NATIONAL OPTICAL ASTRONOMY OBSERVATORIES NATIONAL OPTICAL ASTRONOMY OBSERVATORIES QUARTERLY REPORT OCTOBER - DECEMBER 1995 July 16,1996 TABLE OF CONTENTS I. INTRODUCTION 1 II. SCIENTIFIC HIGHLIGHTS 1 A. Cerro Tololo Inter-American Observatory 1 1. The Faintest Known White Dwarf 1 2. Evidence for a Black Hole in the X-ray Nova GRO J1655-40 2 3. The Stellar Metallicity Distribution in Omega Centauri 3 B. Kitt Peak National Observatory 3 1. Infrared Colors and Pre-Main-Sequence Evolution 3 2. Bargain Redshifts for Faint Galaxies 4 3. Weighing the Dwarf Spheroidal Companions to the Milky Way 5 C. National Solar Observatory 6 1. Fine Dark Threads and the Temperature of the Solar Corona 6 2. An Objective Test of Magnetic Shear as a Flare Predictor 7 3. Solar Particle Dynamics 7 D. US Gemini Program 8 m. PERSONNEL AND BUDGET STATISTICS, NOAO 9 A. Visiting Scientists 9 B. Hired 9 C. Completed Employment 9 D. Changed Status 9 E. Gemini 8-mTelescopes Project 10 F. Chilean Economic Statistics 10 G. NSFForeign Travel Fund 10 Appendices Appendix A: Telescope Usage Statistics Appendix B: Observational Programs Appendix C: US Sites Safety Report I. INTRODUCTION This document covers scientific highlights and personnel changes for the period 1 October - 31 December 1995. Highlights emphasize concluded projects rather than work in progress. The March 1996 NOAO Newsletter Number 45 contains information on major projects, new instrumentation, and operations. The appendices to this report summarize telescope usagestatistics and observational programs. II. SCIENTIFIC HIGHLIGHTS A. Cerro Tololo Inter-American Observatory 1. The Faintest Known White Dwarf The white dwarf luminosity function incorporates information on the age and star formation history of the Galactic disk which is independent of other techniques. In particular, the least luminous white dwarfs set lower limits on the disk age, and thus the age of the Universe. The faintest known white dwarfs have Mv ~ 16.2. Age estimates for these stars are in the range 6.5-11 Gyr, with the variation largely due to uncertainty in the core composition. The white dwarf luminosity function declines abruptly at this absolute magnitude, and this decline has been interpreted as beingdue to the age of the Galactic disk. The question remains, do even less luminous and older white dwarfs—however rare—exist? The search for very low luminosity white dwarfs is still very active. A concerted effort has been carried out for a number of years by Maria Teresa Ruiz and her colleagues at Cerro Calan Observatory (U. of Chile). Selection of promising candidates is done with a classic technique, blinking plate pairs from the ESO Schmidt telescope to identify faint stars with substantial proper motion. The magnitude limit is about mR ~ 21 and motions as small as 0.1 arcsec yr"1 are detectable with a few years' time base. Followup observations at CTIO and other facilities in Chile help winnow the most interesting objects out of the hundreds found on a plate pair. One such object is the cold DC type white dwarf ESO 439-26. Following its initial discovery in 1988, preliminary spectroscopic and astrometric data suggested it to be of very low luminosity. Several years' subsequent investigation at CTIO have now established this definitively. This result is based on another classic technique, the determination of trigonometric parallax, applied with modern CCD detectors in a program led by Claudio Anguita (U. of Chile). Observations were carried out on the CTIO 1.5-m telescope overa five-year period, using initially a 312 x 508 RCA CCDand morerecently a Tek IK device. With 0.3 arcsec pixels and seeing at 1.2 arcsec or better, 40 frames yield a parallax of 0.024± 0.003 arcsec, or Mv = 17.4—more than one magnitude fainter than the faintest previously known white dwarfs. The existence of such an extreme low-luminosity white dwarf may imply a very large value for the age of the local Galactic disk. Alternatively, the low luminosity of ESO439-26 couldbe accounted for if it is a massive whitedwarfwith a correspondingly small radius—also a rare beast, but not one implying greatage. Onlya detailed comparison of the observed absolute flux distribution with theoretical models can resolve the ambiguity. Ruiz and Anguita, together withS. K. Leggett (IRTF) and P. Bergeron (U. of Montreal), haveused BVRIphotometry obtained on the CTIO 1.5-m and 0.9-m telescopes and model atmosphere calculations by Bergeron and colleagues to make this comparison. They find the high mass interpretation to be the correct one, independent of details of atmospheric composition. The best fit to the photometiy gives Teff=4560 K, log g = 9.0, and M= 1.2 M solar masses—twice that of a typical white dwarf. Comparison with carbon core evolutionary models yields an upper age limit of 6.4 Gyr for the best fit solution. More complex interiors models would tend to reduce the age significantly, but are not yet available at appropriately cool temperatures. Infrared JHK photometry, in progress, will help constrain the atmospheric composition. Interestingly, spectroscopy does not. Even though Ha can be detected in low mass white dwarfs of comparable effective temperature, the large surface gravity of this small, massive object collisionally broadens hydrogen lines to the level of undetectability. Although not of great age, the high mass of ESO 439-26 makes it unique in one respect. Comparison withmodel isochrones indicate it is in an advanced stateof crystallization. 2. Evidence for a Black Hole in the X-ray Nova GRO J1655-40 X-ray novae are binary systems in which a compact object accretes gas from a companion star. The observed velocity of the secondary starcan be used to determine the minimum mass of thecompact object. In six cases to date, the mass function of the compact object has been found to exceed 3 solar masses, confirming thepresence of black holes in these systems. These black hole X-ray novae (orBHXNs) provide a unique opportunity to study black holes, since one can observe both thesecondary starvia the absorption lines and the accretion disk surrounding the black hole via the emission lines. The unusual X-ray nova GRO J1655-40 is the newest and one of the most interesting of the BHXNs. Following its discovery in August 1994 by the Compton Gamma-Ray Observatory, Charles Bailyn (Yale U.) and collaborators located the optical counterpart using the CTIO 0.9-m telescope. Soon after, superluminal radio jets were discovered by Tingay et al. (1995, Nature, 374, 141) and Hjellming et al. (1995, Nature, 375, 464), making GRO J1655-40 the only galactic superluminal source which can be observed optically. The jet ejection, at 92% of the speed of light, appears to be episodic and asymmetric, and is so rapid that distinct motions can be seen from one hour to the next. Bailyn and collaborators returned to CTIO in 1995 to make further optical photometric and spectroscopic measurements of GRO J1655-40. Photometry was obtained on 18-25 March and 5-24 April with the 0.9-m telescope and CCD imager, and on 28 March to 2 April with the CTIO 1.5-m telescope and CCD imager. In addition, spectra were obtained with the RC spectrograph on the CTIO 4-m telescope on the nights of 30 April and 2-4 May. As the photometric data were obtained, it became clear that the light curve showed eclipses and was periodic on a timescale of approximately three days. The overall light curve shape was found to consist of a broad triangular primary minimum, and a shorter secondary minimum displaced by about 0.5 in phase from the primary minimum. By analogy with the eclipsing X-ray binary CAL87, it seems likely that aneclipse of a large accretion disk by the secondary staris responsible for theprimary minimum, and an eclipse of the secondary star by the disk explains the secondaryminimum. The spectra of GRO J1655-40 obtained by Bailyn et al. had less prominent Balmer emission lines and stronger high-excitation lines than X-ray novae in quiescence, presumably because the outburst was still underway. Nevertheless, it became immediately apparent from the individual spectra that a stellar F-type absorption spectrum was present. Cross-correlating the spectra against an F5 subgiant of known radial velocity revealed sinusoidal radial velocity variations with a period of 2.6days and a semi-amplitude of 227 km s"1. These parameters yield a minimum possible mass of the compact object of 3.16 ±0.15 solar masses. The maximum stable mass of a neutron star is 3 solarmasses for all possible equations of state, and below 2.5 solar masses for most plausible equations of state. Thus, in the absence of exotic matter and/or non-Einsteinian gravity, a mass function above 3 solarmasses requires thecompact object in GRO J1655-40 to be a black hole. The eclipsing geometry of GRO J1655-40 allows the possibility of using eclipse mapping techniques to explore the geometry of the system in further detail. Changes in the flux and spectral line shape during eclipse can be used to map the inner regions of the accretion disk. Hence, a multiwavelength campaign involving GRO, ROSAT, ASCA, XTE, and a variety of ground-based optical and radio observations (including 0.9-m time at CTIO) was organized for March 1996. One of the primary goals of this campaign was to use the eclipses to map the high-energy, optical, and radio emission. When successful, these observations allow the emission geometry near an accreting, jet-producing black hole to be explored in unprecendented detail. 3. The Stellar Metallicity Distribution in Omega Centauri Suntzeff (CTIO) and Kraft (UCO/Lick) have finished a project on stellar abundances and radial velocities for giant stars in the Galactic globular cluster co Centauri based on the Call infrared triplet.