356 research outputs found
High Resolution CO Observations of Massive Star Forming Regions
Context. To further understand the processes involved in the formation of
massive stars, we have undertaken a study of the gas dynamics surrounding three
massive star forming regions. By observing the large scale structures at high
resolution, we are able to determine properties such as driving source, and
spatially resolve the bulk dynamical properties of the gas such as infall and
outflow. Aims. With high resolution observations, we are able to determine
which of the cores in a cluster forming massive stars is responsible for the
large scale structures. Methods. We present CO observations of three massive
star forming regions with known HII regions and show how the CO traces both
infall and outflow. By combining data taken in two SMA configurations with JCMT
observations, we are able to see large scale structures at high resolution.
Results. We find large (0.26-0.40 pc), massive (2-3 M_sun) and energetic (13-17
\times 10^44 erg) outflows emanating from the edges of two HII regions
suggesting they are being powered by the protostar(s) within. We find infall
signatures in two of our sources with mass infall rates of order 10-4 M_sun/yr.
Conclusions. We suggest that star formation is ongoing in these sources despite
the presence of HII regions. We further conclude that the source(s) within a
single HII region are responsible for the observed large scale structures; that
these large structures are not the net effect of multiple outflows from
multiple HII regions and hot cores.Comment: 8 pages,2 figures, accepted for publication in A&
Combining radiative transfer and diffuse interstellar medium physics to model star formation (article)
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.The dataset associated with this journal article can be found in ORE at http://hdl.handle.net/10871/17067We present a method for modelling star-forming clouds that combines two different models of the thermal evolution of the interstellar medium (ISM). In the combined model, where the densities are low enough that at least some part of the spectrum is optically thin, a model of the thermodynamics of the diffuse ISM is more significant in setting the temperatures. Where the densities are high enough to be optically thick across the spectrum, a model of flux-limited diffusion is more appropriate. Previous methods either model the low-density ISM and ignore the thermal behaviour at high densities (e.g. inside collapsing molecular cloud cores), or model the thermal behaviour near protostars but assume a fixed background temperature (e.g. ≈10 K) on large scales. Our new method treats both regimes. It also captures the different thermal evolution of the gas, dust, and radiation separately. We compare our results with those from the literature, and investigate the dependence of the thermal behaviour of the gas on the various model parameters. This new method should allow us to model the ISM across a wide range of densities and, thus, develop a more complete and consistent understanding of the role of thermodynamics in the star formation process.European Research Council - European Community's Seventh Framework Programme (FP7/2007-2013
Analysis of Hydrogen Cyanide Hyperfine Spectral Components towards Star Forming Cores
Although hydrogen cyanide has become quite a common molecular tracing species
for a variety of astrophysical sources, it, however, exhibits dramatic non-LTE
behaviour in its hyperfine line structure. Individual hyperfine components can
be strongly boosted or suppressed. If these so-called hyperfine line anomalies
are present in the HCN rotational spectra towards low or high mass cores, this
will affect the interpretation of various physical properties such as the line
opacity and excitation temperature in the case of low mass objects and infall
velocities in the case of their higher mass counterparts. This is as a
consequence of the direct effects that anomalies have on the underlying line
shape, be it with the line structural width or through the inferred line
strength. This work involves the first observational investigation of these
anomalies in two HCN rotational transitions, J=1!0 and J=3!2, towards both low
mass starless cores and high mass protostellar objects. The degree of anomaly
in these two rotational transitions is considered by computing the ratios of
neighboring hyperfine lines in individual spectra. Results indicate some degree
of anomaly is present in all cores considered in our survey, the most likely
cause being line overlap effects among hyperfine components in higher
rotational transitions.Comment: 8th Serbian Conference on Spectral Line Shapes in Astrophysics,
Divicibare; 8 pages, 5 figure
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
The Mid-infrared Fine-structure Lines of Neon as an Indicator of Star For mation Rate in Galaxies
The fine-structure lines of singly ([Ne II] 12.8 micron) and doubly ([Ne III]
15.6 micron) ionized neon are among the most prominent features in the
mid-infrared spectra of star-forming regions, and have the potential to be a
powerful new indicator of the star formation rate in galaxies. Using a sample
of star-forming galaxies with measurements of the fine-structure lines
available from the literature, we show that the sum of the [Ne II] and [Ne III]
luminosities obeys a tight, linear correlation with the total infrared
luminosity, over 5 orders of magnitude in luminosity. We discuss the formation
of the lines and their relation with the Lyman continuum luminosity. A simple
calibration between star formation rate and the [Ne II]+[Ne III] luminosity is
presented.Comment: To appear in ApJ. 8 page
Flickering of 1.3 cm Sources in Sgr B2: Towards a Solution to the Ultracompact HII Region Lifetime Problem
Accretion flows onto massive stars must transfer mass so quickly that they
are themselves gravitationally unstable, forming dense clumps and filaments.
These density perturbations interact with young massive stars, emitting
ionizing radiation, alternately exposing and confining their HII regions. As a
result, the HII regions are predicted to flicker in flux density over periods
of decades to centuries rather than increasing monotonically in size as
predicted by simple Spitzer solutions. We have recently observed the Sgr B2
region at 1.3 cm with the VLA in its three hybrid configurations (DnC, CnB and
BnA) at a resolution of 0.25''. These observations were made to compare in
detail with matched continuum observations from 1989. At 0.25'' resolution, Sgr
B2 contains 41 UC HII regions, 6 of which are hypercompact. The new
observations of Sgr B2 allow comparison of relative peak flux densites for the
HII regions in Sgr B2 over a 23 year time baseline (1989-2012) in one of the
most source-rich massive star forming regions in the Milky Way. The new 1.3 cm
continuum images indicate that four of the 41 UC HII regions exhibit
significant changes in their peak flux density, with one source (K3) dropping
in peak flux density, and the other 3 sources (F10.303, F1 and F3) increasing
in peak flux density. The results are consistent with statistical predictions
from simulations of high mass star formation, suggesting that they offer a
solution to the lifetime problem for ultracompact HII regions.Comment: 12 pages, 3 figures, Accepted for publication in the Astrophysical
Journal Letter
Multi-density model of the ionised gas in NGC 253 using radio recombination lines
We have imaged the H92alpha (8.3 GHz), H75alpha (15 GHz), and H166alpha (1.4
GHz) Radio Recombination Lines (RRLs) from NGC 253 at resolutions of 4.5 pc
(0.4"), 2.5 pc (0.2") and 53 pc (4.5") respectively. The H92alpha line arises
from individual compact sources, most of which possess radio continuum
counterparts. The line widths range from ~200 km/s for the sources near the
radio nucleus to 70-100 km/s for the extranuclear ones. These lines are emitted
by gas at a density ~10000 /cc. The remainder of the cm-wave RRLs arise in
lower density gas (~500 /cc) with a higher area filling factor and with ten
times highermass. A third component of higher density gas (>10000 /cc) is
required to explain the mm-wave RRLs.Comment: Accepted by A&A; Changed to fit all figures within pag
Massive star-formation toward G28.87+0.07 (IRAS 18411-0338) investigated by means of maser kinematics and radio to infrared, continuum observations
We used the Very Long Baseline Array (VLBA) and the European VLBI Network
(EVN) to perform phase-referenced VLBI observations of the three most powerful
maser transitions associated with the high-mass star-forming region
G28.87+0.07: the 22.2 GHz HO, 6.7 GHz CHOH, and 1.665 GHz OH lines.
We also performed VLA observations of the radio continuum emission at 1.3 and
3.6 cm and Subaru observations of the continuum emission at 24.5 m. Two
centimeter continuum sources are detected and one of them (named "HMC") is
compact and placed at the center of the observed distribution of HO,
CHOH and OH masers. The bipolar distribution of line-of-sight (l.o.s)
velocities and the pattern of the proper motions suggest that the water masers
are driven by a (proto)stellar jet interacting with the dense circumstellar
gas. The same jet could both excite the centimeter continuum source named "HMC"
(interpreted as free-free emission from shocked gas) and power the molecular
outflow observed at larger scales -- although one cannot exclude that the
free-free continuum is rather originating from a hypercompact \ion{H}{2}
region. At 24.5 m, we identify two objects separated along the north-south
direction, whose absolute positions agree with those of the two VLA continuum
sources. We establish that 90% of the luminosity of the region
(\times10^{5} L_\sun$) is coming from the radio source "HMC", which
confirms the existence of an embedded massive young stellar object (MYSO)
exciting the masers and possibly still undergoing heavy accretion from the
surrounding envelope.Comment: Accepted for publication in Ap
Infall of gas as the formation mechanism of stars up to 20 times more massive than the Sun
Theory predicts and observations confirm that low-mass stars (like the Sun)
in their early life grow by accreting gas from the surrounding material. But
for stars ~ 10 times more massive than the Sun (~10 M_sun), the powerful
stellar radiation is expected to inhibit accretion and thus limit the growth of
their mass. Clearly, stars with masses >10 M_sun exist, so there must be a way
for them to form. The problem may be solved by non-spherical accretion, which
allows some of the stellar photons to escape along the symmetry axis where the
density is lower. The recent detection of rotating disks and toroids around
very young massive stars has lent support to the idea that high-mass (> 8
M_sun) stars could form in this way. Here we report observations of an ammonia
line towards a high-mass star forming region. We conclude from the data that
the gas is falling inwards towards a very young star of ~20 M_sun, in line with
theoretical predictions of non-spherical accretion.Comment: 11 pages, 2 figure
- …