1999 REU/PIA Projects

A Comprehensive Look at LH72 in the Context of Constellation III
Jeremy Buss - University of Wisconsin, Oshkosh 

Abstract

We present results obtained from a multi-faceted study of the LMC OB association LH 72. LH 72 lies near the geometrical center of Constellation III, a kpc-sized complex of stars and gas comprising one of the largest known superbubbles. Our study combines ultraviolet and optical spectra of the brightest blue stars, UBV photometry of the entire association, narrow-band H-alpha and H-beta images of the surrounding H II region, and will include analysis of available data of the H I gas. These data provide the necessary ingredients for characterizing the star formation history and initial mass function of the association; for mapping the distribution of dust and measuring the ultraviolet reddening law; and for exploring the interaction of the stars with the interstellar medium.

To date, we have analyzed optical spectra of 13 stars in the association in addition to IUE spectra of 7 of the same stars. We have assigned spectral and luminosity classes to the stars on the basis of the optical spectra, from which we generate an H-R diagram of the bright stars of LH 72. By fitting isochrones to the H-R diagram, we find stellar ages ranging from ~2-18 Myr. LH 72 thus contains stars as young and as old as any found in Constellation III. The observed age range spans the proposed expansion age of the surrounding superbubble, suggesting that star formation may proceed long after an initial triggering event. We also find continued evidence for propagating star formation on the scale of the association, as the center of star formation activity has drifted by ~100 pc over the past 10-15 Myr.

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LMC X-2: The Search for the Orbital Period
Jenny Greene - Yale University

Abstract

LMC X-2 is the most X-ray luminous low mass X-ray binary (LMXB) known, as well as one of the five brightest X-ray sources in the LMC. It has been tentatively classified as a Z source based on its high luminosity and X-ray flaring activity. Its location in the LMC provides a known distance and low line of sight absorption, neither of which are easy to determine for Galactic LMXBs. In addition, the low metallicity of the LMC offers the opportunity to compare the physical properties of LMXBs in different environments. However, despite extensive observation, no orbital period has yet been established for this system. Orbital period searches have produced contradictory results, with evidence for photometric and/or spectroscopic periods of 6.4 hrs (Bonnet-Bidaud et al. 1989), 8.15 hrs (Callanan et al. 1990) and 12.5 days (Crampton et al. 1990).

We present synoptic V band photometry of LMC X-2 obtained with the YALO 1-m telescope at CTIO. LMC X-2 was observed for 1 hr each night for 45 nights from 9 Nov 1998 to 8 Jan 1999. We find LMC X-2 to be variable over a range of V=17.9-18.9. Our observed light curve is very complex, exhibiting modulations of up to 0.5 mag in the brightness level from night to night in addition to ~0.1 mag variability during the nightly observations. Using the CLEAN and PDM algorithms we find the strongest periodic signal at 8.18 hrs. If LMC X-2 is indeed a Z source with an ~8 hr period, this would challenge current evolutionary scenarios in which Z sources contain an evolved mass donor and thus a long orbital period. Due to the random variability component apparent in our lightcurve, additional photometry is unlikely to yield a more accurate determination of the orbital period. Therefore, radial velocity observations of LMC X-2 are urgently needed to confirm a probable 8 hr orbital period.

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The Orbital Period of the Double-lined Cataclysmic Variable Phe 1
Jessica Kim-Quijano - Towson University

Abstract

We present an orbital period for the double-lined cataclysmic variable (CV) Phe 1, derived from time-resolved CCD photometry using the 0.9-m telescope at the Cerro Tololo Inter-American Observatory. The CV nature of Phe 1 was first confirmed spectroscopically in 1997; in addition to emission lines of H I and He I, TiO bands characteristic of the late type secondary star were observed. A period of 0.27 d has been determined with the Phase Dispersion Minimization algorithm from the differential V and I light curves taken over 6 consecutive nights in 1998. The light curves show a full range of variation of approximately 0.7 and 0.4 mag in V and I, respectively. When the data are folded on this period, two orbital minima of different depths are observed. Three mechanisms may contribute to the variability observed in Phe 1: primary and secondary eclipses, ellipsoidal variation of the distorted secondary star, and/or viewing of its irradiation-heated inner face. A radial velocity study to determine the component masses is currently underway.

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The Evolution of SN1987A Debris: 12 Year Light Curves in UBVRIJHK
Alicia Soderberg - Bates College

Abstract

HST observations show that the nitrogen-rich circumstellar material surrounding SN1987A is a significant source of contamination in ground-based photometry for the SN debris. We analyze six epochs of WFPC2 PC images (1994.8--1999.0) in broad-band filters F336W, F439W, F555W, F675W, and F814W (HST UBVRI) and four epochs in narrow-band filters F656N, F658N, and F502N. Our corrected light curves demonstrate that flux from the circumstellar ring dominates flux from the debris in the ground-based data set after day 1195 (JD = 2448047). On day 3268, the magnitudes for the SN debris were U,B,V,R,I = 19.75, 19.85, 19.97, 18.91, 19.30. The magnitudes for the ring (including the faint star superimposed on the ring) were U,B,V,R,I = 18.29, 19.21, 18.36, 16.44, 18.41.

We have fit the slow change in the ring magnitudes over the last five years to provide an extrapolation back to the epochs of ground-based observations. These extrapolations are used to remove the ring flux from the ground-based photometric measurements, which yields a uniform set of UBVRIJHK light curves for the debris of SN1987A over its past 12 year evolution.

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Supernovae Type Ia and Their Host Galaxies
Ricardo Demarco - Pontificia Universidad Católica de Chile, Santiago

Abstract

The study of the absolute luminosities of 29 supernovae (SNe) type Ia in the Calan/Tololo survey confirms a relation between the peak luminosity of the SNe and the decline rate as measured by the light curve. Moreover, there is a relation between the luminosities of the SNe and their environment: late-type galaxies contain the brightest SNe and their light curves show a slow-decline rate, whereas elliptical galaxies have not produced slow-decline SNe. Our project has as a goal to verify the relation between the peak of the light curve and the morphological type of the host galaxy by updating the morphological type of the host galaxy for 4 SNe of the Calan/Tololo survey and considering as well 8 high-z SNe. However, the focus of the project was not the SNe themselves, but rather their host galaxies. The data set consisted of 12 SNe type Ia (SN1992ae, SN1992au, SN1992bi, SN1992bp, SN1992br, SN1995K, SN1995ao, SN1995ap, SN1996F SN1996G, SN1996H, SN1996U), four of which (SN1992ae, SN1992au, SN1992bp, SN1992br) were from the Calan/Tololo survey and the rest from the high-z search. All the images used for this project were taken with the WFPC2 of the HST with the filters F450W (for Calan/Tololo) and F702W (for high-z). The redshift ranges from 0.01 < z < 0.62 within the sample.

First of all, the calibration files for each image were updated. Then, the position of the host galaxy in the HST image was found for each SNe. Because of the resolution of the HST images, it was possible to update the morphological type of the host galaxy for the 4 SNe in the Calan/Tololo survey. Moreover, it was possible to determine roughly the morphological type of the host galaxy for the high-z SNe. In some cases, however, this work was quite difficult, and in others impossible, because of the low surface brightness of the galaxies. Finally, the galaxy types and the delta-mag(15) vs. galaxy type plot were updated. The results obtained from these updated galaxy types are in agreement with the relation between the peak luminosity of SNe Ia and the host galaxy type. We also obtained the position of the SN with respect to the center of its host galaxy and a reference star. Finally, we performed aperture photometry on each galaxy, to determine the instrumental magnitudes. Because of the low surface brightness of some of the galaxies, picking the correct aperture was difficult. Furthermore, in some cases the photometry was complicated by the presence of a bright nearby object, such as a star or galaxy. In the future we plan to determine the physical distance to each galaxy, assuming a universe with a positive cosmological constant. In addition, we will make a better estimate of the errors in the distance measurements and determine the standard magnitudes for the host galaxies.

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