88 research outputs found
An irradiated brown dwarf companion to an accreting white dwarf
No full textBrown dwarfs and giant planets orbiting close to a host star are subjected to significant irradiation that can modify the properties of their atmospheres. In order to test the atmospheric models that are used to describe these systems, it is necessary to obtain accurate observational estimates of their physical properties (masses, radii, temperatures, albedos). Interacting compact binary systems provide a natural laboratory for studying strongly irradiated sub-stellar objects. As the mass-losing secondary in these systems makes a critical, but poorly understood transition from the stellar to the sub-stellar regime, it is also strongly irradiated by the compact accretor. In fact, the internal and external energy fluxes are both expected to be comparable in these objects, providing access to an unexplored irradiation regime. However, the atmospheric properties of such donors have so far remained largely unknown. Here, we report the direct spectroscopic detection and characterisation of an irradiated sub-stellar donor in an accreting white dwarf binary system. Our near-infrared observations allow us to determine a model-independent mass estimate for the donor of and an average spectral type of , supporting both theoretical predictions and model-dependent observational constraints. Our time-resolved data also allow us to estimate the average irradiation-induced temperature difference between the day and night sides on the sub-stellar donor, ~K, and the maximum difference between the hottest and coolest parts of its surface, of ~K. The observations are well described by a simple geometric reprocessing model with a bolometric (Bond) albedo of A_B < 0.54 at the 2- confidence level, consistent with high reprocessing efficiency, but poor lateral heat redistribution in the donor's atmosphere
A magnetic white dwarf in a detached eclipsing binary
No full textSDSS J030308.35+005444.1 is a close, detached, eclipsing white dwarf plus M dwarf binary which shows a large infrared excess which has been interpreted in terms of a circumbinary dust disc. In this paper, we present optical and near-infrared photometric and spectroscopic data for this system. At optical wavelengths, we observe heated pole caps from the white dwarf caused by accretion of wind material from the main-sequence star on to the white dwarf. At near-infrared wavelengths, we see the eclipse of two poles on the surface of the white dwarf by the main-sequence star indicating that the white dwarf is magnetic. Our spectroscopic observations reveal Zeeman-split emission lines in the hydrogen Balmer series, which we use to measure the magnetic field strength as 8 MG. This measurement indicates that the cyclotron lines are located in the infrared, naturally explaining the infrared excess without the need for a circumbinary dust disc. We also detect magnetically confined material located roughly midway between the two stars. Using measurements of the radial velocity amplitude and rotational broadening of the M star, we constrain the physical parameters of the system, a first for a magnetic white dwarf, and the location of the poles on the surface of the white dwarf. SDSS J030308.35+005444.1 is a pre-cataclysmic variable that will likely evolve into an intermediate polar in ∼1 Gyr
The planets around NN Serpentis : still there
No full textWe 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
The shocking transit of WASP-12b : modelling the observed early ingress in the near ultraviolet
No full text4 pages, 2 figuresNear ultraviolet observations of WASP-12b have revealed an early ingress compared to the optical transit lightcurve. This has been interpreted as due to the presence of a magnetospheric bow shock which forms when the relative velocity of the planetary and stellar material is supersonic. We aim to reproduce this observed early ingress by modelling the stellar wind (or coronal plasma) in order to derive the speed and density of the material at the planetary orbital radius. From this we determine the orientation of the shock and the density of compressed plasma behind it. With this model for the density structure surrounding the planet we perform Monte Carlo radiation transfer simulations of the near UV transits of WASP-12b with and without a bow shock. We find that we can reproduce the transit lightcurves with a wide range of plasma temperatures, shock geometries and optical depths. Our results support the hypothesis that a bow shock could explain the observed early ingress.Peer reviewe
Using Gaussian Processes to detect AGN flares
No full textA key feature of active galactic nuclei (AGN) is their variability across all
wavelengths. Typically, AGN vary by a few tenths of a magnitude or more over
periods lasting from hours to years. By contrast, extreme variability of AGN --
large luminosity changes that are a significant departure from the baseline
variability -- are known as AGN flares. These events are rare and their
timescales poorly constrained, with most of the literature focusing on
individual events. It has been suggested that extreme AGN variability including
flares can provide insights into the accretion processes in the disk. With
surveys such as the Legacy Survey of Space and Time (LSST) promising millions
of transient detections per night in the coming decade, there is a need for
fast and efficient classification of AGN flares. The problem with the
systematic detection of AGN flares is the requirement to detect them against a
stochastically variable baseline; the ability to define a signal as a
significant departure from the ever-present variability is a statistical
challenge. Recently, Gaussian Processes (GPs) have revolutionised the analysis
of time-series data in many areas of astronomical research. They have, however,
seen limited uptake within the field of transient detection and classification.
Here we investigate the efficacy of Gaussian Processes to detect AGN flares in
both simulated and real optical light curves. We show that GP analysis can
successfully detect AGN flares with a false-positive rate of less than seven
per cent, and we present examples of AGN light curves that show extreme
variability.Comment: 14 pages, 25 figures, accepted for publication in MNRA
A telescope control and scheduling system for the Gravitational-wave Optical Transient Observer (GOTO)
No full textThe Gravitational-wave Optical Transient Observer (GOTO) is a wide-field telescope project aimed at detecting optical counterparts to gravitational wave sources. The prototype instrument was inaugurated in July 2017 on La Palma in the Canary Islands. We describe the GOTO Telescope Control System (G-TeCS), a custom robotic control system written in Python which autonomously manages the telescope hardware and nightly operations. The system comprises of multiple independent control daemons, which are supervised by a master control program known as the "pilot". Observations are decided by a "just-in-time" scheduler, which instructs the pilot what to observe in real time and provides quick follow-up of transient events
Measuring the Initial-Final Mass-Relation using wide double white dwarf binaries from Gaia DR3
No full textThe Initial-Final Mass-Relation (IFMR) maps the masses of main sequence stars
to their white dwarf descendants. The most common approach to measure the IFMR
has been to use white dwarfs in clusters. However, it has been shown that wide
double white dwarfs can also be used to measure the IFMR using a Bayesian
approach. We have observed a large sample of 90 Gaia double white dwarfs using
FORS2 on the VLT. Considering 52 DA+DA, DA+DC, and DC+DC pairs, we applied our
extended Bayesian framework to probe the IFMR in exquisite detail. Our
monotonic IFMR is well constrained by our observations for initial masses of
1-5 Msun, with the range 1-4 Msun mostly constrained to a precision of 0.03
Msun or better. We add an important extension to the framework, using a
Bayesian mixture-model to determine the IFMR robustly in the presence of
systems departing from single star evolution. We find a large but uncertain
outlier fraction of 5921 percent, with outlier systems requiring an
additional Gyr uncertainty in their cooling age
differences. However, we find that this fraction is dominated by a few systems
with massive components near 0.9 Msun, where we are most sensitive to outliers,
but are also able to establish four systems as merger candidatesComment: Accepted for publication in MNRA
Hot DAVs: a probable new class of pulsating white dwarf stars
No full textWe have discovered a pulsating DA white dwarf at the lower end of the temperature range 45 000–30 000 K where a few helium atmosphere white dwarfs are known. There are now three such pulsators known, suggesting that a new class of theoretically predicted pulsating white dwarf stars exists. We name them the hot DAV stars. From high-speed photometric observations with the ULTRACAM photometer on the 4.2-m William Herschel Telescope, we show that the hydrogen atmosphere white dwarf star WD1017−138 pulsates in at least one mode with a frequency of 1.62 mHz (a period of 624 s). The amplitude of that mode was near 1 mmag at a 10σ confidence level on one night of observation and an 8.4σ confidence level on a second night. The combined data have a confidence level of 11.8σ. This supports the two other detections of hot DAV stars previously reported. From three Very Large Telescope Ultraviolet and Visual Echelle Spectrograph spectra we confirm also that WD1017−138 is a hydrogen atmosphere white dwarf with no trace of helium or metals with Teff = 32 600 K, log g = 7.8 (cgs) and M = 0.55 M⊙. The existence of pulsations in these DA white dwarfs at the cool edge of the 45 000–30 000 K temperature range supports the thin hydrogen layer model for the deficit of helium atmosphere white dwarfs in this range. DA white dwarfs with thick hydrogen layers do not have the superadiabatic, chemically inhomogeneous (μ-gradient) zone that drives pulsation in this temperature range. The potential for higher amplitude hot DAV stars exists; their discovery would open the possibility of a direct test of the explanation for the deficit of helium atmosphere white dwarfs at these temperatures by asteroseismic probing of the atmospheric layers of the hot DAV stars. A search for pulsation in a further 22 candidates with ULTRACAM on the European Southern Observatory New Technology Telescope gave null results for pulsation at precisions in the range 0.5–3 mmag, suggesting that the pulsation amplitudes in such stars are relatively low, hence near the detection limit with the ground-based telescopes used in the survey
An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system
No full textRelativistic plasma jets are observed in many accreting black holes. According to theory, coiled magnetic fields close to the black hole accelerate and collimate the plasma, leading to a jet being launched. Isolating emission from this acceleration and collimation zone is key to measuring its size and understanding jet formation physics. But this is challenging because emission from the jet base cannot be easily disentangled from other accreting components. Here, we show that rapid optical flux variations from a Galactic black-hole binary are delayed with respect to X-rays radiated from close to the black hole by ~0.1 seconds, and that this delayed signal appears together with a brightening radio jet. The origin of these sub-second optical variations has hitherto been controversial. Not only does our work strongly support a jet origin for the optical variations, it also sets a characteristic elevation of <~10^3 Schwarzschild radii for the main inner optical emission zone above the black hole, constraining both internal shock and magnetohydrodynamic models. Similarities with blazars suggest that jet structure and launching physics could potentially be unified under mass-invariant models. Two of the best-studied jetted black-hole binaries show very similar optical lags, so this size scale may be a defining feature of such systems
No wide spread of stellar ages in the Orion Nebula Cluster
No full textThis is the author accepted manuscript. The final version is available from Oxford University Press via the DOI in this record.The wide luminosity dispersion seen for stars at a given effective temperature in the H-R diagrams of young clusters and star forming regions is often interpreted as due to significant (~10 Myr) spreads in stellar contraction age. In the scenario where most stars are born with circumstellar discs, and that disc signatures decay monotonically (on average) over timescales of only a few Myr, then any such age spread should lead to clear differences in the age distributions of stars with and without discs. We have investigated large samples of stars in the Orion Nebula Cluster (ONC) using three methods to diagnose disc presence from infrared measurements. We find no significant difference in the mean ages or age distributions of stars with and without discs, consistent with expectations for a coeval population. Using a simple quantitative model we show that any real age spread must be smaller than the median disc lifetime. For a log-normal age distribution, there is an upper limit of <0.14 dex (at 99% confidence) to any real age dispersion, compared to the ~=0.4 dex implied by the H-R diagram. If the mean age of the ONC is 2.5 Myr, this would mean at least 95% of its low-mass stellar population has ages between 1.3--4.8 Myr. We suggest that the observed luminosity dispersion is caused by a combination of observational uncertainties and physical mechanisms that disorder the conventional relationship between luminosity and age for pre main-sequence stars. This means that individual stellar ages from the H-R diagram are unreliable and cannot be used to directly infer a star formation history. Irrespective of what causes the wide luminosity dispersion, the finding that any real age dispersion is less than the median disc lifetime argues strongly against star formation scenarios for the ONC lasting longer than a few Myr.RDJ and NJM acknowledge the support of the Science and Technology Facilities Council. SPL is supported by a Research Councils UK Academic Fellowship. This research has made use of NASA’s Astrophysics Data System Bibliographic Services. We thank Tom Megeath for supplying tables of the ONC Spitzer data and a referee for several useful suggestions
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