arXiv subjects

G. Torres

Publications and source records attributed to G. Torres.

At least 19 recordsLinked to original sources

arXiv2026

Eight new, triply eclipsing triple star systems

We have identified numerous triply eclipsing hierarchical triple star systems from observations made with the space telescope TESS. For many of these systems we can potentially determine their dynamical and astrophysical parameters with high precision, in some cases even without the use of radial velocity data. In the present paper we report the comprehensive study of eight new, triply eclipsing triple star systems, taken from this larger sample: TICs 77177148, 220456364, 232874003, 306690194, 343763382, 358795247, 379084660, 385933895. Besides the photometric data from the TESS observations, we obtained supplementary ground-based photometric measurements, as well as radial velocity measurements for some of the systems. The eclipse timing variation curves extracted from the TESS data and from the ground-based follow up data, the photometric light curves, radial velocity data, and the spectral energy distribution are combined in a complex photodynamical analysis to yield the stellar and orbital parameters for all of the investigated systems. The outer periods are in the range of 63-294 days, including the currently sixth tightest known triple system, TI 385933895 with an outer to inner period ratio of 9.82. We find that all of the triples are nearly coplanar, with mutual inclination angles of <=2.5deg. Despite such flatness, we still detect eclipse depth variations in TIC 220456364 that are induced by orbital plane precession. Moreover, we find that TIC 358795247 is likely a (2+1)+1-type hierarchical quadruple system. Finally we make some statistical assessments of a number of the orbital elements, stellar parameters, and distances of these systems.

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arXiv2026

TIC 366240660 - A Dynamically Active Triple With a Very Short Outer Period of 30.5 Days

Methods: We analyze TESS photometric data, radial velocity measurements, and eclipse timing variation curves for the triple star system, TIC 366240660, with a photodynamical code to infer the properties of all three stars and orbital motions. Results: With an outer orbital period of only 30.5 d, this is the second most compact triple reported in the literature. We have determined the masses and $T_{\rm eff}$ values of the three stars with an accuracy of about 1% and the radii to about 2%. The inclination angles of the two planes, and the mutual inclination angle are found with an accuracy of better than 1/3$^\circ$. The system is determined to be quite flat (to within 1/2$^\circ$), but with an outer eccentricity of $e_{\rm out} \simeq 0.33$. We find that the properties of the secondary star of the inner binary are completely inconsistent with the evolution of a single star in isolation, and it must have transferred most of its envelope to the current primary star in the inner binary during a prior episode of mass transfer. The eclipse timing variation (ETV) curve derived from the TESS photometric data shows that, in addition to a typical dynamical delay curve at the period of the outer orbit, there is also a much larger amplitude anomalous sinusoidal-like delay with a period of 840 d. This latter feature in the ETV curve is not yet describable in simple analytic terms, but it appears certain that it can be explained by the near integer ($\simeq$13:1) ratio of the outer to the inner orbital periods. Conclusions: We have extensively explored the exotic and anomalous ETVs of this highly compact triple system and found that they can exceed hours for the inner binary eclipses, and several days for the outer orbital eclipses. We provide a preliminary explanation for how these are related to the 13:1 mean motion resonance of the outer to inner orbits.

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arXiv2025

Gravity-darkening exponents for MESA stellar evolution models

In this Research Note we present new gravity-darkening exponents ($β$) for several stellar evolution models from the ZAMS up to the giant phase. The models were computed using the MESA code (version 7385) for the composition $X = 0.70$ and $Z = 0.02$, adopting A09 opacities and a mixing length parameter of $α_{\rm MLT} = 1.84$. Results were calculated using the triangle strategy, for initial stellar masses of 1.0, 1.5, 2.0, 2.5, 3.0, 5.0, 7.0, 10.0, 15.0, and 20.0 $M_{\odot}$, and provide an update on previously available $β$ values that used older models.

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arXiv2025

Orbital and Physical Properties of the Pleiades Binary 27 Tau (Atlas)

We report new spectroscopic and interferometric observations of the Pleiades binary star Atlas, which played an important role nearly three decades ago in settling the debate over the distance to the cluster from ground-based and space-based determinations. We use the new measurements, together with other published and archival astrometric observations, to improve the determination of the 291-day orbit and the distance to Atlas ($136.2 \pm 1.4$ pc). We also derive the main properties of the components, including their absolute masses ($5.04 \pm 0.17 M_{\odot}$ and $3.64 \pm 0.12 M_{\odot}$), sizes, effective temperatures, projected rotational velocities, and chemical composition. We find that the more evolved primary star is rotationally distorted, and are able to estimate its oblateness and the approximate orientation of its spin axis from the interferometric observations. The spin axis may well be aligned with the orbital axis. Models of stellar evolution from MESA that account for rotation provide a good match to all of the primary's global properties, and point to an initial angular rotation rate on the zero-age main sequence of about 55% of the breakup velocity. The current location of the star in the H-R diagram is near the very end of the hydrogen-burning main sequence, at an age of about 105 Myr, according to these models. Our spectroscopic analysis of the more slowly-rotating secondary indicates that it is a helium-weak star, with other chemical anomalies.

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arXiv2025

Limb-Darkening Coefficients for the 4-Term and Power-2 Laws for the JWST Space Mission, Adopting PHOENIX Spherical Models at High Resolution

Modeling observations of transiting exoplanets or close binary systems by comparing the observations with theoretical light curves requires precise knowledge of the distribution of specific intensities across the stellar disk. We aim to facilitate this type of research by providing extensive tabulations of limb-darkening coefficients for 11 frequently used near- and mid-infrared passbands on the NIRCam, NIRISS, and NIRSpec instruments installed on board the James Webb Space Telescope. The calculation of the limb-darkening coefficients was based on spherically symmetric atmosphere models from the PHOENIX series, with high spectral resolution (approximately $10^{6}$ wavelengths), and covering the wavelength range $0.1-6.0~μ$m. The models were computed for solar composition, and a microturbulent velocity of 1.0 km s$^{-1}$. We adopted two of the more accurate parametrizations for the coefficients: the 4-term law, and the power-2 law. We applied the Levenberg-Marquardt least-squares minimization method, with a strategy to determine the critical value $μ_{\rm crit}$ of the cosine of the viewing angle near the limb that is designed to improve numerical accuracy. The limb-darkening coefficients were derived based on a total of 306 atmosphere models covering an effective temperature range of $2400-7800$ K, and a $\log g$ interval between 3.0 and 5.5. We discuss the quality of the fits to the specific intensities provided by the power-2 and 4-term laws, as well as by the often used quadratic law. Based on a comparison, we recommend the use of the 4-term or power-2 laws, in that order of preference.

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arXiv2024

Revisiting the warm sub-Saturn TOI-1710b

The Transiting Exoplanet Survey Satellite (TESS) provides a continuous suite of new planet candidates that need confirmation and precise mass determination from ground-based observatories. This is the case for the G-type star TOI-1710, which is known to host a transiting sub-Saturn planet ($\mathrm{M_p}=$28.3$\pm$4.7$\mathrm{M}_\oplus$) in a long-period orbit (P=24.28\,d). Here we combine archival SOPHIE and new and archival HARPS-N radial velocity data with newly available TESS data to refine the planetary parameters of the system and derive a new mass measurement for the transiting planet, taking into account the impact of the stellar activity on the mass measurement. We report for TOI-1710b a radius of $\mathrm{R_p}$$=$5.15$\pm$0.12$\mathrm{R}_\oplus$, a mass of $\mathrm{M_p}$$=$18.4$\pm$4.5$\mathrm{M}_\oplus$, and a mean bulk density of $ρ_{\rm p}$$=$0.73$\pm$0.18$\mathrm{g \, cm^{-3}}$, which are consistent at 1.2$σ$, 1.5$σ$, and 0.7$σ$, respectively, with previous measurements. Although there is not a significant difference in the final mass measurement, we needed to add a Gaussian process component to successfully fit the radial velocity dataset. This work illustrates that adding more measurements does not necessarily imply a better mass determination in terms of precision, even though they contribute to increasing our full understanding of the system. Furthermore, TOI-1710b joins an intriguing class of planets with radii in the range 4-8 $\mathrm{R}_\oplus$ that have no counterparts in the Solar System. A large gaseous envelope and a bright host star make TOI-1710b a very suitable candidate for follow-up atmospheric characterization.

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arXiv2023

TOI-1736 and TOI-2141: two systems including sub-Neptunes around solar analogs revealed by TESS and SOPHIE

Planetary systems around solar analogs inform us about how planets form and evolve in Solar System-like environments. We report the detection and characterization of two planetary systems around the solar analogs TOI-1736 and TOI-2141 using TESS photometry data and spectroscopic data obtained with the SOPHIE instrument on the 1.93 m telescope at the Observatoire de Haute-Provence (OHP). We performed a detailed spectroscopic analysis of these systems to obtain the precise radial velocities (RV) and physical properties of their host stars. TOI-1736 and TOI-2141 each host a transiting sub-Neptune with radii of $2.44\pm0.18$ R$_{\oplus}$ and $3.05\pm0.23$ R$_{\oplus}$, orbital periods of $7.073088(7)$ d and $18.26157(6)$ d, and masses of $12.8\pm1.8$ M$_{\oplus}$ and $24\pm4$ M$_{\oplus}$, respectively. TOI-1736 shows long-term RV variations that are consistent with a two-planet solution plus a linear trend of $-0.177$ ms$^{-1}$d$^{-1}$. We measured an RV semi-amplitude of $201.1\pm0.7$ ms$^{-1}$ for the outer companion, TOI-1736 c, implying a projected mass of $m_{c}\sin{i}=8.09\pm0.20$ M$_{\rm Jup}$. From the GAIA DR3 astrometric excess noise, we constrained the mass of TOI-1736 c at $8.7^{+1.5}_{-0.6}$ M$_{\rm Jup}$. This planet is in an orbit of $570.2\pm0.6$ d with an eccentricity of $0.362\pm0.003$ and a semi-major axis of $1.381\pm0.017$ au, where it receives a flux of $0.71\pm0.08$ times the bolometric flux incident on Earth, making it an interesting case of a supergiant planet that has settled into an eccentric orbit in the habitable zone of a solar analog. Our analysis of the mass-radius relation for the transiting sub-Neptunes shows that both TOI-1736 b and TOI-2141 b likely have an Earth-like dense rocky core and a water-rich envelope.

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arXiv2023

The SOPHIE search for northern extrasolar planets-XIX. A system including a cold sub-Neptune potentially transiting a V = 6.5 star HD88986

Transiting planets with orbital periods longer than 40 d are extremely rare among the 5000+ planets discovered so far. The lack of discoveries of this population poses a challenge to research into planetary demographics, formation, and evolution. Here, we present the detection and characterization of HD88986b, a potentially transiting sub-Neptune, possessing the longest orbital period among known transiting small planets (< 4 R$_{\oplus}$) with a precise mass measurement ($σ_M/M$ > 25%). Additionally, we identified the presence of a massive companion in a wider orbit around HD88986. Our analysis reveals that HD88986b, based on two potential single transits on sector 21 and sector 48 which are both consistent with the predicted transit time from the RV model, is potentially transiting. The joint analysis of RV and photometric data show that HD88986b has a radius of 2.49$\pm$0.18 R$_{\oplus}$, a mass of 17.2$^{+4.0}_{-3.8}$ M$_{\oplus}$, and it orbits every 146.05$^{+0.43}_{-0.40}$ d around a subgiant HD88986 which is one of the closest and brightest exoplanet host stars (G2V type, R=1.543 $\pm$0.065 R$_{\odot}$, V=$6.47\pm 0.01$ mag, distance=33.37$\pm$0.04 pc). The nature of the outer, massive companion is still to be confirmed; a joint analysis of RVs, Hipparcos, and Gaia astrometric data shows that with a 3$σ$ confidence interval, its semi-major axis is between 16.7 and 38.8 au and its mass is between 68 and 284 M$_{Jup}$. HD88986b's wide orbit suggests the planet did not undergo significant mass loss due to extreme-ultraviolet radiation from its host star. Therefore, it probably maintained its primordial composition, allowing us to probe its formation scenario. Furthermore, the cold nature of HD88986b (460$\pm$8 K), thanks to its long orbital period, will open up exciting opportunities for future studies of cold atmosphere composition characterization.

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arXiv2023

A super-Earth and a mini-Neptune near the 2:1 MMR straddling the radius valley around the nearby mid-M dwarf TOI-2096

Several planetary formation models have been proposed to explain the observed abundance and variety of compositions of super-Earths and mini-Neptunes. In this context, multitransiting systems orbiting low-mass stars whose planets are close to the radius valley are benchmark systems, which help to elucidate which formation model dominates. We report the discovery, validation, and initial characterization of one such system, TOI-2096, composed of a super-Earth and a mini-Neptune hosted by a mid-type M dwarf located 48 pc away. We first characterized the host star by combining different methods. Then, we derived the planetary properties by modeling the photometric data from TESS and ground-based facilities. We used archival data, high-resolution imaging, and statistical validation to support our planetary interpretation. We found that TOI-2096 corresponds to a dwarf star of spectral type M4. It harbors a super-Earth (R$\sim1.2 R_{\oplus}$) and a mini-Neptune (R$\sim1.90 R_{\oplus}$) in likely slightly eccentric orbits with orbital periods of 3.12 d and 6.39 d, respectively. These orbital periods are close to the first-order 2:1 mean-motion resonance (MMR), which may lead to measurable transit timing variations (TTVs). We computed the expected TTVs amplitude for each planet and found that they might be measurable with high-precision photometry delivering mid-transit times with accuracies of $\lesssim$2 min. Moreover, measuring the planetary masses via radial velocities (RVs) is also possible. Lastly, we found that these planets are among the best in their class to conduct atmospheric studies using the James Webb Space Telescope (JWST). The properties of this system make it a suitable candidate for further studies, particularly for mass determination using RVs and/or TTVs, decreasing the scarcity of systems that can be used to test planetary formation models around low-mass stars.

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arXiv2023

A Study of Nine Triply Eclipsing Triples

In this work we report the independent discovery and analysis of nine new compact triply eclipsing triple star systems found with the TESS mission: TICs 47151245, 81525800, 99013269, 229785001, 276162169, 280883908, 294803663, 332521671, and 356324779. Each of these nine systems exhibits distinct third-body eclipses where the third (`tertiary') star occults the inner eclipsing binary (EB), or vice versa. We utilize a photodynamical analysis of the TESS photometry, archival photometric data, TESS eclipse timing variations of the EBs, available archival spectral energy distribution curves (SED), and, in some cases, newly acquired radial velocity observations, to solve for the parameters of all three stars, as well as most of the orbital elements. From these analyses we find that the outer orbits of all nine systems are viewed nearly edge on (i.e., within $\lesssim 4^\circ$), and 6 of the systems are coplanar to within $5^\circ$; the others have mutual inclination angles of $20^\circ$, $41^\circ$, and possibly $179^\circ$ (i.e., a retrograde outer orbit). The outer orbital periods range from 47.8 days to 604 days, with eccentricities spanning 0.004 to 0.61. The masses of all 18 EB stars are in the range of 0.9-2.6 M$_\odot$ and are mostly situated near the main sequence. By contrast, the masses and radii of the tertiary stars range from 1.4-2.8 M$_\odot$ and 1.5-13 R$_\odot$, respectively. We make use of the system parameters from these 9 systems, plus those from a comparable number of compact triply eclipsing triples published previously, to gain some statistical insight into their properties.

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arXiv2022

Six New Compact Triply Eclipsing Triples Found With TESS

In this work we report the discovery and analysis of six new compact triply eclipsing triple star systems found with the TESS mission: TICs 37743815, 42565581, 54060695, 178010808, 242132789, and 456194776. All of these exhibit distinct third body eclipses where the inner eclipsing binary (EB) occults the third (`tertiary') star, or vice versa. We utilized the TESS photometry, archival photometric data, and available archival spectral energy distribution curves (SED) to solve for the properties of all three stars, as well as many of the orbital elements. We describe in detail our SED fits, search of the archival data for the outer orbital period, and the final global photodynamical analyses. From these analyses we find that all six systems are coplanar to within $0^\circ$ - $5^\circ$, and are viewed nearly edge on (i.e., within a couple of degrees). The outer orbital periods and eccentricities of the six systems are {$P_{\rm out}$ (days), $e$}: {68.7, 0.36}, {123, 0.16}, {60.7, 0.01}, {69.0, 0.29}, {41.5, 0.01}, {93.9, 0.29}, respectively, in the order the sources are listed above. The masses of all 12 EB stars were in the range of 0.7-1.8 M$_\odot$ and were situated near the main sequence. By contrast, the masses and radii of the tertiary stars ranged from 1.5-2.3 M$_\odot$ and 2.9-12 R$_\odot$, respectively. We use this information to estimate the occurrence rate of compact flat triple systems.

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arXiv2021

Identification of additional young nearby runaway stars based on Gaia data release 2 observations and the lithium test

Runaway stars are characterised by their remarkably high space velocities, and the study of their formation mechanisms has attracted considerable interest. Young, nearby runaway stars are the most favorable for identifying their place of origin, and for searching for possible associated objects such as neutron stars. Usually the research field of runaway stars focuses on O- and B-type stars, because these objects are better detectable at larger distances than late-type stars. Early-type runaway stars have the advantage, that they evolve faster and can therefore better be confirmed to be young. In contrast to this, the catalogue of young runaway stars within 3 kpc by Tetzlaff, Neuhäuser, & Hohle (2011) contains also stars of spectral type A and later. The objects in this catalogue were originally classified as young ($\le 50$ Myr) runaway stars by using Hipparcos data to estimate the ages from their location in the Hertzsprung-Russell diagram and evolutionary models. In this article, we redetermine and/or constrain their ages not only by using the more precise second data release of the Gaia mission, but also by measuring the equivalent width of the lithium (6708 $\unicode{xC5}$) line, which is a youth indicator. Therefore, we searched for lithium absorption in the spectra of 51 target stars, taken at the University Observatory Jena between March and September 2020 with the Échelle spectrograph FLECHAS, and within additional TRES-spectra from the Fred L. Whipple Observatory. The main part of this campaign with its 308 reduced spectra, accessible at VizieR, was already published. In this work, which is the continuation and completion of the in 2015 initiated observing campaign, we found three additional young runaway star candidates.

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arXiv2020

Identification of young nearby runaway stars based on Gaia data and the lithium test

Young nearby runaway stars are suitable to search for their place of origin and possibly associated objects, for example neutron stars. Tetzlaff, Neuhäuser & Hohle (2011) selected young ($\le 50$ Myr) runaway star candidates from Hipparcos, for which they had estimated the ages from the location in the Hertzsprung-Russell diagram and evolutionary models. Here, we redetermine or constrain their young ages more precisely not only by using the new Gaia DR2 data, but also by measuring lithium, which is a youth indicator. For 308 stars, we took spectra to search for the strong resonance doublet of the lithium-7 isotope at 6708 $\unicode{xC5}$. The spectra were taken with the Échelle spectrograph FLECHAS at the University Observatory Jena between February 2015 and June 2018 and with TRES between April 2011 and June 2017 at the Fred L. Whipple Observatory. We found 208 stars with significant occurrence of lithium in their spectra, and five possess a possible age younger or about 50 Myr. Three of these targets are even closer than GJ 182, the nearest known runaway star at about 24 pc. Theses stars are young runaway stars suitable for further investigation of their origin from either a dynamical or supernova ejection.

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arXiv2020

HAT-P-58b -- HAT-P-64b: Seven Planets Transiting Bright Stars

We report the discovery and characterization of 7 transiting exoplanets from the HATNet survey. The planets, which are hot Jupiters and Saturns transiting bright sun-like stars, include: HAT-P-58b (with mass Mp = 0.37 MJ, radius Rp = 1.33 RJ, and orbital period P = 4.0138 days), HAT-P-59b (Mp = 1.54 MJ, Rp = 1.12 RJ, P = 4.1420 days), HAT-P-60b (Mp = 0.57 MJ, Rp = 1.63 RJ, P = 4.7948 days), HAT-P-61b (Mp = 1.06 MJ, Rp = 0.90 RJ, P = 1.9023 days), HAT-P-62b (Mp = 0.76 MJ, Rp = 1.07 RJ, P = 2.6453 days), HAT-P-63b (Mp = 0.61 MJ, Rp = 1.12 RJ, P = 3.3777 days), and HAT-P-64b (Mp = 0.58 MJ, Rp = 1.70 RJ, P = 4.0072 days). The typical errors on these quantities are 0.06 MJ, 0.03 RJ, and 0.2seconds, respectively. We also provide accurate stellar parameters for each of the hosts stars. With V = 9.710+/-0.050mag, HAT-P-60 is an especially bright transiting planet host, and an excellent target for additional follow-up observations. With Rp = 1.703+/-0.070 RJ, HAT-P-64b is a highly inflated hot Jupiter around a star nearing the end of its main-sequence lifetime, and is among the largest known planets. Five of the seven systems have long-cadence observations by TESS which are included in the analysis. Of particular note is HAT-P-59 (TOI-1826.01) which is within the Northern continuous viewing zone of the TESS mission, and HAT-P-60, which is the TESS candidate TOI-1580.01.

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arXiv2019

Characterization of the L 98-59 multi-planetary system with HARPS: two confirmed terrestrial planets and a mass upper limit on the third

L 98-59 (TIC 307210830, TOI-175) is a nearby M3 dwarf around which TESS revealed three terrestrial-sized transiting planets (0.80, 1.35, 1.57 Earth radii) in a compact configuration with orbital periods shorter than 7.5 days. Here we aim to measure the masses of the known transiting planets in this system using precise radial velocity (RV) measurements taken with the HARPS spectrograph. We consider both trained and untrained Gaussian process regression models of stellar activity to simultaneously model the RV data with the planetary signals. Our RV analysis is then supplemented with dynamical simulations to provide strong constraints on the planets' orbital eccentricities by requiring long-term stability. We measure the planet masses of the two outermost planets to be $2.46\pm 0.31$ and $2.26\pm 0.50$ Earth masses which confirms their bulk terrestrial compositions. We are able to place an upper limit on the mass of the smallest, innermost planet of $<0.98$ Earth masses with 95% confidence. Our RV plus dynamical stability analysis places strong constraints on the orbital eccentricities and reveals that each planet's orbit likely has $e<0.1$ to ensure a dynamically stable system. The L 98-59 compact system of three likely rocky planets offers a unique laboratory for studies of planet formation, dynamical stability, and comparative atmospheric planetology. Continued RV monitoring will help refine the characterization of the innermost planet and potentially reveal additional planets in the system at wider separations.

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arXiv2018

Occultations from an active accretion disk in a 72 day detached post-Algol system detected by K2

Disks in binary systems can cause exotic eclipsing events. MWC 882 (BD-22 4376, EPIC 225300403) is such a disk-eclipsing system identified from observations during Campaign 11 of the K2 mission. We propose that MWC 882 is a post-Algol system with a B7 donor star of mass $0.542\pm0.053\,M_\odot$ in a 72 day period orbit around an A0 accreting star of mass $3.24\pm0.29\,M_\odot$. The $59.9\pm6.2\,R_\odot$ disk around the accreting star occults the donor star once every orbit, inducing 19 day long, 7% deep eclipses identified by K2, and subsequently found in pre-discovery ASAS and ASAS-SN observations. We coordinated a campaign of photometric and spectroscopic observations for MWC 882 to measure the dynamical masses of the components and to monitor the system during eclipse. We found the photometric eclipse to be gray to $\approx 1$%. We found the primary star exhibits spectroscopic signatures of active accretion, and observed gas absorption features from the disk during eclipse. We suggest MWC 882 initially consisted of a $\approx 3.6\,M_\odot$ donor star transferring mass via Roche lobe overflow to a $\approx 2.1\,M_\odot$ accretor in a $\approx 7$ day initial orbit. Through angular momentum conservation, the donor star is pushed outward during mass transfer to its current orbit of 72 days. The observed state of the system corresponds with the donor star having left the Red Giant Branch ~0.3 Myr ago, terminating active mass transfer. The present disk is expected to be short-lived ($10^2$ years) without an active feeding mechanism, presenting a challenge to this model.

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arXiv2018

HAT-TR-318-007: a double-lined M-dwarf binary with total secondary eclipses discovered by HATNet and observed by K2

We report the discovery by the HATNet survey of HAT-TR-318-007, a $P = 3.34395390\pm0.00000020$ d period detached double-lined M-dwarf binary with total secondary eclipses. We combine radial velocity (RV) measurements from TRES/FLWO 1.5 m, and time-series photometry from HATNet, FLWO 1.2 m, BOS 0.8 m and NASA K2 Campaign 5, to determine the masses and radii of the component stars: $M_{A} = 0.448\pm0.011$ $M_{\odot}$, $M_{B} = 0.2721^{+0.0041}_{-0.0042}$ $M_{\odot}$, $R_{A} = 0.4548^{+0.0035}_{-0.0036}$ $R_{\odot}$, and $R_{B} = 0.2913^{+0.0023}_{-0.0024}$ $R_{\odot}$. We obtained a FIRE/Magellan near-infrared spectrum of the primary star during a total secondary eclipse, and use this to obtain disentangled spectra of both components. We determine spectral types of ST$_{A} = {\rm M}3.71\pm0.69$ and ST$_{B} = {\rm M}5.01\pm0.73$, and effective temperatures of T$_{\rm eff,A} = 3190\pm110$ K and T$_{\rm eff,B} = 3100\pm110$ K, for the primary and secondary star, respectively. We also measure a metallicity of [Fe/H]$=+0.298\pm0.080$ for the system. We find that the system has a small, but significant, non-zero eccentricity of $0.0136\pm0.0026$. The K2 light curve shows a coherent variation at a period of $3.41315^{+0.00030}_{-0.00032}$ d, which is slightly longer than the orbital period, and which we demonstrate comes from the primary star. We interpret this as the rotation period of the primary. We perform a quantitative comparison between the Dartmouth stellar evolution models and the seven systems, including HAT-TR-318-007, that contain M dwarfs with $0.2 M_{\odot} < M < 0.5 M_{\odot}$, have metallicity measurements, and have masses and radii determined to better than 5% precision. Discrepancies between the predicted and observed masses and radii are found for three of the systems.

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arXiv2017

HAT-P-67b: An Extremely Low Density Saturn Transiting an F-Subgiant Confirmed via Doppler Tomography

We report the discovery of HAT-P-67b, a hot-Saturn transiting a rapidly rotating F-subgiant. HAT-P-67b has a radius of Rp = 2.085 -0.071/+0.096 RJ,, orbiting a M* = 1.642 -0.072/+0.155 Msun, R* = 2.546 -0.084/+0.099 Rsun host star in a ~4.81-day period orbit. We place an upper limit on the mass of the planet via radial velocity measurements to be Mp < 0.59 MJ, and lower limit of > 0.056 MJ by limitations on Roche lobe overflow. Despite being a subgiant, the host star still exhibits relatively rapid rotation, with a projected rotational velocity of v sin I* = 35.8 +/- 1.1 km/s, making it difficult to precisely determine the mass of the planet using radial velocities. We validated HAT-P-67b via two Doppler tomographic detections of the planetary transit, which eliminated potential eclipsing binary blend scenarios. The Doppler tomographic observations also confirmed that HAT-P-67b has an orbit that is aligned to within 12 degrees, in projection, with the spin of its host star. HAT-P-67b receives strong UV irradiation, and is amongst the one of the lowest density planets known, making it a good candidate for future UV transit observations to search for an extended hydrogen exosphere.

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