19 research outputs found
Global properties of the light curves of magnetic, chemically peculiar stars as a testbed for the existence of dipole-like symmetry in surface structures
Magnetic, chemically peculiar stars are known for exhibiting surface
abundance inhomogeneities (chemical spots) that lead to photometric and
spectroscopic variability with the rotation period. It is commonly assumed that
the surface structures are causally connected with the global magnetic field
that dominates the photospheric and subphotospheric layers of these stars. As a
rule, the observed magnetic fields show a simple dipole-like geometry, with the
magnetic axis being noncollinear to the rotational one. The present study aims
at detecting underlying patterns in the distribution of photometric spots in a
sample of 650 magnetic, chemically peculiar stars and examines their link to
the magnetic field topology. Photometric time-series observations from the
ASAS-3 archive were employed to inspect the light-curve morphology of our
sample stars and divide them into representative classes described using a
principal component analysis. Theoretical light curves were derived from
numerous simulations assuming different spot parameters and following the
symmetry of a simple dipole magnetic field. These were subsequently compared
with the observed light curves. The results from our simulations are in
contradiction with the observations and predict a much higher percentage of
doublewave light curves than is actually observed. We thereby conclude that the
distribution of the chemical spots does not follow the magnetic field topology,
which indicates that the role of the magnetic field in the creation and
maintenance of the surface structures may be more subsidiary than what is
predicted by theoretical studies.Comment: 19 pages, 9 figures, 2 tables, 1 Appendix table; accepted for
publication in Astronomy & Astrophysic
Photometric variability of the LAMOST sample of magnetic chemically peculiar stars as seen by TESS
High-quality light curves from space missions have opened up a new window on
the rotational and pulsational properties of magnetic chemically peculiar (mCP)
stars and have fuelled asteroseismic studies. They allow the internal effects
of surface magnetic fields to be probed and numerous astrophysical parameters
to be derived with great precision. We present an investigation of the
photometric variability of a sample of 1002 mCP stars discovered in the LAMOST
archival spectra with the aims of measuring their rotational periods and
identifying interesting objects for follow-up studies. TESS photometry was
available for 782 mCP stars and was analysed using a Fourier two-term frequency
fit to determine the stars' rotational periods. The rotational signal was then
subtracted from the light curve to identify non-rotational variability. A
pixel-level blending analysis was performed to check whether the variability
originates in the target star or a nearby blended neighbour. We investigated
correlations between the rotational periods, fractional age on the main
sequence, mass, and several other observables. We present rotational periods
and period estimates for 720 mCP stars. In addition, we identified four
eclipsing binary systems that likely host an mCP star, as well as 25 stars with
additional signals consistent with pulsation (12 stars with frequencies above
10 d and 13 stars with frequencies below 10 ). We find that more
evolved stars have longer rotation periods, in agreement with the assumption of
the conservation of angular momentum during main-sequence evolution. With our
work, we increase the sample size of mCP stars with known rotation periods and
identify prime candidates for detailed follow-up studies. This enables two
paths towards future investigations: population studies of even larger samples
of mCP stars and the detailed characterisation of high-value targets.Comment: 30 pages, 9 figures, 1 table. Accepted for publication in the Journal
of Astronomy and Astrophysics (A&A
Rotational modulation and single g-mode pulsation in the B9pSi star HD 174356?
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 and. 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).Fil: Mikulaek, Z. Masaryk University; RepĂșblica ChecaFil: Paunzen, E.. Masaryk University; RepĂșblica ChecaFil: HĂŒmmerich, S.. Masaryk University; RepĂșblica ChecaFil: Niemczura, E.. University of WrocĆaw; PoloniaFil: Walczak, P.. University of WrocĆaw; PoloniaFil: Fraga, L.. Masaryk University; RepĂșblica ChecaFil: Bernhard, K.. American Association of Variable Star Observers ; Estados UnidosFil: Janik, J.. Masaryk University; RepĂșblica ChecaFil: Hubrig, S.. Leibniz-Institut fĂŒr Astrophysik Potsdam; AlemaniaFil: JĂ€rvinen, S.. Masaryk University; RepĂșblica ChecaFil: Jagelka, M.. Leibniz Institute For Astrophysics Potsdam; AlemaniaFil: Pintado, Olga Ines. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - TucumĂĄn. Instituto Superior de CorrelaciĂłn GeolĂłgica. Universidad Nacional de TucumĂĄn. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de GeologĂa. CĂĄtedra GeologĂa Estructural. Instituto Superior de CorrelaciĂłn GeolĂłgica; Argentina. Universidad San Pablo TucumĂĄn; ArgentinaFil: Krticka, J.. Masaryk University; RepĂșblica ChecaFil: Prisegen, M.. Masaryk University; RepĂșblica ChecaFil: Skarka, M.. Masaryk University; RepĂșblica ChecaFil: Zejda, M.. Masaryk University; RepĂșblica ChecaFil: Ilyin, I.. Leibniz-Institut fĂŒr Astrophysik Potsdam; AlemaniaFil: Pribulla, T.. Masaryk University; RepĂșblica ChecaFil: Kaminski, K.. Adam Mickiewicz University; PoloniaFil: Kaminska, M. K.. Adam Mickiewicz University; PoloniaFil: Tokarek, J.. Adam Mickiewicz University; PoloniaFil: Zielinski, P.. Astronomical Observatory University of Warsaw; Poloni
A radio-pulsing white dwarf binary star
White dwarfs are compact stars, similar in size to Earth but ~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 delta-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56 hr period close binary, pulsing in brightness on a period of 1.97 min. The pulses are so intense that AR Sco's optical flux can increase by a factor of four within 30 s, and they are detectable at radio frequencies, the first such detection for any white dwarf system. They reflect the spin of a magnetic white dwarf which we find to be slowing down on a 10^7 yr 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 originate in large part from the cool star. AR Sco's broad-band 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
Context. 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 α2 Canum Venaticorum (ACV) variables. It is important to increase the sample of known rotational periods among CP stars by discovering new ACV variables. An increased sample size will contribute to the understanding of the CP starsâ evolution in time.
Aims. We aim at discovering new ACV variables in the public data of the third phase of the All Sky Automated Survey (ASAS-3). Furthermore, by analysis of the available photometric data, we intend to derive rotational periods of the stars.
Methods. 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. Objects exhibiting periodic signals well above the noise level were considered and visually inspected, whereas borderline cases were rejected.
Results. 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.
Conclusions. The next steps have to include a compilation of all available rotational periods from the literature and a detailed investigation of the astrophysical parameters of these stars. This includes a determination of the individual masses, luminosities, ages, and inclination angles. However, this information cannot be straightforwardly determined from photometric data alone
Pulsational properties of ten new slowly pulsating B stars
Context. Slowly pulsating B (SPB) stars are upper main-sequence multi-periodic pulsators that show non-radial g-mode oscillations driven by the Îș mechanism acting on the iron bump. These multi-periodic pulsators have great asteroseismic potential and can be employed for the calibration of stellar structure and evolution models of massive stars.
Aims. We collected a sample of ten hitherto unidentified SPB stars with the aim of describing their pulsational properties and identifying pulsational modes.
Methods. Photometric time series data from various surveys were collected and analyzed using diverse frequency search algorithms. We calculated astrophysical parameters and investigated the location of our sample stars in the log Teff vs. log L/Lâ diagram. Current pulsational models were calculated and used for the identification of pulsational modes in our sample stars. An extensive grid of stellar models along with their g-mode eigenfrequencies was calculated and subsequently cross-matched with the observed pulsational frequencies. The best-fit models were then used in an attempt to constrain stellar parameters such as mass, age, metallicity, and convective overshoot.
Results. We present detected frequencies, corresponding g-mode identifications, and the masses and ages of the stellar models producing the best frequency cross-matches. We partially succeeded in constraining stellar parameters, in particular concerning mass and age. Where applicable, rotation periods have been derived from the spacing of triplet component frequencies. No evolved SPB stars are present in our sample. We identify two candidate high-metallicity objects (HD 86424 and HD 163285), one young SPB star (HD 36999), and two candidate young SPB stars (HD 61712 and HD 61076).
Conclusions. We demonstrate the feasibility of using ground-based observations to perform basic asteroseismological analyses of SPB stars. Our results significantly enlarge the sample of known SPB stars with reliable pulsational mode identifications, which provides important input parameters for modeling attempts aiming to investigate the internal processes at work in upper main-sequence stars
PM 1-322: new variable planetary nebula
Spectra of planetary nebulae (PNe) are characterised by strong forbidden
emission lines and often also by an infrared (IR) excess. A few PNe show dust
obscuration events and/or harbour long-period binaries. Some post-asymptotic
giant branch stars, symbiotic stars, or B[e] stars may feature similar
characteristics. Recently, dust clouds eclipsing white dwarfs were also
detected. We report the discovery of an object with a very peculiar variability
pattern that bears signatures compatible with the above-mentioned classes of
objects. The object is ZTFJ201451.59+120353.4 and identifies with PM 1-322. The
object was discovered in Zwicky Transient Facility archival data and
investigated with historical and newly obtained photometric and spectroscopic
observations. The ZTF r and g data show a one magnitude deep, eclipse-like
event with a duration of about half a year that occurred in 2022. The
variability pattern of the star is further characterised by several dimming
events in the optical region that are accompanied by simultaneous brightenings
in the red and IR regions. Apart from that, two fast eruption-like events were
recorded in ZTF r data. Archival data from WISE indicate long-term variability
with a possible period of 6 or 12 yr. Our follow-up time series photometry
reveals a stochastic short-term variability with an amplitude of about 0.1 mag
on a timescale of about one hour. The spectral energy distribution is dominated
by IR radiation. Our high-resolution spectroscopy shows strong forbidden
emission lines from highly ionised species and symmetric double-peaked emission
in Halpha, which is very different from what is seen in earlier spectra
obtained in 2007. Several explanatory scenarios are presented. Our most likely
interpretation is that our target object involves a hot central star surrounded
by gaseous and dusty disks, an extended nebula, and a possible companion star.Comment: 10 pages, 8 figures, 2 tables, accepted for publication in Astronomy
& Astrophysic
A case study of ACV variables discovered in the Zwicky Transient Facility survey
Context. Magnetic chemically peculiar (mCP) stars exhibit complex atmospheres that allow the investigation of the interplay of atomic diffusion, magnetic fields, and stellar rotation. A non-uniform surface distribution of chemical elements and the non-alignment of the rotational and magnetic axes result in the variability of several observables. Photometrically variable mCP stars are referred to as α2 Canum Venaticorum (ACV) variables.
Aims. The present work presents a case study of known variables from the Zwicky Transient Facility (ZTF) survey, with the aim of investigating the surveyâs suitability for the detection and study of new ACV variables.
Methods. Using suitable selection criteria based on the known characteristics of ACV variables, candidate ACV stars were selected from the ZTF Catalog of Periodic Variable Stars. All light curves were inspected in detail to select the most promising candidates. Where available, low-resolution spectra from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) were employed to classify the stars on the MK system and confirm their status as mCP stars.
Results. We have identified 86 new promising ACV star candidates. Fifteen of these stars have LAMOST spectra available that, in all cases, confirm them as classical mCP stars, which highlights the viability of our approach. We present astrophysical parameters for all sample stars that can be sorted into four subgroups characterized by distinct light curve shapes. Anti-phase variations in different photometric passbands, in particular, are a unique characteristic of a subset of ACV stars readily usable for their identification. The availability of data in three different passbands (g, r, and i) is a major advantage of the ZTF survey.
Conclusions. On the basis of our experience with other photometric surveys and the analysis of light curves, we conclude that the ZTF is well suited to the search for, and the analysis of, ACV variables that are not considered in the available ZTF variable star catalogs. Further work will focus on the development and refinement of a search algorithm to correctly identify these stars in ZTF data and, subsequently, in massive photometric time series databases in general
HD 60431, the CP2 star with the shortest rotational period
Context. CP2 stars show periodic photometric, spectroscopic, and magnetic variations with the rotational period. They are generally slow rotators, with rotational periods exceeding half a day, except for the late B-type star HD 60431, which has an unusually short rotational period of 0.4755 days. As slow rotation is deemed a necessary criterion for the establishment of chemical peculiarities, this characteristic renders HD 60431 a special object that might offer valuable insight into, and constraints on, the formation and evolution of CP2 stars.
Aims. The aims of our study are to analyse the light variability, derive atmospheric abundances, and determine the detailed physical parameters of HD 60431 to confirm its status as the CP2 star with the shortest known rotational period, with special emphasis on the rotational period evolution.
Methods. Photometric indices and high-resolution spectroscopy were employed to derive the physical parameters, evolutionary status, and atmospheric abundances of our target star. A light variability study was carried out using combined sets of photometric data from ground- and space-based facilities. A circularly polarised spectrum was employed to check the presence of a longitudinal magnetic field in the star.
Results. With an age of only 10 Myr, an effective temperature of Teffâ=â13â000â
屉
300 K, surface gravity log gâ=â4.10â
屉
0.10, radius Râ=â1.97â
屉
0.09âRâ, and mass Mâ=â3.1â
屉
0.1âMâ, HD 60431 is situated close to the zero age main sequence; it is a member of the open cluster NGC 2547 in the Vela OB2 complex. We confirm its status as a classical late B-type CP2 star showing strong overabundances of Mg (1.8 dex), Si (1.9 dex), Ca (1.6 dex), Ti (2.2 dex), and Fe (1.8 dex). No conclusive evidence for the presence of a strong magnetic field was found in the available spectroscopic data. The light curve of HD 60431 has remained constant over the last four decades. The available photometric time series data confirm the short rotational period and indicate a slight secular increase in the rotational period of áč = 2.36(19) Ă 10â10 = 7.5(6) ms yrâ1. The following quadratic ephemeris has been derived: HJDmin(E) = 2â459â212.969â35â
+â
0. d475â516â64âEâ
+â
5. d62â
Ăâ
10â11E2.
Conclusions. HD 60431 is indeed the CP2 star with the shortest known rotational period. Theory needs to explain the establishment and maintenance of chemical peculiarities in such a young and fast-rotating object. Our results furthermore raise the question of whether period variability on timescales significantly shorter than stellar evolution is inherent to all magnetic chemically peculiar stars