492 research outputs found
Spitzer White Dwarf Planet Limits
We present preliminary limits on the presence of planets around white dwarf
stars using the IRAC photometer on the Spitzer space telescope. Planets emit
strongly in the mid-infrared which allows their presence to be detected as an
excess at these wavelengths. We place limits of for 8 stars assuming
ages of , and for 23 stars.We describe our survey, present our
results and comment on approaches to improve our methodology.Comment: 4 pages, 3 figures, to appear in Proceedings of 15th European White
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A Status Report On A Planet Search Around White Dwarf Stars
We have continued monitoring a pilot sample of 15 isolated, pulsating DA white dwarfs for center-of-mass motion caused by a planetary companion. Roughly 7 years into our survey, we have preliminary evidence for periodic variations in pulse arrival times for at least two white dwarfs in our sample. The variations in these systems are unlikely to be caused by secular evolution and are possibly the result of motion of the white dwarf around a center of mass. We have yet to claim confirmation of a planet. GD66 is a previously published candidate system, with a modulation in pulse arrival times that could be caused by a 2.0 M-J sin i planetary companion with an 8.3 year orbital period. Another candidate system, WD1354+0108, has a phase modulation consistent with a 0.7 M-J sin i planet at 2.3 AU (a 4.5 year orbit). We see similar behavior in two independent frequencies within this star, and while a sinusoid is currently a marginally better fit to the data than a straight line (as we might expect from cooling alone in a DAV), we are hesitant to over-interpret our results. Finally, we have a third system, WD0018+0031, that shows a change in pulse arrival times inconsistent with cooling alone; a 2.7 M-J planet at an orbit of about 5 AU could cause the observed trend. Observations of these candidate systems are ongoing in order to constrain any planetary companions that may be present.Astronom
Nonlinear asteroseismology of RR Lyrae
The observations of the Kepler space telescope revealed that fundamental-mode
RR Lyrae stars may show various radial overtones. The presence of multiple
radial modes may allow us to conduct nonlinear asteroseismology: comparison of
mode amplitudes and frequency shifts between observations and models. Here we
report the detection of three radial modes in the star RR Lyr, the eponym of
the class, using the Kepler short cadence data: besides the fundamental mode,
both the first and the ninth overtones can be derived from the data set. RR
Lyrae shows period doubling, but switches occasionally to a state where a
pattern of six pulsation cycles repeats instead of two. We found hydrodynamic
models that show the same three modes and the period-six state, allowing for
comparison with the observations.Comment: 5 pages, 4 figures, accepted for publication in ApJ Letter
Scene construction in developmental amnesia: An fMRI study.
Amnesic patients with bilateral hippocampal damage sustained in adulthood are generally unable to construct scenes in their imagination. By contrast, patients with developmental amnesia (DA), where hippocampal damage was acquired early in life, have preserved performance on this task, although the reason for this sparing is unclear. One possibility is that residual function in remnant hippocampal tissue is sufficient to support basic scene construction in DA. Such a situation was found in the one amnesic patient with adult-acquired hippocampal damage (P01) who could also construct scenes. Alternatively, DA patients' scene construction might not depend on the hippocampus, perhaps being instead reliant on non-hippocampal regions and mediated by semantic knowledge. To adjudicate between these two possibilities, we examined scene construction during functional MRI (fMRI) in Jon, a well-characterised patient with DA who has previously been shown to have preserved scene construction. We found that when Jon constructed scenes he activated many of the regions known to be associated with imagining scenes in control participants including ventromedial prefrontal cortex, posterior cingulate, retrosplenial and posterior parietal cortices. Critically, however, activity was not increased in Jon's remnant hippocampal tissue. Direct comparisons with a group of control participants and patient P01, confirmed that they activated their right hippocampus more than Jon. Our results show that a type of non-hippocampal dependent scene construction is possible and occurs in DA, perhaps mediated by semantic memory, which does not appear to involve the vivid visualisation of imagined scenes
A Spitzer White Dwarf Infrared Survey
We present mid-infrared photometry of 124 white dwarf stars with the Spitzer Space Telescope. Objects were observed simultaneously at 4.5 and 8.0 ÎĽm with sensitivities better than 0.1 mJy. This data set can be used to test models of white dwarf atmospheres in a new wavelength regime, as well as to search for planetary companions and debris disks
Planetary Candidates Observed by Kepler. VI. Planet Sample from Q1--Q16 (47 Months)
We present the sixth catalog of Kepler candidate planets based on nearly four years of high precision photometry. This catalog builds on the legacy of previous catalogs released by the Kepler project and includes 1493 new Kepler Objects of Interest (KOIs) of which 554 are planet candidates, and 131 of these candidates have best-fit radii > 1.5 R_⊕. This brings the total number of KOIs and planet candidates to 7348 and 4175 respectively. We suspect that many of these new candidates at the low signal-to-noise ratio limit may be false alarms created by instrumental noise, and discuss our efforts to identify such objects. We re-evaluate all previously published KOIs with orbital periods of > 50 days to provide a consistently vetted sample that can be used to improve planet occurrence rate calculations. We discuss the performance of our planet detection algorithms, and the consistency of our vetting products. The full catalog is publicly available at the NASA Exoplanet Archive
Echo-Mapping of Swift J1753.5-0127
We present two epochs of coordinated X-ray-optical timing observations of the
black hole candidate Swift J1753.5-0127 during its 2005 outburst. The first
epoch in July occurred at outburst peak. Two consecutive nights of observations
using the McDonald Observatory Argos camera with the Rossi X-ray Timing
Explorer show a consistent correlation with an immediate response and an
extended tail lasting ~5s. The properties of the variability and the
correlation are consistent with thermal reprocessing in an accretion disk. The
shortness of the lag suggests a short orbital period consistent with that
recently claimed. The second epoch in August used the VLT FORS2 HIT mode again
in conjunction with RXTE. Again a repeatable correlation is seen between two
independent subsets of the data. In this case, though, the cross-correlation
function has an unusual structure comprising a dip followed by a double-peak.
We suggest that this may be equivalent to the dip plus single peak structure
seen by Kanbach et al. (2001) in XTE J1118+480 and attributed there to
synchrotron emission; a similar structure was seen during later activity of
Swift J1753.5-0127 by Durant et al. (2008).Comment: 7 pages, accepted for publication in Monthly Notices of the Royal
Astronomical Societ
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