288 research outputs found
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 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
Dichotomy in the Dynamical Status of Massive Cores in Orion
To study the evolution of high mass cores, we have searched for evidence of
collapse motions in a large sample of starless cores in the Orion molecular
cloud. We used the Caltech Submillimeter Observatory telescope to obtain
spectra of the optically thin (\H13CO+) and optically thick (\HCO+) high
density tracer molecules in 27 cores with masses 1 \Ms. The red- and
blue-asymmetries seen in the line profiles of the optically thick line with
respect to the optically thin line indicate that 2/3 of these cores are not
static. We detect evidence for infall (inward motions) in 9 cores and outward
motions for 10 cores, suggesting a dichotomy in the kinematic state of the
non-static cores in this sample. Our results provide an important observational
constraint on the fraction of collapsing (inward motions) versus non-collapsing
(re-expanding) cores for comparison with model simulations.Comment: 9 pages, 2 Figures. To appear in ApJ(Letters
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
High Resolution Molecular Gas Maps of M33
New observations of CO (J=1->0) line emission from M33, using the 25 element
BEARS focal plane array at the Nobeyama Radio Observatory 45-m telescope, in
conjunction with existing maps from the BIMA interferometer and the FCRAO 14-m
telescope, give the highest resolution (13'') and most sensitive (RMS ~ 60 mK)
maps to date of the distribution of molecular gas in the central 5.5 kpc of the
galaxy. A new catalog of giant molecular clouds (GMCs) has a completeness limit
of 1.3 X 10^5 M_sun. The fraction of molecular gas found in GMCs is a strong
function of radius in the galaxy, declining from 60% in the center to 20% at
galactocentric radius R_gal ~ 4 kpc. Beyond that radius, GMCs are nearly
absent, although molecular gas exists. Most (90%) of the emission from low mass
clouds is found within 100 pc projected separation of a GMC. In an annulus 2.1<
R_gal <4.1 kpc, GMC masses follow a power law distribution with index -2.1.
Inside that radius, the mass distribution is truncated, and clouds more massive
than 8 X 10^5 M_sun are absent. The cloud mass distribution shows no
significant difference in the grand design spiral arms versus the interarm
region. The CO surface brightness ratio for the arm to interarm regions is 1.5,
typical of other flocculent galaxies.Comment: 14 pages, 14 figures, accepted in ApJ. Some tables poorly typeset in
emulateapj; see source files for raw dat
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
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