1,382 research outputs found
What is the Shell Around R Coronae Borealis?
The hydrogen-deficient, carbon-rich R Coronae Borealis (RCB) stars are known
for being prolific producers of dust which causes their large iconic declines
in brightness. Several RCB stars, including R CrB, itself, have large extended
dust shells seen in the far-infrared. The origin of these shells is uncertain
but they may give us clues to the evolution of the RCB stars. The shells could
form in three possible ways. 1) they are fossil Planetary Nebula (PN) shells,
which would exist if RCB stars are the result of a final, helium-shell flash,
2) they are material left over from a white-dwarf merger event which formed the
RCB stars, or 3) they are material lost from the star during the RCB phase.
Arecibo 21-cm observations establish an upper limit on the column density of H
I in the R CrB shell implying a maximum shell mass of 0.3
M. A low-mass fossil PN shell is still a possible source of the shell
although it may not contain enough dust. The mass of gas lost during a
white-dwarf merger event will not condense enough dust to produce the observed
shell, assuming a reasonable gas-to-dust ratio. The third scenario where the
shell around R CrB has been produced during the star's RCB phase seems most
likely to produce the observed mass of dust and the observed size of the shell.
But this means that R CrB has been in its RCB phase for 10 yr.Comment: 5 pages, 2 figures, 2 tables, Accepted for publication in A
The Identification of Extreme Asymptotic Giant Branch Stars and Red Supergiants in M33 by 24 {\mu}m Variability
We present the first detection of 24 {\mu}m variability in 24 sources in the
Local Group galaxy M33. These results are based on 4 epochs of MIPS
observations, which are irregularly spaced over ~750 days. We find that these
sources are constrained exclusively to the Holmberg radius of the galaxy, which
increases their chances of being members of M33. We have constructed spectral
energy distributions (SEDs) ranging from the optical to the sub-mm to
investigate the nature of these objects. We find that 23 of our objects are
most likely heavily self-obscured, evolved stars; while the remaining source is
the Giant HII region, NGC 604. We believe that the observed variability is the
intrinsic variability of the central star reprocessed through their
circumstellar dust shells. Radiative transfer modeling was carried out to
determine their likely chemical composition, luminosity, and dust production
rate (DPR). As a sample, our modeling has determined an average luminosity of
(3.8 0.9) x 10 L and a total DPR of (2.3 0.1) x
10 M yr. Most of the sources, given the high DPRs and
short wavelength obscuration, are likely "extreme" AGB (XAGB) stars. Five of
the sources are found to have luminosities above the classical AGB limit
(M 54,000 L), which classifies them as probably
red supergiants (RSGs). Almost all of the sources are classified as oxygen
rich. As also seen in the LMC, a significant fraction of the dust in M33 is
produced by a handful of XAGB and RSG stars.Comment: 36 pages, 14 figures, 4 tables, Accepted for publication in A
PT-symmetry from Lindblad dynamics in a linearized optomechanical system
We analyze a lossy linearized optomechanical system in the red-detuned regime under the rotating wave approximation. This so-called optomechanical state transfer protocol provides effective lossy frequency converter (quantum beam-splitter-like) dynamics where the strength of the coupling between the electromagnetic and mechanical modes is controlled by the optical steady-state amplitude. By restricting to a subspace with no losses, we argue that the transition from mode-hybridization in the strong coupling regime to the damped-dynamics in the weak coupling regime, is a signature of the passive parity-time (PT) symmetry breaking transition in the underlying non-Hermitian quantum dimer. We compare the dynamics generated by the quantum open system (Langevin or Lindblad) approach to that of the PT-symmetric Hamiltonian, to characterize the cases where the two are identical. Additionally, we numerically explore the evolution of separable and correlated number states at zero temperature as well as thermal initial state evolution at room temperature. Our results provide a pathway for realizing non-Hermitian Hamiltonians in optomechanical systems at a quantum level
Early Dust Formation and a Massive Progenitor for SN 2011ja?
SN 2011ja was a bright (I = -18.3) Type II supernova occurring in the nearby
edge on spiral galaxy NGC 4945. Flat-topped and multi-peaked H-alpha and H-beta
spectral emission lines appear between 64 - 84 days post-explosion, indicating
interaction with a disc-like circumstellar medium inclined 30-45 degrees from
edge-on. After day 84 an increase in the H- and K-band flux along with heavy
attenuation of the red wing of the emission lines are strong indications of
early dust formation, likely located in the cool dense shell created between
the forward shock of the SN ejecta and the reverse shock created as the ejecta
plows into the existing CSM. Radiative transfer modeling reveals both ~1.5 x
10^-4 Msun of pre-existing dust located ~ 10^16.7 cm away and ~ 5 x 10^-5 Msun
of newly formed dust. Spectral observations after 1.5 years reveal the
possibility that the fading SN is located within a young (3-6 Myr) massive
stellar cluster, which when combined with tentative 56Ni mass estimates of 0.2
Msun may indicate a massive (> 25 Msun) progenitor for SN 2011ja.Comment: 13 pages, 8 figures, submitted to MNRAS awaiting final referee repor
Detection of the S(1) Rotational Line of H2 toward IRC+10216: A Simultaneous Measurement of Mass-Loss Rate and CO Abundance
We report the first detection of the S(1) pure rotational line of ortho-H2 at
17.04 um in an asymptotic giant branch star, using observations of IRC+10216
with the Echelon-cross-echelle Spectrograph (EXES) mounted on the Stratospheric
Observatory for Infrared Astronomy (SOFIA). This line, which was observed in a
very high sensitivity spectrum (RMS noise ~0.04% of the continuum), was
detected in the wing of a strong telluric line and displayed a P Cygni profile.
The spectral ranges around the frequencies of the S(5) and S(7) ortho-H2
transitions were observed as well but no feature was detected in spectra with
sensitivities of 0.12% and 0.09% regarding the continuum emission,
respectively. We used a radiation transfer code to model these three lines and
derived a mass-loss rate of 2.43(0.21)E-05 M_sun/yr without using the CO
abundance. The comparison of this rate with previous estimates derived from CO
observations suggests that the CO abundance relative to H2 is 6.7(1.4)E-04.
From this quantity and previously reported molecular abundances, we estimate
the O/H and C/H ratios to be 3.3(0.7)E-04 and >5.2(0.9)E-04, respectively. The
C/O ratio is >1.5(0.4). The absence of the S(5) and S(7) lines of ortho-H2 in
our observations can be explained by the opacity of hot dust within 5R* from
the center of the star. We estimate the intensity of the S(0) and S(2) lines of
para-H2 to be ~0.1% and 0.2% of the continuum, respectively, which are below
the detection limit of EXES.Comment: 7 pages, 4 figures, accepted for publication in ApJ Letter
Non-standard grain properties, dark gas reservoir, and extended submillimeter excess, probed by Herschel in the Large Magellanic Cloud
Context. Herschel provides crucial constraints on the IR SEDs of galaxies, allowing unprecedented accuracy on the dust mass estimates. However, these estimates rely on non-linear models and poorly-known optical properties.
Aims. In this paper, we perform detailed modelling of the Spitzer and Herschel observations of the LMC, in order to: (i) systematically study the uncertainties and biases affecting dust mass estimates; and to (ii) explore the peculiar ISM properties of the LMC.
Methods. To achieve these goals, we have modelled the spatially resolved SEDs with two alternate grain compositions, to study the impact of different submillimetre opacities on the dust mass. We have rigorously propagated the observational errors (noise and calibration) through the entire fitting process, in order to derive consistent parameter uncertainties.
Results. First, we show that using the integrated SED leads to underestimating the dust mass by ≃50% compared to the value obtained with sufficient spatial resolution, for the region we studied. This might be the case, in general, for unresolved galaxies. Second, we show that Milky Way type grains produce higher gas-to-dust mass ratios than what seems possible according to the element abundances in the LMC. A spatial analysis shows that this dilemma is the result of an exceptional property: the grains of the LMC have on average a larger intrinsic submm opacity (emissivity index β ≃ 1.7 and opacity κ_(abs)(160 μm) = 1.6 m^2 kg^(-1)) than those of the Galaxy. By studying the spatial distribution of the gas-to-dust mass ratio, we are able to constrain the fraction of unseen gas mass between ≃10, and ≃100% and show that it is not sufficient to explain the gas-to-dust mass ratio obtained with Milky Way type grains. Finally, we confirm the detection of a 500 μm extended emission excess with an average relative amplitude of ≃15%, varying up to 40%. This excess anticorrelates well with the dust mass surface density. Although we do not know the origin of this excess, we show that it is unlikely the result of very cold dust, or CMB fluctuations
Biocatalytic Synthesis of Polyglycerol Polyricinoleate: A Comparison of Different Commercial Lipases
This paper describes the studies carried out to select the most suitable lipase as catalyst for the esterification of polyglycerol with polyricinoleic acid to yield polyglicerol polyricinoleate (PGPR), a value-added, bio-based food emulsifier. The enzymes assayed
were lipases from Rhizopus arrhizus, Rhizopus oryzae and Mucor javanicus, previously selected because of their suitable activity and moderate cost. First, the reaction was catalyzed by free lipases in a batch reactor and the influence of different operating conditions
(initial water content, amount of enzyme and temperature) on the progress of the reaction was studied. Next, the three lipases were immobilized by physical adsorption on the anion exchange resin, Lewatit MonoPlus MP 64, providing derivatives with a high activity and stability. Recovery of the immobilized derivative from the reaction medium was conducted with very good yields (≥ 99 %) and no loss of activity of the derivative with successive uses was proved. Finally, a high performance reactor, operating at low pressure and a dry atmosphere, was used to synthesise PGPR using the immobilized enzymes. Both Rhizopus arrhizus and Rhizopus oryzae lipases allowed the production of a PGPR which fulfils the “specific purity criteria on food additives other than colours and sweeteners” established by the Commission of the European Communities (AV ≤ 6 mg KOH/g), with an acid value of 4.91 and 5.31 mg KOH/g respectively
- …