253 research outputs found
Photospheric and coronal abundances in solar-type stars: the peculiar case of Tau Bootis
Chemical abundances in solar-type stars are a much debated topic.
Planet-hosting stars are known to be metal-rich, but whether or not this
peculiarity applies also to the chemical composition of the outer stellar
atmospheres is still to be clarified. More in general, coronal and photospheric
abundances in late-type stars appear to be different in many cases, but
understanding how chemical stratification effects work in stellar atmospheres
requires an observational base larger than currently available. We obtained
XMM-Newton high-resolution X-ray spectra of Tau Bootis, a well known nearby
star with a Jovian-mass close-in planet. We analyzed these data with the aim to
perform a detailed line-based emission measure analysis and derive the
abundances of individual elements in the corona with two different methods
applied independently. We compared the coronal abundances of Tau Bootis with
published photospheric abundances based on high-resolution optical spectra and
with those of other late-type stars with different magnetic activity levels,
including the Sun. We find that the two methods provide consistent results
within the statistical uncertainties for both the emission measure distribution
of the hot plasma and for the coronal abundances, with discrepancies at the
2-sigma level limited to the amount of plasma at temperatures of 3-4 MK and to
the O and Ni abundances. In both cases, the elements for which both coronal and
photospheric measurements are available (C, N, O, Si, Fe, and Ni) result
systematically less abundant in the corona by a factor 3 or more, with the
exception of the coronal Ni abundance, which is similar to the photospheric
value. Comparison with other late-type stars of similar activity level shows
that these coronal/photospheric abundance ratios are peculiar to Tau Bootis and
possibly related to the characteristic over-metallicity of this planet-hosting
star.Comment: 9 pages, 7 figures. A&A, in press. Language-edited version, one
reference update
Coronal properties of active G-type stars in different evolutionary phases
We report on the analysis of XMM-Newton observations of three G-type stars in very different evolutionary phases: the "weak-line" T Tauri star HD 283572, the Zero Age Main Sequence star EK Dra and the Hertzsprung-gap giant star 31 Corn. The X-ray luminosities of the three stars are all in the range 10(30) - 10(31) erg/s. We compare the Emission Measure Distributions of these bright sources, derived from high-resolution X-ray spectra, as well as the pattern of elemental abundances vs. First Ionization Potential (FIP). The results of our analysis suggest that the coronae of these stars are very similar in terms of dominant coronal magnetic structures, in spite of differences in their evolutionary phases, surface gravities and metallicities
Coronal abundances of X-ray bright pre-main sequence stars in the Taurus Molecular Cloud
We studied the thermal properties and chemical composition of the X-ray
emitting plasma of a sample of bright members of the Taurus Molecular Cloud to
investigate possible differences among classical and weak-lined T Tauri stars
and possible dependences of the abundances on the stellar activity level and/or
on the presence of accretion/circumstellar material. We used medium-resolution
X-ray spectra obtained with the sensitive EPIC/PN camera in order to analyse
the possible sample. The PN spectra of 20 bright (L_X ~ 10^30 - 10^31 erg/s)
Taurus members, with at least ~ 4500 counts, were fitted using thermal models
of optically thin plasma with two components and variable abundances of O, Ne,
Mg, Si, S, Ar, Ca, and Fe. Extensive preliminary investigations were employed
to study the performances of the PN detectors regarding abundance
determinations, and finally to check the results of the fittings. We found that
the observed X-ray emission of the studied stars can be attributed to coronal
plasma having similar thermal properties and chemical composition both in the
classical and in the weak-lined T Tauri stars. The results of the fittings did
not show evidence for correlations of the abundance patterns with activity or
accretion/disk presence. The iron abundance of these active stars is
significantly lower than (~ 0.2 of) the solar photospheric value. An indication
of slightly different coronal properties in stars with different spectral type
is found from this study. G-type and early K-type stars have, on average,
slightly higher Fe abundances (Fe ~ 0.24 solar) with respect to stars with
later spectral type (Fe ~ 0.15 solar), confirming previous findings from
high-resolution X-ray spectroscopy; stars of the former group are also found to
have, on average, hotter coronae.Comment: 14 pages, 11 figures, to be published in Astronomy & Astrophysic
Optical spectroscopy of X-ray sources in the Taurus molecular cloud: discovery of ten new pre-main sequence stars
We have analyzed optical spectra of 25 X-ray sources identified as potential
new members of the Taurus molecular cloud (TMC), in order to confirm their
membership in this SFR. Fifty-seven candidates were previously selected among
the X-ray sources in the XEST survey, having a 2MASS counterpart compatible
with a PMS star based on color-magnitude and color-color diagrams. We obtained
high-resolution optical spectra for 7 of these candidates with the SARG
spectrograph at the TNG telescope, which were used to search for Li absorption
and to measure the Ha line and the radial and rotational velocities; 18
low-resolution optical spectra obtained with DOLORES for other candidate
members were used for spectral classification, for Ha measurements, and to
assess membership together with IR color-color and color-magnitude diagrams and
additional information from the X-ray data. We found that 3 sources show Li
absorption, with equivalent widths of ~500 mA, broad spectral line profiles,
indicating v sin i ~20-40 km/s, radial velocities consistent with those for
known members, and Ha emission. Two of them are classified as new WTTSs, while
the EW (~ -9 Ang) of the Ha line and its broad asymmetric profile clearly
indicate that the third star (XEST-26-062) is a CTTS. Fourteen sources observed
with DOLORES are M-type stars. Fifteen sources show Ha emission; 6 of them have
spectra that indicate surface gravity lower than in MS stars, and their
de-reddened positions in IR color-magnitude diagrams are consistent with their
derived spectral type and with PMS models at the distance of the TMC. The
K-type star XEST-11-078 is confirmed as a new member from the strength of its
Ha emission line. Overall, we confirm membership to the TMC for 10 out of 25
X-ray sources observed in the optical. Three sources remain uncertain.Comment: 15 pages, 7 figures, accepted by Astronomy & Astrophysic
A comprehensive analysis of methods for assessing polygenic burden on Alzheimerâs disease pathology and risk beyond APOE
Genome-wide association studies have identified dozens of loci that alter the risk to develop Alzheimerâs disease. However, with the exception of the APOE-Δ4 allele, most variants bear only little individual effect and have, therefore, limited diagnostic and prognostic value. Polygenic risk scores aim to collate the disease risk distributed across the genome in a single score. Recent works have demonstrated that polygenic risk scores designed for Alzheimerâs disease are predictive of clinical diagnosis, pathology confirmed diagnosis and changes in imaging biomarkers. Methodological innovations in polygenic risk modelling include the polygenic hazard score, which derives effect estimates for individual single nucleotide polymorphisms from survival analysis, and methods that account for linkage disequilibrium between genomic loci. In this work, using data from the Alzheimerâs disease neuroimaging initiative, we compared different approaches to quantify polygenic disease burden for Alzheimerâs disease and their association (beyond the APOE locus) with a broad range of Alzheimerâs disease-related traits: cross-sectional CSF biomarker levels, cross-sectional cortical amyloid burden, clinical diagnosis, clinical progression, longitudinal loss of grey matter and longitudinal decline in cognitive function. We found that polygenic scores were associated beyond APOE with clinical diagnosis, CSF-tau levels and, to a minor degree, with progressive atrophy. However, for many other tested traits such as clinical disease progression, CSF amyloid, cognitive decline and cortical amyloid load, the additional effects of polygenic burden beyond APOE were of minor nature. Overall, polygenic risk scores and the polygenic hazard score performed equally and given the ease with which polygenic risk scores can be derived; they constitute the more practical choice in comparison with polygenic hazard scores. Furthermore, our results demonstrate that incomplete adjustment for the APOE locus, i.e. only adjusting for APOE-Δ4 carrier status, can lead to overestimated effects of polygenic scores due to APOE-Δ4 homozygous participants. Lastly, on many of the tested traits, the major driving factor remained the APOE locus, with the exception of quantitative CSF-tau and p-tau measures
Coronal properties of G-type stars in different evolutionary phases
We report on the analysis of XMM-Newton observations of three G-type stars in
very different evolutionary phases: the weak-lined T Tauri star HD 283572, the
Zero Age Main Sequence star EK Dra and the Hertzsprung-gap giant star 31 Com.
They all have high X-ray luminosity (10^31 erg/s for HD 283572 and 31 Com and
10^30 erg/s for EK Dra). We compare the Emission Measure Distributions (EMDs)
of these active coronal sources, derived from high-resolution XMM-Newton
grating spectra, as well as the pattern of elemental abundances vs. First
Ionization Potential (FIP). We also perform time-resolved spectroscopy of a
flare detected by XMM from EK Dra. We interpret the observed s as the
result of the emission of ensembles of magnetically confined loop-like
structures with different apex temperatures. Our analysis indicates that the
coronae of HD 283572 and 31 Com are very similar in terms of dominant coronal
magnetic structures, in spite of differences in the evolutionary phase, surface
gravity and metallicity. In the case of EK Dra the distribution appears to be
slightly flatter than in the previous two cases, although the peak temperature
is similar.Comment: 15 pages, 13 Postscript figures, to be published in A&
The X-ray activity-rotation relation of T Tauri stars in Taurus-Auriga
The Taurus-Auriga star-forming complex hosts the only population of T Tauri
stars in which an anticorrelation of X-ray activity and rotation period has
been observed. We have used XMM-Newton's European Photon Imaging Cameras to
perform the most sensitive survey to date of X-ray emission (0.3-10 keV) from
young stars in Taurus-Auriga and investigate the dependences of X-ray activity
measures -- X-ray luminosity, Lx, its ratio with the stellar luminosity,
Lx/Lstar, and the surface-averaged X-ray flux, Fxs -- on rotation period. We
tested for differences in the distributions of Lx/Lstar of fast and slow
rotators, accretors and non-accretors, and compared the dependence of Lx/Lstar
on the ratio of the rotation period and the convective turnover timescale, the
Rossby number, with that of late-type main-sequence stars. We found significant
anticorrelations of Lx and Fxs with rotation period, but these could be
explained by the typically higher stellar luminosity and effective temperature
of fast-rotators in Taurus-Auriga and a near-linear dependence of Lx on Lstar.
We found no evidence for a dependence of Lx/Lstar on rotation period, but for
accretors to have lower Lx/Lstar than non-accretors at all rotation periods.
The Rossby numbers of accretors and non-accretors were found to be the same as
those of late-type main-sequence stars showing saturated X-ray emission. We
conclude that non-accreting T Tauri stars show X-ray activity entirely
consistent with the saturated activity of late-type main-sequence stars.
Accreting T Tauri stars show lower X-ray activity, which cannot be attributed
to their slower rotation.Comment: 13 pages, 8 figures (14 eps files). Accepted by A&A, to appear in a
special section/issue dedicated to the XMM-Newton Extended Survey of the
Taurus Molecular Clou
Spectral properties of X-ray bright variable sources in the Taurus Molecular Cloud
We analyze 19 bright variable X-ray sources detected in the XMM-Newton
Extended Survey of the Taurus Molecular Cloud (XEST), in order to characterize
the variations with time of their coronal properties and to derive informations
on the X-ray emitting structures. We performed time-resolved spectroscopy of
the EPIC PN and MOS spectra of the XEST sources, using a model with one or two
thermal components, and we used the time evolution of the temperatures and
emission measures during the decay phase of flares to derive the size of the
flaring loops. The light curves of the selected sources show different types of
variability: flares, long-lasting decay or rise through the whole observation,
slow modulation or complex flare-like variability. Spectral analysis shows
typical quiescent plasma temperatures of 5-10 MK and 15-35 MK; the cool
component generally remains constant, while the observed flux changes are due
to variations of the hot component. During flares the plasma reaches
temperatures up to 100 MK and luminosities up to erg s.
Loop sizes inferred from flare analysis are generally smaller than or
comparable to the stellar radius.Comment: 18 pages, 11 figures, 3 tables. Accepted by A&A, to appear in a
special section/issue dedicated to the XMM-Newton Extended Survey of the
Taurus Molecular Cloud (XEST
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