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Ansgar Reiners

Publications and source records attributed to Ansgar Reiners.

At least 109 records · Page 6Linked to original sources
arXiv2009

Evidence for accretion in a nearby, young brown dwarf

We report on the discovery of the young, nearby, brown dwarf 2MASS J0041353$-$562112. The object has a spectral type of M7.5, it shows Li absorption and signatures of accretion, which implies that it still has a disk and suggests an age below 10 Myr. The space motion vector and position on the sky indicate that the brown dwarf is probably a member of the $\sim$20 Myr old Tuc-Hor association, or that it may be an ejected member of the $\sim$12 Myr old $β$ Pic association, both would imply that 2MASS J0041353$-$562112 may in fact be older than 10 Myr. No accreting star or brown dwarf was previously known in these associations. Assuming an age of 10 Myr, the brown dwarf has a mass of about 30 M$_{\rm Jup}$ and is located at 35 pc distance. The newly discovered object is the closest accreting brown dwarf known. Its membership to an association older than 10 Myr implies that either disks in brown dwarfs can survive as long as in more massive stars, perhaps even longer, or that star formation in Tuc-Hor or $β$ Pic occured more recently than previously thought. The history and evolution of this object can provide new fundamental insight into the formation process of stars, brown dwarfs, and planets.

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arXiv2009

Activity-induced radial velocity jitter in a flaring M dwarf

We investigate the effect of stellar activity and flares on short-term radial velocity measurements in the mid-M flare star CN Leo. Radial velocity variations are calculated from 181 UVES spectra obtained during three nights. We searched for spectral orders that contain very few atmospheric absorption lines and calibrated them against the telluric A-band from O$_2$ in the Earth's atmosphere. One giant flare occurred during our observations, which has a very strong effect on radial velocity. The apparent radial velocity shift due to the flare is several hundred m s$^{-1}$ and clearly correlated with H$α$ emission. Outside the flare, only spectral orders containing the most prominent emission lines of H, He, and Ca show a correlation to chromospheric activity together with a radial velocity jitter exceeding a few 10 m s$^{-1}$. We identify a number of spectral orders that are free of strong emission lines and show no flaring-related radial velocity jitter, although flares occurred as strong as 0.4 dex in normalized H$α$ luminosity. The mean radial velocity jitter due to moderate flaring is less than 10 m s$^{-1}$. Strong flares are easily recognized directly in the spectra and should be neglected for planet searches.

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arXiv2009

Surprisingly Weak Magnetism on Young Accreting Brown Dwarfs

We have measured the surface magnetic flux on four accreting young brown dwarfs and one non-accreting young very low-mass star utilizing high resolution spectra of absorption lines of the FeH molecule. A magnetic field of 1-2 kG had been proposed for one of the brown dwarfs, 2MASS J1207334$-$393254, because of its similarities to higher mass T Tauri stars as manifested in accretion and the presence of a jet. We do not find clear evidence for a kilo-Gauss field in any of our young brown dwarfs but do find a 2 kG field on the young VLM star. Our 3-$σ$ upper limit for the magnetic flux in 2MASS J1207334$-$393254 just reaches 1 kG. We estimate the magnetic field required for accretion in young brown dwarfs given the observed rotations, and find that fields of only a few hundred Gauss are sufficient for magnetospheric accretion. This predicted value is less than our observed upper limit. We conclude that magnetic fields in young brown dwarfs are a factor of five or more lower than in young stars of about one solar mass, and in older stars with spectral types similar to our young brown dwarfs. It is interesting that, during the first few million years, the fields scale down with mass in line with what is needed for magnetospheric accretion, yet no such scaling is observed at later ages within the same effective temperature range. This scaling is opposite to the trend in rotation, with shorter rotation periods for very young accreting brown dwarfs compared with accreting solar-mass objects (and very low Rossby numbers in all cases). We speculate that in young objects a deeper intrinsic connection may exist between magnetospheric accretion and magnetic field strength, or that magnetic field generation in brown dwarfs may be less efficient than in stars. Neither of these currently have an easy physical explanation.

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arXiv2009

Magnetic field observations of low-mass stars

Direct measurements of magnetic fields in low-mass stars of spectral class M have become available during the last years. This contribution summarizes the data available on direct magnetic measurements in M dwarfs from Zeeman analysis in integrated and polarized light. Strong magnetic fields at kilo-Gauss strength are found throughout the whole M spectral range, and so far all field M dwarfs of spectral type M6 and later show strong magnetic fields. Zeeman Doppler images from polarized light find weaker fields, which may carry important information on magnetic field generation in partially and fully convective stars.

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arXiv2009

On the magnetic topology of partially and fully convective stars

We compare the amount of magnetic flux measured in Stokes V and Stokes I in a sample of early- and mid-M stars around the boundary to full convection (~M3.5). Early-M stars possess a radiative core, mid-M stars are fully convective. While Stokes V is sensitive to the net polarity of magnetic flux arising mainly from large-scale configurations, Stokes I measurements can see the total mean flux. We find that in early-M dwarfs, only ~6% of the total magnetic flux is detected in Stokes V. This ratio is more than twice as large, ~14%, in fully convective mid-M dwarfs. The bulk of the magnetic flux on M-dwarfs is not seen in Stokes V. This is presumably because magnetic flux is mainly stored in small scale components. There is also more to learn about the effect of the weak-field approximation on the accuracy of strong field detections. In our limited sample, we see evidence for a change in magnetic topology at the boundary to full convection. Fully convective stars store a 2-3 times higher fraction of their flux in fields visible to Stokes V. We estimate the total magnetic energy detected in Stokes I and compare it to results from Stokes V. We find that in early-M dwarfs only ~0.5% of the total magnetic energy is detected in Stokes V while this fraction is ~2.5% in mid-M dwarfs.

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arXiv2008

Activity and rotation of low mass stars in young open clusters

We present first results from a multi-object spectroscopy campaign in IC2602, the Hyades, the Pleiades, and the Coma cluster using VLT/FLAMES. We analysed the data for radial velocity, rotational velocity, and H-alpha activity. Here, we highlight three aspects of this study in the context of rotational braking and the rotation-activity relationship among low mass stars. Finally we discuss the cluster membership of sources in IC2602.

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arXiv2008

The moderate magnetic field on the flare star Proxima Centauri

We report moderate magnetic flux of 450G < Bf < 750G (3sigma) on the nearby M5.5 flare star Proxima Centauri. A high resolution UVES spectrum was used to measure magnetic flux from Zeeman broadening in absorption lines of molecular FeH around 1mu. The magnetic flux we find is relatively weak compared with classical strong flare stars, but so are Proxima's flaring rates and actual emission levels. We compare what is known about the rotation rate, Rossby number, and activity levels in this star to relations between these quantities that have been recently being developed more generally for M dwarfs. We conclude that the magnetic flux is higher than the best estimates of the Rossby number from period measurements. On the other hand, the activity levels on Proxima Centauri are at the high end of what could be expected based on the measured field, but not so high as to exceed the natural scatter in these relations (other stars lie along this high envelope as well).

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arXiv2008

Chromospheric Activity, Rotation, and Rotational Braking in M and L Dwarfs

We present results from a high-resolution spectroscopic survey of 45 L dwarfs, which includes both very low-mass stars and brown dwarfs. Our spectra allow us to derive a significant number of new rotational velocities, and discover a slowly rotating (in projected velocity) L dwarf that allows more accurate measurement of spectroscopic rotations for these objects. We measure chromospheric activity (and often its variability) through the H$α$ emission line. Our primary new result is good evidence that magnetic braking dominates the angular momentum evolution of even brown dwarfs, although spindown times appear to increase as mass decreases. We confirm that activity decreases as effective temperature decreases, though a larger fraction of L dwarfs are active than has previously been reported. Essentially all active objects are also variable. We confirm the lack of a rotation-activity connection for L dwarfs. We find a minimum limit for rotational velocities that increases with later spectral types, rising from near zero in older mid-M stars to more than 20 km s$^{-1}$ for mid-L objects. There is strong evidence that all L dwarfs are rapid rotators. We derive a braking law that can depend on either temperature or mass which can explain all the rotational results and provides an age dependence for the angular momentum evolution. It is clear that angular momentum loss mechanisms in smaller and cooler objects become more inefficient starting at the fully convective boundary.

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arXiv2007

At the Bottom of the Main Sequence: Activity and magnetic fields beyond the threshold to complete convection

The bottom of the main sequence hosts objects with fundamentally different properties. At masses of about 0.3 M$_{\odot}$, stars become fully convective and at about 0.08 M$_{\odot}$ the hydrogen-burning main sequence ends; less massive objects are brown dwarfs. While stars and brown dwarfs experience very different evolutions, their inner structure has relatively little impact on the atmospheres. The generation of magnetic fields and activity is obviously connected to the threshold between partial and complete convection, because dynamo mechanisms involving a layer of shear like the solar $αΩ$-dynamo must cease. Hence a change in stellar activity can be expected there. Observations of stellar activity do not confirm a rapid break in activity at the convection boundary, but the fraction of active stars and rapid rotators is higher on the fully convective side. I summarize the current picture of stellar activity and magnetic field measurements at the bottom of the main sequence and present recent results on rotational braking beyond.

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arXiv2007

Ultra-Cool and Extra-Vigorous: Rotation and Activity in M and L dwarfs

The study of rotation and activity in low-mass stars or brown dwarfs of spectral classes M and L has seen enormous progress during the last years. I summarize the results from different works that measured activity, rotation, and sometimes magnetic fields. The generation of magnetic activity seems to be unchanged at the threshold to completely convective stars, i.e. no change in the efficiency of the magnetic dynamos is observed. On the other hand, a sudden change in the strength of rotational braking appears at the threshold mass to full convection, and strong evidence exists for rotational braking weakening with lower mass. A probable explanation is that the field topology changes from dipolar to small scale structure as the objects become fully convective.

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arXiv2007

Spectral type dependent rotational braking and strong magnetic flux in three components of the late-M multiple system LHS 1070

We show individual high resolution spectra of components A, B, and C of the nearby late-M type multiple system LHS 1070. Component A is a mid-M star, B and C are known to have masses at the threshold to brown dwarfs. From our spectra we measure rotation velocities and the mean magnetic field for all three components individually. We find magnetic flux on the order of several kilo-Gauss in all components. The rotation velocities of the two late-M objects B and C are similar (vsini = 16km/s), the earlier A component is spinning only at about half that rate. This suggests weakening of net rotational braking at late-M spectral type, and that the lack of slowly rotating late-M and L dwarfs is real. Furthermore, we found that magnetic flux in the B component is about twice as strong as in component C at similar rotation rate. This indicates that rotational braking is not proportional to magnetic field strength in fully convective objects, and that a different field topology is the reason for the weak braking in low mass objects.

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arXiv2007

The Narrowest M-dwarf Line Profiles and the Rotation-Activity Connection at Very Slow Rotation

The rotation-activity connection explains stellar activity in terms of rotation and convective overturn time. It is well established in stars of spectral types F--K and in M-type stars of young clusters. It is not established in field M-dwarfs, because they rotate very slowly, and detecting rotation periods or rotational line broadening is a challenge. In field M-dwarfs, saturation sets in below v_rot = 5 km/s, hence they are expected to populate the non-saturated part of the rotation-activity connection. This work for the first time shows intrinsically resolved spectral lines of slowly rotating M-dwarfs and makes a first comparison to estimates of convective velocities. By measuring rotation velocities in a sample of mostly inactive M-dwarfs, the unsaturated part of the rotation-activity connection is followed into the regime of very low activity. Spectra of ten M-dwarfs are taken at a resolving power of R = 200 000 in the molecular FeH absorption band. The intrinsically narrow lines are compared to model calculations and rotational broadening is measured. In one star, an upper limit of vsini = 1 km/s was found, significant rotation was detected in the other nine objects. All inactive stars show rotation below or equal to 2 km/s. In the two active stars AD Leo and YZ CMi, rotation velocities are found to be 40-50% below the results from earlier studies. The rotation activity connection holds in field early-M stars, too. Activity and rotation velocities of the sample stars are well in agreement with the relation found in earlier and younger stars. The intrinsic absorption profiles of molecular FeH lines are consistent with calculations from atomic Fe lines. Investigation of FeH line profiles is a very promising tool to measure convection patterns at the surfaces of M-stars.

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arXiv2006

A Survey for Spectroscopic Binaries Among Very Low-Mass Stars

We report on the results of a survey for radial velocity variability in a heterogeneous sample of very low-mass stars and brown dwarfs. One distinguishing characteristic of the survey is its timespan, which allows an overlap between spectroscopic binaries and those which can be found by high angular-resolution imaging. We are able to place a new constraint on the total binary fraction in these objects, which suggests that they are more likely the result of extending the same processes at work at higher masses into this mass range, rather than a distinct mode of formation. Our basic result is that there are $6 \pm 2$ out of 53, or $11^{+0.07}_{-0.04}$% spectroscopic binaries in the separation range 0-6 AU, nearly as many as resolved binaries. This leads to an estimate of an upper limit of $26 \pm 10$% for the binary fraction of VLM objects (it is an upper limit because of the possible overlap between the spectroscopic and resolved populations). A reasonable estimate for the very low-mass binary fraction is $20 - 25$%. We consider several possible separation and frequency distributions, including the same one as found for GK stars, a compressed version of that, a version of the compressed distribution truncated at 15 AU, and a theoretical distribution which considers the evaporation of small-N clusters. We conclude that the latter two bracket the observations, which may mean that these systems form with intrinsically smaller separations due to their smaller mass, and then are truncated due to their smaller binding energy. We do not find support for the ``ejection hypothesis'' as their dominant mode of formation, particularly in view of the similarity in the total binary fraction compared with slightly more massive stars, and the difficulty this mechanism has in producing numerous binary systems.

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arXiv2006

Measuring Magnetic Fields in Ultracool Stars and Brown Dwarfs

We present a new method for direct measurement of magnetic fields on ultracool stars and brown dwarfs. It takes advantage of the Wing-Ford band of FeH, which are seen throughout the M and L spectral types. These molecular features are not as blended as other optical molecular bands, are reasonably strong through most of the spectral range, and exhibit a response to magnetic fields which is easier to detect than other magnetic diagnostics, including the usual optical and near-infrared atomic spectral lines that have heretofore been employed. The FeH bands show a systematic growth as the star gets cooler. We do not find any contamination by CrH in the relevant spectral region. We are able to model cool and rapidly-rotating spectra from warmer, slowly-rotating spectra utilizing an interpolation scheme based on optical depth scaling. We show that the FeH features can distinguish between negligible, moderate, and high magnetic fluxes on low-mass dwarfs, with a current accuracy of about one kilogauss. Two different approaches to extracting the information from the spectra are developed and compared. Which one is superior depends on a number of factors. We demostrate the validity of our new procedures by comparing the spectra of three M stars whose magnetic fluxes are already known from atomic line analysis. The low and high field stars are used to produce interpolated moderate-strength spectra which closely resemble the moderate-field star. The assumption of linear behavior for the magnetic effects appears to be reasonable, but until the molecular constants are better understood the method is subject to that assumption, and rather approximate. Nonetheless, it opens a new regime of very low-mass objects to direct confirmation and testing of their magnetic dynamos.

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arXiv2005

The First High-Resolution Spectra of 1.3 L Subdwarfs

We present the first high-resolution (R ~ 31,000) spectra of the cool sdL 2MASS0532, and what was originally identified as an early-type L subdwarf (sdL) LSR1610-0040. Our work, in combination with contemporaneous work by Cushing and Vacca, makes it clear that the latter object is more probably a mid-M dwarf with an unusual composition that gives it some sub-dwarf spectral features. We use the data to derive precise radial velocities for both objects and to estimate space motion; both are consistent with halo kinematics. We measure the projected rotational velocities, revealing very slow rotation for the old sd?M6 object \lsr. \twom exhibits rapid rotation of vsini = 65 +- 15km/s, consistent with the behavior of L dwarfs. This means that the braking time for L dwarfs is extremely long, or that perhaps they never slow down. A detailed comparison of the atomic Rb and Cs lines to spectra of field L dwarfs shows the spectral type \twom is consistent with being mid- to late-L. The Rb I and K I lines of \lsr\ are like an early-L dwarf, but the Cs I line is like a mid-M dwarf. The appearance of the Ca II triplet in absorption in this object is very hard to understand if it is not as least as warm as M6. We explain these effects in a consistent way using a mildly metal-poor mid-M model. M subdwarfs have weak metal-oxides and enhanced metal-hydrides relative to normal M dwarfs. \lsr\ exhibits metal-hydrides like an M dwarf but metal-oxides like a subdwarf. The same explanation that resolves the atomic line discrepancy explains this as well. We identify atomic lines of Ti around 9600 Åand a small contribution of FeH, but we cannot confirm a detection of TiH in the spectra of cool L subdwarfs. High resolution spectroscopy has aided in beginning to understand the complex molecular chemistry in metal-deficient and ultralow-mass objects.

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arXiv2005

Calibrating models of ultralow-mass stars

Evolutionary and atmospheric models have become available for young ultralow-mass objects. These models are being used to determine fundamental parameters from observational properties. TiO bands are used to determine effective temperatures in ultralow-mass objects, and together with Na- and K-lines to derive gravities at the substellar boundary. Unfortunately, model calibrations in (young) ultralow-mass objects are rare. As a first step towards a calibration of synthetic spectral features, I show molecular bands of TiO, which is a main opacity source in late M-dwarfs. The TiO $ε$-band at 8450Åis systematically too weak. This implies that temperatures determined from that band are underestimated, and I discuss implications for determining fundamental parameters from high resolution spectra.

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arXiv2005

Rotation- and temperature-dependence of stellar latitudinal differential rotation

More than 600 high resolution spectra of stars with spectral type F and later were obtained in order to search for signatures of differential rotation in line profiles. In 147 stars, the rotation law could be measured, 28 of them are found to be differentially rotating. Comparison to rotation laws in stars of spectral type A reveals that differential rotation sets in at the convection boundary in the HR-diagram; no star that is significantly hotter than the convection boundary exhibits the signatures of differential rotation. Four late A-/early F-type stars close to the convection boundary and at vsini~100 km/s show extraordinarily strong absolute shear at short rotation periods around one day. It is suggested that this is due to their small convection zone depth and that it is connected to a narrow range in surface velocity. Detection frequencies of differential rotation were analyzed in stars with varying temperature and rotation velocity. Measurable differential rotation is more frequent in late-type stars and slow rotators. The strength of absolute shear and differential rotation are examined as functions of the stellar effective temperature and rotation period. The strongest shear is found at rotation periods between two and three days. In slower rotators, the strongest shear at a given rotation rate is given approximately by DOmega_max ~ P^{-1}. In faster rotators, alpha_max and DOmega_max diminish less rapidly. A comparison with differential rotation measurements in stars of later spectral type shows that F-stars exhibit stronger shear than cooler stars do, the upper boundary in absolute shear DOmega with temperature is consistent with the temperature scaling law found in Doppler Imaging measurements.

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