COOL STARS, STELLAR SYSTEMS AND THE SUN: Proceedings of the 15th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun
1094(2009); http://dx.doi.org/10.1063/1.3099131View Description Hide Description
Our view of the Sun has changed dramatically over the past 10 years due mainly to a series of space satellites such as Yohkoh, SoHO and TRACE. This state of ferment will continue with the coming onto line last year of two other satellites, Hinode and STEREO, and next year SDO. Here we give a brief overview of the progress made in answering fundamental questions about the nature of the Sun which may have profound implications for other stars.
In the interior, helioseismology has revealed the internal rotation structure and suggested that the main solar dynamo responsible for active regions is located at the tachocline, although the details are highly uncertain and there may be a second dynamo responsible for generating small‐scale ephemeral regions. In the photosphere, flux is mainly concentrated at the edges of supergranule cells, but recent high‐resolution observations have suggested that extra flux is also located at granulation boundaries and Hinode has discovered much horizontal flux.
The solar corona is likely to be heated in myriads of tiny current sheets by reconnection, according to the Coronal Tectonics Model. Observations suggest that all the coronal field lines reconnect every 1.5 hours. Theory has shown that reconnection in 3D has many features that are completely different from the standard 2D picture. The solar wind is highly dynamic and complex and its acceleration mechanism may possibly be high‐frequency ion‐cyclotron waves. Many new features of solar flares and coronal mass ejections have been discovered, but it is not known whether the cause of the eruption is an instability or a lack of equilibrium.
1094(2009); http://dx.doi.org/10.1063/1.3099083View Description Hide Description
Cosmic dust particles play an important role for the thermal, dynamical and chemical conditions in many astrophysical environments, especially for the star and planet formation process and the late stages of stellar evolution. Dust particles determine the spectral appearance of proto‐stars as well as evolved stars with circumstellar envelopes, and they also dominate the extinction curves of galaxies.
The most efficient site of dust formation in the present universe is the cool extended atmospheres of carbon‐rich and oxygen‐rich AGB stars. Dust production also seems likely to occur in supernova remnants, especially in the early universe, however, the true nature of such supernova dust is not as well described as for AGB stars either observationally or theoretically.
Once the dust is formed it will enter the interstellar medium and be an important ingredient for further star‐ and planet‐formation. During star‐formation, dust seems to be an important coolant which can lower the temperature in the centre of the molecular clouds and thereby lead to the formation of smaller stars than in a non‐dusty cloud. If the star is formed with an accretion disk containing dust grains, the subsequent planet‐formation will occur more effectively than in a dustless disk.
1094(2009); http://dx.doi.org/10.1063/1.3099097View Description Hide Description
Multiwavelength studies of evolved protoplanetary disks around solar‐type and low‐mass stars reveal a general trend of changes in the IR excesses, accretion rates, and silicate features, suggesting grain growth/settling, photoevaporation, and maybe the formation of planetesimals and planets. Nevertheless, within the average‐behavior picture of disk evolution, we observe strong variations between individuals, that may be related to different initial conditions, different environments, and the presence of companions.
1094(2009); http://dx.doi.org/10.1063/1.3099106View Description Hide Description
Establishing the origin of accretion powered winds from forming stars is critical for understanding angular momentum evolution in the star‐disk interaction region. Here, the high velocity component of accretion powered winds is launched and accreting stars are spun down, in defiance of the expected spin‐up during magnetospheric accretion. T Tauri stars in the final stage of disk accretion offer a unique opportunity to study the connection between accretion and winds and their relation to stellar spindown. Although spectroscopic indicators of high velocity T Tauri winds have been known for decades, the line of He I 10830 offers a promising new diagnostic to probe the magnetically controlled star‐disk interaction and wind‐launching region. The high opacity and resonance scattering properties of this line offer a powerful probe of the geometry of both the funnel flow and the inner wind that, together with other atomic and molecular spectral lines covering a wide range of excitation and ionization states, suggests that the magnetic interaction between the star and disk, and the subsequent launching of the inner high velocity wind, is sensitive to the disk accretion rate.
Searching for Optical Outflows Driven by Young Brown Dwarfs with the ESO UV‐Visual Echelle Spectrometer UVES1094(2009); http://dx.doi.org/10.1063/1.3099130View Description Hide Description
Here we discuss our work to date on the topic of outflows driven by young brown dwarfs. Soon after the discovery of large numbers of young brown dwarfs in star forming regions it became apparent that many of these sub‐stellar objects were accreting and had accretion disks, analogous to the T Tauri stars. As outflow activity in low mass protostars is strongly connected to accretion it was reasonable to expect these accreting brown dwarfs to also be driving outflows. In the last three years we have searched for brown dwarf outflows using high quality optical spectra obtained with UVES on the VLT and the technique of spectro‐astrometry. To date we have uncovered 5 brown dwarf outflows. Here we discuss our method and outline our results to date. The long‐term aim of this work is to make a comprehensive comparison between proto‐stellar outflows and those driven by sub‐stellar mass objects.
1094(2009); http://dx.doi.org/10.1063/1.3099147View Description Hide Description
To date we have discovered over 300 extrasolar planets and although we are starting to detect some with masses a few times that of the Earth, most have much higher masses and are generally regarded as gas giants, similar to Jupiter or Saturn. Since these planets are predominantly gaseous, we can say—with some confidence—that they must have formed before the dispersal of the gas disc which occurs on a timescale of years. What we are still uncertain about is exactly how these planets form. They also have some extremely interesting and unexpected properties. Some, known as hot Jupiters, orbit very close to their parents stars, and although we expect these planets to form from material on circular orbits, the range of eccentricities is extremely high. Some of this can be understood by considering the interaction of planets with the surrounding protoplanetary disc, but this can also introduce new problems such as rapid inward migration of small planetesimals, through gas drag, and planetary cores, through gravitational interaction with the surrounding disc. In this paper I will review the various planet forming models and discuss how these planets evolve through planet‐disc interactions. I will also discuss how recent work is starting to understand how these planets form and is beginning to explain the various properties of the current population of exoplanets.
Studying Planet Formation around Low‐Mass Stars and Brown Dwarfs through Observations of their Circumstellar Disks1094(2009); http://dx.doi.org/10.1063/1.3099173View Description Hide Description
Observations of circumstellar accretion disks around young stars provide fundamental constraints on the process of planet formation. Disks around low‐mass stars and brown dwarfs have been studied extensively in recent years, primarily through infrared imaging and spectroscopy with the Spitzer Space Telescope . I review the observations of these disks that are relevant to planet formation, including disk fractions and lifetimes, the presence of inner holes, grain growth and dust settling, and the abundances of organic molecules. In particular, I summarize the dependence of these properties on stellar mass to compare the prospects for planet formation between stars like the Sun, low‐mass stars, and brown dwarfs.
1094(2009); http://dx.doi.org/10.1063/1.3099189View Description Hide Description
Rotation is a key parameter in the evolution of stars. From 1 Myr (the age of the ONC) to 4.5 Gyr (the age of the Sun), solar‐like stars lose about 1–2 orders of specific angular momentum. The main agents for this rotational braking are believed to be star‐disk interaction and magnetically powered stellar winds. Over the last decade, the observational fundament to probe the stellar spindown has dramatically improved. Significant progress has been made in exploring the underlying physical causes of the rotational braking. Parameterized models combining the effects of star‐disk interaction, winds, and pre‐main sequence contraction are able to reproduce the main features of the rotational data for stars spanning more than 3 orders of magnitude in age. This has allowed us to constrain stellar ages based on the rotation rates (‘gyrochronology’). One main challenge for future work is to extend this type of analysis to the substellar mass range, where the rotational database is still sparse. More theoretical and observational work is required to explore the physics of the braking processes, aiming to explain rotational evolution from first principles. In this review for Cool Stars 15, I will summarize the status quo and the recent developments in the field.
1094(2009); http://dx.doi.org/10.1063/1.3099215View Description Hide Description
Models of magnetospheric accretion on to classical T Tauri stars often assume that the stellar magnetic field is a simple dipole. Recent Zeeman‐Doppler imaging studies of V2129 Oph and BP Tau have shown however that their magnetic fields are more complex. V2129 Oph is a high mass T Tauri star and despite its young age is believed to have already developed a radiative core. In contrast to this, the lower mass BP Tau is likely to be completely convective. As the internal structure and therefore the magnetic field generation process is different in both stars, it is of particular interest to compare the structure of their magnetic fields obtained by field extrapolation from magnetic surface maps. We compare both field structures to mulitpole magnetic fields, and calculate the disk truncation radius for both systems. We find that by considering magnetic fields with a realistic degree of complexity, the disk is truncated at, or within, the radius obtained for dipole fields.
1094(2009); http://dx.doi.org/10.1063/1.3099231View Description Hide Description
The large‐scale magnetic field and activity cycle of the Sun are believed to be the result of an advection‐dominated dynamo, which is an extension of the well‐known dynamo in which a large‐scale meridional flow plays a key part. We present a model of stellar rotation based on the mean field approach of magnetohydrodynamics. The model reproduces the observed solar internal rotation and surface meridional flow. We compare its predictions with the observations available so far. We also address the generation of magnetic fields in lower main sequence stars.
1094(2009); http://dx.doi.org/10.1063/1.3099257View Description Hide Description
We present an overview of the new insight provided by the CoRoT satellite on stellar rotation. Thanks to its ultra‐high precision, high duty cycle, long photometric monitoring of thousands of stars, CoRoT gives us a powerful tool to study stellar rotational modulation, and therefore to measure stellar rotational periods and to study active structures at the surface of stars. This paper presents preliminary results concerning this type of study.
CoRoT will also provide us with an insight of internal stellar rotation via the measurement and exploitation of rotational splittings of oscillation modes. This approach to stellar rotation with CoRoT will require a careful analysis of the oscillation power spectra, which is in progress, but prospects for such measurements are presented.
1094(2009); http://dx.doi.org/10.1063/1.3099279View Description Hide Description
We present the results of a combination of new stellar rotation periods and extensive information about membership in the young open clusters M 35 and M 34. The observations show that late‐type members of both M 35 and M 34 divide into two distinct groups, each with a different dependence of rotation on mass (color). We discuss these new results in the context of existing rotation data for cool stars in older clusters, with a focus on the dependence of rotation on mass and age. We mention briefly tests of rotation as an “astronomical clock” (gyrochronology), and our plans to use the Kepler space mission to push observations of stellar rotation periods beyond the age of the Hyades and the Sun.
1094(2009); http://dx.doi.org/10.1063/1.3099078View Description Hide Description
In this review, we summarize our present knowledge of the behaviour of the mass‐radius relationship from solar‐type stars down to terrestrial planets, across the regime of substellar objects, brown dwarfs and giant planets. Particular attention is paid to the identification of the main physical properties or mechanisms responsible for this behaviour. Indeed, understanding the mechanical structure of an object provides valuable information about its internal structure, composition and heat content as well as its formation history. Although the general description of these properties is reasonably well mastered, disagreement between theory and observation in certain cases points to some missing physics in our present modelling of at least some of these objects. The mass‐radius relationship in the overlaping domain between giant planets and low‐mass brown dwarfs is shown to represent a powerful diagnostic to distinguish between these two different populations and shows once again that the present IAU distinction between these two populations at a given mass has no valid foundation.
1094(2009); http://dx.doi.org/10.1063/1.3099079View Description Hide Description
We present new dynamical mass estimates for a sample of brown dwarf binaries with sufficient precision to distinguish between current theoretical evolutionary models. This survey, which has targeted 20 sources between spectral types M8 and T5, is enabled by the advent of laser guide star adaptive optics (LGS AO) on the Keck II 10 m telescope. The LGS AO system allows for both high precision astrometry and spatially resolved, high‐resolution spectroscopy A striking result from this work thus far is that for two systems with precisely determined dynamical masses and similar spectral types, both evolutionary models considered correctly predict the mass in one case but underpredict the mass by several sigma in the other case. We postulate that this discrepancy may either stem from a disjoint between evolutionary and atmospheric models at the M/L transition due to atmospheric dust treatment or potentially from magnetic activity inhibiting convection.
1094(2009); http://dx.doi.org/10.1063/1.3099080View Description Hide Description
Rotational studies at different ages and masses are important for constraining the angular momentum evolution of young stellar objects (YSO). Of particular interest are the rotational studies of very low mass (VLM) stars and brown dwarfs (BDs), because few rotational periods are known in that mass range. We aim to extend these studies well down into the substellar regime, providing for the first time information on rotational periods for a large sample of VLM stars and BDs. This extensive rotational period study of YSOs in the 1 Myr old Orion Nebula Cluster (ONC) is based on a deep photometric monitoring campaign using the Wide Field Imager (WFI) camera on the ESO/MPG 2.2 m telescope on La Silla, Chile. Time series data with about 95 data points were taken over 19 nights. Accurate I‐band photometry of 2908 stars was obtained within a magnitude range between 13 and 21 mag, i.e. three magnitudes deeper than the previous studies in the ONC (). Two different power spectral analysis techniques were used to search for periodic variability. In addition, the variability test was used for the detection of irregular variables. We measured rotational periods of 487 objects with estimated masses between and 124 of which are BD candidates. This is by far the most extensive and complete rotational period data set for young VLM stars and BDs. Besides the periodic variables, 808 objects show strong non‐periodic (i.e. irregular) brightness variations. We studied the dependence of the period distribution on the magnitude (mass) and variability level and compared the found period distribution with that of higher‐mass objects in the ONC () and with the rotational data set existing for the twice as old cluster NGC 2264 (). We found that substellar objects rotate on average faster than the VLM stars, a trend which was already observed for higher mass stars. In addition, we found a clear dependence of the rotational periods on position within the field. Objects located inside the so‐called rotate on average slower, which can be explained by an age spread in the ONC, with a somewhat younger central region. The results of a comparison between the period distributions of the ONC and NGC 2264 strongly favours this hypothesis. Interesting correlations between rotational period and variability level were also found in both clusters, probably explained by different magnetic field topologies.
1094(2009); http://dx.doi.org/10.1063/1.3099081View Description Hide Description
Magnetic fields play a central role in the atmospheric properties and variability of active M dwarfs. Information on the strength and structure of magnetic fields in these objects is vital for understanding dynamo mechanisms and magnetically‐driven activity of low‐mass stars, and for constraining theories of star formation and evolution. We have initiated the first systematic high‐resolution survey of magnetically sensitive infrared spectral lines in M dwarf stars using the CRIRES instrument at the ESO VLT. We have completed observations for a sample of 35 active and inactive M dwarfs. Here we report first results of our project, demonstrating a clear detection of magnetic splitting of lines in the spectra of several M dwarfs. We assess diagnostic potential of different Zeeman‐sensitive lines in the observed spectral region and apply spectrum synthesis modelling to infer magnetic field properties of selected M dwarfs.
1094(2009); http://dx.doi.org/10.1063/1.3099082View Description Hide Description
Spectropolarimetry is the optimal tool to investigate large‐scale magnetic topologies of cool low‐mass stars. From phase resolved spectropolarimetric data sets, tomographic imaging can be used to obtain a spherical harmonics decomposition of magnetic fields at the surfaces of stars, and thus reveal, e.g., how strong and complex such fields are, to what degree they are axisymmetric and how they split between their poloidal and toroidal components.
In this paper, we review the current knowledge on large‐scale magnetic topologies of cool stars, with particular emphasis on the latest results obtained with the new spectropolarimeters ESPaDOnS@CFHT and NARVAL@TBL. Observations show that stars with masses larger than host significant toroidal fields whenever their Rossby number Ro is smaller than about 1, with their poloidal field getting stronger and more non‐axisymmetric as Ro decreases. On the opposite, stars with masses lower than host strong poloidal fields in a mostly axisymmetric dipolar configuration.
Spectropolarimetric studies can also reveal how differential rotation and magnetic cycles vary with stellar parameters.
1094(2009); http://dx.doi.org/10.1063/1.3099084View Description Hide Description
We present here the first results of a spectropolarimetric analysis of a small sample of active stars ranging from spectral type M0 to M8, which are either fully‐convective or possess a very small radiative core. This study aims at providing new constraints on dynamo processes in fully‐convective stars.
Results for stars with spectral types M0–M4—i.e. with masses above or just below the full convection threshold —are presented. Tomographic imaging techniques allow us to reconstruct the surface magnetic topologies from the rotationally modulated time‐series of circularly polarised profiles.
We find strong differences between partly and fully convective stars concerning magnetic field topology and characteristic scales, and differential rotation. Our results suggest that magnetic field generation in fully convective stars relies on different dynamo processes than those acting in the Sun and other partly convective stars, in agreement with theoretical expectations.
1094(2009); http://dx.doi.org/10.1063/1.3099085View Description Hide Description
The pulsing radio emission detected from ultracool dwarfs can be used as a powerful diagnostic of magnetic field strengths and topologies at and below the substellar boundary. Studies thus far have confirmed magnetic field strengths of 3 kG for two late M dwarfs and 1.7 kG for an L3.5 dwarf, the latter being the first confirmation of kG magnetic fields for an L dwarf. Ongoing long term monitoring of the radio pulses will also investigate the stability of the associated large‐scale magnetic fields over timescales We also present the preliminary results of a lengthy radio monitoring campaign of the rapidly rotating M4 star V374 Peg, with the resulting light curves phased with magnetic maps previously obtained through Zeeman Doppler Imaging. The radio emission from V374 Peg is strongly modulated by the large scale dipolar magnetic field, with two clear peaks in the radio light curve per period of rotation, occurring when the dipolar field lies in the plane of the sky. These results provide strong evidence that the electron cyclotron maser instability plays a pivotal role in the production of quiescent radio emission from V374 Peg, representing a significant departure from the accepted model of gyrosynchrotron emission as the dominant source of quiescent radio emission from active M dwarfs.