No Open Cluster in the Ruprecht 93 Region

UBVI CCD photometry has been obtained for the Ruprecht 93 (Ru 93) region. We were unable to confirm the existence of an intermediate-age open cluster in Ru 93 from the spatial distribution of blue stars. On the other hand, we found two young star groups in the observed field: the nearer one (Ru 93 group) comprises the field young stars in the Sgr-Car arm at d ~ 2.1 kpc, while the farther one (WR 37 group) is the young stars around WR 37 at d ~ 4.8 kpc. We have derived an abnormal extinction law (Rv = 3.5) in the Ruprecht 93 region.Comment: 6 pages, 6 figures, JKAS 2010, in press (Aug issue

UBVI CCD Photometry of the Open Cluster NGC 4609 and Hogg 15

UBVI CCD photometry is obtained for the open clusters NGC 4609 and Hogg 15 in Crux. For NGC 4609, CCD data are presented for the first time. From new photometry we derive the reddening, distance modulus and age of each cluster - NGC 4609 : E(B-V) = 0.37 +/- 0.03, V_0 - M_V = 10.60 +/- 0.08, log tau = 7.7 +/- 0.1; Hogg 15 : E(B-V) = 1.13 +/- 0.11, V_0 - M_V = 12.50 +/- 0.15, log tau <= 6.6. The young age of Hogg 15 strongly implies that WR 47 is a member of the cluster. We have also determined the mass function of these clusters and have obtained a normal slope (Gamma = -1.2 +/- 0.3) for NGC 4609 and a somewhat shallow slope (Gamma = -0.95 +/- 0.5) for Hogg 15.Comment: 12 pages, 14 figures, JKAS, in pres

Probing the Low Mass Stellar End of the eta Chamaeleontis Cluster

We have identified three faint new members of the eta Chamaeleontis cluster. Spectral types of the new members are estimated to be ~M5 based on their TiO band strengths and broadband colors. With an age of 5-8 Myr for the cluster, masses of these new members are estimated to be ~0.08 Msun. All three display strong Li 6708A absorption and Halpha emission features including one with Halpha emission equivalent width ~60A along with HeI 6678 & 7605 A emission features that are characteristics of classical T Tauri stars.Comment: 11 pages, 4 figures, 2 tables, accepted in Ap

Lithium Depletion Boundary in a Pre-Main Sequence Binary System

A lithium depletion boundary is detected in HIP 112312 (GJ 871.1 A and B), a \~12 Myr old pre-main sequence binary system. A strong (EW 300 mA) Li 6708 A absorption feature is seen at the secondary (~M4.5) while no Li 6708 A feature is detected from the primary (~M4). The physical companionship of the two stars is confirmed from common proper motions. Current theoretical pre-main sequence evolutionary models cannot simultaneously match the observed colors, brightnesses, and Li depletion patterns of this binary system. At the age upper limit of 20 Myr, contemporary theoretical evolutionary models predict too slow Li depletion. If true Li depletion is a faster process than predicted by theoretical models, ages of open clusters (Pleiades, alpha Persei, and IC 2391) estimated from the Li depletion boundary method are all overestimated. Because of the importance of the open cluster age scale, development of self-consistent theoretical models to match the HIP 112312 data is desirable.Comment: Accepted in ApJL. 5 pages total (3 tables, 3 figures

SkyMapper and the Southern Sky Survey - a resource for the southern sky

SkyMapper is amongst the first of a new generation of dedicated, wide-field survey telescopes. The 1.3m SkyMapper telescope features a 5.7 square degree field-of-view Cassegrain imager and will see first light in late 2007. The primary goal of the facility is to conduct the Southern Sky Survey a six colour, six epoch survey of the southern sky. The survey will provide photometry for objects between 8th and 23rd magnitude with global photometric accuracy of 0.03 magnitudes and astrometry to 50 mas. This will represent a valuable scientific resource for the southern sky and in addition provide a basis for photometric and astrometric calibration of imaging data.Comment: 6 pages, 4 figures, proceedings of ESO Calibration Workshop 200

CHORIZOS: a CHi-square cOde for parameteRized modelIng and characteriZation of phOtometry and Spectrophotometry

We have developed a CHi-square cOde for parameteRized modelIng and characteriZation of phOtometry and Spectrophotometry (CHORIZOS). CHORIZOS can use up to two intrinsic free parameters (e.g. temperature and gravity for stars; type and redshift for galaxies; or age and metallicity for stellar clusters) and two extrinsic ones (amount and type of extinction). The code uses chi-square minimization to find all models compatible with the observed data in the model N-dimensional (N=1,2,3,4) parameter space. CHORIZOS can use either correlated or uncorrelated colors as input and is especially designed to identify possible parameter degeneracies and multiple solutions. The code is written in IDL and is available to the astronomical community. Here we present the techniques used, test the code, apply it to a few well-known astronomical problems, and suggest possible applications. As a first scientific result from CHORIZOS, we confirm from photometry the need for a revised temperature-spectral type scale for OB stars previously derived from spectroscopy.Comment: 32 pages, 13 figures. To appear in the September 2004 issue of PAS

Testing the Disk-Locking Paradigm: An Association Between U-V Excess and Rotation in NGC 2264

We present some results from a UVI photometric study of a field in the young open cluster NGC 2264 aimed, in part, at testing whether accretion in pre-main sequence stars is linked to rotation. We confirm that U-V excess is well correlated with H-alpha equivalent width for the stars in our sample. We show that for the more massive stars in the cluster sample (roughly 0.4-1.2 M_sun) there is also a significant association between U-V excess and rotation, in the sense that slow rotators are more likely to show excess U-band emission and variability. This constitutes significant new evidence in support of the disk-locking paradigm.Comment: Accepted by ApJ Letter