870 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
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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
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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
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Motivated monitors: the importance of institutional investorsâ portfolio weights
Studies of institutional monitoring focus on the fraction of the firm held by institutions. We focus on the fraction of the institutionâs 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 institutionâs 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
Star formation in the giant HII regions of M101
The molecular components of three giant HII regions (NGC 5461, NGC 5462, NGC
5471) in the galaxy M101 are investigated with new observations from the James
Clerk Maxwell Telescope, the NRAO 12-meter, and the Owens Valley millimeter
array. Of the three HII regions, only NGC 5461 had previously been detected in
CO emission.
We calculate preliminary values for the molecular mass of the GMCs in NGC
5461 by assuming a CO-to-H_2 factor (X factor) and then compare these values
with the virial masses. We conclude that the data in this paper demonstrate for
the first time that the value of X may decrease in regions with intense star
formation.
The molecular mass for the association of clouds in NGC 5461 is approximately
3x10^7 Mo and is accompanied by 1-2 times as much atomic mass. The observed CO
emission in NGC 5461 is an order of magnitude stronger than in NGC 5462, while
it was not possible to detect molecular gas toward NGC 5471 with the JCMT. An
even larger ratio of atomic to molecular gas in NGC 5471 was observed, which
might be attributed to efficient conversion of molecular to atomic gas.
The masses of the individual clouds in NGC 5461, which are gravitationally
bound, cover a range of (2-8) x 10^5 Mo, comparable with the masses of Galactic
giant molecular clouds (GMCs). Higher star forming efficiencies, and not
massive clouds, appear to be the prerequisite for the formation of the large
number of stars whose radiation is required to produce the giant HII regions in
M101.Comment: 32 pages, 5 figures, accepted for publication in the Astrophysical
Journa
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 New Technique for Detecting Supersymmetric Dark Matter
We estimate the event rate for excitation of atomic transition by
photino-like dark matter. For excitations of several eV, this event rate can
exceed naive cross-section by many orders of magnitude. Although the event rate
for these atomic excitation is smaller than that of nuclear recoil off of
non-zero spin nuclei, the photons emitted by the deexcitation are easier to
detect than low-energy nuclear recoils. For many elements, there are several
low-lying states with comparable excitation rates, thus, spectral ratios could
be used to distinguish signal from background.Comment: 6 pages plain te
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