38 research outputs found
Dust composition and mass-loss return from the luminous blue variable R71 in the LMC
We present an analysis of mid-and far-infrared (IR) spectrum and spectral
energy distribution (SED) of the LBV R71 in the LMC.This work aims to
understand the overall contribution of high-mass LBVs to the total dust-mass
budget of the interstellar medium (ISM) of the LMC and compare this with the
contribution from low-mass asymptotic giant branch (AGB) stars. As a case
study, we analyze the SED of R71. We compiled all the available photometric and
spectroscopic observational fluxes from various telescopes for a wide
wavelength range (0.36 -- 250\,m). We determined the dust composition from
the spectroscopic data, and derived the ejected dust mass, dust mass-loss rate,
and other dust shell properties by modeling the SED of R71. We noted nine
spectral features in the dust shell of R71 by analyzing Spitzer spectroscopic
data. Among these, we identified three new crystalline silicate features. We
computed our model spectrum by using 3D radiative transfer code MCMax. Our
model calculation shows that dust is dominated by amorphous silicates, with
some crystalline silicates, metallic iron, and a very tiny amount of polycyclic
aromatic hydrocarbon (PAH) molecules. The presence of both silicates and PAHs
indicates that the dust has a mixed chemistry. We derived a dust mass of 0.01
M, from which we arrive at a total ejected mass of 5
M. This implies a time-averaged dust mass-loss rate of
2.510 M\,yr with an explosion about 4000 years
ago. We assume that the other five confirmed dusty LBVs in the LMC loose mass
at a similar rate, and estimate the total contribution to the mass budget of
the LMC to be 10 M\,yr, which is comparable to
the contribution by all the AGB stars in the LMC. Based on our analysis on R71,
we speculate that LBVs as a class may be an important dust source in the ISM of
the LMC.Comment: 10 pages, 6 figures, 2 table
Bioinformatics and Functional Analysis of an Entamoeba histolytica Mannosyltransferase Necessary for Parasite Complement Resistance and Hepatical Infection
The glycosylphosphatidylinositol (GPI) moiety is one of the ways by which many cell surface proteins, such as Gal/GalNAc lectin and proteophosphoglycans (PPGs) attach to the surface of Entamoeba histolytica, the agent of human amoebiasis. It is believed that these GPI-anchored molecules are involved in parasite adhesion to cells, mucus and the extracellular matrix. We identified an E. histolytica homolog of PIG-M, which is a mannosyltransferase required for synthesis of GPI. The sequence and structural analysis led to the conclusion that EhPIG-M1 is composed of one signal peptide and 11 transmembrane domains with two large intra luminal loops, one of which contains the DXD motif, involved in the enzymatic catalysis and conserved in most glycosyltransferases. Expressing a fragment of the EhPIG-M1 encoding gene in antisense orientation generated parasite lines diminished in EhPIG-M1 levels; these lines displayed reduced GPI production, were highly sensitive to complement and were dramatically inhibited for amoebic abscess formation. The data suggest a role for GPI surface anchored molecules in the survival of E. histolytica during pathogenesis
Structural characterization of a partially arabinosylated lipoarabinomannan variant isolated from a Corynebacterium glutamicum ubiA mutant
Arabinan polysaccharide side-chains are present in both Mycobacterium tuberculosis and Corynebacterium glutamicum in the heteropolysaccharide arabinogalactan (AG), and in M. tuberculosis in the lipoglycan lipoarabinomannan (LAM). This study shows by quantitative sugar and glycosyl linkage analysis that C. glutamicum possesses a much smaller LAM version, Cg-LAM, characterized by single t-Araf residues linked to the α(1→6)-linked mannan backbone. MALDI-TOF MS showed an average molecular mass of 13 800–15 400 Da for Cg-LAM. The biosynthetic origin of Araf residues found in the extracytoplasmic arabinan domain of AG and LAM is well known to be provided by decaprenyl-monophosphoryl-d-arabinose (DPA). However, the characterization of LAM in a C. glutamicum : : ubiA mutant devoid of prenyltransferase activity and devoid of DPA-dependent arabinan deposition into AG revealed partial formation of LAM, albeit with a slightly altered molecular mass. These data suggest that in addition to DPA utilization as an Araf donor, alternative pathways exist in Corynebacterianeae for Araf delivery, possibly via an unknown sugar nucleotide
The Dust Properties of Two Hot R Coronae Borealis Stars and a Wolf-Rayet Central Star of a Planetary Nebula: in Search of a Possible Link
We present new Spitzer/IRS spectra of two hot R Coronae Borealis (RCB) stars,
one in the Galaxy,V348 Sgr, and one lying in the LMC, HV 2671. These two
objects may constitute a link between the RCB stars and the late Wolf-Rayet
([WCL]) class of central stars of planetary nebula (CSPNe) such as CPD -56 8032
that has little or no hydrogen in their atmospheres. HV 2671 and V348 Sgr are
members of a rare subclass that has significantly higher effective temperatures
than most RCB stars, but sharing the traits of hydrogen deficiency and dust
formation that define the cooler RCB stars. The [WC] CSPNe star, CPD -56 8032,
displays evidence for dual-dust chemistry showing both PAHs and crystalline
silicates in its mid-IR spectrum. HV 2671 shows strong PAH emission but shows
no sign of having crystalline silicates. The spectrum of V348 Sgr is very
different from those of CPD -56 8032 and HV 2671. The PAH emission seen
strongly in the other two stars is not present. Instead, the spectrum is
dominated by a broad emission centered at about 8.2 micron. The mid-IR spectrum
of CPD -56 8032 shows emission features that may be associated with C60. The
other two stars do not show evidence for C60. HV 2671 has also been detected by
Herschel/PACS and SPIRE. V348 Sgr and CPD -56 8032 have been detected by
AKARI/FIS. These data were combined with Spitzer, IRAS, 2MASS and other
photometry to produce their spectral energy distributions from the visible to
the far-IR. Monte Carlo radiative transfer modeling was used to study the
circumstellar dust around these stars. HV 2671 and CPD -56 8032 require both a
flared inner disk with warm dust and an extended diffuse envelope with cold
dust to to fit their SEDs. The SED of V348 Sgr can be fit with a much smaller
disk and envelope.Comment: 20 pages, 5 figures, accepted for publication in The Astronomical
Journa
Approachability in Stackelberg Stochastic Games with Vector Costs
The notion of approachability was introduced by Blackwell [1] in the context
of vector-valued repeated games. The famous Blackwell's approachability theorem
prescribes a strategy for approachability, i.e., for `steering' the average
cost of a given agent towards a given target set, irrespective of the
strategies of the other agents. In this paper, motivated by the multi-objective
optimization/decision making problems in dynamically changing environments, we
address the approachability problem in Stackelberg stochastic games with vector
valued cost functions. We make two main contributions. Firstly, we give a
simple and computationally tractable strategy for approachability for
Stackelberg stochastic games along the lines of Blackwell's. Secondly, we give
a reinforcement learning algorithm for learning the approachable strategy when
the transition kernel is unknown. We also recover as a by-product Blackwell's
necessary and sufficient condition for approachability for convex sets in this
set up and thus a complete characterization. We also give sufficient conditions
for non-convex sets.Comment: 18 Pages, Submitted to Dynamic Games and Application