813 research outputs found
The origin of organic emission in NGC 2071
Context: The physical origin behind organic emission in embedded low-mass
star formation has been fiercely debated in the last two decades. A multitude
of scenarios have been proposed, from a hot corino to PDRs on cavity walls to
shock excitation.
Aims: The aim of this paper is to determine the location and the
corresponding physical conditions of the gas responsible for organics emission
lines. The outflows around the small protocluster NGC 2071 are an ideal testbed
to differentiate between various scenarios.
Methods: Using Herschel-HIFI and the SMA, observations of CH3OH, H2CO and
CH3CN emission lines over a wide range of excitation energies were obtained.
Comparisons to a grid of radiative transfer models provide constraints on the
physical conditions. Comparison to H2O line shape is able to trace gas-phase
synthesis versus a sputtered origin.
Results: Emission of organics originates in three spots: the continuum
sources IRS 1 ('B') and IRS 3 ('A') as well as a outflow position ('F').
Densities are above 10 cm and temperatures between 100 to 200 K.
CH3OH emission observed with HIFI originates in all three regions and cannot be
associated with a single region. Very little organic emission originates
outside of these regions.
Conclusions: Although the three regions are small (<1,500 AU), gas-phase
organics likely originate from sputtering of ices due to outflow activity. The
derived high densities (>10 cm) are likely a requirement for organic
molecules to survive from being destroyed by shock products. The lack of
spatially extended emission confirms that organic molecules cannot (re)form
through gas-phase synthesis, as opposed to H2O, which shows strong line wing
emission. The lack of CH3CN emission at 'F' is evidence for a different history
of ice processing due to the absence of a protostar at that location and recent
ice mantle evaporation.Comment: 10 Pages, 8 figures, Accepted for Astronomy and Astrophysic
Recommended from our members
Stock option grants to target CEOs during private merger negotiations
Unscheduled stock options to target chief executive officers (CEOs) are a nontrivial phenomenon during private merger negotiations. In 920 acquisition bids during 1999-2007, over 13% of targets grant them. These options substitute for golden parachutes and compensate target CEOs for the benefits they forfeit because of the merger. Targets granting unscheduled options are more likely to be acquired but they earn lower premiums. Consequently, deal value drops by '62 for every dollar target CEOs receive from unscheduled options. Conversely, acquirers of targets offering these awards experience higher returns. Therefore, deals involving unscheduled grants exhibit a transfer of wealth from target shareholders to bidder shareholders
Recommended from our members
Contractual revisions in compensation: evidence from merger bonuses to target CEOs
Do merger bonuses to target CEOs facilitate a wealth transfer from target to acquirer shareholders? We test this hypothesis against an alternative that bonuses enable a useful contractual revision in compensation contracts when takeovers generate small synergies. When target CEOs get a merger bonus, acquirers pay lower premiums, but they also typically get less in the form of low synergies. Moreover, both stock and accounting returns to the acquirers are lower on average in deals with target CEO bonuses. These results support the contractual revision alternative. Nevertheless, wealth transfer occurs when merger bonuses are present in deals where targets exhibit high pre-takeover abnormal accruals or are subject to SEC enforcement actions
Recommended from our members
Motivated monitors: the importance of institutional investors’ portfolio weights
Studies of institutional monitoring focus on the fraction of the firm held by institutions. We focus on the fraction of the institution’s portfolio represented by the firm. In the context of acquisitions, we hypothesize that institutional monitoring will be greatest when the target firm represents a significant allocation of funds in the institution’s portfolio. We show that this measure is important in reconciling mixed findings for total institutional ownership in the prior literature. The results indicate that our measure of institutional holdings leads to greater bid completion rates, higher premiums and lower acquirer returns. This empirical evidence provides support for theories predicting a beneficial effect of blockholders in monitoring the firm in general and in enhancing the gains to takeover targets in particular
Node Labels in Local Decision
The role of unique node identifiers in network computing is well understood
as far as symmetry breaking is concerned. However, the unique identifiers also
leak information about the computing environment - in particular, they provide
some nodes with information related to the size of the network. It was recently
proved that in the context of local decision, there are some decision problems
such that (1) they cannot be solved without unique identifiers, and (2) unique
node identifiers leak a sufficient amount of information such that the problem
becomes solvable (PODC 2013).
In this work we give study what is the minimal amount of information that we
need to leak from the environment to the nodes in order to solve local decision
problems. Our key results are related to scalar oracles that, for any given
, provide a multiset of labels; then the adversary assigns the
labels to the nodes in the network. This is a direct generalisation of the
usual assumption of unique node identifiers. We give a complete
characterisation of the weakest oracle that leaks at least as much information
as the unique identifiers.
Our main result is the following dichotomy: we classify scalar oracles as
large and small, depending on their asymptotic behaviour, and show that (1) any
large oracle is at least as powerful as the unique identifiers in the context
of local decision problems, while (2) for any small oracle there are local
decision problems that still benefit from unique identifiers.Comment: Conference version to appear in the proceedings of SIROCCO 201
The hot core towards the intermediate mass protostar NGC7129 FIRS 2: Chemical similarities with Orion KL
NGC 7129 FIRS 2 (hereafter FIRS 2) is an intermediate-mass (2 to 8 Msun)
protostar located at a distance of 1250 pc. High spatial resolution
observations are required to resolve the hot core at its center. We present a
molecular survey from 218200 MHz to 221800 MHz carried out with the IRAM
Plateau de Bure Interferometer. These observations were complemented with a
long integration single-dish spectrum taken with the IRAM 30m telescope. We
used a Local Thermodynamic Equilibrium (LTE) single temperature code to model
the whole dataset. The interferometric spectrum is crowded with a total of ~300
lines from which a few dozens remain unidentified yet. The spectrum has been
modeled with a total of 20 species and their isomers, isotopologues and
deuterated compounds. Complex molecules like methyl formate (CH3OCHO), ethanol
(CH3CH2OH), glycolaldehyde (CH2OHCHO), acetone (CH3COCH3), dimethyl ether
(CH3OCH3), ethyl cyanide (CH3CH2CN) and the aGg' conformer of ethylene glycol
(aGg'-(CH2OH)_2) are among the detected species. The detection of vibrationally
excited lines of CH3CN, CH3OCHO, CH3OH, OCS, HC3N and CH3CHO proves the
existence of gas and dust at high temperatures. In fact, the gas kinetic
temperature estimated from the vibrational lines of CH3CN, ~405 K, is similar
to that measured in massive hot cores. Our data allow an extensive comparison
of the chemistry in FIRS~2 and the Orion hot core. We find a quite similar
chemistry in FIRS 2 and Orion. Most of the studied fractional molecular
abundances agree within a factor of 5. Larger differences are only found for
the deuterated compounds D2CO and CH2DOH and a few molecules (CH3CH2CN, SO2,
HNCO and CH3CHO). Since the physical conditions are similar in both hot cores,
only different initial conditions (warmer pre-collapse phase in the case of
Orion) and/or different crossing time of the gas in the hot core can explain
this behavior.Comment: 30 pages, 9 figure
Triggered massive-star formation on the borders of Galactic HII regions. IV- Star formation at the periphery of Sh2-212
Aims: We wish to establish whether sequential star formation is taking place
at the periphery of the Galactic HII region Sh2-212.
Methods: We present CO millimetre observations of this region obtained at the
IRAM 30-m telescope to investigate the distribution of associated molecular
material. We also use deep JHK observations obtained at the CFHT to study the
stellar content of the region, and radio observations obtained at the VLA to
look for the presence of an ultra-compact (UC) HII region and for maser
emission.
Results: In the optical, Sh2-212 is spherically symmetric around its central
exciting cluster. This HII region is located along a molecular filament. A
thin, well-defined half ring of molecular material surrounds the brightest part
of the HII region at the rear and is fragmented. The most massive fragment
(~200 solar masses) contains a massive young stellar object displaying a
near-IR excess; its spectral energy distribution indicates a high-mass
(~14solar masses), high-temperature (~30000K), and high-luminosity (~17000
solar luminosities) source. This object ionizes a UC HII region.
Conclusions: Sh2-212 is a good example of massive-star formation triggered
via the collect and collapse process. The massive YSO observed at its periphery
is a good candidate for a massive star formed in isolation.Comment: 12 pages, 14 figures. To be published in A&
The Unusual Distributions of Ionized Material and Molecular Hydrogen in NGC 6881: Signposts of Multiple Events of Bipolar Ejection in a Planetary Nebula
The planetary nebula NGC 6881 displays in the optical a quadrupolar
morphology consisting of two pairs of highly collimated bipolar lobes aligned
along different directions. An additional bipolar ejection is revealed by the
hydrogen molecular emission, but its wide hourglass morphology is very
different from that of the ionized material. To investigate in detail the
spatial distribution of molecular hydrogen and ionized material within NGC
6881, and to determine the prevalent excitation mechanism of the H2 emission,
we have obtained new near-IR Br-gamma and H2 and optical H-alpha and [N II]
images, as well as intermediate resolution JHK spectra. These observations
confirm the association of the H2 bipolar lobes to NGC 6881 and find that the
prevalent excitation mechanism is collisional. The detailed morphology and very
different collimation degree of the H2 and ionized bipolar lobes of NGC 6881
not only imply that multiple bipolar ejections have occurred in this nebula,
but also that the dominant shaping agent is different for each bipolar
ejection: a bipolar stellar wind most likely produced the H2 lobes, while
highly collimated outflows are carving out the ionized lobes into the thick
circumstellar envelope. The asymmetry between the southeast and northwest H2
bipolar lobes suggests the interaction of the nebula with an inhomogeneous
interstellar medium. We find evidence that places NGC 6881 in the H II region
Sh 2-109 along the Orion local spiral arm.Comment: 9 pages, 7 figures, 4 table
A young double stellar cluster in a HII region, emerging from its parent molecular cloud
We report the properties of a new young double stellar cluster in the region
towards IRAS 07141-0920 contained in the HII region Sh2-294. High-resolution
optical UBVRI and Halpha images, near-infrared JHKs and H2 filter images were
used to make photometric and morphological studies of the point sources and the
nebula seen towards Sh2-294. The optical images reveal an emission nebula with
very rich morphological details, composed mainly of UV scattered light and of
Halpha emission. Contrasting with the bright parts of the nebula, opaque,
elongated patches are seen. Our optical photometry confirms that the
illuminator of the nebula is likely to be a B0.5V star located at a distance of
about 3.2 kpc. Our near-IR images reveal an embedded cluster, extending for
about 2 pc and exhibiting sub-clustering: a denser, more condensed, sub-cluster
surrounding the optical high-mass B0.5V illuminator star; and a more embedded,
optically invisible, sub-cluster located towards the eastern, dark part of the
nebula and including the luminous MSX source G224.1880+01.2407, a massive
protostellar candidate that could be the origin of jets and extended features
seen at 2.12 micron. The double cluster appears to be clearing the remaining
molecular material of the parent cloud, creating patches of lower extinction
and allowing some of the least reddened members to be detected in the optical
images. We find 12 MS and 143 PMS members using 3 different methods: comparison
with isochrones in optical colour-magnitude diagrams, detection of near-IR
excess, and presence of Halpha emission. The most massive star fits a 4 Myr
post-MS isochrone. The age of the optically selected PMS population is
estimated to be 7-8 Myr. The IR-excess population shows sub-clustering on
scales as small as 0.23 pc and is probably much younger.Comment: 15 pages, 16 figure
Recommended from our members
On the importance of golden parachutes
In acquisitions, target chief executive officers (CEOs) face a moral hazard: Any personal gain from the deal could be offset by the loss of the future compensation stream associated with their jobs. Larger, more important parachutes provide greater relief for these losses. To explicitly measure the moral hazard target CEOs face, we standardize the parachute payment by the expected value of their acquisition-induced lost compensation. We examine 851 acquisitions from 1999–2007, finding that more important parachutes benefit target shareholders through higher completion probabilities. Conversely, as parachute importance increases, target shareholders receive lower takeover premia, while acquirer shareholders capture additional rents from target shareholders
- …