7 research outputs found
A Search For Eclipsing Companions To White Dwarf Stars
In this Thesis, I present a search for eclipsing companions to white dwarf stars, with the aim of studying and more accurately constraining the properties of evolved planetary systems. I first undertake an independent, ground-based survey of metal-rich white dwarfs with similar physical properties to WD1145+017, the first discovered white dwarf hosting a disrupting planetesimal. While no transit-like features are detected in the survey, photometric variability is discovered in two objects. I investigate the nature of such variability in both objects. At the time of writing this work, I am awaiting spectroscopic data for one of the targets in order to search for evidence of an unseen companion. The second object reveals no radial velocity variations, thus the most likely source of variability is determined to be accretion events from the surrounding environment. Moreover, I take part in analysing the white dwarf sample in Campaign 14 of the Kepler K2 mission, sampling 368 white dwarfs brighter than 20 mag. In the sample, I ?nd a brown dwarf totally eclipsing a white dwarf with a period of ~133 minutes. Only two other eclipsing white dwarf + brown dwarf systems were known previous to this discovery. Finally, I present the discovery of two post common-envelope binary systems in the Next Generation Transit Survey. The ?rst system consists of an M4 star orbiting a cool (~7500 K) white dwarf with a period of ~13.85 hours. Only one other system similar to this had been previously discovered. The second binary is a younger, hotter system, in which an ~18,000 K white dwarf is eclipsed by a late-K or early-M main sequence companion.</div
Remnant planetary systems around bright white dwarfs
We cross-correlate several sources of archival photometry for 1265 bright (V ∼ 16 mag) white dwarfs (WDs) with available high signal-to-noise spectroscopy. We find 381 WDs with archival Spitzer+IRAC data and investigate this subsample for infrared excesses due to circumstellar dust. This large data set reveals 15 dusty WDs, including three new debris discs and the hottest WD known to host dust (WD 0010+280). We study the frequency of debris discs at WDs as function of mass. The frequency peaks at 12.5 per cent for 0.7–0.75 M⊙ WDs (with 3 M⊙ main-sequence star progenitors) and falls off for stars more massive than this, which mirrors predicted planet occurrence rates for stars of different masses
NGTS clusters survey -- II. White-light flares from the youngest stars in Orion
We present the detection of high energy white-light flares from pre-main sequence stars associated with the Orion complex, observed as part of the Next Generation Transit Survey (NGTS). With energies up to 5.2 × 1035 erg these flares are some of the most energetic white-light flare events seen to date. We have used the NGTS observations of flaring and non-flaring stars to measure the average flare occurrence rate for 4 Myr M0-M3 stars. We have also combined our results with those from previous studies to predict average rates for flares above 1 × 1035 ergs for early M stars in nearby young associations
NGTS clusters survey - I. Rotation in the young benchmark open cluster Blanco 1
We determine rotation periods for 127 stars in the ~115 Myr old Blanco 1 open
cluster using ~200 days of photometric monitoring with the Next Generation
Transit Survey (NGTS). These stars span F5-M3 spectral types (1.2 0.3 M) and increase the number of known rotation periods in
Blanco 1 by a factor of four. We determine rotation periods using three
methods: Gaussian process (GP) regression, generalised autocorrelation (G-ACF)
and Lomb-Scargle (LS) periodograms, and find that GPs and G-ACF are more
applicable to evolving spot modulation patterns. Between mid-F and mid-K
spectral types, single stars follow a well-defined rotation sequence from ~2 to
10 days, whereas stars in photometric multiple systems typically rotate faster.
This may suggest that the presence of a moderate-to-high mass ratio companion
inhibits angular momentum loss mechanisms during the early pre-main sequence,
and this signature has not been erased at ~100 Myr. The majority of mid-F to
mid-K stars display evolving modulation patterns, whereas most M stars show
stable modulation signals. This morphological change coincides with the shift
from a well-defined rotation sequence (mid-F to mid-K stars) to a broad
rotation period distribution (late-K and M stars). Finally, we compare our
rotation results for Blanco 1 to the similarly-aged Pleiades: the single star
populations in both clusters possess consistent rotation period distributions,
which suggests that the angular momentum evolution of stars follows a
well-defined pathway that is, at least for mid-F to mid-K stars, strongly
imprinted by ~100 Myr
Simultaneous TESS and NGTS Transit Observations of WASP-166b
We observed a transit of WASP-166 b using nine NGTS telescopes simultaneously
with TESS observations of the same transit. We achieved a photometric precision
of 152 ppm per 30 minutes with the nine NGTS telescopes combined, matching the
precision reached by TESS for the transit event around this bright (T=8.87)
star. The individual NGTS light curve noise is found to be dominated by
scintillation noise and appears free from any time-correlated noise or any
correlation between telescope systems. We fit the NGTS data for and
. We find to be consistent to within 0.25 of the result
from the TESS data, and the difference between the TESS and NGTS measured
values is 0.9. This experiment shows that multi-telescope
NGTS photometry can match the precision of TESS for bright stars, and will be a
valuable tool in refining the radii and ephemerides for bright TESS candidates
and planets. The transit timing achieved will also enable NGTS to measure
significant transit timing variations in multi-planet systems
NGTS and WASP photometric recovery of a single-transit candidate from TESS
The Transiting Exoplanet Survey Satellite (TESS) produces a large number of single-transit event candidates, since the mission monitors most stars for only ∼27d. Such candidates correspond to long-period planets or eclipsing binaries. Using the TESS Sector 1 full-frame images, we identified a 7750 ppm single-transit event with a duration of 7 h around the moderately evolved F-dwarf star TIC-238855958 (Tmag = 10.23, Teff = 6280 ± 85 K). Using archival WASP photometry we constrained the true orbital period to one of three possible values. We detected a subsequent transit-event with NGTS, which revealed the orbital period to be 38.20 d. Radial velocity measurements from the CORALIE Spectrograph show the secondary object has a mass of M2 = 0.148 ± 0.003M⊙, indicating this system is an F-M eclipsing binary. The radius of the M-dwarf companion is R2 = 0.171 ± 0.003 R⊙, making this one of the most well characterized stars in this mass regime. We find that its radius is 2.3σ lower than expected from stellar evolution models
An ultrahot Neptune in the Neptune desert
About 1 out of 200 Sun-like stars has a planet with an orbital period shorter than one day: an ultrashort-period planet1,2. All of the previously known ultrashort-period planets are either hot Jupiters, with sizes above 10 Earth radii (R⊕), or apparently rocky planets smaller than 2 R⊕. Such lack of planets of intermediate size (the ‘hot Neptune desert’) has been interpreted as the inability of low-mass planets to retain any hydrogen/helium (H/He) envelope in the face of strong stellar irradiation. Here we report the discovery of an ultrashort-period planet with a radius of 4.6 R⊕ and a mass of 29 M⊕, firmly in the hot Neptune desert. Data from the Transiting Exoplanet Survey Satellite3 revealed transits of the bright Sun-like star LTT 9779 every 0.79 days. The planet’s mean density is similar to that of Neptune, and according to thermal evolution models, it has a H/He-rich envelope constituting 9.0+2.7−2.9% of the total mass. With an equilibrium temperature around 2,000 K, it is unclear how this ‘ultrahot Neptune’ managed to retain such an envelope. Follow-up observations of the planet’s atmosphere to better understand its origin and physical nature will be facilitated by the star’s brightness (Vmag = 9.8).<br