S. G. Parsons

The planets around NN serpentis: Still there

We present 25 new eclipse times of the white dwarf binary NN Ser taken with the high-speed camera ULTRACAM on the William Herschel Telescope and New Technology Telescope, the RISE camera on the Liverpool Telescope and HAWK-I on the Very Large Telescope to test the two-planet model proposed to explain variations in its eclipse times measured over the last 25 yr. The planetary model survives the test with flying colours, correctly predicting a progressive lag in eclipse times of 36 s that has set in since 2010 compared to the previous 8 yr of precise times. Allowing both orbits to be eccentric, we find orbital periods of 7.9 ± 0.5 and 15.3 ± 0.3 yr, and masses of 2.3 ± 0.5 and 7.3 ± 0.3 MJ. We also find dynamically long-lived orbits consistent with the data, associated with 2:1 and 5:2 period ratios. The data scatter by 0.07 s relative to the best-fitting model, by some margin the most precise of any of the proposed eclipsing compact object planet hosts. Despite the high precision, degeneracy in the orbit fits prevents a significant measurement of a period change of the binary and of N-body effects. Finally, we point out a major flaw with a previous dynamical stability analysis of NN Ser, and by extension, with a number of analyses of similar systems.

Variable emission from a gaseous disc around a metal-polluted white dwarf

We present the discovery of strongly variable emission lines from a gaseous disc around the DA white dwarf SDSS J1617+1620, a star previously found to have an infrared excess indicative of a dusty debris disc formed by the tidal disruption of a rocky planetary body. Time series spectroscopy obtained during the period 2006–2014 has shown the appearance of strong double-peaked Ca II emission lines in 2008. The lines were weak, at best, during earlier observations, and monotonically faded through the remainder of our monitoring. Our observations represent unambiguous evidence for short-term variability in the debris environment of evolved planetary systems. Possible explanations for this extraordinary variability include the impact on to the dusty disc of either a single small rocky planetesimal, or of material from a highly eccentric debris tail. The increase in flux from the emission lines is sufficient that similar events could be detected in the broad-band photometry of ongoing and future large-area time domain surveys.

The Not-so-massive Black Hole in the Microquasar GRS1915+105

We present a new dynamical study of the black hole X-ray transient GRS1915+105 making use of near-infrared spectroscopy obtained with X-shooter at the VLT. We detect a large number of donor star absorption features across a wide range of wavelengths spanning the H and K bands. Our 24 epochs covering a baseline of over 1 year permit us to determine a new binary ephemeris including a refined orbital period of P = 33.85±0.16d. The donor star radial velocity curves deliver a significantly improved determination of the donor semi-amplitude which is both accurate (K2 = 126±1 km/s) and robust against choice of donor star template and spectral features used. We furthermore constrain the donor star’s rotational broadening to v sin i = 21 ± 4 km/s, delivering a binary mass ratio of q = 0.042±0.024. If we combine these new constraints with distance and inclination estimates derived from modeling the radio emission, a black hole mass of MBH = 10.1± 0.6M⊙ is inferred, paired with an evolved mass donor of M2 = 0.47± 0.27M⊙. Our analysis suggests a more typical black hole mass for GRS1915+105 rather than the unusually high values derived in the pioneering dynamical study by Greiner et al. (2001). Our data demonstrate that high-resolution infrared spectroscopy of obscured accreting binaries can deliver dynamical mass determinations with a precision on par with optical studies. Subject headings: stars: individual (GRS1915+105) – X-rays: binaries – binaries: close – Techniques: radial velocitiesWe present a new dynamical study of the black hole X-ray transient GRS1915+105 making use of near-infrared spectroscopy obtained with X-shooter at the Very Large Telescope. We detect a large number of donor star absorption features across a wide range of wavelengths spanning the H and K bands. Our 24 epochs covering a baseline of over 1 yr permit us to determine a new binary ephemeris including a refined orbital period of P = 33.85 ± 0.16 days. The donor star radial velocity curves deliver a significantly improved determination of the donor semi-amplitude which is both accurate (K 2 = 126 ± 1 km s–1) and robust against choice of donor star template and spectral features used. We furthermore constrain the donor stars rotational broadening to vsin i = 21 ± 4 km s–1, delivering a binary mass ratio of q = 0.042 ± 0.024. If we combine these new constraints with distance and inclination estimates derived from modeling the radio emission, a black hole mass of M BH = 10.1 ± 0.6 M ☉ is inferred, paired with an evolved mass donor of M 2 = 0.47 ± 0.27 M ☉. Our analysis suggests a more typical black hole mass for GRS1915+105 rather than the unusually high values derived in the pioneering dynamical study by Greiner et al. Our data demonstrate that high-resolution infrared spectroscopy of obscured accreting binaries can deliver dynamical mass determinations with a precision on par with optical studies.

Doppler imaging of the planetary debris disc at the white dwarf SDSS J122859.93+104032.9

Debris discs which orbit white dwarfs are signatures of remnant planetary systems. We present 12 yr of optical spectroscopy of the metal-polluted white dwarf SDSS J1228+1040, which shows a steady variation in the morphology of the 8600 A Ca II triplet line profiles from the gaseous component of its debris disc. We identify additional emission lines of O I, Mg I, Mg II, Fe II and Ca II in the deep co-added spectra. These emission features (including Ca H & K) exhibit a wide range in strength and morphology with respect to each other and to the Ca II triplet, indicating different intensity distributions of these ionic species within the disc. Using Doppler tomography, we show that the evolution of the Ca II triplet profile can be interpreted as the precession of a fixed emission pattern with a period in the range 24–30 yr. The Ca II line profiles vary on time-scales that are broadly consistent with general relativistic precession of the debris disc.

Eclipsing post-common envelope binaries from the Catalina surveys

We analyse the Catalina Real-time Transient Survey light curves of 835 spectroscopically confirmed white dwarf plus main-sequence binaries from the Sloan Digital Sky Survey (SDSS) with g < 19, in search of new eclipsing systems. We identify 29 eclipsing systems, 12 of which were previously unknown. This brings the total number of eclipsing white dwarf plus main-sequence binaries to 49. Our set of new eclipsing systems contains two with periods of 1.9 and 2.3 d, making them the longest period eclipsing white dwarf binaries known. We also identify one system which shows very large ellipsoidal modulation (almost 0.3 mag), implying that the system is both very close to Roche lobe overflow and at high inclination. However, our follow-up photometry failed to firmly detect an eclipse, meaning that either this system contains a cool white dwarf and hence the eclipse is very shallow and undetectable in our red-sensitive photometry or that it is non-eclipsing. Radial velocity measurements for the main-sequence stars in three of our newly identified eclipsing systems imply that their white dwarf masses are lower than those inferred from modelling their SDSS spectra. 13 non-eclipsing post-common envelope binaries were also identified, from either reflection or ellipsoidal modulation effects. The white dwarfs in our newly discovered eclipsing systems span a wide range of parameters, including low-mass (∼0.3 M_⊙), very hot (80 000 K) and a DC white dwarf. The spectral types of the main-sequence stars range from M2 to M6. This makes our sample ideal for testing white dwarf and low-mass star mass–radius relationships as well as close binary evolution.

An accurate mass and radius measurement for an ultracool white dwarf

Studies of cool white dwarfs in the solar neighbourhood have placed a limit on the age of the Galactic disc of 8–9 billion years. However, determining their cooling ages requires the knowledge of their effective temperatures, masses, radii and atmospheric composition. So far, these parameters could only be inferred for a small number of ultracool white dwarfs for which an accurate distance is known, by fitting their spectral energy distributions in conjunction with a theoretical mass–radius relation. However, the mass–radius relation remains largely untested, and the derived cooling ages are hence model dependent. Here we report direct measurements of the mass and radius of an ultracool white dwarf in the double-lined eclipsing binary SDSS J013851.54−001621.6. We find MWD = 0.529 ± 0.010 M⊙ and RWD = 0.0131 ± 0.0003 R⊙. Our measurements are consistent with the mass–radius relation and we determine a robust cooling age of 9.5 billion years for the 3570 K white dwarf. We find that the mass and radius of the low-mass companion star, Msec = 0.132 ± 0.003 M⊙ and Rsec = 0.165 ± 0.001 R⊙, are in agreement with evolutionary models. We also find evidence that this >9.5 Gyr old M5 star is still active, far beyond the activity lifetime for a star of its spectral type. This is likely caused by the high tidally enforced rotation rate of the star. The companion star is close to filling its Roche lobe and the system will evolve into a cataclysmic variable in only 70 Myr. Our direct measurements demonstrate that this system can be used to calibrate ultracool white dwarf atmospheric models.

Post Common Envelope Binaries from SDSS. XV: Accurate stellar parameters for a cool 0.4-solar mass white dwarf and a 0.16-solar mass M-dwarf in a 3 hour eclipsing binary

We identify SDSSJ121010.1+334722.9 as an eclipsing post-common-envelope binary, with an orbital period of P ~ 3 hrs, containing a very cool, low-mass, DAZ white dwarf and a low-mass main-sequence star of spectral type M5. A model atmosphere analysis of the metal absorption lines detected in the blue part of the optical spectrum, along with the GALEX near-ultraviolet flux, yields a white dwarf temperature of 6000 +/- 200 K and a metallicity value of log(Z/H)= -2.0 +/- 0.3. The sodium absorption doublet is used to measure the radial velocity of the secondary star, K2 ~ 252 km/s and iron absorption lines in the blue part of the spectrum provide the radial velocity of the white dwarf, K1 ~ 95 km/s, yielding a mass ratio of q ~ 0.38. Light curve model fitting, using the Markov Chain Monte Carlo (MCMC) method, gives the inclination angle as i = (79.05 - 79.36) +/- 0.15 degrees, and the stellar masses as M1 = 0.415 +/- 0.010 solar-masses and M2 = 0.158 +/- 0.006 solar-masses. Systematic uncertainties in the absolute calibration of the photometric data influence the determination of the stellar radii. The radius of the white dwarf is found to be R1 = (0.0157 - 0.0161) +/- 0.0003 solar-radii and the volume-averaged radius of the tidally distorted secondary is R2 = (0.197 - 0.203) +/- 0.003 solar-radii. The white dwarf in J1210+3347 is a very strong He-core candidate.

Precise parameters for both white dwarfs in the eclipsing binary CSS 41177

We present ULTRACAM photometry and X-Shooter spectroscopy of the eclipsing double white dwarf binary CSS 41177, the only such system that is also a double-lined spectroscopic binary. Combined modelling of the light curves and radial velocities yield masses and radii for both white dwarfs without the need to assume mass–radius relations. We find that the primary white dwarf has a mass of M1 = 0.38 ± 0.02 M⊙ and a radius of R1 = 0.0222 ± 0.0004 R⊙. The secondary white dwarfs mass and radius are M2 = 0.32 ± 0.01 M⊙ and R2 = 0.0207 ± 0.0004 R⊙, and its temperature and surface gravity (T2 = 11678 ± 313 K, log(g2) = 7.32 ± 0.02) put it close to the white dwarf instability strip. However, we find no evidence for pulsations to roughly 0.5 per cent relative amplitude. Both masses and radii are consistent with helium white dwarf models with thin hydrogen envelopes of ≤10−4 M*. The two stars will merge in 1.14 ± 0.07 Gyr due to angular momentum loss via gravitational wave emission.

A radio-pulsing white dwarf binary star.

White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a δ-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56-hour period close binary, pulsing in brightness on a period of 1.97 minutes. The pulses are so intense that AR Sco’s optical flux can increase by a factor of four within 30 seconds, and they are also detectable at radio frequencies. They reflect the spin of a magnetic white dwarf, which we find to be slowing down on a 107-year timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf’s spin, they mainly originate from the cool star. AR Sco’s broadband spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf’s magnetosphere.

Optical transmission photometry of the highly inflated exoplanet WASP-17b

We present ground-based high-precision observations of the transit of WASP-17b using the multiband photometer ULTRACAM on ESOs New Technology Telescope (NTT) in the context of performing transmission spectrophotometry of this highly inflated exoplanet. Our choice of filters (SDSS u′, g′ and r′ bands) is designed to probe for the presence of opacity sources in the upper atmosphere. We find evidence for a wavelength dependence in the planet radius in the form of enhanced absorption in the SDSS r′ band, consistent with a previously detected broad sodium feature. We present a new independent measurement of the planetary radius at Rpl = 1.97 ± 0.06RJ, which confirms this planet as the most inflated exoplanet known to date. Our measurements are most consistent with an atmospheric profile devoid of enhanced TiO opacity, previously predicted to be present for this planet.