YY Gem Research Articles

Magnetic Field of the Eclipsing M-dwarf Binary YY Gem

Abstract YY Gem is a short-period eclipsing binary system containing two nearly identical, rapidly rotating, very active early M dwarfs. This binary represents an important benchmark system for calibrating empirical relations between fundamental properties of low-mass stars and for testing theories of interior structure and evolution of these objects. Both components of YY Gem exhibit inflated radii, which has been attributed to poorly understood magnetic activity effects. Despite a long history of magnetic activity studies of this system, no direct magnetic field measurements have been made for it. Here we present a comprehensive characterization of the surface magnetic field in both components of YY Gem. We reconstructed the global field topologies with the help of a tomographic inversion technique applied to high-resolution spectropolarimetric data. This analysis revealed moderately complex global fields with a typical strength of 200–300 G and anti-aligned dipolar components. A complementary Zeeman intensification analysis of the disentangled intensity spectra showed that the total mean field strength reaches 3.2–3.4 kG in both components of YY Gem. We used these results together with other recent magnetic field measurements of M dwarfs to investigate the relation between the global and small-scale fields in these stars. We also assessed predictions of competing magnetoconvection interior structure models developed for YY Gem, finding that only one of them anticipated the surface field strength compatible with our observations. Results of our starspot mapping of YY Gem do not support the alternative family of theoretical stellar models, which attempts to explain the radius inflation by postulating a large spot filling factor.

Possible substellar companions in low-mass eclipsing binaries: GU Bootis and YY Geminorum

We present the next results of our long-term observational project to analyze the variations in the orbital periods of low-mass eclipsing binaries. About 70 new precise mid-eclipse times recorded with a CCD were obtained for two eclipsing binaries with short orbital periods: GU Boo (P = 0.​d49) and YY Gem (0.​d81). Observed-minus-calculated diagrams of the stars were analyzed using all reliable timings, and new parameters of the light-time effect were obtained. We derived for the first time or improved the short orbital periods of possible third bodies of 11 and 54 years for these low-mass binaries, respectively. We calculated that the minimum masses of the third components are close to 50 MJup, which corresponds to the mass of brown dwarfs. The multiplicity of these systems also plays an important role in the precise determination of their physical parameters.

Magnetoconvective models of red dwarfs: constraints imposed by the lithium abundance

Magnetic fields impede the onset of convection, thereby altering the thermal structure of a convective envelope in a low mass star: this has an effect on the amount of lithium depletion in a magnetized star. In order to quantify this effect, we have applied a magneto-convective model to two low mass stars for which lithium abundances and precise structural parameters are known: YY Gem and CU Cnc. For both stars, we have obtained models which satisfy empirical constraints on the following parameters: R, L, surface magnetic field strength, and Li abundance. In the case of YY Gem, we have obtained a model which satisfies the empirical constraints with an internal magnetic field of several megagauss: such a field strength is within the range of a dynamo where the field energy is in equipartition with rotational energy deep inside the convection zone. However, in the case of CU Cnc, the Li requires an internal magnetic field which is probably too strong for a dynamo origin: we suggest possible alternatives which might account for the reported Li abundance in CU Cnc.

SURFACE MAGNETIC FIELD STRENGTHS: NEW TESTS OF MAGNETOCONVECTIVE MODELS OF M DWARFS

Precision modeling of M dwarfs has become worthwhile in recent years due to the increasingly precise values of masses and radii which can be obtained from eclipsing binary studies. In a recent paper, Torres has identified four prime M dwarf pairs with the most precise empirical determinations of masses and radii. The measured radii are consistently larger than standard stellar models predict by several percent. These four systems potentially provide the most challenging tests of precision evolutionary models of cool dwarfs at the present time. We have previously modeled M dwarfs in the context of a criterion due to Gough & Tayler in which magnetic fields inhibit the onset of convection according to a physics-based prescription. In the present paper, we apply our magnetoconvective approach to the four prime systems in the Torres list. Going a step beyond what we have already modeled in CM Dra (one of the four Torres systems), we note that new constraints on magnetoconvective models of M dwarfs are now available from empirical estimates of magnetic field strengths on the surfaces of these stars. In the present paper, we consider how well our magnetoconvective models succeed when confronted with this new test of surface magnetic field strengths. Among the systems listed by Torres, we find that plausible magnetic models work well for CM Dra, YY Gem, and CU Cnc. (The fourth system in Torres's list does not yet have enough information to warrant magnetic modeling.) Our magnetoconvection models of CM Dra, YY Gem, and CU Cnc yield predictions of the magnetic fluxes on the stellar surface which are consistent with the observed correlation between magnetic flux and X-ray luminosity.

MAGNETIC INHIBITION OF CONVECTION AND THE FUNDAMENTAL PROPERTIES OF LOW-MASS STARS. I. STARS WITH A RADIATIVE CORE

Magnetic fields are hypothesized to inflate the radii of low-mass stars---defined as less massive than 0.8$M_\odot$---in detached eclipsing binaries (DEBs). We investigate this hypothesis using the recently introduced magnetic Dartmouth stellar evolution code. In particular, we focus on stars thought to have a radiative core and convective outer envelope by studying in detail three individual DEBs: UV Psc, YY Gem, and CU Cnc. The results suggest that the stabilization of thermal convection by a magnetic field is a plausible explanation for the observed model-radius discrepancies. However, surface magnetic field strengths required by the models are significantly stronger than those estimated from the observed coronal X-ray emission. Agreement between model predicted surface magnetic field strengths and those inferred from X-ray observations can be found by assuming that the magnetic field sources its energy from convection. This approach makes the transport of heat by convection less efficient and is akin to reduced convective mixing length methods used in other studies. Predictions for the metallicity and magnetic field strengths of the aforementioned systems are reported. We also develop an expression relating a reduction in the convective mixing length to a magnetic field strength in units of the equipartition value. Our results are compared with those from previous investigations to incorporate magnetic fields to explain the low-mass DEB radius inflation. Finally, we explore how the effects of magnetic fields might affect mass determinations using asteroseismic data and the implication of magnetic fields on exoplanet studies.

Chandra study of the eclipsing M dwarf binary, YY Gem

The eclipsing M dwarf binary system, YY Gem, was observed using Chandra covering 140 ks (2Prot) in total, split into two even exposures separated by 0.76 d (0.94 Prot). The system was extremely active: three energetic flares were observed over the course of these observations. The flaring and non-flaring states of the system are analysed in this paper. The activity level increased between the first and second observations even during the quiescent (non-flaring) phases. An analysis of the dynamics of the X-ray-emitting plasma suggests that both components are significantly active. Contemporaneous Hα spectra also suggest that both components show similar levels of activity. The primary star is the likely source of at least two of the flares. From a detailed analysis of the flare emission at the maximum temperature and maximum density with single loop flare models, we find loop lengths of ∼0.7R*, 1.5R* and 1.8R*. All of these flares are strongly associated with hot (>10 MK) X-ray emission which appears to predominantly trace the orbital motion of the primary star. The two largest flaring loops are similar to the largest sizes reported in other active M stars and span nearly half the interbinary system; this may indicate magnetospheric interaction between the binary star coronae. We discuss the time and spectral resolution requirements that are necessary to recover detailed information about coronal structure from the X-ray spectra in similar cool star systems.

Simulations of the periodic flaring rate on YY Gem

The binary YY Gem shows many interesting properties, one of which is the periodicity in its flaring rate. The period, which is about 48 ± 3 min, was ever interpreted in terms of the oscillation of a filament. In this paper, we propose a new model to explain this phenomenon by means of 2.5D magnetohydrodynamic (MHD) numerical simulations. It is found that magnetic reconnection is induced as the coronal loops rooted on both stars inflate and approach each other, which is driven by the differential stellar rotation. The magnetic reconnection is modulated by fast-mode magnetoacoustic waves which are trapped between the surfaces of the two stars, so that the reconnection rate presents a periodic behaviour. With the typical parameters for the binary system, the observed period can be reproduced. We also derive an empirical formula to relate the period of the flaring rate to the coronal temperature and density, as well as the magnetic field.

GU Bootis: A New 0.6M⊙Detached Eclipsing Binary

We have found a new low-mass, double-lined, detached eclipsing binary, GU Boo, among a sample of new variables from the ROTSE-I database. The binary has an orbital period of 0.488728 ± 0.000002 days and estimated apparent magnitudes Vrotse 13.7 and I 11.8. Our analysis of the light and radial velocity curves of the system yields individual masses and radii of M1 = 0.610 ± 0.007 M☉, M2 = 0.599 ± 0.006 M☉, R1 = 0.623 ± 0.016 R☉, and R2 = 0.620 ± 0.020 R☉. The stars in GU Boo are therefore very similar to the components of the eclipsing binary YY Gem. For this study we have adopted a mean effective temperature for the binary of Teff = 3870 ± 130 K. Based on its space velocities we suggest that GU Boo is a main-sequence binary, possibly with an age of several Gyr. The metallicity of the binary is not well constrained at this point, but we speculate that it should not be very different from solar. We have compared the physical parameters of GU Boo with current low-mass stellar models, accounting for uncertainties in age and metallicity by considering a wide range of values for those parameters. Our comparisons reveal that all the models underestimate the radii of the components of GU Boo by at least 10%-15%. This result is in agreement with the recent studies of YY Gem and CU Cnc.

Coronal density diagnostics with Helium-like triplets: CHANDRA–LETGS observations of Algol, Capella, Procyon,ϵ Eri,α Cen A&B, UX Ari, AD Leo, YY Gem, and HR 1099

We present an analysis of ten cool stars (Algol, Capella, Procyon, Eri, Cen A&B, UX Ari, AD Leo, YY Gem, and HR 1099) observed with the Low Energy Transmission Grating Spectrometer (LETGS) on board the Chandra X-ray Observatory. This sample contains all cools stars observed with the LETGS presently available to us with integration times suciently long to warrant a meaningful spectral analysis. Our sample comprises inactive, moderately active, and hyperactive stars and samples the bulk part of activity levels encountered in coronal X-ray sources. We use the LETGS spectra to carry out density and temperature diagnostics with an emphasis on the H-like and the He-like ions. We find a correlation between line flux ratios of the Ly and He-like resonance lines with the mean X-ray surface flux. We determine densities using the He-like triplets. For all stars we find no significant deviations from the low-density limit for the ions of Ne, Mg, and Si, while the measured line ratios for the ions of C, N, and O do show evidence for departures from the low-density limit in the active stars, but not in the inactive stars. Best measurements can be made for the Ovii triplet where we find significant deviations from the low-density limit for the stars Algol, Procyon, YY Gem, Eri, and HR 1099. We discuss the influence of radiation fields on the interpretation of the He-like triplet line ratios in the low-Z ions, which is relevant for Algol, and the influence of dielectronic satellite lines, which is relevant for Procyon. For the active stars YY Gem, Eri, and HR 1099 the low f=i ratios can unambiguously be attributed to high densties in the range 1-3 10 10 cm 3 at Ovii temperatures. We find our LETGS spectra to be an extremely useful tool for plasma diagnostics of stellar coronae.

The EUVE point of view of AD Leo

All the Extreme Ultraviolet Explorer (EUVE) observations of AD Leo, totalling 1.1 Ms of exposure time, have been employed to analyze the corona of this single M dwarf. The light curves show a well defined quiescent stage, and a distribution of amplitude of variability following a power law with a ~-2.4 index. The flaring behavior exhibits much similarity with other M active stars like FK Aqr or YY Gem, and flares behave differently from late type active giants and subgiants. The Emission Measure Distribution (EMD) of the summed spectrum, as well as that of quiescent and flaring stages, were obtained using a line-based method. The average EMD is dominated by material at log T(K) ~ 6.9, with a second peak around log T(K) ~ 6.3, and a large increase in the amount of material with log T(K) 7.1 during flares, material almost absent during quiescence. The results are interpreted as the combination of three families of loops with maximum temperatures at log T(K) ~ 6.3, ~6.9 and somewhere beyond log T(K) 7.1. A value of the abundance of [Ne/Fe] = 1.05 ± 0.08 was measured at log T(K) ~ 5.9. No significative increment of neon abundance was detected between quiescence and flaring states.

Simultaneous X-ray spectroscopy of YY Gem withChandraandXMM-Newton

We report on a detailed study of the X-ray spectrum of the nearby eclipsing spectroscopic binary YY Gem. Observations were obtained simultaneously with both large X-ray observatories, XMM-Newton and Chandra. We compare the high-resolution spectra acquired with the Reflection Grating Spectrometer onboard XMM-Newton and with the Low Energy Transmission Grating Spectrometer onboard Chandra, and evidence in direct comparison the good performance of both instruments in terms of wavelength and flux calibration. The strongest lines in the X-ray spectrum of YY Gem are from oxygen. Oxygen line ratios indicate the presence of a low-temperature component ( MK) with density . The X-ray lightcurve reveals two flares and a dip corresponding to the secondary eclipse. An increase of the density during phases of high activity is suggested from time-resolved spectroscopy. Time-resolved global fitting of the European Photon Imaging Camera CCD spectrum traces the evolution of temperature and emission measure during the flares. These medium-resolution spectra show that temperatures K are relevant in the corona of YY Gem although not as dominant as the lower temperatures represented by the strongest lines in the high-resolution spectrum. Magnetic loops with length on the order of 109 cm, i.e., about 5% of the radius of each star, are inferred from a comparison with a one-dimensional hydrodynamic model. This suggests that the flares did not erupt in the (presumably more extended) inter-binary magnetosphere but are related to one of the components of the binary.

YY Geminorum: A Very Late Type Close Binary with Possible Magnetic Stellar Wind

The O-C curve of the very late type close binary (dMle+dMle)YY Geminorum is formed and analyzed based on all available times of minimum light. It is found that the orbital period shows a secular decrease with rate dP/dt = -1.08x10(-8) days yr(-1). There is weak evidence indicating that a small-amplitude period oscillation is superposed on the period decrease. YY Gem is a well-detached eclipsing binary containing two very active components. The secular period decrease may suggest that the system is undergoing secular mass and angular momentum loss via a magnetic stellar wind. If the small-amplitude oscillation is real, it can be explained either by magnetic activity cycles or by the presence of a giant planet or brown dwarf.

Absolute Dimensions of the M‐Type Eclipsing Binary YY Geminorum (Castor C): A Challenge to Evolutionary Models in the Lower Main Sequence

We present new spectroscopic observations of the detached late-type double-lined eclipsing binary YY Geminorum (P = 0.814 day), a member of the Castor sextuple system and one of the benchmarks for the comparison between observations and stellar evolution theory in the lower main sequence. In addition, we have reanalyzed existing light curves in several passbands using modern techniques that account for the conspicuous presence of spots. This, combined with the spectroscopy, has yielded a very precise determination of the absolute dimensions of the components, which are virtually identical to each other. We obtain for the mean mass, radius, and effective temperature the values M = 0.5992 ± 0.0047 M☉, R = 0.6191 ± 0.0057 R☉, and Teff = 3820 ± 100 K, respectively. Both the mass and the radius determinations are good to better than 1%, which in the case of the radius represents a fourfold improvement over previous results and significantly enhances the value of this quantity for testing the models. We discuss the importance of systematic effects in these measurements by comparison with another high-precision determination of the mass by Segransan and coworkers. A reanalysis of the Hipparcos transit data for Castor AB that accounts for the relative motion of the pair in its 467 yr period orbit has yielded an improved parallax for the system of 66.90 ± 0.63 mas. With this, we have estimated the age (~370 Myr) and metal abundance ([Fe/H] ~ 0.0) of YY Gem from isochrone fits to Castor A and B under the assumption of a common origin. This, along with the other physical properties, allow for an unusually stringent test of the models for low-mass stars. We have compared the observations of YY Gem with a large number of recent theoretical calculations, and we show that all models underestimate the radius by up to 20% and that most overestimate the effective temperature by 150 K or more. Both of these trends are confirmed by observations of another similar system in the Hyades (V818 Tau). Consequently, theoretical ages for relatively low mass objects such as T Tauri stars derived by placing them on the H-R diagram may be considerably biased. If the radius is used directly as a measure of evolution, ages could be underestimated by as much as a factor of 10 in this mass regime. In view of these discrepancies, absolute ages from essentially all current models for the lower main sequence must be viewed with at least some measure of skepticism. Finally, we derive a new and very accurate ephemeris based on all available times of eclipse, and we lay to rest previous claims of sudden changes in the orbital period of the binary, which we show to be spurious.

Period Changes in Four Short-Period Spotted Binaries: UV Piscium, YY Geminorum, CG Cygni, and XY Ursae Majoris

Recently acquired times of minimum for three RS CVn systems and one BY Draconis binary, all with periods under a day, are presented. Times of minimum for UV Psc, YY Gem, CG Cyg, and XY UMa have been measured for decades. These observations have been collected, and the O-C curves for each system have been examined for period changes. The O-C curves for UV Psc and YY Gem showed a linear deviation, and improved periods have been computed. CG Cyg and XY UMa, though, appear to have experienced true changes in their periods. An additional 11.0 yr periodic variation has been found in the O-C data of XY UMa. The mechanism for the period variations remains uncertain.

The XMM-Newton view of stellar coronae: Coronal structure in the Castor X-ray triplet

We present results of a comprehensive X-ray investigation of the multiple stellar system Castor AB + YY Gem. The observations were obtained with the XMM-Newton observatory. We report the first spatially separated detection and identification of both Castor A and B as frequently flaring X-ray sources. Spectral modeling of the YY Gem and Castor coronal sources indicates a broad plasma distribution between 2-15 MK. Elemental abundances are found to be below solar photospheric values, with the abundance of Ne being highest (~0.7 times solar photospheric). The abundances increase during a large flare, most notably for Fe. The Ovii He-like line triplet indicates average coronal densities of the cooler plasma of a few times 1010 cm-3 for YY Gem and Castor. The YY Gem light curve shows three deep eclipses, indicating that the coronae of both binary components are similarly active and are relatively compact. A 3-D light curve inversion shows that coronal active regions follow density scale heights compatible with spectroscopically measured temperatures. The dominant densities also agree with spectroscopic values. Most active regions are located between degrees latitude, in agreement with findings from previous Doppler imaging. We also report the tentative detection of line broadening due to orbital motion.

ROSAT All-Sky Survey X-ray and EUV observations of YY Gem and AU Mic

Simultaneous X-ray and extreme ultraviolet (EUV) (ROSAT XRT and WFC All-Sky Survey) observations of the highly active dMe flare stars YY Gem and AU Mic show that the two stars displayed an unusual type of flaring behaviour. We detect several X-ray and EUV flares superimposed on an enhanced and smoothly varying quiescent background. The two large impulsive-type X-ray flares on YY Gem reach peak X-ray luminosities of and we estimate that they had similar integrated luminosities (∼6–8×1033 erg). AU Mic also produced several X-ray and EUV flares, with one very impulsive flare producing a 10-fold increase in XRT count rate. This flare was even larger than the YY Gem flares (peak LX of and integrated LX of The ratio for both stars is at the ‘saturation’ limit found in rapidly rotating dwarfs and the most active RS CVn stars. We suggest that the gradually varying components are the result of a period of continuous, unresolved flaring activity. Alternatively, they may be the result of the emergence and subsequent decay of a new magnetic active region on the stellar surface of these stars.

X‐Ray Spectroscopy of Rapidly Rotating, Late‐Type Dwarf Stars

We present an X-ray spectroscopic analysis of two active dwarf stars with extremely short rotation periods, HD 197890 (Speedy Mic) and Gliese 890, using X-ray data acquired with the ASCA observatory. We analyze the X-ray spectrum of Speedy Mic in two separate time intervals, one corresponding to an apparent stellar flare, the other to quiescence or the nonflaring state. We also present a reanalysis of the ASCA spectrum of the M dwarf stellar binary YY Gem, during both quiescent and flaring states. We use the recently updated MEKAL plasma code to model these spectra and find that a minimum of two temperature components are required to obtain good fits for all three stars. The inferred emission measures of the hot components in YY Gem and Speedy Mic, and the temperature of the hot component in the case of YY Gem, increase significantly during the observed X-ray flares, in agreement with previous studies of similar stellar X-ray flares. In addition, metal abundances that are low compared to the solar photospheric values are preferred (but not required) for the quiescent coronae of Gliese 890 and Speedy Mic, while for the quiescent corona of YY Gem, subsolar metal abundances are required and solar abundance models can be ruled out with high confidence. We combine the results of this study with those of previous ASCA and ROSAT analyses of late-type dwarf stars with known rotation rates to obtain a sample of 17 G, K, and M dwarfs. We find a strong correlation of the X-ray to bolometric luminosity ratio, L X /L bol , both with the rotation period and the Rossby number (R o ), in agreement with previous studies. Using a homogeneous subset of 10 dwarfs for which we have purely ASCA-derived temperatures and elemental abundances, we find that the temperature of the hottest component present in their coronae is correlated with this luminosity ratio, as expected. Finally, we note that the Fe abundances for all active dwarf stars having normalized X-ray luminosities log (L X /L bol ) > −3.7 are significantly subsolar, whereas the Fe abundances in the less active stars are within a factor of 2 of the solar value.

X-Ray Coronae from Single Late-Type Dwarf Stars

Study of X-ray coronae from late-type stars with moderate resolution X-ray spectroscopy with ASCA, has led to the characterization of temperatures and measurements of elemental abundances in their coronae. Several RS CVn and Algol-type binary systems, and single late-type stars have been observed. We present here the results obtained from X-ray spectroscopy of recently observed single F-G-K-M type dwarfs. The sample observed with ASCA contains αCen (Mewe et al. 1997), π1UMa (Drake et al. 1994), YY Gem, Speedy Mic, GJ 890 (Singh et al. 1997), EK Dra, HN Peg, κ1Cet (Guedel et al. 1997), AB Dor (Mewe et al. 1996) and HD 35850 (Tagliaferri et al. 1997).

Basic stellar parameter determination for active cool eclipsing binaries

We examine procedures for the determination of reliable parameter sets characterizing stars in short period eclipsing binary systems of later dwarf, generally sunlike, spectral types. These stars have magnetodynamically active components and form a useful “extension to the solar laboratory”. Their much more rapid rotation than that of the Sun implies correspondingly enhanced indications of activity-particularly with relatively large maculation effects in broadband data. We concentrate on five such systems, for which we discuss new data and analyses-CG Cyg, ER Vul, YY Gem, BH Vir and UV Psc. Our main aim is to enable more reliable absolute comparisons between the basic properties of such stars and the Sun. Some indications of the ages of these binaries can also provide further pointers to the relative importance of different parameters (e.g. rotation, age, etc.), in determining stellar activity.

An ASCA observation of the Castor system

We report on a day-long ASCA broadband (1-10 keV) spectro-imaging observation of the X-ray emission from the Castor multibinary system. Significant flares were detected from both the flare star system YY Gem (Castor C) and from Castor AB located 73 sec away. Using an optimal viewing geometry and image restoration techniques, we are able to spatially resolve the emission from the two X-ray components. Broadband flare activity from Castor AB is confirmed, and quiescent flux is detected. The quiescent spectrum of YY Gem is a complex blend of emission lines across the ASCA bandpass which requires multitemperature components or two-temperature variable metal-poor abundances (approximately 5-10 below solar) to obtain a satisfactory fit to both the Mewe-Kaastra and Raymond-Smith models. The flare spectrum is consistent with an increase in the emissivity of the hotter component.