256 research outputs found
Late Quaternary Vegetation History of Sulphur Lake, Southwest Yukon Territory, Canada
Paleoecological studies based on the analysis of pollen in lake sediments offer the potential for high resolution and well-dated independent records of past vegetation and climate. A 5 m sediment core was raised from the deepest section of Sulphur Lake, located in the southwest Yukon (60.95°N, 137.95°W; 847 m a.s.l.). The pollen spectra indicate that before 11250 yr BP, the vegetation was a herbaceous tundra marked by the presence of Artemisia. However, the date of the establishment of this initial vegetation cannot be secured because of problems with the basal radiocarbon date and the lack of a reliable chronology of regional deglaciation. A birch shrub tundra prevailed between 11250 and 10250 yr BP and was then replaced by a discontinuous poplar woodland. Juniperus populations expanded at 9500 yr BP, and by 8400 yr BP, Picea invaded the region. The white spruce forest that occupies the region today was established by approximately 8000 yr BP. Alnus crispa increased at 6000 yr BP, but the simultaneous increase in Picea mariana found at most sites in the Yukon was not present at Sulphur Lake. Black spruce was never a dominant component of the vegetation in the southwest Yukon, as it was in the south-central Yukon between 6100 and 4100 yr BP.Les Ă©tudes palĂ©oĂ©cologiques fondĂ©es sur l'analyse de pollens de sĂ©diments lacustres offrent la possibilitĂ© d'obtenir une chronologie de la palĂ©ovĂ©gĂ©tation et du palĂ©oclimat Ă haute rĂ©solution et avec une datation prĂ©cise. Une carotte de sĂ©diment de 5 m a Ă©tĂ© prĂ©levĂ©e dans la section la plus profonde de Sulphur Lake, situĂ© au sud-ouest du Yukon (60,95° N., 137,95° O.; 847 m alt.). D'aprĂšs les spectres polliniques, la vĂ©gĂ©tation a Ă©tĂ© une toundra herbacĂ©e marquĂ©e par la prĂ©sence d'Artemisia avant 11 250 ans B.P. Cependant, la date de colonisation de cette vĂ©gĂ©tation ne peut ĂȘtre dĂ©terminĂ©e de façon dĂ©finitive Ă cause des problĂšmes de datation du dĂ©but de la sĂ©quence sĂ©dimentologique et l'absence d'une chronologie fiable de la dĂ©glaciation rĂ©gionale. Une toundra arbustive Ă bouleau a prĂ©dominĂ© entre 11250 B.P. et 10250 ans B.P., et a ensuite Ă©tĂ© remplacĂ©e par une rĂ©gion boisĂ©e de peuplier discontinu. Les populations de Juniperus ont augmentĂ© vers 9500 ans B.P. et, vers 8400 ans B.P., Picea a colonisĂ© la rĂ©gion. La forĂȘt relativement fermĂ©e d'Ă©pinettes blanches qui occupe la rĂ©gion aujourd'hui s'est Ă©tablie vers 8000 ans B.P. Alnus crispa s'est rĂ©pandu il y a environ 6000 ans B.P. Alors qu'on retrouve une augmentation de Picea mariana Ă cette Ă©poque dans la plupart des sites dans le Territoire du Yukon, celle-ci n'a pas eu lieu Ă Sulphur Lake. L'Ă©pinette noire n'a jamais Ă©tĂ© une composante dominante de la vĂ©gĂ©tation au sud-ouest du Yukon comme elle l'a Ă©tĂ© entre 6100 et 4100 ans B.P. au centre-sud du Yukon
Young "Dipper" Stars in Upper Sco and Oph Observed by K2
We present ten young (10 Myr) late-K and M dwarf stars observed in
K2 Campaign 2 that host protoplanetary disks and exhibit quasi-periodic or
aperiodic dimming events. Their optical light curves show 10-20 dips in
flux over the 80-day observing campaign with durations of 0.5-2 days and
depths of up to 40%. These stars are all members of the Ophiuchus
(1 Myr) or Upper Scorpius (10 Myr) star-forming regions. To
investigate the nature of these "dippers" we obtained: optical and
near-infrared spectra to determine stellar properties and identify accretion
signatures; adaptive optics imaging to search for close companions that could
cause optical variations and/or influence disk evolution; and
millimeter-wavelength observations to constrain disk dust and gas masses. The
spectra reveal Li I absorption and H emission consistent with stellar
youth (<50 Myr), but also accretion rates spanning those of classical and
weak-line T Tauri stars. Infrared excesses are consistent with protoplanetary
disks extending to within 10 stellar radii in most cases; however, the
sub-mm observations imply disk masses that are an order of magnitude below
those of typical protoplanetary disks. We find a positive correlation between
dip depth and WISE-2 excess, which we interpret as evidence that the dipper
phenomenon is related to occulting structures in the inner disk, although this
is difficult to reconcile with the weakly accreting aperiodic dippers. We
consider three mechanisms to explain the dipper phenomenon: inner disk warps
near the co-rotation radius related to accretion; vortices at the inner disk
edge produced by the Rossby Wave Instability; and clumps of circumstellar
material related to planetesimal formation.Comment: Accepted to ApJ, 19 pages, 10 figure
EPIC 219217635: A Doubly Eclipsing Quadruple System Containing an Evolved Binary
We have discovered a doubly eclipsing, bound, quadruple star system in the
field of K2 Campaign 7. EPIC 219217635 is a stellar image with that
contains an eclipsing binary (`EB') with d and a second EB with
d. We have obtained followup radial-velocity (`RV')
spectroscopy observations, adaptive optics imaging, as well as ground-based
photometric observations. From our analysis of all the observations, we derive
good estimates for a number of the system parameters. We conclude that (1) both
binaries are bound in a quadruple star system; (2) a linear trend to the RV
curve of binary A is found over a 2-year interval, corresponding to an
acceleration, cm s; (3) small
irregular variations are seen in the eclipse-timing variations (`ETVs')
detected over the same interval; (4) the orbital separation of the quadruple
system is probably in the range of 8-25 AU; and (5) the orbital planes of the
two binaries must be inclined with respect to each other by at least
25. In addition, we find that binary B is evolved, and the cooler and
currently less massive star has transferred much of its envelope to the
currently more massive star. We have also demonstrated that the system is
sufficiently bright that the eclipses can be followed using small ground-based
telescopes, and that this system may be profitably studied over the next decade
when the outer orbit of the quadruple is expected to manifest itself in the ETV
and/or RV curves.Comment: Accepted for publication in MNRA
EPIC 220204960: A Quadruple Star System Containing Two Strongly Interacting Eclipsing Binaries
We present a strongly interacting quadruple system associated with the K2
target EPIC 220204960. The K2 target itself is a Kp = 12.7 magnitude star at
Teff ~ 6100 K which we designate as "B-N" (blue northerly image). The host of
the quadruple system, however, is a Kp = 17 magnitude star with a composite
M-star spectrum, which we designate as "R-S" (red southerly image). With a 3.2"
separation and similar radial velocities and photometric distances, 'B-N' is
likely physically associated with 'R-S', making this a quintuple system, but
that is incidental to our main claim of a strongly interacting quadruple system
in 'R-S'. The two binaries in 'R-S' have orbital periods of 13.27 d and 14.41
d, respectively, and each has an inclination angle of >89 degrees. From our
analysis of radial velocity measurements, and of the photometric lightcurve, we
conclude that all four stars are very similar with masses close to 0.4 Msun.
Both of the binaries exhibit significant ETVs where those of the primary and
secondary eclipses 'diverge' by 0.05 days over the course of the 80-day
observations. Via a systematic set of numerical simulations of quadruple
systems consisting of two interacting binaries, we conclude that the outer
orbital period is very likely to be between 300 and 500 days. If sufficient
time is devoted to RV studies of this faint target, the outer orbit should be
measurable within a year.Comment: 20 pages, 18 figures, 7 tables; accepted for publication in MNRA
Analytic Quantization of the QCD String
We perform an analytic semi-classical quantization of the straight QCD string
with one end fixed and a massless quark on the other, in the limits of orbital
and radial dominant motion. We compare our results to the exact numerical
semi-classical quantization. We observe that the numerical semi-classical
quantization agrees well with our exact numerical canonical quantization.Comment: RevTeX, 10 pages, 9 figure
Planet Hunters X: Searching for Nearby Neighbors of 75 Planet and Eclipsing Binary Candidates from the K2 Kepler Extended Mission
We present high-resolution observations of a sample of 75 K2 targets from
Campaigns 1-3 using speckle interferometry on the Southern Astrophysical
Research (SOAR) telescope and adaptive optics (AO) imaging at the Keck II
telescope. The median SOAR -band and Keck -band detection limits at 1"
were ~mag and ~mag, respectively. This
sample includes 37 stars likely to host planets, 32 targets likely to be
eclipsing binaries (EBs), and 6 other targets previously labeled as likely
planetary false positives. We find nine likely physically bound companion stars
within 3" of three candidate transiting exoplanet host stars and six likely
EBs. Six of the nine detected companions are new discoveries; one of the six,
EPIC 206061524, is associated with a planet candidate. Among the EB candidates,
companions were only found near the shortest period ones ( days), which is
in line with previous results showing high multiplicity near short-period
binary stars. This high-resolution data, including both the detected companions
and the limits on potential unseen companions, will be useful in future planet
vetting and stellar multiplicity rate studies for planets and binaries.Comment: Accepted in A
Zodiacal Exoplanets In Time (ZEIT) I: A Neptune-sized planet orbiting an M4.5 dwarf in the Hyades Star Cluster
Studying the properties of young planetary systems can shed light on how the dynamics and structure of planets evolve during their most formative years. Recent K2 observations of nearby young clusters (10â800 Myr) have facilitated the discovery of such planetary systems. Here we report the discovery of a Neptune-sized planet transiting an M4.5 dwarf (K2-25) in the Hyades cluster (650â800 Myr). The light curve shows a strong periodic signal at 1.88 days, which we attribute to spot coverage and rotation. We confirm that the planet host is a member of the Hyades by measuring the radial velocity of the system with the high-resolution near-infrared spectrograph Immersion Grating Infrared Spectrometer. This enables us to calculate a distance based on K2-25's kinematics and membership to the Hyades, which in turn provides a stellar radius and mass to â 5%â10%, better than what is currently possible for most Kepler M dwarfs (12%â20%). We use the derived stellar density as a prior on fitting the K2 transit photometry, which provides weak constraints on eccentricity. Utilizing a combination of adaptive optics imaging and high-resolution spectra, we rule out the possibility that the signal is due to a bound or background eclipsing binary, confirming the transits' planetary origin. K2-25b has a radius (3.43}_(-0.31)^(+0.95)R_â) much larger than older Kepler planets with similar orbital periods (3.485 days) and host-star masses (0.29 M_â). This suggests that close-in planets lose some of their atmospheres past the first few hundred million years. Additional transiting planets around the Hyades, Pleiades, and Praesepe clusters from K2 will help confirm whether this planet is atypical or representative of other close-in planets of similar age
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