Bright Variable Stars in NGC 6819 - An Open Cluster in the Kepler Field (original) (raw)
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Kepler observations of the open cluster NGC 6819
Using photometry from the Kepler Input Catalogue, we determine the distance and age of NGC 6819 using several different methods. From isochrone fitting we find the age of the cluster to be about 2.5 Gyr. There are many different types of variables in the field, including eight δ Scuti stars, at least three γ Dor variables and several eclipsing binaries of various types. We derive masses, radii and luminosities from 63 red giants with solar-like oscillations, including 28 stars with no previous measurements, from which we obtain an unreddened distance modulus of 12.20 ± 0.06 mag. We measured rotation periods of 129 stars which showed variations attributable to starspots. There is a correlation between the rotation period and the colour which extends to stars with temperatures equivalent to late A-type. This implies that starspots are present in stars which are much hotter than previously supposed. We also discuss several eclipsing binaries and the variability of stars which are blue straggler candidates and discuss X-ray sources found in the cluster.
Variable stars in the field of open cluster NGC 6819
Monthly Notices of the Royal Astronomical Society, 2002
We report on the discovery of 141 further variable stars found in the field of the open cluster NGC 6819. The stars were identified from time-series photometric data obtained on the Isaac Newton Telescope, La Palma, during two observing runs covering the 19 nights between 1999 June 22-30 and 1999 July 22-31. The variables found include 53 eclipsing binaries, of which eight stars appear to be RS CVns, in addition to 70 stars showing spot activity, 13 showing long-period variability and five variables of other types.
Photometric study and detection of variable stars in the open clusters - I. NGC 6866
Monthly Notices of the Royal Astronomical Society, 2012
We present results of a variability search in the intermediate-age open cluster NGC 6866 from 29 nights over two observing seasons. We find 28 periodic variables, of which 19 are newly identified. The periods of these variables, which have V magnitudes from 11.5 to 19.3 mag, range from ∼ 48 min to 37 d. We detected several δ-Scuti stars, some of which are of high amplitude, as well as γ-Doradus, rotational variables and eclipsing binaries. In order to study the physical properties of the cluster, we obtained U BV RI photometry of all the stars on a good photometric night. The radial distribution of the stellar surface density shows that the cluster has a radial extent of about 7 arcmin (∼ 3 pc) with a peak density of 5.7 ± 0.7 stars/arcmin 2 at the cluster center. The colour-colour diagram indicates a reddening of E(B − V ) = 0.10 mag towards NGC 6866. A distance of ∼1.47 kpc and an age of ∼ 630 Myr is estimated from the colour-magnitude diagram using the theoretical isochrones of solar metallicity.
The Astronomical Journal, 2016
As part of our study of the old (∼2.5 Gyr) open cluster NGC 6819 in the Kepler field, we present photometric (Kepler and ground-based BV R C I C) and spectroscopic observations of the detached eclipsing binary WOCS 24009 (Auner 665; KIC 5023948) with a short orbital period of 3.6 days. WOCS 24009 is a triple-lined system, and we verify that the brightest star is physically orbiting the eclipsing binary using radial velocities and eclipse timing variations. The eclipsing binary components have masses M B = 1.090±0.010M and M C = 1.075±0.013M , and radii R B = 1.095±0.007R and R C = 1.057 ± 0.008R. The bright non-eclipsing star resides at the cluster turnoff, and ultimately its mass will directly constrain the turnoff mass: our preliminary determination is M A = 1.251 ± 0.057M. A careful examination of the light curves indicates that the fainter star in the eclipsing binary undergoes a very brief period of total eclipse, which enables us to precisely decompose the light of the three stars and place them in the color-magnitude diagram. We also present improved analysis of two previously discussed detached eclipsing stars in NGC 6819 (WOCS 40007 and WOCS 23009) en route to a combined determination of the cluster's distance modulus (m − M) V = 12.38 ± 0.04. Because this paper significantly increases the number of measured stars in the cluster, we can better constrain the age of the color-magnitude diagram to be 2.21±0.10± 0.20 Gyr. Additionally, using all measured eclipsing binary star masses and radii, we constrain the age to 2.38 ± 0.05 ± 0.22 Gyr. The quoted uncertainties are estimates of measurement and systematic uncertainties (due to model physics differences and metal content), respectively.
Variable Blue Straggler Stars in Open Cluster NGC 6819 Observed in the Kepler 'Superstamp' Field
arXiv (Cornell University), 2023
NGC 6819 is an open cluster of age 2.4 Gyr that was in the NASA Kepler spacecraft field of view from 2009 to 2013. The central part of the cluster was observed in a 200 x 200 pixel 'superstamp' during these four years in 30-minute cadence photometry, providing a unique long time-series high-precision data set. The cluster contains 'blue straggler' stars, i.e., stars on the main sequence above the cluster turnoff that should have left the main sequence to become red giants. We present light curves and pulsation frequency analyses derived from custom photometric reductions for five confirmed cluster members-four blue stragglers and one star near the main-sequence turnoff. Two of these stars show a rich spectrum of δ Scuti pulsation modes, with 236 and 124 significant frequencies identified, respectively, while two stars show mainly low-frequency modes, characteristic of γ Doradus variable stars. The fifth star, a known active x-ray binary, shows only several harmonics of two main frequencies. For the two δ Scuti stars, we use a frequency separation-mean-density relation to estimate mean density, and then use this value along with effective temperature to derive stellar mass and radius. For the two stars showing low frequencies, we searched for period-spacing sequences that may be representative of gravity-mode or Rossby-mode sequences, but found no clear sequences. The common age for the cluster members, considered along with the frequencies, will provide valuable constraints for asteroseismic analyses, and may shed light on the origin of the blue stragglers.
The Astrophysical Journal, 2013
We present the discovery of the totally eclipsing long-period (P = 771.8 d) binary system WOCS 23009 in the old open cluster NGC 6819 that contains both an evolved star near central hydrogen exhaustion and a low-mass (0.45M ⊙) star. This system was previously known to be a single-lined spectroscopic binary, but the discovery of an eclipse near apastron using data from the Kepler space telescope makes it clear that the system has an inclination that is very close to 90 •. Although the secondary star has not been identified in spectra, the mass of the primary star can be constrained using other eclipsing binaries in the cluster. The combination of total eclipses and a mass constraint for the primary star allows us
Binarity and Variable Stars in the Open Cluster NGC 2126
2018
We present the results of an analysis of photometric time-series observations for NGC 2126 acquired at the Thai National Observatory (TNO) in Thailand and the Mount Lemmon Optical Astronomy Observatory (LOAO) in USA during the years 2004, 2013 and 2015. The main purpose is to search for new variable stars and to study the light curves of binary systems as well as the oscillation spectra of pulsating stars. NGC 2126 is an intermediate-age open cluster which has a population of stars inside the δ Scuti instability strip. Several variable stars are reported including three eclipsing binary stars, one of which is an eclipsing binary star with a pulsating component (V551 Aur). The Wilson-Devinney technique was used to analyze its light curves and to determine a new set of the system’s parameters. A frequency analysis of the eclipse-subtracted light curve was also performed. Eclipsing binaries which are members of open clusters are capable of delivering strong constraints on the cluster’s...
A census of variability in globular cluster M 68 (NGC 4590)
Astronomy & Astrophysics, 2015
Aims. We analyse 20 nights of CCD observations in the V and I bands of the globular cluster M 68 (NGC 4590) and use them to detect variable objects. We also obtained electron-multiplying CCD (EMCCD) observations for this cluster in order to explore its core with unprecedented spatial resolution from the ground. Methods. We reduced our data using difference image analysis to achieve the best possible photometry in the crowded field of the cluster. In doing so, we show that when dealing with identical networked telescopes, a reference image from any telescope may be used to reduce data from any other telescope, which facilitates the analysis significantly. We then used our light curves to estimate the properties of the RR Lyrae (RRL) stars in M 68 through Fourier decomposition and empirical relations. The variable star properties then allowed us to derive the cluster's metallicity and distance. Results. M 68 had 45 previously confirmed variables, including 42 RRL and 2 SX Phoenicis (SX Phe) stars. In this paper we determine new periods and search for new variables, especially in the core of the cluster where our method performs particularly well. We detect 4 additional SX Phe stars and confirm the variability of another star, bringing the total number of confirmed variable stars in this cluster to 50. We also used archival data stretching back to 1951 to derive period changes for some of the single-mode RRL stars, and analyse the significant number of double-mode RRL stars in M 68. Furthermore, we find evidence for double-mode pulsation in one of the SX Phe stars in this cluster. Using the different classes of variables, we derived values for the metallicity of the cluster of [Fe/H] = −2.07 ± 0.06 on the ZW scale, or −2.20 ± 0.10 on the UVES scale, and found true distance moduli μ 0 = 15.00 ± 0.11 mag (using RR0 stars), 15.00 ± 0.05 mag (using RR1 stars), 14.97 ± 0.11 mag (using SX Phe stars), and 15.00 ± 0.07 mag (using the M V −[Fe/H] relation for RRL stars), corresponding to physical distances of 10.00 ± 0.49, 9.99 ± 0.21, 9.84 ± 0.50, and 10.00 ± 0.30 kpc, respectively. Thanks to the first use of difference image analysis on time-series observations of M 68, we are now confident that we have a complete census of the RRL stars in this cluster.
Variable stars in the open cluster NGC 6738 and its surrounding field
2012
New long-term CCD observations of a eld including the open cluster NGC 6738 done using a robotic telescope at Baja Observatory bring new photometric data for known variables in the eld of studied open cluster as well as unveiling of 14 new variable stars, which classication and parameters of light changes are given.