Thomas P. Greene

Discovery and Characterization of Transiting Super Earths Using an All-Sky Transit Survey and Follow-up by the James Webb Space Telescope

Doppler and transit surveys are finding extrasolar planets of ever smaller mass and radius, and are now sampling the domain of super Earths (1-3R⊕). Recent results from the Doppler surveys suggest that discovery of a transiting super Earth in the habitable zone of a lower main sequence star may be possible. We evaluate the prospects for an all-sky transit survey targeted to the brightest stars, that would find the most favorable cases for photometric and spectroscopic characterization using the James Webb Space Telescope (JWST). We use the pro- posed Transiting Exoplanet Survey Satellite (TESS) as representative of an all-sky survey. We couple the simulated TESS yield to a sensitivity model for the MIRI and NIRSpec instruments on JWST. Our sensitivity model includes all currently known and anticipated sources of random and systematic error for these instruments. We focus on the TESS planets with radii between those of Earth and Neptune. Our simulations consider secondary eclipse filter photometry using JWST/MIRI, comparing the 11 and 15 μm bands to measure CO2 absorption in super Earths, as well as JWST/NIRSpec spectroscopy of water absorption from 1.7-3.0 μm, and CO2 absorption at 4.3 μm. We find that JWSTwill be capable of characterizing dozens of TESS super Earths with temperatures above the habitable range, using both MIRI and NIRspec. We project that TESS will discover about eight nearby habitable transiting super Earths, all orbiting lower-main-sequence stars. The principal sources of uncertainty in the prospective JWST characterization of habitable super Earths are super-Earth frequency and the nature of super-Earth atmospheres. Based on our estimates of these uncertainties, we project that JWST will be able to measure the temperature and identify molecular absorptions (water, CO2) in one to four nearby habitable TESS super Earths orbiting lower-main-sequence stars.

Astrophysics of Young Star Binaries

This paper describes our study of the astrophysics of individual components in close pre-main-sequence binaries. We observed both stars in 17 systems, located in four nearby star-forming regions, using low-resolution (R = 760) infrared spectroscopy and photometry. For 29 components we detected photospheric absorption lines and were able to determine spectral type, extinction, K-band excess, and luminosity. The other five objects displayed featureless or pure emission line spectra. In ~50% of the systems, the extinction and K-band excess of the primary stars dominate those of the secondaries. Masses and ages were determined for these 29 objects by placing them on the H-R diagram, overlaid with theoretical pre-main-sequence tracks. Most of the binaries appear to be coeval. The ages span 5 ? 105 to 1 ? 107 yr. The derived masses range from the substellar, 0.06 M?, to 2.5 M?, and the mass ratios from M2/M1 = 0.04 to 1.0. Fourteen stars show evidence of circumstellar disks. The K-band excess is well correlated with the K-L color for stars with circumstellar material.

Spectroscopy of Brown Dwarf Candidates in the ρ Ophiuchi Molecular Core

We present an analysis of low-resolution infrared spectra for 20 brown dwarf candidates in the core of the ρ Ophiuchi molecular cloud. Fifteen of the sources display absorption-line spectra characteristic of late-type stars. By comparing the depths of water vapor absorption bands in our candidate objects with a grid of M dwarf standards, we derive spectral types that are independent of reddening. Optical spectroscopy of one brown dwarf candidate confirms the spectral type derived from the water bands. Combining their spectral types with published near-infrared photometry, effective temperatures and bolometric stellar luminosities are derived, enabling us to place our sample on the Hertzsprung-Russell diagram. We compare the positions of the brown dwarf candidates in this diagram with two sets of theoretical models in order to estimate their masses and ages. Considering uncertainties in placing the candidates in the H-R diagram, six objects consistently lie in the brown dwarf regime and another five objects lie in the transition region between stellar and substellar objects. The ages inferred for the sample are consistent with those derived for higher mass association members. Three of the newly identified brown dwarfs display infrared excesses at λ = 2.2 μm, suggesting that young brown dwarfs can have active accretion disks. Comparing our mass estimates of the brown dwarf candidates with those derived from photometric data alone suggests that spectroscopy is an essential component of investigations of the mass functions of young clusters.

Optical Spectroscopy of the Surface Population of the ρ Ophiuchi Molecular Cloud: The First Wave of Star Formation

We present the results of optical spectroscopy of 139 stars obtained with the Hydra multiobject spectrograph. The objects extend over a 1.3 deg2 area surrounding the main cloud of the ρ Oph complex. The objects were selected from narrowband images to have Hα in emission. Using the presence of strong Hα emission, lithium absorption, location in the Hertzsprung-Russell diagram, or previously reported X-ray emission, we were able to identify 88 objects as young stars associated with the cloud. Strong Hα emission was confirmed in 39 objects with line widths consistent with their origin in magnetospheric accretion columns. Two of the strongest emission-line objects are young, X-ray-emitting brown dwarf candidates with M8 spectral types. Comparisons of the bolometric luminosities and effective temperatures with theoretical models suggest a median age for this population of 2.1 Myr, which is significantly older than the ages derived for objects in the cloud core. It appears that these stars formed contemporaneously with low-mass stars in the Upper Scorpius subgroup, likely triggered by massive stars in the Upper Centaurus subgroup.

Near-infrared observations of young stellar objects in the Rho Ophiuchi dark cloud

We have conducted an imaging survey of 1.4 sq pc of the Rho Ophiuchi dark cloud in the J, H, and K near-infrared photometric bands. Approximately 337 of our 481 detected sources are associated with the cloud, and we estimate that 48 percent of these have near-infrared excesses, indicative of disks or circumstellar material surrounding these young stellar objects (YSOs). The K-band luminosity function is significantly different in different regions of our survey area, suggesting that YSOs in these regions have different ages or mass functions. We estimate that the entire survey area has a high star-formation efficiency, at roughly 23 percent. Finally, our many newly detected sources provide a relatively large, uniformly sensitive sample of objects for study at longer wavelengths to better determine true source luminosities and evolutionary lifetimes.

MAGNETIC BRAKING FORMULATION FOR SUN-LIKE STARS: DEPENDENCE ON DIPOLE FIELD STRENGTH AND ROTATION RATE

We use two-dimensional axisymmetric magnetohydrodynamic simulations to compute steady-state solutions for solar-like stellar winds from rotating stars with dipolar magnetic fields. Our parameter study includes 50 simulations covering a wide range of relative magnetic field strengths and rotation rates, extending from the slow- and approaching the fast-magnetic-rotator regimes. Using the simulations to compute the angular momentum loss, we derive a semi-analytic formulation for the external torque on the star that fits all of the simulations to a precision of a few percent. This formula provides a simple method for computing the magnetic braking of Sun-like stars due to magnetized stellar winds, which properly includes the dependence on the strength of the magnetic field, mass loss rate, stellar radius, surface gravity, and spin rate, and which is valid for both slow and fast rotators.

CHARACTERIZING TRANSITING EXOPLANET ATMOSPHERES with JWST

Author(s): Greene, Thomas P; Line, Michael R; Montero, Cezar; Fortney, Jonathan J; Lustig-Yaeger, Jacob; Luther, Kyle | Abstract: We explore how well James Webb Space Telescope (JWST) spectra will likely constrain bulk atmospheric properties of transiting exoplanets. We start by modeling the atmospheres of archetypal hot Jupiter, warm Neptune, warm sub-Neptune, and cool super-Earth planets with clear, cloudy, or high mean molecular weight atmospheres. Next we simulate the

A NEAR-INFRARED SPECTROSCOPIC SURVEY OF CLASS I PROTOSTARS

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The Physical Natures of Class I and Flat-Spectrum Protostellar Photospheres: A Near-Infrared Spectroscopic Study

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