206 research outputs found
The formation of active protoclusters in the Aquila Rift: A millimeter continuum view
Abridged -- We present an analysis of the Aquila Rift complex which addresses
the questions of the star formation rate (SFR), star formation efficiency (SFE)
and typical lifetime of the Class 0 protostellar phase in two nearby
cluster-forming clumps: the Serpens South and W40 protoclusters. We carried out
a 1.2 mm dust continuum mapping of the Aquila Rift complex with the MAMBO
bolometer array on the IRAM 30m telescope. We perform a systematic source
extraction in our millimeter continuum map. Based on complementary data from
the Herschel Gould Belt survey and Spitzer maps, we characterize the SEDs of
the 77 mm continuum sources detected with MAMBO and estimate their evolutionary
stages. Taking advantage of the comprehensive dataset available for the Serpens
South region, spanning wavelengths from 2 microns to 1.2 mm, we estimate the
numbers of young stellar objects (YSOs) at different evolutionary stages and
find a ratio of Class 0 to Class I protostars N(0)/N(I)=0.19-0.27. This low
ratio supports a scenario of relatively fast accretion at the beginning of the
protostellar phase, and leads to a Class 0 lifetime of ~4-9x10^{4} yr. We also
show that both the Serpens South and W40 protoclusters are characterized by
large fractions of protostars and high SFRs ~20-50 Msun.Myr^{-1}pc^{-2}, in
agreement with the idea that these two nearby clumps are active sites of
clustered star formation currently undergoing bursts of star formation, and
have the potential ability to form bound star clusters. While the formation of
these two protoclusters is likely to have been initiated in a very different
manner, the resulting protostellar populations are observed to be very similar.
This suggests that after the onset of gravitational collapse, the detailed
manner in which the collapse has been initiated does not affect much the
ability of a clump to form stars.Comment: 11 pages, 5 figures. Abstract has been shortened. Accepted for
publication in A&A. Final version including corrections in section 4.1.2.
Table 1 available upon request, contact the author
Massive Clumps in the NGC 6334 Star Forming Region
We report observations of dust continuum emission at 1.2 mm toward the star
forming region NGC 6334 made with the SEST SIMBA bolometer array. The
observations cover an area of square degrees with approximately
uniform noise. We detected 181 clumps spanning almost three orders of magnitude
in mass (3\Msun \Msun) and with sizes in the range 0.1--1.0 pc.
We find that the clump mass function is well fit with a power law
of the mass with exponent -0.6 (or equivalently ). The
derived exponent is similar to those obtained from molecular line emission
surveys and is significantly different from that of the stellar initial mass
function. We investigated changes in the mass spectrum by changing the
assumptions on the temperature distribution of the clumps and on the
contribution of free-free emission to the 1.2 mm emission, and found little
changes on the exponent. The Cumulative Mass Distribution Function is also
analyzed giving consistent results in a mass range excluding the high-mass end
where a power-law fit is no longer valid. The masses and sizes of the clumps
observed in NGC 6334 indicate that they are not direct progenitors of stars and
that the process of fragmentation determines the distribution of masses later
on or occurs at smaller spatial scales. The spatial distribution of the clumps
in NGC 6334 reveals clustering which is strikingly similar to that exhibited by
young stars in other star forming regions. A power law fit to the surface
density of companions gives .Comment: 16 pages, 11 figures, 4 tables. To appear in the Astrophysical
Journa
Magnetic Fields and Massive Star Formation
Massive stars ( \msun) typically form in parsec-scale molecular clumps
that collapse and fragment, leading to the birth of a cluster of stellar
objects. We investigate the role of magnetic fields in this process through
dust polarization at 870 m obtained with the Submillimeter Array (SMA).
The SMA observations reveal polarization at scales of \lsim 0.1 pc. The
polarization pattern in these objects ranges from ordered hour-glass
configurations to more chaotic distributions. By comparing the SMA data with
the single dish data at parsec scales, we found that magnetic fields at dense
core scales are either aligned within of or perpendicular to the
parsec-scale magnetic fields. This finding indicates that magnetic fields play
an important role during the collapse and fragmentation of massive molecular
clumps and the formation of dense cores. We further compare magnetic fields in
dense cores with the major axis of molecular outflows. Despite a limited number
of outflows, we found that the outflow axis appears to be randomly oriented
with respect to the magnetic field in the core. This result suggests that at
the scale of accretion disks (\lsim 10^3 AU), angular momentum and dynamic
interactions possibly due to close binary or multiple systems dominate over
magnetic fields. With this unprecedentedly large sample massive clumps, we
argue on a statistical basis that magnetic fields play an important role during
the formation of dense cores at spatial scale of 0.01 - 0.1 pc in the context
of massive star and cluster star formation.Comment: Accepted for publication in Astrophysical Journa
The Mass Distribution of Starless and Protostellar Cores in Gould Belt Clouds
Using data from the SCUBA Legacy Catalogue (850 ÎŒm) and Spitzer Space Telescope (3.6-70 ÎŒm), we explore dense cores in the Ophiuchus, Taurus, Perseus, Serpens, and Orion molecular clouds. We develop a new method to discriminate submillimeter cores found by Submillimeter Common-User Bolometer Array (SCUBA) as starless or protostellar, using point source photometry from Spitzer wide field surveys. First, we identify infrared sources with red colors associated with embedded young stellar objects (YSOs). Second, we compare the positions of these YSO candidates to our submillimeter cores. With these identifications, we construct new, self-consistent starless and protostellar core mass functions (CMFs) for the five clouds. We find best-fit slopes to the high-mass end of the CMFs of â1.26 ± 0.20, â1.22 ± 0.06, â0.95 ± 0.20, and â1.67 ± 0.72 for Ophiuchus, Taurus, Perseus, and Orion, respectively. Broadly, these slopes are each consistent with the â1.35 power-law slope of the Salpeter initial mass function at higher masses, but suggest some differences. We examine a variety of trends between these CMF shapes and their parent cloud properties, potentially finding a correlation between the high-mass slope and core temperature. We also find a trend between core mass and effective size, but we are very limited by sensitivity. We make similar comparisons between core mass and size with visual extinction (for A_V â„ 3) and find no obvious trends. We also predict the numbers and mass distributions of cores that future surveys with SCUBA-2 may detect in each of these clouds
Circumstellar Structure around Evolved Stars in the Cygnus-X Star Formation Region
We present observations of newly discovered 24 micron circumstellar
structures detected with the Multiband Imaging Photometer for Spitzer (MIPS)
around three evolved stars in the Cygnus-X star forming region. One of the
objects, BD+43 3710, has a bipolar nebula, possibly due to an outflow or a
torus of material. A second, HBHA 4202-22, a Wolf-Rayet candidate, shows a
circular shell of 24 micron emission suggestive of either a limb-brightened
shell or disk seen face-on. No diffuse emission was detected around either of
these two objects in the Spitzer 3.6-8 micron Infrared Array Camera (IRAC)
bands. The third object is the luminous blue variable candidate G79.29+0.46. We
resolved the previously known inner ring in all four IRAC bands. The 24 micron
emission from the inner ring extends ~1.2 arcmin beyond the shorter wavelength
emission, well beyond what can be attributed to the difference in resolutions
between MIPS and IRAC. Additionally, we have discovered an outer ring of 24
micron emission, possibly due to an earlier episode of mass loss. For the two
shell stars, we present the results of radiative transfer models, constraining
the stellar and dust shell parameters. The shells are composed of amorphous
carbon grains, plus polycyclic aromatic hydrocarbons in the case of
G79.29+0.46. Both G79.29+0.46 and HBHA 4202-22 lie behind the main Cygnus-X
cloud. Although G79.29+0.46 may simply be on the far side of the cloud, HBHA
4202-22 is unrelated to the Cygnus-X star formation region.Comment: Accepted by A
Changes of dust opacity with density in the Orion A molecular cloud
We have studied the opacity of dust grains at submillimeter wavelengths by estimating the optical depth from imaging at 160, 250, 350, and 500 ÎŒm from the Herschel Gould Belt Survey and comparing this to a column density obtained from the Two Micron All Sky Survey derived color excess E(J â Ks). Our main goal was to investigate the spatial variations of the opacity due to "big" grains over a variety of environmental conditions and thereby quantify how emission properties of the dust change with column (and volume) density. The central and southern areas of the Orion A molecular cloud examined here, with NH ranging from 1.5 Ă 1021 cmâ2 to 50 Ă 1021 cmâ2, are well suited to this approach. We fit the multi-frequency Herschel spectral energy distributions (SEDs) of each pixel with a modified blackbody to obtain the temperature, T, and optical depth, Ï1200, at a fiducial frequency of 1200 GHz (250 ÎŒm). Using a calibration of NH/E(J â Ks ) for the interstellar medium (ISM) we obtained the opacity (dust emission cross-section per H nucleon), Ïe(1200), for every pixel. From a value ~1 Ă 10â25 cm2 Hâ1 at the lowest column densities that is typical of the high-latitude diffuse ISM, Ïe(1200) increases as N 0.28H over the range studied. This is suggestive of grain evolution. Integrating the SEDs over frequency, we also calculated the specific power P (emission power per H) for the big grains. In low column density regions where dust clouds are optically thin to the interstellar radiation field (ISRF), P is typically 3.7 Ă 10â31 W Hâ1, again close to that in the high-latitude diffuse ISM. However, we find evidence for a decrease of P in high column density regions, which would be a natural outcome of attenuation of the ISRF that heats the grains, and for localized increases for dust illuminated by nearby stars or embedded protostars
ALMA-IMF VIII -- Combination of Interferometric Continuum Images with Single-Dish Surveys and Structural Analysis of Six Protoclusters
We present the combination of ALMA-IMF and single-dish continuum images from
the Mustang-2 Galactic Plane Survey (MGPS90) at 3 millimeters and the Bolocam
Galactic Plane Survey (BGPS) at 1 millimeter. Six and ten out of the fiffteen
ALMA-IMF fields are combined with MGPS90 and BGPS, respectively. The
combination is made via the feathering technique. We used the dendrogram
algorithm throughout the combined images, and performed further analysis in the
six fields with combination in both bands (G012.80, W43-MM1, W43-MM2, W43-MM3,
W51-E, W51-IRS2). In these fields, we calculated spectral index maps and used
them to separate regions dominated by dust or free-free emission, and then
performed further structural analysis. We report the basic physical parameters
of the dust-dominated (column densities, masses) and ionized (emission
measures, hydrogen ionization photon rates) structures. We also searched for
multi-scale relations in the dust-dominated structures across the analyzed
fields, finding that the fraction of mass in dendrogram leaves (which we label
as "Leaf Mass Eficiency", LME) as a function of molecular gas column density
follows a similar trend: a rapid, exponential-like growth, with maximum values
approaching 100% in most cases. The observed behaviour of the LME with gas
column is tentatively interpreted as an indicator of large star formation
activity within the ALMA-IMF protoclusters. W51-E and G012.80 stand out as
cases with comparatively large and reduced potential for further star
formation, respectively.Comment: Accepted to The Astrophysical Journal Supplemen
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