Ap Stars Research Articles

Chandra X-ray study confirms that the magnetic standard Ap star KQ Vel hosts a neutron star companion

Context. KQ Vel is a peculiar A0p star with a strong surface magnetic field of about 7.5 kG. It has a slow rotational period of nearly 8 years. Bailey et al. (A&A, 575, A115) detected a binary companion of uncertain nature and suggested that it might be a neutron star or a black hole. Aims. We analyze X-ray data obtained by the Chandra telescope to ascertain information about the stellar magnetic field and/or interaction between the star and its companion. Methods. We confirm previous X-ray detections of KQ Vel with a relatively high X-ray luminosity of 2 × 1030 erg s−1. The X-ray spectra suggest the presence of hot gas at > 20 MK and, possibly, of a nonthermal component. The X-ray light curves are variable, but data with better quality are needed to determine a periodicity, if any. Results. We interpret the X-ray spectra as a combination of two components: the nonthermal emission arising from the aurora on the A0p star, and the hot thermal plasma filling the extended shell that surrounds the “propelling” neutron star. Conclusions. We explore various alternatives, but a hybrid model involving the stellar magnetosphere along with a hot shell around the propelling neutron star seems most plausible. We speculate that KQ Vel was originally a triple system and that the Ap star is a merger product. We conclude that KQ Vel is an intermediate-mass binary consisting of a strongly magnetic main-sequence star and a neutron star.

Eleanor Margaret Burbidge

Eleanor Margaret Burbidge

Pulsations of the Rapidly Oscillating Ap Star KIC 10685175 Revisited by TESS

Abstract KIC 10685175 (TIC 264509538) was discovered to be a rapidly oscillating Ap star from Kepler long-cadence data using super-Nyquist frequency analysis. It was reobserved by the Transiting Exoplanet Survey Satellite (TESS) with 2 minute cadence data in Sectors 14 and 15. We analyzed the TESS light curves, finding that the previously determined frequency is a Nyquist alias. The revised pulsation frequency is 191.5151 ± 0.0005 day−1 (P = 7.52 minutes) and the rotation frequency is 0.32229 ± 0.00005 day−1 (P rot = 3.1028 days). The star is an oblique pulsator with a pulsation amplitude modulated by the rotation, reaching a maximum pulsation amplitude at the time of the rotational light minimum. The oblique pulsation generates a frequency quintuplet split by exactly the rotation frequency. The phases of sidelobes, the pulsation phase modulation, and a spherical harmonic decomposition all show this star to be pulsating in a distorted quadrupole mode. Following the oblique pulsator model, we calculated the rotation inclination i and magnetic oblique β of this star, which provide detailed information of pulsation geometry. The i and β derived by the best fit of pulsation amplitude and phase modulation through a theoretical model differ from those calculated for a pure quadrupole, indicating the existence of strong magnetic distortion. The model also predicts the polar magnetic field strength is as high as about 6 kG which is predicted to be observed in a high-resolution spectrum of this star.

The Large Far Ultraviolet Variations of α Cir A and α Cir B as Recorded by IUE and FUSE

Abstract Inspection of archival IUE spectra of the bright rapidly oscillating Ap star α Cir A reveal large flux variations from 1600 to 2000 Å over the 4.4790 day rotational period. At 1965 Å, where the flux reaches a maximum, the flux more than doubles between phases of maximum and minimum flux. These large flux variations are most likely caused by the passage of spots or belts of overabundant elements in the line of sight as the star rotates. The FUSE spectrum of α Cir B, the K5V dwarf close to α Cir A, contains several emission lines of O vi, C iii and C ii which are probably formed in the transition region and the chromosphere of this star. The O vi lines completely disappear over a timescale of a few days which probably reflects rotationaly modulated magnetic activity.

Rotational modulation and single g-mode pulsation in the B9pSi star HD 174356?

ABSTRACT Chemically peculiar (CP) stars of the upper main sequence are characterized by specific anomalies in the photospheric abundances of some chemical elements. The group of CP2 stars, which encompasses classical Ap and Bp stars, exhibits strictly periodic light, spectral, and spectropolarimetric variations that can be adequately explained by the model of a rigidly rotating star with persistent surface structures and a stable global magnetic field. Using observations from the Kepler K2 mission, we find that the B9pSi star HD 174356 displays a light curve variable in both amplitude and shape, which is not expected in a CP2 star. Employing archival and new photometric and spectroscopic observations, we carry out a detailed abundance analysis of HD 174356 and discuss its photometric and astrophysical properties in detail. We employ phenomenological modelling to decompose the light curve and the observed radial velocity variability. Our abundance analysis confirms that HD 174356 is a silicon-type CP2 star. No magnetic field stronger than 110 G was found. The star’s light curve can be interpreted as the sum of two independent strictly periodic signals with $P_1=4{_{.}^{\rm d}}043\, 55(5)$ and $P_2=2{_{.}^{\rm d}}111\, 69(3)$. The periods have remained stable over 17 yr of observations. In all spectra, HD 174356 appears to be single-lined. From the simulation of the variability characteristics and investigation of stars in the close angular vicinity, we put forth the hypothesis that the peculiar light variability of HD 174356 arises in a single star and is caused by rotational modulation due to surface abundance patches (P1) and g-mode pulsation (P2).

The discovery of lambda Bootis stars - the Southern Survey II

ABSTRACT The λ Boo stars are chemically peculiar A-type stars whose abundance anomalies are associated with the accretion of metal-poor material. We searched for λ Boo stars in the Southern hemisphere in a targeted spectroscopic survey of metal-weak and emission-line stars. Obtaining spectra for 308 stars and classifying them on the MK system, we found or co-discovered 24 new λ Boo stars. We also revised the classifications of 11 known λ Boo stars, one of which turned out to be a chemically normal rapid rotator. We show that stars previously classified in the literature as blue horizontal branch stars or emission-line A stars have a high probability of being λ Boo stars, although this conclusion is based on small-number statistics. Using WISE infrared fluxes, we searched our targets for infrared excesses that might be attributable to protoplanetary or debris discs as the source of the accreted material. Of the 34 λ Boo stars in our sample, 21 at various main-sequence ages have infrared excesses, confirming that not all λ Boo stars are young.

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

Context.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 most up-to-date catalogue of these objects was published a decade ago. Since then, no large scale spectroscopic surveys targeting this group of objects have been carried out. An increased sample size of mCP stars, however, is important for statistical studies.Aims.The present work is aimed at identifying new mCP stars using spectra collected by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST).Methods.Suitable candidates were selected by searching LAMOST DR4 spectra for the presence of the characteristic 5200 Å 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 fromGaiaDR2, we investigated the space distribution of our sample stars and their properties in the colour-magnitude diagram.Results.Our final sample consists of 1002 mCP stars, most of which are new discoveries (only 59 common entries with the Catalogue of Ap, HgMn and Am stars). 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⊙and 3M⊙. Our results could be considered as strong evidence for an inhomogeneous age distribution among low-mass (M< 3M⊙) mCP stars; however, we caution that our sample has not been selected on the basis of an unbiased, direct detection of a magnetic field. We identified several astrophysically interesting objects: the mCP stars LAMOST J122746.05+113635.3 and LAMOST J150331.87+093125.4 have distances and kinematical properties in agreement with halo stars; LAMOST J034306.74+495240.7 is an eclipsing binary system (Porb= 5.1435 ± 0.0012 d) hosting an mCP star component; and LAMOST J050146.85+383500.8 was found to be an SB2 system likely comprising of an mCP star and a supergiant component.Conclusions.With our work, we significantly increase the sample size of known Galactic mCP stars, paving the way for future in-depth statistical studies.

The Large Flux Variations of 17 Com A as Recorded by FUSE

Abstract Inspection of archival FUSE spectra of the Ap star 17 Com A (HD 108662) reveals large flux variations from 905 to 1187 Å over the 5.0808 days rotational period. These variations are of larger amplitude than those observed in the ultraviolet range with IUE from 1200 to 2000 Å. At 1161 Å, where the flux reaches a maximum, the variation of flux with rotational phase shows a double-wave structure similar to that of the Transiting Exoplanet Survey Satellite lightcurve. These large flux variations are most likely caused by the passage of spots or belts of overabundant elements in the line of sight as the star rotates The FUSE spectrum of 17 Com A contains mostly blends of low-excitation lines of Mn ii, Cr ii and Fe ii and interesting lines of C i, C iii, P ii, S ii and Cl i.

Erratum: The fifth main-sequence magnetic B-type star showing coherent radio emission: Is this really a rare phenomenon?

We report the discovery of intense, highly directional radio emission from the Bp star HD 35298, which we interpret as the consequence of Electron Cyclotron Maser Emission (ECME). The star was observed with the Giant Metrewave Radio Telescope near the rotational phases of both magnetic nulls in band 4 (550-750 MHz) and one of the nulls in band 5 (1060-1460 MHz). In band 4, we observed flux density enhancement in both circular polarizations near both magnetic nulls. The sequences of arrival of the left and right circularly polarized pulses are opposite near the two nulls. In band 5, we did not have circular polarization information and hence measured only the total intensity lightcurve, which also shows enhancement around the magnetic null. The observed sequence of the circular polarization signs in band 4, compared with the longitudinal magnetic field curve, is able to locate the hemisphere from which ECME arises. This observational evidence supports the scenario of ECME in the ordinary mode, arising in a magnetosphere shaped like an oblique dipole. HD 35298 is the most slowly rotating and most distant main sequence magnetic star from which ECME has been observed.

Long-period Ap stars discovered with TESS data

The TESS space mission’s primary goal is to search for exoplanets around bright, nearby stars. Because of the high-precision photometry required for the main mission, it is also producing superb data for asteroseismology, eclipsing binary stars, gyrochronology, indeed any field of stellar astronomy where the data are variable light curves. In this work we show that the TESS data are excellent for astrophysical inference from peculiar stars that show no variability. Ap stars have the strongest magnetic fields of any main-sequence star. Some Ap stars have also been shown to have rotation periods of months, years, decades, and even centuries. The astrophysical cause of their slow rotation – the braking mechanism – is not known with certainty. These stars are rare: there are currently about three dozen with known periods. Magnetic Ap stars have long-lived spots that allow precise determination of their rotation periods. We argue and show that most Ap stars with TESS data that show no low-frequency variability must have rotation periods longer than, at least, a TESS sector of 27 d. From this we find 60 Ap stars in the southern ecliptic hemisphere TESS data with no rotational variability, of which at most a few can be pole-on, and six likely have nearly aligned magnetic and rotation axes. Of the other 54, 31 were previously known to have long rotation periods or very low projected equatorial velocities, which proves our technique; 23 are new discoveries. These are now prime targets for long-term magnetic studies. We also find that 12 of the 54 (22%) long-period Ap stars are roAp stars, versus only 3% (29 out of 960) of the other Ap stars studied with TESS in Sectors 1–13, showing that the roAp phenomenon is correlated with rotation, although this correlation is not necessarily causal. In addition to probing rotation in Ap stars, these constant stars are also excellent targets to characterise the instrumental behaviour of the TESS cameras, as well as for the CHEOPS and PLATO missions. This work demonstrates astrophysical inference from nonvariable stars – we can get “something for nothing”.

Rotational Variability in the TESS Light Curve of 21 Com

Abstract We present an analysis of the TESS-based light curve of the chemically peculiar A-type (Ap) star 21 Com (HD 108945). We used the simple aperture photometry light curve with a time span of 27 days. A least-squares fit to the data with 10 harmonics yields a frequency of 0.48726 day−1, which corresponds to a rotational period of 2.05229 days. Furthermore, we find no evidence of any short-term variability beyond the pure rotational signature.

Longitudinal drift of Tayler instability eigenmodes as a possible explanation for super-slowly rotating Ap stars

Context. Rotation periods inferred from the magnetic variability of some Ap stars are incredibly long, exceeding ten years in some cases. An explanation for such slow rotation is lacking. Aims. This paper attempts to provide an explanation of the super-slow rotation of the magnetic and thermal patterns of Ap stars in terms of the longitudinal drift of the unstable disturbances of the kink-type (Tayler) instability of their internal magnetic field. Methods. The rates of drift and growth were computed for eigenmodes of Tayler instability using stellar parameters estimated from a structure model of an A star. The computations refer to the toroidal background magnetic field of varied strength. Results. The non-axisymmetric unstable disturbances drift in a counter-rotational direction in the co-rotating reference frame. The drift rate increases with the strength of the background field. For a field strength exceeding the (equipartition) value of equal Alfven and rotational velocities, the drift rate approaches the proper rotation rate of a star. The eigenmodes in an inertial frame show very slow rotation in this case. Patterns of magnetic and thermal disturbances of the slowly rotating eigenmodes are also computed. Conclusions. The counter-rotational drift of Tayler instability eigenmodes is a possible explanation for the observed phenomenon of super-slowly rotating Ap stars.

Evolutionary Status of the Ap Stars HD 110066 and HD 153882

We present the results of our determination of the fundamental atmospheric parameters for the magnetic chemically peculiar Ap stars HD 110066 (AX CVn) and HD 153882 (V451 Her). The determination of the atmospheric parameters ( $$T_{\textrm{eff}}$$ , $$\textrm{log}{g}$$ , $$R/R_{\odot}$$ , and $$L/L_{\odot}$$ ) is based on a self-consistent analysis of high-resolution spectra (ESPaDOnS with $$R=65\,000$$ ) and the observed flux distribution in a wide spectral range. We have derived the radial magnetic field components $$B_{r}=4015\pm 180$$ G for HD 110066 and $$B_{r}=3800\pm 200$$ G for HD 153882 from the observed line splitting and magnetic differential broadening. We have analyzed the chemical composition and the vertical distribution (stratification) of Fe and Cr in atmospheric depth, because the lines of these elements make the greatest contribution to the absorption and stratification affects the distribution of the outgoing flux. We have constructed model atmospheres and determined the evolutionary status of these stars by taking into account the anomalous chemical composition and stratification. The positions of HD 110066 and HD 153882 on the Hertzsprung–Russell diagram confirm the observed decrease in the magnetic field strength with stellar age for stars with masses greater than 2.5 $$M_{\odot}$$ .

The 10.5 year rotation period of the strongly magnetic rapidly oscillating Ap star HD 166473

How magnetic fields contribute to the differentiation of the rotation rates of the Ap stars and affect the occurrence of non-radial pulsation in some of them are important open questions. Valuable insight can be gained into these questions by studying some of the most extreme examples of the processes at play. The super-slowly rotating rapidly oscillating Ap (roAp) star HD 166473 is such an example. We performed the first accurate determination of its rotation period, Prot = (3836 ± 30) d, from the analysis of 56 measurements of the mean magnetic field modulus ⟨B⟩ based on high-resolution spectra acquired between 1992 and 2019 at various observatories and with various instrumental configurations. We complemented this analysis with the consideration of an inhomogeneous set of 21 determinations of the mean longitudinal magnetic field ⟨Bz⟩ spanning the same time interval. This makes HD 166473 one of only four Ap stars with a period longer than 10 years for which magnetic field measurements have been obtained over more than a full cycle. The variation curves of ⟨B⟩ and of ⟨Bz⟩ are well approximated by cosine waves. The magnetic field of HD 166473 only seems to deviate slightly from axisymmetry, but it definitely involves a considerable non-dipolar component. Among the stars with rotation periods longer than 1000 d for which magnetic field measurements with full phase coverage are available, HD 166473 has the strongest field. Its magnetic field is also one of the strongest known among roAp stars. Overall, the magnetic properties of HD 166473 do not seem fundamentally distinct from those of the faster-rotating Ap stars. However, considering as a group the eight Ap stars that have accurately determined periods longer than 1000 d and whose magnetic variations have been characterised over a full cycle suggests that the angles between their magnetic and rotation axes tend to be systematically large.

On the first δ Scuti-like pulsating Ap star in an eclipsing binary

HD99458 was recently discovered to be a first object of its kind: a short-period eclipsing binary hosts a spotted, chemically peculiar A-type primary star that is a delta Scuti pulsator, and an M-dwarf secondary. These phenomena should not co-exist at the same time. Based on new photometric data we find that the previously published period of the main pulsation mode is a Nyquist alias of the true period (0.036 d, and not 0.052 d) given by the Kepler sampling.

Open Cluster IC 1369 and Its Vicinity: Multicolor Photometry and Gaia DR2 Astrometry

Abstract The open cluster IC 1369, located in Cygnus close to the galactic equator at the 89.°6 longitude, and its vicinity are investigated by applying two-dimensional photometric classification of stars down to V = 19 mag measured in the Vilnius seven-color photometric system. Employing the Gaia DR2 coordinates, parallaxes, and proper motions, 143 cluster members are found having cluster membership probabilities ≥0.75. The hottest stars of the evolved main sequence are of spectral class B9. Distances to the cluster, determined by two methods based on the Gaia parallaxes, are 3280 and 3370 pc with an uncertainty of 400–500 pc. Employing the physical HR diagram with the coordinates versus and the Padova isochrones, the cluster age is found to be between 300 and 350 Myr. Among the cluster members we identified six red giants of spectral classes G5–G8 and luminosities II–III, 17 Am stars, one Ap star, and one B5 blue straggler. The extinction shows a steep rise up to ∼2.0 mag at 300–700 pc, caused by the extensions of the dust cloud LDN 970 and the Great Cygnus Rift, and up to ∼2.5 mag at the edge of the Perseus arm at 3.0 kpc. The extinction is almost constant between 0.9 and 2.5 kpc from the Sun. The average extinction of the cluster members is 2.54 mag. A systematic error of color excesses found in the Argonaut calculator results is discussed.

Rotational and pulsational variability in the TESS light curve of HD 27463

ABSTRACT The new photometric data on pulsating Ap star HD 27463 obtained recently with the Transiting Exoplanet Survey Satellite (TESS) are analysed to search for variability. Our analysis shows that HD 27463 exhibits two types of photometric variability. The low-frequency variability with the period P = 2.834 274 ± 0.000 008 d can be explained in terms of axial stellar rotation assuming the oblique magnetic rotator model and presence of surface abundance/brightness spots, while the detected high-frequency variations are characteristics of δ Scuti pulsations. From the analysis of Balmer line profiles visible in two FEROS spectra of HD 27463 we have derived its effective temperature and surface gravity, finding values that are close to those published for this star in the TESS Input Catalogue (TIC). Knowing the rotation period and the v sin i value estimated from the fitting of Balmer line profiles we found that the rotational axis is inclined to the line of sight with an angle of $i=33\pm 8\deg$. Our best-fitting model of the observed pulsation modes results in an overshoot parameter value fov = 0.014 and values of global stellar parameters that are in good agreement with the data reported in the TIC and with the data derived from fitting Balmer line profiles. This model indicates an age of 5.0 ± 0.4 × 108 yr, which corresponds to a core hydrogen fraction of 0.33.

Stellar mergers as the origin of magnetic massive stars.

About ten per cent of 'massive' stars (those of more than 1.5 solar masses) have strong, large-scale surface magnetic fields1-3. It has been suggested that merging of main-sequence and pre-main-sequence stars could produce such strong fields4,5, and the predicted fraction of merged massive stars is also about ten per cent6,7. The merger hypothesis is further supported by a lack of magnetic stars in close binaries8,9, which is as expected if mergers produce magnetic stars. Here we report three-dimensional magnetohydrodynamical simulations of the coalescence of two massive stars and follow the evolution of the merged product. Strong magnetic fields are produced in the simulations, and the merged star rejuvenates such that it appears younger and bluer than other coeval stars. This can explain the properties of the magnetic 'blue straggler' star τ Sco in the Upper Scorpius association that has an observationally inferred, apparent age of less than five million years, which is less than half the age of its birth association10. Such massive blue straggler stars seem likely to be progenitors of magnetars, perhaps giving rise to some of the enigmatic fast radio bursts observed11, and their supernovae may be affected by their strong magnetic fields12.

The fifth main-sequence magnetic B-type star showing coherent radio emission: Is this really a rare phenomenon?

ABSTRACT We report the discovery of intense, highly directional radio emission from the Bp star HD 35298, which we interpret as the consequence of Electron Cyclotron Maser Emission (ECME). The star was observed with the Giant Metrewave Radio Telescope near the rotational phases of both magnetic nulls in band 4 (550–750 MHz) and one of the nulls in band 5 (1060–1460 MHz). In band 4, we observed flux density enhancement in both circular polarizations near both magnetic nulls. The sequences of arrival of the left and right circularly polarized pulses are opposite near the two nulls. In band 5, we did not have circular polarization information and hence measured only the total intensity light curve, which also shows enhancement around the magnetic null. The observed sequence of the circular polarization signs in band 4, compared with the longitudinal magnetic field curve, is able to locate the hemisphere from which ECME arises. This observational evidence supports the scenario of ECME in the ordinary mode, arising in a magnetosphere shaped like an oblique dipole. HD 35298 is the most slowly rotating and most distant main-sequence magnetic star from which ECME has been observed.

HD 42659: the only known roAp star in a spectroscopic binary observed with B photometry, TESS, and SALT

ABSTRACT We present a multi-instrument analysis of the rapidly oscillating Ap (roAp) star HD 42659. We have obtained B photometric data for this star and use these data, in conjunction with the Transiting Exoplanet Survey Satellite (TESS) observations, to analyse the high-frequency pulsation in detail. We find a triplet that is split by the rotation frequency of the star (νrot = 0.3756 d−1; Prot = 2.66 d) and present both distorted dipole and distorted quadrupole mode models. We show that the pulsation frequency, 150.9898 d−1 (Ppuls = 9.54 min), is greater than the acoustic cut-off frequency. We utilize 27 high-resolution ($R\simeq 65\, 000$), high signal-to-noise ratio (∼120) spectra to provide new orbital parameters for this, the only known roAp star to be in a short-period binary (Porb = 93.266 d). We find the system to be more eccentric than previously thought, with e = 0.317, and suggest the companion is a mid-F to early-K star. We find no significant trend in the average pulsation mode amplitude with time, as measured by TESS, implying that the companion does not have an effect on the pulsation in this roAp star. We suggest further photometric observations of this star, and further studies to find more roAp stars in close binaries to characterize how binarity may affect the detection of roAp pulsations.