AA Dor Research Articles

Space observations of AA Doradus provide consistent mass determinations. New HW-Vir systems observed with TESS

ABSTRACT We present an overview of eclipsing systems of the HW-Virginis (HW-Vir) type, based on space observations from the Transiting Exoplanet Survey Satellite (TESS) mission. We perform a detailed analysis of the properties of AA Doradus (AA Dor), which was monitored for almost a full year. This excellent time series data set permitted us to search for both stellar pulsations and eclipse timing variations. In addition, we used the high-precision trigonometric parallax from Gaia Early Data Release 3 to make an independent determination of the fundamental stellar parameters. No convincing pulsations were detected down to a limit of 76 parts per million; however, we detected one peak with false alarm probability of 0.2 per cent. 20 s cadences being collected during Year 3 should confirm or reject our detection. From eclipse timing measurements we were able to confirm that the orbital period is stable, with an upper limit to any period change of 5.75 × 10−13 s s−1. The apparent offset of the secondary eclipse is consistent with the predicted Rømer delay when the primary mass is that of a canonical extended horizontal branch star. Using parallax and a spectral energy distribution corroborates that the mass of the primary in AA Dor is canonical, and its radius and luminosity are consistent with an evolutionary state beyond core helium burning. The mass of the secondary is found to be at the limit of hydrogen burning.

EPIC 216747137: a new HW Vir eclipsing binary with a massive sdOB primary and a low-mass M-dwarf companion

ABSTRACT EPIC 216747137 is a new HW Virginis system discovered by the Kepler spacecraft during its K2 ‘second life’. Like the other HW Vir systems, EPIC 216747137 is a post-common-envelope eclipsing binary consisting of a hot subluminous star and a cool low-mass companion. The short orbital period of 3.87 h produces a strong reflection effect from the secondary (∼9 per cent in the R band). Together with AA Dor and V1828 Aql, EPIC 216747137 belongs to a small subgroup of HW Vir systems with a hot evolved sdOB primary. We find the following atmospheric parameters for the hot component: Teff = 40400 ± 1000 K, log g = 5.56 ± 0.06, and log(N(He)/N(H)) = −2.59 ± 0.05. The sdOB rotational velocity v sin i = 51 ± 10 km s−1 implies that the stellar rotation is slower than the orbital revolution and the system is not synchronized. When we combine photometric and spectroscopic results with the Gaia parallax, the best solution for the system corresponds to a primary with a mass of about 0.62 M⊙ close to, and likely beyond, the central helium exhaustion, while the cool M-dwarf companion has a mass of about 0.11 M⊙.

Looking on the bright side: The story of AA Doradus as revealed by its cool companion

Irradiation effects in close binaries are crucial for a reliable determination of system parameters and understanding the close binary evolution. We study irradiated light originating from the low mass component of an eclipsing system comprising a hot subdwarf primary and a low mass companion, to precisely interpret their high precision photometric and spectroscopic data, and accurately determine their system and surface parameters. We re-analyse the archival VLT/UVES spectra of AA Dor system where irradiation features have already been detected. After removing the predominant contribution of the hot subdwarf primary, the residual spectra reveal more than 100 emission lines from the heated side of the secondary with maximum intensity close to the phases around secondary eclipse. We analyse 22 narrow emission lines of the irradiated secondary, mainly of OII, with a few CII lines. Their phase profiles constrain the emission region of the heated side to a radius $\geq$ 95% of the radius of the secondary. The shape of their velocity profiles reveals two distinct asymmetry features one at the quadrature and the other at the secondary eclipse. We identify more than 70 weaker emission lines originating from HeI, NII, SiIII, CaII and MgII. We correct the radial velocity semi-amplitude of the center-of-light to the centre-of-mass of the secondary and calculate accurate masses of both components. The resulting masses $M_{1}$=0.46 $\pm$ 0.01$M_{\odot}$ and $M_{2}$=0.079 $\pm$ 0.002$M_{\odot}$ are in perfect accordance with those of a canonical hot subdwarf primary and a low mass star just at the substellar limit for the companion. We compute a first generation atmosphere model of the irradiated low mass secondary, which matches the observed spectrum well. We find an indication of an extended atmosphere of the irradiated secondary star.

Period and amplitude variations in post-common-envelope eclipsing binaries observed with SuperWASP

Period or amplitude variations in eclipsing binaries may reveal the presence of additional massive bodies in the system, such as circumbinary planets. Here, we have studied twelve previously-known eclipsing post-common-envelope binaries for evidence of such light curve variations, on the basis of multi-year observations in the SuperWASP archive. The results for HW Vir provided strong evidence for period changes consistent with those measured by previous studies, and help support a two-planet model for the system. ASAS J102322-3737.0 exhibited plausible evidence for a period increase not previously suggested; while NY Vir, QS Vir and NSVS 14256825 afforded less significant support for period change, providing some confirmation to earlier claims. In other cases, period change was not convincingly observed; for AA Dor and NSVS 07826147, previous findings of constant period were confirmed. This study allows us to present hundreds of new primary eclipse timings for these systems, and further demonstrates the value of wide-field high-cadence surveys like SuperWASP for the investigation of variable stars.

On the sdOB primary of the post common-envelope binary AA Doradus (LB 3459)

AA Dor is an eclipsing, post common-envelope binary with an sdOB-type primary and a low-mass secondary. Eleven years ago, an NLTE spectral analysis showed a discrepancy in the surface gravity that was derived by radial-velocity and light-curve analysis, log g = 5.21 +/- 0.1 (cm/sec^2) and log g = 5.53 +/- 0.03, respectively. We aim to determine both the effective temperature and surface gravity of AA Dor precisely from high-resolution, high-S/N observations taken during the occultation of the secondary. We calculated an extended grid of metal-line blanketed, state-of-the-art, non-LTE model atmospheres in the parameter range of the primary of AA Dor. Synthetic spectra calculated from this grid were compared to optical observations. We verify Teff = 42000 +/- 1000 K from our former analyses and determine a higher log g = 5.46 +/- 0.05. The main reason are new Stark-broadening tables that were used for calculating of the theoretical Balmer-line profiles. Our result for the surface gravity agrees with the value from light-curve analysis within the error limits, thereby solving the so-called gravity problem in AA Dor.

The orbital periods of AA Dor and NY Vir

New timings of eclipses made between 2000 and 2010 are presented for two binary systems with hot subdwarf primary stars. In the case of AA Dor, an sdOB star with a very cool secondary, the period is found to be constant at a level of about 10 -14 d per orbit. In the case of NY Vir, a rapidly pulsating sdBV r with a cool companion, the period is discovered to be decreasing at a rate of -11.2 x 10 -13 d per orbit.

The cool companion of AA Doradus—Brown dwarf or late M star?

AA Dor is one of only seven known eclipsing binaries consisting of a hot subdwarf star and a low-mass companion. Although AA Dor has been studied in many investigations, a controversy about the nature of its companion persists. Is it a brown dwarf or a low-mass main sequence star?

The mass ratio and the orbital parameters of the sdOB binary AA Doradus

The time sequence of 105 spectra covering one full orbital period of AA Dor has been analysed. Direct determination of V sin i for the sdOB component from 97 spectra outside of the eclipse for the lines Mg II 4481 A and Si IV 4089 A clearly indicated a substantially smaller value than estimated before. Detailed modelling of line-profile variations for eight spectra during the eclipse for the Mg II 4481 A line, combined with the out-of-eclipse fits, gave V sin i = 31.8 ± 1.8 km s −1 . The previous determinations of V sin i, based on the He II 4686 A line, appear to be invalid because of the large natural broadening of the line. With the assumption of the solid-body, synchronous rotation of the sdOB primary, the measured values of the semi-amplitude K1 and V sin i lead to the mass ratio q = 0.213 ± 0.013 which in turn gives K2 and thus the masses and radii of both components. The sdOB component appears to be less massive than assumed before, M 1 = 0.25 ± 0.05 M� , but the secondary has its mass–radius p):

FUSE spectroscopy of the sdOB primary of the post common-envelope binary LB 3459(AA Doradus)

LB 3459 (AA Dor) is an eclipsing, close, post common-envelope binary consisting of an sdOB primary star and an unseen secondary with an extraordinarly low mass - formally a brown dwarf. A recent NLTE spectral analysis shows a discrepancy with the surface gravity, which is derived from analyses of radial-velocity and lightcurves. We aim at precisely determing of the photospheric parameters of the primary, especially of the surface gravity, and searching for weak metal lines in the far UV. We performed a detailed spectral analysis of the far-UV spectrum of LB 3459 obtained with FUSE by means of state-of-the-art NLTE model-atmosphere techniques. A strong contamination of the far-UV spectrum of LB 3459 by interstellar line absorption hampers a precise determination of the photospheric properties of its primary star. Its effective temperature (42 kK) was confirmed by the evaluation of new ionization equilibria. For the first time, phosphorus and sulfur have been identified in the spectrum of LB 3459. Their photospheric abundances are solar and 0.01 times solar, respectively. From the C III 1174-1177A multiplet, we can measure the rotational velocity of 35 +/- 5 km/sec of the primary of LB 3459 and confirm that the rotation is bound. From a re-analysis of optical and UV spectra, we determine a higher log g = 5.3 (cgs) that reduces the discrepancy in mass determination in comparison to analyses of radial-velocity and lightcurves. However, the problem is not completely solved.

Model Atmospheres for Irradiated Stars in Precataclysmic Variables

Model atmospheres have been computed for M dwarfs that are strongly irradiated by nearby hot companions. A variety of primary and secondary spectral types are explored in addition to models specific to four known systems: GD 245, NN Ser, AA Dor, and UU Sge. This work demonstrates that a dramatic temperature inversion is possible on at least one hemisphere of an irradiated M dwarf and that the emergent spectrum will be significantly different from an isolated M dwarf or a blackbody flux distribution. For the first time, synthetic spectra suitable for direct comparison to high-resolution observations of irradiated M dwarfs in non-mass-transferring post-common-envelope binaries are presented. The effects of departures from local thermodynamic equilibrium on the Balmer line profiles are also discussed.

AA Dor—An Eclipsing sdOB—Brown Dwarf Binary

AA Dor is an eclipsing, close, post common-envelope binary (PCEB) consisting of a sdOB primary star and an unseen secondary with an extraordinary small mass (M2≈ 0.066 M⊙)—formally a brown dwarf. The brown dwarf may have been a former planet which survived a common envelope (CE) phase and has even gained mass.

The Enigma of AA Dor

AA Dor (LB 3459) is an eclipsing, close binary (P = 0.26 d) consisting of a sdOB primary star and an unseen secondary with an extraordinary small mass. The secondary is possibly a former planet which may have survived a common-envelope phase and has even gained mass.In order to investigate on an existing discrepancy between the components' mass derived from NLTE spectral analysis and subsequent comparison to evolutionary tracks and masses derived from radial-velocity and the eclipse curves, we performed phase-resolved high-resolution and high-SN spectroscopy with the UVES attached to the ESO VLT. from the obtained spectra, we have determined AA Dor's orbital parameters (P = 22 600.702±0.005 sec, A = 39.19±0.05 km s–1, T0 = 2451917.152690) and the rotational velocity (vrot = 47±5 km s–1) of the primary.

AA DOR An Eclipsing Subdwarf - Brown Dwarf Binary

AA DOR, is an eclipsing, close, post common-envelope binary (PCEB) consisting of a sdOB primary star and an unseen secondary with an extraordinary small mass (M2 ≈ 0.066M⊙) formally a brown dwarf (see Rauch 2004 for details). In a spectral analysis of AA DOR, Rauch (2000) determined Teff = 42 kK and log g — 5.2 (cgs). The determination of the components' masses by comparison of these results with evolutionary models of Driebe et al. (1998) shows a discrepancy to masses derived from radial-velocity and the eclipse curves (Hilditch et al. 2003) log g = 5.5 would be necessary in order to achieve an intersection at M1 ≈ 0.330M⊙.

New VIC photometry of the sdOB binary AA Dor and an improved photometric model

New VI CCD photometry, obtained with integration times of 20s, of the sdOB+degenerate-dwarf eclipsing binary system AA Dor has provided new complete light curves with an rms scatter about a mean curve of +/-0.004 mag. These data are analysed with an improved Light2 light curve synthesis code to yield more accurate determinations of the radii of both stars, the orbital inclination, and the flux ratio between the two components. These radii are only a little different from the values derived 25 years ago from less complete data, but the uncertainties on these values are improved by a factor of two. The apparent discrepancy remains between the surface gravity of the sdOB primary star obtained from the light-curve solution with the published spectroscopic orbit and that obtained from NLTE analysis of high-resolution spectra of the sdOB star. The substantial reflection effect in the system is adequately represented by the Light2 code with a bolometric albedo of unity in light curves extending from 0.35 microns to 2.2 microns. However there are differences at individual wavelengths in the derived albedo, which may indicate redistribution of flux from shorter wavelengths into the V and I passbands.

The rotational velocity of the sdOB primary of the eclipsing binary system LB 3459 (AA Dor)

We present an analysis of the rotational velocity of the primary of LB 3459 based on 107 new high-resolution and high- ESO VLT UVES spectra. 105 of them cover a complete orbital period (0.26 d) of this binary system. We have determined an orbital period of , a radial velocity amplitude of , and . From simulations of the He II λ 4686 Å line profile (based on NLTE model atmosphere calculations), we derive . We present an animation which shows the orbital movement of the binary system, its synthetic lightcurve, and compares the phase-dependent variation of the predicted with the observed He II λ 4686 Å line profile. The radius of the cool component is almost the same size like Jupiter but its mass is about 70 times higher than Jupiter's mass. Thus, from its present mass (), the secondary of LB 3459 lies formally within the brown-dwarf mass range (). It might be a former planet which has survived the previous common-envelope phase and even has gained mass.

A new B-subdwarf eclipsing binary with an extremely short period

The B-type Subdwarf BD-07°3477 has been found to be an eclipsing binary with a period of about 2h48m. A preliminary analysis of the light curve reveals the system to be fully detached. It appears to be similar in many respects to the O-Subdwarf binary AA Dor which is thought to be the remnant of a system which has undergone a common envelope phase of evolution.

The Origin and Evolution of Cataclysmic Binaries

ABSTRACTSome cataclysmic binaries may be products of Case C evolution of low mass stars (orbital period ~ 1 yr; masses ~ 1 - 4 Mʘ), involving a common envelope phase. Other mechanisms, probably involving late Case B and even early Case B, but with significant loss of angular momentum, may be necessary to account for some evolved binaries such as AA Dor or V Sge. Further angular momentum loss, probably by magnetic braking coupled with tidal friction, causes secular evolution in cataclysmic binaries. It is suggested that tidal friction may account for the shortage of cataclysmics with periods ≲ 1.3 hr; but this cutoff, as well as the gap in the period distribution between 2 and 3 hrs, is hard to explain and imposes more severe constraints on possible theories than is commonly acknowledged.

The Subdwarf Eclipsing Binary LB3459 (AA Dor)

A review was presented of the currently available observational data on this evolved system, namely, uvby photometry (Kilkenny et al. 1978; paper II) radial velocities (Kilkenny et al. 1981; paper III) and a non-LTE atmosphere analysis of the primary component (Kudritski et al. 1981). Kudritski et al. conclude that T(pr) = 40000 ± 2500°K and log g = 5.3 ± 0.2 and from the spectroscopic data and light curve analysis (papers II, III) derive masses and radii for the primary and secondary components of M(pr) ≃ 0.25 M⊙, M(sec) ≃ 0.04 M⊙, R(pr) ≃ 0.16R⊙, R(sec) ≃ 0.09R⊙ respectively. Thus the primary component is a normal sd0 star whilst the secondary component is a most enigmatic object, perhaps a normal composition degenerate dwarf but of too low a mass to be on the main sequence, perhaps an evolved degenerate object but then the radius is too large. The published evolutionary models for this system (Paczynski 1980; Conti et al 1981) do not adequately describe its current status.