Chris Koen

The DA+dMe eclipsing binary EC13471–1258: its cup runneth over … just

The optical spectrum and light curve of EC13471-1258 show that it is an eclipsing binary with an orbital period of 3 h 37 m comprising a DA white dwarf and a dMe dwarf. Total eclipses of the white dwarf are observed lasting 14 min, with the partial phases lasting 54 s. On one occasion, two pre-eclipse dips were seen. Timings of the eclipses over 10 yr show jitter of up to 12 s. Flares from the M dwarf are regularly observed. The M dwarf also shows a large-amplitude ellipsoidal modulation in the V-band light curve. The component stars emit almost equal amounts of light at 5500 A. Hubble Space Telescope (HST) STIS spectra show strong Lyman-a absorption with weak metal lines of C I,II and Si II superimposed. Model atmosphere analysis yielded an effective temperature of 14 220 ′ 300 K and log g of 8.34 ′ 0.20 for the white dwarf with these errors being strongly correlated. Its metal abundance is 1/30th solar with an uncertainty of 0.5 dex, and it is rapidly rotating with V 1 sin i = 400 ′ 100 km s - 1 . The white dwarf also shows radial velocity variations with a semi-amplitude of 138 ′ 10 km s - 1 . The gravitational redshift of the white dwarf was measured as 62 km s - 1 . From optical spectroscopy the spectral type of the M dwarf was found to be M3.5-M4, its temperature 3100 ′ 75 K, its rotational velocity 140 ′ 10 km s - 1 , its radial velocity semi-amplitude 266 ′ 5 km s - 1 , its mean V - I colour 2.86 and its absolute V magnitude 11.82. Intriguingly, its metal abundance is normal solar. The Ha emission line shows at least two distinct components, one of which is uniformly distributed around the centre of mass of the M dwarf and provided the estimate of the rotational velocity of the M dwarf. The other arises from the other side of the binary centre of mass, well within the white dwarf Roche lobe. This behaviour is confirmed by Doppler tomography, which shows the presence of two distinct velocity components within the primary Roche lobe. The interpretation of these features is uncertain. Variations in strength of the components with binary phase can be attributed to optical thickness in the Balmer lines. Similar behaviour is seen in the observations of the other Balmer emission lines, although with a poorer signal-to-noise ratio. Flares in Ha were observed and are consistent with arising from the vicinity of the M dwarf. Dynamical solutions for the binary are discussed and yield an inclination of 75.5° ′ 2.0°, a white dwarf mass and radius of 0.78 ′ 0.04 M O . and 0.011 ′ 0.01 R O ., and an M dwarf mass and radius of 0.43 ′ 0.04 M O . and 0.42 ′ 0.02 R O .. These parameters are consistent with the Wood mass-radius relation for white dwarfs and the Clemens et al. mass-radius relation for M dwarfs; we argue that the M dwarf just fills its Roche lobe. The radius of the white dwarf and the model fit imply a distance of 48 ′ 5 pc and an absolute V magnitude of 11.74. The rapid rotation of the white dwarf strongly suggests that the system has undergone mass transfer in the past, and implies that it is a hibernating cataclysmic variable. The M dwarf shows the properties expected of secondaries in cataclysmic variables: chromospheric activity and angular momentum loss.

Structure of Herbig AeBe disks at the milliarcsecond scale: A statistical survey in the H band using PIONIER-VLTI

Context. It is now generally accepted that the near-infrared excess of Herbig AeBe stars originates in the dust of a circumstellar disk. Aims. The aims of this article are to infer the radial and vertical structure of these disks at scales of order 1 au, and the properties of the dust grains. Methods. The program objects (51 in total) were observed with the H-band (1.6 μm) PIONIER/VLTI interferometer. The largest baselines allowed us to resolve (at least partially) structures of a few tenths of an au at typical distances of a few hundred parsecs. Dedicated UBVRIJHK photometric measurements were also obtained. Spectral and 2D geometrical parameters are extracted via fits of a few simple models: ellipsoids and broadened rings with azimuthal modulation. Model bias is mitigated by parallel fits of physical disk models. Sample statistics were evaluated against similar statistics for the physical disk models to infer properties of the sample objects as a group. Results. We find that dust at the inner rim of the disk has a sublimation temperature T_(sub) ≈ 1800 K. A ring morphology is confirmed for approximately half the resolved objects; these rings are wide δr/r ≥ 0.5. A wide ring favors a rim that, on the star-facing side, looks more like a knife edge than a doughnut. The data are also compatible with the combination of a narrow ring and an inner disk of unspecified nature inside the dust sublimation radius. The disk inner part has a thickness z/r ≈ 0.2, flaring to z/r ≈ 0.5 in the outer part. We confirm the known luminosity-radius relation; a simple physical model is consistent with both the mean luminosity-radius relation and the ring relative width; however, a significant spread around the mean relation is present. In some of the objects we find a halo component, fully resolved at the shortest interferometer spacing, that is related to the HAeBe class.

Sporadic long-term variability in radio activity from a brown dwarf

Context. Radio activity has been observed in a large variety of stellar objects, including in the last few years, ultra-cool dwarfs. Aims. To explore the extent of long-term radio activity in ultra-cool dwarfs. Methods. We use data taken over an extended period of 9 hr from the Very Large Array of the source 2MASS J05233822-1403022 in September 2006, plus data taken in 2004. Results. The observation taken in September 2006 failed to detect any radio activity at 8.46 GHz. A closer inspection of earlier data reveals that the source varied from a null detection on 3 May 2004, to ≈95 µJy on 17 May 2004, to 230 µJy on 18 June 2004. The lack of detection in September 2006 suggests at least a factor of ten flux variability at 8.46 GHz. Three short photometric runs did not reveal any optical variability. Conclusions. In addition to the observed pulsing nature of the radio flux from another ultra-cool source, the present observations suggests that ultra-cool dwarfs may not just be pulsing but can also display long-term sporadic variability in their levels of quiescent radio emission. The lack of optical photometric variability suggests an absence of large-scale spots at the time of the latest VLA observations, although small very high latitude spots combined with a low inclination could cause very low amplitude rotational modulation which may not be measurable. We discuss this large variability in the radio emission within the context of both gyrosynchrotron emission and the electron-cyclotron maser, favoring the latter mechanism.

Long-term waves on Cepheid pulsation period trends

Abstract The secular evolution of the pulsation period of δ Cephei, η Aquilae, and ζ Geminorum during a cumulative total of 577 years suggests wave-like departures about the smooth time trends.

Multi-wavelength observations of the binary system PSR B1259−63/LS 2883 around the 2014 periastron passage

We report on the results of the extensive multi-wavelength campaign from optical to GeV γ-rays of the 2014 periastron passage of PSR B1259−63, which is a unique high-mass γ-ray emitting binary system with a young pulsar companion. Observations demonstrate the stable nature of the post-periastron GeV flare and prove the coincidence of the flare with the start of rapid decay of the Hα equivalent width, usually interpreted as a disruption of the Be stellar disc. Intensive X-ray observations reveal changes in the X-ray spectral behaviour happening at the moment of the GeV flare. We demonstrate that these changes can be naturally explained as a result of synchrotron cooling of monoenergetic relativistic electrons injected into the system during the GeV flare.

UBV(RI)_C photometry of transiting planet host stars

We present new UBV(RI)_C photometry of 22 stars that host transiting planets, 19 of which were discovered by the WASP survey. We use these data together with 2MASS JHK_S photometry to estimate the effective temperature of these stars using the infrared flux method. We find that the effective temperature estimates for stars discovered by the WASP survey based on the analysis of spectra are reliable to better than their quoted uncertainties.

UBV(RI) C photometry of transiting planet hosting stars

We present new UBV(RI)_C photometry of 22 stars that host transiting planets, 19 of which were discovered by the WASP survey. We use these data together with 2MASS JHK_S photometry to estimate the effective temperature of these stars using the infrared flux method. We find that the effective temperature estimates for stars discovered by the WASP survey based on the analysis of spectra are reliable to better than their quoted uncertainties.

The detailed forms of the LMC Cepheid PL and PLC relations

Possible deviations from linearity of the Large Magellanic Cloud Cepheid period‐luminosity (PL) and period‐luminosity‐colour (PLC) relations are investigated. Two data sets are studied, respectively from the Optical Gravitational Lensing Experiment (OGLE) and MACHO projects. A non-parametric test, based on linear regression residuals, suggests that neither PL relation is linear. If colour dependence is allowed for, then the MACHO PL relation is found to deviate more significantly from the linear, while the OGLE PL relation is consistent with linearity. These findings are confirmed by fitting ‘Generalized Additive Models’ (non-parametric regression functions) to the two data sets. Colour dependence is shown to be non-linear in both data sets, distinctly so in the case of the MACHO Cepheids. It is also shown that there is interaction between the period and the colour functions in the MACHO

An Evaluation of the Membership Probability of 212 λ Boo Stars. I. A Catalogue

The literature on the λ Boo stars has grown to become somewhat heterogenous, as different authors have applied different criteria across the UV, optical, and infrared regions to determine the membership status of λ Boo candidates. We aim to clear up the confusion by consulting the literature on 212 objects that have been considered as λ Boo candidates, and subsequently evaluating the evidence in favour of their admission to the λ Boo class. We obtained new spectra of ~ 90 of these candidates and classified them on the MK system to aid in the membership evaluations. The re-evaluation of the 212 objects resulted in 64 members and 103 non-members of the λ Boo class, with a further 45 stars for which membership status is unclear. We suggest observations for each of the stars in the latter category that will allow them to be confidently included or rejected from the class. Our reclassification facilitates homogenous analysis on group members, and represents the largest collection of confirmed λ Boo stars known.

The rapidly pulsating sdO star, SDSS J160043.6+074802.9

A spectroscopic analysis of Sloan Digital Sky Survey (SDSS) J160043.6+074802.9, a binary system containing a pulsating subdwarf-O (sdO) star with a late-type companion, yields T eff = 70 000 ± 5000 K and log g = 5.25 ± 0.30, together with a most likely type of K3 V for the secondary star. We compare our results with atmospheric parameters derived by Fontaine et al. and in the context of existing evolution models for sdO stars. New and more extensive photometry is also presented which recovers most, but not all, frequencies found in an earlier paper. Therefore, it seems probable that some pulsation modes have variable amplitudes. A non-adiabatic pulsation analysis of uniform metallicity sdO models show those having log g > 5.3 to be more likely to be unstable and capable of driving pulsation in the observed frequency range.