1,080 research outputs found
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
Chemical Diversity in High-Mass Star Formation
Massive star formation exhibits an extremely rich chemistry. However, not
much evolutionary details are known yet, especially at high spatial resolution.
Therefore, we synthesize previously published Submillimeter Array
high-spatial-resolution spectral line observations toward four regions of
high-mass star formation that are in various evolutionary stages with a range
of luminosities. Estimating column densities and comparing the spatially
resolved molecular emission allows us to characterize the chemical evolution in
more detail. Furthermore, we model the chemical evolution of massive warm
molecular cores to be directly compared with the data. The four regions reveal
many different characteristics. While some of them, e.g., the detection rate of
CH3OH, can be explained by variations of the average gas temperatures, other
features are attributed to chemical effects. For example, C34S is observed
mainly at the core-edges and not toward their centers because of
temperature-selective desorption and successive gas-phase chemistry reactions.
Most nitrogen-bearing molecules are only found toward the hot molecular cores
and not the earlier evolutionary stages, indicating that the formation and
excitation of such complex nitrogen-bearing molecules needs significant heating
and time to be fully developed. Furthermore, we discuss the observational
difficulties to study massive accretion disks in the young deeply embedded
phase of massive star formation. The general potential and limitations of such
kind of dataset are discussed, and future directions are outlined. The analysis
and modeling of this source sample reveals many interesting features toward a
chemical evolutionary sequence. However, it is only an early step, and many
observational and theoretical challenges in that field lie ahead.Comment: 14 pages, 9 figures, accepted for the Astronomical Journal, a high
resolution version can be found at
http://www.mpia.de/homes/beuther/papers.htm
Relative Evolutionary Time Scale of Hot Molecular Cores with Respect to Ultra Compact HII Regions
Using the Owens Valley and Nobeyama Radio Observatory interferometers, we
carried out an unbiased search for hot molecular cores and ultracompact UC HII
regions toward the high-mass star forming region G19.61--0.23. In addition, we
performed 1.2 mm imaging with SIMBA, and retrieved 3.5 and 2 cm images from the
VLA archive data base. The newly obtained 3 mm image brings information on a
cluster of high-mass (proto)stars located in the innermost and densest part of
the parsec scale clump detected in the 1.2 mm continuum. We identify a total of
10 high-mass young stellar objects: one hot core (HC) and 9 UC HII regions,
whose physical parameters are obtained from model fits to their continuum
spectra. The ratio between the current and expected final radii of the UC \HII
regions ranges from 0.3 to 0.9, which leaves the possibility that all O-B stars
formed simultaneously. Under the opposite assumption -- namely that star
formation occurred randomly -- we estimate that HC lifetime is less than
1/3 of that of UCHII regions on the basis of the source number ratio
between them.Comment: 13 pages, 2 figs, including a color fi
Radio Continuum and Recombination Line Study of UC HII Regions with Extended Envelopes
We have carried out 21 cm radio continuum observations of 16 UC HII regions
using the VLA (D-array) in search of associated extended emission. We have also
observed H76 recombination line towards all the sources and
He76 line at the positions with strong H76 line emission. The
UC HII regions have simple morphologies and large (>10) ratios of single-dish
to VLA fluxes. Extended emission was detected towards all the sources. The
extended emission consists of one to several compact components and a diffuse
extended envelope. All the UC HII regions but two are located in the compact
components, where the UC HII regions always correspond to their peaks. The
compact components with UC HII regions are usually smaller and denser than
those without UC HII regions. Our recombination line observations indicate that
the ultracompact, compact, and extended components are physically associated.
The UC HII regions and their associated compact components are likely to be
ionized by the same sources on the basis of the morphological relations
mentioned above. This suggests that almost all of the observed UC HII regions
are not `real' UC HII regions and that their actual ages are much greater than
their dynamical age (<10000 yr). We find that most of simple UC HII regions
previously known have large ratios of single-dish to VLA fluxes, similar to our
sources. Therefore, the `age problem' of UC HII regions does not seem to be as
serious as earlier studies argued. We present a simple model that explains
extended emission around UC HII regions. Some individual sources are discussed.Comment: 29 pages, 28 postscript figures, Accepted for publication in Ap
A Review of H2CO 6cm Masers in the Galaxy
We present a review of the field of formaldehyde (H2CO) 6cm masers in the
Galaxy. Previous to our ongoing work, H2CO 6cm masers had been detected in the
Galaxy only toward three regions: NGC7538 IRS1, Sgr B2, and G29.96-0.02.
Current efforts by our group using the Very Large Array, Arecibo, and the Green
Bank Telescope have resulted in the detection of four new H2CO 6cm maser
regions. We discuss the characteristics of the known H2CO masers and the
association of H2CO 6cm masers with very young regions of massive star
formation. We also review the current ideas on the pumping mechanism for H2CO
6cm masers.Comment: 10 pages, 5 figures, IAU Symposium 242: Astrophysical Masers and
their Environment
Rotational Structure and Outflow in the Infrared Dark Cloud 18223-3
We examine an Infrared Dark Cloud (IRDC) at high spatial resolution as a
means to study rotation, outflow, and infall at the onset of massive star
formation. Submillimeter Array observations combined with IRAM 30 meter data in
12CO(2--1) reveal the outflow orientation in the IRDC 18223-3 region, and PdBI
3 mm observations confirm this orientation in other molecular species. The
implication of the outflow's presence is that an accretion disk is feeding it,
so using high density tracers such as C18O, N2H+, and CH3OH, we looked for
indications of a velocity gradient perpendicular to the outflow direction.
Surprisingly, this gradient turns out to be most apparent in CH3OH. The large
size (28,000 AU) of the flattened rotating object detected indicates that this
velocity gradient cannot be due solely to a disk, but rather from inward
spiraling gas within which a Keplerian disk likely exists. From the outflow
parameters, we derive properties of the source such as an outflow dynamical age
of ~37,000 years, outflow mass of ~13 M_sun, and outflow energy of ~1.7 x 10^46
erg. While the outflow mass and energy are clearly consistent with a high-mass
star forming region, the outflow dynamical age indicates a slightly more
evolved evolutionary stage than previous spectral energy distribution (SED)
modeling indicates. The calculated outflow properties reveal that this is truly
a massive star in the making. We also present a model of the observed methanol
velocity gradient. The rotational signatures can be modeled via rotationally
infalling gas. These data present evidence for one of the youngest known
outflow/infall/disk systems in massive star formation. A tentative evolutionary
picture for massive disks is discussed.Comment: 11 pages, 9 figures. Accepted for publication in A&A. Figures 2,3,6,
and 9 are available at higher resolution by email or in the journal
publicatio
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