Franklin Alvarado

BVI CCD photometry of the broad main-sequence globular cluster NGC 1851

Three-color CCD C-M diagrams are presented for the globular cluster NGC 1851, showing an extreme breadth of the main-sequence, similar to that of Omega Centauri. It is found that the main-sequence turnoff points are located at V(TO) = 19.44 + or - 0.10, with colors at B-V = 0.54 + or - 0.02, V-I = 0.61 + or - 0.02, and B-I = 1.15 + or - 0.03. The best fit to the VandenBerg and Bell (1985) isochrones is shown to be all C-M diagrams with Y = 0.20, Fe/H abundance ratio = -1.27, and (m-M)v = 15.45. It is concluded that NGC 1851 has a Delta V(TO - HB) = 3.34 + or - 0.10 and an age of 16 + or - 2 Gyr. 29 refs.

BVRI CCD photometry of the globular cluster M55 (NGC 6809)

We present BVRI photometry of two neighboring fields in the low concentration globular cluster M55 (NGC 6809) observed with a CCD camera and the 2.2 m telescope at ESO/La Silla. We compare our BV color-magnitude diagram (CMD) with those of Lee [A & AS, 29, 1 (1977)] and Schade et al. [AJ, 96, 1632 (1988)] and find generally good agreement down to about a magnitude below the main-sequence turnoff. We adopt a distance modulus of (m− M) V =14.10, and find that the fits between our four-color observations and the isochrones published by VandenBerg & Bell [ApJS, 58, 561 (1985)] consistently yield an age of 14-15 Gyr, depending on initial assumptions

BVRI CCD photometry of the metal-poor globular cluster NGC 4372

BVRI CCD photometry is presented in two overlapping fields in the metal-poor globular cluster NGC 4372. The observations extend approximately 2 mag below the main-sequence turnoff to V about 21. By comparing the color-magnitude diagram (CMD) with those of clusters with similar metallicities, it is found that E(B-V) = 0.50 {plus minus} 0.03, and (m-M)v = 14.75 {plus minus} 0.06. Comparison with theoretical isochrones leads to a value E(B-V) = 0.53 {plus minus} 0.03. Comparison of the CMD with that of bright stars published by other authors yields a value for Delta V(TO-HB) = 3.3 {plus minus} 0.3. The weighted mean value of the age of the cluster, derived from the four colors, is 15 {plus minus} 4 Gyr (estimated external uncertainty). 17 refs.

BVRI CCD photometry of the metal-poor globular cluster M68 (NGC 4590)

BVRI photometry of the low metallicity globular cluster M68 (NGC 4590) was obtained with a CCD camera and the 2.2-m ESO telescope. The resulting BV color-magnitude diagrams are compared with the observations of McClure et al. (1987). The observations are also compared with theoretical isochrones, yielding a cluster age of 13 Gyr with a likely external uncertainty of 2 or 3 Gyr. 25 refs.

BVI CCD photometry of the globular cluster M4

CCD BV1 main-sequence (MS) photometry of M4, the globular cluster closest to the sun, is presented. The photometry is matched to the BVI isochrones of VandenBerg and Bell (1985). The MS turnoffs are found to be at V = 16.90 + or - 0.05, B-V = 0.81 + or - 0.02, V-I = 0.96 + or - 0.02, and B - I = 1.77 + or - 0.02. The magnitude difference between the MS turnoff and the horizontal branch is Delta M(V) = 3.52 + or - 0.1 for all three color indices. Using Y = 0.2, (Fe/H) = - 1.27, and alpha = 1.65, with a distance modulus of (m-M)V = 12.7 and E(B-V) = 0.41, a consistent age for M4 is deduced in all three color indices of 17 + or - 1.5 Gyr. 34 references.

BVR CCD photometry of the globular cluster NGC 1261

We present BVR photometry of two overlapping fields in the high latitude, highly concentrated globular cluster NGC 1261, observed with a CCD camera and the 1.54 m telescope at ESO/La Silla. We compare our BV color-magnitude diagram (CMD) with that of Bolte & Marleau [PASP, 101, 1088 (1989)] and find generally good agreement down to the limit of our photometry at V∼22.5. We have observed NGC 1261 in the R-band for the first time, and from our photometry in the three colors we adopt a distance modulus of (m−M) V =16.00±0.23, and determine that the fits between our three-color observations and the isochrones published by VandenBerg and Bell [ApJS, 58, 561 (1985)] yield an age of 15±2 Gyr

Photoelectric UBVRI sequences in the Galactic globular clusters NGC 6752 and NGC 6864

UBVRI photoelectric sequences for the Galactic globular clusters NGC 6752 and NGC 6864 are presented. Both of them include fields suitable for CCD exposures. From five UBV sequences in NGC 6572, only five stars are in common with the previous works. 15 refs.

Photoelectric UBVRI sequences in the Magellanic Cloud clusters Lindsay 1, NGC 339, NGC 361, and NGC 1466

UBVRI sequences in three Small Magellanic Cloud (SMC) clusters Lindsay 1, NGC 339, NGC 361, and in NGC 1466, which lies between the two Magellanic Clouds, are presented. These sequences are appropriate for charge-coupled device (CCD) coverage. Only BV standards have been published in NGC 339 and UBV in NGC 1466; no sequences exist for the two other clusters. 15 refs.

SPECTROSCOPY OF AN EXTREMELY BLUE STAR IN THE GLOBULAR CLUSTER NGC 2298

We present spectroscopy of an extremely blue star found in the Galactic globular cluster NGC 2298. Previous CCD photometry gave a magnitude V = 18.79 ± 0.15 and a color B - V = -0.34 ± 0.10. Our visual spectrum reveals a number of He II and H I absorption lines, indicating that this object is a hot helium-rich subdwarf O star, similar to the He-sdO stars found in the Galactic halo.

BV CCD photometry of the low-latitude globular cluster NGC 5946

BV CCD photometry of the previously unstudied, highly concentrated, low galactic latitude globular cluster NGC 5946 observed with the 2.2 m Max-Planck-Institut telescope at ESO/La Silla is presented. The photometry extends to V ∼ 21.5 reaching about 4 mag below the brighter edge of the horizontal branch. The horizontal branch extends continuously from blue to red color indices in the magnitude range 17.0 < V < 18.5. The adopted reddening of E(B-V) = 0.53 ± 0.05 places the cluster at 10.7 kpc from the Sun and 5.8 kpc from the galactic center, assuming R 0 = 8.5 kpc. Indications are that the cluster is more metal-poor than previously believed