K dwarfs and the chemical evolution of the Solar cylinder

K-dwarfs have life-times older than the present age of the Galactic disc, and are thus ideal stars to investigate the disc's chemical evolution. We have developed several photometric metallicity indicators for K dwarfs, based an a sample of accurate spectroscopic metallicities for 34 disc and halo G and K dwarfs. The photometric metallicities lead us to develop a metallicity index for K dwarfs based only on their position in the colour absolute-magnitude diagram. Metallicities have been determined for 431 single K dwarfs drawn from the Hipparcos catalog, selecting the stars by absolute magnitude and removing multiple systems. The sample is essentially a complete reckoning of the metal content in nearby K dwarfs. We use stellar isochrones to mark the stars by mass, and select a subset of 220 of the stars which is complete in a narrow mass interval. We fit the data with a model of the chemical evolution of the Solar cylinder. We find that only a modest cosmic scatter is required to fit our age metallicity relation. The model assumes two main infall episodes for the formation of the halo-thick disc and thin disc respectively. The new data confirms that the solar neighbourhood formed on a long timescale of order 7 Gyr.Comment: 14 pages, 15 figures, accepted by MNRA

Two planets around Kapteyn's star: a cold and a temperate super-Earth orbiting the nearest halo red dwarf

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society - Letters. ©: 2014 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.Exoplanets of a few Earth masses can be now detected around nearby low-mass stars using Doppler spectroscopy. In this Letter, we investigate the radial velocity variations of Kapteyn's star, which is both a sub-dwarf M-star and the nearest halo object to the Sun. The observations comprise archival and new HARPS (High Accuracy Radial velocity Planet Searcher), High Resolution Echelle Spectrometer (HIRES) and Planet Finder Spectrograph (PFS) Doppler measurements. Two Doppler signals are detected at periods of 48 and 120 d using likelihood periodograms and a Bayesian analysis of the data. Using the same techniques, the activity indices and archival All Sky Automated Survey (ASAS-3) photometry show evidence for low-level activity periodicities of the order of several hundred days. However, there are no significant correlations with the radial velocity variations on the same time-scales. The inclusion of planetary Keplerian signals in the model results in levels of correlated and excess white noise that are remarkably low compared to younger G, K and M dwarfs. We conclude that Kapteyn's star is most probably orbited by two super-Earth mass planets, one of which is orbiting in its circumstellar habitable zone, becoming the oldest potentially habitable planet known to date. The presence and long-term survival of a planetary system seem a remarkable feat given the peculiar origin and kinematic history of Kapteyn's star. The detection of super-Earth mass planets around halo stars provides important insights into planet-formation processes in the early days of the Milky Way.Peer reviewe

Gas flows, star formation and galaxy evolution

In the first part of this article we show how observations of the chemical evolution of the Galaxy: G- and K-dwarf numbers as functions of metallicity, and abundances of the light elements, D, Li, Be and B, in both stars and the interstellar medium (ISM), lead to the conclusion that metal poor HI gas has been accreting to the Galactic disc during the whole of its lifetime, and is accreting today at a measurable rate, ~2 Msun per year across the full disc. Estimates of the local star formation rate (SFR) using methods based on stellar activity, support this picture. The best fits to all these data are for models where the accretion rate is constant, or slowly rising with epoch. We explain here how this conclusion, for a galaxy in a small bound group, is not in conflict with graphs such as the Madau plot, which show that the universal SFR has declined steadily from z=1 to the present day. We also show that a model in which disc galaxies in general evolve by accreting major clouds of low metallicity gas from their surroundings can explain many observations, notably that the SFR for whole galaxies tends to show obvious variability, and fractionally more for early than for late types, and yields lower dark to baryonic matter ratios for large disc galaxies than for dwarfs. In the second part of the article we use NGC 1530 as a template object, showing from Fabry-Perot observations of its Halpha emission how strong shear in this strongly barred galaxy acts to inhibit star formation, while compression acts to stimulate it.Comment: 20 pages, 10 figures, to be presented at the "Penetrating Bars through Masks of Cosmic Dust" conference in South Africa, proceedings published by Kluwer, Eds. D.L. Block, K.C. Freeman, I. Puerari, & R. Groes

The chemical evolution of a Milky Way-like galaxy: the importance of a cosmologically motivated infall law

We aim at finding a cosmologically motivated infall law to understand if the LambdaCDM cosmology can reproduce the main chemical characteristics of a Milky Way-like spiral galaxy. In this work we test several different gas infall laws, starting from that suggested in the two-infall model for the chemical evolution of the Milky Way by Chiappini et al., but focusing on laws derived from cosmological simulations which follows a concordance LambdaCDM cosmology. By means of a detailed chemical evolution model for the solar vicinity, we study the effects of the different gas infall laws on the abundance patterns and the G-dwarf metallicity distribution. The cosmological gas infall law predicts two main gas accretion episodes. By means of this cosmologically motivated infall law, we study the star formation rate, the SNIa and SNII rate, the total amount of gas and stars in the solar neighbourhood and the behaviour of several chemical abundances. We find that the results of the two-infall model are fully compatible with the evolution of the Milky Way with cosmological accretion laws. A gas assembly history derived from a DM halo, compatible with the formation of a late-type galaxy from the morphological point of view, can produce chemical properties in agreement with the available observations.Comment: This paper has 26 pages, 19 figures and 5 table

Galactic archaeology with asteroseismology and spectroscopy: Red giants observed by CoRoT and APOGEE

With the advent of the space missions CoRoT and Kepler, it has recently become feasible to determine precise asteroseismic masses and relative ages for large samples of red giant stars. We present the CoRoGEE dataset, obtained from CoRoT light curves for 606 red giants in two fields of the Galactic disc that have been co-observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). We used the Bayesian parameter estimation code PARAM to calculate distances, extinctions, masses, and ages for these stars in a homogeneous analysis, resulting in relative statistical uncertainties of ≲2% in distance, ~4% in radius, ~9% in mass and ~25% in age. We also assessed systematic age uncertainties stemming from different input physics and mass loss. We discuss the correlation between ages and chemical abundance patterns of field stars over a broad radial range of the Milky Way disc (5 kpc <RGal< 14 kpc), focussing on the [α/Fe]-[Fe/H]-age plane in five radial bins of the Galactic disc. We find an overall agreement with the expectations of pure chemical-evolution models computed before the present data were available, especially for the outer regions. However, our data also indicate that a significant fraction of stars now observed near and beyond the solar neighbourhood migrated from inner regions. Mock CoRoGEE observations of a chemodynamical Milky Way disc model indicate that the number of high-metallicity stars in the outer disc is too high to be accounted for even by the strong radial mixing present in the model. The mock observations also show that the age distribution of the [α/Fe]-enhanced sequence in the CoRoGEE inner-disc field is much broader than expected from a combination of radial mixing and observational errors. We suggest that a thick-disc/bulge component that formed stars for more than 3 Gyr may account for these discrepancies. Our results are subject to future improvements due to (a) the still low statistics, because our sample had to be sliced into bins of Galactocentric distances and ages; (b) large uncertainties in proper motions (and therefore guiding radii); and (c) corrections to the asteroseismic mass-scaling relation. The situation will improve not only upon the upcoming Gaia data releases, but also with the foreseen increase in the number of stars with both seismic and spectroscopic information

Spectrometric composition of nearby K dwarfs

We have obtained relatively high resolution spectra of Northern hemisphere K dwarfs. This is the first spectrometric project dedicated only to K dwarfs. Earlier studies have concentrated on more massive F and G dwarfs. However, these stars have already undergone evolutionary effects, unlike K dwarfs, which offer more accurate information about the evolution of the Solar neighbourhood. We have determined the LTE abundances of 14 elements for 42 stars with initial metallicity range covered by $-1.52 < {\rm [Fe/H]} < 0.48$. We confirm the discrepancy in the abundances derived from neutral and ionized lines. The solution to this problem cannot just be the modification of initial physical parameters, but requires fundamental changes in the modeling of K dwarfs

A study of Kapteyns star

We present a review of the current knowledge of Kapteyn's Star (KS) - a nearby, low-metallicity M-dwarf, with an eccentric and retrograde Galactic orbit. A brief survey of its spectroscopic properties is provided, together with an analysis of its Galactic orbit in a Galaxy model that incorporates resonances. We propose that KS may have once belonged to a dwarf spheroidal galaxy that merged with the Galaxy, and whose present remnant, if it still exists, is a globular cluster similar to ω Cen

A study of Kapteyn's star

We present a review of the current knowledge of Kapteyn's Star (KS) – a nearby, low-metallicity M-dwarf, with an eccentric and retrograde Galactic orbit. A brief survey of its spectroscopic properties is provided, together with an analysis of its Galactic orbit in a Galaxy model that incorporates resonances. We propose that KS may have once belonged to a dwarf spheroidal galaxy that merged with the Galaxy, and whose present remnant, if it still exists, is a globular cluster similar to ω Cen