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At least 289 records · Page 16
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DOE OSTI2015

GLOBAL PROPERTIES OF FULLY CONVECTIVE ACCRETION DISKS FROM LOCAL SIMULATIONS

We present an approach to deriving global properties of accretion disks from the knowledge of local solutions derived from numerical simulations based on the shearing box approximation. The approach consists of a two-step procedure. First, a local solution valid for all values of the disk height is constructed by piecing together an interior solution obtained numerically with an analytical exterior radiative solution. The matching is obtained by assuming hydrostatic balance and radiative equilibrium. Although in principle the procedure can be carried out in general, it simplifies considerably when the interior solution is fully convective. In these cases, the construction is analogous to the derivation of the Hayashi tracks for protostars. The second step consists of piecing together the local solutions at different radii to obtain a global solution. Here we use the symmetry of the solutions with respect to the defining dimensionless numbers—in a way similar to the use of homology relations in stellar structure theory—to obtain the scaling properties of the various disk quantities with radius.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Triggering collapse of the presolar dense cloud core and injecting short-lived radioisotopes with a shock wave. IV. effects of rotational axis orientation

Both astronomical observations of the interaction of Type II supernova remnants (SNRs) with dense interstellar clouds as well as cosmochemical studies of the abundances of daughter products of short-lived radioisotopes (SLRIs) formed by supernova nucleosynthesis support the hypothesis that the Solar System's SLRIs may have been derived from a supernova. This paper continues a series devoted to examining whether or not such a shock wave could have triggered the dynamical collapse of a dense, presolar cloud core and simultaneously injected sufficient abundances of SLRIs to explain the cosmochemical evidence. Here, we examine the effects of shock waves striking clouds whose spin axes are oriented perpendicular, rather than parallel, to the direction of propagation of the shock front. The models start with 2.2 M{sub ⊙} cloud cores and shock speeds of 20 or 40 km s{sup −1}. Central protostars and protoplanetary disks form in all models, although with their disk spin axes aligned somewhat randomly. The disks derive most of their angular momentum not from the initial cloud rotation, but from the Rayleigh-Taylor fingers that also inject shock wave SLRIs. Injection efficiencies, f{sub i}, the fraction of the incident shock wave material injected into the collapsing cloud core, are ∼0.04–0.1 in these models, similar to when the rotation axis is parallel to the shock propagation direction. Evidently, altering the rotation axis orientation has only a minor effect on the outcome, strengthening the case for this scenario as an explanation for the Solar System's SLRIs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

MILLIMETER SOURCE 13 S IN R CrA: OBSERVATIONS OF A PROTO-HERBIG Ae SYSTEM CANDIDATE

Compelled to observe a young, intermediate mass (IM) protostar candidate, we image velocity structure in a variety of molecular lines toward a strong millimeter continuum peak, MMS 13, in the young stellar object cluster R Corona Australis. We report the detection of N{sub 2}H{sup +} (1–0) and C{sup 34}S (2–1) corresponding to a filamentary IM core coincident with a site of turbulent flow convergence and Spitzer infrared emission minimum. Several emission features including a central decay of turbulence, kinematic signatures of bulk infall, and strong winds support mass and age estimates in indicating the presence of a proto-Herbig Ae system in the region. Channeling of center velocity flow with turbulence amplification along the rotation axis drives wind generation in the system candidate.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

YOUNG STELLAR OBJECTS IN THE MASSIVE STAR-FORMING REGION W49

We present the initial results of our investigation of the star-forming complex W49, one of the youngest and most luminous massive star-forming regions in our Galaxy. We used Spitzer/Infrared Array Camera (IRAC) data to investigate massive star formation with the primary objective of locating a representative set of protostars and the clusters of young stars that are forming around them. We present our source catalog with the mosaics from the IRAC data. In this study we used a combination of IRAC, MIPS, Two Micron All Sky Survey, and UKIRT Deep Infrared Sky Survey (UKIDSS) data to identify and classify the young stellar objects (YSOs). We identified 232 Class 0/I YSOs, 907 Class II YSOs, and 74 transition disk candidate objects using color–color and color–magnitude diagrams. In addition, to understand the evolution of star formation in W49, we analyzed the distribution of YSOs in the region to identify clusters using a minimal spanning tree method. The fraction of YSOs that belong to clusters with ≥7 members is found to be 52% for a cutoff distance of 96″, and the ratio of Class II/I objects is 2.1. We compared the W49 region to the G305 and G333 star-forming regions and concluded that W49 has the richest population, with seven subclusters of YSOs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Early structure formation from primordial density fluctuations with a blue, tilted power spectrum

While observations of large-scale structure and the cosmic microwave background (CMB) provide strong constraints on the amplitude of the primordial power spectrum (PPS) on scales larger than 10 Mpc, the amplitude of the power spectrum on sub-galactic length scales is much more poorly constrained. We study early structure formation in a cosmological model with a blue-tilted PPS. We assume that the standard scale-invariant PPS is modified at small length scales as P(k)∼k{sup m{sub s}} with m{sub s} > 1. We run a series of cosmological hydrodynamic simulations to examine the dependence of the formation epoch and the characteristic mass of primordial stars on the tilt of the PPS. In models with m{sub s} > 1, star-forming gas clouds are formed at z > 100 when the formation of hydrogen molecules is inefficient because the intense CMB radiation destroys chemical intermediates. Without efficient coolant, the gas clouds gravitationally contract while retaining a high temperature. The protostars formed in such “hot” clouds grow very rapidly through accretion to become extremely massive stars that may leave massive black holes with a few hundred solar masses at z > 100. The shape of the PPS critically affects the properties and the formation epoch of the first generation of stars. Future experiments on CMB polarization and spectrum distortion may provide important information on the nature of the first stars and their formation epoch, and hence on the shape of the small-scale power spectrum.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

PROPER MOTIONS OF THE OUTER KNOTS OF THE HH 80/81/80N RADIO-JET

The radio-knots of the Herbig–Haro (HH) 80/81/80N jet extend from the HH 80 object to the recently discovered Source 34 and has a total projected jet size of 10.3 pc, constituting the largest collimated radio-jet system known so far. It is powered by the bright infrared source IRAS 18162−2048 associated with a massive young stellar object. We report 6 cm JVLA observations that, compared with previous 6 cm VLA observations carried out in 1989, allow us to derive proper motions of the HH 80, HH 81, and HH 80N radio knots located about 2.5 pc away in projection from the powering source. For the first time, we measure proper motions of the optically obscured HH 80N object providing evidence that this knot, along with HH 81 and HH 80 are associated with the same radio-jet. We also confirm the presence of Source 34, located further north of HH 80N, previously proposed to belong to the jet.We derived that the tangential velocity of HH 80N is 260 km s{sup −1} and has a direction in agreement with the expected direction of a ballistic precessing jet. The HH 80 and HH 81 objects have tangential velocities of 350 and 220 km s{sup −1}, respectively, but their directions are somewhat deviated from the expected jet path. The velocities of the HH objects studied in this work are significantly lower than those derived for the radio knots of the jet close to the powering source (600–1400 km s{sup −1}) suggesting that the jet is slowing down due to a strong interaction with the ambient medium. As a result, since HH 80 and HH 81 are located near the edge of the cloud, the inhomogeneous and low density medium may contribute to skew the direction of their determined proper motions. The HH 80 and HH 80N emission at 6 cm is, at least in part, probably synchrotron radiation produced by relativistic electrons in a magnetic field of 1 mG. If these electrons are accelerated in a reverse adiabatic shock, we estimate a jet total density of ≲1000 cm{sup −3}. All of these features are consistent with a jet emanating from a high-mass protostar and make evident its capability of accelerating particles up to relativistic velocities.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

HIERARCHICAL GRAVITATIONAL FRAGMENTATION. I. COLLAPSING CORES WITHIN COLLAPSING CLOUDS

We investigate the Hierarchical Gravitational Fragmentation scenario through numerical simulations of the prestellar stages of the collapse of a marginally gravitationally unstable isothermal sphere immersed in a strongly gravitationally unstable, uniform background medium. The core developes a Bonnor–Ebert (BE)-like density profile, while at the time of singularity (the protostar) formation the envelope approaches a singular-isothermal-sphere (SIS)-like r{sup −2} density profile. However, these structures are never hydrostatic. In this case, the central flat region is characterized by an infall speed, while the envelope is characterized by a uniform speed. This implies that the hydrostatic SIS initial condition leading to Shu's classical inside-out solution is not expected to occur, and therefore neither should the inside-out solution. Instead, the solution collapses from the outside-in, naturally explaining the observation of extended infall velocities. The core, defined by the radius at which it merges with the background, has a time-variable mass, and evolves along the locus of the ensemble of observed prestellar cores in a plot of M/M{sub BE} versus M, where M is the core's mass and M{sub BE} is the critical BE mass, spanning the range from the “stable” to the “unstable” regimes, even though it is collapsing at all times. We conclude that the presence of an unstable background allows a core to evolve dynamically from the time when it first appears, even when it resembles a pressure-confined, stable BE-sphere. The core can be thought of as a ram-pressure confined BE-sphere, with an increasing mass due to the accretion from the unstable background.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A TWO-TEMPERATURE MODEL OF MAGNETIZED PROTOSTELLAR OUTFLOWS

We explore kinematics and morphologies of molecular outflows driven by young protostars using magnetohydrodynamic simulations in the context of the unified wind model of Shang et al. The model explains the observed high-velocity jet and low-velocity shell features. In this work we investigate how these characteristics are affected by the underlying temperature and magnetic field strength. We study the problem of a warm wind running into a cold ambient toroid by using a tracer field that keeps track of the wind material. While an isothermal equation of state is adopted, the effective temperature is determined locally based on the wind mass fraction. In the unified wind model, the density of the wind is cylindrically stratified and highly concentrated toward the outflow axis. Our simulations show that for a sufficiently magnetized wind, the jet identity can be well maintained even at high temperatures. However, for a high temperature wind with low magnetization, the thermal pressure of the wind gas can drive material away from the axis, making the jet less collimated as it propagates. We also study the role of the poloidal magnetic field of the toroid. It is shown that the wind-ambient interface becomes more resistant to corrugation when the poloidal field is present, and the poloidal field that bunches up within the toroid prevents the swept-up material from being compressed into a thin layer. This suggests that the ambient poloidal field may play a role in producing a smoother and thicker swept-up shell structure in the molecular outflow.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

“Dust, ice, and gas in time” (DIGIT) herschel observations of GSS30-IRS1 in Ophiuchus

As a part of the “Dust, Ice, and Gas In Time” (DIGIT) key program on Herschel, we observed GSS30-IRS1, a Class I protostar located in Ophiuchus (d = 120 pc), with Herschel/Photodetector Array Camera and Spectrometer. More than 70 lines were detected within a wavelength range from 50 to 200 μm, including CO, H{sub 2}O, OH, and two atomic [O i] lines at 63 and 145 μm. The [C ii] line, known as a tracer of externally heated gas by the interstellar radiation field (ISRF), is also detected at 158 μm. All lines, except [O i] and [C ii], are detected only at the central spaxel of 9.″4 × 9.″4. The [O i] emissions are extended along a NE–SW orientation, and the [C ii] line is detected over all spaxels, indicative of an external photodissociation region. The total [C ii] intensity around GSS30 reveals that the far-ultraviolet radiation field is in the range of 3 to 20 G{sub 0}, where G{sub 0} is in units of the Habing Field, 1.6 × 10{sup −3} erg cm{sup −2} s{sup −1}. This enhanced external radiation field heats the envelope of GSS30-IRS1, causing the continuum emission to be extended, unlike the molecular emission. The best-fit continuum model of GSS30-IRS1 with the physical structure including flared disk, envelope, and outflow shows that the internal luminosity is 10 L{sub ⊙}, and the region is externally heated by a radiation field enhanced by a factor of 130 compared to the standard local ISRF.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

THE L1157-B1 ASTROCHEMICAL LABORATORY: MEASURING THE TRUE FORMALDEHYDE DEUTERATION ON GRAIN MANTLES

L1157 is a prototypical chemically active outflow driven by a low-mass class-0 protostar, and B1 is its brightest bow shock. Toward L1157-B1, several emission lines of deuterated molecules have been detected for the first time by Codella et al. The authors suggested that these were formed on grain mantles, and then released into the gas phase by the passage of the shock. In this Letter we report observations obtained at high angular resolution with the Plateau de Bure Interferometer of HDCO and CH{sub 2}DOH. The emission of HDCO perfectly delineates the region of the interface between the fast jet and the slower ambient material, confirming the predictions of the previous work that deuterated species were formed on grain mantles and then released into the gas phase by the passage of the shock. CH{sub 2}DOH emission is fainter and thus its emitting region is not well determined. The deuterated fraction HDCO/H{sub 2}CO is ∼0.1 in the HDCO emitting region, an order of magnitude larger than the upper limit found in the surrounding material, probably dominated by warm-gas chemistry and less affected by grain evaporation. Our study represents the first clear evidence ever found of a deuterated molecule as shock tracer, and yields an indirect but ''clean'' measurement of the deuteration of the ices covering the dust grains during the cold pre-protostellar phase.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

FIRST DETECTION OF [C I] {sup 3}P{sub 1}–{sup 3}P{sub 0} EMISSION FROM A PROTOPLANETARY DISK

We performed single point [C i] {sup 3}P{sub 1}–{sup 3}P{sub 0} and CO J = 4–3 observations toward three T Tauri stars (TTSs), DM Tau, LkCa 15, and TW Hya, using the Atacama Large Millimeter/submillimeter Array Band 8 qualification model receiver installed on the Atacama Submillimeter Telescope Experiment. Two protostars (PSs) in the Taurus L1551 region, L1551 IRS 5 and HL Tau, were also observed. We successfully detected [C i] emission from the protoplanetary disk around DM Tau as well as the protostellar targets. The spectral profile of the [C i] emission from the protoplanetary disk is marginally single-peaked, suggesting that atomic carbon (C) extends toward the outermost disk. The detected [C i] emission is optically thin and the column densities of C are estimated to be ≲10{sup 16} and ∼10{sup 17} cm{sup −2} for the TTS targets and the PSs, respectively. We found a clear difference in the total mass ratio of C to dust, M(C)/M(dust), between the TTSs and protostellar targets; the M(C)/M(dust) ratio of the TTSs is one order of magnitude smaller than that of the PSs. The decrease of the estimated M(C)/M(dust) ratios for the disk sources is consistent with a theoretical prediction that the atomic C can survive only in the near surface layer of the disk and C{sup +}/C/CO transition occurs deeper into the disk midplane.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ENVIRONMENT AND PROTOSTELLAR EVOLUTION

Even today in our Galaxy, stars form from gas cores in a variety of environments, which may affect the properties of the resulting star and planetary systems. Here, we study the role of pressure, parameterized via ambient clump mass surface density, on protostellar evolution and appearance, focusing on low-mass Sun-like stars and considering a range of conditions from relatively low pressure filaments in Taurus, to intermediate pressures of cluster-forming clumps like the Orion Nebula Cluster, to very high pressures that may be found in the densest infrared dark clouds or in the Galactic center. We present unified analytic and numerical models for the collapse of prestellar cores, accretion disks, protostellar evolution, and bipolar outflows, coupled with radiative transfer calculations and a simple astrochemical model to predict CO gas-phase abundances. Prestellar cores in high-pressure environments are smaller and denser and thus collapse with higher accretion rates and efficiencies, resulting in higher luminosity protostars with more powerful outflows. The protostellar envelope is heated to warmer temperatures, affecting infrared morphologies (and thus classification) and astrochemical processes like CO depletion onto dust grain ice mantles (and thus CO morphologies). These results have general implications for star and planet formation, especially via their effect on astrochemical and dust grain evolution during infall to and through protostellar accretion disks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE ENVIRONMENT OF THE STRONGEST GALACTIC METHANOL MASER

The high-mass star-forming site G009.62+00.20 E hosts the 6.7 GHz methanol maser source with the greatest flux density in the Galaxy which has been flaring periodically over the last 10 yr. We performed high-resolution astrometric measurements of the CH{sub 3}OH, H{sub 2}O, and OH maser emission and 7 mm continuum in the region. The radio continuum emission was resolved in two sources separated by 1300 AU. The CH{sub 3}OH maser cloudlets are distributed along two north–south ridges of emission to the east and west of the strongest radio continuum component. This component likely pinpoints a massive young stellar object which heats up its dusty envelope, providing a constant IR pumping for the Class II CH{sub 3}OH maser transitions. We suggest that the periodic maser activity may be accounted for by an independent, pulsating, IR radiation field provided by a bloated protostar in the vicinity of the brightest masers. We also report the discovery of an elliptical distribution of CH{sub 3}OH maser emission in the region of periodic variability.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Radio properties of young stellar objects in the core of the Serpens South Infrared Dark Cloud

We present deep radio continuum observations of the star-forming core of the Serpens South Infrared Dark Cloud with the Karl G. Jansky Very Large Array. Observations were conducted in two bands centered at 7.25 GHz (4.14 cm) and 4.75 GHz (6.31 cm) with a σ{sub rms} of 8.5 and 11.1 μJy/beam, respectively. We also use 2MASS, Spitzer and Herschel data to put our radio observations in the context of young stellar populations characterized by near and far-infrared observations. Within a 5′ × 5′ region of interest around the central cluster, we detect roughly eighteen radio sources, seven of which we determine are protostellar in nature due to their radio spectral indices and their association with infrared sources. We find evidence for a previously undetected embedded Class 0 protostar and reaffirm Class 0 protostellar classifications determined by previous millimeter wavelength continuum studies. We use our infrared data to derive mid-infrared luminosities for three of our protostellar sources and find relative agreement between the known young stellar object (YSO) radio luminosity versus bolometric luminosity correlation. Lastly, we marginally detect an additional six radio sources at the 2–3σ level that lie within two arcseconds of infrared YSO candidates, providing motivation for higher sensitivity studies to clarify the nature of these sources and further probe embedded and/or low luminosity YSOs in Serpens South.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

STRUCTURAL STUDIES OF EIGHT BRIGHT RIMMED CLOUDS IN THE SOUTHERN HEMISPHERE

We carried out deep- and wide-field near- and mid-infrared observations for a sample of eight bright-rimmed clouds (BRCs). Supplemented with the Spitzer archival data, we have identified and classified 44 to 433 young stellar objects (YSOs) associated with these BRCs. The Class I sources are generally located toward the places with higher extinction and are relatively closer to each other than the Class II sources, confirming that the young protostars are usually found in regions with denser molecular material. On the other hand the comparatively older population, Class II objects, are more randomly found throughout the regions, which can be due to their dynamical evolution. Using the minimal sampling tree analyses, we have extracted 13 stellar cores of eight or more members, which contain 60% of the total YSOs. The typical core is ∼0.6 pc in radii and somewhat elongated (aspect ratio of 1.45), of relatively low stellar density (surface density 60 pc{sup −2}), consisting of a small (35) number of YSOs of relatively young sources (66% Class I), and partially embedded (median A{sub K} = 1.1 mag). But the cores show a wide range in their mass distribution (∼20 to 2400 M {sub ⊙}), with a median value of around 130 M {sub ⊙}. We found the star-formation efficiencies in the cores to be between 3% and 30% with an average of ∼14%, which agrees with the efficiencies needed to link the core mass function to the initial mass function. We also found a linear relation between the density of the clouds and the number of YSOs. The peaked nearest neighbor spacing distributions of the YSOs and the ratio of Jeans lengths to the YSO separations indicates a significant degree of non-thermally driven fragmentation in these BRCs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Vertical structure of magnetized accretion disks around young stars

We model the vertical structure of the magnetized accretion disks that are subject to viscous and resistive heating and irradiation by the central star. We apply our formalism to the radial structure of the magnetized accretion disks that are threaded by the poloidal magnetic field dragged during the process of star formation, which was developed by Shu and coworkers. We consider disks around low-mass protostars, T Tauri, and FU Orionis stars, as well as two levels of disk magnetization: λ{sub sys}=4 (strongly magnetized disks) and λ{sub sys}=12 (weakly magnetized disks). The rotation rates of strongly magnetized disks have large deviations from Keplerian rotation. In these models, resistive heating dominates the thermal structure for the FU Ori disk, and the T Tauri disk is very thin and cold because it is strongly compressed by magnetic pressure; it may be too thin compared with observations. Instead, in the weakly magnetized disks, rotation velocities are close to Keplerian, and resistive heating is always less than 7% of the viscous heating. In these models, the T Tauri disk has a larger aspect ratio, which is consistent with that inferred from observations. All the disks have spatially extended hot atmospheres where the irradiation flux is absorbed, although most of the mass (∼90%–95%) is in the disk midplane. With the advent of ALMA one expects direct measurements of magnetic fields and their morphology at disk scales. It will then be possible to determine the mass-to-flux ratio of magnetized accretion disks around young stars, an essential parameter for their structure and evolution. Our models contribute to the understanding of the vertical structure and emission of these disks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

ALMA observation of the 658 GHz vibrationally excited H{sub 2}O maser in Orion KL source I

We present an observational study of the vibrationally excited H{sub 2}O line at 658 GHz (ν{sub 2} = 1, 1{sub 1,0}-1{sub 0,1}) toward Orion KL using the Atacama Large Millimeter/Submillimeter Array (ALMA). This line is clearly detected at the position of the massive protostar candidate, Source I. The spatial structure is compact, with a size of about 100 AU, and is elongated along the northeast–southwest low-velocity (18 km {sup −1}) bipolar outflow traced by 22 GHz H{sub 2}O masers, SiO masers, and thermal SiO lines. A velocity gradient can be seen perpendicular to the bipolar outflow. The overall spatial and velocity structure seems to be analogous to that of the 321 GHz H{sub 2}O maser line previously detected with ALMA and vibrationally excited SiO maser emission. The brightness temperature of the 658 GHz H{sub 2}O line is estimated to be higher than 2 × 10{sup 4} K, implying that it is emitted via maser action. Our results suggest that the 658 GHz H{sub 2}O maser line is emitted from the base of the outflow from a rotating and expanding accretion disk as observed for the SiO masers and the 321 GHz H{sub 2}O maser. We also search for two other H{sub 2}O lines at 646 GHz (9{sub 7,3}-8{sub 8,0} and 9{sub 7,2}-8{sub 8,1}), but they are not detected in Orion KL.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

CS EMISSION NEAR MIR-BUBBLES

We survey 44 young stellar objects located near the edges of mid-IR-identified bubbles in CS (1–0) using the Green Bank Telescope. We detect emission in 18 sources, indicating young protostars that are good candidates for being triggered by the expansion of the bubble. We calculate CS column densities and abundances. Three sources show evidence of infall through non-Gaussian line-shapes. Two of these sources are associated with dark clouds and are promising candidates for further exploration of potential triggered star formation. We obtained on-the-fly maps in CS (1–0) of three sources, showing evidence of significant interactions between the sources and the surrounding environment.

79 ASTRONOMY AND ASTROPHYSICS↗