58 research outputs found
A High Spectral Resolution Study of the Soft X-ray Background with the X-ray Quantum Calorimeter
We present here a combined analysis of four high spectral resolution
observations of the Diffuse X-ray Background (DXRB), made using the University
of Wisconsin-Madison/Goddard Space Flight Center X-ray Quantum Calorimeter
(XQC) sounding rocket payload. The observed spectra support the existence of a
keV Local Hot Bubble and a keV Hot Halo, with discrepancies
between repeated observations compatible with expected contributions of
time-variable emission from Solar Wind Charge Exchange (SWCX). An additional
component of keV emission observed only at low galactic latitudes can
be consistently explained by unresolved dM stars.Comment: 21 pages, 6 figures, accepted for publication in Ap
Does masting scale with plant size? High reproductive variability and low synchrony in small and unproductive individuals
Background and Aims In a range of plant species, the distribution of individual mean fecundity is skewed and dominated by a few highly fecund individuals. Larger plants produce greater seed crops, but the exact nature of the relationship between size and reproductive patterns is poorly understood. This is especially clear in plants that reproduce by exhibiting synchronized quasi-periodic variation in fruit production, a process called masting.
Methods We investigated covariation of plant size and fecundity with individual-plant-level masting patterns and seed predation in 12 mast-seeding species: Pinus pinea, Astragalus scaphoides, Sorbus aucuparia, Quercus ilex, Q. humilis, Q. rubra, Q. alba, Q. montana, Chionochloa pallens, C. macra, Celmisia lyallii and Phormium tenax.
Key Results Fecundity was non-linearly related to masting patterns. Small and unproductive plants frequently failed to produce any seeds, which elevated their annual variation and decreased synchrony. Above a low fecundity threshold, plants had similar variability and synchrony, regardless of their size and productivity.
Conclusions Our study shows that within-species variation in masting patterns is correlated with variation in fecundity, which in turn is related to plant size. Low synchrony of low-fertility plants shows that the failure years were idiosyncratic to each small plant, which in turn implies that the small plants fail to reproduce because of plant-specific factors (e.g. internal resource limits). Thus, the behaviour of these sub-producers is apparently the result of trade-offs in resource allocation and environmental limits with which the small plants cannot cope. Plant size and especially fecundity and propensity for mast failure years play a major role in determining the variability and synchrony of reproduction in plants
EXPRES. II. Searching for Planets Around Active Stars: A Case Study of HD 101501
By controlling instrumental errors to below 10 cm/s, the EXtreme PREcision
Spectrograph (EXPRES) allows for a more insightful study of photospheric
velocities that can mask weak Keplerian signals. Gaussian Processes (GP) have
become a standard tool for modeling correlated noise in radial velocity
datasets. While GPs are constrained and motivated by physical properties of the
star, in some cases they are still flexible enough to absorb unresolved
Keplerian signals. We apply GP regression to EXPRES radial velocity
measurements of the 3.5 Gyr old chromospherically active Sun-like star, HD
101501. We obtain tight constraints on the stellar rotation period and the
evolution of spot distributions using 28 seasons of ground-based photometry, as
well as recent data. Light curve inversion was carried out on both
photometry datasets to reveal the spot distribution and spot evolution
timescales on the star. We find that the m/s rms radial velocity
variations in HD 101501 are well-modeled with a GP stellar activity model
without planets, yielding a residual rms scatter of 45 cm/s. We carry out
simulations, injecting and recovering signals with the GP framework, to
demonstrate that high-cadence observations are required to use GPs most
efficiently to detect low-mass planets around active stars like HD 101501.
Sparse sampling prevents GPs from learning the correlated noise structure and
can allow it to absorb prospective Keplerian signals. We quantify the moderate
to high-cadence monitoring that provides the necessary information to
disentangle photospheric features using GPs and to detect planets around active
stars.Comment: 25 pages, 16 figures, accepted to A
EXPRES IV: Two Additional Planets Orbiting Coronae Borealis Reveal Uncommon System Architecture
Thousands of exoplanet detections have been made over the last twenty-five
years using Doppler observations, transit photometry, direct imaging, and
astrometry. Each of these methods is sensitive to different ranges of orbital
separations and planetary radii (or masses). This makes it difficult to fully
characterize exoplanet architectures and to place our solar system in context
with the wealth of discoveries that have been made. Here, we use the EXtreme
PREcision Spectrograph (EXPRES) to reveal planets in previously undetectable
regions of the mass-period parameter space for the star Coronae
Borealis. We add two new planets to the previously known system with one hot
Jupiter in a 39-day orbit and a warm super-Neptune in a 102-day orbit. The new
detections include a temperate Neptune planet ( M)
in a 281.4-day orbit and a hot super-Earth ( M) in a
12.95-day orbit. This result shows that details of planetary system
architectures have been hiding just below our previous detection limits; this
signals an exciting era for the next generation of extreme precision
spectrographs.Comment: Accepted to AJ; 20 pages, 13 figures, 5 Table
Measured Spin-Orbit Alignment of Ultra-Short Period Super-Earth 55 Cancri e
A planet's orbital alignment places important constraints on how a planet
formed and consequently evolved. The dominant formation pathway of ultra-short
period planets ( day) is particularly mysterious as such planets most
likely formed further out, and it is not well understood what drove their
migration inwards to their current positions. Measuring the orbital alignment
is difficult for smaller super-Earth/sub-Neptune planets, which give rise to
smaller amplitude signals. Here we present radial velocities across two
transits of 55 Cancri e, an ultra-short period Super-Earth, observed with the
Extreme Precision Spectrograph (EXPRES). Using the classical
Rossiter-McLaughlin (RM) method, we measure 55 Cnc e's sky-projected stellar
spin-orbit alignment (i.e., the projected angle between the planet's orbital
axis and its host star's spin axis) to be with an unprojected angle of . The best-fit RM model to the EXPRES data has a radial velocity
semi-amplitude of just . The spin-orbit
alignment of 55 Cnc e favors dynamically gentle migration theories for
ultra-short period planets, namely tidal dissipation through low-eccentricity
planet-planet interactions and/or planetary obliquity tides.Comment: 12 pages, 4 figures, published in Nature Astronom
EXPRES I. HD~3651 an Ideal RV Benchmark
The next generation of exoplanet-hunting spectrographs should deliver up to
an order of magnitude improvement in radial velocity precision over the
standard 1 m/s state of the art. This advance is critical for enabling the
detection of Earth-mass planets around Sun-like stars. New calibration
techniques such as laser frequency combs and stabilized etalons ensure that the
instrumental stability is well characterized. However, additional sources of
error include stellar noise, undetected short-period planets, and telluric
contamination. To understand and ultimately mitigate error sources, the
contributing terms in the error budget must be isolated to the greatest extent
possible. Here, we introduce a new high cadence radial velocity program, the
EXPRES 100 Earths program, which aims to identify rocky planets around bright,
nearby G and K dwarfs. We also present a benchmark case: the 62-d orbit of a
Saturn-mass planet orbiting the chromospherically quiet star, HD 3651. The
combination of high eccentricity (0.6) and a moderately long orbital period,
ensures significant dynamical clearing of any inner planets. Our Keplerian
model for this planetary orbit has a residual RMS of 58 cm/s over a
month time baseline. By eliminating significant contributors to the radial
velocity error budget, HD 3651 serves as a standard for evaluating the long
term precision of extreme precision radial velocity (EPRV) programs.Comment: 11 pages, 6 figures, accepted for publication in Astronomical Journa
In-flight Calibration of Hitomi Soft X-ray Spectrometer (3) Effective Area
We present the result of the in-flight calibration of the effective area of
the Soft X-ray Spectrometer (SXS) onboard the Hitomi X-ray satellite using an
observation of the Crab nebula. We corrected for the artifacts when observing
high count rate sources with the X-ray microcalorimeter. We then constructed a
spectrum in the 0.5-20 keV band, which we modeled with a single power-law
continuum attenuated by an interstellar extinction. We evaluated the systematic
uncertainty upon the spectral parameters by various calibration items. In the
2-12 keV band, the SXS result is consistent with the literature values in flux
(2.20 0.08) 10 erg s cm with a 1
statistical uncertainty) but is softer in the power-law index (2.19
0.11). The discrepancy is attributable to the systematic uncertainty of about
6/7% and 2/5% respectively for the flux and the power-law index.
The softer spectrum is affected primarily by the systematic uncertainty of the
Dewar gate valve transmission and the event screening.Comment: 13 pages, 7 figures.PASJ accepte
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