arXiv subjects

Benjamin F. Cooke

Publications and source records attributed to Benjamin F. Cooke.

35 records · Page 2Linked to original sources

arXiv2020

A long period (P = 61.8-d) M5V dwarf eclipsing a Sun-like star from TESS and NGTS

The Transiting Exoplanet Survey Satellite (TESS) has produced a large number of single transit event candidates which are being monitored by the Next Generation Transit Survey (NGTS). We observed a second epoch for the TIC-231005575 system (Tmag = 12.06, Teff = 5500 +- 85 K) with NGTS and a third epoch with Las Cumbres Observatory's (LCO) telescope in South Africa to constrain the orbital period (P = 61.777 d). Subsequent radial velocity measurements with CORALIE revealed the transiting object has a mass of M2 = 0.128 +- 0.003 M$_\odot$, indicating the system is a G-M binary. The radius of the secondary is R2 = 0.154 +- 0.008 R$_\odot$ and is consistent with models of stellar evolution to better than 1-$σ$.

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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

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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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

HD 213885b: A transiting 1-day-period super-Earth with an Earth-like composition around a bright ($V=7.9$) star unveiled by TESS

We report the discovery of the 1.008-day, ultra-short period (USP) super-Earth HD 213885b (TOI-141b) orbiting the bright ($V=7.9$) star HD 213885 (TOI-141, TIC 403224672), detected using photometry from the recently launched TESS mission. Using FEROS, HARPS and CORALIE radial-velocities, we measure a precise mass of $8.8\pm0.6$ $M_\oplus$ for this $1.74 \pm 0.05$ $R_\oplus$ exoplanet, which provides enough information to constrain its bulk composition, which is similar to Earth's but enriched in iron. The radius, mass and stellar irradiation of HD 213885b are, given our data, very similar to 55 Cancri e, making this exoplanet a good target to perform comparative exoplanetology of short period, highly irradiated super-Earths. Our precise radial-velocities reveal an additional $4.78$-day signal which we interpret as arising from a second, non-transiting planet in the system, HD 213885c (TOI-141c), whose minimum mass of $19.95\pm 1.4$ $M_\oplus$ makes it consistent with being a Neptune-mass exoplanet. The HD 213885 system is very interesting from the perspective of future atmospheric characterization, being the second brightest star to host an ultra-short period transiting super-Earth (with the brightest star being, in fact, 55 Cancri). Prospects for characterization with present and future observatories are discussed.

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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-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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arXiv2018

Single site observations of \textit{TESS} single transit detections

Context: TESS has been successfully launched and has begin data acquisition. To expedite the science that may be performed with the resulting data it is necessary to gain a good understanding of planetary yields. Given the observing strategy employed by TESS the probability of detecting single transits in long period systems is increased. These systems require careful consideration. Aims: To simulate the number of TESS transit detections during its 2 year mission with a particular emphasis on single transits. Additionally, to determine the feasibility of ground-based follow-up observations from a single site. Methods: A distribution of planets is simulated around the $\sim$ 4 million stars in the TESS Candidate Target List. These planets are tested for detectable transits and characterised. Based on simulated parameters the single transit detections are further analysed to determine which are amenable to ground-based follow-up. Results: TESS will discover an approximate lower bound of 4700 planets with around 460 being single transits. A large fraction of these will be observable from a single ground-based site. This paper finds that, in a single year, approximately 1000 transit events of around 320 unique TESS single transit detections are theoretically observable. Conclusions: As we consider longer period exoplanets the need for exploring single transit detections increases. For periods $\gtrsim45$ days the number of single transit detections outnumber multitransits by a factor of 3 (82$\pm$18 and 25$\pm$7 respectively) a factor which only grows as longer period detections are considered. Therefore, it is worth expending the extra effort required to follow-up these more challenging, but potentially very rewarding, discoveries. Additionally, we conclude that a large fraction of these targets can be theoretically observed from just a single ground-based site.

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arXiv2018

NGTS-2b: An inflated hot-Jupiter transiting a bright F-dwarf

We report the discovery of NGTS-2b, an inflated hot-Jupiter transiting a bright F5V star (2MASS J14202949-3112074; $T_{\rm eff}$=$6478^{+94}_{-89}$ K), discovered as part of the Next Generation Transit Survey (NGTS). The planet is in a P=4.51 day orbit with mass $0.74^{+0.13}_{-0.12}$ M$_{J}$, radius $1.595^{+0.047}_{-0.045}$ R$_{J}$ and density $0.226^{+0.040}_{-0.038}$ g cm$^{-3}$; therefore one of the lowest density exoplanets currently known. With a relatively deep 1.0% transit around a bright V=10.96 host star, NGTS-2b is a prime target for probing giant planet composition via atmospheric transmission spectroscopy. The rapid rotation ($vsin$i=$15.2\pm0.8$ km s$^{-1}$) also makes this system an excellent candidate for Rossiter-McLaughlin follow-up observations, to measure the sky-projected stellar obliquity. NGTS-2b was confirmed without the need for follow-up photometry, due to the high precision of the NGTS photometry.

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arXiv2018

NGTS-4b: A sub-Neptune Transiting in the Desert

We report the discovery of NGTS-4b, a sub-Neptune-sized planet transiting a 13th magnitude K-dwarf in a 1.34d orbit. NGTS-4b has a mass M=$20.6\pm3.0$M_E and radius R=$3.18\pm0.26$R_E, which places it well within the so-called "Neptunian Desert". The mean density of the planet ($3.45\pm0.95$g/cm^3) is consistent with a composition of 100% H$_2$O or a rocky core with a volatile envelope. NGTS-4b is likely to suffer significant mass loss due to relatively strong EUV/X-ray irradiation. Its survival in the Neptunian desert may be due to an unusually high core mass, or it may have avoided the most intense X-ray irradiation by migrating after the initial activity of its host star had subsided. With a transit depth of $0.13\pm0.02$%, NGTS-4b represents the shallowest transiting system ever discovered from the ground, and is the smallest planet discovered in a wide-field ground-based photometric survey.

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arXiv2018

Unmasking the hidden NGTS-3Ab: a hot Jupiter in an unresolved binary system

We present the discovery of NGTS-3Ab, a hot Jupiter found transiting the primary star of an unresolved binary system. We develop a joint analysis of multi-colour photometry, centroids, radial velocity (RV) cross-correlation function (CCF) profiles and their bisector inverse slopes (BIS) to disentangle this three-body system. Data from the Next Generation Transit Survey (NGTS), SPECULOOS and HARPS are analysed and modelled with our new blendfitter software. We find that the binary consists of NGTS-3A (G6V-dwarf) and NGTS-3B (K1V-dwarf) at <1 arcsec separation. NGTS-3Ab orbits every P = 1.675 days. The planet radius and mass are R_planet = 1.48+-0.37 R_J and M_planet = 2.38+-0.26 M_J, suggesting it is potentially inflated. We emphasise that only combining all the information from multi-colour photometry, centroids and RV CCF profiles can resolve systems like NGTS-3. Such systems cannot be disentangled from single-colour photometry and RV measurements alone. Importantly, the presence of a BIS correlation indicates a blend scenario, but is not sufficient to determine which star is orbited by the third body. Moreover, even if no BIS correlation is detected, a blend scenario cannot be ruled out without further information. The choice of methodology for calculating the BIS can influence the measured significance of its correlation. The presented findings are crucial to consider for wide-field transit surveys, which require wide CCD pixels (>5 arcsec) and are prone to contamination by blended objects. With TESS on the horizon, it is pivotal for the candidate vetting to incorporate all available follow-up information from multi-colour photometry and RV CCF profiles.

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arXiv2017

The masses of retired A stars with asteroseismology: Kepler and K2 observations of exoplanet hosts

We investigate the masses of "retired A stars" using asteroseismic detections on seven low-luminosity red-giant and sub-giant stars observed by the NASA Kepler and K2 Missions. Our aim is to explore whether masses derived from spectroscopy and isochrone fitting may have been systematically overestimated. Our targets have all previously been subject to long term radial velocity observations to detect orbiting bodies, and satisfy the criteria used by Johnson et al. (2006) to select survey stars that may have had A-type (or early F-type) main-sequence progenitors. The sample actually spans a somewhat wider range in mass, from $\approx 1\,\rm M_{\odot}$ up to $\approx 1.7\,\rm M_{\odot}$. Whilst for five of the seven stars the reported discovery mass from spectroscopy exceeds the mass estimated using asteroseismology, there is no strong evidence for a significant, systematic bias across the sample. Moreover, comparisons with other masses from the literature show that the absolute scale of any differences is highly sensitive to the chosen reference literature mass, with the scatter between different literature masses significantly larger than reported error bars. We find that any mass difference can be explained through use of differing constraints during the recovery process. We also conclude that underestimated uncertainties on the input parameters can significantly bias the recovered stellar masses, which may have contributed to the controversy on the mass scale for retired A stars.

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arXiv2017

Weighing in on the masses of retired A stars with asteroseismology: K2 observations of the exoplanet-host star HD 212771

Doppler-based planet surveys point to an increasing occurrence rate of giant planets with stellar mass. Such surveys rely on evolved stars for a sample of intermediate-mass stars (so-called retired A stars), which are more amenable to Doppler observations than their main-sequence progenitors. However, it has been hypothesised that the masses of subgiant and low-luminosity red-giant stars targeted by these surveys --- typically derived from a combination of spectroscopy and isochrone fitting --- may be systematically overestimated. Here, we test this hypothesis for the particular case of the exoplanet-host star HD 212771 using K2 asteroseismology. The benchmark asteroseismic mass ($1.45^{+0.10}_{-0.09}\:\text{M}_{\odot}$) is significantly higher than the value reported in the discovery paper ($1.15\pm0.08\:\text{M}_{\odot}$), which has been used to inform the stellar mass-planet occurrence relation. This result, therefore, does not lend support to the above hypothesis. Implications for the fates of planetary systems are sensitively dependent on stellar mass. Based on the derived asteroseismic mass, we predict the post-main-sequence evolution of the Jovian planet orbiting HD 212771 under the effects of tidal forces and stellar mass loss.

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