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NASA NTRS1998

Turbulence, Chondrules, and Planetesimals

It has been shown both numerically and experimentally that 3-D turbulence concentrates aerodynamically size-selected particles by orders of magnitude. In a previous review chapter, in "Chondrules and the protoplanetary disk" we illustrated the initial predictions of Turbulent Concentration (TC) as applied to the solar nebula. We predicted the particle size which will be most effectively concentrated by turbulence; it is the particle which has a gas drag stopping time equal to the overturn time of the smallest (Kolmogorov scale) eddy. The primary uncertainty is the level of nebula turbulence, or Reynolds number Re, which can be expressed in terms of the standard nebula eddy viscosity parameter alpha = Re(nu)(sub m)/cH, where nu(sub m) is molecular viscosity, c is sound speed, and H is vertical scale height. Several studies, and observed lifetimes of circumstellar disks, have suggested that the level of nebula turbulence can be described by alpha = 10(exp -2) - 10(exp -4). There is some recent concern about how energy is provided to maintain this turbulence, but the issue remains open. We adopt a canonical minimum mass nebula with a range of alpha > 0. We originally showed that chondrule-sized particles are selected for concentration in the terrestrial planet region if alpha = 10(exp -3) - 10(exp -4). In addition, Paque and Cuzzi found that the size distribution of chondrules is an excellent match for theoretical predictions. One then asks by what concentration factor C these particles can be concentrated; our early numerical results indicated an increase of C with alpha, and were supported by simple scaling arguments, but the extrapolation range was quite large and the predictions (C ~ 10(exp 5) - 10(exp 6) not unlikely) uncertain. The work presented here, which makes use of our recent demonstration that the particle density field is a multifractal with flow-independent properties provides a far more secure ground for such predictions. We also indicate how fine-grained dust rims on chondrules might enter into constraining the situation.

Cuzzi, Jeffrey↗

NASA NTRS2022

A Census of Stars and Disks in Corona Australis

We have performed a census of the young stellar populations near the Corona Australis molecular cloud using photometric and kinematic data from several sources, particularly Gaia EDR3, and spectroscopy of hundreds of candidate members. We have compiled a catalog of 393 members of Corona Australis, (39 at >M6), 293 (36) of which are spectroscopically classified for the first time in this work. We find that Corona Australis can be described in terms of two stellar populations, a younger one (a few megayears) that is partially embedded in the cloud (the Coronet Cluster) and an older one (∼15 Myr) that surrounds and extends beyond the cloud (Upper Corona Australis). These populations exhibit similar space velocities, and we find no evidence for distinct kinematic populations in Corona Australis, in contrast to a recent study based on Gaia DR2. The distribution of spectral types in Corona Australis reaches a maximum at M5 (∼0.15 M⊙), indicating that the initial mass function has a similar characteristic mass as other nearby star-forming regions. Finally, we have compiled mid-infrared photometry from the Wide-field Infrared Survey Explorer and the Spitzer Space Telescope for the members of Corona Australis, and we have used those data to identify and classify their circumstellar disks. Excesses are detected for 122 stars, one-third of which are reported for the first time in this work.

T L Esplin↗

NASA NTRS2005

Image of Fomalhaut Dust Ring at 350 Microns: The Relative Column Density Map Shows Pericenter-Apocenter Asymmetry

We have imaged the circumstellar disk of Fomalhaut at 350 mm wavelength, using SHARC II (Submillimeter High Angular Resolution Camera II) at the Caltech Submillimeter Observatory. The spatial resolution of the raw images (9") has been enhanced by a factor of 3 using the HiRes deconvolution procedure. We find that at this wavelength and signal-to-noise ratio (approx.12), the observed morphology is that of a simple inclined ring (i approx. 70 deg), with little or no other apparent structure--this is the first observation that shows clearly the ring morphology of the disk. We have combined our 350 mm data with Spitzer Space Telescope images at 24, 70, and 160 mm in order to estimate the two-dimensional spatial variation of relative column density ("tau map") using our DISKFIT procedure. The tau map is based on the following physical assumptions: (1) the wavelength variation of opacity is the same throughout the disk, (2) the radial variation of dust temperature is dictated by the energy balance of individual grains in the stellar radiation field, and (3) the vertical scale height of the disk follows a power-law radial variation. The results confirm the ringlike morphology but also show that the geometric center is displaced from the star by about 8 AU and that the ring has an apocentric enhancement of approximately 14% in integrated column density. If we interpret the displacement in terms of elliptical orbital motion due to gravitational perturbation by an unseen planet, then the implied forced eccentricity is ~0.06; dynamical modeling then predicts an apocentric density enhancement consistent with that inferred from the tau map.

Fomalhaut↗

NASA NTRS2001

Identifying Young, Nearby Stars

Young stars have certain characteristics, e.g., high atmospheric abundance of lithium and chromospheric activity, fast rotation, distinctive space motion and strong X-ray flux compared to that of older main sequence stars. We have selected a list of candidate young (<100Myr) and nearby (<60pc) stars based on their space motion and/or strong X-ray flux. To determine space motion of a star, one needs to know its coordinates (RA, DEC), proper motion, distance, and radial velocity. The Hipparcos and Tycho catalogues provide all this information except radial velocities. We anticipate eventually searching approx. 1000 nearby stars for signs of extreme youth. Future studies of the young stars so identified will help clarify the formation of planetary systems for times between 10 and 100 million years. Certainly, the final output of this study will be a very useful resource, especially for adaptive optics and space based searches for Jupiter-mass planets and dusty proto-planetary disks. We have begun spectroscopic observations in January, 2001 with the 2.3 m telescope at Siding Spring Observatory (SSO) in New South Wales, Australia. These spectra will be used to determine radial velocities and other youth indicators such as Li 6708A absorption strength and Hydrogen Balmer line intensity. Additional observations of southern hemisphere stars from SSO are scheduled in April and northern hemisphere observations will take place in May and July at the Lick Observatory of the University of California. AT SSO, to date, we have observed about 100 stars with a high resolution spectrometer (echelle) and about 50 stars with a medium spectral resolution spectrometer (the "DBS"). About 20% of these stars turn out to be young stars. Among these, two especially noteworthy stars appear to be the closest T-Tauri stars ever identified. Interestingly, these stars share the same space motions as that of a very famous star with a dusty circumstellar disk--beta Pictoris. This new finding better constrains the age of beta Pictoris to be approx. 10 Myr.

Webb, Rich↗

NASA NTRS2003

The Star Formation History of Orion and its Environs

During the period of performance, we have obtained the following observations and carried out the analysis of the Orion associations itemized below. Optical Photometry: The Quest camera that had been damaged by lightning on Sept. 2001 was reconfigured to use 10 chips and only 3 filters instead of the original 4 but retaining its spatial coverage of 2.3deg wide in declination. During the Winter and Fall 2002 observing campaigns, we obtained multifilter photometry using the Quest camera on the 1mt Schmidt of the Venezuelan National Observatory, to continue the survey of the Orion clouds. In the Winter 2002 season, we obtained photometry for 10 scans at DEC=+5, 7 scans at DEC=-5. Several additional scans at DEC=-3 were obtained during the Fall 2002 observing campaign. Slit spectroscopy of bright candidates: During 2002 spectra for some 235 candidate PMS stars were obtained with the FAST spectrograph at the SAO 1.5m telescope in Mt. Hopkins. These objects, all brighter than V=16, are located in two strips centered at DEC=+l and -5, and spanning from RA=5h-6h. The spectra are being analyzed. Multifiber spectroscopy: Spectroscopy for our fainter (V less than or equal to 16) candidates has not kept pace with our spectroscopy of brighter candidates, because of the delay in the commissioning of the Hectospec and Hectoechelle multi-object spectrographs at the new 6.5m MMT. We obtained some data with the Hydra spectrograph on the WIYN telescope: Feb 1-4, 2002. Time granted to the project: "A Large Scale Survey of the Low-Mass Young Population in the Orion OB1 Association" (P.I.: C. Briceno). Three half-nights (we used the first half of the night, the second half of the night was used with the Mini-Mosaic imager by another team). Only the first half night was clear. We observed 3 fields centered in a 2.3 deg wide strip centered at DEC=-1 and spanning RA=Sh to 6h. A total of 250 spectra were obtained for the same number of candidate PMS stars. Nov. 13-15, 2002. Time granted to the project "Cluster survey of protoplanetary disk evolution", (P.I.: A. Sicilia-Aguilar). Clear weather throughout. We obtained spectra for some 200 candidate PMS stars in 3 fields located in a 2.3 deg wide strip centered at DEC=+l and spanning from RA=5h20m to 5h35m. Data for both observing runs is partially reduced and is being analyzed. U photometry: During Nov.29-Dec.4, 2002 we were granted time with the 4-shooter CCD Mosaic Camera at the SAO 1.2m telescope, to obtain U-band photometry of a subset of the newly identified T Tauri stars in the strip centered at DEC=-1. This sample is composed of strong H(alpha) emitting PMS stars (Classical T Tauri stars) located mostly in the Orion OB lb association, around the Orion Belt area. Our data will be combined with our calibration of the U-band excess (Gullbring et al. 1998) to derive mass accretion rates. Because of bad weather we observed only 20 stars during 1.5 clear nights. Near IR photometry: During Dec. 14-19, 2002 we used the IR Camera on the SAO 1.2m telescope, to obtain L-band photometry of a set of 17 the newly identified Orion OB 1a and 1b stars, in order to look for IR excess emission coming from the hotter inner parts of circumstellar disks. Again, mediocre weather prevented a larger set to be observed. These stars were also observed with the OSCIR mid-IR camera on Gemini North during later 2001. Our goal is to construct SEDs from the visual out to the mid-IR (by combining our optical/IR data with 2MASS JHK magnitudes) in order to explore disk dissipation at the critical ages of 5-10 Myr spanned by these stars.

Calvet, Nuria↗

NASA NTRS1984

Studies of low-mass star formation with the large deployable reflector

Estimates are made of the far-infrared and submillimeter continuum and line emission from regions of low mass star formation. The intensity of this emission is compared with the sensitivity of the large deployable reflector (LDR), a large space telescope designed for this wavelength range. The proposed LDR is designed to probe the temperature, density, chemical structure, and the velocity field of the collapsing envelopes of these protostars. The LDR is also designed to study the accretion shocks on the cores and circumstellar disks of low-mass protostars, and to detect shock waves driven by protostellar winds.

Hollenbach, D. J.↗

NASA NTRS1985

Protostars and planets II

Various papers on the formation of stars and planets are presented. The general topics addressed include: molecular clouds and star formation, young stellar objects and circumstellar disks, chemistry and grains in molecular clouds and the solar nebula, formation of giant planets, and models of the solar nebula and other planetary systems. Individual papers address: molecular cloud cores, physical conditions in isolated dark globules, rotation in dark clouds, and turbulence in molecular clouds. Also discussed are: fragmentation and hierarchical structure in the interstellar medium, formation of bound stellar clusters, ambient radiation field of young solar systems, and magnetic fields.

Black, D. C.↗

NASA NTRS1985

Infrared studies of the bipolar nebula OHO739

Infrared studies have been made of the M9 giant OHO739, which is embedded in a bipolar nebula, in order to study the structure and density distribution of the circumstellar disk, and preliminary results are reported. The degree of polarization across the 3.1 micron ice band in an 8 arcsec beam centered at the 2.2 micron peak is found to increase with extinction in the ice band, as expected for a bipolar scattering nebula. A distance of 2 kpc is inferred from the tilt angle of the bipolar axis and phase lag measurements of the OH maser. Maps made in the K band and in narrow bands at 2.85, 3.1, and 3.5 microns by raster-scanning OHO739 are presented. In all maps, the object is clearly elongated along the bipolar axis. Patterns of increases and decreases in extinction in the object are inferred to indicate that ice particles are widespread through the nebula and not just concentrated in the disk.

Tielens, A. G. G. M.↗

NASA NTRS1986

Planetary optical and infrared imaging

Images were acquired of a circumstellar disk of orbiting material around the nearby star beta Pictoris. This material is believed to be associated with the formation of planets and provides a first look at a planetary system much younger than our own. Preliminary work was also done to acquire multicolor photometric data and polarimetry on the beta Pictoris disk as well. A coronagraphic search for other proto-planetary systems is also underway. Coronagraphic imaging provided the first clear images and a determination of the geometric albedo of the rings of Uranus. A search for material around the asteroid Amphitrite showed that the region around the asteroid was free of particles larger than 500 meters in radius or diffuse material with an optical depth larger than .000001. NASA IRTF infrared images of Jupiter and Saturn are providing information on the relative heights and cloud top temperatures of atmospheric features.

Terrile, R. J.↗

NASA NTRS1986

Point sources in the Orion complex

Point sources identified in Orion with the IRAS are discussed to illustrate methods of using the IRAS Catalog. The present survey revealed 54 new objects. The Catalog was employed to find companion stars for the observed sources, discern those sources which were predominantly IR cirrus clouds, and separate sources with 12 and 25 microns emissions that were both detected at 60 and 100 microns. Attention is also given to usage of the Catalog to determine which sources were galaxies in Orion and which were galaxies in nearby Gemini. Color-color diagrams are provided for 12, 25 and 60 microns emissions to demonstrate how the data permit identification pre-main sequence stars, stars with circumstellar disks, very young stars, stars with cool photospheres, and hot cirrus clouds.

Beichman, C. A.↗

NASA NTRS1988

Images of star-forming regions. II - The circumstellar environment of L1551 IRS 5

The circumstellar environment of L 1551 IRS 5 is investigated based on high-resolution 8000 A (i) and 9000 A (z) broadband CCD images. A small conical reflection nebula extending from a bright semistellar knot located near the cusp of the nebula is noted. It is suggested that the point-like structure at lambda of less than about 3.7 microns may represent a bright knot of dust-scattered light located on the inner surface of a flattened circumstellar disk surrounding the radio source. Evidence is found of sharp changes in the position angle of the jet emanating from IRS 5, possibly resulting from precession of the jet nozzle provided by a dense inner disk which has dynamically decoupled from the much larger flattened molecular structure.

Campbell, Bel↗

NASA NTRS1988

X-ray sources in regions of star formation. III - Naked T Tauri stars associated with the Taurus-Auriga complex

Ground-based and IRAS optical and IR spectroscopic and photometric observations are reported for 90 stars in or near 59 Einstein Observatory X-ray error circles in the Tau-Aur region. The data are presented in extensive tables and sample spectra and characterized in detail, with particular attention to 28 newly discovered 'naked' T Tau stars, which are shown to be normal stars with no significant IR or UV excess and ages of 1-40 Myr. These stars are found to outnumber normal T Tau stars by a factor of 10 in an area near the Tau-Aur dark clouds, and it is argued that their evolution toward the ZAMS is typical for low-mass stars. The implications of this finding for the time scales of circumstellar-disk dissipation and planet formation are discussed.

Walter, Frederick M.↗

NASA NTRS1988

Research in planetary astronomy

The objective is the continuation of a long-term research program designed to study the composition, structure and processes operating on the surfaces of solar system objects using the Mauna Kea Observatory with techniques and modern instrumentation. Reflectance spectroscopy and multispectral imaging in the spectral region, 0.3 to 5.0 micrometer are the major techniques used, although thermal (10 micrometer and 20 micrometer) radiometry are used in some aspects of the research. Some specific projects include: (1) systematic spectral imaging observations of the Moon; (2) systematic spectral imaging and spectral monitoring of the Martian surface; (3) thermal radiometry of asteroids as part of the IRAS follow-up and other target specific programs; (4) searches for asteroid satellites and dust belts using a stellar coronagraph; and (5) studies of circumstellar disks using a stellar coronagraph. Progress for each of the programs included is discussed. minerals; (2) completed observations of lunar multi-ringed basins and crater deposits in search of high-Ca spectral anomalies; (3) completed data reduction of an additional 5 asteroids observed by the coronagraphic technique in the search for asteroids satellites and debris clouds; and (4) completed the reduction and calibration of 350 asteroids observed at 10 micron and 20 micron using the NASA IRTF.

Mccord, Thomas B.↗

NASA NTRS1989

Additional measurements of pre-main-sequence stellar rotation

New rotational-velocity measurements for pre-main-sequence stars in the Taurus-Auriga molecular cloud are reported. Rotational velocities or upper limits of 10 km/s are now available for 90 percent of the T Tauri stars with V less than 14.7 in the catalog of Cohen and Kuhi. Measurements of 'continuum emission' stars, thought to be accreting high-angular-momentum material from a circumstellar disk, show that these objects are not especially rapid rotators. The results confirm earlier findings that angular-momentum loss proceeds very efficiently in the earliest stages of star formation, and suggest that stars older than about one million yr contract to the main sequence at nearly constant angular momentum. The slow rotation of T Tauri stars probably requires substantial angular-momentum loss via a magnetically coupled wind.

Hartmann, L.↗

NASA NTRS1988

Dust discs around low-mass main-sequence stars

The current understanding of the formation of circumstellar disks as a natural accompaniment to the process of low-mass star formation is examined. Models of the thermal emission from the dust disks around the prototype stars Alpha Lyr, Alpha PsA, Beta Pic, and Epsilon Eri are discussed, which indicate that the central regions of three of these disks are almost devoid of dust within radii ranging between 17 and 26 AU, with the temperature of the hottest zone lying between about 115 and 210 K. One possible explanation of the dust-free zones is the presence of a planet at the inner boundary of each cloud which sweeps up grains crossing its orbit.

Wolstencroft, R. D.↗

NASA NTRS1989

Circumstellar material associated with solar-type pre-main-sequence stars - A possible constraint on the timescale for planet building

The observed frequency distribution of near-IR excesses for a sample of 83 stars in the Taurus-Auriga star-forming complex is used to provide an estimate of the number of solar-type premain-sequence (PMS) stars surrounded by circumstellar disks. The results suggest that if all such PMS stars are initially surrounded by disks, then disk lifetimes must range from much less than 3 x 10 to the 6th yr to about 10 to the 7th yr, providing a constraint on the time available for planet building. PMS stars with small near-IR excesses, but significant mid-IR and far-IR excesses, provide some evidence for changes in disk structure with time.

Strom, Karen M.↗

NASA NTRS1989

Newborn stars and stellar winds in Barnard 228

Imaging and spectroscopic observations of premain-sequence stars in the B228 molecular cloud have identified three sites of extended, shock-excited nebulosity. A highly collimated, blueshifted jet is associated with the T Tauri star Sz68 (CoD -33:10685 deg). A candidate Herbig-Haro object is found near Sz68 and the T Tauri star Sz69. A third region of shock-excited nebulosity is identified near the embedded newborn star IRAS 15398-3359. IR excesses observed toward these stellar sources may imply a relationship between outflow activity and the presence of a circumstellar disk. Only one of the two outflow sources with well-defined orientations is aligned with the large-scale, magnetic-field geometry of the cloud in contrast to the global alignment of outflows in the Taurus cloud complex. Data for three previously unidentified emission-line stars are presented. A faint 18th magnitude dMe star located 48 arcsecs west of Sz68 is identified.

Heyer, Mark H.↗

NASA NTRS1989

Near-IR imaging and imaging polarimetry of OMC 2

NIR and 2.2-micron imaging polarimetry of the molecular cloud region OMC 2, reveals a cluster of low- to intermediate-mass premain-sequence stars embedded in circumstellar disks. The 2.2-micron imaging polarimetry indicates that the compact NIR sources OMC 2 IRS1, IRS2, IRS3 and IRS4 N, are illumination centers for the surrounding extended emission. By application of Hubble's relation to the nebulae illuminated by IRS1, IRS2 and IRS4 N, the illuminating geometry is explained and the intrinsic NIR colors of these objects are estimated.

Rayner, John↗