Protostellar Outflows Bally, John
Annual review of astronomy and astrophysics,
09/2016, Letnik:
54, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Outflows from accreting, rotating, and magnetized systems are ubiquitous. Protostellar outflows can be observed from radio to X-ray wavelengths in the continuum and a multitude of spectral lines that ...probe a wide range of physical conditions, chemical phases, radial velocities, and proper motions. Wide-field visual and near-IR data, mid-IR observations from space, and aperture synthesis with centimeter- and millimeterwave interferometers are revolutionizing outflow studies. Many outflows originate in multiple systems and clusters. Although most flows are bipolar and some contain highly collimated jets, others are wide-angle winds, and a few are nearly isotropic and exhibit explosive behavior. Morphologies and velocity fields indicate variations in ejection velocity, mass-loss rate, and in some cases, flow orientation and degree of collimation. These trends indicate that stellar accretion is episodic and often occurs in a complex dynamical environment. Outflow power increases with source luminosity but decreases with evolutionary stage. The youngest outflows are small and best traced by molecules such as CO, SiO, H
2
O, and H
2
. Older outflows can grow to parsec scales and are best traced by shock-excited atoms and ions such as hydrogen-recombination lines, S
ii
, and O
ii
. Outflows inject momentum and energy into their surroundings and provide an important mechanism in the self-regulation of star formation. However, momentum injection rates remain uncertain with estimates providing lower bounds.
Abstract
Hubble Space Telescope (HST) images obtained in 2018 are combined with archival HST data taken in 1995 to detect changes and measure proper motions in the HH 80/81 shock complex, which is ...powered by the fastest known jet driven by a forming star, the massive object IRAS 18162-2048. Some persistent features close to the radio jet axis have proper motions of >1000 km s
−1
away from IRAS 18162-2048. About 3–5 pc downstream from the IRAS source and beyond HH 80/81, H
α
emission traces the rim of a parsec-scale bubble blown by the jet. Lower speed motions are seen in S
ii
away from the jet axis; these features have a large component of motion at right angles to the jet. We identify new HH objects and H
2
shocks in the counterflow opposite HH 80/81. The northeastern counterflow to HH 80/81 exhibits an extended but faint complex of 2.12
μ
m H
2
shocks. The inner portion of the outflow is traced by dim 1.64
μ
m Fe
ii
emission. The full extent of this outflow is at least 1500″ (∼10 pc in projection at a distance of 1.4 kpc). We speculate about the conditions responsible for the production of the ultrafast jet and the absence of prominent large-scale molecular outflow lobes.
Most massive stars form in dense clusters where gravitational interactions with other stars may be common. The two nearest forming massive stars, the BN object and Source I, located behind the Orion ...Nebula, were ejected with velocities of ∼29 and ∼13 km s−1 about 500 years ago by such interactions. This event generated an explosion in the gas. New ALMA observations show in unprecedented detail, a roughly spherically symmetric distribution of over a hundred 12CO J = 2−1 streamers with velocities extending from VLSR = −150 to +145 km s−1. The streamer radial velocities increase (or decrease) linearly with projected distance from the explosion center, forming a "Hubble Flow" confined to within 50″ of the explosion center. They point toward the high proper-motion, shock-excited H2 and Fe ii "fingertips" and lower-velocity CO in the H2 wakes comprising Orion's "fingers." In some directions, the H2 "fingers" extend more than a factor of two farther from the ejection center than the CO streamers. Such deviations from spherical symmetry may be caused by ejecta running into dense gas or the dynamics of the N-body interaction that ejected the stars and produced the explosion. This ∼1048 erg event may have been powered by the release of gravitational potential energy associated with the formation of a compact binary or a protostellar merger. Orion may be the prototype for a new class of stellar explosiozn responsible for luminous infrared transients in nearby galaxies.
Aims. Adaptive optics (AO) images are used to test the hypothesis that the explosive BN/KL outflow from the Orion OMC1 cloud core was powered by the dynamical decay of a non-hierarchical system of ...massive stars. Methods. Narrow-band H2, Fe ii, and broad-band Ks obtained with the Gemini South multi-conjugate AO system GeMS and near-IR imager GSAOI are presented. The images reach resolutions of 0.08 to 0.10′′, close to the 0.07′′ diffraction limit of the 8-m telescope at 2.12 μm. Comparison with previous AO-assisted observations of sub-fields and other ground-based observations enable measurements of proper motions and the investigation of morphological changes in H2 and Fe ii features with unprecedented precision. The images are compared with numerical simulations of compact, high-density clumps moving ~103 times their own diameter through a lower density medium at Mach 103. Results. Several sub-arcsecond H2 features and many Fe ii “fingertips” on the projected outskirts of the flow show proper motions of ~300 km s-1. High-velocity, sub-arcsecond H2 knots (“bullets”) are seen as far as 140′′ from their suspected ejection site. If these knots propagated through the dense Orion A cloud, their survival sets a lower bound on their densities of order 107 cm-3, consistent with an origin within a few au of a massive star and accelerated by a final multi-body dynamic encounter that ejected the BN object and radio source I from OMC1 about 500 yr ago. Conclusions. Over 120 high-velocity bow-shocks propagating in nearly all directions from the OMC1 cloud core provide evidence for an explosive origin for the BN/KL outflow triggered by the dynamic decay of a non-hierarchical system of massive stars. Such events may be linked to the origin of runaway, massive stars.
The proper motions of the three stars ejected from Orion's OMC1 cloud core are combined with the requirement that their center of mass is gravitationally bound to OMC1 to show that radio source I ...(Src I) is likely to have a mass around 15 M consistent with recent measurements. Src I, the star with the smallest proper motion, is suspected to be either an astronomical-unit-scale binary or a protostellar merger remnant produced by a dynamic interaction ∼550 yr ago. Near-infrared 2.2 m images spanning ∼21 yr confirm the ∼55 km s−1 motion of "source x" (Src x) away from the site of stellar ejection and point of origin of the explosive OMC1 protostellar outflow. The radial velocities and masses of the Becklin-Neugebauer (BN) object and Src I constrain the radial velocity of Src x to be km s−1. Several high proper-motion radio sources near BN, including Zapata 11 (ZRK2004 11) and a diffuse source near IRc 23, may trace a slow bipolar outflow from BN. The massive disk around Src I is likely the surviving portion of a disk that existed prior to the stellar ejection. Though highly perturbed, shocked, and reoriented by the N-body interaction, enough time has elapsed to allow the disk to relax with its spin axis roughly orthogonal to the proper motion.
ABSTRACT We sort 4683 molecular clouds between 10° < < 65° from the Bolocam Galactic Plane Survey based on observational diagnostics of star formation activity: compact 70 m sources, mid-IR ...color-selected YSOs, H2O and CH3OH masers, and UCH ii regions. We also present a combined NH3-derived gas kinetic temperature and H2O maser catalog for 1788 clumps from our own GBT 100 m observations and from the literature. We identify a subsample of 2223 (47.5%) starless clump candidates (SCCs), the largest and most robust sample identified from a blind survey to date. Distributions of flux density, flux concentration, solid angle, kinetic temperature, column density, radius, and mass show strong (>1 dex) progressions when sorted by star formation indicator. The median SCC is marginally subvirial ( ∼ 0.7) with >75% of clumps with known distance being gravitationally bound ( < 2). These samples show a statistically significant increase in the median clump mass of ΔM ∼ 170-370 M from the starless candidates to clumps associated with protostars. This trend could be due to (i) mass growth of the clumps at M ˙ ∼ 200 - 440 M Myr−1 for an average freefall 0.8 Myr timescale, (ii) a systematic factor of two increase in dust opacity from starless to protostellar phases, and/or (iii) a variation in the ratio of starless to protostellar clump lifetime that scales as ∼M−0.4. By comparing to the observed number of CH3OH maser containing clumps, we estimate the phase lifetime of massive (M > 103 M ) starless clumps to be 0.37 0.08 Myr (M/103 M )−1; the majority (M < 450 M ) have phase lifetimes longer than their average freefall time.
High-mass stars form within star clusters from dense, molecular regions (DMRs), but is the process of cluster formation slow and hydrostatic or quick and dynamic? We link the physical properties of ...high-mass star-forming regions with their evolutionary stage in a systematic way, using Herschel and Spitzer data. In order to produce a robust estimate of the relative lifetimes of these regions, we compare the fraction of DMRs above a column density associated with high-mass star formation, N(H2) > 0.4-2.5 × 1022 cm−2, in the "starless" (no signature of stars 10 forming) and star-forming phases in a 2° × 2° region of the Galactic Plane centered at = 30°. Of regions capable of forming high-mass stars on ∼1 pc scales, the starless (or embedded beyond detection) phase occupies about 60%-70% of the DMR lifetime, and the star-forming phase occupies about 30%-40%. These relative lifetimes are robust over a wide range of thresholds. We outline a method by which relative lifetimes can be anchored to absolute lifetimes from large-scale surveys of methanol masers and UCHII regions. A simplistic application of this method estimates the absolute lifetime of the starless phase to be 0.2-1.7 Myr (about 0.6-4.1 fiducial cloud free-fall times) and the star-forming phase to be 0.1-0.7 Myr (about 0.4-2.4 free-fall times), but these are highly uncertain. This work uniquely investigates the star-forming nature of high column density gas pixel by pixel, and our results demonstrate that the majority of high column density gas is in a starless or embedded phase.
Orion SrcI's Disk Is Salty Ginsburg, Adam; McGuire, Brett; Plambeck, Richard ...
The Astrophysical journal,
02/2019, Letnik:
872, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We report the detection of NaCl, KCl, and their 37Cl and 41K isotopologues toward the disk around Orion SrcI. About 60 transitions of these molecules were identified. This is the first detection of ...these molecules in the interstellar medium not associated with the ejecta of evolved stars. It is also the first ever detection of the vibrationally excited states of these lines in the ISM above v = 1, with firm detections up to v = 6. The salt emission traces the region just above the continuum disk, possibly forming the base of the outflow. The emission from the vibrationally excited transitions is inconsistent with a single temperature, implying the lines are not in LTE. We examine several possible explanations of the observed high excitation lines, concluding that the vibrational states are most likely to be radiatively excited via rovibrational transitions in the 25-35 (NaCl) and 35-45 (KCl) range. We suggest that the molecules are produced by destruction of dust particles. Because these molecules are so rare, they are potentially unique tools for identifying high-mass protostellar disks and measuring the radiation environment around accreting young stars.
The star formation rate (SFR) in the Central Molecular Zone (CMZ, i.e. the central 500 pc) of the Milky Way is lower by a factor of ≥10 than expected for the substantial amount of dense gas it ...contains, which challenges current star formation theories. In this paper, we quantify which physical mechanisms could be responsible. On scales larger than the disc scaleheight, the low SFR is found to be consistent with episodic star formation due to secular instabilities or possibly variations of the gas inflow along the Galactic bar. The CMZ is marginally Toomre-stable when including gas and stars, but highly Toomre-stable when only accounting for the gas, indicating a low condensation rate of self-gravitating clouds. On small scales, we find that the SFR in the CMZ may be caused by an elevated critical density for star formation due to the high turbulent pressure. The existence of a universal density threshold for star formation is ruled out. The H i–H2 phase transition of hydrogen, the tidal field, a possible underproduction of massive stars due to a bottom-heavy initial mass function, magnetic fields, and cosmic ray or radiation pressure feedback also cannot individually explain the low SFR. We propose a self-consistent cycle of star formation in the CMZ, in which the effects of several different processes combine to inhibit star formation. The rate-limiting factor is the slow evolution of the gas towards collapse – once star formation is initiated it proceeds at a normal rate. The ubiquity of star formation inhibitors suggests that a lowered central SFR should be a common phenomenon in other galaxies. We discuss the implications for galactic-scale star formation and supermassive black hole growth, and relate our results to the star formation conditions in other extreme environments.
Context. The Galactic center is the closest region where we can study star formation under extreme physical conditions like those in high-redshift galaxies. Aims. We measure the temperature of the ...dense gas in the central molecular zone (CMZ) and examine what drives it. Methods. We mapped the inner 300 pc of the CMZ in the temperature-sensitive J = 3–2 para-formaldehyde (p - H2CO) transitions. We used the 32,1−22,0/ 30,3−20,2 line ratio to determine the gas temperature in n ~ 104−105 cm-3 gas. We have produced temperature maps and cubes with 30′′ and 1 km s-1 resolution and published all data in FITS form. Results. Dense gas temperatures in the Galactic center range from ~60 K to >100 K in selected regions. The highest gas temperatures TG> 100 K are observed around the Sgr B2 cores, in the extended Sgr B2 cloud, the 20 km s-1 and 50 km s-1 clouds, and in “The Brick” (G0.253+0.016). We infer an upper limit on the cosmic ray ionization rate ζCR< 10-14s-1. Conclusions. The dense molecular gas temperature of the region around our Galactic center is similar to values found in the central regions of other galaxies, in particular starburst systems. The gas temperature is uniformly higher than the dust temperature, confirming that dust is a coolant in the dense gas. Turbulent heating can readily explain the observed temperatures given the observed line widths. Cosmic rays cannot explain the observed variation in gas temperatures, so CMZ dense gas temperatures are not dominated by cosmic ray heating. The gas temperatures previously observed to be high in the inner ~75 pc are confirmed to be high in the entire CMZ.
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