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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

DISCOVERY OF A HOT CORINO IN THE BOK GLOBULE B335

We report the first evidence of a hot corino in a Bok globule. This is based on ALMA observations in the 1.2 mm band toward the low-mass Class 0 protostar IRAS 19347+0727 in B335. Saturated complex organic molecules (COMs), CH{sub 3}CHO, HCOOCH{sub 3}, and NH{sub 2}CHO, are detected in a compact region within a few 10 au around the protostar. Additionally, CH{sub 3}OCH{sub 3}, C{sub 2}H{sub 5}OH, C{sub 2}H{sub 5}CN, and CH{sub 3}COCH{sub 3} are tentatively detected. Carbon-chain related molecules, CCH and c-C{sub 3}H{sub 2}, are also found in this source, whose distributions are extended over a scale of a few 100 au. On the other hand, sulfur-bearing molecules CS, SO, and SO{sub 2} have both compact and extended components. Fractional abundances of the COMs relative to H{sub 2} are found to be comparable to those in known hot corino sources. Though the COMs lines are as broad as 5–8 km s{sup −1}, they do not show obvious rotation motion in the present observation. Thus, the COMs mainly exist in a structure whose distribution is much smaller than the synthesized beam (0.″58 × 0.″52).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

An Extraordinary Outburst in the Massive Protostellar System NGC 6334I-MM1: Quadrupling of the Millimeter Continuum

Based on sub-arcsecond Atacama Large Millimeter/submillimeter Array (ALMA) and Submillimeter Array (SMA) 1.3 mm continuum images of the massive protocluster NGC 6334I obtained in 2015 and 2008, we find that the dust emission from MM1 has increased by a factor of 4.0 ± 0.3 during the intervening years, and undergone a significant change in morphology. The continuum emission from the other cluster members (MM2, MM4, and the UCH ii region MM3 = NGC 6334F) has remained constant. Long-term single-dish maser monitoring at HartRAO finds that multiple maser species toward NGC 6334I flared beginning in early 2015, a few months before our ALMA observation, and some persist in that state. New ALMA images obtained in 2016 July–August at 1.1 and 0.87 mm confirm the changes with respect to SMA 0.87 mm images from 2008, and indicate that the (sub)millimeter flaring has continued for at least a year. The excess continuum emission, centered on the hypercompact H ii region MM1B, is extended and elongated (1.″6 × 1.″0 ≈ 2100 × 1300 au) with multiple peaks, suggestive of general heating of the surrounding subcomponents of MM1, some of which may trace clumps in a fragmented disk rather than separate protostars. In either case, these remarkable increases in maser and dust emission provide direct observational evidence of a sudden accretion event in the growth of a massive protostar yielding a sustained luminosity surge by a factor of 70 ± 20, analogous to the largest events in simulations by Meyer et al. This target provides an excellent opportunity to assess the impact of such a rare event on a protocluster over many years.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Unveiling the Role of the Magnetic Field at the Smallest Scales of Star Formation

We report Atacama Large Millimeter/submillimeter Array (ALMA) observations of polarized dust emission from the protostellar source Ser-emb 8 at a linear resolution of 140 au. Assuming models of dust-grain alignment hold, the observed polarization pattern gives a projected view of the magnetic field structure in this source. Contrary to expectations based on models of strongly magnetized star formation, the magnetic field in Ser-emb 8 does not exhibit an hourglass morphology. Combining the new ALMA data with previous observational studies, we can connect magnetic field structure from protostellar core (∼80,000 au) to disk (∼100 au) scales. We compare our observations with four magnetohydrodynamic gravo-turbulence simulations made with the AREPO code that have initial conditions ranging from super-Alfvénic (weakly magnetized) to sub-Alfvénic (strongly magnetized). These simulations achieve the spatial dynamic range necessary to resolve the collapse of protostars from the parsec scale of star-forming clouds down to the ∼100 au scale probed by ALMA. Only in the very strongly magnetized simulation do we see both the preservation of the field direction from cloud to disk scales and an hourglass-shaped field at <1000 au scales. We conduct an analysis of the relative orientation of the magnetic field and the density structure in both the Ser-emb 8 ALMA observations and the synthetic observations of the four AREPO simulations. We conclude that the Ser-emb 8 data are most similar to the weakly magnetized simulations, which exhibit random alignment, in contrast to the strongly magnetized simulation, where the magnetic field plays a role in shaping the density structure in the source. In the weak-field case, it is turbulence—not the magnetic field—that shapes the material that forms the protostar, highlighting the dominant role that turbulence can play across many orders of magnitude in spatial scale.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1995

CCS Observations of the Protostellar Envelope of B335

Knowledge of the density, velocity and chemical profiles around protostars is of fundamental importance for testing dynamical models of protostar evolution and understanding the nature of the material falling onto circumstellar disks. Presented are single dish and interferometric spectral line observations of CCS towards the core of B335, a classic example of a young, low mass stellar object.

protostar formation molecular cloud cores molecule↗
DOE OSTI2022

Magnetic fields in the formation of the first stars – II. Results

ABSTRACT Beginning with cosmological initial conditions at z = 100, we simulate the effects of magnetic fields on the formation of Population III stars and compare our results with the predictions of Paper I. We use gadget-2 to follow the evolution of the system while the field is weak. We introduce a new method for treating kinematic fields by tracking the evolution of the deformation tensor. The growth rate in this stage of the simulation is lower than expected for diffuse astrophysical plasmas, which have a very low resistivity (high magnetic Prandtl number); we attribute this to the large numerical resistivity in simulations, corresponding to a magnetic Prandtl number of order unity. When the magnetic field begins to be dynamically significant in the core of the minihalo at z = 27, we map it on to a uniform grid and follow the evolution in an adaptive mesh refinement, MHD simulation in orion2. The non-linear evolution of the field in the orion2 simulation violates flux-freezing and is consistent with the theory proposed by Xu & Lazarian. The fields approach equipartition with kinetic energy at densities ∼1010–1012 cm−3. When the same calculation is carried out in orion2 with no magnetic fields, several protostars form, ranging in mass from ∼1 to 30 M⊙; with magnetic fields, only a single ∼30 M⊙ protostar forms by the end of the simulation. Magnetic fields thus suppress the formation of low-mass Pop III stars, yielding a top-heavy Pop III initial mass function and contributing to the absence of observed Pop III stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Thermal History of Matrix Forsterite Grains from Murchison Based on High-resolution Tomography

Protoplanetary disks are dust- and gas-rich structures surrounding protostars. Depending on the distance from the protostar, this dust is thermally processed to different degrees and accreted to form bodies of varying chemical compositions. The primordial accretion processes occurring in the early protoplanetary disk such as chondrule formation and metal segregation are not well understood. One way to constrain them is to study the morphology and composition of forsteritic grains from the matrix of carbonaceous chondrites. Here, we present high-resolution ptychographic X-ray nanotomography and multimodal chemical microtomography (X-ray diffraction and X-ray fluorescence) to reveal the early history of forsteritic grains extracted from the matrix of the Murchison CM2.5 chondrite. Here,. the 3D electron density maps revealed, at unprecedented resolution (64 nm), spherical inclusions containing Fe–Ni, very little silica-rich glass and void caps (i.e., volumes where the electron density is consistent with conditions close to vacuum) trapped in forsterite. The presence of the voids along with the overall composition, petrological textures, and shrinkage calculations is consistent with the grains experiencing one or more heating events with peak temperatures close to the melting point of forsterite (~2100 K), and subsequently cooled and contracted, in agreement with chondrule-forming conditions.

58 GEOSCIENCES↗
DOE OSTI2026

The Extent of Solar Energetic Particle Irradiation in the Sun’s Protoplanetary Disk

Solar flares emit X-rays and high-energy (MeV to GeV) ions called solar energetic particles (SEPs). Astronomical observations show solar-mass protostellar fluxes are a factor Φ ≈ 3 × 10 2 –3 × 10 3 times higher than the present-day Sun. Constraining Φ in the early solar system is important for modeling ionization in the Sun’s protoplanetary disk, the extent of magnetorotational instability or magnetocentrifugal outflows, or even production of short-lived radionuclides. Recent interpretations of meteoritic data—including cosmogenic Ne in hibonite grains, initial ( 10 Be/ 9 Be) 0 ratios in Ca-rich, Al-rich inclusions (CAIs), or even inferences of live 7 Be in CAIs—suggest values Φ > 10 5 , reaching as high as Φ ≈ 6 × 10 6 , which would make the young Sun extraordinarily active, even for a protostar. We constrain Φ by reexamining these data. We conclude that cosmogenic Ne was produced in hibonite grains as they resided in the disk; 36 Cl was created in Cl-poor grains after the disk dissipated; 10 Be was inherited from the molecular cloud, with almost no (<1%) 10 Be created in the disk; and there is no evidence whatsoever for any live 7 Be in CAIs. We show these data are consistent with a value Φ ≈ 3 × 10 3 for the first >5 Myr of the solar nebula. The early Sun evidently emitted a flux of X-rays and SEPs not atypical for a protostar.

79 ASTRONOMY AND ASTROPHYSICS↗
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 NTRS1983

The birthline for low-mass stars

Using the results of protostar theory, the locus in the Hertzsprung-Russell diagram is found where pre-main-sequence stars of subsolar mass should begin their quasi-static contraction phase and first appear as visible objects. This 'birthline' is in striking agreement with observations of T Tauri stars, providing a strong confirmation of the fact that these stars are indeed contracting along Hayashi tracks. The assumption that most T Tauri stars first appear along this line forces a recalibration of their ages. This recalibration removes the puzzling dip in present-day star formation seen in age histograms of several cloud complexes. Since the underlying protostar calculation assumes that the parent cloud was only thermally supported prior to its collapse, the observed location of the birthline places severe restrictions on the degree of extrathermal support provided by rotation, magnetic fields, or turbulence. In addition, the hypothesis that the collapse from thermally supported clouds to low-mass stars proceeds through protostellar disks appears untenable, since the disk accretion process almost certainly produces pre-main-sequence stars with radii well below the observed birthline.

Stahler, S. W.↗
NASA NTRS1984

Submillimeter astronomy and the problem of star formation

Sources that have traditionally been called 'protostars,' because they were strong emitters of infrared radiation embedded in dust clouds, are now recognized to be 'newly formed' stars instead. Recent developments in submillimeter astronomy should permit a redoubling of efforts to find bodies that are the actual predecessors of newly formed stars. This renewed search for true protostars will be aided by advances that have occurred in submillimeter spectroscopy; these will permit an analysis of the physical conditions and chemical constitution of cooler protostellar clouds, and may provide insight into circumstances favoring protostellar collapse.

Harwit, M.↗
NASA NTRS1985

Velocity fields in binary protostellar clouds - An alternative to retrograde rotation

Observations of the emission from optically thin molecular species in several dense interstellar clouds have been interpreted as indicating rotating of cloud envelopes in one direction and of cloud cores in the opposite direction (retrograde rotation). This has been taken as evidence for the presence of magnetic fields sufficiently strong to have caused the retrograde rotation. However, it is shown that the velocity fields that are produced when a nonmagnetic interstellar cloud collapses to form a binary protostellar system yield spatial velocity maps that appear to be at least qualitatively consistent with the (C-13)O observations. An embedded star has been detected recently in one of these clouds (Barnard 5). If the binary protostar model is correct, then another protostar, as yet undetected, should also be present in Barnard 5.

Boss, A. P.↗
NASA NTRS1986

Far-infrared investigation of the Taurus star-forming region using the IRAS database

The Infrared Astronomical Satellite (IRAS) has given us the first completely unbiased sky-survey in the far-infrared with wavebands centered at 12, 25, 60 and 100 microns. The Taurus-Auriga complex was selected as the first molecular cloud to be investigated in this study. The Taurus clouds were defined as lying between 04h and 05h in R.A. and +16 to +31 degrees in Dec., then the IRAS point-source catalogue was searched for sources with good or moderate quality fluxes in all three of the shortest IRAS bands. The sources which were selected in this way were then classified into subgroups according to their IRAS colors. Taurus is generally believed to be an area of low-mass star formation, having no luminous O-B associations within or near to the cloud complex. Once field stars, galaxies and planetary nebulae had been removed from the sample only the molecular cloud cores, T Tauri stars and a few emission-line A and B stars remained. The great majority of these objects are pre-main sequence in nature and, as stated by Chester (1985), main sequence stars without excess far-infrared emission would only be seen in Taurus if their spectral types were earlier than about A5 and then not 25 microns. By choosing our sample in this way we are naturally selecting the hotter and thus more evolved sources. To counteract this, the molecular cloud core-criterion was applied to soruces with good or moderate quality flux at 25, 60 and 100 microns, increasing the core sample by about one third. The candidate protostar B335 is only detected by IRAS at 60 and 100 microns while Taurus is heavily contaminated by cirrus at 100 microns. This means that detection at 25 microns is also required with those at 60 and 100 microns to avoid confusing a ridge of cirrus with a genuine protostar. The far-infrared luminosity function of these sources is then calculated and converted to the visual band by a standard method to compare with the field star luminosity function of Miller and Scalo. The eventual aim of this work is to obtain far-infrared luminosity functions for a number of molecular clouds which are known to be forming low-mass stars and to investigate how the slope is affected by changes in the density and turbulence of material.

Hughes, J. D.↗
NASA NTRS1986

Grain processes in massive star formation

Observational evidence suggests that stars greater than 100 M(solar) exist in the Galaxy and Large Magellanic Cloud (LMC), however classical star formation theory predicts stellar mass limits of only approx. 60 M(solar). A protostellar accretion flow consists of inflowing gas and dust. Grains are destroyed as they are near the central protostar creating a dust shell or cocoon. Radiation pressure acting on the grain can halt the inflow of material thereby limiting the amount of mass accumulated by the protostar. We first consider rather general constraints on the initial grain to gas ratio and mass accretion rates that permit inflow. We further constrain these results by constructing a numerical model. Radiative deceleration of grains and grain destruction processes are explicitly accounted for in an iterative solution of the radiation-hydrodynamic equations. Findings seem to suggest that star formation by spherical accretion requires rather extreme preconditioning of the grain and gas environment.

Wolfire, M. G.↗
NASA NTRS1987

Spectral evolution of young stellar objects

An evolutionary sequence, from protostars to pre-main sequence stars, for the classification of young stellar objects is derived by comparing the predictions of the theoretical protostar models of Adams and Shu (AS, 1986) with the morphological classification scheme of Lada and Wilking (1984). It is shown that the AS models adequately explain the emergent spectral energy distributions of unidentified objects with negative spectral indices in the mid-IR and near-IR in both Taurus and Ophiuchus. If the infalling dust envelope is then completely removed, the spectra of the underlying stars and nebular disks used by AS provide a natural explanation for the near-IR and mid-IR excesses and the positive spectral indices of embedded T Tauri stars. It is found that the addition of a simple physical model for residual dust envelopes can reproduce the far-IR excesses found in some of these T Tauri stars.

Adams, Fred C.↗
NASA NTRS1990

Dynamo magnetic field-induced angular momentum transport in protostellar nebulae - The 'minimum mass' protosolar nebula

Magnetic torques can produce angular momentum redistribution in protostellar nebulas. Dynamo magnetic fields can be generated in differentially rotating and turbulent nebulas and can be the source of magnetic torques that transfer angular momentum from a protostar to a disk, as well as redistribute angular momentum within a disk. A magnetic field strength of 100-1000 G is needed to transport the major part of a protostar's angular momentum into a surrounding disk in a time characteristic of star formation, thus allowing formation of a solar-system size protoplanetary nebula in the usual 'minimum-mass' model of the protosolar nebula. This paper examines the possibility that a dynamo magnetic field could have induced the needed angular momentum transport from the proto-Sun to the protoplanetary nebula.

Stepinski, T. F.↗
NASA NTRS1990

N2H(+) in warm and cold clouds

A topic of importance in both cometary and instellar astronomy is the N2/NH3 abundance ratio in star forming regions. A key test for solar system formation models is the N2/NH3 abundance ratio in comets, which are thought to be made up of material from the protostar nebula, and in molecular clouds, from which stars are thought to be approx. 0.1. The N2/NH3 abundance ratio has never been measured in the interstellar medium, but is estimated from theoretical models to be in the range of 100 to 200. If the theoretical models are truly representative of dense molecular clouds then the difference in the abundance ratios of N2/NH3 between comets and the ISM would imply that significant chemical processing must have occurred in the protostar nebula that somehow altered the amount of N2 relative to NH3. A systematic survey was made of star forming regions to establish N2 abundances and N2/NH3 ratios for comparison with cometary data.

Womack, Maria↗
NASA NTRS1991

The formation of molecules in protostellar winds

The production and destruction processes for molecules in very fast protostellar winds are analyzed and modeled with a one-dimensional chemical kinetics code. Radial density and temperature distributions suggested by protostellar theory are explored as are a range of mass-loss rates. The efficiency of in situ formation of heavy molecules is found to be high if the wind temperature falls sufficiently rapidly, as indicated by theory. The degree of molecular conversion is a strong function of the mass-loss rate and of density gradients associated with the acceleration and collimation of the wind. Even in cases where essentially all of the heavy atoms are processed into molecules, a significant fraction of atomic hydrogen remains so that hghly molecular, protostellar winds are able to emit the 21-cm line. Although CO has a substantial abundance in most models relevant to very young protostars, high abundances of other molecules such as SiO and H2O signify more complete association characteristic of winds containing regions of very high density. Although the models apply only to regions close to the protostar, they are in qualitative accord with recent observations at much larger distances of both atomic and molecular emission from extremely high-velocity flow.

Glassgold, A. E.↗
NASA NTRS1991

Interstellar solid CO - Polar and nonpolar interstellar ices

Observations of the solid-CO lines corresponding to particular protostars are compared with respect to peak position and width and contrasted with the solid CO bands derived experimentally for astrophysical mixtures. Most lines of sight reveal narrow and broad observational solid-CO components; the narrow band appears when nonpolar molecules are dominant. When the concentration of CO is greater than 0.3 the 'surface modes' of the protostars are responsible for the position and shape of the CO fundamental. Some variations of the parameters are related to the composition and/or morphology of the grains particularly at the interstellar 2140/cm line. Grain mantles are theorized to be composed of both a polar and a nonpolar mixtures which reflect the chemical variations associated with accretion.

Tielens, A. G. G. M.↗