Ap Stars Research Articles

Absolute Calibration. III. Improved Absolute Calibration for the Visible through the Mid-infrared

We propose Sirius as an improved zero-point-defining star and calibrate its spectrum to an accuracy of ∼0.6% in both the visible and infrared. This result is based on a newly derived independent calibration in the visible of similar accuracy to the previous standard one, with which it is combined. We use a large variety of approaches in the infrared to reach about three times smaller error than for previous absolute calibrations. The results in the two wavelength regimes are in agreement, providing a consistent link from the visible throughout the near- and mid-infrared. The Sirius-based zero-point at 5557.5 Å (in vacuum) is 13.436 ± 0.081 × 10−12 W cm−2 μm−1, based on the improved value for Vega of 3.473 ± 0.018 × 10−12 W cm−2 μm−1 and the measured magnitude difference between the two stars. At 2.1603 μm, the zero-point is 4.225 ± 0.025 × 10−14 W cm−2 μm−1 taking Sirius at a magnitude of −1.395. A jackknife analysis indicates that there are no serious systematic errors in these results. We consider selection of secondary standards that can extend the calibration over the sky. Despite more than a century in this role, normal A-stars are not suitable, although Am and Ap stars may be. G-stars older than ∼1 Gyr are good candidates if accurate temperatures can be measured. White dwarfs are suitable from the visible through the near-infrared, but their properties are unexplored at the necessary level at the longer infrared wavelengths, and for most facilities they are too faint there. Finally, as a further test of the calibration, we demonstrate an upgraded infrared flux method to determine accurate stellar diameters from K-band photometry.

HD 213258: A new rapidly oscillating, super slowly rotating, strongly magnetic Ap star in a spectroscopic binary

We report on HD 213258, an Ap star that we recently identified as presenting a unique combination of rare, remarkable properties. Our study of this star is based on ESPaDOnS Stokes I and V data obtained at seven epochs spanning a time interval slightly shorter than 2 yr, on TESS data, and on radial velocity measurements from the CORAVEL data base. We confirm that HD 213258, which was originally suspected to be an F str λ 4077 star, is definitely an Ap star. We found that, in its spectrum, the Fe IIλ 6149.2 Å line is resolved into its two magnetically split components. The mean magnetic field modulus of HD 213258, ⟨B⟩∼3.8 kG, which we determined from this splitting, does not show significant variations over ∼2 yr. Comparing our mean longitudinal field determinations with a couple of measurements from the literature, we show that the stellar rotation period is likely of the order of 50 yr, with a reversal of the field polarity. Moreover, HD 213258 is a rapidly oscillating Ap (roAp) star, in which high overtone pulsations with a period of 7.58 min are detected. Finally, we confirm that HD 213258 has a mean radial velocity exceeding (in absolute value) that of at least 99% of Ap stars. The radial velocity shows low amplitude variations, which suggests that the star is a single-line spectroscopic binary. It is also a known astrometric binary. While its orbital elements remain to be determined, its orbital period is likely one of the shortest known for a binary roAp star. Its secondary is close to the borderline between stellar and substellar objects. There is a significant probability that it is a brown dwarf. The pair represents a combination that has never been observed before. Most of the above-mentioned properties taken in isolation are only observed in a small fraction of the whole population of Ap stars. Thus, the probability that a single star possesses all of them is extremely low. This makes HD 213258 an exceptionally interesting object that deserves to be studied in detail in the future.

An Ap Star Catalog Based on LAMOST DR9

We present a sample of 2700 Ap stars in the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) DR9 catalog. The candidates are first selected to be in a temperature range typical of Ap stars by using the BP − RP color index from Gaia DR3. Then the 5200 Å flux depression features characteristic of Ap stars are visually checked in LAMOST DR9 spectra. The detailed spectral features are given by applying a modified spectral classification program, MKCLASS. The stellar parameters of these Ap stars, such as the T eff, , [Fe/H], [Si/H], and , are either extracted from a hot star catalog or derived through empirical relations and then a statistical analysis is carried out. The evolutionary stages are also discussed. Finally, we discuss the rotation and pulsation features of those that have TESS or Kepler light curves. Among these Ap stars, we find seven new rotation variables, one new roAp star, and a new δ Scuti pulsation of a previously known roAp star.

Diffusion velocity of metals in Ap star atmospheres: the effect of ambipolar diffusion of H

ABSTRACT Ambipolar diffusion of hydrogen is effective in the atmosphere of stars when hydrogen is partially ionized. This is precisely the case for Ap stars whose atmospheres are stable enough to allow atomic diffusion in building metals stratification. Now, ambipolar diffusion of H induces an additional term to the diffusion velocity usually considered for metals. For some chemical elements, this may significantly affect the diffusion flux, and so, their abundance distribution. This effect decreases for high effective temperatures since protons density becomes very dominant compared to neutral H (that is partial ionization of H may be neglected), and so, this process has been neglected in recent models of abundance distributions in Bp stars. In this study, we evaluate for many metals the role of ambipolar diffusion in view of future detailed modelling of atomic diffusion for chemically peculiar stars with $T_{\rm eff}\lesssim 12\, 000$ K. We discuss a possible effect that we name the ambipolar flooding.

Pulsation of chemically peculiar stars

AbstractThis paper reviews our current knowledge about pulsating chemically peculiar (CP) stars. CP stars are slowly rotating upper main-sequence objects, efficiently employing diffusion in their atmospheres. They can be divided into magnetic and non-magnetic objects. Magnetic activity significantly influence their pulsational characteristics. Only a handful of magnetic, classical pulsating objects are now known. The only exceptions are about 70 rapidly oscillating Ap stars, which seem to be located within a very tight astrophysical parameter space. Still, many observational and theoretical efforts are needed to understand all important physical aspects and their interrelationships. The most important steps to reach these goals are reviewed.

Stability of axially symmetric magnetic fields in stars

Abstract The magnetic fields observed in Ap stars, white dwarfs, and neutron stars are known to be stable for long times. However, the physical conditions inside the stellar interiors that allow these states are still a matter of research. It has been formally demonstrated that both purely toroidal and purely poloidal magnetic fields develop instabilities at some point in the star. On the other hand, numerical simulations have proved the stability of roughly axisymmetric magnetic field configurations inside stably stratified stars. These configurations consist of mutually stabilizing toroidal and poloidal components in a twisted torus shape. Previous studies have proposed rough upper and lower bounds on the ratio of the magnetic energy in the toroidal and poloidal components of the magnetic field. With the purpose of mapping out the parameter space under which such configurations remain stable, we used the Pencil Code to perform 3D magnetohydrodynamic simulations of the evolution of the magnetic field in non-rotating, non-degenerate stars in which viscosity is the only dissipation mechanism, both for stars with a uniform (barotropic) and radially increasing (stably stratified) specific entropy. Furthermore, we considered different conditions regarding the degree of stable stratification and the magnetic energy in each component, roughly confirming the previously suggested stability boundaries for the magnetic field.

Jet from the enigmatic high-latitude star BP Psc and evolutionary status of its driving source

ABSTRACT BP Psc is an active late-type (sp:G9) star with unclear evolutionary status lying at high-galactic latitude b = −57°. It is also the source of the well collimated bipolar jet. We present results of the proper motion and radial velocity study of BP Psc outflow based on the archival Hα imaging with the GMOS camera at 8.1-m Gemini-North telescope as well as recent imaging and long-slit spectroscopy with the SCORPIO multimode focal reducer at 6-m BTA telescope of SAO RAS. The 3D kinematics of the jet revealed the full spatial velocity up to ∼140 km s−1 and allows us to estimate the distance to BP Psc system as D = 135 ± 40 pc. This distance leads to an estimation of the central source luminosity L* ≈ 1.2 L⊙, indicating that it is the ≈1.3M⊙ T Tauri star with an age t ≲ 7 Myrs. We measured the electron density of order Ne ∼ 102 cm−3 and mean ionization fraction f ≈ 0.04 within the jet knots and estimated upper limit of the mass-loss rate in NE lobe as $\dot{M}_{\rm out}\approx 1.2\cdot 10^{-8}{\rm M}_{\odot }\,{\rm yr}^{-1}$. The physical characteristics of the outflow are typical for the low-excitation YSO jets and consistent with the magnetocentrifugal mechanism of its launching and collimation. Prominent wiggling pattern revealed in Hα images allowed us to suppose the existence of a secondary substellar companion in a non-coplanar orbit and estimate its most plausible mass as Mp ≈ 30MJup. We conclude that BP Psc is one of the closest to the Sun young jet-driving systems and its origin is possibly related to the episode of star formation triggered by expanding supershells in second Galactic quadrant.

Rotation of hot normal, peculiar and Be stars from space photometry

ABSTRACT The periods of 34 376 main-sequence stars hotter than 6000 K are derived from Kepler, K2, and TESS light curves. From the effective temperatures and GAIA DR3 luminosities, the radii, and hence the equatorial rotational velocities, can be estimated. Comparison with the projected rotational velocities shows that the photometric periods are indistinguishable from the rotational periods. Rotational light modulation in cool stars appears to continue to the hottest B stars. In fact, it is the most common type of light variation and is seen in 20–50 per cent of F, A, and B stars. The rotation rates of Am, Ap, and Bp stars do not appear to be very different from those of non-peculiar stars. Mixing of the surface layers by rotation cannot be the sole reason why element diffusion is not present in stars with normal surface abundances. The rotation rate of Be stars is well below the rate required to trigger mass-loss by non-radial pulsation. Surface activity leading to ejection of material into co-rotating clouds is suggested as the mass-loss mechanism. The rotational amplitudes of non-Be stars increases rapidly with effective temperature, possibly due to an increasing proportion of incipient Be stars in the sample. The simplest way to explain these results is to assume that surface convection is present in all main-sequence stars, from the coolest M star to the hottest B star.

Magnetic field measurements of sharp-lined Ap stars

ABSTRACT Previous observations suggested that Ap and Bp stars exhibit a bimodal distribution of surface magnetic field strengths and that actually only few or no stars exist with magnetic dipole field strengths below 300 G down to a few Gauss. As the number of Ap and Bp stars currently known to possess weak magnetic fields is not large, it is necessary to carry out additional spectropolarimetric studies of Ap and Bp stars to prove whether the assumption of the existence of a critical value for the stability of magnetic fields is realistic. In this study, we present high-resolution HARPSpol magnetic field measurements for a sample of Ap stars with sharp spectral lines with a view to characterize the strengths of their magnetic fields. Out of the studied seven sharp-lined stars, two stars, HD 174779 and HD 203932, exhibit a rather weak longitudinal magnetic field with <Bz > = − 45 ± 3 G and <Bz > =21 ± 4 G, respectively. Additionally, TESS observations were used to test previous conclusions on the differentiation of rotation periods of Ap and Bp stars. Apart from HD 189832 and HD 203932, all other studied sharp-lined stars have long rotation periods. Since an explanation for the slow rotation of Ap stars is currently missing, additional studies of slowly rotating Ap and Bp stars are necessary to improve our understanding of the formation and evolution of Ap and Bp stars.

Spectroscopic study of five SB1 stars with CP components

ABSTRACT In this paper, we present the results of a spectroscopic campaign devoted to ascertaining the actual nature of a sample of five objects reported as chemically peculiar stars in the Catalogue of Ap, HgMn and Am stars (2009, A&A, 498, 961). Spectroscopic observations were obtained with Catania Astrophysical Observatory Spectropolarimeter. For each of the objects, we derive the effective temperature, gravity, rotational and radial velocities, and chemical abundances using the spectral synthesis method. All the targets were found to be SB1 systems. Our abundance analysis led us to classify HD 40788 and HD 187959 as marginal Am stars and HD 202431 as an Am star, and to confirm HD 134793 as an SrEuCr Ap star and, finally, HD 189652 as a normal star. Finally, we confirm that Am stars can possibly be used as an accurate tool to date stellar populations in galaxies.

Discovery and origin of the radio emission from the multiple stellar system KQ Vel

ABSTRACT KQ Vel is a binary system composed of a slowly rotating magnetic Ap star with a companion of unknown nature. In this paper, we report the detection of its radio emission. We conducted a multifrequency radio campaign using the ATCA interferometer (band-names: 16 cm, 4 cm, and 15 mm). The target was detected in all bands. The most obvious explanation for the radio emission is that it originates in the magnetosphere of the Ap star, but this is shown unfeasible. The known stellar parameters of the Ap star enable us to exploit the scaling relationship for non-thermal gyro-synchrotron emission from early-type magnetic stars. This is a general relation demonstrating how radio emission from stars with centrifugal magnetospheres is supported by rotation. Using KQ Vel’s parameters the predicted radio luminosity is more than five orders of magnitudes lower than the measured one. The extremely long rotation period rules out the Ap star as the source of the observed radio emission. Other possible explanations for the radio emission from KQ Vel, involving its unknown companion, have been explored. A scenario that matches the observed features (i.e. radio luminosity and spectrum, correlation to X-rays) is a hierarchical stellar system, where the possible companion of the magnetic star is a close binary (possibly of RS CVn type) with at least one magnetically active late-type star. To be compatible with the total mass of the system, the last scenario places strong constraints on the orbital inclination of the KQ Vel stellar system.

The magnetic field of the chemically peculiar star V352 Peg

ABSTRACTWe present a spectropolarimetric analysis of the hot star V352 Peg. We have acquired 18 spectropolarimetric observations of the star with ESPaDOnS at the CFHT between 2018 and 2019 and completed our data set with one archival ESPaDOnS measurement obtained in 2011. Our analysis of the spectra shows that the star is on the main sequence and chemically peculiar, i.e. it is a Bp star, with overabundances of iron peak elements (Ti, Cr, and Fe) and underabundance of He and O. Through a Least-Square Deconvolution of each spectrum, we extracted the mean Zeeman signature and mean line profile of thousands of spectral lines and detected a magnetic field in V352 Peg. By modelling the Stokes I and V profiles and using the Oblique Rotator Model, we determined the geometrical configuration of V352 Peg. We also performed Zeeman–Doppler Imaging (ZDI) to provide a more detailed characterization of the magnetic field of V352 Peg and its surface chemical distributions. We find a magnetic field that is mainly dipolar, dominantly poloidal, and largely non-axisymmetric with a dipole field strength of ∼9 kG and a magnetic axis almost perpendicular to the rotation axis. The strong variability of Stokes I profiles also suggests the presence of chemical spots at the stellar surface.

The Behavior of the Bp Star 36 Lynxis in the Far-UV

The analysis of twenty-four high resolution SWP spectra of the magnetic Bp star HD 79158 (36 Lynxis) recorded by the International Ultraviolet Explorer (IUE) reveals that the far-ultraviolet flux of this star mostly remained constant over the various observations with IUE. Only a few spectra show less flux shortwards of 1600 Å and enhanced line absorption compared to the mean spectrum, which could be interpreted as non uniform distribution of certain chemical elements over the surface of 36 Lynxis. The TESS lightcurve of HD 79158 reveals a complex structure with an estimated rotational period of about 3.832 ± 0.019 days, close to previous determinations.

Problems with Root-Mean-Square Values of the Magnetic Field of Ap Stars. I.

Problems with Root-Mean-Square Values of the Magnetic Field of Ap Stars. I.

Long-period Ap stars discovered with TESS data: The northern ecliptic hemisphere

The rotation periods of the magnetic Ap stars span five to six orders of magnitude. While it is well established that period differentiation must have taken place at the pre-main sequence stage, the physical processes that lead to it remain elusive. The existence of Ap stars that have rotation periods of tens to hundreds of years is particularly intriguing, and their study represents a promising avenue to gain additional insight into the origin and evolution of rotation in Ap stars. Historically, almost all the longest period Ap stars known have been found to be strongly magnetic; very few weakly magnetic Ap stars with very long periods have been identified and studied. To remedy that, we showed how a systematic search based on the analysis of TESS photometric data could be performed to identify super-slowly rotating Ap (ssrAp) stars independently of the strengths of their magnetic fields, with the intention to characterise the distribution of the longest Ap star rotation periods in an unbiased manner. We successfully applied this method to the analysis of the TESS 2-min cadence observations of Ap stars of the southern ecliptic hemisphere. For our present study, we applied the same approach to the analysis of the TESS 2-min cadence observations of Ap stars of the northern ecliptic hemisphere. We confirm that the technique leads to the reliable identification of ssrAp star candidates in an unbiased manner. We find 67 Ap stars with no rotational variability in the northern ecliptic hemisphere TESS data. Among them, 46 are newly identified ssrAp star candidates, which is double the number found in the southern ecliptic hemisphere. We confirm that super-slow rotation tends to occur less frequently in weakly magnetic Ap stars than in strongly magnetic stars. We present new evidence of the existence of a gap between ∼2 kG and ∼3 kG in the distribution of the magnetic field strengths of long period Ap stars. We also confirm that the incidence of roAp stars is higher than average in slowly rotating Ap stars. We report the unexpected discovery of nine definite and five candidate δ Sct stars, and of two eclipsing binaries. This work paves the way for a systematic, unbiased study of the longest period Ap stars, with a view to characterise the correlations between their rotational, magnetic, and pulsational properties.

Asteroseismology of the double-radial mode δ Scuti star BP Pegasi

ABSTRACT Using the All-Sky Automated Survey data, we determine the pulsational frequencies of the high-amplitude δ Scuti star BP Pegasi. The analysis revealed only the two known, independent frequencies that we use to perform the seismic analysis of the star. On the basis of multicolour Strömgren photometry, we independently find that both frequencies can only be associated with radial modes which, according to the frequency ratio, are fundamental and first overtone modes. The models fitting the two frequencies depend strongly on the opacity data. For low values of the mixing-length parameter αMLT ≈ 0.5, only the OPAL seismic models in the post-main-sequence phase of evolution are caught within the observed error box. Seismic models computed with the OP and OPLIB data are much less luminous and cooler. They can only reach the error box if we increase the convection efficiency to at least αMLT = 2.0. Then, including the non-adiabatic parameter f into our seismic modelling, we constrain the value of αMLT. Computing an extensive grid of seismic OPAL models and employing Monte Carlo-based Bayesian analysis, we obtain constraints on various parameters. In particular, the efficiency of envelope convection can be parametrized by αMLT ∈ (0.5, 1.0), depending on the atmospheric microturbulent velocity that amounts to ξt = 4 or 8 km s−1.

First detection of a magnetic field in low-luminosity B[e] stars

We report the first detection of the magnetic field in a star of FS CMa type, a subgroup of objects characterized by the B[e] phenomenon. The split of magnetically sensitive lines in IRAS 17449+2320 determines the magnetic field modulus of 6.2 ± 0.2 kG. Spectral lines and their variability reveal the presence of a B-type spectrum and a hot continuum source in the visible. The hot source confirms GALEX UV photometry. Because there is a lack of spectral lines for the hot source in the visible, the spectral fitting gives only the lower temperature limit of the hot source, which is 50 000 K, and the upper limit for the B-type star of 11 100 K. The V∕R ratio of the Hα line shows quasiperiodic behavior on timescale of 800 days. We detected a strong red-shifted absorption in the wings of Balmer and O I lines in some of the spectra. The absorption lines of helium and other metals show no, or very small, variations, indicating unusually stable photospheric regions for FS CMa stars. We detected two events of material infall, which were revealed to be discrete absorption components of resonance lines. The discovery of the strong magnetic field together with the Gaia measurements of the proper motion show that the most probable nature of this star is that of a post-merger object created after the binary left its birth cluster. Another possible scenario is a magnetic Ap star around Terminal-Age Main Sequence. On the other hand, the strong magnetic field defies the hypothesis that IRAS 17449+2320 is an extreme classical Be star. Thus, IRAS 17449+2320 provides a pretext for exploring a new explanation of the nature of FS CMa stars or, at least, a group of stars with very similar spectral properties.

New Determinations of the Rotational Periods of 78 Vir (A2p SrCrEu) and 52 Her (A1p SiSrCr)

Abstract The TESS lightcurves of the two cool Ap stars, 78 Vir and 52 Her, are used to derive new rotational periods for both stars. A frequency analysis of the light curve of 52 Her with the Generalized Lomb–Scargle periodogram reveals a periodic structure with three harmonics. The inverse of the fundamental frequency yields a rotational period of 3.840 ± 0.037 days for 52 Her. The same analysis for 78 Vir yields two harmonics with a rotation period of 3.718 ± 0.037 days, only slightly shorter than previous determinations.

Magnetic field topology, chemical spot distributions, and photometric variability of the Ap star φ Draconis

ABSTRACT The primary component of the multiple star φ Dra is one of the brightest magnetic chemically peculiar stars in the northern sky. Here, we report results of a comprehensive study of the rotational photometric variability, binarity, magnetic field geometry, and surface chemical spot structure for this star. We derived a precise photometric rotational period of 1.71650213(21) d based on 1 yr of TESS nearly continuous space observations and discovered modulation of the stellar light curve with the phase of the 127.9-d binary orbit due to the light time travel effect. We revised parameters of the binary orbit and detected spectroscopic contribution of the secondary. A tomographic mapping technique was applied to the average intensity and circular polarization profiles derived from Narval high-resolution spectropolarimetric observations. This analysis yielded a detailed map of the global magnetic field topology together with the surface distributions of Si, Cr, and Fe abundances. Magnetic mapping demonstrates that the surface field structure of φ Dra is dominated by a distorted dipolar component with a peak field strength of 1.4 kG and a large asymmetry between the poles. Chemical maps show an enhancement of Cr, Fe and, to a lesser extent, Si in a series of spots encircling intersections of the magnetic and rotational equators. These chemical spot geometries do not directly correlate with either the local field strength or the field inclination.

Rapidly oscillating TESS A–F main-sequence stars: are the roAp stars a distinct class?

ABSTRACT From sector 1–44 TESS observations, 19 new rapidly oscillating Ap (roAp) stars, 103 ostensibly non-peculiar stars with roAp-like frequencies, and 617 δ Scuti stars with independent frequencies typical of roAp stars were found. Examination of all chemically peculiar stars observed by TESS resulted in the discovery of 199 Ap stars that pulsate as δ Sct or γ Dor variables. The fraction of pulsating Ap stars is the same as the fraction of pulsating chemically normal stars. There is no distinct separation in frequency or radial order between chemically peculiar δ Sct stars and roAp stars. In fact, all the features that originally distinguished roAp from δ Sct stars in the past have disappeared. There is no reason to assume that the high frequencies in roAp stars are driven by a different mechanism from the high frequencies in chemically normal stars. However, chemically peculiar stars are far more likely to pulsate with high frequencies. The term ‘roAp’ should be dropped: all roAp stars are normal δ Scuti stars.