We have conducted a homogeneous near-infrared (near-IR) spectroscopic survey of 33 objects with varying degrees of similarity to FU Orionis. Common spectroscopic features that are characteristic of ...the three classical FUors, FU Ori, V1057 Cyg, and V1515 Cyg, are strong CO absorption, weak metal absorption, strong water bands, low gravity, strong blueshifted He i absorption, and few (if any) emission lines. Based on these criteria, we classify the 33 objects as either bona fide FUors (eruption observed), FUor-like objects (eruption not observed), or peculiar objects with some FUor-like characteristics, and present a spectral atlas of 14 bona fide FUors, 10 FUor-like objects, and 9 peculiar objects. All objects that we classify as FUors or FUor-like have very similar near-IR spectra. We use this spectral similarity to determine the extinction to each source, and correlate the extinction to the depth of the 3 m ice band. All bona fide FUors still today maintain the spectrum of a FUor, despite the eruption occurring up to 80 years ago. Most FUors and FUor-like objects occupy a unique space on a plot of Na+Ca versus CO equivalent widths, whereas the peculiar objects tend to be found mostly elsewhere. Since most FUors show a reflection nebula, we also present an atlas of K-band images of each target. We found that the near-IR spectra of FUors and young brown dwarfs can be extremely similar, a distinguishing feature being the Paschen β absorption in the spectra of FUors. Although V1647 Ori, AR 6a, and V346 Normae had been previously classified as candidate FUors, we classify them as peculiar objects with some FUor-like properties since their spectra now differ significantly from bona fide FUors. We confirm two new FUor-like objects that were initially identified as candidates based on their near-IR morphology.
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.
We report on the source Gaia 19ajj, identifying it as a young star associated with a little-studied star-forming region seen along a complex line of sight through the Gum Nebula. The optical ...lightcurve recently recorded by Gaia exhibits a slow and unsteady 5.5 mag rise over about 3 yr, while the mid-infrared lightcurve from NEOWISE over the same time period shows a 1.2 mag rise having similar structure. Available color information is inconsistent with pure extinction reduction as the cause for the photometric brightening. Optical spectroscopic characteristics in the current bright phase include: little in the way of absorption except for the hallmark Li i 6707 signature of youth plus weak, e.g., Ca i and notably Ba ii; strong wind/outflow in Ca ii, Mg i b, Na i D, H , K i, and O i; jet signatures in O i, S ii, Ca ii, Fe ii, and Ni ii; and narrow rest-velocity emission in neutral species such as Fe i, Ni i, and Mg i. The infrared spectrum is also characterized by outflow and emission, including: a hot He i wind, jet lines such as Fe ii and H2; and weak narrow rest-velocity atomic line emission. The bandheads are weakly in emission, but there is also broad H2O absorption. Gaia 19ajj exhibited a previous bright state in the 2010-2012 time frame. The body of photometric and spectroscopic evidence suggests that the source bears resemblance to V2492 Cyg (PTF 10nvg) and PV Cep, both of which similarly experience bright phases that recur on long timescales, with large-amplitude photometric variations and emission-dominated spectra. We interpret the behavior of Gaia 19ajj as caused by cycles of enhanced disk accretion accompanied by reduced extinction.
Outflow activity is associated with all stages of early stellar evolution,
from deeply embedded protostellar objects to visible young stars. Herbig-Haro
(HH) objects are the optical manifestations of ...this powerful mass loss.
Analysis of HH flows, and in particular of the subset of highly collimated HH
jets, provides indirect but important insights into the nature of the accretion
and mass-loss processes that govern the formation of stars. The recent
recognition that HH flows may attain parsec-scale dimensions opens up the
possibility of partially reconstructing the mass-ejection history of the newly
born driving sources and, therefore, their mass-accretion history. Furthermore,
HH flows are astrophysical laboratories for the analysis of shock structures,
of hydrodynamics in collimated flows, and of their interaction with the
surrounding environment. HH flows may be an important source of turbulence in
molecular clouds. Recent technological developments have enabled detailed
observations of outflows from young stars at near-infrared, mid-infrared,
submillimeter, millimeter, and centimeter wavelengths, providing a
comprehensive picture of the outflow phenomenon of young stars.
Celotno besedilo
Dostopno za:
CMK, DOBA, FMFMET, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Abstract
Multi-epoch narrowband Hubble Space Telescope images of the bipolar H
ii
region Sh2-106 reveal highly supersonic nebular proper motions that increase with projected distance from the massive ...young stellar object S106 IR, reaching over ∼30 mas yr
−1
(∼150 km s
−1
at D = 1.09 kpc) at a projected separation of ∼1.′4 (0.44 pc) from S106 IR. We propose that S106 IR experienced a ∼10
47
erg explosion ∼3500 yr ago. The explosion may be the result of a major accretion burst or a recent encounter with another star, or a consequence of the interaction of a companion with the bloated photosphere of S106 IR as it grew from ∼10 through ∼15
M
⊙
at a high accretion rate. Near-IR images reveal fingers of H
2
emission pointing away from S106 IR and an asymmetric photon-dominated region surrounding the ionized nebula. Radio continuum and Br
γ
emission reveal a C-shaped bend in the plasma, indicating either the motion of S106 IR toward the east, or the deflection of plasma toward the west by the surrounding cloud. The H
ii
region bends around a ∼1′ diameter dark bay west of S106 IR that may be shielded from direct illumination by a dense molecular clump. Herbig–Haro and Molecular Hydrogen Objects tracing outflows powered by stars in the Sh2-106 protocluster such as the Class 0 source S106 FIR are discussed.
The formation of very wide binary systems, such as the α Centauri system with Proxima (also known as α Centauri C) separated from α Centauri (which itself is a close binary A/B) by 15,000 ...astronomical units (1 AU is the distance from Earth to the Sun), challenges current theories of star formation, because their separation can exceed the typical size of a collapsing cloud core. Various hypotheses have been proposed to overcome this problem, including the suggestion that ultrawide binaries result from the dissolution of a star cluster--when a cluster star gravitationally captures another, distant, cluster star. Recent observations have shown that very wide binaries are frequently members of triple systems and that close binaries often have a distant third companion. Here we report N-body simulations of the dynamical evolution of newborn triple systems still embedded in their nascent cloud cores that match observations of very wide systems. We find that although the triple systems are born very compact--and therefore initially are more protected against disruption by passing stars--they can develop extreme hierarchical architectures on timescales of millions of years as one component is dynamically scattered into a very distant orbit. The energy of ejection comes from shrinking the orbits of the other two stars, often making them look from a distance like a single star. Such loosely bound triple systems will therefore appear to be very wide binaries.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
The HH 24 complex harbors five collimated jets emanating from a small protostellar multiple system. We have carried out a multiwavelength study of the jets, their driving sources, and the ...cloud core hosting the embedded stellar system, based on data from the Hubble Space Telescope, Gemini, Subaru, Apache Point Observatory 3.5 m, Karl G. Jansky Very Large Array, and Atacama Large Millimeter/submillimeter Array (ALMA) telescopes. The data show that the multiple system, SSV 63, contains at least 7 sources, ranging in mass from the hydrogen-burning limit to proto-Herbig Ae stars. The stars are in an unstable nonhierarchical configuration, and one member, a borderline brown dwarf, is moving away from the protostellar system with 25 km s
−1
, after being ejected ∼5800 yr ago as an orphaned protostar. Five of the embedded sources are surrounded by small, possibly truncated, disks resolved at 1.3 mm with ALMA. Proper motions and radial velocities imply jet speeds of 200–300 km s
−1
. The two main HH 24 jets, E and C, form a bipolar jet system that traces the innermost portions of parsec-scale chains of Herbig–Haro and H
2
shocks with a total extent of at least 3 pc. H
2
CO and C
18
O observations show that the core has been churned and continuously fed by an infalling streamer.
13
CO and
12
CO trace compact, low-velocity, cavity walls carved by the jets and an ultracompact molecular outflow from the most embedded object. Chaotic
N
-body dynamics likely will eject several more of these objects. The ejection of stars from their feeding zones sets their masses. Dynamical decay of nonhierarchical systems can thus be a major contributor to establishing the initial mass function.
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