arXiv subjects

Don Pollacco

Publications and source records attributed to Don Pollacco.

At least 55 records · Page 3Linked to original sources

arXiv2020

DebrisWatch I: A survey of faint geosynchronous debris

Recent anomalies exhibited by satellites and rocket bodies have highlighted that a population of faint debris exists at geosynchronous (GEO) altitudes, where there are no natural removal mechanisms. Despite previous optical surveys probing to around 10-20 cm in size, regular monitoring of faint sources at GEO is challenging, thus our knowledge remains sparse. It is essential that we continue to explore the faint debris population using large telescopes to better understand the risk posed to active GEO satellites. To this end, we present photometric results from a survey of the GEO region carried out with the 2.54 m Isaac Newton Telescope in La Palma, Canary Islands. We probe to 21st visual magnitude (around 10 cm, assuming Lambertian spheres with an albedo of 0.1), uncovering 129 orbital tracks with GEO-like motion across the eight nights of dark-grey time comprising the survey. The faint end of our brightness distribution continues to rise until the sensitivity limit of the sensor is reached, suggesting that the modal brightness could be even fainter. We uncover a number of faint, uncatalogued objects that show photometric signatures of rapid tumbling, many of which straddle the limiting magnitude of our survey over the course of a single exposure, posing a complex issue when estimating object size. This work presents the first instalment of DebrisWatch, an ongoing collaboration between the University of Warwick and the Defence Science and Technology Laboratory (UK) investigating the faint population of GEO debris.

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arXiv2020

A remnant planetary core in the hot-Neptune desert

The interiors of giant planets remain poorly understood. Even for the planets in the Solar System, difficulties in observation lead to large uncertainties in the properties of planetary cores. Exoplanets that have undergone rare evolutionary processes provide a route to understanding planetary interiors. Planets found in and near the typically barren hot-Neptune 'desert' (a region in mass-radius space that contains few planets) have proved to be particularly valuable in this regard. These planets include HD149026b, which is thought to have an unusually massive core, and recent discoveries such as LTT9779b and NGTS-4b, on which photoevaporation has removed a substantial part of their outer atmospheres. Here we report observations of the planet TOI-849b, which has a radius smaller than Neptune's but an anomalously large mass of $39.1^{+2.7}_{-2.6}$ Earth masses and a density of $5.2^{+0.7}_{-0.8}$ grams per cubic centimetre, similar to Earth's. Interior structure models suggest that any gaseous envelope of pure hydrogen and helium consists of no more than $3.9^{+0.8}_{-0.9}$ per cent of the total planetary mass. The planet could have been a gas giant before undergoing extreme mass loss via thermal self-disruption or giant planet collisions, or it could have avoided substantial gas accretion, perhaps through gap opening or late formation. Although photoevaporation rates cannot account for the mass loss required to reduce a Jupiter-like gas giant, they can remove a small (a few Earth masses) hydrogen and helium envelope on timescales of several billion years, implying that any remaining atmosphere on TOI-849b is likely to be enriched by water or other volatiles from the planetary interior. We conclude that TOI-849b is the remnant core of a giant planet.

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arXiv2020

The EBLM project. VII. Spin-orbit alignment for the circumbinary planet host EBLM J0608-59 A/TOI-1338 A

A dozen short-period detached binaries are known to host transiting circumbinary planets. In all circumbinary systems so far, the planetary and binary orbits are aligned within a couple of degrees. However, the obliquity of the primary star, which is an important tracer of their formation, evolution, and tidal history, has only been measured in one circumbinary system until now. EBLM J0608-59/TOI-1338 is a low-mass eclipsing binary system with a recently discovered circumbinary planet identified by TESS. Here, we perform high-resolution spectroscopy during primary eclipse to measure the projected stellar obliquity of the primary component. The obliquity is low, and thus the primary star is aligned with the binary and planetary orbits with a projected spin-orbit angle $β= 2.8 \pm 17.1$ deg. The rotation period of $18.1 \pm 1.6$ days implied by our measurement of $v\sin{i_\star}$ suggests that the primary has not yet pseudo-synchronized with the binary orbit, but is consistent with gyrochronology and weak tidal interaction with the binary companion. Our result, combined with the known coplanarity of the binary and planet orbits, is suggestive of formation from a single disc. Finally, we considered whether the spectrum of the faint secondary star could affect our measurements. We show through simulations that the effect is negligible for our system, but can lead to strong biases in $v\sin{i_\star}$ and $β$ for higher flux ratios. We encourage future studies in eclipse spectroscopy test the assumption of a dark secondary for flux ratios $\gtrsim 1$ ppt.

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arXiv2020

NGTS-11 b / TOI-1847 b: A transiting warm Saturn recovered from a TESS single-transit event

We report the discovery of NGTS-11 b (=TOI-1847 b), a transiting Saturn in a 35.46-day orbit around a mid K-type star (Teff=5050 K). We initially identified the system from a single-transit event in a TESS full-frame image light-curve. Following seventy-nine nights of photometric monitoring with an NGTS telescope, we observed a second full transit of NGTS-11 b approximately one year after the TESS single-transit event. The NGTS transit confirmed the parameters of the transit signal and restricted the orbital period to a set of 13 discrete periods. We combined our transit detections with precise radial velocity measurements to determine the true orbital period and measure the mass of the planet. We find NGTS-11 b has a radius of 0.817+0.028-0.032 $R_J$, a mass of 0.344+0.092-0.073 $M_J$, and an equilibrium temperature of just 435+34-32 K, making it one of the coolest known transiting gas giants. NGTS-11 b is the first exoplanet to be discovered after being initially identified as a TESS single-transit event, and its discovery highlights the power of intense photometric monitoring in recovering longer-period transiting exoplanets from single-transit events.

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arXiv2020

Two transiting hot Jupiters from the WASP survey: WASP-150b and WASP-176b

We report the discovery of two transiting exoplanets from the WASP survey, WASP-150b and WASP-176b. WASP-150b is an eccentric ($e$ = 0.38) hot Jupiter on a 5.6 day orbit around a $V$ = 12.03, F8 main-sequence host. The host star has a mass and radius of 1.4 $\rm M_{\odot}$ and 1.7 $\rm R_{\odot}$ respectively. WASP-150b has a mass and radius of 8.5 $\rm M_J$ and 1.1 $\rm R_J$, leading to a large planetary bulk density of 6.4 $\rm ρ_J$. WASP-150b is found to be $\sim3$ Gyr old, well below its circularisation timescale, supporting the eccentric nature of the planet. WASP-176b is a hot Jupiter planet on a 3.9 day orbit around a $V$ = 12.01, F9 sub-giant host. The host star has a mass and radius of 1.3 $\rm M_{\odot}$ and 1.9 $\rm R_{\odot}$. WASP-176b has a mass and radius of 0.86 $\rm M_J$ and 1.5 $\rm R_J$ respectively, leading to a planetary bulk density of 0.23 $\rm ρ_J$.

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arXiv2020

SpecPhot: A Comparison of Spectroscopic and Photometric Exoplanet Follow-Up Methods

We set out a simulation to explore the follow-up of exoplanet candidates. We look at comparing photometric (transit method) and spectroscopic (Doppler shift method) techniques using three instruments: NGTS, HARPS and CORALIE. We take into account precision of follow-up and required observing time in attempt to rank each method for a given set of planetary system parameters. The methods are assessed on two criteria, SNR of the detection and follow-up time before characterisation. We find that different follow-up techniques are preferred for different regions of parameter space. For SNR we find that the ratio of spectroscopic to photometric SNR for a given system goes like $R_p/P^{\frac{1}{3}}$. For follow-up time we find that photometry is favoured for the shortest period systems ($<10$ d) as well as systems with small planet radii. Spectroscopy is then preferred for systems with larger radius, and thus more massive, planets (given our assumed mass-radius relationship). Finally, we attempt to account for availability of telescopes and weight the two methods accordingly.

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arXiv2020

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 $T_C$ and $R_p/R_*$. We find $T_C$ to be consistent to within 0.25$σ$ of the result from the TESS data, and the difference between the TESS and NGTS measured $R_p/R_*$ values is 0.9$σ$. 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.

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arXiv2020

TOI-1338: TESS' First Transiting Circumbinary Planet

We report the detection of the first circumbinary planet found by TESS. The target, a known eclipsing binary, was observed in sectors 1 through 12 at 30-minute cadence and in sectors 4 through 12 at two-minute cadence. It consists of two stars with masses of 1.1 MSun and 0.3 MSun on a slightly eccentric (0.16), 14.6-day orbit, producing prominent primary eclipses and shallow secondary eclipses. The planet has a radius of ~6.9 REarth and was observed to make three transits across the primary star of roughly equal depths (~0.2%) but different durations -- a common signature of transiting circumbinary planets. Its orbit is nearly circular (e ~ 0.09) with an orbital period of 95.2 days. The orbital planes of the binary and the planet are aligned to within ~1 degree. To obtain a complete solution for the system, we combined the TESS photometry with existing ground-based radial-velocity observations in a numerical photometric-dynamical model. The system demonstrates the discovery potential of TESS for circumbinary planets, and provides further understanding of the formation and evolution of planets orbiting close binary stars.

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arXiv2020

CHEOPS observations of TESS primary mission monotransits

We set out to look at the overlap between CHEOPS sky coverage and TESS primary mission monotransits to determine what fraction of TESS monotransits may be observed by CHEOPS. We carry out a simulation of TESS transits based on the stellar population in TICv8 in the primary TESS mission. We then select the monotransiting candidates and determine their CHEOPS observing potential. We find that TESS will discover approximately 433 monotransits during its primary mission. Using a baseline observing efficiency of 40% we then find that 387 of these ($\sim$89%) will be observable by CHEOPS with an average observing time of $\sim$60 days per year. Based on the individual observing times and orbital periods of each system we predict that CHEOPS could observe additional transits for approximately 302 of the 433 TESS primary mission monotransits ($\sim$70%). Given that CHEOPS will require some estimate of period before observing a target we estimate that up to 250 ($\sim$58%) TESS primary mission monotransits could have solved periods prior to CHEOPS observations using a combination of photometry and spectroscopy.

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arXiv2020

NGTS-10b: The shortest period hot Jupiter yet discovered

We report the discovery of a new ultra-short period transiting hot Jupiter from the Next Generation Transit Survey (NGTS). NGTS-10b has a mass and radius of $2.162\,^{+0.092}_{-0.107}$ M$_{\rm J}$ and $1.205\,^{+0.117}_{-0.083}$ R$_{\rm J}$ and orbits its host star with a period of $0.7668944\pm0.0000003$ days, making it the shortest period hot Jupiter yet discovered. The host is a $10.4\pm2.5$ Gyr old K5V star ($T_\mathrm{eff}$=$4400\pm100$\,K) of Solar metallicity ([Fe/H] = $-0.02\pm0.12$\,dex) showing moderate signs of stellar activity. NGTS-10b joins a short list of ultra-short period Jupiters that are prime candidates for the study of star-planet tidal interactions. NGTS-10b orbits its host at just $1.46\pm0.18$ Roche radii, and we calculate a median remaining inspiral time of $38$\,Myr and a potentially measurable transit time shift of $7$\,seconds over the coming decade, assuming a stellar tidal quality factor $Q'_{\rm s}=2\times10^{7}$.

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arXiv2020

Shallow transit follow-up from NGTS: simultaneous observations of HD106315 with 11 identical telescopes

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.

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arXiv2019

TOI-222: a single-transit TESS candidate revealed to be a 34-day eclipsing binary with CORALIE, EulerCam and NGTS

We report the period, eccentricity, and mass determination for the TESS single-transit event candidate TOI-222, which displayed a single 3000 ppm transit in the TESS two-minute cadence data from Sector 2. We determine the orbital period via radial velocity measurements (P=33.9,days), which allowed for ground-based photometric detection of two subsequent transits. Our data show that the companion to TOI-222 is a low mass star, with a radius of $0.18_{-0.10}^{+0.39}$ Rsun and a mass of $0.23\pm0.01$ Msun. This discovery showcases the ability to efficiently discover long-period systems from TESS single transit events using a combination of radial velocity monitoring coupled with high precision ground-based photometry.

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arXiv2019

An examination of the effect of the TESS extended mission on southern hemisphere monotransits

Context: NASA recently announced an extended mission for TESS. As a result it is expected that the southern ecliptic hemisphere will be re-observed approximately two years after the initial survey. Aims: We aim to explore how TESS re-observing the southern ecliptic hemisphere will impact the number and distribution of mono-transits discovered during the first year of observations. This simulation will be able to be scaled to any future TESS re-observations. Methods: We carry out an updated simulation of TESS detections in the southern ecliptic hemisphere. This simulation includes realistic Sector window-functions based on the first 11 sectors of SPOC 2 min SAP lightcurves. We then extend this simulation to cover the expected Year 4 of the mission when TESS will re-observed the southern ecliptic fields. For recovered monotransits we also look at the possibility of predicting the period based on the coverage in the TESS data. Results: We find an updated prediction of 339 monotransits from the TESS Year 1 southern ecliptic hemisphere, and that approximately 80% of these systems (266/339) will transit again in the Year 4 observations. The Year 4 observations will also contribute new monotransits not seen in Year 1, resulting in a total of 149 monotransits from the combined Year 1 and Year 4 data sets. We find that 75% (189/266) of recovered Year 1 monotransits will only transit once in the Year 4 data set. For these systems we will be able to constrain possible periods, but period aliasing due to the large time gap between Year 1 and Year 4 observations means that the true period will remain unknown with further spectroscopic or photometric follow-up.

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arXiv2019

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 $\sim$27\,days. Such candidates correspond to long-period planets or eclipsing binaries. Using the \tess\ Sector 1 full-frame images, we identified a 7750\,ppm single-transit event with a duration of 7\,hours around the moderately evolved F-dwarf star \tic\ (Tmag=10.23, \teff=6280$\pm{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 $M_2$= $0.148\pm{0.003}$\,M$_{\odot}$, indicating this system is an F-M eclipsing binary. The radius of the M-dwarf companion is $R_2$ = $0.171\pm{0.003}$\,R$_{\odot}$, making this one of the most well characterised stars in this mass regime. We find that its radius is 2.3-$σ$ lower than expected from stellar evolution models.

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arXiv2019

NGTS-6b: An Ultra Short Period Hot-Jupiter Orbiting an Old K Dwarf

We report the discovery of a new ultra-short period hot Jupiter from the Next Generation Transit Survey. NGTS-6b orbits its star with a period of 21.17~h, and has a mass and radius of $1.330^{+0.024}_{-0.028}$\mjup\, and $1.271^{+0.197}_{-0.188}$\rjup\, respectively, returning a planetary bulk density of 0.711$^{+0.214}_{-0.136}$~g~cm$^{-3}$. Conforming to the currently known small population of ultra-short period hot Jupiters, the planet appears to orbit a metal-rich star ([Fe/H]$=+0.11\pm0.09$~dex). Photoevaporation models suggest the planet should have lost 5\% of its gaseous atmosphere over the course of the 9.6~Gyrs of evolution of the system. NGTS-6b adds to the small, but growing list of ultra-short period gas giant planets, and will help us to understand the dominant formation and evolutionary mechanisms that govern this population.

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arXiv2019

NGTS-7Ab: An ultra-short period brown dwarf transiting a tidally-locked and active M dwarf

We present the discovery of NGTS-7Ab, a high mass brown dwarf transiting an M dwarf with a period of 16.2 hours, discovered as part of the Next Generation Transit Survey (NGTS). This is the shortest period transiting brown dwarf around a main or pre-main sequence star to date. The M star host (NGTS-7A) has an age of roughly 55 Myr and is in a state of spin-orbit synchronisation, which we attribute to tidal interaction with the brown dwarf acting to spin up the star. The host star is magnetically active and shows multiple flares across the NGTS and follow up lightcurves, which we use to probe the flare-starspot phase relation. The host star also has an M star companion at a separation of 1.13 arcseconds with very similar proper motion and systemic velocity, suggesting the NGTS-7 system is a hierarchical triple. The combination of tidal synchronisation and magnetic braking is expected to drive ongoing decay of the brown dwarf orbit, with a remaining lifetime of only 5-10 Myr.

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arXiv2019

A Discrete Set of Possible Transit Ephemerides for Two Long Period Gas Giants Orbiting HIP 41378

In 2015, K2 observations of the bright (V = 8.9, K = 7.7) star HIP 41378 revealed a rich system of at least five transiting exoplanets, ranging in size from super-Earths to gas giants. The 2015 K2 observations only spanned 74.8 days, and the outer three long-period planets in the system were only detected with a single transit, so their orbital periods and transit ephemerides could not be determined at that time. Here, we report on 50.8 days of new K2 observations of HIP 41378 from summer 2018. These data reveal additional transits of the long-period planets HIP 41378 d and HIP 41378 f, yielding a set of discrete possible orbital periods for these two planets. We identify the most probable orbital periods for these two planets using our knowledge of the planets' transit durations, the host star's properties, the system's dynamics, and data from the ground-based HATNet, KELT, and WASP transit surveys. Targeted photometric follow-up during the most probable future transit times will be able to determine the planets' orbital periods, and will enable future observations with facilities like the James Webb Space Telescope. The methods developed herein to determine the most probable orbital periods will be important for long-period planets detected by the Transiting Exoplanet Survey Satellite, where similar period ambiguities will frequently arise due to the telescope's survey strategy.

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arXiv2019

NGTS-5b: a highly inflated planet offering insights into the sub-Jovian desert

Context: Planetary population analysis gives us insight into formation and evolution processes. For short-period planets, the subJovian desert has been discussed in recent years with regard to the planet population in the mass/period and radius/period parameter space without taking stellar parameters into account. The Next Generation Transit Survey (NGTS) is optimised for detecting planets in this regime, which allows for further analysis of the sub-Jovian desert. Aims: With high-precision photometric surveys (e.g. with NGTS and TESS), which aim to detect short period planets especially around M/K-type host stars, stellar parameters need to be accounted for when empirical data are compared to model predictions. Presenting a newly discovered planet at the boundary of the sub-Jovian desert, we analyse its bulk properties and use it to show the properties of exoplanets that border the sub-Jovian desert. Methods: Using NGTS light curve and spectroscopic follow-up observations, we confirm the planetary nature of planet NGTS-5b and determine its mass. Using exoplanet archives, we set the planet in context with other discoveries. Results: NGTS-5b is a short-period planet with an orbital period of 3.3569866 +- 0.0000026 days. With a mass of 0.229 +- 0.037 MJup and a radius of 1.136 +- 0.023 RJup, it is highly inflated. Its mass places it at the upper boundary of the sub-Jovian desert. Because the host is a K2 dwarf, we need to account for the stellar parameters when NGTS-5b is analysed with regard to planet populations. Conclusions: With red-sensitive surveys (e.g. with NGTS and TESS), we expect many more planets around late-type stars to be detected. An empirical analysis of the sub-Jovian desert should therefore take stellar parameters into account.

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