Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userPeriods of magnetic field variations in the Ap star $gamma$ Equulei (HD 201601)
67
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
This paper presents a series of 95 new measurements of the longitudinal (effective) magnetic field $B_e$ of the Ap star $gamma$ Equ (HD 201601). Observations were obtained at the coude focus of the 1-m reflector at the Special Astrophysical Observatory (SAO RAS) in Russia over a time period of 4190 days (more than 11 years). We compiled a long record of $B_e$ points, adding our measurements to all published data. The time series of magnetic data consists of 395 $B_e$ points extending for 24488 days, or over 67 years. Various methods of period determination were examined for the case in which the length of the observed time series is rather short and amounts only to ~69 percent of the period. We argue that the fitting of a sine wave to the observed $B_e$ points by least squares yields the most reliable period in the case of $gamma$ Equ. Therefore, the best period for long-term magnetic variations of $gamma$ Equ, and hence the rotational period, is $P_{rm rot}=35462.5 pm 1149$ days $= 97.16 pm 3.15$ years.
rate research
Read More
Physical processes working in the stellar interiors as well as the evolution of stars depend on some fundamental stellar properties, such as mass, radius, luminosity, and chemical abundances. A classical way to test stellar interior models is to compare the predicted and observed location of a star on theoretical evolutionary tracks in a H-R diagram. This requires the best possible determinations of stellar mass, radius, luminosity and abundances. To derive its fundamental parameters, we observed the well-known rapidly oscillating Ap star, $gamma$ Equ, using the visible spectro-interferometer VEGA installed on the optical CHARA array. We computed the calibrated squared visibility and derived the limb-darkened diameter. We used the whole energy flux distribution, the parallax and this angular diameter to determine the luminosity and the effective temperature of the star. We obtained a limb-darkened angular diameter of 0.564~$pm$~0.017~mas and deduced a radius of $R$~=~2.20~$pm$~0.12~${rm R_{odot}}$. Without considering the multiple nature of the system, we derived a bolometric flux of $(3.12pm 0.21)times 10^{-7}$ erg~cm$^{-2}$~s$^{-1}$ and an effective temperature of 7364~$pm$~235~K, which is below the effective temperature that has been previously determined. Under the same conditions we found a luminosity of $L$~=~12.8~$pm$~1.4~${rm L_{odot}}$. When the contribution of the closest companion to the bolometric flux is considered, we found that the effective temperature and luminosity of the primary star can be, respectively, up to $sim$~100~K and up to $sim$~0.8~L$_odot$ smaller than the values mentioned above.These new values of the radius and effective temperature should bring further constraints on the asteroseismic modelling of the star.
Context. The existence of a significant population of Ap stars with very long rotation periods (up to several hundred years) has progressively emerged over the past two decades. However, only lower limits of the periods are known for most of them because their variations have not yet been observed over a sufficient timebase. Aims. We determine the rotation period of the slowly rotating Ap star HD 18078 and we derive constraints on the geometrical structure of its magnetic field. Methods. We combine measurements of the mean magnetic field modulus obtained from 1990 to 1997 with determinations of the mean longitudinal magnetic field spanning the 1999-2007 time interval to derive an unambiguous value of the rotation period. We show that this value is consistent with photometric variations recorded in the Stroemgren uvby photometric system between 1995 and 2004. We fit the variations of the two above-mentioned field moments with a simple model to constrain the magnetic structure. Results. The rotation period of HD 18078 is (1358 +/- 12) d. The geometrical structure of its magnetic field is consistent to first order with a colinear multipole model whose axis is offset from the centre of the star. Conclusions. HD 18078 is only the fifth Ap star with a rotation period longer than 1000 days for which the exact value of that period (as opposed to a lower limit) could be determined. The strong anharmonicity of the variations of its mean longitudinal magnetic field and the shift between their extrema and those of the mean magnetic field modulus are exceptional and indicative of a very unusual magnetic structure.
Context. The Ap stars that rotate extremely slowly, with periods of decades to centuries, represent one of the keys to the understanding of the processes leading to the differentiation of stellar rotation. Aims. We characterise the variations of the magnetic field of the Ap star HD 50169 and derive constraints about its structure. Methods. We combine published measurements of the mean longitudinal field <Bz> of HD 50169 with new determinations of this field moment from circular spectropolarimetry obtained at the 6-m telescope BTA of the Special Astrophysical Observatory of the Russian Academy of Sciences. For the mean magnetic field modulus <B>, literature data are complemented by the analysis of ESO spectra, both newly acquired and from the archive. Radial velocities are also obtained from these spectra. Results. We present the first determination of the rotation period of HD 50169, Prot = (29.04+/-0.82) y. HD 50169 is currently the longest-period Ap star for which magnetic field measurements have been obtained over more than a full cycle. The variation curves of both <Bz> and <B> have a significant degree of anharmonicity, and there is a definite phase shift between their respective extrema. We confirm that HD 50169 is a wide spectroscopic binary, refine its orbital elements, and suggest that the secondary is probably a dwarf star of spectral type M. Conclusions. The shapes and mutual phase shifts of the derived magnetic variation curves unquestionably indicate that the magnetic field of HD 50169 is not symmetric about an axis passing through its centre. Overall, HD 50169 appears similar to the bulk of the long-period Ap stars.
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.
Despite of the importance of magnetic fields for the full understanding of the properties of accreting Herbig Ae/Be stars, these fields have scarcely been studied over the rotation cycle until now. One reason for the paucity of such observations is the lack of knowledge of their rotation periods. The sharp-lined young Herbig Ae star HD101412 with a strong surface magnetic field became in the last years one of the most studied targets among the Herbig Ae/Be stars. A few months ago we obtained multi-epoch polarimetric spectra of this star with FORS2 to search for a rotation period and to constrain the geometry of the magnetic field. We measured longitudinal magnetic fields on 13 different epochs distributed over 62 days. These new measurements together with our previous measurements of the magnetic field in this star were combined with available photometric observations to determine the rotation period. The search of the rotation period resulted in P=42.076+-0.01d. According to near-infrared imaging studies the star is observed nearly edge-on. The star exhibits a single-wave variation of the longitudinal magnetic field during the stellar rotation cycle. These observations are usually considered as evidence for a dominant dipolar contribution to the magnetic field topology.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
