160 research outputs found
Small Bites: Star formation recipes in extreme dwarfs
We study the relationship between the gas column density (Sigma_HI) and the
star formation rate surface density (Sigma_SFR) for a sample of extremely small
(M_B ~ -13, Delta V_50 ~ 30 km/s) dwarf irregular galaxies. We find a clear
stochasticity in the relation between the gas column density and star
formation. All gas with Sigma_HI >~ 10 M_sun/pc^2 has some ongoing star
formation, but the fraction of gas with ongoing star formation decreases as the
gas column density decreases, and falls to about 50% at Sigma_HI ~ 3
M_sun/pc^2. Further, even for the most dense gas, the star formation efficiency
is at least a factor of ~ 2 smaller than typical of star forming regions in
spirals. We also find that the ratio of H-alpha emission to FUV emission
increases with increasing gas column density. This is unlikely to be due to
increasing dust extinction because the required dust to gas ratios are too
high. We suggest instead that this correlation arises because massive (i.e.
H-alpha producing) stars are formed preferentially in regions with high gas
density.Comment: 5 pages, 5 figures, 1 table. Accepted for publication in MNRAS
Letters. The definitive version is available at www.blackwell- synergy.co
The Recent Star Formation in NGC 6822: an Ultraviolet Study
We characterize the star formation in the low-metallicity galaxy NGC 6822
over the past few hundred million years, using GALEX far-UV (FUV, 1344-1786 A)
and near-UV (NUV, 1771-2831 A) imaging, and ground-based Ha imaging. From GALEX
FUV image, we define 77 star-forming (SF) regions with area >860 pc^2, and
surface brightness <=26.8 mag(AB)arcsec^-2, within 0.2deg (1.7kpc) of the
center of the galaxy. We estimate the extinction by interstellar dust in each
SF region from resolved photometry of the hot stars it contains: E(B-V) ranges
from the minimum foreground value of 0.22mag up to 0.66+-0.21mag. The
integrated FUV and NUV photometry, compared with stellar population models,
yields ages of the SF complexes up to a few hundred Myr, and masses from 2x10^2
Msun to 1.5x10^6 Msun. The derived ages and masses strongly depend on the
assumed type of interstellar selective extinction, which we find to vary across
the galaxy. The total mass of the FUV-defined SF regions translates into an
average star formation rate (SFR) of 1.4x10^-2 Msun/yr over the past 100 Myr,
and SFR=1.0x10^-2 Msun/yr in the most recent 10 Myr. The latter is in agreement
with the value that we derive from the Ha luminosity, SFR=0.008 Msun/yr. The
SFR in the most recent epoch becomes higher if we add the SFR=0.02 Msun/yr
inferred from far-IR measurements, which trace star formation still embedded in
dust (age <= a few Myr).Comment: Accepted for publication in ApJ, 21 pages, 6 figures, 3 table
Candidate X-ray-Emitting OB Stars in the Carina Nebula Identified Via Infrared Spectral Energy Distributions
We report the results of a new survey of massive, OB stars throughout the
Carina Nebula using the X-ray point source catalog provided by the Chandra
Carina Complex Project (CCCP) in conjunction with infrared (IR) photometry from
the Two Micron All-Sky Survey and the Spitzer Space Telescope Vela--Carina
survey. Mid-IR photometry is relatively unaffected by extinction, hence it
provides strong constraints on the luminosities of OB stars, assuming that
their association with the Carina Nebula, and hence their distance, is
confirmed. We fit model stellar atmospheres to the optical (UBV) and IR
spectral energy distributions (SEDs) of 182 OB stars with known spectral types
and measure the bolometric luminosity and extinction for each star. We find
that the extinction law measured toward the OB stars has two components:
Av=1--1.5 mag produced by foreground dust with a ratio of total-to-selective
absorption Rv=3.1 plus a contribution from local dust with Rv>4.0 in the Carina
molecular clouds that increases as Av increases. Using X-ray emission as a
strong indicator of association with Carina, we identify 94 candidate OB stars
with Lbol\geq10^4 Lsun by fitting their IR SEDs. If the candidate OB stars are
eventually confirmed by follow-up spectroscopic observations, the number of
cataloged OB stars in the Carina Nebula will increase by ~50%. Correcting for
incompleteness due to OB stars falling below the Lbol cutoff or the CCCP
detection limit, these results potentially double the size of the young massive
stellar population.Comment: 19 pages, 8 figures, accepted for the ApJS Special Issue on the
Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011.
All 16 CCCP Special Issue papers, including a version of this article with
high-quality figures, are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html (through 2011
at least
The Massive Star Content of NGC 3603
We investigate the massive star content of NGC 3603, the closest known giant
H II region. We have obtained spectra of 26 stars in the central cluster using
the Baade 6.5-m telescope (Magellan I). Of these 26 stars, 16 had no previous
spectroscopy. We also obtained photometry of all of the stars with previous or
new spectroscopy, primarily using archival HST ACS/HRC images. We use these
data to derive an improved distance to the cluster, and to construct an H-R
diagram for discussing the masses and ages of the massive star content of this
cluster.Comment: Accepted by the Astronomical Journal. This revision updates the
coordinates in Table 1 by (-0.18sec, +0.2") to place them on the UCAC2 syste
Case Report: Insulin hypersensitivity in youth with type 1 diabetes
OBJECTIVE: Immediate type I, type III, and delayed type IV hypersensitivity reactions to insulin are rare, but potentially serious complications of exogenous insulin administration required for the treatment of type 1 diabetes (T1D).
METHODS: We present four cases of insulin hypersensitivity reactions occurring in youth with T1D and a literature review of this topic.
RESULTS: Insulin hypersensitivity reactions included types I, III, and IV with presentations ranging from localized urticaria, erythematous nodules, and eczematous plaques to anaphylaxis with respiratory distress. Reactions occurred in youth with newly diagnosed T1D and in those with long-standing T1D who were using both injection and insulin pump therapy. Multidisciplinary care involving pediatric endocrinology and allergy/immunology utilizing trials of many adjunct therapies yielded minimal improvement. Despite the use of various treatments, including antihistamines, topical therapies, immunosuppressant medications, desensitization trials, and intravenous immune globulin, cutaneous reactions, elevated hemoglobin A1c levels, and negative effects on quality of life remain persistent challenges. One patient became one of the youngest pancreas transplant recipients in the world at age 12 years due to uncontrollable symptoms and intolerable adverse effects of attempted therapies.
CONCLUSION: Although rare, insulin hypersensitivity reactions negatively affect glycemic control and quality of life. These cases demonstrate the varying severity and presentation of insulin hypersensitivity reactions along with the limited success of various treatment approaches. Given the life-sustaining nature of insulin therapy, further studies are needed to better understand the underlying pathophysiology of insulin hypersensitivity and to develop targeted treatment approaches
A GALEX Ultraviolet Imaging Survey of Galaxies in the Local Volume
We present results from a GALEX ultraviolet (UV) survey of a complete sample
of 390 galaxies within ~11 Mpc of the Milky Way. The UV data are a key
component of the composite Local Volume Legacy (LVL), an
ultraviolet-to-infrared imaging program designed to provide an inventory of
dust and star formation in nearby spiral and irregular galaxies. The ensemble
dataset is an especially valuable resource for studying star formation in dwarf
galaxies, which comprise over 80% of the sample. We describe the GALEX survey
programs which obtained the data and provide a catalog of far-UV (~1500
Angstroms) and near-UV (~2200 Angstroms) integrated photometry. General UV
properties of the sample are briefly discussed. We compute two measures of the
global star formation efficiency, the SFR per unit HI gas mass and the SFR per
unit stellar mass, to illustrate the significant differences that can arise in
our understanding of dwarf galaxies when the FUV is used to measure the SFR
instead of H-alpha. We find that dwarf galaxies may not be as drastically
inefficient in coverting gas into stars as suggested by prior H-alpha studies.
In this context, we also examine the UV properties of late-type dwarf galaxies
that appear to be devoid of star formation because they were not detected in
previous H-alpha narrowband observations. Nearly all such galaxies in our
sample are detected in the FUV, and have FUV SFRs that fall below the limit
where the H-alpha flux is robust to Poisson fluctuations in the formation of
massive stars. The UV colors and star formation efficiencies of
H-alpha-undetected, UV-bright dwarf irregulars appear to be relatively
unremarkable with respect to those exhibited by the general population of
star-forming galaxies.Comment: submitted to ApJS, revised per referee's comments; accepted Oct. 30
w/o further revision; 37 pages; figure 6 omitted due to size; figure
available from http://users.obs.carnegiescience.edu/jlee/paper
Photon Dominated Regions in NGC 3603
Aims: We aim at deriving the excitation conditions of the interstellar gas as
well as the local FUV intensities in the molecular cloud surrounding NGC 3603
to get a coherent picture of how the gas is energized by the central stars.
Methods: The NANTEN2-4m submillimeter antenna is used to map the [CI] 1-0, 2-1
and CO 4-3, 7-6 lines in a 2' x 2' region around the young OB cluster NGC 3603
YC. These data are combined with C18O 2-1 data, HIRES-processed IRAS 60 and 100
micron maps of the FIR continuum, and Spitzer/IRAC maps. Results: The NANTEN2
observations show the presence of two molecular clumps located south-east and
south-west of the cluster and confirm the overall structure already found by
previous CS and C18O observations. We find a slight position offset of the peak
intensity of CO and [CI], and the atomic carbon appears to be further extended
compared to the molecular material. We used the HIRES far-infrared dust data to
derive a map of the FUV field heating the dust. We constrain the FUV field to
values of \chi = 3 - 6 \times 10^3 in units of the Draine field across the
clouds. Approximately 0.2 to 0.3 % of the total FUV energy is re-emitted in the
[CII] 158 {\mu}m cooling line observed by ISO. Applying LTE and escape
probability calculations, we derive temperatures (TMM1 = 43 K, TMM2 = 47 K),
column densities (N(MM1) = 0.9 \times 10^22 cm^-2, N(MM2) = 2.5 \times 10^22
cm^-2) and densities (n(MM1) = 3 \times 10^3 cm^-3, n(MM2) = 10^3 -10^4 cm^-3)
for the two observed molecular clumps MM1 and MM2. Conclusions: The cluster is
strongly interacting with the ambient molecular cloud, governing its structure
and physical conditions. A stability analysis shows the existence of
gravitationally collapsing gas clumps which should lead to star formation.
Embedded IR sources have already been observed in the outskirts of the
molecular cloud and seem to support our conclusions.Comment: 13 pages, 10 figures, accepted for publication by A&
Spectrophotometric Distances to Galactic H\,{\sc{ii}} Regions
We present a near infrared study of the stellar content of 35 H\,{\sc{ii}}
regions in the Galactic plane. In this work, we have used the near infrared
domain , and band color images to visually inspect the
sample. Also, color-color and color-magnitude diagrams were used to indicate
ionizing star candidates, as well as, the presence of young stellar objects
such as classical TTauri Stars (CTTS) and massive young stellar objects
(MYSOs). We have obtained {\it Spitzer} IRAC images for each region to help
further characterize them. {\it Spitzer} and near infrared morphology to place
each cluster in an evolutionary phase of development. {\it Spitzer} photometry
was also used to classify the MYSOs. Comparison of the main sequence in
color-magnitude diagrams to each observed cluster was used to infer whether or
not the cluster kinematic distance is consistent with brightnesses of the
stellar sources. We find qualitative agreement for a dozen of the regions, but
about half the regions have near infrared photometry that suggests they may be
closer than the kinematic distance. A significant fraction of these already
have spectrophotometric parallaxes which support smaller distances. These
discrepancies between kinematic and spectrophotometric distances are not due to
the spectrophotometric methodologies, since independent non-kinematic
measurements are in agreement with the spectrophotometric results. For
instance, trigonometric parallaxes of star-forming regions were collected from
the literature and show the same effect of smaller distances when compared to
the kinematic results. In our sample of H\,{\sc{ii}} regions, most of the
clusters are evident in the near infrared images. Finally, it is possible to
distinguish among qualitative evolutionary stages for these objects.Comment: 59 pages, 146 figures and 4 tables. MNRAS accepte
Evaluation of a Mixed Meal Test for Diagnosis and Characterization of PancrEaTogEniC DiabeTes Secondary to Pancreatic Cancer and Chronic Pancreatitis: Rationale and Methodology for the DETECT Study From the Consortium for the Study of Chronic Pancreatitis, Diabetes, and Pancreatic Cancer
Pancreatogenic diabetes mellitus is most commonly the result of chronic pancreatitis but can also occur secondary to pancreatic cancer. The early identification of pancreatogenic diabetes and distinction from the more prevalent type 2 diabetes are clinically significant; however, currently, there is no validated method to differentiate these diabetes subtypes. We describe a study, "Evaluation of a Mixed Meal Test for Diagnosis and Characterization of PancrEaTogEniC DiabeTes Secondary to Pancreatic Cancer and Chronic Pancreatitis: the DETECT study," that seeks to address this knowledge gap. The DETECT study is a multicenter study that will examine differences in hormone and glucose excursions after a mixed meal test. The study will also create a biorepository that will be used to evaluate novel diagnostic biomarkers for differentiating these diabetes subtypes
Type 3c (pancreatogenic) diabetes mellitus secondary to chronic pancreatitis and pancreatic cancer
Diabetes mellitus is a group of diseases defined by persistent hyperglycaemia. Type 2 diabetes, the most prevalent form, is characterised initially by impaired insulin sensitivity and subsequently by an inadequate compensatory insulin response. Diabetes can also develop as a direct consequence of other diseases, including diseases of the exocrine pancreas. Historically, diabetes due to diseases of the exocrine pancreas was described as pancreatogenic or pancreatogenous diabetes mellitus, but recent literature refers to it as type 3c diabetes. It is important to note that type 3c diabetes is not a single entity; it occurs because of a variety of exocrine pancreatic diseases with varying mechanisms of hyperglycaemia. The most commonly identified causes of type 3c diabetes are chronic pancreatitis, pancreatic ductal adenocarcinoma, haemochromatosis, cystic fibrosis, and previous pancreatic surgery. In this Review, we discuss the epidemiology, pathogenesis, and clinical relevance of type 3c diabetes secondary to chronic pancreatitis and pancreatic ductal adenocarcinoma, and highlight several important knowledge gaps
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