1,077 research outputs found
M Dwarf Metallicities and Giant Planet Occurrence: Ironing Out Uncertainties and Systematics
Comparisons between the planet populations around solar-type stars and those
orbiting M dwarfs shed light on the possible dependence of planet formation and
evolution on stellar mass. However, such analyses must control for other
factors, i.e. metallicity, a stellar parameter which strongly influences the
occurrence of gas giant planets. We obtained infrared spectra of 121 M dwarfs
stars monitored by the California Planet Search (CPS) and determined
metallicities with an accuracy of 0.08 dex. The mean and standard deviation of
the sample is -0.05 and 0.20 dex, respectively. We parameterized the
metallicity dependence of the occurrence of giant planets on orbits with period
less than 2 yr around solar-type stars and applied this to our M dwarf sample
to estimate the expected number of giant planets. The number of detected
planets (3) is lower than the predicted number (6.4) but the difference is not
very significant (12% probability of finding as many or fewer planets). The
three M dwarf planet hosts are not especially metal rich and the most likely
value of the power-law index relating planet occurrence to metallicity is 1.06
dex per dex for M dwarfs compared to 1.80 for solar-type stars; this
difference, however, is comparable to uncertainties. Giant planet occurrence
around both types of stars allows, but does not necessarily require, mass
dependence of dex per dex. The actual planet-mass-metallicity relation
may be complex and elucidating it will require larger surveys like those to be
conducted by ground-based infrared spectrographs and the Gaia space astrometry
mission.Comment: Accepted to The Astrophysical Journa
X-ray/optical classification of cluster mergers and the evolution of the cluster merger fraction
We present the results of a simple but robust morphological classification of
a statis- tically complete sample of 108 of the most X-ray luminous clusters at
0.15 < z < 0.7 observed with Chandra. Our aims are to (a) identify the most
disturbed massive clusters to be used as gravitational lenses for studies of
the distant universe and as probes of particle acceleration mechanisms
resulting in non-thermal radio emission, (b) find cluster mergers featuring
subcluster trajectories that make them suitable for quantitative analyses of
cluster collisions, and (c) constrain the evolution with redshift of the
cluster merger fraction. Finally, (d) this paper represents the third public
release of clusters from the MACS sample, adding 24 clusters to the 46
published previously. To classify clusters by degree of relaxation, we use the
projected offset of the brightest cluster galaxy from the peak (or the global
centroid) of the X-ray emission as a measure of the segregation between the
intracluster gas and dark matter. Regarding (a), we identify ten complex
systems likely to have undergone multiple merger events in the recent past.
Regarding (b), we identify eleven systems likely to be post-collision, binary,
head-on mergers (BHOMs), as well as another six mergers that are possible BHOMs
but probably harder to interpret because of non-negligible impact parameters
and merger axes closer to our line of sight. Regarding (c), we find a highly
significant increase with redshift in the fraction of morphologically disturbed
clusters starting at z \sim 0.4, in spite of a detection bias in our sample
against very disturbed systems at high redshift. A larger sample of clusters
with high-quality X-ray data in particular at high redshift will be needed to
trace the evolutionary history of cluster growth and relaxation closer to the
primary epoch of cluster formation z \sim 1.Comment: Accepted to MNRA
An Understanding of the Shoulder of Giants: Jovian Planets around Late K Dwarf Stars and the Trend with Stellar Mass
Analyses of exoplanet statistics suggest a trend of giant planet occurrence
with host star mass, a clue to how planets like Jupiter form. One missing piece
of the puzzle is the occurrence around late K dwarf stars (masses of
0.5-0.75Msun and effective temperatures of 3900-4800K). We analyzed four years
of Doppler radial velocities data of 110 late K dwarfs, one of which hosts two
previously reported giant planets. We estimate that 4.0+/-2.3% of these stars
have Saturn-mass or larger planets with orbital periods <245d, depending on the
planet mass distribution and RV variability of stars without giant planets. We
also estimate that 0.7+/-0.5% of similar stars observed by Kepler have giant
planets. This Kepler rate is significantly (99% confidence) lower than that
derived from our Doppler survey, but the difference vanishes if only the single
Doppler system (HIP 57274) with completely resolved orbits is considered. The
difference could also be explained by the exclusion of close binaries (without
giant planets) from the Doppler but not Kepler surveys, the effect of
long-period companions and stellar noise on the Doppler data, or an intrinsic
difference between the two populations. Our estimates for late K dwarfs bridge
those for solar-type stars and M dwarfs and support a positive trend with
stellar mass. Small sample size precludes statements about finer structure,
e.g. a "shoulder" in the distribution of giant planets with stellar mass.
Future surveys such as the Next Generation Transit Survey and the Transiting
Exoplanet Satellite Survey will ameliorate this deficiency.Comment: Accepted to The Astrophysical Journa
Spectro-Thermometry of M dwarfs and their candidate planets: too hot, too cool, or just right?
We use moderate-resolution spectra of nearby late K and M dwarf stars with
parallaxes and interferometrically determined radii to refine their effective
temperatures, luminosities, and metallicities. We use these revised values to
calibrate spectroscopic techniques to infer the fundamental parameters of more
distant late-type dwarf stars. We demonstrate that, after masking out poorly
modeled regions, the newest version of the PHOENIX atmosphere models accurately
reproduce temperatures derived bolometrically. We apply methods to late-type
hosts of transiting planet candidates in the Kepler field, and calculate
effective temperature, radius, mass, and luminosity with typical errors of 57
K, 7%, 11%, and 13%, respectively. We find systematic offsets between our
values and those from previous analyses of the same stars, which we attribute
to differences in atmospheric models utilized for each study. We investigate
which of the planets in this sample are likely to orbit in the circumstellar
habitable zone. We determine that four candidate planets (KOI 854.01, 1298.02,
1686.01, and 2992.01) are inside of or within 1-sigma of a conservative
definition of the habitable zone, but that several planets identified by
previous analyses are not (e.g. KOI 1422.02 and KOI 2626.01). Only one of the
four habitable-zone planets is Earth sized, suggesting a downward revision in
the occurrence of such planets around M dwarfs. These findings highlight the
importance of measuring accurate stellar parameters when deriving parameters of
their orbiting planets.Comment: 17 pages, 16 figures, accepted to ApJ. Added requisite significant
Figures to Equations 6-8. Fixed a formatting error in the machine readable
tables. All spectra now downloadable from
http://www.as.utexas.edu/~amann/files/th_spec
The Invisible Majority? Evolution and Detection of Outer Planetary Systems without Gas Giants
We present 230 realizations of a numerical model of planet formation in
systems without gas giants. These represent a scenario in which protoplanets
grow in a region of a circumstellar disk where water ice condenses (the "ice
line''), but fail to accrete massive gas envelopes before the gaseous disk is
dispersed. Each simulation consists of a small number of gravitationally
interacting oligarchs and a much larger number of small bodies that represent
the natal disk of planetesimals. We investigate systems with varying initial
number of oligarchs, oligarch spacing, location of the ice line, total mass in
the ice line, and oligarch mean density. Systems become chaotic in ~1 Myr but
settle into stable configurations in 10-100 Myr. We find: (1) runs consistently
produce a 5-9 Earth mass planet at a semimajor axis of 0.25-0.6 times the
position of the ice line, (2) the distribution of planets' orbital
eccentricities is distinct from, and skewed toward lower values than the
observed distribution of (giant) exoplanet orbits, (3) inner systems of two
dominant planets (e.g., Earth and Venus) are not stable or do not form because
of the gravitational influence of the innermost icy planet. The planets
predicted by our model are unlikely to be detected by current Doppler
observations. Microlensing is currently sensitive to the most massive planets
found in our simulations. A scenario where up to 60% of stars host systems such
as those we simulate is consistent with all the available data. We predict
that, if this scenario holds, the NASA Kepler spacecraft will detect about 120
planets by two or more transits over the course of its 3.5 yr mission. Future
microlensing surveys will detect ~130 analogs over a 5 yr survey. Finally, the
Space Interferometry Mission (SIM-Lite) should be capable of detecting 96% of
the innermost icy planets over the course of a 5 yr mission.Comment: 17 pages, 16 figure
Radial Trends in IMF-Sensitive Absorption Features in Two Early-Type Galaxies: Evidence for Abundance-Driven Gradients
Samples of early-type galaxies show a correlation between stellar velocity
dispersion and the stellar initial mass function (IMF) as inferred from
gravity-sensitive absorption lines in the galaxies' central regions. To search
for spatial variations in the IMF, we have observed two early-type galaxies
with Keck/LRIS and measured radial gradients in the strengths of absorption
features from 4000-5500 \AA and 8000-10,000 \AA. We present spatially
resolved measurements of the dwarf-sensitive spectral indices NaI (8190 \AA)
and Wing-Ford FeH (9915 \AA), as well as indices for species of H, C, CN,
Mg, Ca, TiO, and Fe. Our measurements show a metallicity gradient in both
objects, and Mg/Fe consistent with a shallow gradient in \alpha-enhancement,
matching widely observed trends for massive early-type galaxies. The NaI index
and the CN index at 4160 \AA exhibit significantly steeper gradients,
with a break at ( pc). Inside this radius
NaI strength increases sharply toward the galaxy center, consistent with a
rapid central rise in [Na/Fe]. In contrast, the ratio of FeH to Fe index
strength decreases toward the galaxy center. This behavior cannot be reproduced
by a steepening IMF inside if the IMF is a single power law.
While gradients in the mass function above may occur,
exceptional care is required to disentangle these IMF variations from the
extreme variations in individual element abundances near the galaxies' centers.Comment: Accepted for publication in ApJ. Updates from v1 include an expanded
comparison of measured index strengths to SPS models. 20 page body + 7 page
appendix + references. Includes 25 figure
They might be giants: luminosity class, planet frequency, and planet-metallicity relation of the coolest Kepler target stars
We estimate the stellar parameters of late K and early M type Kepler target
stars. We obtain medium resolution visible spectra of 382 stars with Kp-J>2
(~K5 and later spectral type). We determine luminosity class by comparing the
strength of gravity-sensitive indices (CaH, K I, Ca II, and Na I) to their
strength in a sample of stars of known luminosity class. We find that giants
constitute 96+-% of the bright (Kp<14) Kepler target stars, and 7+-3% of dim
(Kp>14) stars, significantly higher than fractions based on the stellar
parameters quoted in the Kepler Input Catalog (KIC). The KIC effective
temperatures are systematically (110 +15 -35} K) higher than temperatures we
determine from fitting our spectra to PHOENIX stellar models. Through Monte
Carlo simulations of the Kepler exoplanet candidate population, we find a
planet occurrence of 0.36+-0.08 when giant stars are properly removed, somewhat
higher than when a KIC log(g)>4 criterion is used (0.27+-0.05). Lastly, we show
that there is no significant difference in g-r color (a probe of metallicity)
between late-type Kepler stars with transiting Earth-to-Neptune sized exoplanet
candidates and dwarf stars with no detected transits. We show that a previous
claimed offset between these two populations is most likely an artifact of
including a large number of misidentified giants.Comment: Accepted to Ap
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