12 research outputs found
Discovering and Characterising Exoplanets with the Next Generation Transit Survey
The Next Generation Transit Survey (NGTS) is a ground-based transiting exoplanet survey, whose aim is to discover Neptune and sub-Neptune sized planets orbiting bright, late-K and early M-type host stars (mv < 13). In fulfilment of the exoplanet and auxiliary science yield of the project, I characterise the mass, radius and orbital parameters of two interesting systems discovered: (1) An M-dwarf binary system with fully convective atmospheres; (2) A Jupiter sized planet in a 4.5 day orbit (Hot-Jupiter). I achieve this via Markov Chain Monte Carlo (MCMC) global modelling of photometric lightcurves and Radial Velocity (RV) curves. In addition, I show that the exoplanet candidate vetting process can be made more efficient by automatically vetting candidates with a Convolutional Neural Network (CNN) prior to human scrutiny.</div
NGTS-1b: A hot Jupiter transiting an M-dwarf
We present the discovery of NGTS-1b, a hot-Jupiter transiting an early M-dwarf host (Teff,∗=3916 +71 −63 K) in a P = 2.647 d orbit discovered as part of the Next Generation Transit Survey (NGTS). The planet has a mass of 0.812 +0.066 −0.075 MJ making it the most massive planet ever discovered transiting an M-dwarf. The radius of the planet is 1.33 +0.61 −0.33 RJ . Since the transit is grazing, we determine this radius by modelling the data and placing a prior on the density from the population of known gas giant planets. NGTS-1b is the third transiting giant planet found around an M-dwarf, reinforcing the notion that close-in gas giants can form and migrate similar to the known population of hot Jupiters around solar type stars. The host star shows no signs of activity, and the kinematics hint at the star being from the thick disk population. With a deep (2.5%) transit around a K = 11.9 host, NGTS-1b will be a strong candidate to probe giant planet composition around M-dwarfs via JWST transmission spectroscop
Statistical Signatures of Nanoflare Activity. II. A Nanoflare Explanation for Periodic Brightenings in Flare Stars Observed by NGTS
Several studies have documented periodic and quasi-periodic signals from the time series of dMe flare stars and other stellar sources. Such periodic signals, observed within quiescent phases (i.e., devoid of larger-scale microflare or flare activity), range in a period from 1 to 1000 s and hence have been tentatively linked to ubiquitous p-mode oscillations generated in the convective layers of the star. As such, most interpretations for the observed periodicities have been framed in terms of magnetohydrodynamic wave behavior. However, we propose that a series of continuous nanoflares, based upon a power-law distribution, can provide a similar periodic signal in the associated time series. Adapting previous statistical analyses of solar nanoflare signals, we find the first statistical evidence for stellar nanoflare signals embedded within the noise envelope of M-type stellar lightcurves. Employing data collected by the Next Generation Transit Survey (NGTS), we find evidence for stellar nanoflare activity demonstrating a flaring power-law index of 3.25 ± 0.20, alongside a decay timescale of 200 ± 100 s. We also find that synthetic time series, consistent with the observations of dMe flare star lightcurves, are capable of producing quasi-periodic signals in the same frequency range as p-mode signals, despite being purely composed of impulsive signatures. Phenomena traditionally considered a consequence of wave behavior may be described by a number of high-frequency but discrete nanoflare energy events. This new physical interpretation presents a novel diagnostic capability, by linking observed periodic signals to given nanoflare model conditions
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
Stellar flares detected with the Next Generation Transit Survey
We present the results of a search for stellar flares in the first data release from the Next Generation Transit Survey (NGTS). We have found 610 flares from 339 stars, with spectral types between F8 and M6, the majority of which belong to the Galactic thin disc. We have used the 13-s cadence NGTS light curves to measure flare properties such as the flare amplitude, duration, and bolometric energy. We have measured the average flare occurrence rates of K and early to mid-M stars and present a generalized method to measure these rates while accounting for changing detection sensitivities. We find that field age K and early M stars show similar flare behaviour, while fully convective M stars exhibit increased white-light flaring activity, which we attribute to their increased spin-down time. We have also studied the average flare rates of pre-main-sequence K and M stars, showing they exhibit increased flare activity relative to their main-sequence counterparts
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
NGTS 15b, 16b, 17b and 18b: four hot Jupiters from the Next Generation Transit Survey
We report the discovery of four new hot Jupiters with the Next Generation
Transit Survey (NGTS). NGTS-15b, NGTS-16b, NGTS-17b, and NGTS-18b are
short-period (d) planets orbiting G-type main sequence stars, with radii
and masses between and . By considering the
host star luminosities and the planets' small orbital separations
( AU), we find that all four hot Jupiters are highly irradiated
and therefore occupy a region of parameter space in which planetary inflation
mechanisms become effective. Comparison with statistical studies and a
consideration of the planets' high incident fluxes reveals that NGTS-16b,
NGTS-17b, and NGTS-18b are indeed likely inflated, although some disparities
arise upon analysis with current Bayesian inflationary models. However, the
underlying relationships which govern radius inflation remain poorly
understood. We postulate that the inclusion of additional hyperparameters to
describe latent factors such as heavy element fraction, as well as the addition
of an updated catalogue of hot Jupiters, would refine inflationary models, thus
furthering our understanding of the physical processes which give rise to
inflated planets
NGTS 15b, 16b, 17b and 18b: four hot Jupiters from the Next Generation Transit Survey
We report the discovery of four new hot Jupiters with the Next Generation
Transit Survey (NGTS). NGTS-15b, NGTS-16b, NGTS-17b, and NGTS-18b are
short-period (d) planets orbiting G-type main sequence stars, with radii
and masses between and . By considering the
host star luminosities and the planets' small orbital separations
( AU), we find that all four hot Jupiters are highly irradiated
and therefore occupy a region of parameter space in which planetary inflation
mechanisms become effective. Comparison with statistical studies and a
consideration of the planets' high incident fluxes reveals that NGTS-16b,
NGTS-17b, and NGTS-18b are indeed likely inflated, although some disparities
arise upon analysis with current Bayesian inflationary models. However, the
underlying relationships which govern radius inflation remain poorly
understood. We postulate that the inclusion of additional hyperparameters to
describe latent factors such as heavy element fraction, as well as the addition
of an updated catalogue of hot Jupiters, would refine inflationary models, thus
furthering our understanding of the physical processes which give rise to
inflated planets