YY Gem Research Articles

Gliese 841A: an EUV-selected chromospherically active binary system⋆

We report the discovery of a new short-period, main-sequence, chromospherically active late-type binary system. EUV observations performed by the ROSAT Wide Field Camera have identified the late-type star GL 841A as a possible very active star by virtue of a large EUV-to-bolometric flux ratio. Subsequent optical observations at the wavelengths of the Hα and Ca II H and K lines have confirmed this, and also the binary nature of the system. An orbital period of 1.124 d, an inclination of 28° and a mass ratio of approximately unity are deduced from radial velocity data. Both components show Hα in emission. The photometric colours and absolute magnitude indicate that the component stars are both spectral class dM3-5e. A flare is observed in the Hα emission profile and is attributed to flares on at least one and possibly both the component stars within the space of a few hours. The GL 841A system is similar in many respects to the young eclipsing binary YY Gem but, because GL 841B (the common proper motion, visual binary companion) is a cool dQ7 white dwarf, an age of at least 2 ×109 yr is deduced.

ECLIPSING BINARIES IN MULTIPLE STAR SYSTEMS

ABSTRACT About 80 eclipsing binaries are known to be components of multiple star systems. This is less than 2% of all known eclipsing binary systems, but the actual total is certainly much high than this, as the majority of stars under review are in fairly bright, well studied systems. Triple star systems are by far the most common of these, but one system in five is a quadruple or higher order system. Quadruple systems include both systems of hierarchy 2 (two widely spaced pairs, e.g., BV + BW Dra) and hierarchy 3 (e.g., Eta Ori). V772 Her and DN UMa are in quintuple systems, and HR 3337 and the system that contains GZ And may also be quintuple. Castor, which contains the eclipsing binary YY Gem, is a very rare example of a sextuple star system. Also considered are the eclipsing binaries in compact star clusters such as SZ Cam and the two binaries in the Trapezium, BM Ori and V1016 Ori. The most frequently encountered type of eclipsing binary to be found in multiple star systems is that composed of two early-type (B5 or earlier) stars. Nearly all of the very close triple systems which have eclipsing binary components are of this type. W UMa-type stars are also well represented among the multiple star systems containing eclipsing binaries, but some other types are not. The RS CVn stars occur less frequently as components of multiple star systems, and cataclysmic binaries appear to occur only very rarely as components of these systems. The physical and orbital properties of the multiple star systems are discussed in some detail in cases where the data are fairly complete, but it must be noted that many of the visual pairs have periods of revolution which are much too long to permit reliable determination of orbits. In triple star systems with eclipsing binaries the ratio of P2/Pt ranges from less than 10 (for lambda Tau) to more than 107 in some cases. The questions of co-planarity of orbits and the membership of components in multiple star systems are also considered.

A model for the observed periodicity in the flaring rate on YY Gem

Four flares were observed on the late-type binary YY Gem in March 1988 during a total monitoring time of 408 min. The flares were unusual in that there is a periodicity in their occurrence, being separated by 48 ± 3 min. Considering the flares to be formed as a stochastic process, we find that the probability of these events occurring by chance is 0.5%. Modelling indicates that for quite reasonable input parameters (e.g. a spot field strength of 1000 G and a filament with mass per unit length of 106g cm-1), the flare periodicity can be explained in terms of filament oscillations. The only requirement is that there should be a filament at these heights where the magnetic field drops inversely proportional to the height.

Polarimetry of Stellar Active Regions and Flares

AbstractObservations of regular and irregular polarimetric variability in late-type stars are reviewed, and the related physical and geometrical effects are discussed. There are indications that the irregular part of the variability could be caused by transient events, possibly associated with flares. Polarimetric observations during flares are reviewed, and preliminary results of new observations of a well-known flare star, YY Geminorum, are presented. The results show that the small flare in YY Gem did not cause any significant variations in linear polarization, while the binary eclipse evidently causes an enhancement in the polarization. The reasons for the difficulties in stellar flare polarimetry are discussed. Finally, future prospects for the observations of flaring stars and for the utilization of linear polarimetry as a complementary method to other techniques of surface imaging of stellar activity and flares are presented.

Properties of main-sequence eclipsing binaries - Into the G stars with HS Aurigae, FL Lyrae, and EW Orionis

Spectrographic and photometric observations of three detached main sequence eclipsing binary systems with one or both components of type G, HS Aur, FL Lyr, and EW Ori, are analyzed. Ephemerides, V and R magnitudes, V-R indices, epochs, periods, e cos omega, photometric solutions, and spectrograms are presented for the systems. The radial velocities are analyzed and the orbital elements are obtained. A preliminary comparison of the systems is made with predictions from evolutionary models. HS Aur now represents a well-analyzed main sequence detached binary with both components having properties lying between those of the sun and the M1 dwarf YY Gem. In the case of FL Lyr, the two components differ significantly in their properties, making it of particular interest for its comparison with interior models and for its potential contribution to the flux and temperature scales. 37 references.

Flares on opposite sides of the star: Observational aspects

The problem of the flare taking place on opposite sides of a star is considered. Such a screened flare, diffused through the star's atmosphere (chromosphere), may also be registered. The theoretical light curve for diffused flare event is derived, which differs strongly from the usual flare light curves. The light curve of diffused flare is characterized first of all by its very slow rise of brightness. This result opens quite a new direction to understand the nature of the so-called ‘slow’ flares, observed often among the UV Cet-type stars as well as flare stars in aggregates. All slow flares can be interpreted as quite ordinary flares of quite ordinary flare stars — taking place, however, on the opposite sides of the star. The results of interpretation of some slow flare events of YY Gem and three flare stars in Orion are presented. An attempt is made for the determination of actual amplitudes of screened flares taking place on the opposite sides of a star.

Microwave emission from the coronae of late-type dwarf stars

VLA microwave observations of 14 late-type dwarf and subgiant stars and binary systems are examined. In this extensive set of observations, four sources at 6 cm (Chi-1 Ori, UV Cet, YY Gem, and Wolf 630AB) were detected and low upper limits for the remaining stars were found. The microwave luminosities of the nondetected F-K dwarfs are as small as 0.01 those of the dMe stars. The detected emission is slowly variable in all cases and is consistent with gyroresonant emission from thermal electrons spiraling in magnetic fields of about 300 gauss if the source sizes are as large as R/R(asterisk) = 3-4. This would correspond to magnetic fields that are probably in the range 0.001-0.0001 gauss at the photospheric level. An alternative mechanism is gyrosynchrotron emission from a relatively small number of electrons with effective temperature.

STELLAR SURFACE PHENOMENA: HALPHA VARIABILITY IN DME STARS.

ABSTRACT Image-tube spectrograms (resolution 1 A) of the Ha emission line in 15 late-type dwarf stars are examined for evidence of equivalent-width variability on a time scale of days, weeks, and years. Scattered data show Ha variations in three known BY Draconis variables: YY Gem, Gl 182, and BD +48" 1958A. A much more extensive series of spectrograms of BY I)ra itself obtained during 1972 and 1976 shows only a marginal correlation between Ha EW and rotational phase. Criteria for detection of such variability are noted. Key words: dMe stars-BY Draconis variables-Ha emission-starspots

The interpretation of cyclical photometric variations in certain dwarf Me-type stars

A method of determination of parameter sets characterizing models of starspots is described. The method makes use of a systematic integral notation in the description of the darkening due to spots and optimization procedures to evaluate appropriate parameters. The method is applied to light curves of YY Gem and CC Eri. In the latter case a comparison is made with the results of Bopp and Evans's (1973) study. The physical meaning of the derived parameter set and possibly correlated effects are considered for YY Gem.

THE BY DRACONIS VARIABLE FF ANDROMEDAE.

ABSTRACT FF And (sp. dM1e), a BY Dra variable and flare star, is shown to be a double-line spectroscopic binary of period 2d,17 The masses of the components are nearly , and both stars show variation in the intensity of the Balmer and Ca ii H and K emission lines, The system is very similar to YY Gem, though no eclipses have been seen. Using reasonable values for the orbital inclination, we formulate a model for the size and location of the active region observed photometrically on FF And in 1967. Differing photometric periods in 1967 and 1971 may indicate the presence of differential rotation in the system. Key words: variable stars - BY Draconis variables - spectroscopic binaries

THE SURFACE DISTRIBUTION OF BALMER EMISSION ON YY GEMINORUM.

ABSTRACT The Ha and Hfi emission lines in the eclipsing flare star YY Gem have been photoelectrically scanned during primary and secondary eclipse. An increase of 25% in the equivalent width of the emission lines is seen near = 0. We interpret this change as due to a plage region on the near side of the secondary (uneclipsed) star. Under some circumstances the disk of one star may occult a plage region on the more distant component. Decreases of emission-line intensity would then be seen, and spectrophotometric observations could determine the size and longitude extent of a plage on YY Gem. Key words: eclipsing binaries - flare stars - BY Dra variable

Starspots on flare stars.

Sizes of starspots on flare stars can be derived from the author's convection-cell hypothesis. The sizes are in fair agreement with those observed on YY Gem, CC Eri, and BY Dra by Bopp and Evans (1973). The hypothesis predicts that periodic brightness variations due to starspots are restricted to stars brighter than a critical absolute visual magnitude. A convective model of a starspot on YY Gem has been computed, assuming that the missing flux is in the form of Alfven waves. It is found that the surface field must exceed 10,000 G, and is probably less than about 30,000 G. With a surface field of 20,000 G, the effective temperature of the spot is in the range from 1590 to 1890 K, depending on the field gradient. These figures are to be compared with an effective temperature of 2000 K estimated from observations by Bopp and Evans. Efficient dynamo action is shown to be a possible mechanism for generating such large surface fields. There is a possibility that tidal effects may influence starspot formation.

Emission-Line Variability in Quiescent Flare Stars

Spectroscopic monitoring of five flare stars (UV Ceti, YZ CMi, BY Dra, YY Gem, and BD + 48 deg 1958A) reveals that the stars exhibit marked decreases in the Balmer and Ca II emission line strengths over intervals of one day or less. The variations can be explained by a nonuniform, patchy distribution of emitting regions on a rotating star. This is consistent with the starspot model from which in some other cases stellar rotation periods were inferred. Equatorial rotational velocities inferred are significantly greater than might be expected for ordinary stars at the same positions in the H-R diagram. (auth)

Analysis of eclipsing binary light curves on automatic computers

A direct method of analysis of the light curves of eclipsing binary systems is proposed, which, as an alternative to numerical integration methods, adopts the associated alpha functions of Kopal as the means of description of eclipses. Alpha functions can be internally generated, corresponding to given trial parameters and orbital phase, in a computer program which then goes on to optimize the trial parameter set. The method has been applied to a number of light curves including those of YZ Cas, WW Aur, YY Gem and SZ Cam. Second order coefficient of limb darkening have been determined for the YZ Cas stars. Finally, a comparative discussion is given of various methods of light curve analysis.

Remarks on Bolometric Corrections and Effective Temperatures.

view Abstract Citations (69) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Remarks on Bolometric Corrections and Effective Temperatures. Popper, Daniel M. Abstract The usually accepted systems of bolometric corrections and effective temperatures, based essentially on Kuiper's discussion of 20 years ago, are examined in the light of current knowledge. Bolometric corrections for stars cooler than type FO can be improved with the use of photoelectric magnitudes and colors. For hotter stars, improved atmospheric models are available. Photoelectric colors rather than spectral types are used as argument for both bolometric corrections and temperatures. Stars redder than about B - V $ 1.4 mag. are excluded from the discussion. A new fundamental determination of an effective temperature is that of Sirius from its angular diameter by Hanbury Brown and Twiss. The value is about 1000 lower than earlier determinadons and is in agreement with the value from a model-atmosphere investigation of Vega by Hunger. Model-atmosphere computations for 0 and early B stars by various authors lead generally to much higher temperatures than Kuiper adopted. For stars cooler than the sun, the reliability of temperatures based upon interferometer measures of angular diameters is questioned. Moreover, the effective temperature of YY Gem is somewhat uncertain because its bolometric correction has not been measured. Thus, except for the sun, the scale of effective temperatures cannot be considered definitively established for any spectral class. Temperature differences between giants and main-sequence stars of tbe same spectral type are found to be smaller than those usually adopted. Observations for improving the temperature scale are discussed briefly. Publication: The Astrophysical Journal Pub Date: May 1959 DOI: 10.1086/146663 Bibcode: 1959ApJ...129..647P full text sources ADS | data products SIMBAD (25) GCPD (1)

Discussion

Following the papers on instability in binary systems, Dr S. Gaposchkin described a classification scheme that he had devised to describe the eclipsing binaries having emission lines, gas streams, or thick atmospheres. Such stars Gaposchkin calls the ‘camouflaged eclipsing variables’, and divides them into five groups: (1)Typical members are UX Mon and SX Cas: the spectral type of the brighter star is B or A, that of the fainter is G or later. The minima are of equal depth. Balmer emission is present.(2)Prototypes are β Lyr and ν Sgr: only one component can be seen distinctly, but two minima are present in the light curve. The spectroscopic behaviour is most complicated.(3)Prototypes are RT And and YY Gem: the spectral class is G or later. Ca 11 emission is present. The depths of the minima are roughly equal, and the dimensions of the components are about the same.(4)The prototype is DN Ori: both components are so well camouflaged that no definite velocity variation has been observed.(5)Prototypes are V444 Cyg or UX UMa: the bright lines are strong, and their displacements and intensities do not vary in a simple manner with phase.

New emission lines in M dwarfs.

view Abstract Citations (3) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS New emission lines in M dwarfs. Wildt, Rupert Abstract A number of ultraviolet spectra of K and M dwarfs were obtained with the Qf/2 camera, dispersion about 65A/mm at X325o, attached to the spectrograph of the 8 -inch McDonald reflector. The exposure times were chosen as long as feasible, in order to record the ultraviolet at wave lengths less than X35oo. A cursory inspection of the first exposure of the star BD 2002465 (dM4e) revealed 3 emission lines below X33oo, where the continuum of the star had already faded out; their existence was confirmed by a second exposure. Bright lines of HI and CaiI had been noticed in this star by Swings and Struve,1 and recently it has become famous as a flare-star.2 No photoelectric magnitudes were recorded at Lick during the nights the spectrograms were taken at McDonald, 1950 February 5, 8:oI U.T. and February 7, 6:56 U.T. Since Swings and Struve had stressed the similarity of the emission spectra of BD 2002465 and YY Gem, this star too was examined for emissions in the far untraviolet. Only one spectrogram with an exposure time of 6 hours is available. It shows the continuum extending to about 3250, making it more difficult to disentangle faint emission lines. Omitting two doubtful ones, 6 emission lines were recognized in BD 2002465 and 4 of these also appear in YY Gem. However, their character is quite different in these two stars. In BD 2002465 the emission lines stand out 556 1/2 5.56 3/2 13/2,2 4.63 5.53 7/53/2 z4D0 4.62 549 z6D0 5/2 4.60 7/2 ~4175 9/2 i5 1.69 Figure. 1/32/ 1.09 1.66 ~3/29 4 2 1.07 a4D 3/71/3 1.04 0.96 Fell emissions in M dwarfs. lines observed in BD +20e2465, Q lines observed in VY Gem (Castor C). sharply against the background fog, and there is no doubt about their nature as "lines"; an exception is the rather diffuse emission at X3228 where there is some vignetting by the spectrograph. In YY Gem the emissions are quite diffuse. In particular, X3256 looks nearly 2A wide; and X3278 appears unsymmetrical, the continuous background being of different density to the red and ultraviolet of this line, as if the emission line coincided with the edge of an absorption band degraded toward th ultraviolet. It is fair to say that the existence of these emissions in YY Gem could hardly have been established from a single spectrogram if it had not been for for their previous discovery in BD 2002465. All the lines were identified with the strongest members of two low-excitation multiplets in the spectrum of Fell; they are entered in the term diagram shown in the figure, which also contains the appropriate excitation potentials. The same two multiplets of Feii, including their fainter members, appear in emission in a Herculis (M ) and a Scorpii (MI), according to G. Herzberg's observations.1 He proved that Feii emission does not originate in the atmospheres of the early-type companions of these stars, but is characteristic of the giants themselves, which also have bright H and K lines. Herzberg also lists 8 unidentified emissions; one of them, at X329o.I5, may be identical with a doubtful emission at X3290.o in BD 2002465. Both a Herculis and a Scorpii have also bright H and K lines. Thus there is suggested remarkable parallelism in emission characteristics between dwarfs and supergiants of spectral type M. Further investiagtion of this point seems 1n- dicated. This investigation was supported by the Office of Naval Research. I.Pub. A.S.P. 53, 244, 1941. 2.K. C. Gordon and G. E. Kron, Pub. A.S.P. 6i, 210, 1949. 3.Ap. J. 107, 94, 1948. Yale University Observatory, New Haven, Conn. Publication: The Astronomical Journal Pub Date: 1951 DOI: 10.1086/106629 Bibcode: 1951AJ.....56S..51W full text sources ADS | data products SIMBAD (4)