Kenneth A. Janes

Metallicities of Old Open Clusters

We present radial velocities and metallicities for a sample of 39 open clusters with ages greater than about 700 million years. For 24 clusters new moderate-resolution spectroscopic data obtained with multiobject spectrographs on the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory 4 m telescopes are used to determine radial velocities and mean cluster metallicities. These new results are combined with data published previously by Friel & Janes to provide a sample of 459 giants in 39 old open clusters, which are used to investigate radial abundance gradients in the Galactic disk. Based on an updated abundance calibration of spectroscopic indices measuring Fe and Fe-peak element blends, this larger sample yields an abundance gradient of -0.06 ± 0.01 dex kpc-1 over a range in Galactocentric radius of 7 to 16 kpc. There is a slight suggestion of a steepening of the abundance gradient with increasing cluster age in this sample, but the significance of the result is limited by the restricted distance range for the youngest clusters. The clusters show no correlation of metallicity with age in the solar neighborhood. Consistent with the evidence for a steepening of the gradient with age, the clusters in the outer disk beyond 10 kpc show a suggestion at the 1.5 σ level of a dependence of metallicity on age.

CCD Photometry of the Old Open Cluster M67

We present a CCD photometric survey of the central one-half degree of the old open cluster, M67, in U, B, V, and I colors to magnitude V=20. Extensive comparison of our photometry with other published datasets shows excellent agreement, indicating that CCD photometry is capable of producing a uniform set of measurements consistent with the photometric system defined primarily by the Landolt standard sequence. The color-magnitude diagram of the cluster shows a well-defined main sequence extending at least to the limit of the photometry at M V =10.55 and a substantial binary sequence. At least 38% of cluster stars are binaries. The current generation of theoretical isochrones cannot be fit to the observed sequences within the observational errors

A TRANSITING PLANET OF A SUN-LIKE STAR

A planet transits an 11th magnitude, G1 V star in the constellation Corona Borealis. We designate the planet XO-1b and the star XO-1, also known as GSC 02041-01657. XO-1 lacks a trigonometric distance; we estimate it to be 200 ± 20 pc. Of the 10 stars currently known to host extrasolar transiting planets, the star XO-1 is the most similar to the Sun in its physical characteristics: its radius is 1.0 ± 0.08 R☉, its mass is 1.0 ± 0.03 M☉, its V sin i < 3 km s-1, and its metallicity [Fe/H] is 0.015 ± 0.04. The orbital period of the planet XO-1b is 3.941534 ± 0.000027 days, one of the longer ones known. The planetary mass is 0.90 ± 0.07MJ, which is marginally larger than that of other transiting planets with periods between 3 and 4 days. Both the planetary radius and the inclination are functions of the spectroscopically determined stellar radius. If the stellar radius is 1.0 ± 0.08 R☉, then the planetary radius is 1.30 ± 0.11RJ and the inclination of the orbit is 877 ± 12. We have demonstrated a productive international collaboration between professional and amateur astronomers that was important to distinguishing this planet from many other similar candidates.

XO-3b: A Massive Planet in an Eccentric Orbit Transiting an F5 V Star

We report the discovery of a massive planet (Mpsin i = 13.02 ± 0.64 MJ; total mass = 13.25 ± 0.64 MJ), large (1.95 ± 0.16 RJ) planet in a transiting, eccentric orbit (e = 0.260 ± 0.017) around a 10th magnitude F5 V star in the constellation Camelopardalis. We designate the planet XO-3b and the star XO-3, also known as GSC 03727–01064. The orbital period of XO-3b is 3.1915426 ± 0.00014 days. XO-3 lacks a trigonometric parallax; we estimate its distance to be 260 ± 23 pc. The radius of XO-3 is 2.13 ± 0.21 R☉, its mass is 1.41 ± 0.08 M☉, its vsin i = 18.54 ± 0.17 km s−1, and its metallicity is [ Fe/H ] = − 0.177 ± 0.027. This system is unusual for a number of reasons. XO-3b is one of the most massive planets discovered around any star for which the orbital period is less than 10 days. The mass is near the deuterium-burning limit of 13 MJ, which is a proposed boundary between planets and brown dwarfs. Although Burrows et al. propose that formation in a disk or formation in the interstellar medium in a manner similar to stars is a more logical way to differentiate planets and brown dwarfs, our current observations are not adequate to address this distinction. XO-3b is also unusual in that its eccentricity is large given its relatively short orbital period. Both the planetary radius and the inclination are functions of the spectroscopically determined stellar radius. Analysis of the transit light curve of XO-3b suggests that the spectroscopically derived parameters may be overestimated. Though relatively noisy, the light curves favor a smaller radius in order to better match the steepness of the ingress and egress. The light curve fits imply a planetary radius of 1.25 ± 0.15 RJ, which would correspond to a mass of 12.03 ± 0.46 MJ. A precise trigonometric parallax measurement or a very accurate light curve is needed to resolve the uncertainty in the planetary mass and radius.

The Kepler Cluster Study: Stellar Rotation in NGC?6811

We present rotation periods for 71 single dwarf members of the open cluster NGC?6811 determined using photometry from NASAs Kepler mission. The results are the first from The Kepler Cluster Study, which combines Keplers photometry with ground-based spectroscopy for cluster membership and binarity. The rotation periods delineate a tight sequence in the NGC?6811 color-period diagram from ~1?day at mid-F to ~11?days at early-K spectral type. This result extends to 1?Gyr similar prior results in the ~600?Myr Hyades and Praesepe clusters, suggesting that rotation periods for cool dwarf stars delineate a well-defined surface in the three-dimensional space of color (mass), rotation, and age. It implies that reliable ages can be derived for field dwarf stars with measured colors and rotation periods, and it promises to enable further understanding of various aspects of stellar rotation and activity for cool stars.

The XO Project: Searching for Transiting Extrasolar Planet Candidates

The XO project’s first objective is to find hot Jupiters transiting bright stars, i.e. V < 12, by precision differential photometry. Two XO cameras have been operating since September 2003 on the 10,000-foot Haleakala summit on Maui. Each XO camera consists of a 200-mm f/1.8 lens coupled to a 1024x1024 pixel, thinned CCD operated by drift scanning. In its first year of routine operation, XO has observed 6.6% of the sky, within six 7 ◦ -wide strips scanned from 0 ◦ to +63 ◦ of declination and centered at RA=0, 4, 8, 12, 16, and 20 hours. Autonomously operating, XO records 1 billion pixels per clear night, calibrates them photometrically and astrometrically, performs aperture photometry, archives the pixel data and transmits the photometric data to STScI for further analysis. From the first year of operation, the resulting database consists of photometry of �100,000 stars at more than 1000 epochs per star with differential photometric precision better than 1% per epoch. Analysis of the light curves of those stars produces transiting-planet candidates requiring detailed follow up, described elsewhere, culminating in spectroscopy to measure radial-velocity variation in order to differentiate genuine planets from the more numerous impostors, primarily eclipsing binary and multiple stars.

Properties of the open cluster system

A system of weights corresponding to the precision of open cluster data is described. Using these weights, some properties of open clusters can be studied more accurately than was possible earlier. It is clear that there are three types of objects: unbound clusters, bound clusters in the thin disk, and older bound clusters. Galactic gradients of metallicity, longevity, and linear diameter are studied. Distributions at right angles to the galactic plane are discussed in the light of the different cluster types. The clumping of clusters in complexes is studied. An estimate of the selection effects influencing the present material of open cluster data is made in order to evaluate the role played by open clusters in the history of the galactic disk. 58 references.

The luminosity scale of RR Lyrae stars with the Baade-Wesselink method. II. The absolute magnitudes of 13 field RR Lyrae stars

The surface brightness version of the Baade-Wesselink method has been applied to 13 field RR Lyrae stars to derive their absolute magnitudes from the photometry and radial velocities presented in a previous paper. Using static model atmospheres to provide metallicity- and gravity-dependent color-temperature transformations, it is found that the RR Lyrae temperature scale can be relatively well defined using the infrared color index V-K except for a small phase interval where strong atmospheric acceleration and shock waves are present, while the temperatures determined from bluer color indices are systematically overestimated. The results show a clear correlation between the luminosities of the RR Lyraes and their metallicities. 99 refs.

The same frequency of planets inside and outside open clusters of stars

Most stars and their planets form in open clusters. Over 95 per cent of such clusters have stellar densities too low (less than a hundred stars per cubic parsec) to withstand internal and external dynamical stresses and fall apart within a few hundred million years. Older open clusters have survived by virtue of being richer and denser in stars (1,000 to 10,000 per cubic parsec) when they formed. Such clusters represent a stellar environment very different from the birthplace of the Sun and other planet-hosting field stars. So far more than 800 planets have been found around Sun-like stars in the field. The field planets are usually the size of Neptune or smaller. In contrast, only four planets have been found orbiting stars in open clusters, all with masses similar to or greater than that of Jupiter. Here we report observations of the transits of two Sun-like stars by planets smaller than Neptune in the billion-year-old open cluster NGC6811. This demonstrates that small planets can form and survive in a dense cluster environment, and implies that the frequency and properties of planets in open clusters are consistent with those of planets around field stars in the Galaxy.

Young open clusters as probes of the star-formation process. II: Mass and luminosity functions of young open clusters

This paper is the second in a series devoted to the use of young, optically revealed open clusters as probes of the star-formation process. The first paper in the series [Phelps & Janes, ApJS (1993) (in press)] presented the dataset used for the study, and discussed some of the results from the survey of 23 open clusters in the Cassipoeia region of the Perseus spiral arm of our Galaxy. This paper presents an analysis of the stellar content of a subset of the clusters, including determinations of the luminosity and mass functions of 8 of the clusters to ∼1M ○. . The slope of the average mass function Γ is found to be −1.40±0.13 over the mass range 1.4<M/M ○. <7.9. Significant variations from this average value are found for two of the clusters, NGC 581 and NGC 663