1,582 research outputs found
Relationship between the column density distribution and evolutionary class of molecular clouds as viewed by ATLASGAL
We present the first study of the relationship between the column density
distribution of molecular clouds within nearby Galactic spiral arms and their
evolutionary status as measured from their stellar content. We analyze a sample
of 195 molecular clouds located at distances below 5.5 kpc, identified from the
ATLASGAL 870 micron data. We define three evolutionary classes within this
sample: starless clumps, star-forming clouds with associated young stellar
objects, and clouds associated with HII regions. We find that the N(H2)
probability density functions (N-PDFs) of these three classes of objects are
clearly different: the N-PDFs of starless clumps are narrowest and close to
log-normal in shape, while star-forming clouds and HII regions exhibit a
power-law shape over a wide range of column densities and log-normal-like
components only at low column densities. We use the N-PDFs to estimate the
evolutionary time-scales of the three classes of objects based on a simple
analytic model from literature. Finally, we show that the integral of the
N-PDFs, the dense gas mass fraction, depends on the total mass of the regions
as measured by ATLASGAL: more massive clouds contain greater relative amounts
of dense gas across all evolutionary classes.Comment: Accepted for publication in A&A (25th June 15) 23 pages, 12 figures.
Additional appendix figures will appear in the journal version of this pape
High-fidelity view of the structure and fragmentation of the high-mass, filamentary IRDC G11.11-0.12
Star formation in molecular clouds is intimately linked to their internal
mass distribution. We present an unprecedentedly detailed analysis of the
column density structure of a high-mass, filamentary molecular cloud, namely
IRDC G11.11-0.12 (G11). We use two novel column density mapping techniques:
high-resolution (FWHM=2", or ~0.035 pc) dust extinction mapping in near- and
mid-infrared, and dust emission mapping with the Herschel satellite. These two
completely independent techniques yield a strikingly good agreement,
highlighting their complementarity and robustness. We first analyze the dense
gas mass fraction and linear mass density of G11. We show that G11 has a top
heavy mass distribution and has a linear mass density (M_l ~ 600 Msun pc^{-1})
that greatly exceeds the critical value of a self-gravitating, non-turbulent
cylinder. These properties make G11 analogous to the Orion A cloud, despite its
low star-forming activity. This suggests that the amount of dense gas in
molecular clouds is more closely connected to environmental parameters or
global processes than to the star-forming efficiency of the cloud. We then
examine hierarchical fragmentation in G11 over a wide range of size-scales and
densities. We show that at scales 0.5 pc > l > 8 pc, the fragmentation of G11
is in agreement with that of a self-gravitating cylinder. At scales smaller
than l < 0.5 pc, the results agree better with spherical Jeans' fragmentation.
One possible explanation for the change in fragmentation characteristics is the
size-scale-dependent collapse time-scale that results from the finite size of
real molecular clouds: at scales l < 0.5 pc, fragmentation becomes sufficiently
rapid to be unaffected by global instabilities.Comment: 8 pages, 8 figures, accepted to A&
Dust-temperature of an isolated star-forming cloud: Herschel observations of the Bok globule CB244
We present Herschel observations of the isolated, low-mass star-forming Bok
globule CB244. It contains two cold sources, a low-mass Class 0 protostar and a
starless core, which is likely to be prestellar in nature, separated by 90
arcsec (~ 18000 AU). The Herschel data sample the peak of the Planck spectrum
for these sources, and are therefore ideal for dust-temperature and column
density modeling. With these data and a near-IR extinction map, the MIPS 70
micron mosaic, the SCUBA 850 micron map, and the IRAM 1.3 mm map, we model the
dust-temperature and column density of CB244 and present the first measured
dust-temperature map of an entire star-forming molecular cloud. We find that
the column-averaged dust-temperature near the protostar is ~ 17.7 K, while for
the starless core it is ~ 10.6K, and that the effect of external heating causes
the cloud dust-temperature to rise to ~ 17 K where the hydrogen column density
drops below 10^21 cm^-2. The total hydrogen mass of CB244 (assuming a distance
of 200 pc) is 15 +/- 5 M_sun. The mass of the protostellar core is 1.6 +/- 0.1
M_sun and the mass of the starless core is 5 +/- 2 M_sun, indicating that ~ 45%
of the mass in the globule is participating in the star-formation process.Comment: Accepted for A&A Herschel Special Issue; 5 pages, 2 figure
The structured environments of embedded star-forming cores. PACS and SPIRE mapping of the enigmatic outflow source UYSO 1
The intermediate-mass star-forming core UYSO 1 has previously been found to
exhibit intriguing features. While deeply embedded and previously only
identified by means of its (sub-)millimeter emission, it drives two powerful,
dynamically young, molecular outflows. Although the process of star formation
has obviously started, the chemical composition is still pristine. We present
Herschel PACS and SPIRE continuum data of this presumably very young region.
The now complete coverage of the spectral energy peak allows us to precisely
constrain the elevated temperature of 26 - 28 K for the main bulge of gas
associated with UYSO1, which is located at the interface between the hot HII
region Sh 2-297 and the cold dark nebula LDN 1657A. Furthermore, the data
identify cooler compact far-infrared sources of just a few solar masses, hidden
in this neighbouring dark cloud.Comment: accepted contribution for the forthcoming Herschel Special Issue of
A&A, 5 pages (will appear as 4-page letter in the journal), 6 figure file
Anomaluos RR Lyrae (V-I)_0 colors in Baade's Window
We compare (V-I)_0-(V-K)_0 color-color and (V-I)_0-log P period-color
diagrams for Baade's Window and local RRab Lyrae stars. We find that for a
fixed log P the Baade's Window RR Lyrae stars are ~0.17 magnitudes redder in
(V-I)_0 than the local RR Lyrae stars. We also show that there is no such
effect observed in (V-K)_0. We argue that an extinction misestimate towards
Baade's Window is not a plausible explanation of the discrepancy. Unlike
Baade's Window RR Lyrae stars, the local ones follow a black-body color-color
relation and are well approximated by theoretical models. We test two
parameters, metallicity and surface gravity, and find that their effects are
too small to explain the (V-I)_0 discrepancy between the two groups of stars.
We do not provide any explanation for the anomalous (V-I)_0 behavior of the
Baade's Window RR Lyrae stars. We note that a similar effect for clump giant
stars has been recently reported by Paczynski and we caution that RR Lyrae
stars and clump giants, often used as standard candles, can be subject to the
same type of systematics.Comment: 10 pages, 7 figures, submitted to Ap
Restriction and modification in Bacillus subtilis: inducibility of a DNA methylating activity in nonmodifying cells
The nonrestricting/nonmodifying strain Bacillus subtilis 222 (r-m-) can be induced to synthesize a DNA-modifying activity upon treatment with either mitomycin C (MC) or UV light. This is shown by the following facts. (i) Infection of MC-pretreated 222 cells with unmodified SPP1 phage yields about 3% modified phage that are resistant to restriction in B. subtilis R (r+m+). The induced modifying activity causes the production of a small fraction of fully modified phage in a minority class of MC-treated host cells. (ii) The MC-pretreated host cells contain a DNA cytosine methylating activity: both bacterial and phage DNAs have elevated levels of 5-methylcytosine. (iii) The MC-induced methylation of SPP1 DNA takes place at the recognition nucleotide sequences of restriction endonuclease R from B. subtilis R. (iv) Crude extracts of MC-pretreated 222 cells have enhanced DNA methyltransferase activities, with a substrate specificity similar to that found in modification enzymes present in (constitutively) modifying strains
Hier ist wahrhaftig ein Loch im Himmel - The NGC 1999 dark globule is not a globule
The NGC 1999 reflection nebula features a dark patch with a size of ~10,000
AU, which has been interpreted as a small, dense foreground globule and
possible site of imminent star formation. We present Herschel PACS far-infrared
70 and 160mum maps, which reveal a flux deficit at the location of the globule.
We estimate the globule mass needed to produce such an absorption feature to be
a few tenths to a few Msun. Inspired by this Herschel observation, we obtained
APEX LABOCA and SABOCA submillimeter continuum maps, and Magellan PANIC
near-infrared images of the region. We do not detect a submillimer source at
the location of the Herschel flux decrement; furthermore our observations place
an upper limit on the mass of the globule of ~2.4x10^-2 Msun. Indeed, the
submillimeter maps appear to show a flux depression as well. Furthermore, the
near-infrared images detect faint background stars that are less affected by
extinction inside the dark patch than in its surroundings. We suggest that the
dark patch is in fact a hole or cavity in the material producing the NGC 1999
reflection nebula, excavated by protostellar jets from the V 380 Ori multiple
system.Comment: accepted for the A&A Herschel issue; 7 page
On the nature of the deeply embedded protostar OMC-2 FIR 4
We use mid-infrared to submillimeter data from the Spitzer, Herschel, and
APEX telescopes to study the bright sub-mm source OMC-2 FIR 4. We find a point
source at 8, 24, and 70 m, and a compact, but extended source at 160, 350,
and 870 m. The peak of the emission from 8 to 70 m, attributed to the
protostar associated with FIR 4, is displaced relative to the peak of the
extended emission; the latter represents the large molecular core the protostar
is embedded within. We determine that the protostar has a bolometric luminosity
of 37 Lsun, although including more extended emission surrounding the point
source raises this value to 86 Lsun. Radiative transfer models of the
protostellar system fit the observed SED well and yield a total luminosity of
most likely less than 100 Lsun. Our models suggest that the bolometric
luminosity of the protostar could be just 12-14 Lsun, while the luminosity of
the colder (~ 20 K) extended core could be around 100 Lsun, with a mass of
about 27 Msun. Our derived luminosities for the protostar OMC-2 FIR 4 are in
direct contradiction with previous claims of a total luminosity of 1000 Lsun
(Crimier et al 2009). Furthermore, we find evidence from far-infrared molecular
spectra (Kama et al. 2013, Manoj et al. 2013) and 3.6 cm emission (Reipurth et
al 1999) that FIR 4 drives an outflow. The final stellar mass the protostar
will ultimately achieve is uncertain due to its association with the large
reservoir of mass found in the cold core.Comment: Accpeted by ApJ, 17 pages, 11 figure
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