122 research outputs found
Metal Abundance Calibration of the Ca II Triplet Lines in RR Lyrae Stars
The GAIA satellite is likely to observe thousands of RR Lyrae stars within a
small spectral window, between 8470A and 8750A, at a resolution of 11,500. In
order to derive the metallicity of RR Lyrae stars from Gaia, we have obtained
numerous spectra of RR Lyrae stars at a resolution of 35,000 with the Apache
Point Observatory 3.5 m echelle spectrograph. We have correlated the Ca II
triplet line strengths with metallicity as derived from Fe II abundances,
analogous to Preston's (1959) use of the Ca II K line to estimate the
metallicity of RR Lyrae stars. The Ca II line at 8498A is the least blended
with neighboring Paschen lines and thus provides the best correlation.Comment: Accepted for publication in Astrophysics & Space Scienc
Statistics of Magnetic Fields for OB Stars
Based on an analysis of the catalog of magnetic fields, we have investigated
the statistical properties of the mean magnetic fields for OB stars. We show
that the mean effective magnetic field of a star can be used as a
statistically significant characteristic of its magnetic field. No correlation
has been found between the mean magnetic field strength and
projected rotational velocity of OB stars, which is consistent with the
hypothesis about a fossil origin of the magnetic field. We have constructed the
magnetic field distribution function for B stars, , that has a
power-law dependence on with an exponent of . We have
found a sharp decrease in the function F for {\cal B}\lem 400 G
that may be related to rapid dissipation of weak stellar surface magnetic
fields.Comment: 22 pages, 7 figures, accepted Astronomy Letters, 2010, vol.36, No.5,
pp.370-379, contact E-mail: [email protected]
Comprehensive study of the magnetic stars HD 5797 and HD 40711 with large chromium and iron overabundances
We present the results of a comprehensive study of the chemically peculiar
stars HD 5797 and HD 40711. The stars have the same effective temperature, Teff
= 8900 K, and a similar chemical composition with large iron (+1.5 dex) and
chromium (+3 dex) overabundances compared to the Sun. The overabundance of
rare-earth elements typically reaches +3 dex. We have measured the magnetic
field of HD 5797. The longitudinal field component Be has been found to vary
sinusoidally between -100 and +1000 G with a period of 69 days. Our estimate of
the evolutionary status of the stars suggests that HD 5797 and HD 40711, old
objects with an age t \approx 5 \times 108 yr, are near the end of the core
hydrogen burning phase.Comment: 26 pages, 5 Encapsulated Postscript figure
Effects of oral meal feeding on whole body protein breakdown and protein synthesis in cachectic pancreatic cancer patients
Background:
Pancreatic cancer is often accompanied by cachexia, a syndrome of severe weight loss and muscle wasting. A suboptimal response to nutritional support may further aggravate cachexia, yet the influence of nutrition on protein kinetics in cachectic patients is poorly understood.
Methods:
Eight cachectic pancreatic cancer patients and seven control patients received a primed continuous intravenous infusion of lâ[ringâ2H5]phenylalanine and lâ[3,3â2H2]tyrosine for 8âh and ingested sips of water with lâ[1â13C]phenylalanine every 30âmin. After 4âh, oral feeding was started. Whole body protein breakdown, protein synthesis, and net protein balance were calculated. Results are given as median with interquartile range.
Results:
Baseline protein breakdown and protein synthesis were higher in cachectic patients compared with the controls (breakdown: 67.1 (48.1â79.6) vs. 45.8 (42.6â46.3)â”mol/kg lean body mass/h, Pâ=â0.049; and synthesis: 63.0 (44.3â75.6) vs. 41.8 (37.6â42.5)â”mol/kg lean body mass/h, Pâ=â0.021). During feeding, protein breakdown decreased significantly to 45.5 (26.9â51.1)â”mol/kg lean body mass/h (Pâ=â0.012) in the cachexia group and to 33.7 (17.4â37.1)â”mol/kg lean body mass/h (Pâ=â0.018) in the control group. Protein synthesis was not affected by feeding in cachectic patients: 58.4 (46.5â76.1)â”mol/kg lean body mass/h, but was stimulated in controls: 47.9 (41.8â56.7)â”mol/kg lean body mass/h (Pâ=â0.018). Both groups showed a comparable positive net protein balance during feeding: cachexia: 19.7 (13.1â23.7) and control: 16.3 (13.6â25.4)â”mol/kg lean body mass/h (Pâ=â0.908).
Conclusion:
Cachectic pancreatic cancer patients have a higher basal protein turnover. Both cachectic patients and controls show a comparable protein anabolism during feeding, albeit through a different pattern of protein kinetics. In cachectic patients, this is primarily related to reduced protein breakdown, whereas in controls, both protein breakdown and protein synthesis alterations are involved
The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets
This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sunâs centre, equal to half of Mercuryâs perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics
Recommended from our members
Erratum: "On the Use of Field RR Lyrae as Galactic Probes. V. Optical and Radial Velocity Curve Templates" (2021, ApJ, 919, 85)
[no abstract available
Magnetic Field Generation in Stars
Enormous progress has been made on observing stellar magnetism in stars from
the main sequence through to compact objects. Recent data have thrown into
sharper relief the vexed question of the origin of stellar magnetic fields,
which remains one of the main unanswered questions in astrophysics. In this
chapter we review recent work in this area of research. In particular, we look
at the fossil field hypothesis which links magnetism in compact stars to
magnetism in main sequence and pre-main sequence stars and we consider why its
feasibility has now been questioned particularly in the context of highly
magnetic white dwarfs. We also review the fossil versus dynamo debate in the
context of neutron stars and the roles played by key physical processes such as
buoyancy, helicity, and superfluid turbulence,in the generation and stability
of neutron star fields.
Independent information on the internal magnetic field of neutron stars will
come from future gravitational wave detections. Thus we maybe at the dawn of a
new era of exciting discoveries in compact star magnetism driven by the opening
of a new, non-electromagnetic observational window.
We also review recent advances in the theory and computation of
magnetohydrodynamic turbulence as it applies to stellar magnetism and dynamo
theory. These advances offer insight into the action of stellar dynamos as well
as processes whichcontrol the diffusive magnetic flux transport in stars.Comment: 41 pages, 7 figures. Invited review chapter on on magnetic field
generation in stars to appear in Space Science Reviews, Springe
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
Extent and Causes of Chesapeake Bay Warming
Coastal environments such as the Chesapeake Bay have long been impacted by eutrophication stressors resulting from human activities, and these impacts are now being compounded by global warming trends. However, there are few studies documenting long-term estuarine temperature change and the relative contributions of rivers, the atmosphere, and the ocean. In this study, Chesapeake Bay warming, since 1985, is quantified using a combination of cruise observations and model outputs, and the relative contributions to that warming are estimated via numerical sensitivity experiments with a watershedâestuarine modeling system. Throughout the Bayâs main stem, similar warming rates are found at the surface and bottom between the late 1980s and late 2010s (0.02 +/- 0.02C/year, mean +/- 1 standard error), with elevated summer rates (0.04 +/- 0.01C/year) and lower rates of winter warming (0.01 +/- 0.01C/year). Most (~85%) of this estuarine warming is driven by atmospheric effects. The secondary influence of ocean warming increases with proximity to the Bay mouth, where it accounts for more than half of summer warming in bottom waters. Sea level rise has slightly reduced summer warming, and the influence of riverine warming has been limited to the heads of tidal tributaries. Future rates of warming in Chesapeake Bay will depend not only on global atmospheric trends, but also on regional circulation patterns in mid-Atlantic waters, which are currently warming faster than the atmosphere.
Supporting model data available at: https://doi.org/10.25773/c774-a36
The impact of breastfeeding patterns on regional differences in infant mortality in Germany, 1910
This paper examines the impact of breastfeeding practices on the large regional differences in infant mortality in Germany around 1910. Breastfeeding is strongly negatively associated with infant mortality and remains so after controlling for public health measures and for demographic, economic, and social factors that also affect infant mortality. But it contributes much less to regional differences in infant mortality than do access to medical care, percentage illegitimate and marital fertility. Breastfeeding is less important than these other factors because it affects fewer causes of death and has a smaller impact on cause-specific infant mortality rates. L'auteur Ă©tudie l'impact des pratiques d'allaitement sur les grandes diffĂ©rences rĂ©gionales de mortalitĂ© infantile observĂ©es en Allemagne aux alentours de 1910. Il existe une association fortement nĂ©gative entre l'allaitement et la mortalitĂ© infantile, mĂȘme quand on contrĂŽle les facteurs dĂ©mographiques, Ă©conomiques, sociaux et de politique sanitaire, qui, eux aussi, affectent la mortalitĂ© infantile. Mais les diffĂ©rences rĂ©gionales de mortalitĂ© infantile s'expliquent nettement moins par l'allaitement que par l'accessibilitĂ© des soins mĂ©dicaux, le taux d'illĂ©gitimitĂ© des naissances et la fĂ©conditĂ© lĂ©gitime. L'allaitement est un facteur de moindre importance que ceux-ci parce qu'il n'a d'impact que sur un petit nombre de causes de dĂ©cĂšs, et un faible impact sur les taux de mortalitĂ© infantile par cause.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42730/1/10680_2005_Article_BF01796777.pd
- âŠ