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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
NASA NTRS2012

Debris Disks as Tracers of Nearby Planetary Systems

Many main-sequence stars possess tenuous circumstellar dust clouds believed to trace extrasolar analogs of the Sun's asteroid and Kuiper Belts. While most of these "debris disks" are known only from far-infrared photometry, dozens are now spatially resolved. In this talk, I'll review the observed structural properties of debris disks as revealed by imaging with the Hubble, Spitzer, and Herschel Space Telescopes. I will show how modeling of the far-infrared spectral energy distributions of resolved disks can be used to constrain their dust particle sizes and albedos. I will review cases of disks whose substructures suggest planetary perturbations, including a newly-discovered eccentric ring system. I'll conclude with thoughts on the potential of upcoming and proposed facilities to resolve similar structures around a greatly expanded sample of nearby debris systems.

Stapelfeldt, Karl↗
NASA NTRS2013

The SEEDS Direct Imaging Survey for Planets and Scattered Dust Emission in Debris Disk Systems

Debris disks around young main-sequence stars often have gaps and cavities which for a long time have been interpreted as possibly being caused by planets. In recent years, several giant planet discoveries have been made in systems hosting disks of precisely this nature, further implying that interactions with planets could be a common cause of such disk structures. As part of the SEEDS high-contrast imaging survey, we are surveying a population of debris disk-hosting stars with gaps and cavities implied by their spectral energy distributions, in order to attempt to spatially resolve the disk as well as to detect any planets that may be responsible for the disk structure. Here we report on intermediate results from this survey. Five debris disks have been spatially resolved, and a number of faint point sources have been discovered, most of which have been tested for common proper motion, which in each case has excluded physical companionship with the target stars. From the detection limits of the 50 targets that have been observed, we find that beta Pic b-like planets (approximately 10M(sub jup) planets around G-A-type stars) near the gap edges are less frequent than 15-30%, implying that if giant planets are the dominant cause of these wide (27 AU on average) gaps, they are generally less massive than beta Pic b.

high-contrast imaging↗
NASA NTRS2013

Herschel - PACS Survey Of Protoplanetary Disks In Taurus - Auriga Observations Of [O I] And [C Ii], And Far-Infrared Continuum

The Herschel Space Observatory was used to observe approx. 120 pre-main-sequence stars in Taurus as part of the GASPS Open Time Key project. Photodetector Array Camera and Spectrometer was used to measure the continuum as well as several gas tracers such as [O I] 63 micron, [O I] 145 micron, [C II] 158, micron OH, H2O, and CO. The strongest line seen is [O I] at 63 micron. We find a clear correlation between the strength of the [O I] 63 micron line and the 63 micron continuum for disk sources. In outflow sources, the line emission can be up to 20 times stronger than in disk sources, suggesting that the line emission is dominated by the outflow. The tight correlation seen for disk sources suggests that the emission arises from the inner disk (<50 AU) and lower surface layers of the disk where the gas and dust are coupled. The [O I] 63 micron is fainter in transitional stars than in normal Class II disks. Simple spectral energy distribution models indicate that the dust responsible for the continuum emission is colder in these disks, leading to weaker line emission. [C II] 158 micron emission is only detected in strong outflow sources. The observed line ratios of [O I] 63 micron to [O I] 145 micron are in the regime where we are insensitive to the gas-to-dust ratio, neither can we discriminate between shock or photodissociation region emission. We detect no Class III object in [O I] 63 micron and only three in continuum, at least one of which is a candidate debris disk.

GASPS Open Time Key project↗
NASA NTRS2016

Making High Accuracy Null Depth Measurements for the LBTI ExoZodi Survey

The characterization of exozodiacal light emission is both important for the understanding of planetary systems evolution and for the preparation of future space missions aiming to characterize low mass planets in the habitable zone of nearby main sequence stars. The Large Binocular Telescope Interferometer (LBTI) exozodi survey aims at providing a ten-fold improvement over current state of the art, measuring dust emission levels down to a typical accuracy of approximately 12 zodis per star, for a representative ensemble of approximately 30+ high priority targets. Such measurements promise to yield a final accuracy of about 2 zodis on the median exozodi level of the targets sample. Reaching a 1 sigma measurement uncertainty of 12 zodis per star corresponds to measuring interferometric cancellation (null) levels, i.e visibilities at the few 100 ppm uncertainty level. We discuss here the challenges posed by making such high accuracy mid-infrared visibility measurements from the ground and present the methodology we developed for achieving current best levels of 500 ppm or so. We also discuss current limitations and plans for enhanced exozodi observations over the next few years at LBTI.

exozodiacal light emission↗
NASA NTRS2017

First Scattered-Light Images of the Gas-Rich Debris Disk Around 49 Ceti

We present the first scattered-light images of the debris disk around 49 Ceti, a approximately 40 Myr A1 main-sequence star at 59 pc, famous for hosting two massive dust belts as well as large quantities of atomic and molecular gas. The outer disk is revealed in reprocessed archival Hubble Space Telescope NICMOS-F110W images, as well as new coronagraphic H-band images from the Very Large Telescope SPHERE instrument. The disk extends from 1."1 (65 au) to 4." 6 (250 au) and is seen at an inclination of 73 deg, which refines previous measurements at lower angular resolution. We also report no companion detection larger than 3 MJup at projected separations beyond 20 au from the star (0." 34). Comparison between the F110W and H-band images is consistent with a gray color of 49 Ceti's dust, indicating grains larger than approximately greater than 2 micrometers. Our photometric measurements indicate a scattering efficiency/infrared excess ratio of 0.2-0.4, relatively low compared to other characterized debris disks. We find that 49 Ceti presents morphological and scattering properties very similar to the gas-rich HD 131835 system. From our constraint on the disk inclination we find that the atomic gas previously detected in absorption must extend to the inner disk, and that the latter must be depleted of CO gas. Building on previous studies, we propose a schematic view of the system describing the dust and gas structure around 49 Ceti and hypothetical scenarios for the gas nature and origin.

49 Ceti↗
NASA NTRS2016

Making High Accuracy Null Depth Measurements for the LBTI Exozodi Survey

The characterization of exozodiacal light emission is both important for the understanding of planetary systems evolution and for the preparation of future space missions aiming to characterize low mass planets in the habitable zone of nearby main sequence stars. The Large Binocular Telescope Interferometer (LBTI) exozodi survey aims at providing a ten-fold improvement over current state of the art, measuring dust emission levels down to a typical accuracy of 12 zodis per star, for a representative ensemble of 30+ high priority targets. Such measurements promise to yield a final accuracy of about 2 zodis on the median exozodi level of the targets sample. Reaching a 1 sigma measurement uncertainty of 12 zodis per star corresponds to measuring interferometric cancellation (null) levels, i.e visibilities at the few 100 ppm uncertainty level. We discuss here the challenges posed by making such high accuracy mid-infrared visibility measurements from the ground and present the methodology we developed for achieving current best levels of 500 ppm or so. We also discuss current limitations and plans for enhanced exozodi observations over the next few years at LBTI.

exozodiacal light emission↗
NASA NTRS2018

Hi-5: A Potential High-Contrast Thermal Near-Infrared Imager for the VLTI

Hi-5 is a high-contrast (or high dynamic range) infrared imager project for the VLTI. Its main goal is to characterize young extra-solar planetary systems and exozodiacal dust around southern main-sequence stars. In this paper, we present an update of the project and key technology pathways to improve the contrast achieved by the VLTI. In particular, we discuss the possibility to use integrated optics, proven in the near-infrared, in the thermal near-infrared (L and M bands, 3-5 μm) and advanced fringe tracking strategies. We also address the strong exoplanet science case (young exoplanets, planet formation, and exozodiacal disks) offered by this wavelength regime as well as other possible science cases such as stellar physics (fundamental parameters and multiplicity) and extragalactic astrophysics (active galactic nuclei and fundamental constants). Synergies and scientific preparation for other potential future instruments such as the Planet Formation Imager are also briefly discussed.

exozodiacal dust↗
NASA NTRS2018

Relative Sputtering Rates of FeS, MgS, and Mg Silicates: Implications for ISM Gas Phase Depletions of Rock-Forming Elements

Astronomical measurements of S abundances in the diffuse interstellar medium (ISM) indicate ionized S is a dominant species with little (< 5%) S residing in grains (e.g. Jenkins 2009). This is an enigmatic result, given that abundant Fe-sulfide grains are observed in dust around pre- and post-main sequence stars and are also observed as major components of primitive meteoritic and cometary samples. These disparate observations suggest that the lifetime of sulfide grains in the ISM is short because of destruction processes. Our previous work has shown that FeS and MgS retain their crystallinity and do not amorphize during radiation processing, whereas enstatite and forsterite are readily amorphized. We have extended this study to measure the relative sputtering rates of FeS and MgS compared to enstatite and forsterite. Irradiation of FeS with 4 keV He+ results in preferential sputtering of S and the formation of a thin 2-3 nm, compact Fe metal layer that armors the surface. The zone of S loss extends to a depth of ~8-10 nm below the exposed surface. Despite this S loss, the FeS retains its crystallinity and shows no sign of incipient amorphization. Irradiation of FeS with 5kV Ga+ in a focused ion beam (FIB) instrument resulted in preferential sputtering of S and the formation of a 5-8-nm thick surface layer of nanophase Fe metal. X-ray mapping shows that the zone of S sputtering extends to a depth of nearly 20 nm, but there is no evidence for FeS amorphization, consistent with our previous work. The irradiation experiments show that the relative sputtering rate of FeS and MgS are much higher than olivine or enstatite. Sputtering experiments utilizing 30 kV and 5 kV Ga ions in the FIB produced volume loss in troilite that was ~4X greater than in enstatite or forsterite. The sputter yield under these conditions is such that for every Si atom sputtered from enstatite, ~14 S atoms are sputtered from FeS. We have performed similar sputtering experiments on Fe-bearing niningerite (MgS) and co-existing enstatite from the ALH 84170 EH3 chondrite. MgS also sputters much more rapidly than enstatite with a relative Si:S sputter yield of 1:8. For MgS, sulfur is highly depleted at the surface and the S-depletion zone extends to a depth of ~15 nm (using 5 kV Ga+). There is a corresponding zone of Mg and especially Fe enrichment that extends from 5 to ~10 nm below the surface, respectively. The dominant grain destruction mechanism in the ISM is sputtering from passage of supernova-generated shock waves. This process also results in the amorphization of crystalline silicates in the ISM. Our results indicate that FeS and MgS grains produced in evolved stars and injected into the ISM will be destroyed more rapidly than crystalline silicates. This process may account for the lack of significant depletion of S from the gas phase in the ISM. However, rare nanophase FeS grains occur as inclusions in circumstellar amorphous silicate grains found in comet dust particles analyzed in the laboratory. These results show that a finite amount of S in the ISM is sequestered in solid grains.

SPUTTERING RATES↗
NASA NTRS2018

Detection of Photospheric Features in the Near-Infrared Spectrum of a Class 0 Protostar

We present a near-infrared K-band R ≃ 1500 Keck spectrum of S68N, a Class 0 protostar in the Serpens molecular cloud. The spectrum shows a very red continuum, CO absorption bands, weak or non-existent atomic metal absorptions, and H2 emission lines. The near-IR H2 emission is consistent with excitation in shocks or by X-rays but not by UV radiation. We model the absorption component as a stellar photosphere plus circumstellar continuum emission with wavelength-dependent extinction. A Markov Chain Monte Carlo analysis shows that the most likely model parameters are consistent with a low-temperature, low-gravity photosphere with signifi cant extinction and no more than modest continuum veiling. Its T(eff) ≃ 3260 K effective temperature is similar to that of older, more evolved pre-main-sequence stars, but its surface gravity log g ≃ 2:4 cm/sq.s is approximately 1 dex lower. This implies that the radius of this protostar is a factor of ~ 3 larger than that of 106 yr old T Tauri stars. Its low veiling is consistent with a circumstellar disk having intrinsic near-IR emission that is less than or equal to that of more evolved Class I protostars. Along with the high extinction, this suggests that most of the circumstellar material is in a cold envelope, as expected for a Class 0 protostar. This is the first known detection and analysis of a Class 0 protostar absorption spectrum.

Thomas P Greene↗
NASA NTRS2021

Multiwavelength Observations of the RV Tauri Variable System U Monocerotis: Long-term Variability Phenomena That Can Be Explained by Binary Interactions with a Circumbinary Disk

We present an X-ray through submillimeter observations of the classical RV Tauri(RVb-type)variable U Mon, apost-asymptotic giant branch(AGB)binary with a circumbinary disk(CBD). Our SMA observations indicate aCBD diameter of550 au. Our XMM-Newton observations make U Mon thefirst RV Tauri variable detected inX-rays. The X-ray emission is characteristic of a hot plasma(∼10 MK), withLX=5×1030erg s−1, and weconsider its possible origin from U Mon, its companion, and/or binary system interactions. Combining DASCHand AAVSO data, we extend the time-series photometric baseline back to the late 1880s andfind evidence thatU Mon has secular changes that appear to recur on a timescale of∼60 yr, possibly caused by a feature in the CBD.From literature radial velocities wefind that the binary companion is a∼2MeA-type main-sequence star. Theorientation of the binary’s orbit lies along our line of sight(ω=95°), such that apastron corresponds to photometricRVb minima, consistent with the post-AGB star becoming obscured by the near side of the CBD. In addition, wefind the size of the inner-CBD hole(∼4.5–9au)to be comparable to the binary separation, implying that one orboth stars may interact with the CBD at apastron. The obscuration of the post-AGB star implicates the companionas the likely source of the enhanced Hαobserved at RVb minima and of the X-ray emission that may arise fromaccreted material.

Laura D Vega↗
NASA NTRS2016

Making high accuracy null depth measurements for the LBTI Exozodi survey

The characterization of exozodiacal light emission is both important for the understanding of planetary systems evolution and for the preparation of future space missions aiming to characterize low mass planets in the habitable zone of nearby main sequence stars. The Large Binocular Telescope Interferometer (LBTI) exozodi survey aims at providing a ten-fold improvement over current state of the art, measuring dust emission levels down to a typical accuracy of ~12 zodis per star, for a representative ensemble of ~30+ high priority targets. Such measurements promise to yield a final accuracy of about 2 zodis on the median exozodi level of the targets sample. Reaching a 1σ measurement uncertainty of 12 zodis per star corresponds to measuring interferometric cancellation (“null”) levels, i.e visibilities at the few 100 ppm uncertainty level. We discuss here the challenges posed by making such high accuracy mid-infrared visibility measurements from the ground and present the methodology we developed for achieving current best levels of 500 ppm or so. We also discuss current limitations and plans for enhanced exozodi observations over the next few years at LBTI.

Mennesson, Bertrand↗
NASA NTRSDate not supplied

Atmospheric Escape from Magnetized Rocky Exoplanets

Recent Kepler and TESS observations discovered many rocky exoplanets in habitable zones around active main-sequence stars. The upper atmospheres of exoplanets are subject to two important energy sources derived from their host stars. First, the stellar photon flux in the X-ray and XUV bands ionizes and heats the upper atmosphere, driving atmospheric heating, affecting the conductance, and enhancing atmospheric escape. Second, the stellar wind’s interaction with the exoplanet’s intrinsic magnetic field transfers energy to the atmosphere through field aligned currents and Poynting flux. That energy is dissipated in the high latitude cusp and auroral regions through Joule heating which can inflate the atmosphere and also enhance the atmospheric escape rate. This presentation will discuss recent advances in modeling these energy inputs and their consequences for exoplanetary habitability. Also, I will discuss the development of a new model, the (exo) PLANETary Ionosphere-Thermosphere Tool for Research (PLANET-ITTR) and some early results from it.

William Danchi↗
NASA NTRS2021

A Deep Search for Five Molecules in the 49 Ceti Debris Disk

Surprisingly strong CO emission has been observed from more than a dozen debris disks around nearby main-sequence stars. The origin of this CO is unclear, in particular whether it is left over from the protoplanetary disk phase or is second-generation material released from collisions between icy bodies like debris dust. The primary unexplored avenue for distinguishing the origin of the material is understanding its molecular composition. Here we present a deep search for five molecules (CN, HCN, HCO+, SiO, and CH3OH) in the debris disk around 49 Ceti. We take advantage of the high sensitivity of the Atacama Large Millimeter/submillimeter Array at Band 7 to integrate for 3.2 hr at modest spatial (1′′) and spectral (0.8 km s−1) resolution. Our search yields stringent upper limits on the flux of all surveyed molecular lines, which imply abundances relative to CO that are orders of magnitude lower than those observed in protoplanetary disks and solar system comets, and also those predicted in outgassing models of second-generation material. However, if CI shielding is responsible for extending the lifetime of any CO produced in second-generation collisions as proposed by Kral et al., then the line ratios do not reflect true ice phase chemical abundances but rather imply that CO is shielded by its own photodissociation product, CI, and other molecules are rapidly photodissociated by the stellar and interstellar radiation field.

Debris disks↗
NASA NTRS1988

Lithium and chromospherically active single giants

Nine chromospherically active single K giants were identified from surveys of chromospherically active stars. The stars have v sin i's ranging from 6 to 46 km/sec. Such large velocities are not explained by scenarios of main sequence to giant star evolution. Fluxes of the ultraviolet emission lines of these stars are substantially less than those of FK Comae. Many of these giants have a moderate or strong lithium line strongly suggesting that these stars recently evolved from rapidly rotating A or early F stars as is suggested by their space motions. Thus, they are not spun down FK Com stars. The characteristics of these stars are such that they may be confused with pre-main sequence stars. The primary difference may be that the post main sequence stars have strong H alpha absorption lines while the pre-main sequence stars appear to have a weak H alpha absorption line or possibly H alpha in emission above the continuum.

Fekel, Francis C.↗
NASA NTRS1982

Observations of normal main-sequence and giant B stars

When interpreting the continuous and line spectra of B stars, it is helpful to think in terms of a model consisting of a photosphere and a mantle which is the outer part of the atmosphere where the effects of nonradiative heating are seen. A survey of the spectra of these stars shows that conditions in the photosphere determine most of what is seen, and in the case of most B stars, the presence of the mantle can be detected only by a special effort. The shape of the visible continuum spectrum and the shape and absolute value of the UV continuous spectrum as determined from low resolution spectra are discussed. Effective temperature for B stars in the main sequence, including corrections for interstellar extinction and bolometric corrections are explored. The major constituents of B-type spectra, variation of the strength of line along the main sequence band, the UV spectra, UV line blocking, intrinsic colors, and variations in light and spectra are also examined.

Source record↗
NASA NTRS1989

Active phenomena in the pre-main sequence Herbig Ae star HD 163296

Observations by IUE of the short-term variability of the Mg II and Ca II resonance lines in the Herbig Ae star HD 163296 are presented. Evidence that these lines show a phenomenon of rotational modulation, similar to the one observed in AB Aur, another Herbig Ae star is found. The variations in the spectrum of HD 163296 are even more conspicuous than in the spectrum of AB Aur. Magnetically structured winds may thus be a widespread phenomenon among the pre-main sequence Herbig Ae/Be stars.

Catala, C.↗