2,204 research outputs found
An Investigation of the Biosynthesis of Citromycetin in PenicWium frequentans using IT-and I4C-Labelled Precursors
The alternating oxygenation pattern of the carbon skeleton of these metabolites suggests a polyketide biosynthesis but the unusual branched structure cannot be derived straightforwardly from a single unbranched polyketone chain. The biosynthesis of citromycetin (1) has therefore been the subject of much speculation. Four representative proposals which have guided our planning in the present investigation are presented in Scheme 1. Pathways A and B are 'two chain pathways,'6 according to which the citromycetin skeleton might be formed by the combination of two polyketide intermediates, each formed separately from a different polyketide chain. Pathways C7 and D8 are of a more conventional type in which the skeleton might arise by modification of an intermediate derived from a single chain. The various structural types represented by intermediates (5)-(lo), are well represented among polyketide natural products, with the notable exception of compound (7), which is unusual in having its carboxy terminus as the longer of the two uncyclised residues attached to its aromatic ring.g For clarity, it is assumed in Scheme 1 that a compound such as (ll), an uncyclised analogue of (l), is formed as an intermediate
Sexual Differentiation of the Surge Mode of Gonadotropin Secretion: Prenatal Androgens Abolish the GonadotropinâReleasing Hormone Surge in the Sheep
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106142/1/j.1365-2826.1996.tb00698.x.pd
Protostellar holes: Spitzer Space Telescope observations of the protostellar binary IRAS16293-2422
Mid-infrared (23-35 micron) emission from the deeply embedded "Class 0"
protostar IRAS16293-2422 is detected with the Spitzer Space Telescope infrared
spectrograph. A detailed radiative transfer model reproducing the full spectral
energy distribution (SED) from 23 micron to 1.3 mm requires a large inner
cavity of radius 600 AU in the envelope to avoid quenching the emission from
the central sources. This is consistent with a previous suggestion based on
high angular resolution millimeter interferometric data. An alternative
interpretation using a 2D model of the envelope with an outflow cavity can
reproduce the SED but not the interferometer visibilities. The cavity size is
comparable to the centrifugal radius of the envelope and therefore appears to
be a natural consequence of the rotation of the protostellar core, which has
also caused the fragmentation leading to the central protostellar binary. With
a large cavity such as required by the data, the average temperature at a given
radius does not increase above 60-80 K and although hot spots with higher
temperatures may be present close to each protostar, these constitute a small
fraction of the material in the inner envelope. The proposed cavity will also
have consequences for the interpretation of molecular line data, especially of
complex species probing high temperatures in the inner regions of the envelope.Comment: Accepted for publication in ApJ Letter
From Molecular Cores to Planet-forming Disks: An SIRTF Legacy Program
Crucial steps in the formation of stars and planets can be studied only at midâ to farâinfrared wavelengths, where the Space Infrared Telescope (SIRTF) provides an unprecedented improvement in sensitivity. We will use all three SIRTF instruments (Infrared Array Camera [IRAC], Multiband Imaging Photometer for SIRTF [MIPS], and Infrared Spectrograph [IRS]) to observe sources that span the evolutionary sequence from molecular cores to protoplanetary disks, encompassing a wide range of cloud masses, stellar masses, and starâforming environments. In addition to targeting about 150 known compact cores, we will survey with IRAC and MIPS (3.6â70 Îźm) the entire areas of five of the nearest large molecular clouds for new candidate protostars and substellar objects as faint as 0.001 solar luminosities. We will also observe with IRAC and MIPS about 190 systems likely to be in the early stages of planetary system formation (ages up to about 10 Myr), probing the evolution of the circumstellar dust, the raw material for planetary cores. Candidate planetâforming disks as small as 0.1 lunar masses will be detectable. Spectroscopy with IRS of new objects found in the surveys and of a select group of known objects will add vital information on the changing chemical and physical conditions in the disks and envelopes. The resulting data products will include catalogs of thousands of previously unknown sources, multiwavelength maps of about 20 deg^2 of molecular clouds, photometry of about 190 known young stars, spectra of at least 170 sources, ancillary data from groundâbased telescopes, and new tools for analysis and modeling. These products will constitute the foundations for many followâup studies with groundâbased telescopes, as well as with SIRTF itself and other space missions such as SIM, JWST, Herschel, and TPF/Darwin
The Double Dust Envelopes of R Coronae Borealis Stars
The study of extended, cold dust envelopes surrounding R Coronae Borealis (RCB) stars began with their discovery by the Infrared Astronomical Satellite. RCB stars are carbon-rich supergiants characterized by their extreme hydrogen deficiency and their irregular and spectacular declines in brightness (up to 9 mag). We have analyzed new and archival Spitzer Space Telescope and Herschel Space Observatory data of the envelopes of seven RCB stars to examine the morphology and investigate the origin of these dusty shells. Herschel, in particular, has revealed the first-ever bow shock associated with an RCB star with its observations of SU Tauri. These data have allowed the assembly of the most comprehensive spectral energy distributions (SEDs) of these stars with multiwavelength data from the ultraviolet to the submillimeter. Radiative transfer modeling of the SEDs implies that the RCB stars in this sample are surrounded by an inner warm (up to 1200 K) and an outer cold (up to 200 K) envelope. The outer shells are suggested to contain up to 10-3 M o of dust and have existed for up to 105 years depending on the expansion rate of the dust. This age limit indicates that these structures have most likely been formed during the RCB phase
The Spitzer c2d survey of Large, Nearby, Interstellar Clouds. V. Chamaeleon II Observed with IRAC
We present IRAC (3.6, 4.5, 5.8, and 8.0 micron) observations of the
Chamaeleon II molecular cloud. The observed area covers about 1 square degree
defined by . Analysis of the data in the 2005 c2d catalogs reveals a
small number of sources (40) with properties similar to those of young
stellaror substellar objects (YSOs). The surface density of these YSO
candidates is low, and contamination by background galaxies appears to be
substantial, especially for sources classified as Class I or flat SED. We
discuss this problem in some detail and conclude that very few of the candidate
YSOs in early evolutionary stages are actually in the Cha II cloud. Using a
refined set of criteria, we define a smaller, but more reliable, set of 24 YSO
candidates.Comment: 19 pages, 10 figures, in press Ap
The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. VIII. Serpens Observed with MIPS
We present maps of 1.5 deg^2 of the Serpens dark cloud at 24, 70, and 160 Îźm observed with the Spitzer Space Telescope MIPS camera. We describe the observations and briefly discuss the data processing carried out by the c2d team on these data. More than 2400 compact sources have been extracted at 24 Îźm, nearly 100 at 70 Îźm, and four at 160 Îźm. We estimate completeness limits for our 24 Îźm survey from Monte Carlo tests with artificial sources inserted into the Spitzer maps. We compare source counts, colors, and magnitudes in the Serpens cloud to two reference data sets: a 0.50 deg^2 set on a low-extinction region near the dark cloud, and a 5.3 deg^2 subset of the SWIRE ELAIS N1 data that was processed through our pipeline. These results show that there is an easily identifiable population of young stellar object candidates in the Serpens cloud that is not present in either of the reference data sets. We also show a comparison of visual extinction and cool dust emission illustrating a close correlation between the two and find that the most embedded YSO candidates are located in the areas of highest visual extinction
The Spitzer c2d Survey of Weak-line T Tauri Stars II: New Constraints on the Timescale for Planet Building
One of the central goals of the Spitzer Legacy Project ``From Molecular Cores
to Planet-forming Disks'' (c2d) is to determine the frequency of remnant
circumstellar disks around weak-line T Tauri stars (wTTs) and to study the
properties and evolutionary status of these disks. Here we present a census of
disks for a sample of over 230 spectroscopically identified wTTs located in the
c2d IRAC (3.6, 4.5, 4.8, and 8.0 um) and MIPS (24 um) maps of the Ophiuchus,
Lupus, and Perseus Molecular Clouds. We find that ~20% of the wTTs in a
magnitude limited subsample have noticeable IR-excesses at IRAC wavelengths
indicating the presence of a circumstellar disk. The disk frequencies we find
in these 3 regions are ~3-6 times larger than that recently found for a sample
of 83 relatively isolated wTTs located, for the most part, outside the highest
extinction regions covered by the c2d IRAC and MIPS maps. The disk fractions we
find are more consistent with those obtained in recent Spitzer studies of wTTs
in young clusters such as IC 348 and Tr 37. From their location in the H-R
diagram, we find that, in our sample, the wTTs with excesses are among the
younger part of the age distribution. Still, up to ~50% of the apparently
youngest stars in the sample show no evidence of IR excess, suggesting that the
circumstellar disks of a sizable fraction of pre-main-sequence stars dissipate
in a timescale of ~1 Myr. We also find that none of the stars in our sample
apparently older than ~10 Myrs have detectable circumstellar disks at
wavelengths < 24 um. Also, we find that the wTTs disks in our sample exhibit a
wide range of properties (SED morphology, inner radius, L_DISK/L*, etc) which
bridge the gaps observed between the cTTs and the debris disk regimes.Comment: 54 pages, 13 figures, Accepted by Ap
Resident macrophages influence stem cell activity in the mammary gland
Introduction Macrophages in the mammary gland are essential for morphogenesis of the ductal epithelial tree and have been implicated in promoting breast tumor metastasis. Although it is well established that macrophages influence normal mammopoiesis, the mammary cell types that these accessory cells influence have not been determined. Here we have explored a role for macrophages in regulating mammary stem cell (MaSC) activity, by assessing the ability of MaSCs to reconstitute a mammary gland in a macrophage-depleted fat pad. Methods Two different in vivo models were used to deplete macrophages from the mouse mammary fat pad, allowing us to examine the effect of macrophage deficiency on the mammary repopulating activity of MaSCs. Both the Csf1(op/op) mice and clodronate liposome-mediated ablation models entailed transplantation studies using the MaSC-enriched population. Results We show that mammary repopulating ability is severely compromised when the wild-type MaSC-enriched subpopulation is transplanted into Csf1(op/op) fat pads. In reciprocal experiments, the MaSC-enriched subpopulation from Csf1(op/op) glands had reduced regenerative capacity in a wildtype environment. Utilizing an alternative strategy for selective depletion of macrophages from the mammary gland, we demonstrate that co-implantation of the MaSC-enriched subpopulation with clodronate-liposomes leads to a marked decrease in repopulating frequency and outgrowth potential. Conclusions Our data reveal a key role for mammary gland macrophages in supporting stem/progenitor cell function and suggest that MaSCs require macrophage-derived factors to be fully functional. Macrophages may therefore constitute part of the mammary stem cell nich
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