Ivan R. King

Catalogue of Galactic globular-cluster surface-brightness profiles

We present a catalogue of surface-brightness profiles (SBPs) of 125 Galactic globular clusters, the largest such collection ever gathered. The SPBs are constructed from generally inhomogeneous data, but are based heavily on the Berkeley Global Cluster Survey of Djorgovski & King. All but four of the SBPs have photometric zero points. We derive central surface brightness, King-model concentrations, core radii, half-light, and other fraction-of-light radii where data permit, and we briefly discuss their use.

HST color-magnitude diagrams of 74 galactic globular clusters in the HST F439W and F555W bands ⋆

We present the complete photometric database and the color-magnitude diagrams for 74 Galactic globular clusters observed with the HST/WFPC2 camera in the F439W and F555W bands. A detailed discussion of the various reduction steps is also presented, and of the procedures to transform instrumental magnitudes into both the HST F439W and F555W flight system and the standard Johnson B and V systems. We also describe the artificial star experiments which have been performed to derive the star count completeness in all the relevant branches of the color magnitude diagram. The entire photometric database and the completeness function will be made available on the Web immediately after the publication of the present paper.

Toward High‐Precision Astrometry with WFPC2. I. Deriving an Accurate Point‐Spread Function

The first step toward doing high-precision astrometry is the measurement of individual stars in individual images, a step that is fraught with dangers when the images are undersampled. The key to avoiding systematic positional error in undersampled images is to determine an extremely accurate point-spread function (PSF). We apply the concept of the effective PSF (ePSF) and show that in images that consist of pixels it is the ePSF, rather than the often-used instrumental PSF, that embodies the information from which accurate star positions and magnitudes can be derived. We show how, in a rich star field, one can use the information from dithered exposures to derive an extremely accurate effective PSF by iterating between the PSF itself and the star positions that we measure with it. We also give a simple but effective procedure for representing spatial variations of the Hubble Space Telescope PSF. With such attention to the PSF, we find that we are able to measure the position of a single reasonably bright star in a single image with a precision of 0.02 pixel (2 mas in WF frames, 1 mas in PC), but with a systematic accuracy better than 0.002 pixel (0.2 mas in WF, 0.1 mas in PC), so that multiple observations can reliably be combined to improve the accuracy by .

The Luminosity Function of the Globular Cluster NGC 6397 near the Limit of Hydrogen Burning

Second-epoch Hubble Space Telescope observations of NGC 6397 have led to the measurement of proper motions accurate enough to separate the faintest cluster stars from the field, thus extending the luminosity function of this globular cluster far enough to approach the limit of hydrogen burning on the main sequence. We isolate a sample of 1385 main-sequence stars, from just below the turnoff down to I = 24.5 (MI 12.5), which corresponds to a mass of less than 0.1 m☉ for the metallicity of this cluster. Below I = 22 (MI 10), the main-sequence luminosity function drops rapidly, in a manner similar to that predicted by theoretical models of low-mass stars near the hydrogen-burning limit.

HUBBLE SPACE TELESCOPE Observations of Galactic Globular Cluster Cores. II. NGC 6273 and the Problem of Horizontal-Branch Gaps*

We present observations of the center of the Galactic globular cluster NGC 6273, obtained with the Hubble Space Telescope Wide Field Planetary Camera 2 as part of the snapshot program GO-7470. A BV color-magnitude diagram (CMD) for ~28,000 stars is presented and discussed. The most prominent feature of the CMD, identified for the first time in this paper, is the extended horizontal-branch blue tail (EBT) with a clear double-peaked distribution and a significant gap. The EBT of NGC 6273 is compared with the EBTs of seven other globular clusters for which we have a CMD in the same photometric system. From this comparison, we conclude that all the globular clusters in our sample with an EBT show at least one gap along the horizontal branch, which could have similar origins. A comparison with theoretical models suggests that at least some of these gaps may be occurring at a particular value of the stellar mass, common to a number of different clusters. From the CMD of NGC 6273 we obtain a distance modulus (m - M)V = 16.27 ± 0.20. We also estimate an average reddening E(B-V) = 0.47 ± 0.03, though the CMD is strongly affected by differential reddening, with the relative reddening spanning a ΔE(B-V) ~ 0.2 mag in the WFPC2 field. A luminosity function for the evolved stars in NGC 6273 is also presented and compared with the most recent evolutionary models.

Peculiar Multimodality on the Horizontal Branch of the Globular Cluster NGC 2808

We present distributions of colors of stars along the horizontal branch (HB) of the globular cluster NGC 2808, from Hubble Space Telescope Wide Field Planetary Camera 2 imaging in B, V, and an ultraviolet filter (F218W). This clusters HB is already known to be strongly bimodal, with approximately equal-sized HB populations widely separated in the color-magnitude diagram. Our images reveal a long blue tail with two gaps, for a total of four nearly distinct HB groups. These gaps are very narrow, corresponding to envelope-mass differences of only ~0.01 M?. This remarkable multimodality may be a signature of mass-loss processes, subtle composition variations, or dynamical effects; we briefly summarize the possibilities. The existence of narrow gaps between distinct clumps on the HB presents a challenge for models that attempt to explain HB bimodality or other peculiar HB structures.

Astrometric and Photometric Corrections for the 34th Row Error in HST’s WFPC2 Camera

ABSTRACT A small manufacturing defect, known but rarely noted, occurs at approximately every 34th row of each of the chips of WFPC2. This defect induces errors in photometry at the 0.01–0.02 mag level for about 6% of all stars, and periodic errors in astrometry for all stars, with an amplitude of 0.03 pixel. We derive simple corrections for these errors. The errors exist in all astrometry and photometry previously done with WFPC2, but they can be avoided in future work by applying the corrections that we give here.

Far-UV properties of the nuclear region of M31

Comparison of the Hubble Space Telescope (HST) far-UV and visible images of the nucleus of M31 deepens the mystery of the two brightness peaks recently discovered by Lauer et al. At 175 mm the brightest point is the optically fainter peak (P2) that is close to the dynamical center. The very center of P2 has a UV upturn that is much greater than that of the bulge light, while the UV upturn of the optically brighter but off-center P1 is very similar to that of its surroundings. The excess FUV radiation form P2 is closely confined to its center and has a total flux density of 3 micro-joules, a level that is less likely to come from stellar radiation than from a high-frequency extension of the radio source at the center of M31. A surrounding region of 1.8 pc radius has a somewhat smaller Uv upturn than the rest of the bulge, but there is some lingering possibility that this depression might be an artifact of our correction of the spherical aberration. Our improvement decomposition of the V image removes the need to postulate a dust lane near the center. We confirm that P1 is very compact, and we derive a luminosity for it similar to that found by Lauer et al. The implications of all of this are briefly discussed.

Color-Magnitude Diagram and Luminosity Function of M4 Near the Hydrogen-Burning Limit

A proper-motion separation of M4 members from field stars, using deep Hubble Space Telescope observations separated by a time baseline of 5 yr, allows us to study a pure sample of cluster main-sequence stars almost to the minimum mass for hydrogen burning. High-precision photometry shows how badly current theoretical models fail to reproduce the color-magnitude diagram of low-mass stars of moderate metallicity ([M/H] -1). This inability of theory to reproduce the luminosity-color relation casts doubt on the theoretical mass-luminosity relation, which is needed to convert the observed luminosity function (LF) into a mass function (MF) as well as to convert our locally determined LF into a global MF. To the extent that we trust theoretical M-L relations for such transformations, we obtain a flat MF from the LF, and some indication that theoretical masses might be too low at a given luminosity, near the H-burning limit.

Hubble Space Telescope Observations of Galactic Globular Cluster Cores. I. NGC 6362 and NGC 6934

We present observations of the centers of the Galactic globular clusters NGC 6362 and NGC 6934 obtained with the Hubble Space Telescope/Wide Field Planetary Camera 2 as part of the snapshot program GO-7470. The B, V color-magnitude diagrams (CMDs) are presented for 4104 stars in NGC 6362 and for 8187 stars in NGC 6934. From the CMDs, we obtain (m - M)V = 14.67 ± 0.20 and E(B-V) = 0.06 ± 0.03 for NGC 6362 and (m - M)V = 16.37 ± 0.20 and E(B-V) = 0.05 ± 0.02 for NGC 6934. Each cluster shows a well-defined sequence of blue stragglers. Both the CMD and the luminosity function (LF) of the blue stragglers differ in the two clusters, suggesting two different origins for these stars. NGC 6362 has a strikingly narrow blue straggler sequence. The data have also been used to derive a complete LF, which extends from the red giant branch tip to 2–3 mag below the turnoff. In the case of NGC 6362, which has the deepest LF, the theoretical LF cannot match both the evolved LF and that of the main sequence with any reasonable mass-function slope. The LF has also been used to estimate the ages of the two clusters. A comparison with the theoretical LF, the more classical isochrone fitting, and the ΔV suggest an age between 13 and 16 Gyr for both clusters. No significant age difference between NGC 6362 and NGC 6934 is observed.