Stefan Hümmerich

A radio-pulsing white dwarf binary star.

White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a δ-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56-hour period close binary, pulsing in brightness on a period of 1.97 minutes. The pulses are so intense that AR Sco’s optical flux can increase by a factor of four within 30 seconds, and they are also detectable at radio frequencies. They reflect the spin of a magnetic white dwarf, which we find to be slowing down on a 107-year timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf’s spin, they mainly originate from the cool star. AR Sco’s broadband spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf’s magnetosphere.

A search for photometric variability in magnetic chemically peculiar stars using ASAS-3 data

The (magnetic) chemically peculiar (CP) stars of the upper main sequence are well-suited laboratories for investigating the influence of magnetic fields on the stellar surface because they produce abundance inhomogeneities (spots), which results in photometric variability that is explained in terms of the oblique rotator model. CP stars exhibiting this phenomenon are normally classified as alpha2 Canum Venaticorum (ACV) variables. It is important to increase the sample of known rotational periods among CP stars by discovering new ACV variables. The ASAS-3 data were cross-correlated with the Catalogue of Ap, HgMn, and Am stars in order to analyse the light curves of bona fide CP and related stars. The light curves were downloaded and cleaned of outliers and data points with a flag indicating bad quality. Promising candidates showing a larger scatter than observed for constant stars in the corresponding magnitude range were searched for periodic signals using a standard Fourier technique. In total, we found 323 variables, from which 246 are reported here for the first time, and 77 were probably wrongly classified before. The observed variability pattern of most stars is in accordance with an ACV classification. For some cases, it is difficult to distinguish between the light curves of double-waved ACVs and the variability induced by orbital motion (ellipsoidal variables/eclipsing variables), especially for objects exhibiting very small amplitudes and/or significant scatter in their light curves. Thus, some eclipsing or rotating ellipsoidal variables might be present. However, we are confident that the given periods are the correct ones. There seems to be a possible weak correlation between the rotational period and colour, in the sense that cooler magnetic CP stars rotate more slowly. However, this correlation seems to disappear when correcting for the interstellar reddening.

NEW PHOTOMETRICALLY VARIABLE MAGNETIC CHEMICALLY PECULIAR STARS IN THE ASAS-3 ARCHIVE

The magnetic Ap or CP2 stars are natural atomic and magnetic laboratories and ideal testing grounds for the evaluation of model atmospheres. CP2 stars exhibiting photometric variability are traditionally referred to as alpha2 Canum Venaticorum (ACV) variables. Strictly periodic changes are observed in the spectra and brightness of these stars, which allow the derivation of rotational periods. Related to this group of objects are the He-weak (CP4) and He-rich stars, some of which are also known to undergo brightness changes due to rotational modulation. Increasing the sample size of known rotational periods among CP2/4 stars is an important task, which will contribute to our understanding of these objects and their evolution in time. We have compiled an extensive target list of magnetic chemically peculiar (CP2/4) stars. In addition to that, a systematic investigation of early-type (spectral types B/A) variable stars of undetermined type in the International Variable Star Index of the AAVSO (VSX) yielded additional ACV candidates, which were included in our sample. We investigated our sample stars using publicly available observations from the ASAS-3 archive. We were able to identify another 360 stars exhibiting photometric variability in the accuracy limit of the ASAS-3 data, thereby concluding our search for photometrically variable magnetic chemically peculiar stars in the ASAS-3 archive. Summary data, folded light curves and, if available, information from the literature are presented for all variable stars of our sample, which is composed of 334 bona-fide ACV variables, 23 ACV candidates and three eclipsing binary systems. In particular, we call attention to HD 66051 (V414 Pup), which was identified as an eclipsing binary system showing obvious rotational modulation of the light curve due to the presence of an ACV variable in the system.

Magnetic, chemically peculiar (CP2) stars in the SuperWASP survey

The magnetic chemically peculiar (CP2) stars of the upper main sequence are well-suited for investigating the impact of magnetic fields on the surface layers of stars, which leads to abundance inhomogeneities (spots) resulting in photometric variability. The light changes are explained in terms of the oblique rotator model; the derived photometric periods thus correlate with the rotational periods of the stars. CP2 stars exhibiting this kind of variability are classified as alpha(2) Canum Venaticorum (ACV) variables. We have analysed around 3 850 000 individual photometric WASP measurements of magnetic chemically peculiar (CP2) stars and candidates selected from the catalogue of Ap, HgMn, and Am stars, with the ultimate goal of detecting new ACV variables. In total, we found 80 variables, from which 74 are reported here for the first time. The data allowed us to establish variability for 23 stars which had been reported as probably constant in the literature before. Light curve parameters were obtained for all stars by a least-squares fit with the fundamental sine wave and its first harmonic. Because of the scarcity of Stromgren uvby beta measurements and the lack of parallax measurements with an accuracy better than 20%, we are not able to give reliable astrophysical parameters for the investigated objects

HD 240121-An ACV variable showing anti-phase variations of the B and V light curves

Abstract The variability of HD 240121 = BD+59 2602 was first suspected by Sarg and Wramdemark (1970) and later confirmed by Grobel R. (1992a,b). Because of the observed anti-phase variations of the B and V light curves, the latter author tentatively suggested an ACV type. Apart from its inclusion in the catalog of New Suspected Variables (NSV 25977), no further investigations of the star have been published. HD 240121 was included into our target list of ACV candidates and investigated in order to determine the reason for the observed brightness variations. All available information on HD 240121 were collected via an exhaustive data mining procedure. Data from Grobel (1992a,b) were re-analysed and photometric observations from the NSVS and Hipparcos archives were procured and investigated. Line-of-sight reddening and stellar parameters were calculated from archival photometric data. HD 240121 is a young, late B-type CP2 star of the silicon subgroup. The observed period, amplitude of light variations and variability pattern (anti-phase variations) are typical of ACV variables. The occurrence of anti-phase variations of the B and V light curves is rarely observed and points to the existence of a null wavelength in the visual spectrum. We therefore strongly encourage further multi-colour photometric observations of this star.

HD 66051, an eclipsing binary hosting a highly peculiar, HgMn-related star

HD 66051 is an eclipsing system with an orbital period of about 4.75 d that exhibits out-of-eclipse variability with the same period. New multicolour photometric observations confirm the longevity of the secondary variations, which we interpret as a signature of surface inhomogeneities on one of the components. Using archival and newly acquired high-resolution spectra, we have performed a detailed abundance analysis. The primary component is a slowly rotating late B-type star (Teff = 12500 ± 200 K; log g = 4.0, v sin i = 27 ± 2 km s−1) with a highly peculiar composition reminiscent of the singular HgMn-related star HD 65949, which seems to be its closest analogue. Some light elements as He, C, Mg, Al are depleted, while Si and P are enhanced. Except for Ni, all the iron-group elements, as well as most of the heavy elements, and in particular the REE elements, are overabundant. The secondary component was estimated to be a slowly rotating A-type star (Teff ~ 8000 K; log g = 4.0, v sin i ~ 18 km s−1). The unique configuration of HD 66051 opens up intriguing possibilities for future research, which might eventually and significantly contribute to the understanding of such diverse phenomena as atmospheric structure, mass transfer, magnetic fields, photometric variability and the origin of chemical anomalies observed in HgMn stars and related objects.

A spectroscopic and photometric investigation of the mercury-manganese star KIC 6128830

The advent of space-based photometry provides the opportunity for the first precise characterizations of variability in mercury-manganese (HgMn/CP3) stars, which might advance our understanding of their internal structure. We have carried out a spectroscopic and photometric investigation of the candidate CP3 star KIC 6128830. A detailed abundance analysis based on newly acquired high-resolution spectra was performed, which confirms that the stars abundance pattern is fully consistent with its proposed classification. Photometric variability was investigated using 4 yr of archival Kepler data. In agreement with results from the literature, we have identified a single significant and independent frequency f(1) = 0.2065424 d(-1) with a peak-to-peak amplitude of similar to 3.4 mmag and harmonic frequencies up to 5f(1). Drawing on the predictions of state-of-the-art pulsation models and information on evolutionary status, we discuss the origin of the observed light changes. Our calculations predict the occurrence of g-mode pulsations at the observed variability frequency. On the other hand, the strictly mono-periodic nature of the variability strongly suggests a rotational origin. While we prefer the rotational explanation, the present data leave some uncertainty.

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 6.4-12.6 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 analysed 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 +/- 5 per cent of stars), long-term variations (36 +/- 6 per cent), periodic variations on intermediate time-scales (1 +/- 1 per cent), and short-term periodic variations (6 +/- 3 per cent). 24 +/- 6 per cent 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.

Orbital parameters and evolutionary status of the highly peculiar binary system HD 66051

Context. The spectroscopic binary system HD66051 (V414 Pup) consists of a highly peculiar CP3 (HgMn) star and an A-type component. It also shows out-of-eclipse variability that is due to chemical spots. This combination allows the derivation of tight constraints for the testing of time-dependent diffusion models. Aims. We aim at deriving astrophysical parameters, information on age, and an orbital solution of the system. Methods. We analysed radial velocity and photometric data using two different methods to determine astrophysical parameters and the orbit of the system. Appropriate isochrones were used to derive the age of the system. Results. The orbital solution and the estimates from the isochrones are in excellent agreement with the estimates from a prior spectroscopic study. The system is very close to the zero-age main sequence and younger than 120 Myr. Conclusions. HD66051 is a most important spectroscopic binary system that can be used to test the predictions of the diffusion theory explaining the peculiar surface abundances of CP3 stars.

NSV 1907 - a new eclipsing, nova-like cataclysmic variable

NSV 1907, formerly listed as an irregular variable in variability catalogues, was classified as an Algol-type eclipsing binary in the Catalina Surveys Periodic Variable Star Catalogue. We have identified NSV 1907 as an ultraviolet (UV) bright source using measurements from the GALEX space telescope and detected obvious out-of-eclipse variability in archival photometric data from the Catalina Sky Survey, which instigated a closer examination of the object. A spectrum and extensive multicolour photometric observations were acquired, from which we deduce that NSV 1907 is a deeply eclipsing, nova-like cataclysmic variable. Apart from the orbital variations (deep eclipses with a period of P ≈ 6.63 hours), changes in mean brightness and irregular short-term variability (flickering) were observed. The presence of a secondary minimum at phase φ ≈ 0.5 was established, which indicates a significant contribution of the companion star to the optical flux of the system. We find possible evidence for sinusoidal variations with a period of P ≈ 4.2 d, which we interpret as the nodal precession period of the accretion disc. No outbursts or VY Scl-like drops in brightness were detected either by the CSS or during our photometric monitoring. Because of its spectral characteristics and the observed variability pattern, we propose NSV 1907 as a new moderately bright long-period SW Sextantis star. Further photometric and spectroscopic observations are encouraged.