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 period from \(1-1000\) seconds 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 magneto-hydrodynamic 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 \(3.25 \pm 0.20 \), alongside a decay timescale of \(200 \pm 100\) 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 comprised of impulsive signatures. Phenomena traditionally considered a consequence of wave behaviour 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.
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 second cadence NGTS lightcurves 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 generalised 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.
We present the discovery of NGTS J0930-18, an extreme mass ratio eclipsing M-dwarf binary system with an early M-dwarf primary and a late M-dwarf secondary close to the hydrogen burning limit. Global ...modelling of photometry and radial velocities reveals that the secondary component (NGTS J0930-18 B) has a mass of M=\(0.0818 ^{+0.0040}_{-0.0015}\) \(M_*\) and radius of R=\(0.1059 ^{+0.0023}_{-0.0021}\) \(R_*\), making it one of the lowest mass stars with direct mass and radius measurements. With a mass ratio of q =\(0.1407 ^{+0.0065}_{-0.017}\), NGTS J0930-18 has the lowest mass ratio of any known eclipsing M-dwarf binary system, posing interesting questions for binary star formation and evolution models. The mass and radius of NGTS J0930-18 B is broadly consistent with stellar evolutionary models. NGTS J0930-18 B lies in the sparsely populated mass radius parameter space close to the substellar boundary. Precise measurements of masses and radii from single lined eclipsing binary systems of this type are vital for constraining the uncertainty in the mass-radius relationship - of importance due to the growing number of terrestrial planets being discovered around low mass stars.
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 \(5.2\times10^{35}\) 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 \(1\times10^{35}\) ergs for early M stars in nearby young associations.
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 \(T_C\) and \(R_p/R_*\). We find \(T_C\) to be consistent to within 0.25\(\sigma\) of the result from the TESS data, and the difference between the TESS and NGTS measured \(R_p/R_*\) values is 0.9\(\sigma\). 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.
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 (\(P<5\)d) planets orbiting G-type main ...sequence stars, with radii and masses between \(1.10-1.30\) \(R_J\) and \(0.41-0.76\) \(M_J\). By considering the host star luminosities and the planets' small orbital separations (\(0.039-0.052\) 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.
We present the detection of a \(\Delta V\sim\) -10 flare from the ultracool L2.5 dwarf ULAS J224940.13-011236.9 with the Next Generation Transit Survey (NGTS). The flare was detected in a targeted ...search of late-type stars in NGTS full-frame images and represents one of the largest flares ever observed from an ultracool dwarf. This flare also extends the detection of white-light flares to stars with temperatures below 2000 K. We calculate the energy of the flare to be \(3.4^{+0.9}_{-0.7}\times10^{33}\)erg, making it an order of magnitude more energetic than the Carrington event on the Sun. Our data show how the high-cadence NGTS full-frame images can be used to probe white-light flaring behaviour in the latest spectral types.
The Next Generation Transit Survey (NGTS) is a photometric survey for transiting exoplanets, consisting of twelve identical 0.2-m telescopes. We report a measurement of the transit of HD106315c using ...a novel observing mode in which multiple NGTS telescopes observed the same target with the aim of increasing the signal-to-noise. Combining the data allows the robust detection of the transit, which has a depth less than 0.1 per cent, rivalling the performance of much larger telescopes. We demonstrate the capability of NGTS to contribute to the follow-up of K2 and TESS discoveries using this observing mode. In particular, NGTS is well-suited to the measurement of shallow transits of bright targets. This is particularly important to improve orbital ephemerides of relatively long-period planets, where only a small number of transits are observed from space.
We report the discovery of the massive hot Jupiter NGTS-13b by the Next Generation Transit Survey (NGTS). The V = 12.7 host star is likely in the subgiant evolutionary phase with log g\(_{*}\) = 4.04 ...\(\pm\) 0.05, T\(_{eff}\) = 5819 \(\pm\) 73 K, M\(_{*}\) = 1.30\(^{+0.11}_{-0.18}\) M\(_{\odot}\), and R\(_{*}\) = 1.79 \(\pm\) 0.06 R\(_{\odot}\). NGTS detected a transiting planet with a period of P = 4.12 days around the star, which was later validated with the Transiting Exoplanet Survey Satellite (TESS; TIC 454069765). We confirm the planet using radial velocities from the CORALIE spectrograph. Using NGTS and TESS full-frame image photometry combined with CORALIE radial velocities we determine NGTS-13b to have a radius of R\(_{P}\) = 1.142 \(\pm\) 0.046 R\(_{Jup}\), mass of M\(_{P}\) = 4.84 \(\pm\) 0.44 M\(_{Jup}\) and eccentricity e = 0.086 \(\pm\) 0.034. Some previous studies suggest that \(\sim\)4 M\(_{Jup}\) may be a border between two separate formation scenarios (e.g., core accretion and disk instability) and that massive giant planets share similar formation mechanisms as lower-mass brown dwarfs. NGTS-13b is just above 4 M\(_{Jup}\) making it an important addition to the statistical sample needed to understand the differences between various classes of substellar companions. The high metallicity, Fe/H = 0.25 \(\pm\) 0.17, of NGTS-13 does not support previous suggestions that massive giants are found preferentially around lower metallicity host stars, but NGTS-13b does support findings that more massive and evolved hosts may have a higher occurrence of close-in massive planets than lower-mass unevolved stars.
We present the discovery of NGTS J214358.5-380102, an eccentric M-dwarf binary discovered by the Next Generation Transit Survey. The system period of 7.618 days is greater than many known eclipsing ...M-dwarf binary systems. Its orbital eccentricity of \(0.323^{+0.0014}_{-0.0037}\), is large relative to the period and semi-major axis of the binary. Global modelling of photometry and radial velocities indicate stellar masses of \(M_A\)=\(0.426 ^{+0.0056}_{-0.0049}\), \(M_B\)=\(0.455 ^{+0.0058}_{-0.0052}\) and stellar radii \(R_A\)=\(0.461 ^{+0.038}_{-0.025}\) \(R_B\)=\(0.411 ^{+0.027}_{-0.039}\), respectively. Comparisons with stellar models for low mass stars show that one star is consistent with model predictions whereas the other is substantially oversized. Spectral analysis of the system suggests a primary of spectral type M3V, consistent with both modelled masses and radii, and with SED fitting of NGTS photometry. As the most eccentric eclipsing M-dwarf binary known, NGTS J214358.5-380102 provides an interesting insight into the strength of tidal effects in the circularisation of stellar orbits.
