James L. Clem

Empirically Constrained Color-Temperature Relations. I. BV(RI)C

This investigation presents a set of transformations to Johnson B-V, Cousins V-R, and Cousins V-I, as well as bolometric corrections to V, for [Fe/H] = -3, -2, -1, -0.5, 0.0, and +0.3 and, in each case, values of log g from -0.5 to 5.0 for 3000 K ≤ Teff ≤ 5500 K and from 2.0 to 5.0 for 6000 K ≤ Teff ≤ 40,000 K. These transformations employ the predictions from Kurucz model atmospheres at high temperatures (Teff ≥ 8000 K) and from MARCS model atmospheres at intermediate temperatures (from 7000 K down to a temperature in the range 4000 K ≤ Teff ≤ 5500 K, depending on [Fe/H], where adjustments to satisfy observational constraints become necessary). Thus, theoretical color-Teff relations are used exclusively down to a minimum temperature that is cooler than the temperatures of turnoff stars in open and globular star clusters. To better represent the color transformations obeyed by cool stars (down to 3000 K), corrections to the synthetic transformations have been determined from a careful consideration of observations for a few globular clusters (M92, M68, and 47 Tucanae), the color-magnitude diagrams (CMDs) of several open clusters (M67, the Pleiades, the Hyades, and NGC 6791), the CMDs and mass-luminosity diagram for solar neighborhood stars having good distance measurements from Hipparcos, empirical (B-V)-Teff and (V-K)-Teff relations, and color-color diagrams for field giants. The semiempirical color transformations that have been produced as a result of our analysis are also compared with several others that have been published in recent years: some of the deficiencies of the latter are revealed.

THE GALACTIC BULGE SURVEY: OUTLINE AND X-RAY OBSERVATIONS

We introduce the Galactic Bulge Survey (GBS) and we provide the Chandra source list for the region that has been observed to date. Among the goals of the GBS are constraining the neutron star (NS) equation of state and the black hole (BH) mass distribution via the identification of eclipsing NS and BH low-mass X-ray binaries (LMXBs). The latter goal will, in addition, be obtained by significantly enlarging the number of BH systems for which a BH mass can be derived. Further goals include constraining X-ray binary formation scenarios, in particular the common envelope phase and the occurrence of kicks, via source-type number counts and an investigation of the spatial distribution of X-ray binaries, respectively. The GBS targets two strips of 6° × 1° (12 deg2 in total), one above (1° < b < 2°) and one below (-2° < b < -1°) the Galactic plane in the direction of the Galactic center at both X-ray and optical wavelengths. By avoiding the Galactic plane (-1° < b < 1°) we limit the influence of extinction on the X-ray and optical emission but still sample relatively large number densities of sources. The survey is designed such that a large fraction of the X-ray sources can be identified from their optical spectra. The X-ray survey, by design, covers a large area on the sky while the depth is shallow using 2 ks per Chandra pointing. In this way we maximize the predicted number ratio of (quiescent) LMXBs to cataclysmic variables. The survey is approximately homogeneous in depth to a 0.5-10 keV flux of 7.7 × 10-14 erg cm-2 s-1. So far, we have covered about two-thirds (8.3 deg2) of the projected survey area with Chandra providing over 1200 unique X-ray sources. We discuss the characteristics and the variability of the brightest of these sources.

THE EFFECTS OF DIFFERENTIAL REDDENING AND STELLAR ROTATION ON THE APPEARANCE OF MULTIPLE POPULATIONS IN STAR CLUSTERS: THE CASE OF TRUMPLER 20

We present a detailed analysis of the upper main sequence of the ~1.3 Gyr old open cluster Trumpler 20. High-accuracy BVI photometry combined with the Very Large Telescope/FLAMES medium-resolution spectroscopy of 954 stars is essential to understanding the unusual appearance of the color-magnitude diagram (CMD), initially suggesting multiple populations in Trumpler 20. We show that differential reddening is a dominant contributor to the apparent splitting/widening of the main-sequence turnoff region. At its extreme, the excess differential reddening reaches Δ(B – V) ~ 0.1 while the adopted minimum reddening for the cluster is E(B – V) = 0.36. A unique sample of measured projected rotational velocities indicates that stellar rotation is high near the main-sequence turnoff, reaching vsin i ~ 180 km s–1. By dividing the upper main-sequence stars into two equal groups of slow and fast rotators, we find that fast rotators have a marginal blueshift of δ(V – I) ~ –0.01, corresponding to a difference in the median vsin i of ~60 km s–1 between these subsamples. We conclude that stellar rotation has an insignificant effect on the morphology of the upper main sequence of this intermediate-age open cluster. Trumpler 20 appears to contain a single coeval population of stars but there is evidence that the red clump is extended.

BRIGHT VARIABLE STARS IN NGC 6819: AN OPEN CLUSTER IN THE KEPLER FIELD

We describe a variability study of the moderately old open cluster NGC 6819. We have detected four new detached eclipsing binaries near the cluster turnoff (one of which may be in a triple system). Several of these systems should be able to provide mass and radius information, and can therefore constrain the age of the cluster. We have also newly detected one possible detached binary member about 3.5 mag below the turnoff. One EW-type binary (probably not a cluster member) shows unusually strong night-to-night light curve variations in sets of observations separated by eight years. According to the best current information, the three brightest variables we detected (two of them new) are cluster members, making them blue stragglers. The first one is a δ Scu pulsating variable, the second one is a close but detached binary, and the third one contains a detached short-period binary that shows total eclipses. In each case, however, there is evidence hinting that the system may have been produced through the interaction of more than two stars.

FAINT UBVRI STANDARD STAR FIELDS

High-quality CCD-based UBVRI photometry is presented for ∼45, 000 stars distributed among 60 different fields centered on the celestial equator and at δ ≈ –50°. This photometry has been calibrated to the standard Johnson UBV and Kron-Cousins RI systems via observations of the UBVRI standard stars presented in the works of Landolt. The majority of the stars in our photometric catalog fall in the magnitude range 12 ∼< V ∼< 22 and in the color range –0.3 ∼< (B – V) ∼< 1.8. Each star averages 67 measures in each UBVRI filter from data taken on 250 different photometric nights over a period of ∼6.5 yr from two different telescopes. Our final photometric database effectively extends the UBVRI standard star network defined by Landolt to much fainter magnitudes and increases the number density of stars within pre-existing standard fields. Hence, these new, fainter standards serve as suitable calibrators for investigators who employ either small or large-aperture telescopes for their observational projects.

DEEP, WIDE-FIELD CCD PHOTOMETRY FOR THE OPEN CLUSTER NGC 3532

We present the results of a deep, wide-field CCD survey for the open cluster NGC 3532. Our new BV(RI) c photometry effectively covers a one square degree area and reaches an unprecedented depth of V ~ 21 to reveal that NGC 3532 is a rich open cluster that harbors a large number of faint, low-mass stars. We employ a number of methods to reduce the impact of field star contamination in the cluster color-magnitude diagrams (CMDs), including supplementing our photometry with JHK_s data from the 2MASS catalog. These efforts allow us to define a robust sample of candidate main-sequence stars suitable for a purely empirical determination of the clusters parameters by comparing them to the well-established Hyades main sequence. Our results confirm previous findings that NGC 3532 lies fairly near to the Sun [(m – M)_0 = 8.46 ± 0.05; 492^(+12)_(–11) pc] and has an extremely low reddening for its location near the Galactic plane [E(B – V) = 0.028 ± 0.006]. Moreover, an age of ~300 Myr has been derived for the cluster by fitting a set of overshooting isochrones to the well-populated upper main sequence. This new photometry also extends faint enough to reach the cluster white dwarf sequence, as confirmed by our photometric recovery of eight spectroscopically identified members of the cluster. Using the location of these eight members, along with the latest theoretical cooling tracks, we have identified ~30 additional white dwarf stars in the [V, (B – V)] CMD that have a high probability of belonging to NGC 3532. Reassuringly, the age we derive from fitting white dwarf isochrones to the locus of these stars, 300 ± 100 Myr, is consistent with the age derived from the turnoff. Our analysis of the photometry also includes an estimation of the binary star fraction as well as a determination of the clusters luminosity and mass functions.

THE 2010 ERUPTION OF THE RECURRENT NOVA U SCORPII: THE MULTI-WAVELENGTH LIGHT CURVE

The recurrent nova U Scorpii most recently erupted in 2010. Our collaboration observed the eruption in bands ranging from the Swift XRT and UVOT w2 (193 nm) to K-band (2200 nm), with a few serendipitous observations stretching down to WISE W2 (4600 nm). Considering the time and wavelength coverage, this is the most comprehensively observed nova eruption to date. We present here the resulting multi-wavelength light curve covering the two months of the eruption as well as a few months into quiescence. For the first time, a U Sco eruption has been followed all the way back to quiescence, leading to the discovery of new features in the light curve, including a second, as-yet-unexplained, plateau in the optical and near-infrared. Using this light curve we show that U Sco nearly fits the broken power law decline predicted by Hachisu & Kato, with decline indices of −1.71 ± 0.02 and −3.36 ± 0.14. With our unprecedented multi-wavelength coverage, we construct daily spectral energy distributions and then calculate the total radiated energy of the eruption, E_(rad) = 6.99_(-0.57)^(+0.83) x 10^(44) erg. From that, we estimate the total amount of mass ejected by the eruption to be m_(ej) = 2.10_(-0.17^(+0.24) x 10^(-6) M_☉. We compare this to the total amount of mass accreted by U Sco before the eruption, to determine whether the white dwarf undergoes a net mass loss or gain, but find that the values for the amount of mass accreted are not precise enough to make a useful comparison.

Monitoring R CrB at Minimum Light

The star R CrB has been monitored at its faintest known minimum brightness in a search for possible light variations. No short-term variations were identified. In addition, UBVRI photometry is presented for stars in the surrounding neighborhood.

FAINT UBVRI STANDARD STAR FIELDS AT +50° DECLINATION

Precise and accurate CCD-based UBVRI photometry is presented for ~2000 stars distributed around the sky in a declination zone centered approximately at +50°. Their photometry has been calibrated to the standard Johnson UBV and Kron–Cousins RI systems through observations of the UBVRI standard stars presented in the various works of Landolt. The magnitude and color range for these stars are 12 V 22 and −0.3 (B − V) 1.8, respectively. Each star averages 13 measures in each UBVRI filter from data taken on 41 different photometric nights obtained over a 21 month period. Hence, there now exists a network of faint UBVRI photometric standard stars centered on the declination zones δ = −50°, 0°, and +50°.

The Eclipsing Binary PG 1323−086A (GSC 05544−00493)

The star PG 1323−086A (GSC 05544−00493) is shown to be a short-period eclipsing binary system. Photometric elements for the system are computed and reveal that the stellar components are in a marginal degree of contact (f ∼ 8%) with a small temperature difference of about 700 K. During the study of the starfield, a nearby object, HE 1323-0830, was also discovered to be variable in light. This star has been cataloged as a candidate blue straggler and is most likely a pulsating variable. Online material: extended tables