37 research outputs found
An IR Study of the Velocity Structure of the Cometary Compact HII region G29.96-0.02
We have mapped the velocity structure of the cometary compact HII region
G29.96-0.02 using long-slit echelle spectra of the HI Br gamma line. This
technique detects line emission over a much wider area at the necessary spatial
resolution compared to radio recombination line observations. Significant
structure in both the velocity centroids and the line widths is seen over the
entire nebula. Large line widths are seen ahead of the bow and in the tail
which may be due to turbulent motions in shocked and interface regions
respectively. We construct analytic models of the density and velocity
structure in order to attempt to distinguish between the bow shock and
champagne flow models which have been put forward to explain the cometary
morphology of many compact HII regions. The bow shock model is unable to
explain the large velocity gradient that we see right across the tail of the
cometary region which can only be explained by the streaming motions towards
low density regions in the champagne model. However, our approximation to the
champagne model is also not able to fit all of the features of the data. More
realistic versions of this model which include the effects of stellar winds and
density gradients may be able to provide a better match to these data.Comment: 19 pages Latex source, 9 postscript figures and macros. gzipped tar
set. To appear in Astrophysical Journal, June 20. Also available by anonymous
ftp from ftp://aaoepp.aao.gov.au/local/sll/g29.uu (uuencoded gzipped tar
file
Disks around massive young stellar objects: are they common?
We present K-band polarimetric images of several massive young stellar
objects at resolutions 0.1-0.5 arcsec. The polarization vectors around
these sources are nearly centro-symmetric, indicating they are dominating the
illumination of each field. Three out of the four sources show elongated
low-polarization structures passing through the centers, suggesting the
presence of polarization disks. These structures and their surrounding
reflection nebulae make up bipolar outflow/disk systems, supporting the
collapse/accretion scenario as their low-mass siblings. In particular, S140
IRS1 show well defined outflow cavity walls and a polarization disk which
matches the direction of previously observed equatorial disk wind, thus
confirming the polarization disk is actually the circumstellar disk. To date, a
dozen massive protostellar objects show evidence for the existence of disks;
our work add additional samples around MYSOs equivalent to early B-type stars.Comment: 9 pages, including 2 figures, 1 table, to appear on ApJ
The circumstellar disk, envelope, and bi-polar outflow of the Massive Young Stellar Object W33A
The Young Stellar Object (YSO) W33A is one of the best known examples of a
massive star still in the process of forming. Here we present Gemini North
ALTAIR/NIFS laser-guide star adaptive-optics assisted K-band integral-field
spectroscopy of W33A and its inner reflection nebula. In our data we make the
first detections of a rotationally-flattened outer envelope and fast bi-polar
jet of a massive YSO at near-infrared wavelengths. The predominant spectral
features observed are Br-gamma, H_2, and a combination of emission and
absorption from CO gas. We perform a 3-D spectro-astrometric analysis of the
line emission, the first study of its kind. We find that the object's Br-gamma
emission reveals evidence for a fast bi-polar jet on sub-milliarcsecond scales,
which is aligned with the larger-scale outflow. The hybrid CO features can be
explained as a combination of hot CO emission arising in a disk close to the
central star, while cold CO absorption originates in the cooler outer envelope.
Kinematic analysis of these features reveals that both structures are rotating,
and consistent with being aligned perpendicularly to both the ionised jet and
the large-scale outflow. Assuming Keplerian rotation, we find that the
circumstellar disk orbits a central mass of >10Msun, while the outer envelope
encloses a mass of ~15Msun. Our results suggest a scenario of a central star
accreting material from a circumstellar disk at the centre of a cool extended
rotating torus, while driving a fast bi-polar wind. These results therefore
provide strong supporting evidence for the hypothesis that the formation
mechanism for high-mass stars is qualitatively similar to that of low-mass
stars.Comment: 13 pages, 18 figs. Accepted for publication in MNRA
Observing protoplanetary discs with the Square Kilometre Array -- I. Characterising pebble substructure caused by forming planets
High angular resolution observations of discs at mm wavelengths (on scales of
a few au) are now commonplace, but there is a current lack of a comparable
angular resolution for observations at cm wavelengths. This presents a
significant barrier to improving our understanding of planet formation, in
particular how dust grains grow from mm to cm sizes. In this paper, we examine
the ability of the Square Kilometre Array (SKA) to observe dust substructure in
a young, planet-forming disc at cm wavelengths. We use dusty hydrodynamics and
continuum radiative transfer to predict the distribution and emission of 1 cm
dust grains (or pebbles) within the disc, and simulate continuum observations
with the current SKA1-MID design baseline at frequencies of 12.5 GHz (Band 5b,
~2.4 cm) on 5-10 au scales. The SKA will provide high-fidelity observations of
the cm dust emission substructure in discs for integration times totalling
100's of hours. Radial structure can be obtained at a sufficient resolution and
S/N from shorter (10's of hours) integration times by azimuthal averaging in
the image plane. By modelling the intensity distribution directly in the
visibility plane, it is possible to recover a similar level of (axisymmetric)
structural detail from observations with integration times 1-2 orders of
magnitude lower than required for high-fidelity imaging. Our results
demonstrate that SKA1-MID will provide crucial constraints on the distribution
and morphology of the raw material for building planets, the pebbles in
protoplanetary discs.Comment: 12 pages, 8 figures, accepted for publication in MNRA
An equatorial wind from the massive young stellar object S140 IRS 1
The discovery of the second equatorial ionized stellar wind from a massive
young stellar object is reported. High resolution radio continuum maps of S140
IRS 1 reveal a highly elongated source that is perpendicular to the larger
scale bipolar molecular outflow. This picture is confirmed by location of a
small scale monopolar near-IR reflection nebula at the base of the blueshifted
lobe. A second epoch of observations over a five year baseline show little
ordered outward proper motion of clumps as would have been expected for a jet.
A third epoch, taken only 50 days after the second, did show significant
changes in the radio morphology. These radio properties can all be understood
in the context of an equatorial wind driven by radiation pressure from the
central star and inner disc acting on the gas in the surface layers of the disc
as proposed by Drew et al. (1998). This equatorial wind system is briefly
compared with the one in S106IR, and contrasted with other massive young
stellar objects that drive ionized jets.Comment: 19 pages, 5 figures, accepted by ApJ, minor changes in light of
referees repor
Tomography of Galactic star-forming regions and spiral arms with the Square Kilometer Array
© 2014 Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence https://creativecommons.org/licenses/by-nc-sa/3.0/us/. Published by Proceedings of Science http://pos.sissa.it/Very Long Baseline Interferometry (VLBI) at radio wavelengths can provide astrometry accurate to 10 micro-arcseconds or better (i.e. better than the target GAIA accuracy) without being limited by dust obscuration. This means that unlike GAIA, VLBI can be applied to star-forming regions independently of their internal and line-of-sight extinction. Low-mass young stellar objects (particularly T Tauri stars) are often non-thermal compact radio emitters, ideal for astrometric VLBI radio continuum experiments. Existing observations for nearby regions (e.g. Taurus, Ophiuchus, or Orion) demonstrate that VLBI astrometry of such active T Tauri stars enables the reconstruction of both the regions' 3D structure (through parallax measurements) and their internal kinematics (through proper motions, combined with radial velocities). The extraordinary sensitivity of the SKA telescope will enable similar "tomographic mappings" to be extended to regions located several kpc from Earth, in particular to nearby spiral arm segments. This will have important implications for Galactic science, galactic dynamics and spiral structure theories
The RMS Survey: Critical Tests of Accretion Models for the Formation of Massive Stars
There is currently no accepted theoretical framework for the formation of the
most massive stars, and the manner in which protostars continue to accrete and
grow in mass beyond \sim10Msun is still a controversial topic. In this study we
use several prescriptions of stellar accretion and a description of the
Galactic gas distribution to simulate the luminosities and spatial distribution
of massive protostellar population of the Galaxy. We then compare the
observables of each simulation to the results of the Red MSX Source (RMS)
survey, a recently compiled database of massive young stellar objects. We find
that the observations are best matched by accretion rates which increase as the
protostar grows in mass, such as those predicted by the turbulent core and
competitive accretion (i.e. Bondi-Hoyle) models. These 'accelerating accretion'
models provide very good qualitative and quantitative fits to the data, though
we are unable to distinguish between these two models on our simulations alone.
We rule out models with accretion rates which are constant with time, and those
which are initially very high and which fall away with time, as these produce
results which are quantitatively and/or qualitatively incompatible with the
observations. To simultaneously match the low- and high-luminosity YSO
distribution we require the inclusion of a 'swollen-star' pre-main-sequence
phase, the length of which is well-described by the Kelvin-Helmholz timescale.
Our results suggest that the lifetime of the YSO phase is \sim 10^5yrs, whereas
the compact Hii-region phase lasts between \sim 2 - 4 \times 10^5yrs depending
on the final mass of the star. Finally, the absolute numbers of YSOs are best
matched by a globally averaged star-formation rate for the Galaxy of
1.5-2Msun/yr.Comment: 22 pages, 24 figures. Accepted for publication in MNRA
VISTA Variables in the <i>VĂa LĂĄctea</i> (VVV): Halfway Status and Results
The VISTA Variables in the VĂa LĂĄctea (VVV) survey is one of six near-infrared ESO public surveys, and is now in its fourth year of observing. Although far from being complete, the VVV survey has already delivered many results, some directly connected to the intended science goals (detection of variable stars, microlensing events, new star clusters), others concerning more exotic objects, e.g., novae. Now, at the end of the fourth observing period, and comprising roughly 50% of the proposed observations, the status of the survey, as well some of results based on the VVV data, are presented.Facultad de Ciencias AstronĂłmicas y GeofĂsica