589 research outputs found
Current and secular accretion rates of EX Hydrae
No full textWe report an observed accretion rate of Ṁ 1 = (3.86 ± 0.60) × 10 −11 M ⊙ yr −1 for the white dwarf in the short-period, intermediate polar EX Hya . This result is based upon the accretion-induced 4 π -averaged energy flux from 2.45 μm to 100 keV and the corresponding luminosity at the Gaia distance of 56.77 pc. Our result is in perfect agreement with the theoretical mass transfer rate from the secondary star induced by gravitational radiation (GR) and the spin-up of the white dwarf, − Ṁ 2 = (3.90 ± 0.35) × 10 −11 M ⊙ yr −1 ; 24% of it is caused by the spin-up. The agreement indicates that mass transfer is conservative. The measured Ṁ 1 obviates the need for angular momentum loss (AML) by any process other than GR. We complemented this result with an estimate of the mean secular mass transfer rate over ∼10 7 yr by interpreting the non-equilibrium radius of the secondary star in EX Hya based on published evolutionary calculations. This suggests a time-averaged mass transfer rate enhanced over GR by a factor f GR ≳ 2. Combined with the present-day lack of such an excess, we suggest that an enhanced secular AML is due to an intermittently active process, such as the proposed frictional motion of the binary in the remnants of nova outbursts. We argue that EX Hya , despite its weakly magnetic nature, has evolved in a very similar way to non-magnetic CVs. We speculate that the discontinuous nature of an enhanced secular AML may similarly apply to the latter
The high-field magnetic white dwarf LP790-29: Not a fast rotator
No full textWe have investigated the nature of the magnetic white dwarf LP 790-29 = LHS 2293 by polarimetric monitoring, searching for short-term variability. No periodicity was found and we can exclude rotation periods between 4 s and 1.5 hour with a high confidence. Maximum amplitudes of sinusoidal variations are DeltaR < 0.009 mag and Delta V-R < 0.7% for a mean value of the R-band circular polarization of V-R = -9.1 +/- 0.3%. Combined with earlier results by other authors, our observation suggests that LP 790-29 is, in fact, an extremely slowly rotating single white dwarf and not an unrecognized fast rotator and/or disguised cataclysmic variable
Soft X-ray spectral variability of AM Herculis
No full textContext.
Polars (AM Herculis binaries) are a prominent class of bright soft X-ray
sources, many of which were discovered with ROSAT.
Aims.
We present a homogenous analysis of all the pointed ROSAT PSPC
observations of polars subdivided into two papers that discuss
the prototype polar AM He
A new soft X-ray spectral model for polars with an application to AM Herculis
No full textWe present a simple heuristic model for the time-averaged soft X-ray temperature distribution in the accretion spot on the white dwarf in polars. The model is based on the analysis of the Chandra LETG spectrum of the prototype polar AM Her and involves an exponential distribution of the emitting area vs. blackbody temperature a(T) = a0 exp( − T/T0). With one free parameter besides the normalization, it is mathematically as simple as the single blackbody, but is physically more plausible and fits the soft X-ray and far-ultraviolet spectral fluxes much better. The model yields more reliable values of the wavelength-integrated flux of the soft X-ray component and the implied accretion rate than reported previously
The secondary star and distance of the polar V1309 Orionis
No full textContext.
The first phase-resolved JHK light curves of the
eclipsing polar (AM Herculis binary) V1309 Or
Neglected X-ray discovered polars - II. The peculiar eclipsing binary HY Eridani
No full textWe report on the X-ray observations of the eclipsing polar HY Eri (RX J0501–0359), along with its photometric, spectrophotometric, and spectropolarimetric optical variations, collected over 30 years. With an orbital period of 2.855 h, HY Eri falls near the upper edge of the 2–3 h period gap. After 2011, the system went into a prolonged low state, continuing to accrete at a low level. We present an accurate alias-free long-term orbital ephemeris and report a highly significant period change by 10 ms that took place over the time interval from 2011 to 2018. We acquired a high-quality eclipse spectrum that shows the secondary star as a dM5–6 dwarf at a distance d = 1050 ± 110 pc. Based on phase-resolved cyclotron and Zeeman spectroscopy, we identify the white dwarf (WD) in HY Eri as a two-pole accretor with nearly opposite accretion spots of 28 and 30 MG. The Zeeman analysis of the low state spectrum reveals a complex magnetic field structure, which we fit by a multipole model. We detected narrow emission lines from the irradiated face of the secondary star, of which Mg Iλ5170 with a radial velocity amplitude of K′2 = 139 ± 10 km s−1 (90% confidence) tracks the secondary more reliably than the narrow Hα line. Based on the combined dynamical analysis and spectroscopic measurement of the angular radius of the WD, we obtain a primary mass of M1 = 0.42 ± 0.05 M⊙ (90% confidence errors), identifying it as a probable He WD or hybrid HeCO WD. The secondary is a main sequence star of M2 = 0.24 ± 0.04 M⊙ that seems to be slightly inflated. The large distance of HY Eri and the lack of similar systems suggest a very low space density of polars with low-mass primary. According to current theory, these systems are destroyed by induced runaway mass transfer, suggesting that HY Eri may be doomed to destruction. Over the last 30 years, HY Eri experienced high and low states with mass transfer rates that differed by three orders of magnitude, varying between Ṁ ≃ 10−9 M⊙ yr−1 and 10−12 M⊙ yr−1. At a galactic latitude of −26.1°, it is located about 500 pc below the galactic plane
Zeeman tomography of magnetic white dwarfs. II, The quadrupole-dominated magnetic field of HE 1045-0908
No full textWe report time-resolved optical flux and circular polarization spectroscopy of the magnetic DA white dwarf HE 1045−0908 obtained with FORS1 at the ESO VLT. Considering published results, we estimate a likely rotational period of P rot 2.7 h, but cannot exclude values as high as about 9 h. Our detailed Zeeman tomographic analysis reveals a field structure which is dominated by a quadrupole and contains additional dipole and octupole contributions, and which does not depend strongly on the assumed value of the period. A good fit to the Zeeman flux and polarization spectra is obtained if all field components are centred and inclinations of their magnetic axes with respect to each other are allowed for. The fit can be slightly improved if an offset from the centre of the star is included. The prevailing surface field strength is 16 MG, but values between 10 and ∼ 75 MG do occur. We derive an effective photospheric temperature of HE 1045−0908 of T eff = 10 000 ± 1000 K. The tomographic code makes use of an extensive database of pre-computed Zeeman spectra (Paper I)
The IAG solar flux atlas: Accurate wavelengths and absolute convective blueshift in standard solar spectra
No full textWe present a new solar flux atlas with the aim of understanding wavelength precision and accuracy in solar benchmark data. The atlas covers the wavelength range 405−2300 nm and was observed at the Institut für Astrophysik, Göttingen (IAG), with a Fourier transform spectrograph (FTS). In contrast to other FTS atlases, the entire visible wavelength range was observed simultaneously using only one spectrograph setting. We compare the wavelength solution of the new atlas to the Kitt Peak solar flux atlases and to the HARPS frequency-comb calibrated solar atlas. Comparison reveals systematics in the two Kitt Peak FTS atlases resulting from their wavelength scale construction, and shows consistency between the IAG and the HARPS atlas. We conclude that the IAG atlas is precise and accurate on the order of ± 10 m s-1 in the wavelength range 405−1065 nm, while the Kitt Peak atlases show deviations as large as several ten to 100 m s-1. We determine absolute convective blueshift across the spectrum from the IAG atlas and report slight differences relative to results from the Kitt Peak atlas that we attribute to the differences between wavelength scales. We conclude that benchmark solar data with accurate wavelength solution are crucial to better understand the effect of convection on stellar radial velocity measurements, which is one of the main limitations of Doppler spectroscopy at m s -1 precision
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