253 research outputs found
The standard model of star formation applied to massive stars: accretion disks and envelopes in molecular lines
We address the question of whether the formation of high-mass stars is
similar to or differs from that of solar-mass stars through new molecular line
observations and modeling of the accretion flow around the massive protostar
IRAS20126+4104. We combine new observations of NH3(1,1) and (2,2) made at the
Very Large Array, new observations of CHCN(13-12) made at the Submillimeter
Array, previous VLA observations of NH(3,3), NH(4,4), and previous Plateau de
Bure observations of C34S(2-1), C34S(5-4), and CHCN(12-11) to obtain a data set
of molecular lines covering 15 to 419 K in excitation energy. We compare these
observations against simulated molecular line spectra predicted from a model
for high-mass star formation based on a scaled-up version of the standard
disk-envelope paradigm developed for accretion flows around low-mass stars. We
find that in accord with the standard paradigm, the observations require both a
warm, dense, rapidly-rotating disk and a cold, diffuse infalling envelope. This
study suggests that accretion processes around 10 M stars are similar to those
of solar mass stars.Comment: Accepted MNRA
Is protostellar heating sufficient to halt fragmentation? A case study of the massive protocluster G8.68-0.37
If star formation proceeds by thermal fragmentation and the subsequent
gravitational collapse of the individual fragments, how is it possible to form
fragments massive enough for O and B stars in a typical star-forming molecular
cloud where the Jeans mass is about 1Msun at the typical densities (10^4 cm^-3)
and temperatures (10K)? We test the hypothesis that a first generation of
low-mass stars may heat the gas enough that subsequent thermal fragmentation
results in fragments >=10Msun, sufficient to form B stars. We combine ATCA and
SMA observations of the massive star-forming region G8.68-0.37 with radiative
transfer modeling to derive the present-day conditions in the region and use
this to infer the conditions in the past, at the time of core formation.
Assuming the current mass/separation of the observed cores equals the
fragmentation Jeans mass/length and the region's average density has not
changed, requires the gas temperature to have been 100K at the time of
fragmentation. The postulated first-generation of low-mass stars would still be
around today, but the number required to heat the cloud exceeds the limits
imposed by the observations. Several lines of evidence suggest the observed
cores in the region should eventually form O stars yet none have sufficient raw
material. Even if feedback may have suppressed fragmentation, it was not
sufficient to halt it to this extent. To develop into O stars, the cores must
obtain additional mass from outside their observationally defined boundaries.
The observations suggest they are currently fed via infall from the very
massive reservoir (~1500Msun) of gas in the larger pc scale cloud around the
star-forming cores. This suggests that massive stars do not form in the
collapse of individual massive fragments, but rather in smaller fragments that
themselves continue to gain mass by accretion from larger scales.Comment: 23 pages, 14 figures. Accepted for publication in Ap
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&
High Velocity Molecular Outflows In Massive Cluster Forming Region G10.6-0.4
We report the arcsecond resolution SMA observations of the CO (2-1)
transition in the massive cluster forming region G10.6-0.4. In these
observations, the high velocity CO emission is resolved into individual
outflow systems, which have a typical size scale of a few arcseconds. These
molecular outflows are energetic, and are interacting with the ambient
molecular gas. By inspecting the shock signatures traced by CHOH, SiO,
and HCN emissions, we suggest that abundant star formation activities are
distributed over the entire 0.5 pc scale dense molecular envelope. The star
formation efficiency over one global free-fall timescale (of the 0.5 pc
molecular envelope, years) is about a few percent. The total
energy feedback of these high velocity outflows is higher than 10 erg,
which is comparable to the total kinetic energy in the rotational motion of the
dense molecular envelope. From order-of-magnitude estimations, we suggest that
the energy injected from the protostellar outflows is capable of balancing the
turbulent energy dissipation. No high velocity bipolar molecular outflow
associated with the central OB cluster is directly detected, which can be due
to the photo-ionization.Comment: 42 pages, 14 figures, accepted by Ap
Spitzer-IRAC GLIMPSE of high mass protostellar objects. I Infrared point sources and nebulae
The GLIMPSE archive was used to obtain 3.6--8.0micron, point source
photometry and images for 381 massive protostellar candidates lying in the
Galactic mid-plane. The colours, magnitudes and spectral indicies of sources in
each of the 381 target fields were analysed and compared with the predictions
of 2D radiative transfer model simulations. Although no discernable embedded
clusters were found in any targets, multiple sources or associations of
redenned young stellar objects were found in many sources indicating
multiplicity at birth. The spectral index () of these point sources in
3.6--8.0mum bands display large values of =2--5. A color-magnitude
analog plot was used to identify 79 infrared counterparts to the HMPOs. Compact
nebulae are found in 75% of the detected sources with morphologies that can be
well described by core-halo, cometary, shell-like and bipolar geometries
similar to those observed in ultra-compact HII regions. The IRAC band SEDs of
the IR counterparts of HMPOs are best described to represent YSOs with a mass
range of 8--20\msun in their Class I stages when compared with 2D radiative
transfer models. They also suggest that the high values represent
reprocessed star/star+disk emission that is arising in the dense envelopes.
Thus we are witnessing the luminous envelopes around the protostars rather than
their photospheres or disks. We argue that the compact infrared nebulae likely
reflect the underlying physical structure of the dense cores and are found to
imitate the morphologies of known UCHII regions. Our results favour models of
continuuing accretion involving both molecular and ionised accretion components
to build the most massive stars rather than purely molecular rapid accretion
flows.Comment: 13 pages, 7 figures, accepted by A&
Hot high-mass accretion disk candidates
To better understand the physical properties of accretion disks in high-mass
star formation, we present a study of a 12 high-mass accretion disk candidates
observed at high spatial resolution with the Australia Telescope Compact Array
(ATCA) in the NH3 (4,4) and (5,5) lines. Almost all sources were detected in
NH3, directly associated with CH3OH Class II maser emission. From the remaining
eleven sources, six show clear signatures of rotation and/or infall motions.
These signatures vary from velocity gradients perpendicular to the outflows, to
infall signatures in absorption against ultracompact HII regions, to more
spherical infall signatures in emission. Although our spatial resolution is
~1000AU, we do not find clear Keplerian signatures in any of the sources.
Furthermore, we also do not find flattened structures. In contrast to this, in
several of the sources with rotational signatures, the spatial structure is
approximately spherical with sizes exceeding 10^4 AU, showing considerable
clumpy sub-structure at even smaller scales. This implies that on average
typical Keplerian accretion disks -- if they exist as expected -- should be
confined to regions usually smaller than 1000AU. It is likely that these disks
are fed by the larger-scale rotating envelope structure we observe here.
Furthermore, we do detect 1.25cm continuum emission in most fields of view.Comment: 21 pages, 32 figures, accepted for ApJS. A high-resolution version
can be found at http://www.mpia.de/homes/beuther/papers.htm
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 high-mass disk candidates NGC7538IRS1 and NGC7538S
Context: The nature of embedded accretion disks around forming high-mass
stars is one of the missing puzzle pieces for a general understanding of the
formation of the most massive and luminous stars. Methods: Using the Plateau de
Bure Interferometer at 1.36mm wavelengths in its most extended configuration we
probe the dust and gas emission at ~0.3",corresponding to linear resolution
elements of ~800AU. Results: NGC7538IRS1 remains a single compact and massive
gas core with extraordinarily high column densities, corresponding to visual
extinctions on the order of 10^5mag, and average densities within the central
2000AU of ~2.1x10^9cm^-3 that have not been measured before. We identify a
velocity gradient across in northeast-southwest direction that is consistent
with the mid-infrared emission, but we do not find a gradient that corresponds
to the proposed CH3OH maser disk. The spectral line data toward NGC7538IRS1
reveal strong blue- and red-shifted absorption toward the mm continuum peak
position. The red-shifted absorption allows us to estimate high infall rates on
the order of 10^-2 Msun/yr. Although we cannot prove that the gas will be
accreted in the end, the data are consistent with ongoing star formation
activity in a scaled-up low-mass star formation scenario. Compared to that,
NGC7538S fragments in a hierarchical fashion into several sub-sources. While
the kinematics of the main mm peak are dominated by the accompanying jet, we
find rotational signatures from a secondary peak. Furthermore, strong spectral
line differences exist between the sub-sources which is indicative of different
evolutionary stages within the same large-scale gas clump.Comment: 15 pages, 12 figures, accepted for A&
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