17 research outputs found
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On the discovery of K-enhanced and possibly Mg-depleted stars throughout the Milky Way
Stars with unusual elemental abundances offer clues about rare astrophysical
events or nucleosynthetic pathways. Stars with significantly depleted magnesium
and enhanced potassium ([Mg/Fe] 1) have to date only been
found in the massive globular cluster NGC 2419 and, to a lesser extent, NGC
2808. The origin of this abundance signature remains unknown, as does the
reason for its apparent exclusivity to these two globular clusters. Here we
present 112 field stars, identified from 454,180 LAMOST giants, that show
significantly enhanced [K/Fe] and possibly depleted [Mg/Fe] abundance ratios.
Our sample spans a wide range of metallicities (-1.5 < [Fe/H] < 0.3), yet none
show abundance ratios of [K/Fe] or [Mg/Fe] that are as extreme as those
observed in NGC 2419. If confirmed, the identified sample of stars represents
evidence that the nucleosynthetic process producing the anomalous abundances
ratios of [K/Fe] and [Mg/Fe] probably occurs at a wide range of metallicities.
This would suggest that pollution scenarios that are limited to early epochs
(such as Population III supernovae) are an unlikely explanation, although they
cannot be ruled out entirely. This sample is expected to help guide modelling
attempts to explain the origin of the Mg-K abundance signature
Hot Jupiters from Secular Planet--Planet Interactions
About 25 per cent of `hot Jupiters' (extrasolar Jovian-mass planets with
close-in orbits) are actually orbiting counter to the spin direction of the
star. Perturbations from a distant binary star companion can produce high
inclinations, but cannot explain orbits that are retrograde with respect to the
total angular momentum of the system. Such orbits in a stellar context can be
produced through secular (that is, long term) perturbations in hierarchical
triple-star systems. Here we report a similar analysis of planetary bodies,
including both octupole-order effects and tidal friction, and find that we can
produce hot Jupiters in orbits that are retrograde with respect to the total
angular momentum. With distant stellar mass perturbers, such an outcome is not
possible. With planetary perturbers, the inner orbit's angular momentum
component parallel to the total angular momentum need not be constant. In fact,
as we show here, it can even change sign, leading to a retrograde orbit. A
brief excursion to very high eccentricity during the chaotic evolution of the
inner orbit allows planet-star tidal interactions to rapidly circularize that
orbit, decoupling the planets and forming a retrograde hot Jupiter.Comment: accepted for publication by Nature, 3 figures (version after proof -
some typos corrected
Planet Populations as a Function of Stellar Properties
Exoplanets around different types of stars provide a window into the diverse
environments in which planets form. This chapter describes the observed
relations between exoplanet populations and stellar properties and how they
connect to planet formation in protoplanetary disks. Giant planets occur more
frequently around more metal-rich and more massive stars. These findings
support the core accretion theory of planet formation, in which the cores of
giant planets form more rapidly in more metal-rich and more massive
protoplanetary disks. Smaller planets, those with sizes roughly between Earth
and Neptune, exhibit different scaling relations with stellar properties. These
planets are found around stars with a wide range of metallicities and occur
more frequently around lower mass stars. This indicates that planet formation
takes place in a wide range of environments, yet it is not clear why planets
form more efficiently around low mass stars. Going forward, exoplanet surveys
targeting M dwarfs will characterize the exoplanet population around the lowest
mass stars. In combination with ongoing stellar characterization, this will
help us understand the formation of planets in a large range of environments.Comment: Accepted for Publication in the Handbook of Exoplanet
The Rossiter-McLaughlin effect in Exoplanet Research
The Rossiter-McLaughlin effect occurs during a planet's transit. It provides
the main means of measuring the sky-projected spin-orbit angle between a
planet's orbital plane, and its host star's equatorial plane. Observing the
Rossiter-McLaughlin effect is now a near routine procedure. It is an important
element in the orbital characterisation of transiting exoplanets. Measurements
of the spin-orbit angle have revealed a surprising diversity, far from the
placid, Kantian and Laplacian ideals, whereby planets form, and remain, on
orbital planes coincident with their star's equator. This chapter will review a
short history of the Rossiter-McLaughlin effect, how it is modelled, and will
summarise the current state of the field before describing other uses for a
spectroscopic transit, and alternative methods of measuring the spin-orbit
angle.Comment: Review to appear as a chapter in the "Handbook of Exoplanets", ed. H.
Deeg & J.A. Belmont
Circumstellar disks and planets. Science cases for next-generation optical/infrared long-baseline interferometers
We present a review of the interplay between the evolution of circumstellar
disks and the formation of planets, both from the perspective of theoretical
models and dedicated observations. Based on this, we identify and discuss
fundamental questions concerning the formation and evolution of circumstellar
disks and planets which can be addressed in the near future with optical and
infrared long-baseline interferometers. Furthermore, the importance of
complementary observations with long-baseline (sub)millimeter interferometers
and high-sensitivity infrared observatories is outlined.Comment: 83 pages; Accepted for publication in "Astronomy and Astrophysics
Review"; The final publication is available at http://www.springerlink.co
Extremely metal-poor stars from the cosmic dawn in the bulge of the Milky Way
This document is the Accepted Manuscript version of the following article: L. M. Howes, et al, âExtremely metal-poor stars from the cosmic dawn in the bulge of the Milky Wayâ, Nature, Vol. 527, November 2015. This manuscript version is made available under the Nature Researchâs Conditions of Use, http://www.nature.com/authors/policies/license.html#Self_archiving_policy. The final, published version is available online at DOI: http://www.nature.com/doifinder/10.1038/nature15747. © 2015 Macmillan Publishers Limited. All rights reservedThe first stars are predicted to have formed within 200 million years after the Big Bang, initiating the cosmic dawn. A true first star has not yet been discovered, although stars with tiny amounts of elements heavier than helium ('metals') have been found in the outer regions ('halo') of the Milky Way. The first stars and their immediate successors should, however, preferentially be found today in the central regions ('bulges') of galaxies, because they formed in the largest over-densities that grew gravitationally with time. The Milky Way bulge underwent a rapid chemical enrichment during the first 1-2 billion years, leading to a dearth of early, metal-poor stars. Here we report observations of extremely metal-poor stars in the Milky Way bulge, including one star with an iron abundance about 10,000 times lower than the solar value without noticeable carbon enhancement. We confirm that the most metal-poor bulge stars are on tight orbits around the Galactic Centre, rather than being halo stars passing through the bulge, as expected for stars formed at redshifts greater than 15. Their chemical compositions are in general similar to typical halo stars of the same metallicity although intriguing differences exist, including lower abundances of carbon.Peer reviewedFinal Accepted Versio
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Absence of a metallicity effect for ultra-short-period planets
Ultra-short-period (USP) planets are a newly recognized class of planets with periods shorter than one day and radii smaller than about 2 R â. It has been proposed that USP planets are the solid cores of hot Jupiters that have lost their gaseous envelopes due to photo-evaporation or Roche lobe overflow. We test this hypothesis by asking whether USP planets are associated with metal-rich stars, as has long been observed for hot Jupiters. We find the metallicity distributions of USP-planet and hot-Jupiter hosts to be significantly different (p = 310-4) based on Keck spectroscopy of Kepler stars. Evidently, the sample of USP planets is not dominated by the evaporated cores of hot Jupiters. The metallicity distribution of stars with USP planets is indistinguishable from that of stars with short-period planets with sizes between 2 and 4 R â. Thus, it remains possible that the USP planets are the solid cores of formerly gaseous planets that are smaller than Neptune
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Constraints on the Obliquities of Kepler Planet-hosting Stars
Stars with hot Jupiters have obliquities ranging from 0° to 180°, but relatively little is known about the obliquities of stars with smaller planets. Using data from the California-Kepler Survey, we investigate the obliquities of stars with planets spanning a wide range of sizes, most of which are smaller than Neptune. First, we identify 156 planet hosts for which measurements of the projected rotation velocity () and rotation period are both available. By combining estimates of v and , we find nearly all the stars to be compatible with high inclination, and hence, low obliquity (âČ20°). Second, we focus on a sample of 159 hot stars ( K) for which is available but not necessarily the rotation period. We find six stars for which is anomalously low, an indicator of high obliquity. Half of these have hot Jupiters, even though only 3% of the stars that were searched have hot Jupiters. We also compare the distribution of the hot stars with planets to that of 83 control stars selected without prior knowledge of planets. The mean of the control stars is lower than that of the planet hosts by a factor of approximately , as one would expect if the planet hosts have low obliquities. All these findings suggest that the Kepler planet-hosting stars generally have low obliquities, with the exception of hot stars with hot Jupiters
Tidal Interactions between Binary Stars Can Drive Lithium Production in Low-mass Red Giants
Theoretical models of stellar evolution predict that most of the lithium
inside a star is destroyed as the star becomes a red giant. However,
observations reveal that about 1% of red giants are peculiarly rich in lithium,
often exceeding the amount in the interstellar medium or predicted from the Big
Bang. With only about 150 lithium-rich giants discovered in the past four
decades, and no distinguishing properties other than lithium enhancement, the
origin of lithium-rich giant stars is one of the oldest problems in stellar
astrophysics. Here we report the discovery of 2,330 low-mass (1 to
3) lithium-rich giant stars, which we argue are consistent with
internal lithium production that is driven by tidal spin-up by a binary
companion. Our sample reveals that most lithium-rich giants have helium-burning
cores (), and that the frequency of lithium-rich giants rises
with increasing stellar metallicity. We find that while planet accretion may
explain some lithium-rich giants, it cannot account for the majority that have
helium-burning cores. We rule out most other proposed explanations as the
primary mechanism for lithium-rich giants, including all stages related to
single star evolution. Our analysis shows that giants remain lithium-rich for
only about two million years. A prediction from this lithium depletion
timescale is that most lithium-rich giants with a helium-burning core have a
binary companion