Spin-resolved spectroscopy of the intermediate polar DQ Her

We present high-speed spectroscopic observations of the intermediate polar (IP) DQ Herculis. Doppler tomography of two He I lines reveals a spiral density structure in the accretion disc around the white dwarf (WD) primary. The spirals look very similar to the spirals seen in dwarf novae during outburst. DQ Her is the first well-established IP in which spirals are seen, which are in addition likely persistent because of the system's high mass transfer rate. Spiral structures give an alternative explanation for sidebands of the WD spin frequency that are found in IP light curves. The Doppler tomogram of He II lambda 4686 indicates that a large part of the emission is not disc-like. Spin trails of spectra reveal a pulsation in the He II lambda 4686 emission that is believed to result from reprocessing of X-rays from the WD's magnetic poles in the accretion flow close to the WD. We confirm the previous finding that the pulsation is only visible in the redshifted part of the line when the beam points to the back side of the disc. The absence of reprocessed light from the front side of the disc can be explained by obscuration by the front rim of the disc, but the absence of extra emission from the blueshifted back side of the disc is puzzling. Reprocessing in accretion curtains can be an answer to the problem and can also explain the highly non-Keplerian velocity components that are found in the He II lambda 4686 line. Our spin trails can form a strong test for future accretion curtain models, with the possibility of distinguishing between a spin period of 71 or 142 s. Spin trails of data taken at selected orbital phases show little evidence for a significant contribution of the bright spot to the pulsations and allow us to exclude a recent suggestion that 71 s is the beat period and 70.8 s the spin period

KIC7668647: a 14 day beaming sdB+WD binary with a pulsating subdwarf

The recently discovered subdwarf B (sdB) pulsator KIC7668647 is one of the 18 pulsating sdB stars detected in the Kepler field. It features a rich g-mode frequency spectrum, with a few low-amplitude p-modes at short periods. We use new ground-based low-resolution spectroscopy, and the near-continuous 2.88 year Kepler lightcurve, to reveal that KIC7668647 consists of a subdwarf B star with an unseen white-dwarf companion with an orbital period of 14.2d. An orbit with a radial-velocity amplitude of 39km/s is consistently determined from the spectra, from the orbital Doppler beaming seen by Kepler at 163ppm, and from measuring the orbital light-travel delay of 27 by timing of the many pulsations seen in the Kepler lightcurve. The white dwarf has a minimum mass of 0.40 M_sun. We use our high signal-to-noise average spectra to study the atmospheric parameters of the sdB star, and find that nitrogen and iron have abundances close to solar values, while helium, carbon, oxygen and silicon are underabundant relative to the solar mixture. We use the full Kepler Q06--Q17 lightcurve to extract 132 significant pulsation frequencies. Period-spacing relations and multiplet splittings allow us to identify the modal degree L for the majority of the modes. Using the g-mode multiplet splittings we constrain the internal rotation period at the base of the envelope to 46-48d as a first seismic result for this star. The few p-mode splittings may point at a slightly longer rotation period further out in the envelope of the star. From mode-visibility considerations we derive that the inclination of the rotation axis of the sdB in KIC7668647 must be around ~60 degrees. Furthermore, we find strong evidence for a few multiplets indicative of degree 3 <= L <= 8, which is another novelty in sdB-star observations made possible by Kepler.Comment: arXiv admin note: text overlap with arXiv:1206.387

Kepler detection of a new extreme planetary system orbiting the subdwarf-B pulsator KIC10001893

KIC10001893 is one out of 19 subdwarf-B (sdB) pulsators observed by the Kepler spacecraft in its primary mission. In addition to tens of pulsation frequencies in the g-mode domain, its Fourier spectrum shows three weak peaks at very low frequencies, which is too low to be explained in terms of g modes. The most convincing explanation is that we are seeing the orbital modulation of three Earth-size planets (or planetary remnants) in very tight orbits, which are illuminated by the strong stellar radiation. The orbital periods are P1=5.273, P2=7.807, and P3=19.48 hours, and the period ratios P2/P1=1.481 and P3/P2=2.495 are very close to the 3:2 and 5:2 resonances, respectively. One of the main pulsation modes of the star at 210.68 {\mu}Hz corresponds to the third harmonic of the orbital frequency of the inner planet, suggesting that we see, for the first time in an sdB star, g-mode pulsations tidally excited by a planetary companion. The extreme planetary system that emerges from the Kepler data is very similar to the recent discovery of two Earth-size planets orbiting the sdB pulsator KIC05807616 (Charpinet et al. 2011a).Comment: 6 pages, 5 figures, accepted for publication in Astronomy and Astrophysic

Asteroseismic analysis of subdwarf B variable stars of KIC 10001893 and EPIC 220641886

KIC10001893 and EPIC220641886 are V1093Her type pulsating subdwarf-B stars, which were observed by the Kepler spacecraft during nominal and K2 mission respectively. The amplitude spectrum of both stars show similar characteristics that the majority of the pulsation frequencies lay in the gravity g-mode domain. While the g-mode region contains 248 frequencies, the p-mode region contains just 15, altogether indicating the hybrid nature of both stars. We used one of the seismic tools, asymptotic period spacing effectively to identify the modal degree of the majority of the modes. We could not find rotational multiplets for both stars which is likely due to pole-on orientation. We assigned dipole and quadrupole modes for KIC10001893, while for EPIC220641886 we defined modal degrees ranging from l=1 to l=12, apart from l=3 and l=11 modes, which are not seen

Time-series Spectroscopy and Photometry of the Pulsating Subdwarf B Star PG 1219+534 (KY UMa)

We present observations and analysis of time-series spectroscopy and photometry of the pulsating subdwarf B star PG 1219+534 (KY UMa). Subdwarf B stars are blue horizontal branch stars which have shed most of their hydrogen envelopes. Pulsating subdwarf B stars allow a probe into this interesting phase of evolution. Low resolution spectra were obtained at the Nordic Optical Telescope and Kitt Peak National Observatory, and photometric observations were obtained at MDM and Baker observatories in 2006. We extracted radial velocity and equivalent width variations from several Balmer and He I lines in individual spectra. The pulsation frequencies were separated via phase binning to detect line-profile variations in Balmer and helium lines, which were subsequently matched to atmospheric models to infer effective temperature and gravity changes throughout the pulsation cycle. From the photometry we recovered the four previously observed frequencies and detected a new fifth frequency. From the spectra we directly measured radial velocity and equivalent width variations for the four main frequencies and from atmospheric models we successfully inferred temperature and gravity changes for these four frequencies. We compared amplitude ratios and phase differences of these quantities and searched for outliers which could be identified as high-degree modes. These are the first such measurements for a "normal" amplitude pulsating subdwarf B star, indicating that spectroscopic studies can benefit the majority of pulsating subdwarf B stars.Comment: 9 pages, 13 figures accepted for publication in Astronomy & Astrophysic

Cosmological Microlensing Statistics: Variability rates for Quasars and GRB Afterglows, and implications for macrolensing magnification bias and flux ratios

The fraction of quasar's and gamma-ray burst (GRB) afterglows that vary due to microlensing by the stellar populations of intervening elliptical/S0 galaxies is computed by combining the joint distribution of effective microlensing convergence (kappa) and shear (gamma) with microlensing magnification patterns. Microlensing is common in multiply imaged sources. We find that 1 in 3 multiply imaged quasars should vary by more than 0.5 magnitudes per decade due to microlensing, while 10% of macrolensed GRB afterglows should show a departure of more than 0.5 magnitudes from their intrinsic light-curve during the first 30 days. However microlensing by stars is rare in general, with only 1 source in ~500 varying by more than 0.5 magnitudes during the same periods. We find that most microlensing by stars will be observed in a regime where gamma>0.1. Thus point-mass lenses do not provide an adequate description for most microlensing events. If dark matter halos contain a large fraction of mass in compact objects, the fraction of microlensed (by 0.5 magnitudes) images rises significantly to ~1 in 10 for quasars and ~1 in 5 for GRB afterglows. Comparison of variability between macrolensed and normal quasar images, and a moderate number of well sampled GRB afterglow light-curves should therefore discover or refute the existence of stellar mass compact objects in galaxy halos. While microlensing results in departures of the distribution of magnifications from that of a smooth model, the effect on the macrolensing magnification bias for the discovery of lenses in quasar surveys is small. On the other hand, microlensing significantly broadens the distribution of macrolensed image flux ratios.Comment: 35 pages, 15 figures. Submitted to Ap

A pulsation analysis of K2 observations of the subdwarf B star PG 1142-037 during Campaign 1 : A subsynchronously rotating ellipsoidal variable

We report a new subdwarf B pulsator, PG 1142-037, discovered during the first full-length campaign of K2, the two-gyro mission of the Kepler space telescope. 14 periodicities have been detected between 0.9 and 2.5 hr with amplitudes below 0.35 parts-per-thousand. We have been able to associate all of the pulsations with low-degree, 1Peer reviewe