Search NASA⌕ Search

SEARCH · Search NASA

Results for “young stellar objects”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

A 2.2 micron survey in the L1630 molecular cloud

A 2.2 micron survey of a significant portion of the L1630 molecular cloud was carried out using the NOAO infrared array camera on the Kitt Peak 1.3 m telescope. This survey is estimated to be complete to 13th magnitude at K, which corresponds to a main-sequence star of about 0.6 solar mass. Therefore this survey has provided a sensitive census of the young stellar objects within this molecular cloud. As a result, 912 sources having m(K) less than 13.0 have been detected. It is estimated that 50 percent of these sources are associated with the molecular cloud. These sources are not distributed uniformly throughout the surveyed region but rather appear grouped or clustered. In fact, four embedded infrared clusters were identified. These clusters contain at least 58 percent but possibly as much as 96 percent of the total number of sources associated with this molecular cloud suggesting that the dominant mode of star formation in this region is in clusters.

Lada, Elizabeth A.↗
NASA NTRS1991

Direct VLBI detection of the magnetosphere surrounding the young star S1 in Rho Ophiuchi

VLBI 6-mm data are presently used to investigate the circularly polarized radio core previously identified around the young B3 star S1 in Rho Ophiuchi. The measured angular diameter and brightness temperature are found to be consistent with gyrosynchrotrom radiation emission from mildly relativistic electrons. A simple model based on a pole-on dipolar magnetic field of about 2 kG at the stellar surface suggests itself as consistent with the main observed features of the S1 magnetosphere; an important feature of the model is its taking the influence of the X-ray-emitting plasma into account. S1 may represent a new type of young stellar object, characterized by very extended magnetic fields.

Andre, Philippe↗
NASA NTRS1990

Global star formation in the L1630 molecular cloud

This paper presents the preliminary results of a study of star formation in the L1630 molecular cloud. An unbiased well sampled systematic CS (2 to 1) survey for density condensations and a 2.2-micron survey for embedded infrared sources in L1630 have been completed. These surveys have provided a complete census of the dense cores and young stellar objects within this molecular cloud. As a result, four embedded stellar clusters have been identified in this cloud. These clusters are located near the most massive CS cores implying that the most active sites of star formation are located within the most massive dense cores.

Lada, Elizabeth A.↗
NASA NTRS1992

Infrared morphology of mass outflow from GL 2591

Results are presented of a spectrophotometric study of a young stellar object, GL 2591, at wavelengths covering 2.10 to 2.27 microns, employing the KPNO long-slit cryogenic spectrometer. H2 emission was discovered in this source. A map of the v = 1-0 S(1) emission line shows a bipolar structure extending over 0.9 d (d/2 kpc) pc in an east-west direction roughly centered around GL 2591 IRS, with four bright knotty peaks. Br-gamma emission was detected in this source. A map of the Br-gamma emission line delineates a monopolar jetlike structure emanating over 0.2 d (d/2 kpc)pc from NIRS 1 to the southwest. The map of 2-micron continuum emission shows a multilooplike structure extending predominantly to the west. The dust distribution derived from this continuum map reveals a monopolar shell structure composed of several clumps. The spatial distribution of the H2, Br-gamma and 2-micron continuum emission is consistently explained by an outflow and dust disk model around GL 2591, if this emission is interpreted as shocked gas, ionized, gas, and swept-up dust, respectively, associated with the outflow from GL 2591.

Tamura, Motohide↗
NASA NTRS1992

Global star formation in the L1630 molecular cloud

The first systematic and coordinated surveys for both dense gas and young stellar objects within a single molecular cloud, the L1630 molecular cloud are compared. It is found that (1) star formation in the L1630 molecular cloud occurs almost exclusively within the dense gas; (2) star formation does not occur uniformly throughout the dense gas and is strongly favored in a few very massive dense cores, where efficient conversion of molecular gas into stars has resulted in the production of rich stellar clusters; and (3) high gas densities and high gas mass may be necessary but not sufficient conditions for the formation of star clusters since two of the five most massive dense cores in the cloud have very low levels of star formation activity.

Lada, Elizabeth A.↗
NASA NTRS1992

Water masers in W49N - The youngest stellar jet?

Observations by Gwinn et al. (1992) of the proper motions of water masers in W49N show that they have an elongated distribution expanding from a common center. Features with high space velocity only occur far from the center, while low-velocity features occur at all distances. It is proposed that water masers in star-forming regions occur in expanding shells swept up by high-velocity winds from young, massive stars during the early phases of the expansion. In W49N, confinement of the bubble by a density distribution with an axial cavity can explain both the velocity field and the shape of the maser distribution. A fully dynamical calculation of the expanding bubble is presented which provides a satisfactory fit for the observations and suggests that this system is only about 250 yr old. Thus these observations may show the very first stages of the formation of a jet from a young stellar object.

Mac Low, MORDECAI-M.↗
NASA NTRS1992

Measurement of CO overtone line profiles in SVS 13

High-resolution spectra of the CO overtone emission in the infrared source SVS 13 are obtained and the individual vibration-rotation lines are resolved for the first time in a young stellar object. The observed FWHM velocity of the lines in the v = 2-0 band is 40 km/s. A fit of the v = 2-0 band head with a single temperature LTE slab model yields an optical depth of 0.3 in the R(51) line and a rotational temperature of about 3500 K. Given the best estimates for the orientation of the SVS 13 outflow, the observed line widths are much smaller than expected for either of the two most favored interpretations for the CO emission: a circumstellar disk or a high-velocity neutral wind.

Carr, John S.↗
NASA NTRS1993

Structure and chemistry of Orion S

We present interferometric observations of the SiO J = 2-1, H13CO(+) J = 1-0, HC3N J = 11-10, CH3OH J(K) = 2(0)-1(0), and SO2 J(KpK0) = 8(17)-8(08) transitions along with the 3.1 mm continuum toward the young stellar object Orion S. The HC3N and H13CO(+) emission trace similar spatial and velocity distributions which are extended and follow the Orion molecular ridge. The SiO emission is more spatially confined, peaking to the west of the 3.1 mm continuum source, while the CH3OH emission peaks to the southwest. Weak SO2 emission was detected southeast of the continuum source position. Column densities and fractional abundances are derived for each species at different positions in the region. In general, the molecular abundances near the continuum source are similar to those in the quiescent material near IRc 2, but the abundances decrease toward the continuum source position indicating localized depletions of at least a factor of three. The presence of strong SiO emission with much weaker SO2 emission is interpreted as resulting from high-velocity shock interactions between the outflow from Orion S and the surrounding cloud.

Mcmullin, Joseph P.↗
NASA NTRS1993

Infrared and optical imaging of IRAS sources with CO outflow - A snapshot of early star formation

We present multiband imaging of three IRAS sources associated with CO molecular outflows. We find stellar density enhancements around all three IRAS sources. Optical and near-IR photometry indicates that at least 60 percent of the near-IR sources in the vicinity of the IRAS sources are pre-main-sequence stars. Using the photometric data at nbL and M, we are able to identify candidates for the near-IR counterparts of the IRAS sources. We also find that (1) the spectral energy distribution of the deeply embedded sources could be complicated by source confusion and scattered light from the young stellar objects; (2) star formation in the vicinity of the IRAS sources is a continuous process with an age span of 0.5-3 Myr; and (3) stellar density enhancement is probably a phenomenon found at the earliest stage of star formation.

Chen, H.↗
NASA NTRS1993

The structure of protostellar accretion disks and the origin of bipolar flows

Equations are obtained which govern the disk-wind structure and identify the physical parameters relevant to circumstellar disks. The system of equations is analyzed in the thin-disk approximation, and it is shown that the system can be consistently reduced to a set of ordinary differential equations in z. Representative solutions are presented, and it is shown that the apparent paradox discussed by Shu (1991) is resolved when the finite thickness of the disk is taken into account. Implications of the results for the origin of bipolar flows in young stellar objects and possible application to active galactic nuclei are discussed.

Wardle, Mark↗
NASA NTRS1993

IRAS variables as galactic structure tracers - Classification of the bright variables

The characteristics of the 'bright infrared variables' (BIRVs), a sample consisting of the 300 brightest stars in the IRAS Point Source Catalog with IRAS variability index VAR of 98 or greater, are investigated with the purpose of establishing which of IRAS variables are AGB stars (e.g., oxygen-rich Miras and carbon stars, as was assumed by Weinberg (1992)). Results of the analysis of optical, infrared, and microwave spectroscopy of these stars indicate that, out of 88 stars in the BIRV sample identified with cataloged variables, 86 can be classified as Miras. Results of a similar analysis performed for a color-selected sample of stars, using the color limits employed by Habing (1988) to select AGB stars, showed that, out of 52 percent of classified stars, 38 percent are non-AGB stars, including H II regions, planetary nebulae, supergiants, and young stellar objects, indicating that studies using color-selected samples are subject to misinterpretation.

Allen, L. E.↗
NASA NTRS1993

IRAS 21391 + 5802 - A study in intermediate mass star formation

We present infrared and millimeter wavelength observations of the cold IRAS source 21391 + 5802 and its associated molecular core. Infrared observations at lambda = 3.5 microns reveal a heavily obscured, central point source which is coincident with a compact lambda = 2.7 mm continuum and C18O emission region. The source radiates about 310 solar luminosities, primarily at FIR wavelengths, suggesting that it is a young stellar object of intermediate mass. The steeply rising spectral energy distribution and the large fraction of the system mass residing in circumstellar material imply that IRAS 21391 + 5802 is in an early stage of evolution. The inferred dust temperature indicates a temperature gradient in the core. A comprehensive model for the surrounding core of dust and gas is devised to match the observed dust continuum emission and multitransition CS emission from this and previous studies. We find a r exp -1.5 +/- 0.2 density gradient consistent with that of a gravitationally evolved core and a total core mass of 380 solar masses. The observed dust emission is most consistent with a lambda exp -1.5 - lambda exp -2 dust emissivity law; for a lambda exp -2 law, the data are best fit by a mass opacity coefficient of 3.6 x 10 exp -3 sq cm/g at lambda = 1.25 mm.

Wilking, Bruce↗
NASA NTRS1991

The nature, origin and evolution of embedded star clusters

The recent development of imaging infrared array cameras has enabled the first systematic studies of embedded protoclusters in the galaxy. Initial investigations suggest that rich embedded clusters are quite numerous and that a significant fraction of all stars formed in the galaxy may begin their lives in such stellar systems. These clusters contain extremely young stellar objects and are important laboratories for star formation research. However, observational and theoretical considerations suggest that most embedded clusters do not survive emergence from molecular clouds as bound clusters. Understanding the origin, nature, and evolution of embedded clusters requires understanding the intimate physical relation between embedded clusters and the dense molecular cloud cores from which they form.

Lada, Charles J.↗
NASA NTRS1994

Multiwavelength search for protoplanetary disks

Infrared emission of circumstellar dust was observed for almost one hundred T Tauri stars. This dust is interpreted to be part of a protoplanetary disk orbiting the central star. T Tauri stars are young stellar objects and evolve into solar type stars. Planets are believed to form in these disks. The spectral energy distribution of a disk depends on its temperature profile. Different disk regions emit at different wavelengths. The disk-star boundary layer is hot and emits H(alpha) radiation. Inner disk regions at around 1 AU with a temperature of a few hundred Kelvin can be probed in near infrared wavelength regimes. Outer disk regions at around 100 AU distance from the star are colder and emit far infrared and sub-millimeter radiation. Also, X-ray emission from the stellar surface can reveal information on disk properties. Emission from the stellar surface and the boundary layer may be shielded by circumstellar gas and dust. T Tauri stars with low H(alpha) emission, i.e. no boundary layer, show stronger X-ray emission than classical T Tauri stars, because the inner disk regions of weak emission-line T Tauri stars may be clear of material. In this paper, first ROSAT all sky survey results on the X-ray emission of T Tauri stars and correlations between X-ray luminosity and properties of T Tauri disks are presented. Due to atmospheric absorption, X-ray and most infrared observations cannot be carried out on Earth, but from Earth orbiting satellites (e.g. IRAS, ROSAT, ISO) or from lunar based observatories, which would have special advantages such as a stable environment.

Neuhaeuser, Ralph↗
NASA NTRS1994

Molecular abundances and low-mass star formation. 1: Si- and S-bearing species toward IRAS 16293-2422

Results from millimeter and submillimeter spectral line surveys of the protobinary source Infrared Astronomical Satellite (IRAS) 16293-2422 are presented. Here we outline the abundances of silicon- and sulfur-containing species. A combination of rotation diagram and full statistical equilibrium/radiative transfer calculations is used to constrain the physical conditions toward IRAS 16293 and to construct its beam-averaged chemical composition over a 10 to 20 sec (1600 to 3200 AU) scale. The chemical complexity as judged by species such as SiO, OCS, and H2S, is intermediate between that of dark molecular clouds such as L134N and hot molecular cloud cores such as Orion KL. From the richness of the spectra compared to other young stellar objects of similar luminosity, it is clear that molecular abundances do not scale simply with mass; rather, the chemistry is a strong function of evolutionary state, i.e., age.

Blake, Geoffrey A.↗
NASA NTRS1994

High resolution spectroscopy with CSHELL

The NASA Infrared Telescope Facility Cryogenic Echelle Spectrograph (CSHELL) was designed to fill a need for high sensitivity, high resolution, long slit near-infrared spectroscopy. Scientific programs in the areas of comets, planetary atmospheres, young stellar objects, the interstellar medium, and galactic dynamics have been pursued with CSHELL and are described herein. The future of the instrument is also discussed.

Greene, T. P.↗
NASA NTRS1994

Formation, early evolution, and gravitational stability of protoplanetary disks

The formation, viscous evolution, and gravitational stability of protoplanetary disks are investigated. The formation process is parameterized by the angular velocity of the molecular cloud core omega, while the viscous evolution is parameterized by the viscosity parameter alpha in the disk; in this study we consider a range of (0.4-6) x 10(exp -14)/s for omega and from 10(exp -5) to 10(exp -1) for alpha. The axisymmetric gravitational stabilities of the disks are checked using Toomre's criterion. The resulting disk surface temperature distribution, (d log T(sub s)/d log R) approximately = -0.6 (R is the cylindrical radius), can be attributed to two heating sources: the viscous heating dominant in the inner disk region, and the accretion shock heating dominant in the outer disk region. This surface temperature distribution matches that observed in many disks around young stellar objects. During the infall stage, disks with alpha less than 10(exp -1.5) become gravitationally unstable independent of omega. The gravitational instabilities occur at radii ranging from 5 to 40 AU. The ratio of the disk mass to the central star mass ranges from 0.2 to 0.5 at the times of instability, about 4 x 10(exp -5) x (omega/10(exp -14)/s)(exp -0.67) yr. Most disks with low alpha and high omega become gravitationally unstable during their formation phase.

Nakamoto, Taishi↗
NASA NTRS1995

The evolutionary status of the stellar population in the rho Ophiuchi cloud core

This contribution reports the results of an infrared imaging survey aimed at characterizing the stellar populations associated with the three densest star-forming cores in the Ophiuchus molecular cloud complex. The survey has sufficient sensitivity at J, H, and K (at 5 sigma limits of 16.5, 15.4, and 14.2) to provide a complete census of embedded young stellar objects (YSOs) with masses greater than the hydrogen-burning limit, provided that their ages are less than 3 Myr and that they are obscured by no more than approximately 18 mag of visual extinction. Our data suggest (1) a large fraction (greater than 70%) of the sources located within the cores are still surrounded by circumstellar disks and/or envelopes; and (2) the shape of the initial mass function for masses, M less than 1 solar mass, appears to be consistent with that derived from the solar neighborhood. We also report the results of a deeper imaging survey of centimeter continuum sources (14 sources) in these star-forming cores and in the larger Ophiuchus complex (eight sources). A large fraction (11/14) of the radio sources associated with the cores appear to have infrared excesses diagnostic of circumstellar accretion disks and/or infalling circumstellar envelopes. In these cases, the centimeter continuum radiation most likely diagnoses the ionized component of energetic winds or jets which characterizes YSOs during the disk accretion phase. By contrast, of the eight radio sources located outside dense cores, only two show infrared excesses.

Strom, Karen M.↗