Cool Stars Research Articles

Editor's note: Astron. Nachr. 1–2/2013

Astronomische NachrichtenVolume 334, Issue 1-2 p. 3-3 Editor's NoteFree Access Editor's note: Astron. Nachr. 1–2/2013 Mercedes López-Morales, Mercedes López-MoralesSearch for more papers by this authorKlaus G. Strassmeier, Klaus G. StrassmeierSearch for more papers by this author Mercedes López-Morales, Mercedes López-MoralesSearch for more papers by this authorKlaus G. Strassmeier, Klaus G. StrassmeierSearch for more papers by this author First published: 06 February 2013 https://doi.org/10.1002/asna.201390002AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume334, Issue1-2Special Issue: Cool Stars 17February 2013Pages 3-3 RelatedInformation

Non‐thermal processes in coronae and beyond

AbstractThis contribution summarizes the splinter session “Non‐thermal processes in coronae and beyond” held at the Cool Stars 17 workshop in Barcelona in 2012. It covers new developments in high energy non‐thermal effects in the Earth's exosphere, solar and stellar flares, the diffuse emission in star forming regions and reviews the state and the challenges of the underlying atomic databases. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cover Picture: Astron. Nachr. 1–2/2013

AbstractA section of the advertisement poster of the 17th Cool Stars, Stellar systems, and the Sun meeting in Barcelona in June 2012 showing the Barcelona sky line and the many stellar connections. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Multiplicity of cool dwarfs

AbstractMultiple systems have long been used to probe the origin and evolution of stars of all masses. Only in the past 10–15 years have such studies been extended to brown dwarfs and the lowest mass stars through binary surveys of both young star forming regions and the older field population. In addition, a groundswell of interest in M dwarfs in recent years has resulted in large, modern datasets for these most common stars in the Galaxy, thereby enabling renewed perspectives on their multiplicity properties. These latest observational results have in turn fueled the many theories competing to explain the formation of low‐mass stars and brown dwarfs. This Cool Stars 17 splinter session examined the current state of this field by reviewing results from the numerous observational techniques – radial velocities, astrometry, direct imaging, and synoptic surveys – that have been used to study multiplicity from the earliest embedded protostars to objects in young star forming regions, old and intermediate‐age clusters, as well as the more heterogeneous field population. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

M dwarf stars in the light of (future) exoplanet searches

AbstractWe present a brief overview of a splinter session on M dwarf stars as planet hosts that was organized as part of the Cool Stars 17 conference. The session was devoted to reviewing our current knowledge of M dwarf stars and exoplanets in order to prepare for current and future exoplanet searches focusing in low mass stars. We review the observational and theoretical challenges to characterize M dwarf stars and the importance of accurate fundamental parameters for the proper characterization of their exoplanets and our understanding on planet formation. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Measuring the ages of low‐mass stars and brown dwarfs

AbstractAge is among the most elusive, yet important, fundamental properties of low–mass stars and brown dwarfs. M dwarfs have main‐sequence lifetimes that are estimated to be trillions of years, with little change in luminosity. In contrast, brown dwarfs cool and dim with time, resulting in a significant degeneracy between mass, age, and luminosity. Despite these inherent challenges, there have been recent efforts on both observational and theoretical fronts that may yield precise ages for low‐mass stars and brown dwarfs. We feature some current observational efforts focused on estimating ages of these objects as presented in our Cool Stars 17 splinter session. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The new Toulouse-Geneva stellar evolution code including radiative accelerations of heavy elements

Atomic diffusion has been recognized as an important process that has to be considered in any computations of stellar models. In solar-type and cooler stars, this process is dominated by gravitational settling, which is now included in most stellar evolution codes. In hotter stars, radiative accelerations compete with gravity and become the dominant ingredient in the diffusion flux for most heavy elements. Introducing radiative accelerations into the computations of stellar models modifies the internal element distribution and may have major consequences on the stellar structure. Coupling these processes with hydrodynamical stellar motions has important consequences that need to be investigated in detail. We aim to include the computations of radiative accelerations in a stellar evolution code (here the TGEC code) using a simplified method (SVP) so that it may be coupled with sophisticated macroscopic motions. We also compare the results with those of the Montreal code in specific cases for validation and study the consequences of these coupled processes on accurate models of A- and early-type stars. We implemented radiative accelerations computations into the Toulouse-Geneva stellar evolution code following the semi-analytical prescription proposed by Alecian and LeBlanc. This allows more rapid computations than the full description used in the Montreal code. We present results for A-type stellar models computed with this updated version of TGEC and compare them with similar published models obtained with the Montreal evolution code. We discuss the consequences for the coupling with macroscopic motions, including thermohaline convection.

A southern hemisphere survey of the 5780 and 6284 Å diffuse interstellar bands: correlation with the extinction

We present a new database of 5780.5 and 6283.8 {\AA} DIB measurements and also study their correlation with the reddening. The database is based on high-resolution, high-quality spectra of early-type nearby stars located in the southern hemisphere at an average distance of 300 pc. Equivalent widths of the two DIBs were determined by means of a realistic continuum fitting and synthetic atmospheric transmissions. For all stars that possess a precise measurement of their color excess, we compare the DIBs and the extinction. We find average linear relationships of the DIBS and the color excess that agree well with those of a previous survey of northern hemisphere stars closer than 550 pc. This similarity shows that there is no significant spatial dependence of the average relationship in the solar neighborhood within $\simeq$ 600 pc. A noticeably different result is our higher degree of correlation of the two DIBs with the extinction. We demonstrate that it is simply due to the lower temperature and intrinsic luminosity of our targets. Using cooler target stars reduces the number of outliers, especially for nearby stars, confirming that the radiation field of UV bright stars has a significant influence on the DIB strength. We have used the cleanest data to compute updated DIB shapes.

Emission-line stars discovered in the UKST Hα survey of the Large Magellanic Cloud - I. Hot stars

We present new, accurate positions, spectral classifications, radial and rotational velocities, H-alpha fluxes, equivalent widths and B,V,I,R magnitudes for 579 hot emission-line stars (classes B0 - F9) in the Large Magellanic Cloud which include 469 new discoveries. Candidate emission line stars were discovered using a deep, high resolution H-alpha map of the central 25 deg2 of the LMC obtained by median stacking a dozen 2 hour H-alpha exposures taken with the UK Schmidt Telescope. Spectroscopic follow-up observations on the AAT, UKST, VLT, the SAAO 1.9m and the MSSSO 2.3m telescope have established the identity of these faint sources down to magnitude R~23 for H-alpha (4.5 x 10^-17 ergs cm^2 s^-1 Ang). Confirmed emission-line stars have been assigned an underlying spectral classification through cross-correlation against 131 absorption line template spectra covering the range O1 to F8. We confirm 111 previously identified emission line stars and 64 previously known variable stars with spectral types hotter than F8. The majority of hot stars identified (518 stars or 89%) are class B. Of all the hot emission-line stars in classes B-F, 130 or 22% are type B[e], characterised by the presence of forbidden emission lines such as [SII], [NII] and [OII]. We report on the physical location of these stars with reference to possible contamination from ambient HII emission. Along with flux calibration of the H-alpha emission we provide the first H-alpha luminosity function for selected sub-samples after correction for any possible nebula or ambient contamination. We find a moderate correlation between the intensity of H-alpha emission and the V magnitude of the central star based on SuperCOSMOS magnitudes and OGLE-II photometry where possible. Cool stars from classes G-S, with and without strong H-alpha emission, will be the focus of part 2 in this series.

SOLVING THE MODE IDENTIFICATION PROBLEM IN ASTEROSEISMOLOGY OF F STARS OBSERVED WITH KEPLER

Asteroseismology of F-type stars has been hindered by an ambiguity in identification of their oscillation modes. The regular mode pattern that makes this task trivial in cooler stars is masked by increased linewidths. The absolute mode frequencies, encapsulated in the asteroseismic variable epsilon, can help solve this impasse because the values of epsilon implied by the two possible mode identifications are distinct. We find that the correct epsilon can be deduced from the effective temperature and the linewidths and we apply these methods to a sample of solar-like oscillators observed with Kepler.

Observations of Cool-Star Magnetic Fields

Cool stars like the Sun harbor convection zones capable of producing substantial surface magnetic fields leading to stellar magnetic activity. The influence of stellar parameters like rotation, radius, and age on cool-star magnetism, and the importance of the shear layer between a radiative core and the convective envelope for the generation of magnetic fields are keys for our understanding of low-mass stellar dynamos, the solar dynamo, and also for other large-scale and planetary dynamos. Our observational picture of cool-star magnetic fields has improved tremendously over the last years. Sophisticated methods were developed to search for the subtle effects of magnetism, which are difficult to detect particularly in cool stars. With an emphasis on the assumptions and capabilities of modern methods used to measure magnetism in cool stars, I review the different techniques available for magnetic field measurements. I collect the analyses on cool-star magnetic fields and try to compare results from different methods, and I review empirical evidence that led to our current picture of magnetic fields and their generation in cool stars and brown dwarfs.

The Observational View

Within less than two decades, the study of low-mass stars and brown dwarfs has bloomed into one of the most active fields in astronomy. The M, L, T and Y dwarfs sequences includes objects spawning more than an order of magnitude in absolute temperature, from 4000 K down to room temperature, and nearly fills the entire temperature gap between the coolest stars and our Solar System’s giant planets. I present an overview of the large-scale surveys that led to the discovery of a population of ultracool dwarfs in our immediate galactic vicinity, their classification and various noteworthy spectroscopic features found only in these objects. I provide an outline of photometric variability study of L and T dwarfs, which opens a unique window on the atmospheric phenomenon at play in their atmospheres. Finally, I summarize the capabilities of an upcoming instrument, the SPIRou near-infrared, high-resolution spectropolarimeter, that will be available to the CFHT communities in 2015. SPIRou will be a unique tool for the study of cool dwarfs, and will be used to undertake an ambitious survey of habitable Earth-sized planets around nearby M dwarfs.

Magnetic Fields from Low-Mass Stars to Brown Dwarfs

Magnetic fields have been detected on stars across the H-R diagram and substellar objects either directly by their effect on the formation of spectral lines, or through the activity phenomena they power which can be observed across a large part of the electromagnetic spectrum. Stars show a very wide variety of magnetic properties in terms of strength, geometry or variability. Cool stars generate their magnetic fields by dynamo effect, and their properties appear to correlate – to some extent – with stellar parameters such as mass, rotation, and age. With the improvements of instrumentation and data analysis techniques, magnetic fields can now be detected and studied down to the domain of very-low-mass stars and brown dwarfs, triggering new theoretical works aimed, in particular, at modelling dynamo action in these objects. After a brief discussion on the importance of magnetic field in stellar physics, the basics of dynamo theory and magnetic field measurements are presented. The main results stemming from observational and theoretical studies of magnetism are then detailed in two parts: the fully-convective transition, and the very-low mass stars and brown dwarfs domain.

Recent Results for Widths of Lines Important in the Spectra of Cool Stars

Abstract The broadening of spectral lines by neutral perturbers is an important factor in the interpretation of the observed spectra of cool stars and is often included in the analysis by using a simple van der Waals formula. Detailed calculations are carried out for selected lines and are used to examine the validity of this formula for a range of temperatures.

Mixing at young ages: beryllium abundances in cool main-sequence stars of the open clusters IC 2391 and IC 2602

The determination of lithium abundances in stars of young clusters have shown that they deplete Li by different degrees during their pre-main sequence phase. Beryllium abundances are complementary to the lithium ones, and can help tracing the mixing processes in the stellar interiors. Our aim is to derive beryllium abundances in a sample of G- and K-type stars of two young pre-main sequence open clusters, IC 2391 and IC 2602. The Be abundances are used to investigate the mixing of internal material in these stars. The reliability of the Be lines as abundance indicators in low-temperatures is also investigated in detail. We derived Be abundances from high-resolution, high signal-to-noise UVES/VLT spectra using spectrum synthesis and model atmospheres. Atmospheric parameters and other elemental abundances are adopted from a previous work. The sample stars have masses in the range between 0.80 < M/Msun < 1.20. They have been shown to differ in lithium abundance by about 0.60 dex, with lower A(Li) in cooler and lower mass stars. Here, we find that all the stars have the same Be abundance within the uncertainties. These observations show that the Be abundance is not affected by the mixing events in the pre-main sequence, in this mass range, in agreement with the expectation of evolutionary models. A comparison with Be abundances in older clusters shows that, contrary to the models, cool stars deplete Be during their main-sequence lifetime, confirming what has been previously suggested in the literature.

The winds of hydrogen-rich central stars of planetary nebulae

We have modelled far-UV, UV and optical spectra of a sample of 10 hydrogen-rich central stars of planetary nebulae (CSPN) using stellar atmosphere codes to derive their photospheric and wind parameters. The resulting stellar temperatures range from 40 to 120 kK, well spanning the CSPN evolutionary phase and allowing certain trends to be discerned. In particular, an inhomogeneous wind structure and X-ray emission in the wind are required to match spectral diagnostic lines in many cases. For the majority of the sample, a wind clumping factor of 0.1 ≤f≤ 0.04 is derived (mainly from the P vλλ1118, 28 and O vλ1371 lines). Such factors correspond to clump densities of ∼10–25 times that of the smooth wind density, with resulting mass-loss rates one-third to one-fifth the smooth wind values, which is of significant consequence to nebular dynamics, stellar and galactic evolution. Furthermore, we find clumping to begin at small radii (∼1.2 R*), as has been found when modelling the winds of (massive) O stars. The inclusion of X-ray fluxes, presumably from shocks, in the model atmosphere calculations is found to improve the fit of the O viλλ1032, 38 line (and other features) for stars with 55 ≤ Teff≤ 80 kK, and to be absolutely necessary to match this feature for the coolest stars in our sample (Teff≲ 45 kK). These findings suggest that shocks originating from line-driven wind instabilities leading to the formation of clumped winds and X-rays may be a common characteristic of CSPN, as has been found for the winds of massive O-type stars. We also find interesting results for some individual stars. NGC 1360 (Teff≃ 105 kK) displays the signature of a (previously undetected) weak stellar wind in its O vi 1032, 38 profile, and probably has the lowest mass-loss rate ( M⊙ yr−1) of any known CSPN. In contrast, we find the wind terminal velocity of NGC 2392 ( Teff≃ 45 kK) to be v∞≃ 300 km s−1, one of the slowest CSPN wind known, probably related to its subsolar metallicity. We have included in the model calculations many elements and high-ionization species previously neglected in analyses of this type, providing additional wind diagnostics such as Ne viiλ973 and Ar viiλ1064. The effects of including these as well as other line-blanketing elements are discussed.

Photometric variability of five candidates for protoplanetary nebulae

We analyze long-term UBV observations and ASAS-3 photometry for five candidates for protoplanetary nebulae—F and G supergiants with infrared excesses at highG alactic latitudes—V340 Ser, IRAS 05113+1347, V552 Pup, V448 Lac, and RV Col. These stars exhibit quasi-periodic multifrequency light variations caused by pulsations with characteristic time scales from 83 to 139 days, depending on the stellar temperature. Cooler stars undergo variations with larger amplitudes and periods. The variations at close frequencies with a period ratio of 1.03–1.09 are responsible for the amplitude modulation revealed for most program stars.

ON THE RELIABILITY OF STELLAR AGES AND AGE SPREADS INFERRED FROM PRE-MAIN-SEQUENCE EVOLUTIONARY MODELS

We revisit the problem of low-mass pre-main-sequence (PMS) stellar evolution and its observational consequences for where stars fall on the Hertzsprung-Russell diagram (HRD). In contrast to most previous work, our models follow stars as they grow from small masses via accretion, and we perform a systematic study of how the stars' HRD evolution is influenced by their initial radius, by the radiative properties of the accretion flow, and by the accretion history, using both simple idealized accretion histories and histories taken from numerical simulations of star cluster formation. We compare our numerical results to both non-accreting isochrones and to the positions of observed stars in the HRD, with a goal of determining whether both the absolute ages and the age dispersions inferred from non-accreting isochrones are reliable. We show that non-accreting isochrones can sometimes overestimate stellar ages for more massive stars (those with effective temperatures above \sim 3500 K), thereby explaining why non-accreting isochrones often suggest a systematic age difference between more and less massive stars in the same cluster. However, we also find the only way to produce a similar overestimate for the ages of cooler stars is by selecting parameters that are strongly inconsistent with both theoretical models and observational constraints. We therefore conclude that inferred ages and age spreads for cool stars are reliable, at least to the extent that the observed bolometric luminosities and temperatures are accurate. Finally, we note that the time-dependence of the mass accretion rate has remarkably little effect on low-mass stars' evolution on the HRD, and that such time-dependence may be neglected for all stars except those with effective temperatures above \sim 4000 K.

Li and Be Depletion in Stars with Exoplanets?

AbstractIt is well known that stars with orbiting giant planets have a higher metallic content than stars without detected planets. In addition, we have found that solar-type stars with planets present an extra Li depletion when compared with field stars. On the other hand, Be needs a greater temperature to be destroyed, so we may find such a relation in cooler stars, whose convective envelopes are deep enough to carry material to layers where Be can be burned. We present Li and Be abundances for an extensive sample of stars with and without detected planets, covering a wide range of effective temperatures (4700-6500 K) with the aim of studying possible differences between the abundances of both groups. The processes that take place in the formation of planetary systems may affect the mixing of material inside their host stars and hence the abundances of light elements.

The spectroscopic orbit of Capella revisited

Context. Capella is among the few binary stars with two evolved giant components. The hotter component is a chromospherically active star within the Hertzsprung gap, while the cooler star is possibly helium-core burning. Aims. The known inclination of the orbital plane from astrometry in combination with precise radial velocities will allow very accurate masses to be determined for the individual Capella stars. This will constrain their evolutionary stage and possibly the role of the active star's magnetic field on the dynamical evolution of the binary system. Methods. We obtained a total of 438 high-resolution \'echelle spectra during the years 2007-2010 and used the measured velocities to recompute the orbital elements. Our double-lined orbital solution yields average residuals of 64 m/s for the cool component and 297 m/s for the more rapidly rotating hotter component. Results. The semi-amplitude of the cool component is smaller by 0.045 km/s than the orbit determination of Torres et al. from data taken during 1996-1999 but more precise by a factor of 5.5, while for the hotter component it is larger by 0.580 km/s and more precise by a factor of 3.6. This corresponds to masses of 2.573\pm0.009 M_sun and 2.488\pm0.008 M_sun for the cool and hot component, respectively. Their relative errors of 0.34% and 0.30% are about half of the values given in Torres et al. for a combined literature- data solution but with absolute values different by 4% and 2% for the two components, respectively. The mass ratio of the system is therefore q = M_A/M_B = 0.9673 \pm 0.0020. Conclusions. Our orbit is the most precise and also likely to be the most accurate ever obtained for Capella.