558 research outputs found
Cloudlet Capture Model for the Accretion Streamer onto the disk of DG Tau
DG Tau is a nearby T Tauri star associated with a collimated jet, a
circumstellar disk and a streamer a few hundred au long. The streamer connects
to the disk at 50 au from DG Tau. At this location SO emission is
observed, likely due to the release of sulphur from dust grains caused by the
shock of the impact of the accretion streamer onto the disk. We investigate the
possibility that the DG Tau streamer was produced via cloudlet capture on the
basis of hydrodynamic simulations, considering a cloudlet initiating infall at
600 au from DG Tau with low angular momentum so that the centrifugal force is
smaller than the gravitational force, even at 50 au. The elongation of the
cloudlet into a streamer is caused by the tidal force when its initial velocity
is much less than the free-fall velocity. The elongated cloudlet reaches the
disk and forms a high density gas clump. Our hydrodynamic model reproduces the
morphology and line-of-sight velocity of CS () emission from the Northern
streamer observed with ALMA. We discuss the conditions for forming a streamer
based on the simulations. We also show that the streamer should perturb the
disk after impact for several thousands of years.Comment: 12 page, 11 figure
High Resolution 8 mm and 1 cm Polarization of IRAS 4A from the VLA Nascent Disk and Multiplicity (VANDAM) Survey
Magnetic fields can regulate disk formation, accretion and jet launching.
Until recently, it has been difficult to obtain high resolution observations of
the magnetic fields of the youngest protostars in the critical region near the
protostar. The VANDAM survey is observing all known protostars in the Perseus
Molecular Cloud. Here we present the polarization data of IRAS 4A. We find that
with ~ 0.2'' (50 AU) resolution at {\lambda} = 8.1 and 10.3 mm, the inferred
magnetic field is consistent with a circular morphology, in marked contrast
with the hourglass morphology seen on larger scales. This morphology is
consistent with frozen-in field lines that were dragged in by rotating material
entering the infall region. The field morphology is reminiscent of rotating
circumstellar material near the protostar. This is the first polarization
detection of a protostar at these wavelengths. We conclude from our
observations that the dust emission is optically thin with {\beta} ~ 1.3,
suggesting that mm/cm-sized grains have grown and survived in the short
lifetime of the protostar.Comment: Accepted to ApJL. 13 pages, 4 figure
The VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey of Perseus Protostars. VI. Characterizing the Formation Mechanism for Close Multiple Systems
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations
of multiple protostar systems in the Perseus molecular cloud previously
detected by the Karl G. Jansky Very Large Array (VLA). We observed 17 close
(600~AU separation) multiple systems at 1.3~mm in continuum and five
molecular lines (i.e., \twco, \cateo, \thco, HCO, SO) to characterize the
circum-multiple environments in which these systems are forming. We detect at
least one component in the continuum for the 17 multiple systems. In three
systems, one companion is not detected, and for two systems the companions are
unresolved at our observed resolution. We also detect circum-multiple dust
emission toward 8 out of 9 Class 0 multiples. Circum-multiple dust emission is
not detected toward any of the 8 Class I multiples. Twelve systems are detected
in the dense gas tracers toward their disks/inner envelopes. For these 12
systems, we use the dense gas observations to characterize their formation
mechanism. The velocity gradients in the circum-multiple gas are clearly
orthogonal to the outflow directions in 8 out of the 12 systems, consistent
with disk fragmentation. Moreover, only two systems with separations 200~AU
are \textit{inconsistent} with disk fragmentation, in addition to the two
widest systems (500~AU). Our results suggest that disk fragmentation via
gravitational instability is an important formation mechanism for close
multiple systems, but further statistics are needed to better determine the
relative fraction formed via this method.Comment: 48 Pages, 26 Figures, 7 Tables, Accepted by Ap
A protostellar system fed by a streamer of 10,500 au length
Binary formation is an important aspect of star formation. One possible route
for close-in binary formation is disk fragmentation. Recent
observations show small scale asymmetries (<300 au) around young
protostars, although not always resolving the circumbinary disk, are
linked to disk phenomena. In later stages, resolved circumbinary disk
observations (<200 au) show similar asymmetries, suggesting the origin
of the asymmetries arises from binary-disk interactions. We
observed one of the youngest systems to study the connection between disk and
dense core. We find for the first time a bright and clear streamer in
chemically fresh material (Carbon-chain species) that originates from outside
the dense core (>10,500 au). This material connects the outer dense core with
the region where asymmetries arise near disk scales. This new structure type,
10x larger than those seen near disk scales, suggests a different
interpretation of previous observations: large-scale accretion flows funnel
material down to disk scales. These results reveal the under-appreciated
importance of the local environment on the formation and evolution of disks in
early systems and a possible initial condition for the formation of
annular features in young disks.Comment: Published in Nature Astronomy on July 27th 2020. This is the authors'
version before final edits, including methods section. Link to the NatAstro
publication: https://www.nature.com/articles/s41550-020-1150-
Finding substructures in protostellar disks in Ophiuchus
High-resolution, millimeter observations of disks at the protoplanetary stage
reveal substructures such as gaps, rings, arcs, spirals, and cavities. While
many protoplanetary disks host such substructures, only a few at the younger
protostellar stage have shown similar features. We present a detailed search
for early disk substructures in ALMA 1.3 and 0.87~mm observations of ten
protostellar disks in the Ophiuchus star-forming region. Of this sample, four
disks have identified substructure, two appear to be smooth disks, and four are
considered ambiguous. The structured disks have wide Gaussian-like rings
() with low contrasts () above a
smooth disk profile, in comparison to protoplanetary disks where rings tend to
be narrow and have a wide variety of contrasts
( and ranges from ). The four
protostellar disks with the identified substructures are among the brightest
sources in the Ophiuchus sample, in agreement with trends observed for
protoplanetary disks. These observations indicate that substructures in
protostellar disks may be common in brighter disks. The presence of
substructures at the earliest stages suggests an early start for dust grain
growth and, subsequently, planet formation. The evolution of these protostellar
substructures is hypothesized in two potential pathways: (1) the rings are the
sites of early planet formation, and the later observed protoplanetary disk
ring-gap pairs are secondary features, or (2) the rings evolve over the disk
lifetime to become those observed at the protoplanetary disk stage.Comment: Accepted by ApJ, 22 pages, 10 figure
Dendrogram Analysis of Large-Area CARMA Images in Perseus: the Dense Gas in NGC 1333, Barnard 1, and L1451
We present spectral line maps of the dense gas across 400 square arcminutes of the Perseus Molecular Cloud, focused on NGC 1333, Barnard 1, and L1451. We constructed these maps as part of the CARMA Large Area Star-formation Survey (CLASSy), which is a CARMA key project that connects star forming cores to their natal cloud environment. This is achieved by leveraging CARMA's high angular resolution, imaging capability, and high efficiency at mosaicing large areas of the sky. CLASSy maps capture the structure and kinematics of N2H+, HCN, and HCO+ J=1-0 emission from thousand AU to parsec scales in three evolutionarily distinct regions of Perseus (in addition to two regions in Serpens). We show results from a non-binary dendrogram analysis of the Perseus N2H+ emission, which answers questions about the turbulent properties of the dense gas across evolutionary stages and across the range of size scales probed by CLASSy. There is a flat relation between mean internal turbulence and structure size for the dense gas in NGC 1333 and Barnard 1, but the magnitude of internal turbulence increases with nearby protostellar activity; the dense gas in the B1 main core and NGC 1333, which have active young stars, are characterized by mostly transonic to supersonic turbulence, while the filaments and clumps southwest of the B1 main core, which have no active young stars, have mostly subsonic turbulence. We have recently completed the observations of L1451, and results for that region will be revealed at the meeting. Released CLASSy data products can be found on our project website.Fil: Storm, Shaye. University of Maryland; Estados UnidosFil: Mundy, Lee G.. University of Maryland; Estados UnidosFil: Teuben, Peter J.. University of Maryland; Estados UnidosFil: Lee, Katherine. University of Maryland; Estados UnidosFil: Looney, Leslie. University of Illinois at Urbana; Estados UnidosFil: Fernandez Lopez, Manuel. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; ArgentinaFil: Rosolowsky, Erik. University of Alberta; CanadáFil: Arce, Hector G.. University of Yale; Estados UnidosFil: Shirley, Yancy L.. University of Arizona; Estados UnidosFil: Segura Cox, Dominique. University of Illinois; Estados UnidosFil: Isella, Andrea. Caltech; Estados UnidosFil: CLASSy Collaboration. No especifíca;223rd American Astronomical Society MeetingWashingtonEstados UnidosAmerican Astronomical Societ
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