arXiv subjects

Aaron C. Rizzuto

Publications and source records attributed to Aaron C. Rizzuto.

34 records · Page 2Linked to original sources

arXiv2019

How to Constrain Your M dwarf II: the mass-luminosity-metallicity relation from 0.075 to 0.70$M_\odot$

The mass-luminosity relation for late-type stars has long been a critical tool for estimating stellar masses. However, there is growing need for both a higher-precision relation and a better understanding of systematic effects (e.g., metallicity). Here we present an empirical relationship between Mks and mass spanning $0.075M_\odot<M<0.70M_\odot$. The relation is derived from 62 nearby binaries, whose orbits we determine using a combination of Keck/NIRC2 imaging, archival adaptive optics data, and literature astrometry. From their orbital parameters, we determine the total mass of each system, with a precision better than 1% in the best cases. We use these total masses, in combination with resolved Ks magnitudes and system parallaxes, to calibrate the mass-Mks relation. The result can be used to determine masses of single stars with a precision of 2-3%, which we confirm by a comparison to dynamical masses from the literature. The precision is limited by scatter around the best-fit relation beyond mass uncertainties, perhaps driven by intrinsic variation in the mass-Mks relation or underestimated measurement errors. We find the effect of [Fe/H] on the mass-Mks relation is likely negligible for metallicities in the Solar neighborhood (0.0+/-2.2% change in mass per dex change in [Fe/H]). This weak effect is consistent with predictions from the Dartmouth Stellar Evolution Database, but inconsistent with those from MESA Isochrones and Stellar Tracks. A sample of binaries with a wider range of abundances will be required to discern the importance of metallicity in extreme populations (e.g., in the Galactic Halo or thick disk).

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arXiv2018

Orbital Motion of the Wide Planetary-Mass Companion GSC 6214-210 b: No Evidence for Dynamical Scattering

Direct-imaging exoplanet surveys have discovered a class of 5-20 \Mjup\space substellar companions at separations >100 AU from their host stars, which present a challenge to planet and star formation models. Detailed analysis of the orbital architecture of these systems can provide constraints on possible formation mechanisms, including the possibility they were dynamically ejected onto a wide orbit. We present astrometry for the wide planetary-mass companion GSC~6214-210\,b (240 AU; $\approx$14 \Mjup) obtained using NIRC2 with adaptive optics at the Keck telescope over ten years. Our measurements achieved astrometric uncertainties of $\approx$1 mas per epoch. We determined a relative motion of $1.12 \pm 0.15$~mas~yr$^{-1}$ (0.61 $\pm$ 0.09 km s$^{-1}$), the first detection of orbital motion for this companion. We compute the minimum periastron for the companion due to our measured velocity vector, and derive constraints on orbital parameters through our modified implementation of the Orbits for the Impatient rejection sampling algorithm. We find that close periastron orbits, which could indicate the companion was dynamically scattered, are present in our posterior but have low likelihoods. For all orbits in our posterior, we assess the detectability of close-in companions that could have scattered GSC~6214-210\,b from a closer orbit, and find that most potential scatterers would have been detected in previous imaging. We conclude that formation at small orbital separation and subsequent dynamical scattering through interaction with another potential close-in object is an unlikely formation pathway for this companion. We also update stellar and substellar properties for the system due to the new parallax from \textit{Gaia} DR2.

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arXiv2018

Zodiacal Exoplanets in Time (ZEIT) VIII: A Two Planet System in Praesepe from K2 Campaign 16

Young planets offer a direct view of the formation and evolution processes that produced the diverse population of mature exoplanet systems known today. The repurposed Kepler mission K2 is providing the first sample of young transiting planets by observing populations of stars in nearby, young clusters or stellar associations. We report the detection and confirmation of two planets transiting K2-264, an M2.5 dwarf in the 650 Myr old Praesepe open cluster. Using our notch-filter search method on the K2 lightcurve, we identify planets with periods of 5.84 d and 19.66 d. This is currently the second known multi-transit system in open clusters younger than 1 Gyr. The inner planet has a radius of 2.27$_{-0.16}^{+0.20}$ R$_\oplus$ and the outer planet has a radius of 2.77$_{-0.18}^{+0.20}$ R$_\oplus$. Both planets are likely mini-Neptunes. These planets are expected to produce radial velocity signals of 3.4 and 2.7 m/s respectively, which is smaller than the expected stellar variability in the optical ($\simeq$30 m/s), making mass measurements unlikely in the optical, but possible with future near-infrared spectrographs. We use an injection-recovery test to place robust limits on additional planets in the system, and find that planets larger than 2 R$_\oplus$ with periods of 1-20 d are unlikely.

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arXiv2018

K2-231 b: A sub-Neptune exoplanet transiting a solar twin in Ruprecht 147

We identify a sub-Neptune exoplanet ($R_p = 2.5 \pm 0.2$ R$_\oplus$) transiting a solar twin in the Ruprecht 147 star cluster (3 Gyr, 300 pc, [Fe/H] = +0.1 dex). The ~81 day light curve for EPIC 219800881 (V = 12.71) from K2 Campaign 7 shows six transits with a period of 13.84 days, a depth of ~0.06%, and a duration of ~4 hours. Based on our analysis of high-resolution MIKE spectra, broadband optical and NIR photometry, the cluster parallax and interstellar reddening, and isochrone models from PARSEC, Dartmouth, and MIST, we estimate the following properties for the host star: $M_\star = 1.01 \pm 0.03$ M$_\odot$, $R_\star= 0.95 \pm 0.03$ R$_\odot$, and $T_{\rm eff} = 5695 \pm 50$ K. This star appears to be single, based on our modeling of the photometry, the low radial velocity variability measured over nearly ten years, and Keck/NIRC2 adaptive optics imaging and aperture-masking interferometry. Applying a probabilistic mass-radius relation, we estimate that the mass of this planet is $M_p = 7 +5 -3$ M$_\oplus$, which would cause a RV semi-amplitude of $K = 2 \pm 1$ m s$^{-1}$ that may be measurable with existing precise RV facilities. After statistically validating this planet with BLENDER, we now designate it K2-231 b, making it the second sub-stellar object to be discovered in Ruprecht 147 and the first planet; it joins the small but growing ranks of 23 other planets found in open clusters.

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arXiv2017

Zodiacal Exoplanets in Time (ZEIT) VI: a three-planet system in the Hyades cluster including an Earth-sized planet

Planets in young clusters are powerful probes of the evolution of planetary systems. Here we report the discovery of three planets transiting K2-136 (EPIC 247589423), a late K dwarf in the Hyades (~800 Myr) cluster, and robust detection limits for additional planets in the system. The planets were identified from their K2 light curves, as part of our survey of young clusters and star forming regions. The smallest planet has a radius comparable to Earth (0.99 +/- 0.05 Earth radii), making it one of the few Earth-sized planets with a known, young age. The two larger planets are likely a mini-Neptune and a super-Earth, with radii of 2.91+/-0.11 and 1.45+/-0.10 Earth radii, respectively. The predicted radial velocity signals from these planets are between 0.4 and 2 m/s, achievable with modern precision RV spectrographs. Because the target star is bright (V=11.2) and has relatively low-amplitude stellar variability for a young star (2-6 mmag), EPIC 247589423 hosts the best planets known in a young open cluster for precise radial velocity follow-up, enabling a robust test of earlier claims that young planets are less dense than their older counterparts.

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arXiv2017

Zodiacal Exoplanets in Time (ZEIT) V: A Uniform Search for Transiting Planets in Young Clusters Observed by K2

Detection of transiting exoplanets around young stars is more difficult than for older systems due to increased stellar variability. Nine young open cluster planets have been found in the K2 data, but no single analysis pipeline identified all planets. We have developed a transit search pipeline for young stars which uses a transit-shaped notch and quadratic continuum in a 12 or 24\,hour window to fit both the stellar variability and the presence of a transit. In addition, for the most rapid rotators (P$_\mathrm{rot}<2$\,days) we model the variability using a linear combination of observed rotations of each star. To maximally exploit our new pipeline, we update the membership for four stellar populations observed by K2 (Upper Scorpius, Pleiades, Hyades, Praesepe), and conduct a uniform search of the members. We identify all known transiting exoplanets in the clusters, 17 eclipsing binaries, one transiting planet candidate orbiting a potential Pleiades member, and three orbiting unlikely members of the young clusters. Limited injection-recovery testing on the known planet hosts indicates that for the older Praesepe systems, we are sensitive to additional exoplanets as small as 1-2\,R$_\oplus$, and for the larger Upper Scorpius planet host (K2-33) our pipeline is sensitive to $\sim$4\,R$_\oplus$ transiting planets. The lack of detected multiple systems in the young clusters is consistent with the expected frequency from the original Kepler sample, within our detection limits. With a robust pipeline that detects all known planets in the young clusters, occurrence rate testing at young ages is now possible.

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arXiv2017

The Factory and the Beehive III: PTFEB132.707+19.810, a Low-Mass Eclipsing Binary in Praesepe Observed by PTF and K2

Theoretical models of stars constitute a fundamental bedrock upon which much of astrophysics is built, but large swaths of model parameter space remain uncalibrated by observations. The best calibrators are eclipsing binaries in clusters, allowing measurement of masses, radii, luminosities, and temperatures, for stars of known metallicity and age. We present the discovery and detailed characterization of PTFEB132.707+19.810, a P=6.0 day eclipsing binary in the Praesepe cluster ($τ$~600--800 Myr; [Fe/H]=0.14$\pm$0.04). The system contains two late-type stars (SpT$_P$=M3.5$\pm$0.2; SpT$_S$=M4.3$\pm$0.7) with precise masses ($M_p=0.3953\pm0.0020$~$M_{\odot}$; $M_s=0.2098\pm0.0014$~$M_{\odot}$) and radii ($R_p=0.363\pm0.008$~$R_{\odot}$; $R_s=0.272\pm0.012$~$R_{\odot}$). Neither star meets the predictions of stellar evolutionary models. The primary has the expected radius, but is cooler and less luminous, while the secondary has the expected luminosity, but is cooler and substantially larger (by 20%). The system is not tidally locked or circularized. Exploiting a fortuitous 4:5 commensurability between $P_{orb}$ and $P_{rot,prim}$, we demonstrate that fitting errors from the unknown spot configuration only change the inferred radii by <1--2%. We also analyze subsets of data to test the robustness of radius measurements; the radius sum is more robust to systematic errors and preferable for model comparisons. We also test plausible changes in limb darkening, and find corresponding uncertainties of ~1%. Finally, we validate our pipeline using extant data for GU Boo, finding that our independent results match previous radii to within the mutual uncertainties (2--3%). We therefore suggest that the substantial discrepancies are astrophysical; since they are larger than for old field stars, they may be tied to the intermediate age of PTFEB132.707+19.810.

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arXiv2017

The Greater Taurus-Auriga Ecosystem I: There Is A Distributed Older Population

The Taurus-Auriga association and its associated molecular cloud are a benchmark population for studies of star and planet formation. The census of Taurus-Auriga has been assembled over seven decades and has inherited the biases, incompleteness, and systematic uncertainties of the input studies. The notably unusual shape of the inferred IMF and the existence of several isolated disk-bearing stars suggest that additional (likely disk-free) members might remain to be discovered. We therefore have begun a global reassessment of the membership of Taurus-Auriga that exploits new data and better definitions of youth and kinematic membership. As a first step, we reconsider the membership of all disk-free candidate members from the literature with spectral type $\ge$F0, $3^h50^m<α<5^h40^m$, and $14^{\circ}<δ<34^{\circ}$. We combine data from the literature with Keck/HIRES and UH88/SNIFS spectra to test the membership of these candidates using HR diagram positions, proper motions, RVs, H$α$, lithium, and surface gravity. We find 218 confirmed or likely Taurus members, 160 confirmed or likely interlopers, and only 18 that still lack sufficient evidence to draw firm conclusions. A significant fraction of these stars (81/218=37%) are not included in the most recent canonical member lists. Intriguingly, there are few additional members in the immediate vicinity of the molecular clouds, preserving the IMFs that have been deemed anomalous in past work. Many of the likely Taurus members are distributed broadly across the search area. When combined with known disk hosts, our updated census reveals two regimes: a high-density population with a high disk fraction (indicative of youth) that broadly traces the molecular clouds, and a low-density population with low disk fraction (hence likely older) that most likely represents previous generations of star formation.

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arXiv2016

Zodiacal Exoplanets in Time (ZEIT) IV: seven transiting planets in the Praesepe cluster

Open clusters and young stellar associations are attractive sites to search for planets and to test theories of planet formation, migration, and evolution. We present our search for, and characterization of, transiting planets in the ~800 Myr old Praesepe (Beehive, M44) Cluster from K2 light curves. We identify seven planet candidates, six of which we statistically validate to be real planets, the last of which requires more data. For each host star we obtain high-resolution NIR spectra to measure its projected rotational broadening and radial velocity, the latter of which we use to confirm cluster membership. We combine low-resolution spectra with the known cluster distance and metallicity to provide precise temperatures, masses, radii, and luminosities for the host stars. Combining our measurements of rotational broadening, rotation periods, and our derived stellar radii, we show that all planetary orbits are consistent with alignment to their host star's rotation. We fit the K2 light curves, including priors on stellar density to put constraints on the planetary eccentricities, all of which are consistent with zero. The difference between the number of planets found in Praesepe and Hyades (8 planets) and a similar dataset for Pleiades (0 planets, ~125 Myr) suggests a trend with age, but may be due to incompleteness of current search pipelines for younger, faster-rotating stars. We see increasing evidence that some planets continue to lose atmosphere past 800 Myr, as now two planets at this age have radii significantly larger than their older counterparts from Kepler.

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arXiv2016

Zodiacal Exoplanets in Time (ZEIT) III: A short-period planet orbiting a pre-main-sequence star in the Upper Scorpius OB Association

We confirm and characterize a close-in ($P_{\rm{orb}}$ = 5.425 days), super-Neptune sized ($5.04^{+0.34}_{-0.37}$ Earth radii) planet transiting K2-33 (2MASS J16101473-1919095), a late-type (M3) pre-main sequence (11 Myr-old) star in the Upper Scorpius subgroup of the Scorpius-Centaurus OB association. The host star has the kinematics of a member of the Upper Scorpius OB association, and its spectrum contains lithium absorption, an unambiguous sign of youth (<20 Myr) in late-type dwarfs. We combine photometry from K2 and the ground-based MEarth project to refine the planet's properties and constrain the host star's density. We determine \name's bolometric flux and effective temperature from moderate resolution spectra. By utilizing isochrones that include the effects of magnetic fields, we derive a precise radius (6-7%) and mass (16%) for the host star, and a stellar age consistent with the established value for Upper Scorpius. Follow-up high-resolution imaging and Doppler spectroscopy confirm that the transiting object is not a stellar companion or a background eclipsing binary blended with the target. The shape of the transit, the constancy of the transit depth and periodicity over 1.5 years, and the independence with wavelength rules out stellar variability, or a dust cloud or debris disk partially occulting the star as the source of the signal; we conclude it must instead be planetary in origin. The existence of K2-33b suggests close-in planets can form in situ or migrate within $\sim 10$ Myr, e.g., via interactions with a disk, and that long-timescale dynamical migration such as by Lidov-Kozai or planet-planet scattering is not responsible for all short-period planets.

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arXiv2016

Testing the Binary Trigger Hypothesis in FUors

We present observations of three FU Orionis objects (hereafter, FUors) with nonredundant aperture-mask interferometry (NRM) at 1.59 um and 2.12 um that probe for binary companions on the scale of the protoplanetary disk that feeds their accretion outbursts. We do not identify any companions to V1515 Cyg or HBC 722, but we do resolve a close binary companion to V1057 Cyg that is at the diffraction limit (rho = 58.3 +/- 1.4 mas or 30 +/- 5 AU) and currently much fainter than the outbursting star (delta(K') = 3.34 +/- 0.10 mag). Given the flux excess of the outbursting star, we estimate that the mass of the companion (M ~ 0.25 Msun) is similar to or slightly below that of the FUor itself, and therefore it resembles a typical T Tauri binary system. Our observations only achieve contrast limits of delta(K') ~ 4 mag, and hence we are only sensitive to companions that were near or above the pre-outburst luminosity of the FUors. It remains plausible that FUor outbursts could be tied to the presence of a close binary companion. However, we argue from the system geometry and mass reservoir considerations that these outbursts are not directly tied to the orbital period (i.e., occurring at periastron passage), but instead must only occur infrequently.

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arXiv2015

Dynamical Masses of Young Stars I: Discordant Model Ages of Upper Scorpius

We present the results of a long term orbit monitoring program, using sparse aperture masking observations taken with NIRC2 on the Keck-II telescope, of seven G to M-type members of the Upper Scorpius subgroup of the Sco-Cen OB association. We present astrometry and derived orbital elements of the binary systems we have monitored, and also determine the age, component masses, distance and reddening for each system using the orbital solutions and multi-band photometry, including Hubble Space Telescope photometry, and a Bayesian fitting procedure. We find that the models can be forced into agreement with any individual system by assuming an age, but that age is not consistent across the mass range of our sample. The G-type binary systems in our sample have model ages of ~11.5 Myr, which is consistent with the latest age estimates for Upper Scorpius, while the M-type binary systems have significantly younger model ages of ~7 Myr. Based on our fits, this age discrepancy in the models corresponds to a luminosity under-prediction of 0.8-0.15 dex, or equivalently an effective temperature over-prediction of 100-300 K for M-type stars at a given premain-sequence age. We also find that the M-type binary system RXJ 1550.0-2312 has an age (~16 Myr) and distance (~90 pc) indicating that it is either a nearby young binary system or a member of the Upper-Centaurus-Lupus subgroup with a 57% probability of membership.

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arXiv2015

Mapping the shores of the brown dwarf desert. IV. Ophiuchus

We conduct a multiplicity survey of members of the rho Ophiuchus cloud complex with high resolution imaging to characterize the multiple star population of this nearby star forming region and investigate the relation between stellar multiplicity and star and planet formation. Our aperture masking survey reveals the presence of 5 new stellar companions beyond the reach of previous studies, but does not result in the detection of any new substellar companions. We find that 43+/-6% of the 114 stars in our survey have stellar mass companions between 1.3-780AU, while 7 (+8 -5)% host brown dwarf companions in the same interval. By combining this information with knowledge of disk-hosting stars, we show that the presence of a close binary companion (separation < 40 AU) significantly influences the lifetime of protoplanetary disks, a phenomenon previously seen in older star forming regions. At the ~1-2Myr age of our Ophiuchus members ~2/3 of close binary systems have lost their disks, compared to only ~30% of single stars and wide binaries. This has significant impact on the formation of giant planets, which are expected to require much longer than 1 Myr to form via core accretion and thus planets formed via this pathway should be rare in close binary systems.

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arXiv2015

The Mass-Radius Relation of Young Stars, I: UScoCTIO 5, An M4.5 Eclipsing Binary in Upper Scorpius Observed By K2

Evolutionary models of pre-main sequence stars remain largely uncalibrated, especially for masses below that of the Sun, making each new dynamical mass and radius measurement a valuable test of theoretical models. Stellar mass dependent features of star formation (such as disk evolution, planet formation, and even the IMF) are fundamentally tied to these models, which implies a systematic uncertainty that can only be improved with precise measurements of calibrator stars. We present the discovery that UScoCTIO 5, a known spectroscopic binary (P = 34 days, Mtot sin(i) = 0.64 Msun), is an eclipsing system with both primary and secondary eclipses apparent in K2 light curves obtained during Campaign 2. We have simultaneously fit the eclipse profiles from the K2 light curves and the existing RV data to demonstrate that UScoCTIO 5 consists of a pair of nearly identical M4.5 stars with M_A = 0.329 +/- 0.002 Msun, R_A = 0.834 +/- 0.006 Rsun, M_B = 0.317 +/- 0.002 Msun, and R_B = 0.810 +/- 0.006 Rsun. The radii are broadly consistent with pre-main sequence ages predicted by stellar evolutionary models, but none agree to within the uncertainties. All models predict systematically incorrect masses at the 25--50% level for the HR diagram position of these mid-M dwarfs, suggesting significant modifications for any trend of other properties that vary with stellar mass. The form of the discrepancy for most model sets is not that they predict luminosities that are too low, but rather that they predict temperatures that are too high, suggesting that the models do not fully encompass the physics of energy transport (via convection and/or missing opacities) and/or a miscalibration of the SpT-Teff scale. The simplest modification to the models (changing Teff to match observations) would yield an older age for this system, in line with the recently proposed older age of Upper Scorpius (~11 Myr).

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arXiv2013

Science and Technology Progress at the Sydney University Stellar Interferometer

This paper presents an overview of recent progress at the Sydney University Stellar Interferometer (SUSI). Development of the third-generation PAVO beam combiner has continued. The MUSCA beam combiner for high-precision differential astrometry using visible light phase referencing is under active development and will be the subject of a separate paper. Because SUSI was one of the pioneering interferometric instruments, some of its original systems are old and have become difficult to maintain. We are undertaking a campaign of modernization of systems: (1) an upgrade of the Optical Path Length Compensator IR laser metrology counter electronics from a custom system which uses an obsolete single-board computer to a modern one based on an FPGA interfaced to a Linux computer - in addition to improving maintainability, this upgrade should allow smoother motion and higher carriage speeds; (2) the replacement of the aged single-board computer local controllers for the siderostats and the longitudinal dispersion compensator has been completed; (3) the large beam reducing telescope has been replaced with a pair of smaller units with separate accessible foci. Examples of scientific results are also included.

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arXiv2011

WISE Circumstellar Disks in the Young Sco-Cen Association

We present an analysis of the WISE photometric data for 829 stars in the Sco-Cen OB2 association, using the latest high-mass membership probabilities. We detect infrared excesses associated with 135 BAF-type stars, 99 of which are secure Sco-Cen members. There is a clear increase in excess fraction with membership probability, which can be fitted linearly. We infer that 41+-5% of Sco-Cen OB2 BAF stars to have excesses, while the field star excess fraction is consistent with zero. This is the first time that the probability of non-membership has been used in the calculation of excess fractions for young stars. We do not observe any significant change in excess fraction between the three subgroups. Within our sample, we have observed that B-type association members have a significantly smaller excess fraction than A and F-type association members.

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