36 research outputs found
X-rays Trace the Volatile Content of Interstellar Objects
The non-detection of a coma surrounding 1I/`Oumuamua, the first discovered
interstellar object (ISO), has prompted a variety of hypotheses to explain its
nongravitational acceleration. Given that forthcoming surveys are poised to
identify analogues of this enigmatic object, it is prudent to devise
alternative approaches to characterization. In this study, we posit X-ray
spectroscopy as a surprisingly effective probe of volatile ISO compositions.
Heavily ionized metals in the solar wind interact with outgassed neutrals and
emit high-energy photons in a process known as charge exchange, and charge
exchange induced X-rays from comets and planetary bodies have been observed
extensively in our Solar System. We develop a model to predict the X-ray flux
of an ISO based on its chemical inventory and ephemeris. We find that while
standard cometary constituents, such as HO, CO, CO, and dust are best
probed via optical or infrared observations, we predict strong X-ray emission
generated by charge exchange with extended comae of H and N -- species
which lack strong infrared fluorescence transitions. We find that XMM-Newton
would have been sensitive to charge exchange emission from 1I/`Oumuamua during
the object's close approach to Earth, and that constraints on composition may
have been feasible. We argue for follow-up X-ray observations of newly
discovered ISOs with close-in perihelia. Compositional constraints on the
general ISO population could reconcile the apparently self-conflicting nature
of 1I/`Oumuamua, and provide insight into the earliest stages of planet
formation in extrasolar systems.Comment: Accepted to The Astrophysical Journal. 18 pages, 6 figure
Transmission spectroscopy of the lowest-density gas giant: metals and a potential extended outflow in HAT-P-67b
Extremely low-density exoplanets are tantalizing targets for atmospheric
characterization because of their promisingly large signals in transmission
spectroscopy. We present the first analysis of the atmosphere of the
lowest-density gas giant currently known, HAT-P-67 b. This inflated Saturn-mass
exoplanet sits at the boundary between hot and ultrahot gas giants, where
thermal dissociation of molecules begins to dominate atmospheric composition.
We observed a transit of HAT-P-67 b at high spectral resolution with CARMENES
and searched for atomic and molecular species using cross-correlation and
likelihood mapping. Furthermore, we explored potential atmospheric escape by
targeting H and the metastable helium line. We detect Ca II and Na I
with significances of 13.2 and 4.6, respectively. Unlike in
several ultrahot Jupiters, we do not measure a day-to-night wind. The large
line depths of Ca II suggest that the upper atmosphere may be more ionized than
models predict. We detect strong variability in H and the helium
triplet during the observations. These signals suggest the possible presence of
an extended planetary outflow that causes an early ingress and late egress. In
the averaged transmission spectrum, we measure redshifted absorption at the
and level in the H and He I triplet lines,
respectively. From an isothermal Parker wind model, we derive a mass loss rate
of and an outflow temperature of . However, due to the lack of a longer out-of-transit baseline in
our data, additional observations are needed to rule out stellar variability as
the source of the H and He signals.Comment: The Astronomical Journal, in press. 17 pages, 9 figure
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. 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
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
An Extreme Precision Radial Velocity Pipeline: First Radial Velocities from EXPRES
The EXtreme PREcision Spectrograph (EXPRES) is an environmentally stabilized,
fiber-fed, , optical spectrograph. It was recently commissioned at
the 4.3-m Lowell Discovery Telescope (LDT) near Flagstaff, Arizona. The
spectrograph was designed with a target radial-velocity (RV) precision of
30. In addition to instrumental innovations, the EXPRES
pipeline, presented here, is the first for an on-sky, optical, fiber-fed
spectrograph to employ many novel techniques---including an "extended flat"
fiber used for wavelength-dependent quantum efficiency characterization of the
CCD, a flat-relative optimal extraction algorithm, chromatic barycentric
corrections, chromatic calibration offsets, and an ultra-precise laser
frequency comb for wavelength calibration. We describe the reduction,
calibration, and radial-velocity analysis pipeline used for EXPRES and present
an example of our current sub-meter-per-second RV measurement precision, which
reaches a formal, single-measurement error of 0.3 for an
observation with a per-pixel signal-to-noise ratio of 250. These velocities
yield an orbital solution on the known exoplanet host 51 Peg that matches
literature values with a residual RMS of 0.895