408 research outputs found
Tests of In-Situ Formation Scenarios for Compact Multiplanet Systems
Kepler has identified over 600 multiplanet systems, many of which have
several planets with orbital distances smaller than that of Mercury -- quite
different from the Solar System. Because these systems may be difficult to
explain in the paradigm of core accretion and disk migration, it has been
suggested that they formed in situ within protoplanetary disks with high solid
surface densities. The strong connection between giant planet occurrence and
stellar metallicity is thought to be linked to enhanced solid surface densities
in disks around metal-rich stars, so the presence of a giant planet can be a
detectable sign of planet formation in a high solid surface density disk. I
formulate quantitative predictions for the frequency of long-period giant
planets in these in situ models of planet formation by translating the proposed
increase in disk mass into an equivalent metallicity enhancement. I rederive
the scaling of giant planet occurrence with metallicity as P_gp =
0.05_{-0.02}^{+0.02} x 10^{(2.1 +/- 0.4) [M/H]} = 0.08_{-0.03}^{+0.02} x
10^{(2.3 +/- 0.4) [Fe/H]} and show that there is significant tension between
the frequency of giant planets suggested by the minimum mass extrasolar nebula
scenario and the observational upper limits. This fact suggests that high-mass
disks alone cannot explain the observed properties of the close-in Kepler
multiplanet systems and that migration is still a necessary contributor to
their formation. More speculatively, I combine the metallicity scaling of giant
planet occurrence with recently published small planet occurrence rates to
estimate the number of Solar System analogs in the Galaxy. I find that in the
Milky Way there are perhaps 4 x 10^6 true Solar System analogs with an FGK star
hosting both a terrestrial planet in the habitable zone and a long-period giant
planet companion.Comment: 16 pages, 7 figures, and 2 tables in emulateapj format; submitted to
Ap
Chemistry of the Most Metal-poor Stars in the Bulge and the z > 10 Universe
Metal-poor stars in the Milky Way are local relics of the epoch of the first
stars and the first galaxies. However, a low metallicity does not prove that a
star formed in this ancient era, as metal-poor stars form over a range of
redshift in different environments. Theoretical models of Milky Way formation
have shown that at constant metallicity, the oldest stars are those closest to
the center of the Galaxy on the most tightly-bound orbits. For that reason, the
most metal-poor stars in the bulge of the Milky Way provide excellent tracers
of the chemistry of the high-redshift universe. We report the dynamics and
detailed chemical abundances of three stars in the bulge with [Fe/H]
, two of which are the most metal-poor stars in the bulge in the
literature. We find that with the exception of scandium, all three stars follow
the abundance trends identified previously for metal-poor halo stars. These
three stars have the lowest [Sc II/Fe] abundances yet seen in -enhanced
giant stars in the Galaxy. Moreover, all three stars are outliers in the
otherwise tight [Sc II/Fe]-[Ti II/Fe] relation observed among metal-poor halo
stars. Theoretical models predict that there is a 30% chance that at least one
of these stars formed at , while there is a 70% chance that at
least one formed at . These observations imply that
by , the progenitor galaxies of the Milky Way had both reached [Fe/H]
and established the abundance pattern observed in extremely
metal-poor stars.Comment: Submitted to ApJ on 2014 December 23, accepted 2015 May 4th after
minor revisions. ArXiv tarball includes referee report and respons
The Best and Brightest Metal-Poor Stars
The chemical abundances of large samples of extremely metal-poor (EMP) stars
can be used to investigate metal-free stellar populations, supernovae, and
nucleosynthesis as well as the formation and galactic chemical evolution of the
Milky Way and its progenitor halos. However, current progress on the study of
EMP stars is being limited by their faint apparent magnitudes. The acquisition
of high signal-to-noise spectra for faint EMP stars requires a major telescope
time commitment, making the construction of large samples of EMP star
abundances prohibitively expensive. We have developed a new, efficient
selection that uses only public, all-sky APASS optical, 2MASS near-infrared,
and WISE mid-infrared photometry to identify bright metal-poor star candidates
through their lack of molecular absorption near 4.6 microns. We have used our
selection to identify 11,916 metal-poor star candidates with V < 14, increasing
the number of publicly-available candidates by more than a factor of five in
this magnitude range. Their bright apparent magnitudes have greatly eased
high-resolution follow-up observations that have identified seven previously
unknown stars with [Fe/H] <~ -3.0. Our follow-up campaign has revealed that
3.8^{+1.3}_{-1.1}% of our candidates have [Fe/H] <~ -3.0 and
32.5^{+3.0}_{-2.9}% have -3.0 <~ [Fe/H] <~ -2.0. The bulge is the most likely
location of any existing Galactic Population III stars, and an infrared-only
variant of our selection is well suited to the identification of metal-poor
stars in the bulge. Indeed, two of our confirmed metal-poor stars with [Fe/H]
<~ -2.7 are within about 2 kpc of the Galactic Center. They are among the most
metal-poor stars known in the bulge.Comment: 28 pages, 6 figures, and 4 tables in emulateapj format; accepted for
publication in Ap
Evidence for the Tidal Destruction of Hot Jupiters by Subgiant Stars
Tidal transfer of angular momentum is expected to cause hot Jupiters to
spiral into their host stars. Although the timescale for orbital decay is very
uncertain, it should be faster for systems with larger and more evolved stars.
Indeed, it is well established that hot Jupiters are found less frequently
around subgiant stars than around main-sequence stars. However, the
interpretation of this finding has been ambiguous, because the subgiants are
also thought to be more massive than the F- and G-type stars that dominate the
main-sequence sample. Consequently it has been unclear whether the absence of
hot Jupiters is due to tidal destruction, or inhibited formation of those
planets around massive stars. Here we show that the Galactic space motions of
the planet-hosting subgiant stars demand that on average they be similar in
mass to the planet-hosting main-sequence F- and G-type stars. Therefore the two
samples are likely to differ only in age, and provide a glimpse of the same
exoplanet population both before and after tidal evolution. As a result, the
lack of hot Jupiters orbiting subgiants is clear evidence for their tidal
destruction. Questions remain, though, about the interpretation of other
reported differences between the planet populations around subgiants and
main-sequence stars, such as their period and eccentricity distributions and
overall occurrence rates.Comment: 12 pages and 6 figures in emulateapj format; accepted for publication
in Ap
Shock formation around planets orbiting M-dwarf stars
Bow shocks can be formed around planets due to their interaction with the
coronal medium of the host stars. The net velocity of the particles impacting
on the planet determines the orientation of the shock. At the Earth's orbit,
the (mainly radial) solar wind is primarily responsible for the formation of a
shock facing towards the Sun. However, for close-in planets that possess high
Keplerian velocities and are frequently located at regions where the host
star's wind is still accelerating, a shock may develop ahead of the planet. If
the compressed material is able to absorb stellar radiation, then the signature
of bow shocks may be observed during transits. Bow-shock models have been
investigated in a series of papers (Vidotto et al. 2010, 2011,a,b; Llama et al.
2011) for known transiting systems. Once the signature of a bow-shock is
observed, one can infer the magnetic field intensity of the transiting planet.
Here, we investigate the potential to use this model to detect magnetic fields
of (hypothetical) planets orbiting inside the habitable zone of M-dwarf stars.
For these cases, we show, by means of radiative transfer simulations, that the
detection of bow-shocks of planets surrounding M-dwarf stars may be more
difficult than for the case of close-in giant planets orbiting solar-type
stars.Comment: Published in Astronomische Nachrichten, Vol. 9-10/2011, page
1055-1061. 7 pages, 5 figure
The Oblique Orbit of the Super-Neptune HAT-P-11b
We find the orbit of the Neptune-sized exoplanet HAT-P-11b to be highly
inclined relative to the equatorial plane of its host star. This conclusion is
based on spectroscopic observations of two transits, which allowed the
Rossiter-McLaughlin effect to be detected with an amplitude of 1.5 m/s. The
sky-projected obliquity is 103_{-10}^{+26} degrees. This is the smallest
exoplanet for which spin-orbit alignment has been measured. The result favors a
migration scenario involving few-body interactions followed by tidal
dissipation. This finding also conforms with the pattern that the systems with
the weakest tidal interactions have the widest spread in obliquities. We
predict that the high obliquity of HAT-P-11 will be manifest in transit light
curves from the Kepler spacecraft: starspot-crossing anomalies will recur at
most once per stellar rotation period, rather than once per orbital period as
they would for a well-aligned system.Comment: ApJ Letters, in press [5 pages
A short-period censor of sub-Jupiter mass exoplanets with low density
Despite the existence of many short-period hot Jupiters, there is not one hot
Neptune with an orbital period less than 2.5 days. Here we discuss a cluster
analysis of the currently known 106 transiting exoplanets to investigate a
possible explanation for this observation. We find two distinct clusters in the
mass-density space, one with hot Jupiters with a wide range of orbital periods
(0.8--114 days) and a narrow range of planet radii (1.2 +- 0.2 R_J); and
another one with a mixture of super-Earths, hot Neptunes and hot Jupiters,
exhibiting a surprisingly narrow period distribution (3.7 +- 0.8 days). These
two clusters follow different distributions in the period-radius parameter
plane. The branch of sub-Jupiter mass exoplanets is censored by the orbital
period at large-radii: no planets with mass between 0.02--0.8 M_J or with
radius between 0.25--1.0 R_J are known with P_orb<2.5 days. This clustering is
not predicted by current theories of planet formation and evolution that we
also review briefly.Comment: 5 pages, 2 figures (5 panels), accepted by ApJ
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