370 research outputs found
Discovery of a TiO emission band in the infrared spectrum of the S star NP Aurigae
We report on the discovery of an infrared emission band in the Spitzer
spectrum of the S-type AGB star NP Aurigae that is caused by TiO molecules in
the circumstellar environment. We modelled the observed emission to derive the
temperature of the TiO molecules (\approx 600 K), an upper limit on the column
density (\approx 10^17.25 cm^{-2}) and a lower limit on the spatial extent of
the layer that contains these molecules. (\approx 4.6 stellar radii). This is
the first time that this TiO emission band is observed. A search for similar
emission features in the sample of S-type stars yielded two additional
candidates. However, owing to the additional dust emission, the identification
is less stringent. By comparing the stellar characteristics of NP Aur to those
of the other stars in our sample, we find that all stars with TiO emission show
large-amplitude pulsations, s-process enrichment, and a low C/O ratio. These
characteristics might be necessary requirements for a star to show TiO in
emission, but they are not sufficient.Comment: 4 pages, 4 figures, letter to the edito
Sphagnum bleaching:Bicarbonate ‘toxicity’ and tolerance for seven Sphagnum species
Growth and functioning of Sphagnum mosses are closely linked to water level and chemistry. Sphagnum mosses occur in wet, generally acidic conditions, and when buffered, alkaline water is known to negatively impact Sphagnum. The effects of time, dose and species-specific responses of buffered, alkaline water on Sphagnum are largely unknown. We investigated the effects of bicarbonate and calcium on the survival, growth and physiological functioning of seven Sphagnum species occurring in contrasting environments, from raised bogs to (rich) fens. Mosses were submerged in different concentrations of bicarbonate and calcium solutions for 10 weeks under climate-controlled circumstances. After 2 weeks, all species exposed to the high bicarbonate treatment (2.0 mM) showed severe potassium leakage and swift discoloration. In contrast, species showed differential responses to the intermediate bicarbonate treatment (0.8 mM), some with a later onset of potassium leakage. S. squarrosum, S. teres & S. contortum generally persisted the longest, with all species dying after 6 to 10 weeks. Calcium alone, in contrast, negatively affected S. squarrosum, S. teres & S. contortum, causing discoloration and potassium leakage. Our study shows enrichment with bicarbonate, but not calcium, is detrimental for most Sphagnum species tested. A mechanistic model was developed that is consistent with dose and duration dependence and the species specificity. Future conservation and restoration measures for Sphagnum-dominated habitats and Sphagnum farming (cultivation, production and harvest of Sphagnum moss biomass) should limit flooding with bicarbonate-rich waters while investigating new management options, like acidifying surface waters to lower bicarbonate levels
When an old star smolders: On the detection of hydrocarbon emission from S-type AGB stars
Polycyclic aromatic hydrocarbons (PAHs) produce characteristic infrared
emission bands that have been observed in a wide range of astrophysical
environments, where carbonaceous material is subjected to ultraviolet (UV)
radiation. Although PAHs are expected to form in carbon-rich AGB stars, they
have up to now only been observed in binary systems where a hot companion
provides a hard radiation field. In this letter, we present low-resolution
infrared spectra of four S-type AGB stars, selected from a sample of 90 S-type
AGB stars observed with the infrared spectrograph aboard the Spitzer satellite.
The spectra of these four stars show the typical infrared features of PAH
molecules. We confirm the correlation between the temperature of the central
star and the centroid wavelength of the 7.9 {\mu}m feature, present in a wide
variety of stars spanning a temperature range from 3 000 to 12 000 K. Three of
four sources presented in this paper extend this relation towards lower
temperatures. We argue that the mixture of hydrocarbons we see in these S-stars
has a rich aliphatic component. The fourth star, BZ CMa, deviates from this
correlation. Based on the similarity with the evolved binary TU Tau, we predict
that BZ CMa has a hot companion as well.Comment: 5 pages, 2 figures, 2 table
The Spitzer Spectroscopic Survey of S-type Stars
S-type AGB stars are thought to be in the transitional phase between M-type
and C-type AGB stars. Because of their peculiar chemical composition, one may
expect a strong influence of the stellar C/O ratio on the molecular chemistry
and the mineralogy of the circumstellar dust. In this paper, we present a large
sample of 87 intrinsic galactic S-type AGB stars, observed at infrared
wavelengths with the Spitzer Space Telescope, and supplemented with
ground-based optical data. On the one hand, we derive the stellar parameters
from the optical spectroscopy and photometry, using a grid of model
atmospheres. On the other, we decompose the infrared spectra to quantify the
flux-contributions from the different dust species. Finally, we compare the
independently determined stellar parameters and dust properties. For the stars
without significant dust emission, we detect a strict relation between the
presence of SiS absorption in the Spitzer spectra and the C/O ratio of the
stellar atmosphere. These absorption bands can thus be used as an additional
diagnostic for the C/O ratio. For stars with significant dust emission, we
define three groups, based on the relative contribution of certain dust species
to the infrared flux. We find a strong link between group-membership and C/O
ratio. We show that these groups can be explained by assuming that the
dust-condensation can be cut short before silicates are produced, while the
remaining free atoms and molecules can then form the observed magnesium
sulfides or the carriers of the unidentified 13 and 20 micron features.
Finally, we present the detection of emission features attributed to molecules
and dust characteristic to C-type stars, such as molecular SiS, hydrocarbons
and magnesium sulfide grains. We show that we often detect magnesium sulfides
together with molecular SiS and we propose that it is formed by a reaction of
SiS molecules with Mg.Comment: Accepted for publication in A&
Inherited Variation in Vitamin D Genes Is Associated With Predisposition to Autoimmune Disease Type 1 Diabetes
Objective: Vitamin D deficiency (25-hydroxyvitamin D [25(OH)D] <50 nmol/L) is commonly reported in both children and adults worldwide, and growing evidence indicates that vitamin D deficiency is associated with many extraskeletal chronic disorders, including the autoimmune diseases type 1 diabetes and multiple sclerosis. Research Design and Methods: We measured 25(OH)D concentrations in 720 case and 2,610 control plasma samples and genotyped single nucleotide polymorphisms from seven vitamin D metabolism genes in 8,517 case, 10,438 control, and 1,933 family samples. We tested genetic variants influencing 25(OH)D metabolism for an association with both circulating 25(OH)D concentrations and disease status. Results: Type 1 diabetic patients have lower circulating levels of 25(OH)D than similarly aged subjects from the British population. Only 4.3 and 18.6% of type 1 diabetic patients reached optimal levels (75 nmol/L) of 25(OH)D for bone health in the winter and summer, respectively. We replicated the associations of four vitamin D metabolism genes (GC, DHCR7, CYP2R1, and CYP24A1) with 25(OH)D in control subjects. In addition to the previously reported association between type 1 diabetes and CYP27B1 (P = 1.4 × 10), we obtained consistent evidence of type 1 diabetes being associated with DHCR7 (P = 1.2 × 10) and CYP2R1 (P = 3.0 × 10). Conclusions: Circulating levels of 25(OH)D in children and adolescents with type 1 diabetes vary seasonally and are under the same genetic control as in the general population but are much lower. Three key 25(OH)D metabolism genes show consistent evidence of association with type 1 diabetes risk, indicating a genetic etiological role for vitamin D deficiency in type 1 diabetes
Rediscovering vitamin D
Over the past 2 years there has been a radical change in standard clinical practice with respect to vitamin D. As a result of a growing body of knowledgeable physicians are assessing the vitamin D nutritional status of their patients and prescribing aggressive repletion regimens of a vitamin D supplement. The present paper summarizes some basic information about this essential nutrient and reviews some of the more recent data implicating vitamin D deficiency in disease etiology with an emphasis on cardiovascular disease and cancer. Finally a rational approach to the dosing of vitamin D in different patient populations is provided
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