650 research outputs found
Kepler-91b: a planet at the end of its life. Planet and giant host star properties via light-curve variations
The evolution of planetary systems is intimately linked to the evolution of
their host star. Our understanding of the whole planetary evolution process is
based on the large planet diversity observed so far. To date, only few tens of
planets have been discovered orbiting stars ascending the Red Giant Branch.
Although several theories have been proposed, the question of how planets die
remains open due to the small number statistics. In this work we study the
giant star Kepler-91 (KOI-2133) in order to determine the nature of a
transiting companion. This system was detected by the Kepler Space Telescope.
However, its planetary confirmation is needed. We confirm the planetary nature
of the object transiting the star Kepler-91 by deriving a mass of and a planetary radius of
. Asteroseismic analysis produces a
stellar radius of and a mass of
. We find that its eccentric orbit
() is just away
from the stellar atmosphere at the pericenter. Kepler-91b could be the previous
stage of the planet engulfment, recently detected for BD+48 740. Our
estimations show that Kepler-91b will be swallowed by its host star in less
than 55 Myr. Among the confirmed planets around giant stars, this is the
planetary-mass body closest to its host star. At pericenter passage, the star
subtends an angle of , covering around 10% of the sky as seen from
the planet. The planetary atmosphere seems to be inflated probably due to the
high stellar irradiation.Comment: 21 pages, 8 tables and 11 figure
Is there really a debris disc around ?
The presence of a debris disc around the Gyr-old solar-type star
was suggested by the infrared
excess detection. Follow-up observations with /PACS revealed
a double-lobed feature, that displayed asymmetries both in brightness and
position. Therefore, the disc was thought to be edge-on and significantly
eccentric. Here we present ALMA/ACA observations in Band 6 and 7 which
unambiguously reveal that these lobes show no common proper motion with
. In these observations, no flux has been detected
around that exceeds the levels. We
conclude that surface brightness upper limits of a debris disc around
are at 1.3 mm, and
at 870 microns. Our results overall demonstrate
the capability of the ALMA/ACA to follow-up observations of
debris discs and clarify the effects of background confusion.Comment: 6 pages, 2 figures, 2 table
YSOVAR: Mid-IR variability in the star forming region Lynds 1688
The emission from young stellar objects (YSOs) in the mid-IR is dominated by
the inner rim of their circumstellar disks. We present an IR-monitoring survey
of about 800 objects in the direction of the Lynds 1688 (L1688) star forming
region over four visibility windows spanning 1.6 years using the \emph{Spitzer}
space telescope in its warm mission phase. Among all lightcurves, 57 sources
are cluster members identified based on their spectral-energy distribution and
X-ray emission. Almost all cluster members show significant variability. The
amplitude of the variability is larger in more embedded YSOs. Ten out of 57
cluster members have periodic variations in the lightcurves with periods
typically between three and seven days, but even for those sources, significant
variability in addition to the periodic signal can be seen. No period is stable
over 1.6 years. Non-periodic lightcurves often still show a preferred timescale
of variability which is longer for more embedded sources. About half of all
sources exhibit redder colors in a fainter state. This is compatible with
time-variable absorption towards the YSO. The other half becomes bluer when
fainter. These colors can only be explained with significant changes in the
structure of the inner disk. No relation between mid-IR variability and stellar
effective temperature or X-ray spectrum is found.Comment: accepted by ApJ, 24 pages, 17 figure
Age-dependent differences in human brain activity using a face- and location-matching task: An fMRI study
Purpose: To evaluate the differences of cortical activation patterns in young and elderly healthy subjects for object and spatial visual processing using a face- and location-matching task. Materials and Methods: We performed a face- and a location-matching task in 15 young (mean age: 28 +/- 9 years) and 19 elderly (mean age: 71 +/- 6 years) subjects. Each experiment consisted of 7 blocks alternating between activation and control condition. For face matching, the subjects had to indicate whether two displayed faces were identical or different. For location matching, the subjects had to press a button whenever two objects had an identical position. For control condition, we used a perception task with abstract images. Functional imaging was performed on a 1.5-tesla scanner using an EPI sequence. Results: In the face-matching task, the young subjects showed bilateral (right 1 left) activation in the occipito-temporal pathway (occipital gyrus, inferior and middle temporal gyrus). Predominantly right hemispheric activations were found in the fusiform gyrus, the right dorsolateral prefrontal cortex (inferior and middle frontal gyrus) and the superior parietal gyrus. In the elderly subjects, the activated areas in the right fronto-lateral cortex increased. An additional activated area could be found in the medial frontal gyrus (right > left). In the location-matching task, young subjects presented increased bilateral (right > left) activation in the superior parietal lobe and precuneus compared with face matching. The activations in the occipito-temporal pathway, in the right fronto-lateral cortex and the fusiform gyrus were similar to the activations found in the face-matching task. In the elderly subjects, we detected similar activation patterns compared to the young subjects with additional activations in the medial frontal gyrus. Conclusion: Activation patterns for object-based and spatial visual processing were established in the young and elderly healthy subjects. Differences between the elderly and young subjects could be evaluated, especially by using a face-matching task. Copyright (c) 2007 S. Karger AG, Basel
Temperature constraints on the coldest brown dwarf known WISE 0855-0714
Context. Nearby isolated planetary mass objects are beginning to be
discovered, but their individual properties are poorly constrained because
their low surface temperatures and strong molecular self-absorption make them
extremely faint.
Aims. We aimed to detect the near infrared emission of the coldest brown
dwarf (BD) found so far, WISE08550714, located 2.2 pc away, and to
improve its temperature estimate (T= 225-260 K) from a comparison
with state-of-the-art models of BD atmospheres.
Methods. We observed the field containing WISE0855-0714 with HAWK-I at the
VLT in the band. For BDs with T500\,K theoretical models
predict strong signal (or rather less molecular absorption) in this band.
Results. WISE0855-0714 was not detected in our Y-band images, thus placing an
upper limit on its brightness to Y>24.4 mag at 3- level, leading to
Y-[4.5]>10.5. Combining this limit with previous detections and upper limits at
other wavelengths, WISE08550714 is confirmed as the reddest BD detected,
further supporting its status as the coldest known brown dwarf. We applied
spectral energy distribution fitting with collections of models from two
independent groups for extremely cool BD atmospheres leading to an effective
temperature of T250\,K,.Comment: 4 pages, 4 figures. A&A letter Accepte
<i>Herschel</i> observations of the debris disc around HIP 92043
Context. Typical debris discs are composed of particles ranging from several micron sized dust grains to km sized asteroidal bodies, and their infrared emission peaks at wavelengths 60-100 μm. Recent Herschel DUNES observations have identified several debris discs around nearby Sun-like stars (F, G and K spectral type) with significant excess emission only at 160 μm.
Aims. We observed HIP 92043 (110 Her, HD 173667) at far-infrared and sub-millimetre wavelengths with Herschel PACS and SPIRE. Identification of the presence of excess emission from HIP 92043 and the origin and physical properties of any excess was undertaken through analysis of its spectral energy distribution (SED) and the PACS images.
Methods. The PACS and SPIRE images were produced using the HIPE photProject map maker routine. Fluxes were measured using aperture photometry. A stellar photosphere model was scaled to optical and near infrared photometry and subtracted from the far-infared and sub-mm fluxes to determine the presence of excess emission. Source radial profiles were fitted using a 2D Gaussian and compared to a PSF model based on Herschel observations of α Boo to check for extended emission.
Results. Clear excess emission from HIP 92043 was observed at 70 and 100 μm. Marginal excess was observed at 160 and 250 μm. Analysis of the images reveals that the source is extended at 160 μm. A fit to the source SED is inconsistent with a photosphere and single temperature black body.
Conclusions. The excess emission from HIP 92043 is consistent with the presence of an unresolved circumstellar debris disc at 70 and 100 μm, with low probability of background contamination. The extended 160 μm emission may be interpreted as an additional cold component to the debris disc or as the result of background contamination along the line of sight. The nature of the 160 μm excess cannot be determined absolutely from the available data, but we favour a debris disc interpretation, drawing parallels with previously identified cold disc sources in the DUNES sample
Incidence of debris discs around FGK stars in the solar neighbourhood
Debris discs are a consequence of the planet formation process and constitute
the fingerprints of planetesimal systems. Their solar system's counterparts are
the asteroid and Edgeworth-Kuiper belts. The aim of this paper is to provide
robust numbers for the incidence of debris discs around FGK stars in the solar
neighbourhood. The full sample of 177 FGK stars with d<20 pc proposed for the
DUNES survey is presented. Herschel/PACS observations at 100 and 160 micron
complemented with data at 70 micron, and at 250, 350 and 500 micron SPIRE
photometry, were obtained. The 123 objects observed by the DUNES collaboration
were presented in a previous paper. The remaining 54 stars, shared with the
DEBRIS consortium and observed by them, and the combined full sample are
studied in this paper. The incidence of debris discs per spectral type is
analysed and put into context together with other parameters of the sample,
like metallicity, rotation and activity, and age.
The subsample of 105 stars with d<15 pc containing 23 F, 33 G and 49 K stars,
is complete for F stars, almost complete for G stars and contains a substantial
number of K stars to draw solid conclusions on objects of this spectral type.
The incidence rates of debris discs per spectral type 0.26 (6 objects with
excesses out of 23 F stars), 0.21 (7 out of 33 G stars) and 0.20 (10 out of 49
K stars), the fraction for all three spectral types together being 0.22 (23 out
of 105 stars). Uncertainties corresponding to a 95% confidence level are given
in the text for all these numbers. The medians of the upper limits of
L_dust/L_* for each spectral type are 7.8E-7 (F), 1.4E-6 (G) and 2.2E-6 (K);
the lowest values being around 4.0E-7. The incidence of debris discs is similar
for active (young) and inactive (old) stars. The fractional luminosity tends to
drop with increasing age, as expected from collisional erosion of the debris
belts.Comment: 31 pages, 15 figures, 10 tables, 2 appendice
Young Stellar Object Variability (YSOVAR): Long Timescale Variations in the Mid-Infrared
The YSOVAR (Young Stellar Object VARiability) Spitzer Space Telescope
observing program obtained the first extensive mid-infrared (3.6 & 4.5 um)
time-series photometry of the Orion Nebula Cluster plus smaller footprints in
eleven other star-forming cores (AFGL490, NGC1333, MonR2, GGD 12-15, NGC2264,
L1688, Serpens Main, Serpens South, IRAS 20050+2720, IC1396A, and Ceph C).
There are ~29,000 unique objects with light curves in either or both IRAC
channels in the YSOVAR data set. We present the data collection and reduction
for the Spitzer and ancillary data, and define the "standard sample" on which
we calculate statistics, consisting of fast cadence data, with epochs about
twice per day for ~40d. We also define a "standard sample of members",
consisting of all the IR-selected members and X-ray selected members. We
characterize the standard sample in terms of other properties, such as spectral
energy distribution shape. We use three mechanisms to identify variables in the
fast cadence data--the Stetson index, a chi^2 fit to a flat light curve, and
significant periodicity. We also identified variables on the longest timescales
possible of ~6 years, by comparing measurements taken early in the Spitzer
mission with the mean from our YSOVAR campaign. The fraction of members in each
cluster that are variable on these longest timescales is a function of the
ratio of Class I/total members in each cluster, such that clusters with a
higher fraction of Class I objects also have a higher fraction of long-term
variables. For objects with a YSOVAR-determined period and a [3.6]-[8] color,
we find that a star with a longer period is more likely than those with shorter
periods to have an IR excess. We do not find any evidence for variability that
causes [3.6]-[4.5] excesses to appear or vanish within our data; out of members
and field objects combined, at most 0.02% may have transient IR excesses.Comment: Accepted to AJ; 38 figures, 93 page
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