Cool Ap Star Research Articles

The Far Ultraviolet Light Curve of 78 Vir (A2p SrCrEu)

All IUE spectra obtained shortwards of 2000 Å with the SWP camera are used to construct a far-ultraviolet (FUV) light curve at 1500 Å for 78 Vir over its rotational period. Setting an arbitrary phase 0.00 for the first spectrum recorded on 1987 May 10, the minimum and maximum occur respectively near phases 0.20–0.25 and 0.60–0.70. The Fine Error Sensor counts, which measure the brightness near the V band, vary in phase with the FUV flux, which is the opposite situation to that observed in another cool Ap star, 21 Com.

The complex magnetic field topology of the cool Ap star 49 Cam

49 Cam is a cool magnetic chemically peculiar star which has been noted for showing strong, complex Zeeman linear polarisation signatures. This paper describes magnetic and chemical surface maps obtained for 49 Cam using the INVERS10 magnetic Doppler imaging code and high-resolution spectropolarimetric data in all four Stokes parameters collected with the ESPaDOnS and Narval spectropolarimeters at the Canada-France-Hawaii Telescope and Pic du Midi Observatory. The reconstructed magnetic field maps of 49 Cam show a relatively complex structure. Describing the magnetic field topology in terms of spherical harmonics, we find significant contributions of modes up to l=3, including toroidal components. Observations cannot be reproduced using a simple low-order multipolar magnetic field structure. 49 Cam exhibits a level of field complexity that has not been seen in magnetic maps of other cool Ap stars. Hence we concluded that relatively complex magnetic fields are observed in Ap stars at both low and high effective temperatures. In addition to mapping the magnetic field, we also derive surface abundance distributions of nine chemical elements, including Ca, Sc, Ti, Cr, Fe, Ce, Pr, Nd, Eu. Comparing these abundance maps with the reconstructed magnetic field geometry, we find no clear relationship of the abundance distributions with the magnetic field for some elements. However, for other elements some distinct patterns are found. We discuss these results in the context of other recent magnetic mapping studies and theoretical predictions of radiative diffusion.

Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712

Aims: We present a magnetic Doppler imaging study from all Stokes parameters of the cool, chemically peculiar star HD 24712. This is the very first such analysis performed at a resolving power exceeding 10^5. Methods: The analysis is performed on the basis of phase-resolved observations of line profiles in all four Stokes parameters obtained with the HARPSpol instrument attached at the 3.6-m ESO telescope. We use the magnetic Doppler imaging code, INVERS10, which allows us to derive the magnetic field geometry and surface chemical abundance distributions simultaneously. Results: We report magnetic maps of HD 24712 recovered from a selection of FeI, FeII, NdIII, and NaI lines with strong polarization signals in all Stokes parameters. Our magnetic maps successfully reproduce most of the details available from our observation data. We used these magnetic field maps to produce abundance distribution map of Ca. This new analysis shows that the surface magnetic field of HD 24712 has a dominant dipolar component with a weak contribution from higher-order harmonics. The surface abundance distributions of Fe and Ca show enhancements near the magnetic equator with an underabundant patch at the visible (positive) magnetic pole; Nd is highly abundant around the positive magnetic pole. The Na abundance map shows a high overabundance around the negative magnetic pole. Conclusions: Based on our investigation and similar recent four Stokes parameter magnetic mapping studies, we present a tentative evidence for the hypothesis that Ap stars with dipole-like fields are older than stars with magnetic fields that have more small-scale structures. We find that our abundance maps are inconsistent with recent theoretical calculations of atomic diffusion in presence of magnetic fields.

Time resolved spectroscopy of the cool Ap star HD 213637*

We present an analysis of high time resolution spectra of the chemically peculiar Ap star HD 213637. The star shows rapid radial velocity variations with a period close to the photometric pulsation period. Radial velocity pulsation amplitudes vary significantly for different rare earth elements. The highest pulsation amplitudes belong to lines of TbIII ~360 m/s, PrII ~250 m/s and PrIII ~230 m/s . We did not detect any pulsations from spectral lines of EuII and in Halpha, in contrast to many other roAp stars. We also did not find radial velocity pulsations using spectral lines of other chemical elements, including Mg, Si, Ca, Sc, Cr, Fe, Ni, Y and Ba. There are phase shifts between the maxima of pulsation amplitudes of different rare earth elements and ions, which is evidence of an outwardly running magneto-acoustic wave propagating through the upper stellar atmosphere.

Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712

High-resolution spectropolarimetric observations provide simultaneous information about stellar magnetic field topologies and three-dimensional distributions of chemical elements. Here we present analysis of a unique full Stokes vector spectropolarimetric data set, acquired for the cool magnetic Ap star HD 24712. The goal of our work is to examine circular and linear polarization signatures inside spectral lines and to study variation of the stellar spectrum and magnetic observables as a function of rotational phase. HD 24712 was observed with the HARPSpol instrument at the 3.6-m ESO telescope over a period of 2010-2011. The resulting spectra have S/N ratio of 300-600 and resolving power exceeding 100000. The multiline technique of least-squares deconvolution (LSD) was applied to combine information from the spectral lines of Fe-peak and rare-earth elements. We used the HARPSPol spectra of HD 24712 to study the morphology of the Stokes profile shapes in individual spectral lines and in LSD Stokes profiles corresponding to different line masks. From the LSD Stokes V profiles we measured the longitudinal component of the magnetic field, <Bz>, with an accuracy of 5-10 G. We also determined the net linear polarization from the LSD Stokes Q and U profiles. We determined an improved rotational period of the star, P_rot = 12.45812 +/- 0.00019d. We measured <Bz> from the cores of Halpha and Hbeta lines. The analysis of <Bz> measurements showed no evidence for a significant radial magnetic field gradient in the atmosphere of HD 24712. We used our <Bz> and net linear polarization measurements to determine parameters of the dipolar magnetic field topology. We found that magnetic observables can be reasonably well reproduced by the dipolar model. We discovered rotational modulation of the Halpha core and related it a non-uniform surface distribution of rare-earth elements.

Orbital parameters, chemical composition and magnetic field of the Ap binary HD 98088★

HD 98088 is a synchronised, double-lined spectroscopic binary system with a magnetic Ap primary component and an Am secondary component. We study this rare system using high-resolution MuSiCoS spectropolarimetric data, to gain insight into the effect of binarity on the origin of stellar magnetism and the formation of chemical peculiarities in A-type stars. Using a new collection of 29 high-resolution Stokes VQU spectra we re-derive the orbital and stellar physical parameters and conduct the first disentangling of spectroscopic observations of the system to conduct spectral analysis of the individual stellar components. From this analysis we determine the projected rotational velocities of the stars and conduct a detailed chemical abundance analysis of each component using both the SYNTH3 and ZEEMAN spectrum synthesis codes. The surface abundances of the primary component are typical of a cool Ap star, while those of the secondary component are typical of an Am star. We present the first magnetic analysis of both components using modern data. Using Least-Squares Deconvolution, we extract the longitudinal magnetic field strength of the primary component, which is observed to vary between +1170 and -920 G with a period consistent with the orbital period. There is no field detected in the secondary component. The magnetic field in the primary is predominantly dipolar, with the positive pole oriented approximately towards the secondary.

14 Ceti: a probable Ap-star-descendant entering the Hertzsprung gap

14 Ceti is a subgiant star of F spectral class that displays variations in the S-index of its CaII H & K lines and an X-ray emission that is stronger than the mean observed for its spectral class, which may be due to some magnetic activity. We attempt to Zeeman-detect and study the magnetic field of 14 Ceti and to infer its origin. We used the spectropolarimeter Narval at the Telescope Bernard Lyot, Pic du Midi Observatory, and the least squares deconvolution method to create high signal-to-noise ratio Stokes V profiles. We derived the surface-averaged longitudinal magnetic field Bl. We also measured the S-index, and the radial velocity for each observation. 14 Ceti is Zeeman-detected for the 30 observed dates spanning from August 2007 to January 2012. The average longitudinal magnetic field does not reverse its sign, reaches about -35 G, and shows some month-long-timescale variations in our 2008 and 2011-2012 observations. The S-index follows the same long-term trend as Bl. 14 Ceti is confirmed as a single star without H-K emission cores. The strength of the observed surface magnetic field of 14 Ceti is one order of magnitude greater than the observed one for late F main-sequence stars, and is comparable to the values measured in the active late F pre-main-sequence star HR 1817. On the other hand, taking into account the post-main-sequence evolution of an Ap star, an oblique rotator model can explain the strength of the magnetic field of 14 Ceti. The variations with a timescale of months observed for both the Bl and S-index could be due to the rotation. The most probable scenario to explain our observations appears to be that 14 Ceti is the descendant of a cool Ap star .

SIMULATIONS OF MAGNETOACOUSTIC PULSATIONS IN ATMOSPHERES OF RAPIDLY OSCILLATING Ap STARS

Rapidly oscillating Ap (roAp) stars exhibit an astrophysically interesting combination of strong, dipolar-like magnetic fields and high-overtone p-mode pulsations similar to the Sun. Recent time-resolved spectroscopy of these stars unravelled a complex picture of propagating magnetoacoustic pulsation waves, with amplitude and phase strongly changing as a function of atmospheric height. To interpret these observations and gain a new insight into the atmospheric dynamics of roAp stars we have carried out two-dimensional time-dependent, non-linear magnetohydrodynamical simulations of waves for a realistic atmospheric stratification of a cool Ap star. We explore a grid of simulations in a wide parameter space, treating oscillations of the velocity, magnetic field, and thermodynamic quantities in a self-consistent manner. Our simulations foster a new understanding of the influence of the atmosphere and the magnetic field on the propagation and reflection properties of magnetoacoustic waves, formation of node surfaces, and relative variation of different quantities. Our simulations reproduce all main features of the observed pulsational behavior of roAp stars. We show, for the first time, that the overall depth dependence of the pulsations in roAp atmospheres is strongly influenced by the density inversion at the photospheric base.

Discovery of very low amplitude 9-minute multiperiodic pulsations in the magnetic Ap star HD 75445

We present our discovery of pulsational radial-velocity variations in the cool Ap star HD75445, an object spectroscopically similar to the bright, rapidly-oscillating Ap (roAp) star gamma Equ. Based on high-resolution time-series spectroscopy obtained with the HARPS spectrometer at the European Southern Observatory 3.6-m telescope, we detected oscillations in Nd II and Nd III lines with a period close to 9 min and amplitudes of 20-30 m s(-1). Substantial variation in the pulsational amplitude during our 3.8 h observing run reveals the presence of at least three excited non-radial modes. The detection of extremely low amplitude pulsations in HD75445 indicates that the roAp excitation mechanism produces variability in the radial velocity amplitude of between a few tens ms(-1) and several km s(-1). This supports the idea that many, if not all, cool Ap stars occupying the roAp instability strip may harbour non-radial pulsations, which currently remain undetected due to their small photometric and radial-velocity amplitudes.

Discovery of pulsational line profile variations in the δ Scuti star HD 21190 and in the Ap Sr star HD 218994★

Asteroseismology has the potential to provide new insights into the physics of stellar interiors. We have obtained Ultraviolet and Visual Echelle Spectrograph high time-resolution observations of the δ Scuti star HD 21190 and of the Ap Sr star HD 218994 to search for pulsational line profile variations. We report the discovery of a new roAp star, HD 218994, with a pulsation period of 14.2 min. This is one of the most evolved roAp stars. No rapid pulsations have been found in the spectra of the cool Ap star –δ Scuti star HD 21190. However, we detect with unprecedented clarity for a δ Scuti star moving peaks in the cores of spectral lines that indicate the presence of high-degree non-radial pulsations in this star.

HD 178892 – a cool Ap star with extremely strong magnetic field

We report a discovery of the Zeeman resolved spectral lines, corresponding to the extremely large magnetic field modulus $\langle B_{\rm s}\rangle$ = 17.5 kG, in the cool Ap star HD 178892. The mean longitudinal field of this star reaches 7.5 kG, and its rotational modulation implies the strength of the dipolar magnetic component $B_{\rm p}\ge$ 23 kG. We have revised rotation period of the star using the All Sky Automated Survey photometry and determined P = 8.2478 d. Rotation phases of the magnetic and photometric maxima of the star coincide with each other. We obtained Geneva photometric observation of HD 178892 and estimated T eff = $7700\pm250$ K using photometry and the hydrogen Balmer lines. Preliminary abundance analysis reveals abundance pattern typical of rapidly oscillating Ap stars.

High time resolution spectroscopy and magnetic variability of the cool Ap star HD 965

We present the results of an investigation of the magnetic Ap star HD 965 with high spectral and time resolution. We determine precise radial velocities using spectra obtained with the Ultraviolet‐Visual Echelle Spectrograph (UVES) on the European Southern Observatory Very Large Telescope. Special attention is given to spectral lines of rare-earth elements which in rapidly oscillating Ap (roAp) stars exhibit the strongest radial velocity variations with pulsation period. Careful time series analysis did not detect any convincing evidence of pulsation in HD 965 with an upper limit in amplitude of 15‐20 m s −1 . All properties of HD 965 are similar to the majority of roAp stars. A likely reason for the apparent lack of pulsation could be connected with the geometrical structure of the magnetic field and the aspect of the star at the time of observation. Longitudinal magnetic field measurements for HD 965 showed that our UVES spectral observations were carried out when the longitudinal field was near zero and therefore, according to the oblique rotator model, near a time when the star was viewed from the magnetic equator. For a dipole oscillation aligned with the magnetic field, as is typical of roAp stars, no variation can be detected at this aspect. We may, therefore, expect to detect rapid oscillations in HD 965 in the future, when the star will present one of the magnetic poles. Ke yw ords: stars: individual: HD 965 ‐ stars: magnetic fields ‐ stars: oscillations ‐ stars: variables: other.

Radial velocity variations in pulsating Ap stars – III. The discovery of 16.21-min oscillations in β CrB

We present the analysis of 3 h of a rapid time series of precise stellar radial velocity (RV) measurements (a = 4.5 m s -1 ) of the cool Ap star β CrB. The integrated RV measurements spanning the wavelength interval 5000-6000 A show significant variations (false alarm probability = 10 -5 ) with a period of 16.21 min (v = 1028.17 μHz) and an amplitude of 3.54 ± 0.56 m s -1 The RV measured over a much narrower wavelength interval reveals one spectral feature at λ6272.0 A pulsating with the same 16.21-min period and an amplitude of 138 ± 23 m s -1 . These observations establish β CrB to be a low-amplitude rapidly oscillating Ap star.

On the possible presence of promethium in the spectra of HD 101065 (Przybylski's star) and HD 965

Traditional and statistical line-identification methods indicate the presence of Pm I and II, Tc I, and perhaps Tc II in the spectrum of the roAp star HD 101065. These methods also lead to the presence of Pm II and probably also Pm I in a related cool Ap star, HD 965. The spectroscopic evidence is strong enough that we would declare promethium to be present without hesitation, if any of its isotopes were stable. The longest-lived promethium isotope has a half-life of only 17.7 years. The presence of this element would mean that unrecognized processes – perhaps flare activities – are taking place in the atmospheres of these stars. The significance of such processes for galactic chemical evolution cannot be ruled out. We discuss the possibility that the highly improbable wavelength coincidences are due to chance, or due to contamination of the laboratory sources.

Search for the Companion in the β CrB Spectrum

Spectroscopic analyses of the β CrB spectrum have been performed in the past without taking into consideration the double nature of this well-known cool Ap star (e.g. Hiltner, 1945; Hack, 1958; Adelman, 1973). Recently, Kamper et al. (1990) have re-examined the astrometric (interferometric and visual) and spectroscopic (radial velocity) data and have given an improved orbit for the β CrB system (P=10.55 yr) and the passage at periastron at 1991.02.

A photometric search for rapid oscillations in the cool Ap star CrB near magnetic maximum

Photoelectric photometry has been obtained on 8 nights, all within one day of the magnetic maximum of β CrB based on Kurtz' ephemeris, in order to attempt to clarify whether this cool magnetic oblique rotator is or is not a rapidly oscillating Ap (roAp) star. Coherent oscillations were not detected down to a level of 0.34 millimagnitude using narrow-band filters close in wavelength to Johnson B. This implies that β CrB may not be an roAp star, in spite of possessing characteristics very similar to other known roAp stars.

The discovery and analysis of oscillations with a period of 7.4 min in the cool Ap star HD 80316

La decouverte de variations de lumiere a faibles amplitudes avec une periode de 7.4 mn dans l'etoile Ap magnetique HD 80316 est rapportee. Les observations photometriques obtenues en 1989 a l'Observatoire SAAO sont presentees. Une analyse de frequence des donnees photometriques est realisee

Frequency Analysis of the Rapidly Oscillating Ap Star HD 60435

AbstractThe cool Ap star HD 60435 was monitored in a programme of rapid B photometry during 18 nights in January/February 1984, from two stations widely spaced in longitude (the University of Toronto 0.6-m telescope at the Carnegie Southern Observatory (CARSO) on Las Campanas, Chile, and the 0.5-m telescope of the South African Astronomical Observatory ( SAAO)). On six of those nights, contiguous light curves from both sites were obtained.Fourier analysis of these data confirms the rapid variability first reported by Kurtz (1984) and reveals several additional transient oscillations. HD 60435 exhibits persistent - but modulated - oscillations at a frequency near 1.4 IRHZ (period = 11.9 minutes), and short-lived oscillations at frequencies near 1.1 and 4.2 mHz (periods of 15.2 and 4.0 minutes, respectively). These latter two periods represent the longest and shortest yet observed in the class of rapidly oscillating Ap stars.We have applied the oblique pulsator model (Kurtz 1982) to the fine-scale splittings detected in the frequency spectra of the 1.4 and 1.1 mHz oscillations. Also, the series of frequencies close to 1.4 mHz which fall into a pattern of roughly equal spacing is compared to such spacings predicted for overtones in pulsating main-sequence A stars (Shibahashi and Saio 1984). Both approaches suggest that HD 60435 is undergoing non-radial pulsations of odd and even degree (probably with ℓ ≲ 3).The oblique pulsator interpretation of the splittings in the frequency spectrum and the amplitude modulation of the 1.4 mHz oscillations also predict: a rotation period of approximately eight days for this star. Mean photometry of HD 60435, collected by the authors, supports a similar value of 7.7 days for the period.Analysis of the oscillations is hampered by ambiguities due to daily aliases present in the data, and by the complicated structure and time-dependence of the frequency spectrum. Further observations of HD 60435 are essential if we are to fully understand its rapid variations.

SPECTRAL MORPHOLOGY IN THE OPEN CLUSTER NGC 2287.

ABSTRACT Spectral classifications in NGC 2287 have revealed the presence of an He-weak star, three Hg-Mn stars, one star with a weak K line, one cool Ap star and one star with a composite spectrum. Spectroscopic evidence determined eight double-line binaries as well as five suspected double-line binaries.

THE RELATIONSHIP BETWEEN SPECTRUM AND LIGHT VARIATIONS IN THE MAGNETIC AP STAR HR 1217 (HD 24712).

ABSTRACT Measurements of equivalent widths and line-blocking coefficients have been made from 3850 Å to 5650 Å in the spectrum of HR 1217, using photographic data. The variation of the equivalent widths of the Eu II lines suggests a revised period of 12d.460 in order to phase the present data with those of Preston (1972). The variation of equivalent widths shows a dependence of amplitude on atomic number which resembles that found by Preston, but there are clear discontinuities in the relationship. The line blocking coefficient was measured in unweighted 50 Å bands throughout the spectral region, except between 4750 Å and 5200 Å, and weighted by the sensitivity functions of the υ, b, and y bands of the Strömgren photometric system. The line blocking variation in the filter bands is found to account for approximately one-half of the light variation observed by Wolff and Morrison (1973); the spectroscopy and photometry considered together demonstrate the need for continuous opacity variation in the filter bands to account for the light variation. Redistribution of flux from the ultraviolet appears to have no significant role in the light variation, which is not unexpected in this cool Ap star.