Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe long term polarimetric variability of the strongly magnetic white dwarf Grw+70$^circ$8247
121
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Some of the white dwarfs exhibit among the strongest magnetic fields in the universe. Many of these degenerate magnetic stars are also rotating very slowly. Among these objects, Grw+70$^circ$,8247, with its century-long suspected rotation period and its 400,MG magnetic field, stands as a particularly interesting object. Surprisingly, for this star, the first white dwarf in which a magnetic field was discovered, no spectropolarimetric observations have been discussed in the literature in the last 40 years. Here we present two sets of linear and circular polarisation spectra taken in 2015 and 2018, and we compare them with spectropolarimetric data obtained in the 1970s. Polarisation shows variability over a time interval of four decades, but some subtle changes may have been detected even over a three year time interval. Using the variation of the polarisation position angle as a proxy for the rotation of the magnetic axis in the plane of the sky, we conclude that the stars rotation period probably lies in the range of $10^2$ to $10^3$ years. Our data analysis is accompanied by a description of our various calibrations and tests of the ISIS instrument at the William Herschel Telescope that may be of general interest for linear spectropolarimetric measurements. We also found discrepancies in the sign of circular polarisation as reported in the literature, and made explicit the definitions that we have adopted.
rate research
Read More
We present and interpret simultaneous new photometric and spectroscopic observations of the peculiar magnetic white dwarf WD1953-011. The flux in the V-band filter and intensity of the Balmer spectral lines demonstrate variability with the rotation period of about 1.45 days. According to previous studies, this variability can be explained by the presence of a dark spot having a magnetic nature, analogous to a sunspot. Motivated by this idea, we examine possible physical relationships between the suggested dark spot and the strong-field magnetic structure (magnetic spot, or tube) recently identified on the surface of this star. Comparing the rotationally-modulated flux with the variable spectral observables related to the magnetic spot we establish their correlation, and therefore their physical relationship. Modeling the variable photometric flux assuming that it is associated with temperature variations in the stellar photosphere, we argue that the strong-field area and dark, low-temperature spot are comparable in size and located at the same latitudes, essentially overlapping each other with a possible slight longitudinal shift. In this paper we also present a new, improved value of the stars rotational period and constrain the characteristics of the thermal inhomogeneity over the degenerates surface.
We present a long-term programme for timing the eclipses of white dwarfs in close binaries to measure apparent and/or real variations in their orbital periods. Our programme includes 67 close binaries, both detached and semi-detached and with M-dwarfs, K-dwarfs, brown dwarfs or white dwarfs secondaries. In total, we have observed more than 650 white dwarf eclipses. We use this sample to search for orbital period variations and aim to identify the underlying cause of these variations. We find that the probability of observing orbital period variations increases significantly with the observational baseline. In particular, all binaries with baselines exceeding 10 yrs, with secondaries of spectral type K2 -- M5.5, show variations in the eclipse arrival times that in most cases amount to several minutes. In addition, among those with baselines shorter than 10 yrs, binaries with late spectral type (>M6), brown dwarf or white dwarf secondaries appear to show no orbital period variations. This is in agreement with the so-called Applegate mechanism, which proposes that magnetic cycles in the secondary stars can drive variability in the binary orbits. We also present new eclipse times of NN Ser, which are still compatible with the previously published circumbinary planetary system model, although only with the addition of a quadratic term to the ephemeris. Finally, we conclude that we are limited by the relatively short observational baseline for many of the binaries in the eclipse timing programme, and therefore cannot yet draw robust conclusions about the cause of orbital period variations in evolved, white dwarf binaries.
We present preliminary results of our analysis on the long-term variations observed in the optical spectrum of the LBV star Eta Carinae. Based on the hydrogen line profiles, we conclude that the physical parameters of the primary star did not change in the last 15 years.
Massive star winds are important contributors to the energy, momentum and chemical enrichment of the interstellar medium. Strong, organized and predominantly dipolar magnetic fields have been firmly detected in a small subset of massive O-type stars. Magnetic massive stars are known to exhibit phase-locked variability of numerous observable quantities that is hypothesized to arise due to the presence of an obliquely rotating magnetosphere formed via the magnetic confinement of their strong outflowing winds. Analyzing the observed modulations of magnetic O-type stars is thus a key step towards the better understanding of the physical processes that occur within their magnetospheres. The dynamical processes that lead to the formation of a magnetosphere are formally solved utilizing complex MHD simulations. Recently, an Analytic Dynamical Magnetosphere (ADM) model has been developed that can quickly be employed to compute the time-averaged density, temperature and velocity gradients within a dynamical magnetosphere. Here, we exploit the ADM model to compute photometric and polarimetric observables of magnetic Of?p stars, to test geometric models inferred from magnetometry. We showcase important results on the prototypical Of?p-type star HD 191612, that lead to a better characterization of massive star wind and magnetic properties.
We report the detection of variable stars within a 11.5 x 11.5 region near the Galactic centre (GC) that includes the Arches and Quintuplet clusters, as revealed by the VISTA Variables in the Via Lactea (VVV) survey. There are 353 sources that show Ks-band variability, of which the large majority (81%) correspond to red giant stars, mostly in the asymptotic giant branch (AGB) phase. We analyze a population of 52 red giants with long-term trends that cannot be classified into the typical pulsating star categories. Distances and extinctions are calculated for 9 Mira variables, and we discuss the impact of the chosen extinction law on the derived distances. We also report the presence of 48 new identified young stellar object (YSO) candidates in the region.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
