migration, such as via scattering off another undiscovered companion in the system. YSES 2b is an ideal target for follow-up observations to further the understanding of the physical and chemical formation mechanisms of wide-orbit Jovian planets.

astro-ph.EP↗

arXiv2020

How many suns are in the sky? A SPHERE multiplicity survey of exoplanet host stars I -- Four new close stellar companions including a white dwarf

We are studying the influence of stellar multiplicity on exoplanet systems, in particular systems that have been detected via radial-velocity searches. We are in particular interested in the closest companions as they would have a strong influence on the evolution of the original planet forming disks. In this study we present new companions detected during our ongoing survey of exoplanet hosts with VLT/SPHERE. We are using the extreme adaptive optics imager SPHERE at the ESO/VLT to search for faint (sub)stellar companions. We utilized the classical coronagraphic imaging mode to perform a snapshot survey (3-6 min integration time) of exoplanet host stars in the Ks-band. We detected new stellar companions to the exoplanet host stars HD1666, HIP68468, HIP107773, and HD109271. With an angular separation of only 0.38" (40 au of projected separation) HIP107773 is among the closest companions found to exoplanet host stars. The presence of the stellar companion explains the linear radial-velocity trend seen in the system. At such a small separation the companion likely had significant influence on the evolution of the planet forming disk around the primary star. We find that the companion in the HD1666 system may well be responsible for the high orbit eccentricity (0.63) of the detected Jupiter class planet, making this system one of only a few where such a connection can be established. A cross-match with the Gaia DR2 catalog showed furthermore that the near infrared faint companion around HD109271 had been detected in the optical and is significantly brighter than in the near infrared making it a white dwarf companion.

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arXiv2020

A Search for the modern counterparts of the Far Eastern guest stars 369 CE, 386 CE, and 393 CE

In this study, we apply our previously developed method to investigate ancient transient sightings in order to derive consequences for modern astrophysical problems.We present case studies of three observations of so called 'guest stars' in the 4th century CE which lasted several months each. These three observations had been suggested and discussed as possible supernovae but slow novae are also viable alternatives. Our careful re-interpretation of the historical texts and the currently known objects in the given fields shed new light on this topic. In particular, for the two events in 386 and 393 CE we suggest possible supernova identifications, while in all three cases there are interesting candidates for past classical or recurrent nova eruptions among known cataclysmic variables (CVs) and/or symbiotic stars. The transient of 369, we suggest to explain as classical and possibly recurrent nova instead of a supernova. The most plausible candidates are BZ Cam, a CV with a possible nova shell, or CQ Dra, a naked eye multiple system perhaps able to permit an overwhelmingly bright nova with day-time visibility.

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arXiv2020

Counterparts of Far Eastern Guest Stars: Novae, supernovae, or something else?

Historical observations of transients are crucial for studies of their long-term evolution. This paper forms part of a series of papers in which we develop methods for the analysis of ancient data of transient events and their usability in modern science. Prior research on this subject by other authors has focused on looking for historical supernovae and our earlier work focused on cataclysmic binaries as classical novae. In this study we consider planetary nebulae, symbiotic stars, supernova remnants and pulsars in the search fields of our test sample. We present the possibilities for these object types to flare up visually, give a global overview on their distribution and discuss the objects in our search fields individually. To summarise our results, we provide a table of the most likely identifications of the historical sightings in our test sample and outline our method in order to apply it to further historical records in future works. Highlights of our results include a re-interpretation of two separate sightings as one supernova observation from May 667 to June 668 CE, the remnant of which could possibly be SNR G160.9+02.6. We also suggest the recurrent nova U Sco as a candidate for the appearance observed between Scorpius and Ophiuchus in 891, which could point towards a long-term variability of eruption amplitudes. In addition, we find that the `shiny bright' sighting in 1431 can be linked to the symbiotic binary KT Eri, which erupted as a naked eye classical nova in 2009.

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arXiv2020

Cataclysmic variables as possible counterparts of ancient Far Eastern guest stars

Continuing our efforts to select possible classical nova candidates among Far Eastern guest stars and to identify them with modern cataclysmic variables (CVs), we present a search for counterparts in 24 promising areas of the sky corresponding to ancient observations between 204 BCE and 1690 CE. These areas have been derived by us in a previous paper. Based on physical entities of the CVs in our areas and reasonable magnitude limits compatible with the distribution of known eruption amplitudes of telescopic classical novae, we present a catalogue of a total of 80 CVs and related targets which could possibly have caused the historical sightings. This list could potentially be reduced by additionally discussing further information more vaguely given in the text. In some cases, we present a detailed discussion of the interpretation of ancient sources confronting them with properties of the brightest CVs in the field. In order to estimate whether this list is representative, we discuss the distribution of CV types in our catalogue of counterparts for the historical events. Compared to the entire sky, the surface density of most CV subtypes in our search fields reveals similar values, except for polars and intermediate polars, i. e. strongly magnetic CVs, for which a significant excess in our search fields was detected. Finally, we give an outlook towards future research in this topic, and add in an Appendix a complete atlas of the celestial maps of all 24 guest star events, displaying the search areas and locations of CVs within them.

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arXiv2019

A new approach to generate a catalogue of potential historical novae

Ancient Chinese, Korean and Vietnamese observers left us records of celestial sightings, the so-called `guest stars' dated up to $\sim2500$ years ago. Their identification with modern observable targets could open interesting insights into the long-term behavior of astronomical objects, as shown by the successful identification of 8 galactic supernovae (SNe). Here we evaluate the possibility to identify ancient classical novae with presently known cataclysmic variables (CVs). For this purpose, we have developed a method which reconsiders in detail positions and sizes of ancient asterisms, in order to define areas on the sky that should be used for a search of modern counterparts. These areas range from a few to several hundred square degrees, depending on the details given in ancient texts; they should replace the single coordinate values given by previous authors. Any appropriate target (CVs, X-ray binaries etc.) within these areas can be considered as a valid candidate for identification with the corresponding ancient event. Based on the original descriptions of several hundred old events, we selected those without movement and without a tail (to exclude comets) and which did not only visible within a certain hour (to exclude meteors). This way, we present a shortlist of 24 most promising events which could refer to classical nova eruptions. Our method is checked by applying it to the known SN identifications, leading to a margin of error between 0 and 4.5 degrees, meaning that some SN remnants lay exactly inside the areas given by the historical reports while in some other cases they are laying at considerable distances.

astro-ph.IM↗

arXiv2019

On the possibilities of classical nova identifications among historical Far Eastern guest star observations

More than 100 guest star observations have been obtained by Chinese, Korean, Japanese and Vietnamese astronomers between ~600 BCE and ~1690 CE. Comparing the coordinates from the information given in old texts for eight supernova recoveries with modern supernova remnant positions, we estimate a typical positional accuracy of the order of 0.3 to 7 deg for these supernovae. These values turn out to be also a start for the expected deviation angle between a classical nova observed as a guest star and its modern counterpart among known cataclysmic variables (CVs). However, there are considerable disagreements among modern authors in the interpretation of ancient Far Eastern texts, emphasizing the need to consult again the original historic sources, in order to improve the positioning reliability. We also discuss the typical amplitudes of well observed classical novae and find that modern counterparts of nova guest stars should be V = 18 mag and thus easily observable. In this context we also consider the "hibernation scenario" and conclude that it is impossible to decide from currently-available observations whether hibernation is common. In addition to the limiting magnitude around 2 mag for ancient guest star detections mentioned in the literature, we consider the possibility that also fainter guest stars (4-5 mag) could have been detected by ancient observers and give arguments in favor of this possibility. For these limits we compare the expected nova detection rate of ancient naked-eye observers with that during modern times, and conclude that they coincide in order of magnitude, which implies that, indeed, a considerable number of classical nova remnants should be hidden among the Far Eastern guest star reports. Finally, we present a statistical analysis of the probability of casual misidentifications based on frequency and galactic distribution of CVs in the AAVSO-VSX catalogue.

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arXiv2018

Revival of an abandoned telescope: the Boller and Chivens Bochum 0.61-metre telescope of Universidad de Valparaiso

In 2015 the Institute of Physics and Astronomy of the Universidad de Valparaíso in Chile received as a donation the Bochum 0.61-meter telescope. Here we preset the ongoing project to convert this senior member of La Silla Observatory to modern standards aiming at performing state-of-art science, as well as teaching and outreach. Firstly, the site characterization was performed in order to verify the observing conditions. The preliminary results were auspicious in relation to the nights available for observation. In early 2016 began the transfer work form La Silla Observatory to the new site of operations. The actual status of the telescope was analyzed and an upgrade plan was proposed to make it usable remotely using a web-based telescope control system developed in Chile by ObsTech SpA. Future upgrade and scientific collaboration will be discussed based on the site characterization and technical studies regarding the potential for new instrumentation.

astro-ph.IM↗

arXiv2014

A direct imaging search for close stellar and sub-stellar companions to young nearby stars

A total of 28 young nearby stars (ages $\leq 60$,Myr) have been observed in the K$_{\rm s}$-band with the adaptive optics imager Naos-Conica of the Very Large Telescope at the Paranal Observatory in Chile. Among the targets are ten visual binaries and one triple system at distances between 10 and 130 pc, all previously known. During a first observing epoch a total of 20 faint stellar or sub-stellar companion-candidates were detected around seven of the targets. These fields, as well as most of the stellar binaries, were re-observed with the same instrument during a second epoch, about one year later. We present the astrometric observations of all binaries. Their analysis revealed that all stellar binaries are co-moving. In two cases (HD 119022 AB and FG Aqr B/C) indications for significant orbital motions were found. However, all sub-stellar companion-candidates turned out to be non-moving background objects except PZ Tel which is part of this project but whose results were published elsewhere. Detection limits were determined for all targets, and limiting masses were derived adopting three different age values; they turn out to be less than 10 Jupiter masses in most cases, well below the brown dwarf mass range. The fraction of stellar multiplicity and of the sub-stellar companion occurrence in the star forming regions in Chamaeleon are compared to the statistics of our search, and possible reasons for the observed differences are discussed.

astro-ph.SR↗

arXiv2011

The post-nova population

We here present our on-going project to unveil the post-nova population by re-discovering old novae that have been lost after the initial outburst and of which the binary itself is unobserved. We take UBVR photometry for the candidate selection, long-slit spectroscopy to confirm these candidates, and time-resolved spectroscopy to measure the orbital period of the newly confirmed post-novae. Some preliminary results are shown as examples.

astro-ph.SR↗

arXiv2008

Astrometric and photometric monitoring of GQ Lup and its sub-stellar companion

Neuhaeuser et al. (2005) presented direct imaging evidence for a sub-stellar companion to the young T Tauri star GQ Lup. Common proper motion was highly significant, but no orbital motion was detected. Faint luminosity, low gravity, and a late-M/early-L spectral type indicated that the companion is either a planet or a brown dwarf. We have monitored GQ Lup and its companion in order to detect orbital and parallactic motion and variability in its brightness. We also search for closer and fainter companions. We have taken six more images with the VLT Adaptive Optics instrument NACO from May 2005 to Feb 2007, always with the same calibration binary from Hipparcos for both astrometric and photometric calibration. By adding up all the images taken so far, we search for additional companions. The position of GQ Lup A and its companion compared to a nearby non-moving background object varies as expected for parallactic motion by about one pixel (2 πwith parallax π). We could not find evidence for variability of the GQ Lup companion in the K-band (standard deviation being \pm 0.08 mag), which may be due to large error bars. No additional companions are found with deep imaging. There is now exceedingly high significance for common proper motion of GQ Lup A and its companion. In addition, we see for the first time an indication for orbital motion (about 2 to 3 mas/yr decrease in separation, but no significant change in the position angle), consistent with a near edge-on or highly eccentric orbit. We measured the parallax for GQ Lup A to be π= 6.4 \pm 1.9 mas (i.e. 156 \pm 50 pc) and for the GQ Lup companion to be 7.2 \pm 2.1 mas (i.e. 139 \pm 45 pc), both consistent with being in the Lupus I cloud and bound to each other.

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