51 research outputs found
Pre- and Post-burst Radio Observations of the Class 0 Protostar HOPS 383 in Orion
There is increasing evidence that episodic accretion is a common phenomenon
in Young Stellar Objects (YSOs). Recently, the source HOPS 383 in Orion was
reported to have a mid-infrared -- and bolometric -- luminosity
increase between 2004 and 2008, constituting the first clear example of a class
0 YSO (a protostar) with a large accretion burst. The usual assumption that in
YSOs accretion and ejection follow each other in time needs to be tested. Radio
jets at centimeter wavelengths are often the only way of tracing the jets from
embedded protostars. We searched the Very Large Array archive for the available
observations of the radio counterpart of HOPS 383. The data show that the radio
flux of HOPS 383 varies only mildly from January 1998 to December 2014, staying
at the level of to 300 Jy in the X band ( GHz), with a
typical uncertainty of 10 to 20 Jy in each measurement. We interpret the
absence of a radio burst as suggesting that accretion and ejection enhancements
do not follow each other in time, at least not within timescales shorter than a
few years. Time monitoring of more objects and specific predictions from
simulations are needed to clarify the details of the connection betwen
accretion and jets/winds in YSOs.Comment: ApJ Letters, in pres
Resolving the Structure and Kinematics of the Youngest HII Regions and Radio Jets from Young Stellar Objects
In this contribution we explore the new science that a Next Generation Very
Large Array (ngVLA) would be able to perform on the topics of the youngest HII
regions and (proto)stellar jets. Free-free continuum and radio recombination
line (RRL) emission are often the only way of peering into the dense envelopes
surrounding (proto)stars of all masses, and trace their initial feedback in the
form of `radio jets', `hypercompact HII regions', or photoevaporating,
partially-ionized flows. Properly disentangling free-free from dust emission is
also mandatory in studies of protoplanetary and accretion disks. Current VLA
research has reached an impasse in which a population of faint ionized radio
sources, probably corresponding to the above mentioned objects, is detected,
but their nature is mostly unknown. The ngVLA would allow us to resolve the
density structure and kinematics of such sources, revolutionizing our knowledge
of star formation across the entire stellar-mass spectrum.Comment: An ngVLA Science Book chapter. v2 includes a few modifications in
reference
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
Extreme fragmentation and complex kinematics at the center of the L1287 cloud
The filamentary infrared dark cloud L1287 is actively forming a dense cluster
of low-mass YSOs at its inner 0.1 pc region. To help understand the
origin of this low-mass YSO cluster, the present work aims at resolving the gas
structures and kinematics. We have performed 1 angular resolution
(930 AU) SMA observations at 1.3 mm. From a 2 resolution
1.3 mm continuum image we identified six dense cores, namely SMA1-6 with masses
in the range of M. From a 1 resolution 1.3 mm
continuum image, we find a high fragmentation level, with 14 compact millimeter
sources within 0.1 pc (two of them associated with the known accretion outburst
YSOs RNO 1C and RNO 1B). The dense gas tracer DCN (3--2) traces well the dust
continuum emission and shows a clear velocity gradient along the NW-SE
direction centered at SMA3. There is another velocity gradient with opposite
direction around the most luminous YSO IRAS 00338+6312. The fragmentation
within 0.1 pc in L1287 is very high compared to other regions at the same
spatial scales. The incoherent motions of dense gas flows are sometimes
interpreted by being influenced by (proto)stellar feedback (e.g., outflows),
which is not yet ruled out in this particular target source. The directions of
the velocity gradients traced by DCN are approximately perpendicular to those
of the dominant CO outflow(s). Therefore, we alternatively hypothesize that the
velocity gradients revealed by DCN trace the convergence from the 0.1
pc scales infalling motion towards the rotational motions around the more
compact ( pc) sources. This global molecular gas converging flow may
feed the formation of the dense low-mass YSO cluster. IRAS 00338+6312 is the
most likely powering source of the dominant CO outflow. A compact blue-shifted
outflow from RNO 1C is also identified.Comment: 14 pages, 11 figures. Accepted for publication in Astronomy &
Astrophysic
Time Variability in Simulated Ultracompact and Hypercompact HII Regions
Ultracompact and hypercompact HII regions appear when a star with a mass
larger than about 15 solar masses starts to ionize its own environment. Recent
observations of time variability in these objects are one of the pieces of
evidence that suggest that at least some of them harbor stars that are still
accreting from an infalling neutral accretion flow that becomes ionized in its
innermost part. We present an analysis of the properties of the HII regions
formed in the 3D radiation-hydrodynamic simulations presented by Peters et al.
as a function of time. Flickering of the HII regions is a natural outcome of
this model. The radio-continuum fluxes of the simulated HII regions, as well as
their flux and size variations are in agreement with the available
observations. From the simulations, we estimate that a small but non-negligible
fraction (~ 10 %) of observed HII regions should have detectable flux
variations (larger than 10 %) on timescales of ~ 10 years, with positive
variations being more likely to happen than negative variations. A novel result
of these simulations is that negative flux changes do happen, in contrast to
the simple expectation of ever growing HII regions. We also explore the
temporal correlations between properties that are directly observed (flux and
size) and other quantities like density and ionization rates.Comment: Monthly Notices of the Royal Astronomical Society, in press. The
movie of free-free optical depth can be found at
http://www.ita.uni-heidelberg.de/~tpeters/tau.av
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