6,039 research outputs found
The Unusual Substrate Specificity of a Virulence Associated Serine Hydrolase from the Highly Toxic Bacterium, \u3cem\u3eFrancisella tularensis\u3c/em\u3e
Francisella tularensis is the causative agent of the highly, infectious disease, tularemia. Amongst the genes identified as essential to the virulence of F. tularensis was the proposed serine hydrolase FTT0941c. Herein, we purified FTT0941c to homogeneity and then characterized the folded stability, enzymatic activity, and substrate specificity of FTT0941c. Based on phylogenetic analysis, FTT0941c was classified within a divergent Francisella subbranch of the bacterial hormone sensitive lipase (HSL) superfamily, but with the conserved sequence motifs of a bacterial serine hydrolase. FTT0941c showed broad hydrolase activity against diverse libraries of ester substrates, including significant hydrolytic activity across alkyl ester substrates from 2 to 8 carbons in length. Among a diverse library of fluorogenic substrates, FTT0941c preferred α-cyclohexyl ester substrates, matching with the substrate specificity of structural homologues and the broad open architecture of its modeled binding pocket. By substitutional analysis, FTT0941c was confirmed to have a classic catalytic triad of Ser115, His278, and Asp248 and to remain thermally stable even after substitution. Its overall substrate specificity profile, divergent phylogenetic homology, and preliminary pathway analysis suggested potential biological functions for FTT0941c in diverse metabolic degradation pathways in F. tularensis
Characterizing star formation activity in infrared dark cloud MSXDC G048.65-00.29
Infrared Dark Clouds (IRDCs), condensed regions of the ISM with high column
densities, low temperatures and high masses, are suspected sites of star
formation. Thousands of IRDCs have already been identified. To date, it has not
been resolved whether IRDCs always show star formation activity and, if so, if
massive star formation (> 8 solar masses) is the rule or the exception in
IRDCs. Previous analysis of sub-millimeter cores in the cloud MSXDC
G048.65-00.29 (G48.65) indicates embedded star formation activity. To
characterize this activity in detail, mid-infrared photometry (3-30 micron) has
been obtained with the Spitzer Space Telescope. This paper analyzes the point
sources seen in the 24 micron band, combined with counterparts or upper limits
at shorter and longer wavelengths. Data points in wavelength bands ranging from
1 up to 850 micron are used to compare each 24 micron source to a set of
Spectral Energy Distributions of Young Stellar Object (YSO) models. By
assessing the models that fit the data, an attempt is made to identify YSOs as
such and determine their evolutionary stages and stellar masses. A total of 17
sources are investigated, 13 of which are classified as YSOs, primarily - but
not exclusively - in an early embedded phase of star formation. The modeled
masses of the central stellar objects range from sub-solar to ~8 solar masses.
Every YSO is at less than 1 pc projected distance from its nearest YSO
neighbor. We conclude that IRDC G48.65 is a region of active star formation. We
find YSOs in various evolutionary phases, indicating that the star formation in
this cloud is not an instantaneous process. The inferred masses of the central
objects suggest that this IRDC hosts only low to intermediate mass YSOs and
none with masses exceeding ~8 solar masses.Comment: 10 pages, 6 figures; v2: minor editorial changes to match published
versio
Herschel Observations of a Newly Discovered UX Ori Star in the Large Magellanic Cloud
The LMC star, SSTISAGE1C J050756.44-703453.9, was first noticed during a
survey of EROS-2 lightcurves for stars with large irregular brightness
variations typical of the R Coronae Borealis (RCB) class. However, the visible
spectrum showing emission lines including the Balmer and Paschen series as well
as many Fe II lines is emphatically not that of an RCB star. This star has all
of the characteristics of a typical UX Ori star. It has a spectral type of
approximately A2 and has excited an H II region in its vicinity. However, if it
is an LMC member, then it is very luminous for a Herbig Ae/Be star. It shows
irregular drops in brightness of up to 2 mag, and displays the reddening and
"blueing" typical of this class of stars. Its spectrum, showing a combination
of emission and absorption lines, is typical of a UX Ori star that is in a
decline caused by obscuration from the circumstellar dust. SSTISAGE1C
J050756.44-703453.9 has a strong IR excess and significant emission is present
out to 500 micron. Monte Carlo radiative transfer modeling of the SED requires
that SSTISAGE1C J050756.44-703453.9 has both a dusty disk as well as a large
extended diffuse envelope to fit both the mid- and far-IR dust emission. This
star is a new member of the UX Ori subclass of the Herbig Ae/Be stars and only
the second such star to be discovered in the LMC.Comment: ApJ, in press. 9 pages, 5 figure
Radio Astronomical Polarimetry and the Lorentz Group
In radio astronomy the polarimetric properties of radiation are often
modified during propagation and reception. Effects such as Faraday rotation,
receiver cross-talk, and differential amplification act to change the state of
polarized radiation. A general description of such transformations is useful
for the investigation of these effects and for the interpretation and
calibration of polarimetric observations. Such a description is provided by the
Lorentz group, which is intimately related to the transformation properties of
polarized radiation. In this paper the transformations that commonly arise in
radio astronomy are analyzed in the context of this group. This analysis is
then used to construct a model for the propagation and reception of radio
waves. The implications of this model for radio astronomical polarimetry are
discussed.Comment: 10 pages, accepted for publication in Astrophysical Journa
The youngest massive protostars in the Large Magellanic Cloud
We demonstrate the unique capabilities of Herschel to study very young
luminous extragalactic young stellar objects (YSOs) by analyzing a central
strip of the Large Magellanic Cloud obtained through the HERITAGE Science
Demonstration Program. We combine PACS 100 and 160, and SPIRE 250, 350, and 500
microns photometry with 2MASS (1.25-2.17 microns) and Spitzer IRAC and MIPS
(3.6-70 microns) to construct complete spectral energy distributions (SEDs) of
compact sources. From these, we identify 207 candidate embedded YSOs in the
observed region, ~40% never-before identified. We discuss their position in
far-infrared color-magnitude space, comparing with previously studied,
spectroscopically confirmed YSOs and maser emission. All have red colors
indicating massive cool envelopes and great youth. We analyze four example
YSOs, determining their physical properties by fitting their SEDs with
radiative transfer models. Fitting full SEDs including the Herschel data
requires us to increase the size and mass of envelopes included in the models.
This implies higher accretion rates (greater than or equal to 0.0001 M_sun/yr),
in agreement with previous outflow studies of high-mass protostars. Our results
show that Herschel provides reliable longwave SEDs of large samples of
high-mass YSOs; discovers the youngest YSOs whose SEDs peak in Herschel bands;
and constrains the physical properties and evolutionary stages of YSOs more
precisely than was previously possible.Comment: Main text: 4 pages, 3 figures, 1 table; Online material: 3 figures, 1
table; to appear in the A&A Herschel Special Issu
Electrical Control of Linear Dichroism in Black Phosphorus from the Visible to Mid-Infrared
The incorporation of electrically tunable materials into photonic structures
such as waveguides and metasurfaces enables dynamic control of light
propagation by an applied potential. While many materials have been shown to
exhibit electrically tunable permittivity and dispersion, including transparent
conducting oxides (TCOs) and III-V semiconductors and quantum wells, these
materials are all optically isotropic in the propagation plane. In this work,
we report the first known example of electrically tunable linear dichroism,
observed here in few-layer black phosphorus (BP), which is a promising
candidate for multi-functional, broadband, tunable photonic elements. We
measure active modulation of the linear dichroism from the mid-infrared to
visible frequency range, which is driven by anisotropic quantum-confined Stark
and Burstein-Moss effects, and field-induced forbidden-to-allowed optical
transitions. Moreover, we observe high BP absorption modulation strengths,
approaching unity for certain thicknesses and photon energies
Mid-Infrared Variability of protostars in IC 1396A
We have used Spitzer/IRAC to conduct a photometric monitoring program of the
IC1396A dark globule in order to study the mid-IR (3.6 - 8 micron) variability
of the heavily embedded Young Stellar Objects (YSOs) present in that area. We
obtained light curves covering a 14 day timespan with a twice daily cadence for
69 YSOs, and continuous light curves with approximately 12 second cadence over
7 hours for 38 YSOs. Typical accuracies for our relative photometry were 1-2%
for the long timespan data and a few mmag, corresponding to less than 0.5%, for
the 7 hour continuous "staring-mode" data. More than half of the YSOs showed
detectable variability, with amplitudes from ~0.05 mag to ~0.2 mag. About
thirty percent of the YSOs showed quasi-sinusoidal light curve shapes with
apparent periods from 5-12 days and light curve amplitudes approximately
independent of wavelength over the IRAC bandpasses. We have constructed models
which simulate the time dependent spectral energy distributions of Class I and
I I YSOs in order to attempt to explain these light curves. Based on these
models, the apparently periodic light curves are best explained by YSO models
where one or two high latitude photospheric spots heat the inner wall of the
circumstellar disk, and where we view the disk at fairly large inclination
angle. Disk inhomogeneities, such as increasing the height where the accretion
funnel flows to the stellar hotspot, enhances the light curve modulations. The
other YSOs in our sample show a range of light curve shapes, some of which are
probably due to varying accretion rate or disk shadowing events. One star,
IC1396A-47, shows a 3.5 hour periodic light curve; this object may be a PMS
Delta Scuti star
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