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Klaus Bernhard

Publications and source records attributed to Klaus Bernhard.

At least 19 recordsLinked to original sources
arXiv2026

On the variability of the reflection nebula Van den Bergh 27 surrounding RY Tau

We present an analysis of the short- and long-term optical variability in Van den Bergh 27 (vdB 27), the reflection nebula surrounding RY Tau. The observed variations form a light-echo pattern, with apparent propagation speeds varying across different regions of the nebula and reaching up to approximately 3.6 c, consistent with geometric projection effects. The observed variations in nebular surface brightness are consistent with changes in illumination from the central star.

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arXiv2026

Chemically peculiar stars investigated by the BRITE Mission

We present a comprehensive analysis of BRITE photometry for 85 chemically peculiar stars, aimed at refining or determining their rotational periods. Utilizing a uniform Lomb-Scargle-based pipeline, we derived significant periods for 47 targets. A comparison with existing literature periods reveals generally good agreement, although several stars exhibit discrepant or previously unrecognized behavior. Notably, six targets display clear multiperiodicity, which, when combined with archival TESS data, suggests that these six candidates are likely misclassified, for example, as a magnetic CP2 or a CP4 star and instead exhibit characteristics consistent with a Be/shell star. Furthermore, eleven stars show no detectable periodic variations within the precision limits of BRITE. Our analysis demonstrates the effectiveness of long-term nanosatellite photometry, particularly when complemented by TESS data, in verifying catalogue periods, identifying multiperiodic behavior, and detecting potential misclassifications among bright CP stars.

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arXiv2025

GALEX UV survey of magnetic chemically peculiar stars in the Kepler prime field

Magnetic chemically peculiar (mCP) stars are strongly magnetic upper main-sequence stars that exhibit light rotational variability due to an uneven surface distribution of certain peculiar elements, which may appear in phase at certain wavelengths and in antiphase to the flux at other wavelengths. We present a study of the properties of photometric variability of a sample of confirmed mCP stars (mostly Ap/CP2 stars), mCP star candidates, and several non-CP stars in the near ultraviolet and visible wavelength regions based on observational data from the GALEX and Kepler prime missions. Antiphase variations between the near ultraviolet and optical light curves are observed in the majority of mCP stars. We investigate the presence of a correlation of the variability amplitudes in both wavelength regions with effective temperature, surface gravity, and metallicity and calculate model atmospheres, spectral energy distributions and synthetic light curves to connect our findings to theoretical models. While the theoretical calculations show that, at fixed abundances, a clear correlation between the light curve amplitude ratios and effective temperature is expected, our sample does not show any correlation with the investigated properties. This may be due to the highly individualistic abundance patterns of our sample stars, which are the main contributors to the line blanketing in different wavelength bands.

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arXiv2025

ZTF J172132.75+445851.0: A Possible New Cataclysmic Variable of the VY Sculptoris Type

We report the discovery of ZTF J172132.75+445851.0 as a new possible VY Sculptoris variable, with an orbital period of 0.109765426(44) days. Survey observations from 2005 to the present reveal significant activity in the system, with brightness variations ranging between 18.1 and 21.3 (zr) magnitudes, including deep fades exceeding an amplitude of 3 magnitudes.

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arXiv2024

ZTF J185259.31+124955.2: A new evolved disc-eclipsing binary system

Discs in long-period eclipsing binary systems are rare and can lead to extraordinary eclipsing events. ZTF J185259.31+124955.2 was identified as a candidate disc-eclipsing system through a continuing search programme of ZTF variables with a near-IR excess in the WISE data. Examination of the combined ZTF and ATLAS photometry shows seven eclipses since 2017 with depths of 0\fm34 in all bands on a period of $289.57\pm0.09$\,d. The eclipse width is $\sim 40$\,d but this and the profile evolve over time. Comparison with library spectra shows that the spectral energy distribution from the available photometry is consistent with an early K-type giant, and fitting black-body profiles suggests $T_{eff} \sim 4000$\,K for the stellar component, with a cool component having $T_{eff} < 500$\,K. The reddening and distance, and hence the luminosity place the star within the giant branch. The most likely scenario is that the system is in a state of rapid evolution following Case B/C mass transfer into an extended disc around an unseen companion.

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arXiv2023

High-velocity CP2 stars in the Galactic halo

Context. The second subclass of chemically peculiar stars, the CP2 stars, are early-type stars exhibiting anomalous abundances with variable line strengths possibly also accompanied by photometric variability that typically belong to the Galactic disk. However, a small fraction of these objects were recently found to be located far from the Galactic plane and are thought to belong to the Galactic halo, which is unexpected for this class of objects. Aims. Our study investigates the halo membership of the proposed halo CP2 star candidates based on their velocities and Galactic orbits, to determine their points of origin. In addition, we searched for further halo CP2 star candidates using an as-yet-unpublished sample of CP stars. Methods. Our analysis relied on the astrometry from the early third \textit{Gaia} data release and radial velocities based on the spectroscopy from LAMOST and SDSS, which was also employed in spectral classification. The light variability of the confirmed CP2 stars in our sample was analyzed using data from the ZTF and ATLAS surveys. Results. After filtering our initial sample using kinematic and spectroscopic criteria, we identified six CP2 stars with kinematical properties consistent with a halo membership. The orbits of these stars are in agreement with an origin in the Galactic disk where they were probably ejected through dynamical interactions or in the binary supernova scenario, making them the first runaway CP2 stars known.

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arXiv2022

Two Candidate KH 15D-like Systems from the Zwicky Transient Facility

KH 15D contains a circumbinary disk that is tilted relative to the orbital plane of the central binary. The precession of the disk and the orbital motion of the binary together produce rich phenomena in the photometric light curve. In this work, we present the discovery and preliminary analysis of two objects that resemble the key features of KH 15D from the Zwicky Transient Facility. These new objects, Bernhard-1 and Bernhard-2, show large-amplitude ($>1.5\,$mag), long-duration (more than tens of days), and periodic dimming events. A one-sided screen model is developed to model the photometric behaviour of these objects, the physical interpretation of which is a tilted, warped circumbinary disk occulting the inner binary. Changes in the object light curves suggest potential precession periods over timescales longer than 10 years. Additional photometric and spectroscopic observations are encouraged to better understand the nature of these interesting systems.

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arXiv2021

New magnetic chemically peculiar stars and candidates in the ATLAS First Catalog of Variable Stars

The number of known variable stars has increased by several magnitudes over the last decade, and automated classification routines are becoming increasingly important to cope with this development. Here we show that the "upside-down CBH variables", which were proposed as a potentially new class of variable stars by Heinze et al. (2018) in the ATLAS First Catalogue of Variable Stars, are, at least to a high percentage, made up of alpha2 Canum Venaticorum (ACV) variables - that is, photometrically variable magnetic chemically peculiar (CP2/He-peculiar) stars - with distinct double-wave light curves. Using suitable selection criteria, we identified 264 candidate ACV variables in the ATLAS variable star catalogue. 62 of these objects were spectroscopically confirmed with spectra from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (all new discoveries except for nine stars) and classified on the MK system. The other 202 stars are here presented as ACV star candidates that require spectroscopic confirmation. The vast majority of our sample of stars are main-sequence objects. Derived masses range from 1.4M(Sun) to 5M(Sun), with half our sample stars being situated in the range from 2 M(Sun) to 2.4 M(Sun), in good agreement with the spectral classifications. Most stars belong to the thin or thick disk; four objects, however, classify as members of the halo population. With a peak magnitude distribution at around 14th magnitude, the here presented stars are situated at the faint end of the known Galactic mCP star population. Our study highlights the need to consider rare variability classes, like ACV variables, in automated classification routines.

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arXiv2020

A plethora of new, magnetic chemically peculiar stars from LAMOST DR4

Magnetic chemically peculiar (mCP) stars are important to astrophysics because their complex atmospheres lend themselves perfectly to the investigation of the interplay between such diverse phenomena as atomic diffusion, magnetic fields, and stellar rotation. The present work is aimed at identifying new mCP stars using spectra collected by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). Suitable candidates were selected by searching LAMOST DR4 spectra for the presence of the characteristic 5200A flux depression. Spectral classification was carried out with a modified version of the MKCLASS code and the accuracy of the classifications was estimated by comparison with results from manual classification and the literature. Using parallax data and photometry from Gaia DR2, we investigated the space distribution of our sample stars and their properties in the colour-magnitude diagram. Our final sample consists of 1002 mCP stars, most of which are new discoveries (only 59 previously known). Traditional mCP star peculiarities have been identified in all but 36 stars, highlighting the efficiency of the code's peculiarity identification capabilities. The derived temperature and peculiarity types are in agreement with manually derived classifications and the literature. Our sample stars are between 100 Myr and 1 Gyr old, with the majority having masses between 2M(Sun) and 3M(Sun). Our results could be considered as strong evidence for an inhomogeneous age distribution among low-mass (M < 3M(Sun)) mCP stars. We identified several astrophysically interesting objects: two mCP stars have distances and kinematical properties in agreement with halo stars; an eclipsing binary system hosting an mCP star component; and an SB2 system likely comprising of an mCP star and a supergiant component.

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arXiv2020

High-Amplitude gamma Doradus Variables

According to most literature sources, the amplitude of the pulsational variability observed in gamma Doradus stars does not exceed 0.1 mag in Johnson V. We have analyzed fifteen high-amplitude gamma Doradus stars with photometric peak-to-peak amplitudes well beyond this limit, with the aim of unraveling the mechanisms behind the observed high amplitudes and investigating whether these objects are in any way physically distinct from their low-amplitude counterparts. We have calculated astrophysical parameters and investigated the location of the high-amplitude gamma Doradus stars and a control sample of fifteen low-amplitude objects in the log Teff versus log L diagram. Employing survey data and our own observations, we analyzed the photometric variability of our target stars using discrete Fourier transform. Correlations between the observed primary frequencies, amplitudes and other parameters like effective temperature and luminosity were investigated. The unusually high amplitudes of the high-amplitude gamma Doradus stars can be explained by the superposition of several base frequencies in interaction with their combination and overtone frequencies. Although the maximum amplitude of the primary frequencies does not exceed an amplitude of 0.1 mag, total light variability amplitudes of over 0.3 mag (V) can be attained in this way. Low- and high-amplitude gamma Doradus stars do not appear to be physically distinct in any other respect than their total variability amplitudes but merely represent two ends of the same, uniform group of variables.

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arXiv2020

New and improved rotational periods of magnetic CP stars from ASAS-3, KELT and MASCARA data

Magnetic chemically peculiar (mCP) stars allow the investigation of such diverse phenomena as atomic diffusion, magnetic fields and stellar rotation. The aim of the present investigation is to enhance our knowledge of the rotational properties of mCP stars by increasing the sample of objects with accurately determined rotational periods. To this end, archival photometric time-series data from the ASAS-3, KELT and MASCARA surveys were employed to improve existing rotational period information and derive rotational periods for mCP stars hitherto not known to be photometric variables. Our final sample consists of 294 mCP stars, a considerable amount of which (more than 40%) are presented here as photometric variables for the first time. In addition, we identified 24 mCP star candidates that show light variability in agreement with rotational modulation but lack spectroscopic confirmation. The rotational period distribution of our sample agrees well with the literature.Most stars are between 100Myr and 1Gyr old, with an apparent lack of very young stars. No objects were found on the zero age main sequence; several stars seem to have evolved to the subgiant stage, albeit well before the first dredge-up. We identified four eclipsing binaries (HD 244391, HD 247441, HD 248784 and HD 252519), which potentially host an mCP star. This is of great interest because mCP stars are very rarely found in close binary systems, particularly eclipsing ones. Using archival spectra, we find strong evidence that the HD 252519 system indeed harbours an mCP star component.

astro-ph.GA↗
arXiv2019

A revisit to the enigmatic variable star 21 Comae

The magnetic chemically peculiar (Ap/CP2) star 21 Com has been extensively studied in the past, albeit with widely differing and sometimes contradictory results, in particular concerning the occurrence of short term variability between about 5 to 90 minutes. We have performed a new investigation of 21 Com using MOST satellite and high-cadence ground-based photometry, time series spectroscopy, and evolutionary and pulsational modeling. Our analysis confirms that 21 Com is a classical CP2 star showing increased abundances of, in particular, Cr and Sr. From spectroscopic analysis, we have derived Teff = 8900(200) K, log g = 3.9(2), and vsini = 63(2) km/s. Our modeling efforts suggest that 21 Com is a main sequence (MS) star seen equator-on with a mass of 2.29(10) M(Sun) and a radius of R = 2.6(2) R(Sun). Our extensive photometric data confirm the existence of rotational light variability with a period of 2.05219(2) d. However, no significant frequencies with a semi-amplitude exceeding 0.2 mmag were found in the frequency range from 5 to 399 c/d. Our RV data also do not indicate short-term variability. We calculated pulsational models assuming different metallicities and ages, which do not predict the occurrence of unstable modes. The star 18 Com, often employed as comparison star for 21 Com in the past, has been identified as a periodic variable (P = 1.41645 d). While it is impossible to assess whether 21 Com has exhibited short-term variability in the past, the new observational data and several issues/inconsistencies identified in previous studies strongly suggest that 21 Com is neither a delta Scuti nor a roAp pulsator but a "well-behaved" CP2 star exhibiting its trademark rotational variability.

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arXiv2019

An overview of the properties of a sample of newly-identified magnetic chemically peculiar stars in the Kepler field

We present a comprehensive overview of the properties of a sample of 41 magnetic chemically peculiar stars that have been recently identified in the Kepler field by our team (Hümmerich et al. 2018). The stars populate the whole age range from zero-age to terminal-age main sequence in the mass interval from 1.5 to 4 Ms. Several of the studied objects exhibit a hitherto unobserved wealth of detail in their light curves indicative of persisting complex surface structures. Monoperiodic variability and light curve stability were identified as cardinal criteria for selecting mCP star candidates among early-type objects in photometric surveys. Subsequent studies will be concerned with an exhaustive follow-up analysis of the new mCP stars, which we expect to lead to new insights on the physics of the CP star phenomenon.

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arXiv2018

Interplay between pulsation, mass loss, and third dredge-up: More about Miras with and without technetium

We follow-up on a previous finding that AGB Mira variables containing the 3DUP indicator technetium (Tc) in their atmosphere form a different sequence of K-[22] colour as a function of pulsation period than Miras without Tc. A near- to mid-infrared colour such as K-[22] is a good probe for the dust mass-loss rate of the stars. Contrary to what might be expected, Tc-poor Miras show redder K-[22] colours (i.e. higher dust mass-loss rates) than Tc-rich Miras at a given period. Here, the previous sample is extended and the analysis is expanded towards other colours and dust spectra. The most important aim is to investigate if the same two sequences can be revealed in the gas mass-loss rate. We analysed new optical spectra and expanded the sample by including more stars from the literature. Near- and mid-IR photometry and ISO dust spectra of our stars were investigated. Literature data of gas mass-loss rates of Miras and semi-regular variables were collected and analysed. Our results show that Tc-poor Miras are redder than Tc-rich Miras in a broad range of the mid-IR, suggesting that the previous finding based on the K-[22] colour is not due to a specific dust feature in the 22 micron band. We establish a linear relation between K-[22] and the gas mass-loss rate. We also find that the 13 micron feature disappears above K-[22]~2.17 mag, corresponding to $\dot{M}_{\rm g}\sim2.6\times10^{-7}M_{\sun}yr^{-1}$. No similar sequences of Tc-poor and Tc-rich Miras in the gas mass-loss rate vs. period diagram are found, most probably owing to limitations in the available data. Different hypotheses to explain the observation of two sequences in the P vs. K-[22] diagram are discussed and tested, but so far none of them convincingly explains the observations. Nevertheless, we might have found an hitherto unknown but potentially important process influencing mass loss on the TP-AGB.

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arXiv2018

TYC 3637-1152-1 - a High Amplitude delta Scuti star with peculiar pulsational properties

In some delta Scuti stars, only one or two radial modes are excited (usually the fundamental mode and/or first overtone mode) and the observed peak-to-peak amplitudes exceed 0.3 mag (V). These stars are known as High Amplitude Delta Scuti (HADS) variables. We here present a detailed photometric and spectroscopic analysis of the HADS star TYC 3637-1152-1. We have derived a metallicity close to solar, a spectral type of F4V and an age of log t = 9.1. Employing archival time series data from different sources, two frequencies f0 =10.034 c/d and f1 =12.681 c/d and their harmonics and linear combinations were identified. The period ratio of f0/f1 = 0.791 puts this star into a peculiar position in the Petersen diagram, from which we conclude that TYC 3637-1152-1 is a unique object with peculiar pulsational properties that indicate a transitional state between HADS stars pulsating in the fundamental and first overtone modes and stars pulsating in higher overtones.

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arXiv2018

The Kepler view of magnetic chemically peculiar stars

Magnetic chemically peculiar (mCP) stars exhibit complex atmospheres that allow the investigation of such diverse phenomena as atomic diffusion, magnetic fields, and stellar rotation. The advent of space-based photometry provides the opportunity for the first precise characterizations of the photometric variability properties of these stars. We carried out a search for new mCP stars in the Kepler field with the ultimate aim of investigating their photometric variability properties using Kepler data. As an aside, we describe criteria for selecting mCP star candidates based on light curve properties, and assess the accuracy of the spectral classifications provided by the MKCLASS code. As only very few known mCP stars are situated in the Kepler field, we had to depend largely on alternative (nonspectroscopic) means of identifying suitable candidates that rely mostly on light curve properties; in particular we relied on monoperiodic variability and light curve stability. Newly acquired and archival spectra were used to confirm most of our mCP star candidates. Linear ephemeris parameters and effective amplitudes were derived from detrended Kepler data. Our final sample consists of 41 spectroscopically confirmed mCP stars of which 39 are new discoveries, 5 candidate mCP stars, and 7 stars in which no chemical peculiarities could be established. Our targets populate the whole age range from zero-age main sequence to terminal-age main sequence and are distributed in the mass interval from 1.5 M_sun to 4 M_sun. About 25% of the mCP stars show a hitherto unobserved wealth of detail in their light curves indicative of complex surface structures. We identified light curve stability as a primary criterion for identifying mCP star candidates among early-type stars in large photometric surveys, and prove the reliability of the spectral classifications provided by the MKCLASS code.

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arXiv2018

An investigation of the photometric variability of confirmed and candidate Galactic Be stars using ASAS-3 data

We present an investigation of a large sample of confirmed (N=233) and candidate (N=54) Galactic classical Be stars (mean V magnitude range of 6.4 to 12.6 mag), with the main aim of characterizing their photometric variability. Our sample stars were preselected among early-type variables using light curve morphology criteria. Spectroscopic information was gleaned from the literature, and archival and newly-acquired spectra. Photometric variability was analyzed using archival ASAS-3 time series data. To enable a comparison of results, we have largely adopted the methodology of Labadie-Bartz et al. (2017), who carried out a similar investigation based on KELT data. Complex photometric variations were established in most stars: outbursts on different time-scales (in 73$\pm$5% of stars), long-term variations (36$\pm$6%), periodic variations on intermediate time-scales (1$\pm$1%) and short-term periodic variations (6$\pm$3%). 24$\pm$6% of the outbursting stars exhibit (semi)periodic outbursts. We close the apparent void of rare outbursters reported by Labadie-Bartz et al. (2017) and show that Be stars with infrequent outbursts are not rare. While we do not find a significant difference in the percentage of stars showing outbursts among early-type, mid-type and late-type Be stars, we show that early-type Be stars exhibit much more frequent outbursts. We have measured rising and falling times for well-covered and well-defined outbursts. Nearly all outburst events are characterized by falling times that exceed the rising times. No differences were found between early-, mid- and late-type stars; a single non-linear function adequately describes the ratio of falling time to rising time across all spectral subtypes, with the ratio being larger for short events.

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