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تسجيل مستخدم جديدThe Enigma of the Open Cluster M29 (NGC 6913) Solved
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Determining the distance to the open cluster M29 (NGC 6913) has proven difficult, with distances determined by various authors differing by a factor of two or more. To solve this problem, we have initiated a new photometric investigation of the cluster in the Vilnius seven-color photometric system supplementing it with available data in the BV and JHK_s photometric systems, and spectra of the nine brightest stars of spectral classes O and B. Photometric spectral classes and luminosities of 260 stars in a 15 arcmin x 15 arcmin area down to V = 19 mag are used to investigate the interstellar extinction run with distance and to estimate the distance of the Great Cygnus Rift, 800 pc. The interstellar reddening law in the optical and near-infrared regions is found to be close to normal, with the ratio of extinction to color excess R(BV) = 2.87. The extinction A(V) of cluster members is between 2.5 to 3.8 mag, with a mean value of 2.97 mag or E(B-V) = 1.03. The average distance of eight stars of spectral types O9-B2 is 1.54 +- 0.15 kpc. Two stars from the seven brightest ones are field stars: HDE 229238 is a background B0.5 supergiant and HD 194378 is a foreground F star. In the intrinsic color-magnitude diagram, seven fainter stars of spectral classes B3-B8 are identified as possible members of the cluster. The 15 selected members of the cluster of spectral classes O9--B8 plotted on the log L/L(sun) vs. log T(eff) diagram, together with the isochrones from the Padova database, give the age of the cluster as 5 +- 1 Myr.
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Between 1996 and 2003 we have obtained 226 high resolution spectra of 16 stars in the field of the young open cluster NGC 6913, to the aim of constraining its main properties and study its internal kinematics. Twelve of the program stars turned out t
o be members, one of them probably unbound. Nine are binaries (one eclipsing and another double lined) and for seven of them the observations allowed to derive the orbital elements. All but two of the nine discovered binaries are cluster members. In spite of the young age (a few Myr), the cluster already shows signs that could be interpreted as evidence of dynamical relaxation and mass segregation. However, they may be also the result of an unconventional formation scenario. The dynamical (virial) mass as estimated from the radial velocity dispersion is larger than the cluster luminous mass, which may be explained by a combination of the optically thick interstellar cloud that occults part of the cluster, the unbound state or undetected very wide binary orbit of some of the members that inflate the velocity dispersion and a high inclination for the axis of a possible cluster angular momentum. All discovered binaries are hard enough to survive average close encounters within the cluster and do not yet show sign of relaxation of the orbital elements to values typical of field binaries.
The NASA space telescope Kepler has provided unprecedented time-series observations which have revolutionised the field of asteroseismology, i.e. the use of stellar oscillations to probe the interior of stars. The Kepler-data include observations of
stars in open clusters, which are particularly interesting for asteroseismology. One of the clusters observed with Kepler is NGC 6811, which is the target of the present paper. However, apart from high-precision time-series observations, sounding the interiors of stars in open clusters by means of asteroseismology also requires accurate and precise atmospheric parameters as well as cluster membership indicators for the individual stars. We use medium-resolution (R~25,000) spectroscopic observations, and three independent analysis methods, to derive effective temperatures, surface gravities, metallicities, projected rotational velocities and radial velocities, for 15 stars in the field of the open cluster NGC 6811. We discover two double-lined and three single-lined spectroscopic binaries. Eight stars are classified as either certain or very probable cluster members, and three stars are classified as non-members. For four stars, cluster membership could not been assessed. Five of the observed stars are G-type giants which are located in the colour-magnitude diagram in the region of the red clump of the cluster. Two of these stars are surely identified as red clump stars for the first time. For those five stars, we provide chemical abundances of 31 elements. The mean radial-velocity of NGC 6811 is found to be +6.68$pm$0.08 km s$^{-1}$ and the mean metallicity and overall abundance pattern are shown to be very close to solar with an exception of Ba which we find to be overabundant.
We present results of a search for variable stars in the intermediate-age open cluster NGC 7044. We found 23 variable stars in the observed field. One star turned out to be of the delta Sct type with two pulsational modes excited. From the position i
n the color-magnitude diagram we conclude that this star is a member of the cluster. Moreover, we found 13 eclipsing systems, of which five are W UMa stars, one is a beta Lyr variable, six are beta Per binaries showing detached configuration, and the last one is another probable beta Per system. Using the period-luminosity-color relation for W UMa stars we established the membership of the contact binaries, finding four of them to be very probable cluster members. We estimated from these four stars an apparent distance modulus (m-M)_V of NGC 7044 to be 14.2 +/- 0.4 mag, which is smaller than previous determinations of this parameter. We were able to derive orbital period for only four beta Per systems. For the remaining ones we observed only two or three eclipses. Finally, nine stars we found to show irregular light changes. Most of them are red stars not belonging to the cluster. For the cluster core we determined a reddening map, which allowed us to construct a dereddened color-magnitude diagram of NGC 7044 with a narrow main-sequence. By fitting a theoretical isochrone to this diagram we derived E(V-I_C) = 0.92 mag, (m-M)_V = 14.45 mag and log(age/yr) = 9.2.
We report the results of a search for variable stars in the open cluster NGC 2141. Ten variable stars are detected, among which nine are new variable stars and they are classified as three short period W UMa type eclipsing binaries, two EA type eclip
sing binaries, one EB type eclipsing binary, one very short period RS CVn type eclipsing binary, one d type RR Lyrae variable star, and one unknown type variable star. The membership and physical properties are discussed, based on their light curves, positions in the CMDs, spatial locations and periods. A known EB type eclipsing binary is also identified as a blue struggler candidate of the cluster. Furthermore, we find that all eclipsing contact binaries have prominently asymmetric eclipses and O Connell effect (O Connell 1951) which increases with the decrease of the orbital periods. This suggests that the O Connell effect is probably related to the evolution of the orbital period in short period eclipsing binary systems.
High-dispersion spectra of 333 stars in the open cluster NGC 6819, obtained using the HYDRA spectrograph on the WIYN 3.5m telescope, have been analyzed to determine the abundances of iron and other metals from lines in the 400 A region surrounding th
e Li 6708 A line. Our spectra, with signal-to-noise per pixel (SNR) ranging from 60 to 300, span the luminosity range from the tip of the red giant branch to a point two magnitudes below the top of the cluster turnoff. We derive radial and rotational velocities for all stars, as well as [Fe/H] based on 17 iron lines, [Ca/H], [Si/H], and [Ni/H] in the 247 most probable, single members of the cluster. Input temperature estimates for model atmosphere analysis are provided by (B-V) colors merged from several sources, with individual reddening corrections applied to each star relative to a cluster mean of E(B-V) = 0.16. Extensive use is made of ROBOSPECT, an automatic equivalent width measurement program; its effectiveness on large spectroscopic samples is discussed. From the sample of likely single members, [Fe/H] = -0.03 +/- 0.06, where the error describes the median absolute deviation about the sample median value, leading to an internal precision for the cluster below 0.01 dex. The final uncertainty in the cluster abundance is therefore dominated by external systematics due to the temperature scale, surface gravity, and microturbulent velocity, leading to [Fe/H] = -0.02 +/- 0.02 for a sub-sample restricted to main sequence and turnoff stars. This result is consistent with our recent intermediate-band photometric determination of a slightly subsolar abundance for this cluster. [Ca/Fe], [Si/Fe], and [Ni/Fe] are determined to be solar within the uncertainties. NGC 6819 has an abundance distribution typical of solar metallicity thin disk stars in the solar neighborhood.
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