53 research outputs found
Investigating fragmentation of gas structures in OB cluster-forming molecular clump G33.92+0.11 with 1000 AU resolution observations of ALMA
We report new, 1000 AU spatial resolution observations of 225 GHz dust
continuum emission towards the OB cluster-forming molecular clump G33.92+0.11.
On parsec scales, this molecular clump presents a morphology with several
arm-like dense gas structures surrounding the two central massive (100
) cores. From the new, higher resolution observations, we identified
28 localized, spatially compact dust continuum emission sources, which may be
candidates of young stellar objects. Only one of them is not embedded within
known arm-like (or elongated) dense gas structures. The spatial separations of
these compact sources can be very well explained by Jeans lengths. We found
that G33.92+0.11 may be consistently described by a marginally centrifugally
supported, Toomre unstable accretion flow which is approximately in a face-on
projection. The arm-like overdensities are natural consequence of the Toomre
instability, which can fragment to form young stellar objects in shorter time
scales than the timescale of the global clump contraction. On our resolved
spatial scales, there is not yet evidence that the fragmentation is halted by
turbulence, magnetic field, or stellar feedback.Comment: 24 pages, 18 figures. Accepted to publish on December 04, 2018;
updated to arXiv on December 05, 201
Digging into the Interior of Hot Cores with ALMA (DIHCA). III: The Chemical Link between NHCHO, HNCO, and HCO
We have analyzed the NHCHO, HNCO, HCO, and CHCN
(CHCN) molecular lines at an angular resolution of
obtained by the Atacama Large Millimeter/submillimeter Array (ALMA) Band 6
toward 30 high-mass star-forming regions. The NHCHO emission has been
detected in 23 regions, while the other species have been detected toward 29
regions. A total of 44 hot molecular cores (HMCs) have been identified using
the moment 0 maps of the CHCN line. The fractional abundances of the four
species have been derived at each HMC. In order to investigate pure chemical
relationships, we have conducted a partial correlation test to exclude the
effect of temperature. Strong positive correlations between NHCHO and
HNCO () and between NHCHO and HCO (0.84) have been
found. These strong correlations indicate their direct chemical links;
dual-cyclic hydrogen addition and abstraction reactions between HNCO and
NHCHO and gas-phase formation of NHCHO from HCO. Chemical
models including these reactions can reproduce the observed abundances in our
target sources.Comment: Accepted for The Astrophysical Journal. 27 pages, 10 tables, and 13
figure
Digging into the Interior of Hot Cores with ALMA: Spiral Accretion into the High-mass Protostellar Core G336.01-0.82
We observed the high-mass star-forming core G336.01-0.82 at 1.3 mm and 0.05''
(~150 au) angular resolution with the Atacama Large Millimeter/submillimeter
Array (ALMA) as part of the Digging into the Interior of Hot Cores with ALMA
(DIHCA) survey. These high-resolution observations reveal two spiral streamers
feeding a circumstellar disk at opposite sides in great detail. Molecular line
emission from CHOH shows velocity gradients along the streamers consistent
with infall. Similarly, a flattened envelope model with rotation and infall
implies a mass larger than 10 M for the central source and a
centrifugal barrier of 300 au. The location of the centrifugal barrier is
consistent with local peaks in the continuum emission. We argue that gas
brought by the spiral streamers is accumulating at the centrifugal barrier,
which can result in future accretion burst events. A total high infall rate of
~ M yr is derived by matching models to the
observed velocity gradient along the streamers. Their contribution account for
20-50% the global infall rate of the core, indicating streamers play an
important role in the formation of high-mass stars.Comment: 11 pages, 5 figures, 2 appendices. Accepted for publication in ApJL.
Interactive figure available at https://folguinch.github.io/projects/dihc
Observational signatures of outbursting protostars - I: From hydrodynamic simulations to observations
Accretion onto protostars may occur in sharp bursts. Accretion bursts during the embedded phase of young protostars are probably most intense, but can only be inferred indirectly through long-wavelength observations. We perform radiative transfer calculations for young stellar objects (YSOs) formed in hydrodynamic simulations to predict the long wavelength, sub-mm and mm, flux responses to episodic accretion events, taking into account heating from the young protostar and from the interstellar radiation field. We find that the flux increase due to episodic accretion events is more prominent at sub-mm wavelengths than at mm wavelengths; e.g. a factor of ∼570 increase in the luminosity of the young protostar leads to a flux increase of a factor of 47 at 250 μ
μ
m but only a factor of 10 at 1.3 mm. Heating from the interstellar radiation field may reduce further the flux increase observed at longer wavelengths. We find that during FU Ori-type outbursts the bolometric temperature and luminosity may incorrectly classify a source as a more evolved YSO due to a larger fraction of the radiation of the object being emitted at shorter wavelengths
Observational signatures of outbursting protostars - II: Exploring a wide range of eruptive protostars
Young stars exhibit variability due to changes in the gas accretion rate onto them, an effect that should be quite significant in the early stages of their formation. As protostars are embedded within their natal cloud, this variability may only be inferred through long wavelength observations. We perform radiative transfer simulations of young stellar objects (YSOs) formed in hydrodynamical simulations, varying the structure and luminosity properties in order to estimate the long-wavelength, sub-mm and mm, variations of their flux. We find that the flux increase due to an outburst event depends on the protostellar structure and is more prominent at sub-mm wavelengths than at mm wavelengths; e.g. a factor of 40 increase in the luminosity of the young protostar leads to a flux increase of a factor of 10 at 250 μm but only a factor of 2.5 at 1.3 mm. We find that the interstellar radiation field dilutes the flux increase but that this effect may be avoided if resolution permits the monitoring of the inner regions of a YSO, where the heating is primarily due to protostellar radiation. We also confirm that the bolometric temperature and luminosity of outbursting protostars may result in an incorrect classification of their evolutionary stage
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