arXiv subjects

Ansgar Reiners

Publications and source records attributed to Ansgar Reiners.

At least 91 records · Page 5Linked to original sources

arXiv2012

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.

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arXiv2012

A catalogue of rotation and activity in early-M stars

We present a catalogue of rotation and chromospheric activity in a sample of 334 M dwarfs of spectral types M0--M4.5 populating the parameter space around the boundary to full convection. We obtained high-resolution optical spectra for 206 targets and determined projected rotational velocity, vsini, and Halpha emission. The data are combined with measurements of vsini in field stars of the same spectral type from the literature. Our sample adds 157 new rotation measurements to the existing literature and almost doubles the sample of available vsini. The final sample provides a statistically meaningful picture of rotation and activity at the transition to full convection in the solar neighborhood. We confirm the steep rise in the fraction of active stars at the transition to full convection known from earlier work. In addition, we see a clear rise in rotational velocity in the same stars. In very few stars, no chromospheric activity but a detection of rotational broadening was reported. We argue that all of them are probably spurious detections; we conclude that in our sample all significantly rotating stars are active, and all active stars are significantly rotating. The rotation-activity relation is valid in partially and in fully convective stars. Thus, we do not observe any evidence for a transition from a rotationally dominated dynamo in partially convective stars to a rotation-independent turbulent dynamo in fully convective stars; turbulent dynamos in fully convective stars of spectral types around M4 are still driven by rotation. Finally, we compare projected rotational velocities of 33 stars to rotational periods derived from photometry in the literature and determine inclinations for a few of them.

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arXiv2011

M-dwarf metallicities - A high-resolution spectroscopic study in the near infrared

The relativley large spread in the derived metallicities ([Fe/H]) of M dwarfs shows that various approaches have not yet converged to consistency. The presence of strong molecular features, and incomplete line lists for the corresponding molecules have made metallicity determinations of M dwarfs difficult. Furthermore, the faint M dwarfs require long exposure times for a signal-to-noise ratio sufficient for a detailed spectroscopic abundance analysis. We present a high-resolution (R~50,000) spectroscopic study of a sample of eight single M dwarfs and three wide-binary systems observed in the infrared J-band. The absence of large molecular contributions allow for a precise continuum placement. We derive metallicities based on the best fit synthetic spectra to the observed spectra. To verify the accuracy of the applied atmospheric models and test our synthetic spectrum approach, three binary systems with a K-dwarf primary and an M-dwarf companion were observed and analysed along with the single M dwarfs. We obtain a good agreement between the metallicities derived for the primaries and secondaries of our test binaries and thereby confirm the reliability of our method of analysing M dwarfs. Our metallicities agree well with certain earlier determinations, and deviate from others. We conclude that spectroscopic abundance analysis in the J band is a reliable method for establishing the metallicity scale for M dwarfs. We recommend its application to a larger sample covering lower as well as higher metallicities. Further prospects of the method include abundance determinations for individual elements.

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arXiv2011

Radius Dependent Angular Momentum Evolution in Low-Mass Stars. I

Angular momentum evolution in low-mass stars is determined by initial conditions during star formation, stellar structure evolution, and the behaviour of stellar magnetic fields. Here we show that the empirical picture of angular momentum evolution arises naturally if rotation is related to magnetic field strength instead of to magnetic flux, and formulate a corrected braking law based on this. Angular momentum evolution then becomes a strong function of stellar radius, explaining the main trends observed in open clusters and field stars at a few Gyr: the steep transition in rotation at the boundary to full convection arises primarily from the large change in radius across this boundary, and does not require changes in dynamo mode or field topology. Additionally, the data suggest transient core-envelope decoupling among solar-type stars, and field saturation at longer periods in very low-mass stars. For solar-type stars, our model is also in good agreement with the empirical Skumanich law. Finally, in further support of the theory, we show that the predicted age at which low-mass stars spin down from the saturated to unsaturated field regimes in our model corresponds remarkably well to the observed lifetime of magnetic activity in these stars.

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arXiv2011

MHD simulations reveal crucial differences between solar and very-cool star magnetic structures

We carried out 3D radiative magnetohydrodynamic simulations of the convective and magnetic structure in the surface layers (uppermost part of the convection zone and photosphere) of main-sequence stars of spectral types F3 to M2. The simulation results were analyzed in terms of sizes and properties of the convection cells (granules) and magnetic flux concentrations as well as velocity, pressure, density, and temperature profiles. Our numerical simulations show for the first time a qualitative difference in the magneto-convection between solar-like stars and M dwarfs. Owing to higher surface gravity, lower opacity (resulting in higher density at optical depth unity), and more stable downflows, small-scale magnetic structures concentrate into pore-like configurations of reduced intensity. This implies that in very cool stars magnetic surface structures like plage regions and starspots significantly differ from the solar example. Such a difference would have major impact on the interpretation of Doppler imaging data and the analysis of M dwarf spectra.

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arXiv2010

Zeeman Broadening in Optical Stokes I of Solar-like Stars

We present our analysis aimed at inferring average magnetic fields in slowly-rotating solar-like stars. Using the spectral line inversion code SPINOR, we perform high-accuracy line profile fitting and investigate whether Zeeman broadening can be reliably detected in optical data of unprecedented quality. We argue that our usage of both high- and low-g_{eff} lines does provide a certain sensitivity to magnetic fields that may, indeed, be detected. However, the measurement is subject to a model dependence and prone to ambiguities, e.g. due to spectral blends. Hence, while a field may be successfully recovered, the quantification of this field is subject to large uncertainties, even for the highest-quality optical data.

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arXiv2010

Kepler light curves and stellar rotational periods

The Kepler space telescope monitors over 156.000 stars with an unprecedented photometric precision. We are interested in stellar rotational periods which we find using Lomb-Scargle periodograms. This work focuses on the 306 exoplanet candidate host stars released on June 15, 2010. We present statistics on how many of them show periodic photometric variability, providing preliminary periods and estimates of stellar activity. In the future, our work will focus on spot evolution and differential rotation.

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arXiv2010

Magnetic Fields on Cool Stars

Magnetic fields are an important ingredient to cool star physics, and there is great interest in measuring fields and their geometry in order to understand stellar dynamos and their influence on star formation and stellar evolution. During the last few years, a large number of magnetic field measurements became available. Two main approaches are being followed to measure the Zeeman effect in cool stars; 1) the measurement of polarized light, for example to produce magnetic maps, and 2) the measurement of integrated Zeeman broadening to measure the average magnetic field strength on the stellar surface. This article briefly reviews the two methods and compares results between them that are now available for about a dozen M-type stars. It seems that we see a great variety of magnetic geometries and field strengths with typical average fields of a few kG in active M-type stars. The interpretation of geometries, however, has not yet led to a clear picture of magnetic dynamos and field configuration, and work is needed on more observational data but also on the fundamental understanding of our measurements.

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arXiv2010

Investigation of magnetically sensitive FeH lines

M-type stars exhibit strong magnetic fields towards decreasing effective temperatures. The measurement of these fields is complicated due to missing indicators. Molecular FeH lines provide an excellent means to determine magnetic field strengths from the Zeeman broadening of magnetically sensitive lines. Our aim is the investigation of possible dependencies of the amount of sensitivity to magnetic fields from rotational quantum number, branch, and the projection of the total angular momentum onto the internuclear axis (Ω). We also compare results from com-putations with those from observations. We use high resolution CRIRES spectra of the two M dwarfs GJ1002 (M5.5 inactive) and GJ1224 (M4.5 active). Individual lines are fitted by Gaussians and the obtained line depths and widths from the active and inactive star can be compared with each other. In this way, magnetically sensitive lines can be detected. For test purposes, we do the same with computed spectra of FeH. One with zero magnetic field and the other with a 2 kG magnetic field vector used in the disc integration (i.e. pure radial at the disc center). We found, in agreement with theory, that lines with high Ω show strong sensitivity to magnetic fields. No obvious correlation with branch or J was found, which was also expected for lines formed in intermediate Hund's case. The computations agreed in general well with the observations, but in many cases the individual splitting of certain lines can be very different to observations.

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arXiv2010

Rotation and differential rotation in rapidly rotating field stars

We continue our studies on stellar latitudinal differential rotation. The presented work is a sequel of the work of Reiners et al. who studied the spectral line broadening profile of hundreds of stars of spectral types A through G at high rotational speed (vsini > 12 km/s). While most stars were found to be rigid rotators, only a few tens show the signatures of differential rotation. The present work comprises the rotational study of some 180 additional stars. The overall broadening profile is derived according to Reiners et al. from hundreds of spectral lines by least-squares deconvolution, reducing spectral noise to a minimum. Projected rotational velocities vsini are measured for about 120 of the sample stars. Differential rotation produces a cuspy line shape which is best measured in inverse wavelength space by the first two zeros of its Fourier transform. Rigid and differential rotation can be distinguished for more than 50 rapid rotators (vsini > 12 km/s) among the sample stars from the available spectra. Ten stars with significant differential rotation rates of 10-54 % are identified, which add to the few known rapid differential rotators. Differential rotation measurements of 6 % and less for four of our targets are probably spurious and below the detection limit. Including these objects, the line shapes of more than 40 stars are consistent with rigid rotation.

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arXiv2010

On detectability of Zeeman broadening in optical spectra of F- and G-dwarfs

We investigate the detectability of Zeeman broadening in optical Stokes I spectra of slowly rotating sun-like stars. To this end, we apply the LTE spectral line inversion package SPINOR to very-high quality CES data and explore how fit quality depends on the average magnetic field, Bf . One-component (OC) and two-component (TC) models are adopted. In OC models, the entire surface is assumed to be magnetic. Under this assumption, we determine formal 3σ upper limits on the average magnetic field of 200 G for the Sun, and 150 G for 61 Vir (G6V). Evidence for an average magnetic field of ~ 500 G is found for 59 Vir (G0V), and of ~ 1000 G for HD 68456 (F6V). A distinction between magnetic and non-magnetic regions is made in TC models, while assuming a homogeneous distribution of both components. In our TC inversions of 59 Vir, we investigate three cases: both components have equal temperatures; warm magnetic regions; cool magnetic regions. Our TC model with equal temperatures does not yield significant improvement over OC inversions for 59 Vir. The resulting Bf values are consistent for both. Fit quality is significantly improved, however, by using two components of different temperatures. The inversions for 59 Vir that assume different temperatures for the two components yield results consistent with 0 - 450 G at the formal 3σ confidence level. We thus find a model dependence of our analysis and demonstrate that the influence of an additional temperature component can dominate over the Zeeman broadening signature, at least in optical data. Previous comparable analyses that neglected effects due to multiple temperature components may be prone to the same ambiguities.

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arXiv2010

A magnetic field evolution scenario for brown dwarfs and giant planets

Very little is known about magnetic fields of extrasolar planets and brown dwarfs. We use the energy flux scaling law presented by Christensen et al. (2009) to calculate the evolution of average magnetic fields in extrasolar planets and brown dwarfs under the assumption of fast rotation, which is probably the case for most of them. We find that massive brown dwarfs of about 70 M_Jup can have fields of a few kilo-Gauss during the first few hundred Million years. These fields can grow by a factor of two before they weaken after deuterium burning has stopped. Brown dwarfs with weak deuterium burning and extrasolar giant planets start with magnetic fields between ~100G and ~1kG at the age of a few Myr, depending on their mass. Their magnetic field weakens steadily until after 10Gyr it has shrunk by about a factor of 10. We use observed X-ray luminosities to estimate the age of the known extrasolar giant planets that are more massive than 0.3M_Jup and closer than 20pc. Taking into account the age estimate, and assuming sun-like wind-properties and radio emission processes similar to those at Jupiter, we calculate their radio flux and its frequency. The highest radio flux we predict comes out as 700mJy at a frequency around 150MHz for $τ$Boob, but the flux is below 60mJy for the rest. Most planets are expected to emit radiation between a few Mhz and up to 100MHz, well above the ionospheric cutoff frequency.

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arXiv2010

Discovery of a nearby young brown dwarf binary candidate

In near-infrared NaCo observations of the young brown dwarf 2MASS J0041353-562112, we discovered a companion a little less than a magnitude fainter than the primary. The binary candidate has a separation of 143 mas, the spectral types are M6.5 and M9.0 for the two components. Colors and flux ratios are consistent with the components being located at the same distance minimizing the probability of the secondary being a background object. The brown dwarf is known to show Li absorption constraining the age to less than ~200 Myr, and it was suspected to show ongoing accretion, indicating an age as low as ~10 Myr. We estimate distance and orbital parameters of the binary as a function of age. For an age of 10 Myr, the distance to the system is 50 pc, the orbital period is 126 yr, and the masses of the components are ~30 and ~15 MJup. The binary brown dwarf fills a so far unoccupied region in the parameters mass and age; it is a valuable new benchmark object for brown dwarf atmospheric and evolutionary models.

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arXiv2010

The CRIRES Search for Planets Around the Lowest-Mass Stars. I. High-Precision Near-Infrared Radial Velocities with an Ammonia Gas Cell

Radial velocities measured from near-infrared spectra are a potentially powerful tool to search for planets around cool stars and sub-stellar objects. However, no technique currently exists that yields near-infrared radial velocity precision comparable to that routinely obtained in the visible. We describe a method for measuring high-precision relative radial velocities of these stars from K-band spectra. The method makes use of a glass cell filled with ammonia gas to calibrate the spectrograph response similar to the "iodine cell" technique that has been used very successfully in the visible. Stellar spectra are obtained through the ammonia cell and modeled as the product of a Doppler-shifted template spectrum of the object and a spectrum of the cell, convolved with a variable instrumental profile model. A complicating factor is that a significant number of telluric absorption lines are present in the spectral regions containing useful stellar and ammonia lines. The telluric lines are modeled simultaneously as well using spectrum synthesis with a time-resolved model of the atmosphere over the observatory. The free parameters in the complete model are the wavelength scale of the spectrum, the instrumental profile, adjustments to the water and methane abundances in the atmospheric model, telluric spectrum Doppler shift, and stellar Doppler shift. Tests of the method based on the analysis of hundreds of spectra obtained for late M dwarfs over six months demonstrate that precisions of ~5 m/s are obtainable over long timescales, and precisions of better than 3 m/s can be obtained over timescales up to a week. The obtained precision is comparable to the predicted photon-limited errors, but primarily limited over long timescales by the imperfect modeling of the telluric lines.

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arXiv2010

The CRIRES Search for Planets Around the Lowest-Mass Stars. II. The Proposed Giant Planet Orbiting VB10 Does Not Exist

We present high-precision relative radial velocities of the very low-mass star VB10 that were obtained over a time span of 0.61 yr as part of an ongoing search for planets around stars at the end of the main sequence. The radial velocities were measured from high-resolution near-infrared spectra obtained using the CRIRES instrument on the VLT with an ammonia gas cell. The typical internal precision of the measurements is 10 m/s. These data do not exhibit significant variability and are essentially constant at a level consistent with the measurement uncertainties. Therefore, we do not detect the radial velocity variations of VB10 expected due to the presence of an orbiting giant planet similar to that recently proposed by Pravdo and Shaklan based on apparent astrometric perturbations. In addition, we do not confirm the ~1 km/s radial velocity variability of the star tentatively detected by Zapatero Osorio and colleagues with lower precision measurements. Our measurements rule out planets with M_p > 3 M_Jup and the orbital period and inclination suggested by Pravdo and Shaklan at better than 5 sigma confidence. We conclude that the planet detection claimed by Pravdo and Shaklan is spurious on the basis of this result. Although the outcome of this work is a non-detection, it illustrates the potential of using ammonia cell radial velocities to detect planets around very low-mass stars.

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arXiv2010

On the kinematic age of brown dwarfs: Radial velocities and space motions of 43 nearby L dwarfs

We present radial velocity measurements of a sample of L0-L8 dwarfs observed with VLT/UVES and Keck/HIRES. We combine these measurements with distance and proper motion from the literature to determine space motions for 43 of our targets. We identify nine candidate members of young moving groups, which have ages of 50-600 Myr according to their space motion. From the total velocity dispersion of the 43 L dwarfs, we calculate a kinematic age of ~5 Gyr for our sample. This age is significantly higher than the ~3 Gyr age known for late M dwarfs in the solar neighbourhood. We find that the distributions of the U and V velocity components of our sample are clearly non-Gaussian, placing the age estimate inferred from the full space motion vector into question. The W-component exhibits a distribution more consistent with a normal distribution, and from W alone we derive an age of ~3 Gyr, which is the same age found for late-M dwarf samples. Our brightness-limited sample is probably contaminated by a number of outliers that predominantly bias the U and V velocity components. The origin of the outliers remain unclear, but we suggest that these brown dwarfs may have gained their high velocities by means of ejection from multiple systems during their formation.

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arXiv2009

A Volume-limited Sample of 63 M7-M9.5 Dwarfs II. Activity, magnetism, and the fade of the rotation-dominated dynamo

In a volume-limited sample of 63 ultracool dwarfs of spectral type M7-M9.5, we have obtained high-resolution spectroscopy with UVES at the Very Large Telescope and HIRES at Keck Observatory. In this second paper, we present projected rotation velocities, average magnetic field strengths, and chromospheric emission from the Halpha line. We confirm earlier results that the mean level of normalized Halpha luminosity decreases with lower temperature, and we find that the scatter among Halpha luminosities is larger at lower temperature. We measure average magnetic fields between 0 and 4kG with no indication for a dependence on temperature between M7 and M9.5. For a given temperature, Halpha luminosity is related to magnetic field strength, consistent with results in earlier stars. A few very slowly rotating stars show very weak magnetic fields and Halpha emission, all stars rotating faster than our detection limit show magnetic fields of at least a few hundred Gauss. In contrast to earlier-type stars, we observe magnetic fields weaker than 1kG in stars rotating faster than ~3km/s, but we find no correlation between rotation and magnetic flux generation among them. We interpret this as a fundamental change in the dynamo mechanism; in ultracool dwarfs, magnetic field generation is predominantly achieved by a turbulent dynamo, while other mechanisms can operate more efficiently at earlier spectral type.

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arXiv2009

The Origin of Enhanced Activity in the Suns of M67

We report the results of the analysis of high resolution photospheric line spectra obtained with the UVES instrument on the VLT for a sample of 15 solar-type stars selected from a recent survey of the distribution of H and K chromospheric line strengths in the solar-age open cluster M67. We find upper limits to the projected rotation velocities that are consistent with solar-like rotation (i.e., v sini ~< 2-3 km/s) for objects with Ca II chromospheric activity within the range of the contemporary solar cycle. Two solar-type stars in our sample exhibit chromospheric emission well in excess of even solar maximum values. In one case, Sanders 1452, we measure a minimum rotational velocity of vsini = 4 +/- 0.5 km/s, or over twice the solar equatorial rotational velocity. The other star with enhanced activity, Sanders 747, is a spectroscopic binary. We conclude that high activity in solar-type stars in M67 that exceeds solar levels is likely due to more rapid rotation rather than an excursion in solar-like activity cycles to unusually high levels. We estimate an upper limit of 0.2% for the range of brightness changes occurring as a result of chromospheric activity in solar-type stars and, by inference, in the Sun itself. We discuss possible implications for our understanding of angular momentum evolution in solar-type stars, and we tentatively attribute the rapid rotation in Sanders 1452 to a reduced braking efficiency.

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