V1898 Cygni: An interacting eclipsing binary in the vicinity of North America nebula
aa r X i v : . [ a s t r o - ph . S R ] J un Manuscript for
Revista Mexicana de Astronom´ıa y Astrof´ısica (2007)
V1898 CYGNI : AN INTERACTING ECLIPSINGBINARY IN THE VICINITY OF NORTH AMERICANEBULA. A. Dervi¸soˇglu, ¨O. C¸ akırlı, C. ˙Ibanoˇglu and E. Sipahi Draft version: November 15, 2018
RESUMENABSTRACTWe present spectroscopic observations of the double-lined Algol type eclipsingbinary V1898 Cyg. Analyses of the BV light curves and RVs led to determi-nation of the fundamental stellar parameters of the V1898 Cyg’s components.The absolute parameters for the stars are derived as: M =6.054 ± ⊙ ,M =1.162 ± ⊙ , R =3.526 ± ⊙ , R =2.640 ± ⊙ , T eff =18 000 ± eff =6 200 ±
200 K. The residuals between the observed and com-puted times of mid-eclipses were analysed and a rate of the period change˙
P /P = 6 . × − yr − was obtained and a mass transfer rate of 1 . × − M ⊙ in a year is estimated.We have calculated the distance to the system ofV1898 Cyg as 501 ± Key Words: stars: binaries: close — binaries: eclipsing — binaries: general— binaries: spectroscopic — stars: individual: V1898 Cyg1. INTRODUCTIONV1898 Cyg (HD 200776, BD +45 ◦ m .81, (B-V)=+0 m .01) was discovered to be single-lined spectro-scopic binary by Abt et al. (1972). HD 200776 was included in the list ofbright OB stars to be observed for determination of galactic rotational con-stants and other galactic parameters. Their spectroscopic observations yieldthat HD 200776 is a spectroscopic binary with an orbital period of 2.9258days. They also calculated the preliminary elements and mass function forthe system. McCrosky and Whitney (1982) searched for photometric vari-ations some short-period spectroscopic binaries including HD 200776 givenin the Seventh Catalogue of the Orbital Elements of Spectroscopic Binary Ege University, Science Faculty, Department of Astronomy and Space Sciences, 35100Bornova, ˙Izmir, TURKEY T ¨UB˙ITAK National Observatory, Akdeniz University Campus, 07058 Antalya,TURKEY BASED ON OBSERVATIONS COLLECTED AT CATANIA ASTROPHYSICALOBSERVATORY (ITALY) AND T ¨UB˙ITAK NATIONAL OBSERVATORY (ANTALYA,TURKEY). m .2-0 m .4, in the brightness of the system which were inconsistent withthe orbital period proposed by Abt et al (1972). Photometric observationsin B and V-bandpass made by Halbedel (1985) revealed that HD 200776 isan eclipsing binary with both eclipses are nearly identical, in contrast to thespectroscopic observations. He proposed a new orbital period of 3.0239 days,3 per cent longer than that given by Abt et al (1972). Caton and Smith (here-after CS, 2005) published new light curve and times of mid-eclipses as well asa new orbital period of 1.5131273 days, nearly half that of given by Halbedel(1985). Soon later Dallaporta and Munari (hereafter DM, 2006) presentedcomplete and accurate BV light curves of HD 200776 as well as three timesfor mid-primary eclipse. Fortunately the same comparison star was used inthe photometric observations of Halbedel (1985) and DM. No further spectro-scopic observations were made into this eclipsing-spectroscopic binary afterAbt et al. (1972).The main aims of this study are: (1)to detect some lines of the secondarycomponent;(2)to reveal radial velocities for both components; (3) to solvethe radial velocity curves for the primary and secondary components on thebasis of new observations in order to reveal accurate masses and radii; and(4)to determine the rotational velocities of the components and compare withorbital synchronization.2. SPECTROSCOPIC OBSERVATIONSThe spectra were obtained from several telescopes during the course ofthree years, beginning the year of 2007. Table 1 lists the full set of observa-tions. The first set was observed with the ´echelle spectrograph (FRESCO) atthe 91-cm telescope of Catania Astrophysical Observatory. Spectroscopic ob-servations have been performed with the spectrograph is fed by the telescopethrough an optical fibre ( U V – N IR , 100 µ m core diameter) and is located, ina stable position, in the room below the dome level. Spectra were recorded ona CCD camera equipped with a thinned back–illuminated SITe CCD of 1k × × µ λ / δλ C o r r e l a t i o n Shift (km s -1 ) Fig. 1. Sample of Cross Correlation Functions (CCFs) between V1898 Cyg and theradial velocity template spectrum (Vega) at four different orbital phases.
The electronic bias was removed from all spectra and we used the crre-ject task of IRAF for cosmic ray removal. The ´echelle spectra were extractedand wavelength calibrated by using a Fe-Ar and Th-Ar lamp source with helpof the IRAF echelle package. The stability of the instruments were checked bycross correlating the spectra of the standard star against each other using the fxcor task in IRAF. The standard deviation of the differences between thevelocities measured using fxcor and the velocities in Nidever et al. (2002)was about 1.1 km s − .Twenty-eight spectra of V1898 Cyg were collected during the two differentseasons. Typical exposure times for the V1898 Cyg spectroscopic observationswere between 2400 and 2600 s for Catania telescope and 1200 s for RTT150telescope. The signal-to-noise ratio ( S/N ) achieved was between 70 and 115,and ∼
150 depending on atmospheric condition. α Lyr (A0V), 59 Her (A3IV), ι Psc (F7V), HD 27962 (A2IV), and τ Her (B5IV) were observed during eachrun as radial velocity and/or rotational velocity templates. The average
S/N at continuum in the spectral region of interest was 150–200 for the standardstars. 3. SPECTROSCOPIC ANALYSISDouble-lined spectroscopic binaries reveal two peaks, displacing back andforth, in the cross-correlation function (CCF) between variable and the radialvelocity template spectrum as seen in Fig. 1. The location of the peaks allowsto measure of the radial velocity of each component at the time of observation. IRAF is distributed by the National Optical Observatory, which is operated by theAssociation of the Universities for Research in Astronomy, inc. (AURA) under cooperativeagreement with the National Science Foundation
DERVIS¸O ˇGLU ET AL.
Fig. 2. Radial velocities of the primary (dots) and secondary stars (triangles) arefolded on an orbital period of 1.513126 days. The velocities obtained at the Cataniaobservatory are indicated by filled symbols while those obtained at the NationalObservatory of Turkey by open symbols. The vertical lines show error bars of eachradial velocity. The residuals between the observed and computed RVs are plottedin the lower panel.
HJD
Phase Star 1 Star 2 Remarks2 400 000+ V p σ O-C V s σ O-C54327.55412 0.5489 21.1 10.9 0.0 -85.5 16.9 -3.0 a54328.50480 0.1772 -38.1 3.6 6.9 255.3 11.1 -7.2 a54329.46135 0.8093 55.5 3.9 -0.3 -255.5 11.1 8.0 a54330.47685 0.4805 1.2 11.1 3.5 ... ... ... a54331.44914 0.1231 -38.8 4.2 -4.7 210.1 9.9 4.7 a54335.45358 0.7695 56.7 3.3 -2.5 -285.8 4.3 -4.8 a54336.40281 0.3969 -21.5 5.1 7.3 179.9 14.6 1.9 a54337.46485 0.0988 -21.1 6.6 6.5 ... ... ... a54338.44704 0.7479 55.5 3.1 -4.1 -277.7 6.1 5.5 a54360.40110 0.2571 -44.4 5.1 6.3 288.6 7.2 -3.2 a54361.41090 0.9245 16.6 11.2 -3.0 -121.1 11.6 5.8 a54362.55110 0.6780 55.5 7.7 1.4 -258.6 10.9 -4.3 a54363.50010 0.3052 -47.9 5.9 -0.5 281.1 9.2 6.1 a54364.50527 0.9695 18.0 9.1 3.1 -63.7 18.5 -13.4 a54365.53939 0.6530 51.1 4.5 1.4 -211.1 9.8 20.3 a54366.54137 0.3152 -33.5 4.1 12.6 270.9 10.1 2.6 a55387.53130 0.0777 -11.0 9.0 10.4 143.0 11.0 3.6 b55390.46920 0.0193 13.0 8.0 15.2 77.0 20.0 37.8 b55390.58090 0.0932 -26.0 9.0 0.0 178.0 21.0 14.6 b55391.50380 0.7031 59.0 2.0 1.8 -276.0 8.0 -5.2 b55391.58150 0.7544 67.0 3.0 7.4 -287.0 9.0 -3.9 b55392.41900 0.3079 -43.0 3.0 4.1 265.0 9.0 -8.3 b55393.41980 0.9694 23.0 11.0 8.0 -45.0 9.0 5.5 b55394.57290 0.7314 44.0 3.0 -15.2 -274.0 6.0 7.2 b55396.47080 0.9857 33.0 12.0 23.6 -35.0 19.0 -13.6 b55397.38320 0.5887 25.0 7.0 -8.6 -156.0 11.0 -8.3 b55397.56950 0.7119 41.0 3.0 -17.0 -277.0 8.0 -2.0 b55398.39480 0.2573 -52.0 2.0 -1.4 302.0 8.0 10.2 bRemarks: (a) Based on Catania and (b) on TUG observations.
DERVIS¸O ˇGLU ET AL.The cross-correlation technique applied to digitized spectra is now one of thestandard tools for the measurement of radial velocities in close binary systems.The radial velocities of V1898 Cyg were obtained by cross–correlating of´echelle orders of V1898 Cyg spectra with the spectra of the bright radial veloc-ity standard stars α Lyr (A0V), 59 Her (A3IV) and ι Psc (F7V) (Nordstr¨omet al., 2004). For this purpose the IRAF task fxcor was used.Fig. 1 shows examples of CCFs of V1898 Cyg near the first and secondquadrature. The two non-blended peaks correspond to each component ofV1898 Cyg. We applied the cross-correlation technique to five wavelengthregions with well-defined absorption lines of the primary and secondary com-ponents. These regions include the following lines: Si iii ii i i i =55.2 ± − and K =287.6 ± − for the secondary component with a systemic velocity of 4 . ± − .3.1 . Spectral classification The spectral types of the stars can be found either photometry or spec-troscopy and/or both. The apparent visual magnitude and colours of V1898 Cygwere estimated by Hiltner (1956) as V=7 m .81, B-V=0 m .01, U-B=-0 m .82.However the apparent magnitudes are given by Reed (2003) as 7 m .0, 7 m .80and 7 m .82 in the U, B and V passbands, respectively. On the other hand, TheB-V colour of the system at outside of eclipse was determined as 0 m .01 byHalbedel (1985). We computed the B-V colour of the system at the maximaas 0 m .036 using the data given by Dallaporta and Munari (2006). The com-bined spectral types are given as B1 IVp by Abt et al. (1972) and B2 III byKennedy and Buscombe (1974). The infrared magnitudes of the system aregiven by Cutri et al. (2003) as J=7 m .697, H=7 m .718 and K=7 m .757. Unfor-tunately, intermediate- and narrow-band photometric measurements are notavailable for the system. Bessell et al.(1998) derive reddening-independent Q-parameter from theoretical colours as Q=(U-B)-0.71 (B-V). They also predictinterstellar reddening for the main-sequence OB stars as E(B-V)=(B-V)-((U-B)-0.71(B-V))/3. Using the observed (U-B) and (B-V) colours by Hiltner(1956) we find E(B-V)=0 m .28, while for Dallaporta and Munari (2006) esti-mate the reddening as 0 m .31.1898 CYGNI 7We computed the intrinsic colours of the primary component using theJHK magnitudes as J − H = − m . ± m .
044 and H − K = − m . ± m . E ( J − H ) = 0 m . ± m .
060 and E ( H − K ) = 0 m . ± m . E ( B − V ) = 0 m . m .036 one obtains an intrinsic B-V colour of -0 m .25 whichcorresponds to a B1V star in the calibrations of Papaj et al. (1993).We have used our spectra to determine the spectral type of the primarycomponent of V1898 Cyg. We have followed the procedures of Hern´andez etal. (2004), choosing helium lines in the blue-wavelength region, where the con-tribution of the secondary component to the observed spectrum is almost neg-ligible. From several spectra we measured EW s of HeI λ , , , . ± . , ± . , . ± . , . ± . EW -spectral type diagrams given by Hern´andez et al. (2004).The EW s of the helium lines indicate that the spectral type of the primarycomponent is B . ± . m .21 for a B2V star, 16800 K and-0 m .19 for the same spectral type but for a giant star. Therefore, we esti-mated an effective temperature of 18000 ±
600 K for the primary componentof V1898 Cyg. 3.2 . Reddening
The measurement of reddening is a key step in determining the distanceof stars. V1898 Cyg locates in the direction of the NAP, in which reddeningvaries from one place to other. We estimated the reddening in the B-V colouras 0 m .29 using the infrared colours. On the other hand we find E ( B − V ) =0 m .25 for a star of B2V and E ( B − V ) = 0 m .23 for a star of B2III. Thephotometric and spectroscopic determinations of the interstellar reddeningseem to be in a good agreement within 3 σ error. Our spectra cover theinterstellar Na i (5890 and 5896 ˚A) doublet, which is excellent estimator of thereddening as demonstrated by Munari & Zwitter (1997). They calibrated atight relation linking the Na i D1 (5890 ˚A) equivalent widths with the E(B-V)reddening. On spectra obtained at quadratures, lines from both componentsare un-blended with the interstellar ones, which can therefore be accuratelymeasured. We derive an equivalent width of 0.52 ± i D1 line,which corresponds to E(B-V)= 0 m .33 ± l =87 ◦ .60, b =-0 ◦ .34) and near the edge of North Americanebula (NAN) such a reddening in the optical wavelengths is expected.3.3 . Rotational velocity The width of the cross-correlation profile is a good tool for the measure-ment of v sin i (see, e.g., Queloz et al. 1998). The rotational velocities ( v sin i ) DERVIS¸O ˇGLU ET AL.of the two components were obtained by measuring the FWHM of the CCFpeaks in nine high-S/N spectra of V1898 Cyg acquired close to the quadra-tures, where the spectral lines have the largest Doppler-shifts. In order toconstruct a calibration curve FWHM– v sin i , we have used an average spec-trum of HD 27962, acquired with the same instrumentation. Since the rota-tional velocity of HD 27962 is very low but not zero ( v sin i ≃
11 km s − , e.g.,Royer et al. (2002) and references therein), it could be considered as a usefultemplate for A-type stars rotating faster than v sin i ≃
10 km s − . The spec-trum of HD 27962 was synthetically broadened by convolution with rotationalprofiles of increasing v sin i in steps of 5 km s − and the cross-correlation withthe original one was performed at each step. The FWHM of the CCF peakwas measured and the FWHM- v sin i calibration was established. The v sin i values of the two components of V1898 Cyg were derived from the FWHM oftheir CCF peak and the aforementioned calibration relations, for a few wave-length regions and for the best spectra. This gave values of 110 ± − forthe primary star and 90 ± − for the secondary star.4. TIMES OF MINIMA AND THE ORBITAL PERIODTimes of mid-eclipses were published by Halbedal(1985), CS, DM and Br´atet al. (2008). These times of eclipses were presented in Table 2. The O-C(I)residuals are computed using the light elements given below, M inI ( HJD ) = 2 450 690 . d . × E (1)where the orbital period is adopted from DM. The behaviour of the deviationsfrom the linear light elements O-C(II) with respect to the epoch numberssuggests an upward curved parabola. Therefore, a parabolic fit to the datagives, M inI ( HJD ) = 2 450 690 . d . × E + 1 . − × E . (2)The standard mean errors in the last digits are given in the parenthe-ses. The coefficient of the quadratic term is positive which indicates that theorbital period of the system is increasing with the epoch number. Such aquadratic ephemeris appears to a very good representation of the orbital pe-riod change of V1898 Cyg, as well as other interacting Algols. This quadraticbehaviour of the O-C(II) residuals, plotted in the upper panel of Fig.3, is anindication of the secular period increase for the system. In the bottom panel ofFig.3 the O-C(III) residuals with respect to ephemeris (2) are plotted, whichillustrates a good agreement between the timings and new ephemeris. It isknown that the classical Algols have an evolved less massive component whichfills its Roche lobe and transfers its mass to the more massive primary starthrough Lagrangian L point of the system. The orbital period of the systemis increasing at the average rate of ˙ P /P = 6 . ± . × − yr − whichmeans that the orbital period was increased by about 0.38( ± (1) Halbedel (1985), (2) Dallaporta and Munari (2006), (3) Caton and Smith(2005), (4) Br´at et al. (2008) Fig. 3. The O-C(II) residuals plotted versus the epoch number for V1898 Cyg . Aleast-squares quadratic fit to the residuals was shown by dashed line (upper panel).In the bottom panel the O-C(III)residuals, the deviations from quadratic fit, arealso plotted. in the last 24 years. The sum of squares of residuals for the parabolic fit is3 . × − d . We limited times of mid-eclipses covering about 24 yrs. Thetime span of the observations is very short to reveal any abrubt period changecaused by the fast mass transfer phenomenon. Of course the observationswill be obtained in the coming years indicate some hints about the nature oforbital period change. Assuming a conservative mass transfer from the lessmassive component to the more massive primary star we estimate an amountof 1 . ± . × − M ⊙ in a year.5. ANALYSIS OF THE LIGHT CURVESThree light curves V1898 Cyg were obtained and the observational datawere published. The first photometric observations obtained by Halbedel(1985) between July and November, 1985. The second photometric observa-tions made by CS from August 22, 2001 to October 26, 2004. The star wasobserved by DM from July 22, 2003 to September 17, 2004. Only the V-bandlight curve and observational data of CS were published. However,the B andV light curves of DM were published. The V light curves obtained in twostudies are asymmetric in shape, furthermore differ from each other. The1898 CYGNI 11 Fig. 4. Comparison of the observed and computed light curves of V1898 Cyg. Fromtop to bottom the CS-V, DM-B and DM-V light curves, respectively. In the lowerpanel residuals of the fit have been plotted to show the goodness of the fit. m .03 in theCS light curve than that in DM’s. However, CS report that the brightnessof their primary comparison star showed light variations during the observa-tions. Moreover the depth of the secondary eclipse is larger by about 0 m .007in the light curve of CS than that of DM. We should note that there is alsoa slight asymmetry in both the DM’s B- and V-passband light curves. Thisfeature is attributed to the transferring material from cool secondary to thehigh temperature primary star which occults a small area of the primary starjust before the deeper eclipse.Acerbi and Barani (2007) analysed the DM’s light curves. Since the spec-troscopic mass-ratio was not available at that time they started to the analysisby deriving the photometric mass-ratio.Their preliminary analysis indicatedthat the mass-ratio for the system was about 0.30. Assuming an effectivetemperature of 20 183 K for the primary component and the secondary lessmassive star fills its corresponding Roche-lobe, i.e. semi-detached configura-tion, they arrived at a preliminary elements for the system. Their orbitalparameters were: i = 70 ◦ , r = 0 . r =0.2795 and T =7 500 K. Sincethe light curves were asymmetric they reported that the agreement betweenthe computed and observed light curves were not very satisfactory, the sumof squares of residuals was about 0.814.In order to analyse the light curves we choose the Wilson–Devinney (W–D) code implemented into the PHOEBE package tool by Prsa & Zwitter(2005). Preliminary analysis indicates that the system is a classical Algol, thesecondary component fills its corresponding Roche lobe. Therefore Mode–5 isapplied. WD code is based on Roche geometry which is sensitive to the massratio which is taken from RV analysis as 0.192 ± g = 1, g = 0.32 and bolometric albedos Alb =1, Alb =0.5 wereset, i.e. the more massive star has a radiative envelope while the less massivesecondary convective atmosphere. We used the non-linear square-root limb–darkening and the bolometric limb-darkening coefficients from the tables Diaz-Cordoves, Claret, and Gimenez (1995) and van Hamme(1993).The orbital inclination ( i ), effective temperature of the secondary star(T ), surface potential of the primary (Ω ), phase shift (∆ φ ), and fractionalluminosity of the primary ( L ) were taken as adjustable parameters. Theother parameters were fixed. The iterations were carried out automaticallyuntil convergence and a solution was defined as the set of parameters for whichthe differential corrections were smaller than the probable errors. The finalresults obtained by separate analysis of three light curves are listed in Table3 and the computed light curves are shown as continuous lines in Fig. 4. Theuncertainties assigned to the adjusted parameters are the internal errors pro-1898 CYGNI 13TABLE 3RESULTS OF INDIVIDUAL LIGHT CURVE ANALYSES FORV1898 CYG.Parameter CS V DM B DM V i ( ◦ ) 74.20 ± ± ± T (K) 18 000[ Fix ] T (K) 6582 ±
65 6205 ±
76 6109 ± ± ± ± Fix ] q spec Fix ] L / ( L ) 0.9563 ± ± ± r ± ± ± r ± ± ± φ ± ± ± res ) r , r : Relative volume radii, CS V: Caton and Smith’s (2005) V-band lightcurve, DM B and DM V: The B- and V-band light curves of Dallaporta andMunari (2006). The errors quoted for the adjustable parameters are the formalerrors determined by the WD-code. vided directly by the WD code. As seen in Table 3 the sum of residuals-squaresare 0.0109 and 0.0123 for the B- and V-bandpass light curves, indicating 75-times smaller than the analysis made by Acerbi and Barani (2007) of thesame data. In the bottom panel of Fig.4 the residuals between observed andcomputed intensities are also plotted. The residuals reveal that the binarymodel may represent the observed DM’s light curves successfully. However,the computed light curve differs mostly from fourth contact to beginning ofsecondary eclipse in the CS light curve.6. DISCUSSION AND CONCLUSIONSince the sum-of-squares in the analysis of CS light curve is too largewhen compared to the DM’s light curves, and there is a doubt on the lightconstancy of their primary comparison we take weighted mean orbital pa-rameters obtained by the analysis of the DM’s B and V light curves.Themean parameters obtained from the light curve analysis are: i = 73 ◦ .05 ± r =0.3291 ± r =0.2464 ± =6 200 ±
200 K. The fractional ra-dius of the secondary component exceeds its corresponding Roche lobe radiusby about 4%. Combining the results obtained from RVs analysis we havederived the astrophysical parameters of the components and other proper-ties listed in Table 4. The mass and radius of the massive primary star are4 DERVIS¸O ˇGLU ET AL.TABLE 4FUNDAMENTAL PARAMETERS OF V1898 CYG.Parameter Primary SecondaryMass (M ⊙ ) 6.054 ± ± ⊙ ) 3.526 ± ± T eff (K) 18 000 ±
600 6 200 ± L/L ⊙ ) 3.071 ± ± g ( cgs ) 4.125 ± ± ± ± a (R ⊙ ) 10.714 ± i ( ◦ ) 73.05 ± d (pc) 501 ± vsin i ) obs (km s − ) 110 ± ± vsin i ) calc. (km s − ) 112.8 ± ± J , H , K s (mag) ∗ ± ± ± µ α cosδ , µ δ (mas yr − ) ∗∗ ± ± All-Sky Point Source Catalogue (Cutri et al. 2003)**Newly Reduced Hipparcos Catalogue (van Leeuwen 2007)1898 CYGNI 15derived with an accuracy of 0.6 and 1.4 per cent while for the less massivedonor star 0.4 and 0.5 per cent. The observed and computed rotational ve-locities of the components are in good agreement, showing nearly synchronizerotation. In Fig. 5, we plot the location of V1898 Cyg stellar components inlog T eff − log L/L ⊙ diagram. The evolutionary tracks for masses 6 and1.16 M ⊙ are also shown in this figure. For constructing the solar metallic-ity evolutionary tracks, we used the Cambridge version of the stars code which was originally developed by Eggleton (1971) and substantially updatedby Eldridge & Tout (2004). The continuous and dashed lines from left toright corner show the zero-age and terminal-age main-sequences, respectively.While the primary star locates very close to the ZAMS the secondary, lessmassive star appears to be evolved up to the giant branch as is common insemi-detached Algol-type binaries. This appearance of the components inthe HR diagram is common for the classical Algol-type binaries. The donorseems to higher temperature and luminosity, in which most of the atmosphericmaterial has been transferred to its companion. Since the secondary compo-nent fills its Roche lobe it transfers its mass to the more massive component.Therefore the orbital period change is attributed to the mass transfer. How-ever, the gainer is ascended up to higher effective temperature and luminosity.In addition, V1898 Cyg locates in the diagram between the specific angularmomentum and mass-ratio where angular momentum decreases faster (seeIbanoglu et al. 2006). The average distance to the system was calculatedto be 501 ± +443 − pc from the trigonometric parallax measured by the Hipparcosmission. The distance derived in this study is smaller by about 20 per cent,the most importantly it has very small uncertainty when compared with thatmeasured by Hipparcos mission.The North America (NGC 7000) and Pelican (IC 5070) nebulae (NAP) areknown as the most nearby huge, extended HII regions where star formationwith intermediate mass is still ongoing. The distance to this extended star-forming region has been estimated from 200 to 2000 pc (see for example,Bally and Reipurth, 2003). While Herbig (1958) estimates a distance of 500pc, Laugalys and Straiˇzys (2002) derived a distance to NAP 600 ±
50 pc. Ifthe star is a member of NAP complex the distance to the star derived by usis in a better agreement with that of Herbig, but agrees with that proposedby Laugalys and Straiˇzys (2002) within 2 σ level. Recently, Straiˇzys, Corballyand Laugalys (2008) listed OB stars in the vicinity of NAP, one of which isV1898 Cyg. The distance to the star estimated by us appears to confirm themembership of NAN. Proper motions for V1898 Cyg are given in the SIMBADdatabase as µ α cosδ =2.07 ± − and µ δ =0.81 ± − with aspace velocity of 45.3 km s − . We selected about 20 stars in the vicinity ofV1898 Cyg listed by Laugalys and Straiˇzys (2002) for comparison their meanproper motions with the variable. The velocities of the stars vary from 45 to −
97 kms − , and the mean proper motions are: µ α cosδ = 0 . µ δ = − . − . It seems that one can not definitely classify which stars are actually6 DERVIS¸O ˇGLU ET AL. Fig. 5. Location of the two stellar components of V1898 Cyg in log T eff - log L diagram, together with evolutionary models for the masses of 1.15 and 6.0 M ⊙ . Theopen circle corresponds to the primary and the open square to the secondary witherror bars.The zero-age main-sequence(continuous line) and terminal-age MS (long-dashed-dotted line)are also plotted. The evolutionary tracks are shown by dottedlines. belong to the NAP and which are not.AcknowledgmentsWe thank Prof. G. Strazzulla, director of the Catania Astrophysical Ob-servatory, and Dr. G. Leto, responsible for the M. G. Fracastoro observingstation for their warm hospitality and allowance of telescope time for the ob-servations. We also thank to T ¨UB˙ITAK National Observatory (TUG) fora partial support in using RTT150 with project numbers 10ARTT150-483-0and 09BRTT150-468-0. This research has been also supported by T ¨UB˙ITAKunder project number 109T708 and INAF and Italian MIUR. This researchhas been made use of the ADS and CDS databases, operated at the CDS,Strasbourg, France and ULAKB˙IM S¨ureli Yayınlar Kataloˇgu. The authorsthank to the anonymous referee for his/her valuable comments.REFERENCES Abt H. A., Levy S. G., Gandet T. L., 1972, AJ, 77, 138Acerbi F., Barani C., 2007, OEJV, 77, 1Bally J., Reipurth B., 2003, AJ, 126, 893Batten A. H., Fletcher J. M., Mann P. J., 1978, PDAO, 15, 121Bessell M. S., Castelli F., Plez B., 1998, A&A, 337, 321Br´at L., et al., 2008, OEJV, 94, 1
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Ege University, Science Faculty, Astronomy and Space Sciences Dept., 35100Bornova, ˙Izmir, Turkey. e − mailmail