arXiv subjects

Andrew W. Mann

Publications and source records attributed to Andrew W. Mann.

At least 163 records · Page 9Linked to original sources

arXiv2020

Two intermediate-mass transiting brown dwarfs from the TESS mission

We report the discovery of two intermediate-mass brown dwarfs (BDs), TOI-569b and TOI-1406b, from NASA's Transiting Exoplanet Survey Satellite mission. TOI-569b has an orbital period of $P = 6.55604 \pm 0.00016$ days, a mass of $M_b = 64.1 \pm 1.9 M_J$, and a radius of $R_b = 0.75 \pm 0.02 R_J$. Its host star, TOI-569, has a mass of $M_\star = 1.21 \pm 0.03 M_\odot$, a radius of $R_\star = 1.47 \pm 0.03 R_\odot$, $\rm [Fe/H] = +0.29 \pm 0.09$ dex, and an effective temperature of $T_{\rm eff} = 5768 \pm 110K$. TOI-1406b has an orbital period of $P = 10.57415 \pm 0.00063$ days, a mass of $M_b =46.0 \pm 2.7 M_J$, and a radius of $R_b = 0.86 \pm 0.03 R_J$. The host star for this BD has a mass of $M_\star =1 .18 \pm 0.09 M_\odot$, a radius of $R_\star = 1.35 \pm 0.03 R_\odot$, $ \rm [Fe/H] = -0.08 \pm 0.09$ dex and an effective temperature of $T_{\rm eff} = 6290 \pm 100K$. Both BDs are in circular orbits around their host stars and are older than 3 Gyr based on stellar isochrone models of the stars. TOI-569 is one of two slightly evolved stars known to host a transiting BD (the other being KOI-415). TOI-1406b is one of three known transiting BDs to occupy the mass range of $40-50 M_J$ and one of two to have a circular orbit at a period near 10 days (with the first being KOI-205b).Both BDs have reliable ages from stellar isochrones in addition to their well-constrained masses and radii, making them particularly valuable as tests for substellar isochrones in the BD mass-radius diagram.

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arXiv2020

Very regular high-frequency pulsation modes in young intermediate-mass stars

Asteroseismology is a powerful tool for probing the internal structures of stars by using their natural pulsation frequencies. It relies on identifying sequences of pulsation modes that can be compared with theoretical models, which has been done successfully for many classes of pulsators, including low-mass solar-type stars, red giants, high-mass stars and white dwarfs. However, a large group of pulsating stars of intermediate mass--the so-called delta Scuti stars--have rich pulsation spectra for which systematic mode identification has not hitherto been possible. This arises because only a seemingly random subset of possible modes are excited, and because rapid rotation tends to spoil the regular patterns. Here we report the detection of remarkably regular sequences of high-frequency pulsation modes in 60 intermediate-mass main-sequence stars, allowing definitive mode identification. Some of these stars have space motions that indicate they are members of known associations of young stars, and modelling of their pulsation spectra confirms that these stars are indeed young.

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arXiv2020

A pair of TESS planets spanning the radius valley around the nearby mid-M dwarf LTT 3780

We present the confirmation of two new planets transiting the nearby mid-M dwarf LTT 3780 (TIC 36724087, TOI-732, $V=13.07$, $K_s=8.204$, $R_s$=0.374 R$_{\odot}$, $M_s$=0.401 M$_{\odot}$, d=22 pc). The two planet candidates are identified in a single TESS sector and are validated with reconnaissance spectroscopy, ground-based photometric follow-up, and high-resolution imaging. With measured orbital periods of $P_b=0.77$ days, $P_c=12.25$ days and sizes $r_{p,b}=1.33\pm 0.07$ R$_{\oplus}$, $r_{p,c}=2.30\pm 0.16$ R$_{\oplus}$, the two planets span the radius valley in period-radius space around low mass stars thus making the system a laboratory to test competing theories of the emergence of the radius valley in that stellar mass regime. By combining 63 precise radial-velocity measurements from HARPS and HARPS-N, we measure planet masses of $m_{p,b}=2.62^{+0.48}_{-0.46}$ M$_{\oplus}$ and $m_{p,c}=8.6^{+1.6}_{-1.3}$ M$_{\oplus}$, which indicates that LTT 3780b has a bulk composition consistent with being Earth-like, while LTT 3780c likely hosts an extended H/He envelope. We show that the recovered planetary masses are consistent with predictions from both photoevaporation and from core-powered mass loss models. The brightness and small size of LTT 3780, along with the measured planetary parameters, render LTT 3780b and c as accessible targets for atmospheric characterization of planets within the same planetary system and spanning the radius valley.

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arXiv2020

TESS Hunt for Young and Maturing Exoplanets (THYME) II: A 17 Myr Old Transiting Hot Jupiter in the Sco-Cen Association

We present the discovery of a transiting hot Jupiter orbiting HIP 67522 ($T_{eff}\sim5650$ K; $M_* \sim 1.2 M_{\odot}$) in the 10-20 Myr old Sco-Cen OB association. We identified the transits in the TESS data using our custom notch-filter planet search pipeline, and characterize the system with additional photometry from Spitzer, spectroscopy from SOAR/Goodman, SALT/HRS, LCOGT/NRES, and SMARTS/CHIRON, and speckle imaging from SOAR/HRCam. We model the photometry as a periodic Gaussian process with transits to account for stellar variability, and find an orbital period of 6.9596$^{+0.000016}_{-0.000015}$ days and radius of 10.02$^{+0.54}_{-0.53}$ R$_\oplus$. We also identify a single transit of an additional candidate planet with radius 8.01$^{+0.75}_{-0.71}$ R$_\oplus$ that has an orbital period of $\gtrsim23$ days. The validated planet HIP 67522 b is currently the youngest transiting hot Jupiter discovered and is an ideal candidate for transmission spectroscopy and radial velocity follow-up studies, while also demonstrating that some young giant planets either form in situ at small orbital radii, or else migrate promptly from formation sites farther out in the disk.

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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

Orbital Parameter Determination for Wide Stellar Binary Systems in the Age of Gaia

The orbits of binary stars and planets, particularly eccentricities and inclinations, encode the angular momentum within these systems. Within stellar multiple systems, the magnitude and (mis)alignment of angular momentum vectors among stars, disks, and planets probes the complex dynamical processes guiding their formation and evolution. The accuracy of the \textit{Gaia} catalog can be exploited to enable comparison of binary orbits with known planet or disk inclinations without costly long-term astrometric campaigns. We show that \textit{Gaia} astrometry can place meaningful limits on orbital elements in cases with reliable astrometry, and discuss metrics for assessing the reliability of \textit{Gaia} DR2 solutions for orbit fitting. We demonstrate our method by determining orbital elements for three systems (DS Tuc AB, GK/GI Tau, and Kepler-25/KOI-1803) using \textit{Gaia} astrometry alone. We show that DS Tuc AB's orbit is nearly aligned with the orbit of DS Tuc Ab, GK/GI Tau's orbit might be misaligned with their respective protoplanetary disks, and the Kepler-25/KOI-1803 orbit is not aligned with either component's transiting planetary system. We also demonstrate cases where \textit{Gaia} astrometry alone fails to provide useful constraints on orbital elements. To enable broader application of this technique, we introduce the python tool \texttt{lofti\_gaiaDR2} to allow users to easily determine orbital element posteriors.

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arXiv2020

Temperatures and Metallicities of M dwarfs in the APOGEE Survey

M dwarfs have enormous potential for our understanding of structure and formation on both Galactic and exoplanetary scales through their properties and compositions. However, current atmosphere models have limited ability to reproduce spectral features in stars at the coolest temperatures ($T_{\rm eff} < 4200\,$K) and to fully exploit the information content of current and upcoming large-scale spectroscopic surveys. Here we present a catalog of spectroscopic temperatures, metallicities, and spectral types for 5875 M dwarfs in the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and Gaia DR2 surveys using The Cannon: a flexible, data-driven spectral-modeling and parameter-inference framework demonstrated to estimate stellar-parameter labels ($T_{\rm eff}$, log$g$, [Fe/H], and detailed abundances) to high precision. Using a training sample of 87 M dwarfs with optically derived labels spanning $2860 < T_{\rm eff} < 4130\,$K calibrated with bolometric temperatures, and $-0.5 < $[Fe/H]$ < 0.5\,$dex calibrated with FGK binary metallicities, we train a two-parameter model with predictive accuracy (in cross-validation) to $77\,$K and $0.09\,$dex respectively. We also train a one-dimensional spectral classification model using 51 M dwarfs with Sloan Digital Sky Survey optical spectral types ranging from M0 to M6, to predictive accuracy of 0.7 types. We find Cannon temperatures to be in agreement to within $60\,$K compared to a subsample of 1702 sources with color-derived temperatures, and Cannon metallicities to be in agreement to within $0.08\,$dex metallicity compared to a subsample of 15 FGK+M or M+M binaries. Finally, our comparison between Cannon and APOGEE pipeline (ASPCAP DR14) labels finds that ASPCAP is systematically biased toward reporting higher temperatures and lower metallicities for M dwarfs.

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arXiv2020

TESS spots a hot Jupiter with an inner transiting Neptune

Hot Jupiters are rarely accompanied by other planets within a factor of a few in orbital distance. Previously, only two such systems have been found. Here, we report the discovery of a third system using data from the Transiting Exoplanet Survey Satellite (TESS). The host star, TOI-1130, is an 11th magnitude K-dwarf in the Gaia G band. It has two transiting planets: a Neptune-sized planet ($3.65\pm 0.10$ $R_E$) with a 4.1-day period, and a hot Jupiter ($1.50^{+0.27}_{-0.22}$ $R_J$) with an 8.4-day period. Precise radial-velocity observations show that the mass of the hot Jupiter is $0.974^{+0.043}_{-0.044}$ $M_J$. For the inner Neptune, the data provide only an upper limit on the mass of 0.17 $M_J$ (3$σ$). Nevertheless, we are confident the inner planet is real, based on follow-up ground-based photometry and adaptive optics imaging that rule out other plausible sources of the TESS transit signal. The unusual planetary architecture of and the brightness of the host star make TOI-1130 a good test case for planet formation theories, and an attractive target for future spectroscopic observations.

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arXiv2020

LHS 1815b: The First Thick-Disk Planet Detected By TESS

We report the first discovery of a thick-disk planet, LHS 1815b (TOI-704b, TIC 260004324), detected in the TESS survey. LHS 1815b transits a bright (V = 12.19 mag, K = 7.99 mag) and quiet M dwarf located $ 29.87\pm0.02 pc$ away with a mass of $0.502\pm0.015 M_{\odot}$ and a radius of $0.501\pm0.030 R_{\odot}$. We validate the planet by combining space and ground-based photometry, spectroscopy, and imaging. The planet has a radius of $1.088\pm 0.064 R_{\oplus}$ with a $3 σ$ mass upper-limit of $8.7 M_{\oplus}$. We analyze the galactic kinematics and orbit of the host star LHS1815 and find that it has a large probability ($P_{thick}/P_{thin} = 6482$) to be in the thick disk with a much higher expected maximal height ($Z_{max} = 1.8 kpc$) above the Galactic plane compared with other TESS planet host stars. Future studies of the interior structure and atmospheric properties of planets in such systems using for example the upcoming James Webb Space Telescope (JWST), can investigate the differences in formation efficiency and evolution for planetary systems between different Galactic components (thick and thin disks, and halo).

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arXiv2020

Zodiacal Exoplanets in Time (ZEIT) IX: a flat transmission spectrum and a highly eccentric orbit for the young Neptune K2-25b as revealed by Spitzer

Transiting planets in nearby young clusters offer the opportunity to study the atmospheres and dynamics of planets during their formative years. To this end, we focused on K2-25b -- a close-in ($P$=3.48 days), Neptune-sized exoplanet orbiting a M4.5 dwarf in the 650Myr Hyades cluster. We combined photometric observations of K2-25 covering a total of 44 transits and spanning >2 yr, drawn from a mix of space-based telescopes (Spitzer Space Telescope and K2) and ground-based facilities (Las Cumbres Observatory Global Telescope network and MEarth). The transit photometry spanned 0.6--4.5$μ$m, which enabled our study of K2-25b's transmission spectrum. We combined and fit each dataset at a common wavelength within a Markov Chain Monte Carlo framework, yielding consistent planet parameters. The resulting transit depths ruled out a solar-composition atmosphere for K2-25b for the range of expected planetary masses and equilibrium temperature at a $>4σ$ confidence level, and are consistent with a flat transmission spectrum. Mass constraints and transit observations at a finer grid of wavelengths (e.g., from the Hubble Space Telescope) are needed to make more definitive statements about the presence of clouds or an atmosphere of high mean molecular weight. Our precise measurements of K2-25b's transit duration also enabled new constraints on the eccentricity of K2-25's orbit. We find K2-25b's orbit to be eccentric ($e>0.20$) for all reasonable stellar densities and independent of the observation wavelength or instrument. The high eccentricity is suggestive of a complex dynamical history and motivates future searches for additional planets or stellar companions.

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arXiv2019

TOI 564 b and TOI 905 b: Grazing and Fully Transiting Hot Jupiters Discovered by TESS

We report the discovery and confirmation of two new hot Jupiters discovered by the Transiting Exoplanet Survey Satellite (TESS): TOI 564 b and TOI 905 b. The transits of these two planets were initially observed by TESS with orbital periods of 1.651 d and 3.739 d, respectively. We conducted follow-up observations of each system from the ground, including photometry in multiple filters, speckle interferometry, and radial velocity measurements. For TOI 564 b, our global fitting revealed a classical hot Jupiter with a mass of $1.463^{+0.10}_{-0.096}\ M_J$ and a radius of $1.02^{+0.71}_{-0.29}\ R_J$. TOI 905 b is a classical hot Jupiter as well, with a mass of $0.667^{+0.042}_{-0.041}\ M_J$ and radius of $1.171^{+0.053}_{-0.051}\ R_J$. Both planets orbit Sun-like, moderately bright, mid-G dwarf stars with V ~ 11. While TOI 905 b fully transits its star, we found that TOI 564 b has a very high transit impact parameter of $0.994^{+0.083}_{-0.049}$, making it one of only ~20 known systems to exhibit a grazing transit and one of the brightest host stars among them. TOI 564 b is therefore one of the most attractive systems to search for additional non-transiting, smaller planets by exploiting the sensitivity of grazing transits to small changes in inclination and transit duration over the time scale of several years.

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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

The young planetary system K2-25: constraints on companions and starspots

The abundance of planets with orbital periods of a few to tens of days suggests that exoplanets experience complex dynamical histories. Planets in young stellar clusters or associations have well-constrained ages and therefore provide an opportunity to explore the dynamical evolution of exoplanets. K2-25b is a Neptune-sized planet in an eccentric, 3.48 day orbit around an M4.5 dwarf star in the Hyades cluster (650 Myr). In order to investigate its non-zero eccentricity and tight orbit, we analyze transit timing variations (TTVs) which could reveal clues to the migration processes that may have acted on the planet. We obtain 12 non-consecutive transits using the MEarth Observatories and long-term photometric monitoring, which we combine with 10 transits from the Spitzer Space Telescope and 20 transits from K2. Tables of MEarth photometry accompany this work. We fit each transit lightcurve independently. We first investigate whether inhomogeneities on the stellar surface (such as spots or plages) are differentially affecting our transit observations. The measured transit depth does not vary significantly between transits, though we see some deviations from the fiducial transit model. We then looked for TTVs as evidence of a non-transiting perturber in the system. We find no evidence for > 1 $M_\oplus$ companions within a 2:1 period ratio, or for > 5 $M_\oplus$ planets within a 7:2 period ratio.

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arXiv2019

Magnetic Inflation and Stellar Mass. V. Intensification and saturation of M dwarf absorption lines with Rossby number

In young sun-like stars and field M dwarf stars, chromospheric and coronal magnetic activity indicators such as H$α$, X-ray and radio emission are known to saturate with low Rossby number ($Ro \lesssim 0.1$), defined as the ratio of rotation period to convective turnover time. The mechanism for the saturation is unclear. In this paper, we use photospheric Ti I and Ca I absorption lines in $Y$ band to investigate magnetic field strength in M dwarfs for Rossby numbers between 0.01 and 1.0. The equivalent widths of the lines are magnetically enhanced by photospheric spots, a global field or a combination of the two. The equivalent widths behave qualitatively similar to the chromospheric and coronal indicators: we see increasing equivalent widths (increasing absorption) with decreasing $Ro$ and saturation of the equivalent widths for $Ro \lesssim 0.1$. The majority of M dwarfs in this study are fully convective. The results add to mounting evidence that the magnetic saturation mechanism occurs at or beneath the stellar photosphere.

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arXiv2019

KELT-25b and KELT-26b: A Hot Jupiter and a Substellar Companion Transiting Young A-stars Observed by TESS

We present the discoveries of KELT-25b (TIC 65412605, TOI-626.01) and KELT-26b (TIC 160708862, TOI-1337.01), two transiting companions orbiting relatively bright, early A-stars. The transit signals were initially detected by the KELT survey, and subsequently confirmed by \textit{TESS} photometry. KELT-25b is on a 4.40-day orbit around the V = 9.66 star CD-24 5016 ($T_{\rm eff} = 8280^{+440}_{-180}$ K, $M_{\star}$ = $2.18^{+0.12}_{-0.11}$ $M_{\odot}$), while KELT-26b is on a 3.34-day orbit around the V = 9.95 star HD 134004 ($T_{\rm eff}$ =$8640^{+500}_{-240}$ K, $M_{\star}$ = $1.93^{+0.14}_{-0.16}$ $M_{\odot}$), which is likely an Am star. We have confirmed the sub-stellar nature of both companions through detailed characterization of each system using ground-based and \textit{TESS} photometry, radial velocity measurements, Doppler Tomography, and high-resolution imaging. For KELT-25, we determine a companion radius of $R_{\rm P}$ = $1.64^{+0.039}_{-0.043}$ $R_{\rm J}$, and a 3-sigma upper limit on the companion's mass of $\sim64~M_{\rm J}$. For KELT-26b, we infer a planetary mass and radius of $M_{\rm P}$ = $1.41^{+0.43}_{-0.51}$ $M_{\rm J}$ and $R_{\rm P}$ = $1.940^{+0.060}_{-0.058}$ $R_{\rm J}$. From Doppler Tomographic observations, we find KELT-26b to reside in a highly misaligned orbit. This conclusion is weakly corroborated by a subtle asymmetry in the transit light curve from the \textit{TESS} data. KELT-25b appears to be in a well-aligned, prograde orbit, and the system is likely a member of a cluster or moving group.

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arXiv2019

TOI-132 b: A short-period planet in the Neptune desert transiting a $V=11.3$ G-type star

The Neptune desert is a feature seen in the radius-mass-period plane, whereby a notable dearth of short period, Neptune-like planets is found. Here we report the {\it TESS} discovery of a new short-period planet in the Neptune desert, orbiting the G-type dwarf TYC\,8003-1117-1 (TOI-132). {\it TESS} photometry shows transit-like dips at the level of $\sim$1400 ppm occurring every $\sim$2.11 days. High-precision radial velocity follow-up with HARPS confirmed the planetary nature of the transit signal and provided a semi-amplitude radial velocity variation of $\sim$11.5 m s$^{-1}$, which, when combined with the stellar mass of $0.97\pm0.06$ $M_{\odot}$, provides a planetary mass of 22.83$^{+1.81}_{-1.80}$ $M_{\oplus}$. Modeling the {\it TESS} high-quality light curve returns a planet radius of 3.43$^{+0.13}_{-0.14}$ $R_{\oplus}$, and therefore the planet bulk density is found to be 3.11$^{+0.44}_{-0.450}$ g cm$^{-3}$. Planet structure models suggest that the bulk of the planet mass is in the form of a rocky core, with an atmospheric mass fraction of 4.3$^{+1.2}_{-2.3}$\%. TOI-132 b is a {\it TESS} Level 1 Science Requirement candidate, and therefore priority follow-up will allow the search for additional planets in the system, whilst helping to constrain low-mass planet formation and evolution models, particularly valuable for better understanding the Neptune desert.

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arXiv2019

TOI-677 b: A Warm Jupiter (P=11.2d) on an eccentric orbit transiting a late F-type star

We report the discovery of TOI-677 b, first identified as a candidate in light curves obtained within Sectors 9 and 10 of the Transiting Exoplanet Survey Satellite (TESS) mission and confirmed with radial velocities. TOI-677 b has a mass of M_p = 1.236$^{+0.069}_{-0.067}$ M_J, a radius of R_p = 1.170 +- 0.03 R_J,and orbits its bright host star (V=9.8 mag) with an orbital period of 11.23660 +- 0.00011 d, on an eccentric orbit with e = 0.435 +- 0.024. The host star has a mass of M_* = 1.181 +- 0.058 M_sun, a radius of R_* = 1.28 +- 0.03 R_sun, an age of 2.92$^{+0.80}_{-0.73}$ Gyr and solar metallicity, properties consistent with a main sequence late F star with T_eff = 6295 +- 77 K. We find evidence in the radial velocity measurements of a secondary long term signal which could be due to an outer companion. The TOI-677 b system is a well suited target for Rossiter-Mclaughlin observations that can constrain migration mechanisms of close-in giant planets.

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arXiv2019

TESS Reveals HD 118203 b to be a Transiting Planet

The exoplanet HD 118203 b, orbiting a bright (V = 8.05) host star, was discovered using the radial velocity method by da Silva et al. (2006), but was not previously known to transit. TESS photometry has revealed that this planet transits its host star. Five planetary transits were observed by TESS, allowing us to measure the radius of the planet to be $1.133 \pm 0.031 R_J$, and to calculate the planet mass to be $2.173 \pm 0.078 M_J$. The host star is slightly evolved with an effective temperature of $T_{\rm eff} = 5692 \pm 83$ K and a surface gravity of ${\rm log}(g) = 3.891 \pm 0.019$. With an orbital period of $6.134980 \pm 0.000038$ days and an eccentricity of $0.316 \pm 0.021$, the planet occupies a transitional regime between circularized hot Jupiters and more dynamically active planets at longer orbital periods. The host star is among the ten brightest known to have transiting giant planets, providing opportunities for both planetary atmospheric and asteroseismic studies.

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