2000 REU/PIA Projects

A Search for Dwarf Novae in Globular Cluster 47 Tucanae - Melanie Blackburn - West Virginia University


Until recently, no dwarf novae have been known to exist in globular clusters. Because of the density of stars that form these clusters, one may expect many such binary systems. Dwarf novae are easy to detect because of their relatively short recurrence times, ranging from 20 to 300 days, as compared with the 10,000 (or more) year recurrence times of classical novae. Dwarf novae also have amplitude outbursts of 2-6 in magnitude, making them easily visible. 47 Tucanae has been chosen for observation, as it appears in the right location at the right time of year, and has characteristics of an average globular cluster with moderately poor metallicity. This helps minimize any difference in metallicity of the dwarf novae, while still allowing for study of these differences.

Data obtained over a period of five months from the 1m YALO telescope at Cerro Tololo Inter-American Observatory has been reduced and subtracted. To date, several variable stars have been found through visual means, but no dwarf novae. Further analysis of this data, as well as new data, may provide a more optimistic outcome. If found, study of these dwarf novae will provide a look at the stellar dynamics of globular clusters. For now, we can conclude that dwarf novae are indeed rare in globular clusters.

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Identifying the Globular Cluster Systems of the Sculptor Group - John Bright - Mesa State College, Colorado


We present results from a study to identify the globular cluster systems of six of the brightest galaxies of the Sculptor Group: NGC 45, NGC 55, NGC 247, NGC 253, NGC 300, and NGC 7793. Our observations were made with the CTIO 4-m telescope, Mosaic II camera, and CMR filter set. The 30 arcmin field of Mosaic II captures the entire expected extent of the globular cluster systems of these galaxies in a single pointing of the telescope. We identify the cluster candidates through their morphology and C-R and M-R colors. Selecting objects by isophotal area, isophotal flux, ellipticity, C-R and M-R, we find 100-150 objects per galaxy with properties similar to known globular clusters in NGC 253. We estimate that half of these objects are true clusters, while the other half are contaminating background galaxies. With spectroscopic follow-up, these cluster candidates will provide important clues to understanding the formation of globular clusters in small galaxies like the LMC and M33.


Did halos form before disks in late-type galaxies?

Some evidence FOR:

    * Milky Way's halo globular clusters are among oldest objects in the Universe, with ages 12-15 Gyr
    * Milky Way disk appears to be ~10 Gyr old

Some evidence AGAINST:

    * Halo formation is a continuing process. Example: infall of Sagittarius dwarf into Milky Way (Ibata et al. 1994), which is contributing ~4 globular clusters to the halo (Da Costa & Armandroff 1995)
    * The LMC's oldest globular clusters are as old as the Milky Way's oldest clusters (Mighell et al. 1996, Olsen et al. 1998, Johnson et al. 1999), yet they have disk kinematics (Schommer et al. 1992)
    * M33's clusters have halo kinematics (Schommer et al. 1991), but many have exclusively red horizontal branches, suggesting intermediate age (Sarajedini et al. 1998)

What is the dominant formation site of globular clusters in late-type galaxies?


The identification of clusters in late-type galaxies is difficult because the number of clusters is small compared to the number of contaminating foreground stars and background galaxies. We circumvent this problem by:

    * Observing galaxies which are nearby enough to partially resolve the clusters yet far enough away to capture the bulk of a cluster system in a single image with a large-format CCD camera
    * Using the automated program SExtractor (Bertin & Arnouts 1996) to create a database from which we select likely candidate clusters

Our observations were taken with the CTIO 4-m telescope and Mosaic II camera on the nights of Nov 11-13, 1999, two of which were photometric. We used three filters, Kron-Cousins R and Washington C and M, which combined give improved age-metallicity discrimination over BVR. Exposure times were chosen to achieve S/N of 100 in R for clusters as faint as Mv = -6. The Mosaic II camera delivers excellent image quality over a 30'x30' field of view; combined with good seeing ( 1 arcsec), Mosaic II lets us discriminate candidate clusters from stars and background galaxies over a 20x20 square-kpc area at a distance of 2 Mpc, typical of the nearest Sculptor Group galaxies.

The processed and stacked images were analyzed with SExtractor:

    * Detection and measurement limits were set by the limiting surface brightness, 3-sigma above the summed noise
    * Likely cluster candidates selected by isophotal area, isophotal flux, ellipticity, C-R and M-R colors

Beasley & Sharples (2000) have spectroscopically identified 14 globular clusters in NGC 253. These known clusters were used to test and tune our selection criteria, described in Fig. 1.


Beasley, M. A., Sharples, R. M. 2000, MNRAS, 311, 673
Bertin, E., Arnouts, S. 1996, A&A, 117, 393
Da Costa, G. S., Armandroff, T. E. 1995, AJ, 109, 2533
Ibata, R. A., Gilmore, G., Irwin, M. J. 1994, Nature, 370, 194
Johnson, J. A., Bolte, M., Stetson, P. B., Hesser, J. E., Somerville, R. S. 1999, ApJ, 527, 199
Mighell, K. J., Rich, R. M., Shara, M., Fall, S. M., 1996, AJ, 111, 2314
Olsen, K. A., Hodge, P. W., Mateo, M., Olszewski, E. W., Schommer, R. A., Suntzeff, N. B., Walker, A. R. 1998, MNRAS, 300, 665
Sarajedini, A., Geisler, D., Harding, P., Schommer, R. 1998, ApJ, 508, 37
Schommer, R. A., Christian, C. A., Caldwell, N., Bothum, G. D., Huchra, J. 1991, AJ, 101, 873
Schommer, R. A., Suntzeff, N. B., Olszewski, E. W., Harris, H. C. 1992, AJ, 103, 447


Figure 1: The area within the isophote having surface brightness 3-sigma above the background (per square-arcsec) is plotted vs. the flux within the isophote, for objects measured in a stacked R image of NGC 253 (1 arcsec seeing) using the program SExtractor. Only those objects with e < 0.4, 0.25 < M-R < 0.8, and 0 < C-R < 2.4 are shown. Objects with spectra from Beasley & Sharples (2000) are marked with filled circles for globular clusters, diamonds for background galaxies, and asterisks for stars. A number of the Beasley & Sharples objects fall outside our selection criteria or were masked as lying too close to the galaxy disk, and so are not plotted here. The line shows the locus defined by a scaled PSF measured from a single bright star. The majority of the globular clusters lie above this line but below the sequence defined by the galaxies. The box encloses 100 objects of unknown nature, 50% of which we expect are globular clusters in NGC 253, as estimated from the number of clusters and galaxies within the box. JPG

Figure 2: Color-color diagram of globular clusters (filled circles), background galaxies (diamonds), stars (asterisks), and candidate clusters picked from the box of Fig. 1 (small open circles) in NGC 253. A typical error bar, representing both random photometric error and uncertainty in the calibration, is shown in the lower left-hand corner. The lines represent three composite stellar populations simulated from the models of Schaerer et al. (1993), with ages of 1 Gyr (solid line), 4 Gyr (dotted line), and 12 Gyr (dashed line). The lengths of the lines correspond to the range of metallicities -1.7 < [Fe/H] < 0.0. Although the combination of photometric error and age-metallicity degeneracy prevents us from deriving precise ages and abundances, the bulk of the identified globular clusters appear to be of old or intermediate age, as already noted by Beasley & Sharples (2000). The bluest appears young, perhaps analogous to the blue globular clusters of the Magellanic Clouds. Our candidate clusters, if confirmed through spectra, appear to contain a balanced distribution of young and older clusters. JPG 

Figure 3: The area within the isophote having surface brightness 3 above the background (per arcsec2) is plotted vs. the flux within the isophote, for objects measured in a stacked R image of NGC 45, NGC 55, NGC 247, NGC 300, and NGC 7793 using the program SExtractor. Only those objects with e<0.4 are shown. The box edges are as defined in Fig. 2, after scaling for differences in distance and exposure time. Each box encloses 100-150 candidate clusters. JPG

Figure 4: A GLOBULAR CLUSTER IN NGC 253: This closeup of a globular cluster spectroscopically identified by Beasley & Sharples (2000) shows the object to be round yet noticeably broad compared to the profile of a nearby star. Our analysis of this image has turned up 100 objects with properties similar to previously identified globular clusters. JPG

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A Search for the Orbital Period of LMXB X0614+091 - Ben Johnson - University of California, Los Angeles


We present extensive new photometry of the low mass X-ray binary (LMXB) X0614+091. Discovered in 1974, its X-ray properties have identified it as a member of the so-called Atoll sources. Atoll sources are thought to have main sequence mass donors and relatively short (< 8 hours) orbital periods. Early observations of the faint (V=18.5) optical counterpart revealed an unusual spectrum that only showed the NIII/CIII 4640/50 emission complex. LMXB spectra usually always also exhibit HeII 4686 emission. Previous variability searches detected a 5.2 day modulation in X-rays (Marshall & Millit 1981) and a ~10 day periodicity in the optical (Machin et al. 1990). If either one of these modulations reflects the orbital period, mass transfer via Roche lobe overflow would require a giant secondary, inconsistent with the observed optical properties of the source. An alternative cause for the variations is precession of the accretion disk in the system, though it is not clear how these two periods can be reconciled.

We obtained V band observations of X0614 with the queue scheduled YALO telescope at CTIO between UT 1999 Dec 10 and 2000 Jan 31 in order to confirm the reported ~10 day variability and search for an orbital period. Comparison with Rossi X-ray Timing Explorer (RXTE) All Sky Monitor (ASM) data reveals that some of the optical observations occurred during a rare state of decreased X-ray variability of the source. Comparison of optical observations from the periods of high and low X-ray variability shows several differences. First, optical variability increases during the period of high X-ray variability, with an increase in the amplitude of variability from 0.1 mag to 0.25 mag. In addition, this increased optical variability seems to be over longer time scales than during the X-ray constant period. Second, the average optical magnitude decreases during the period of high X-ray variability, while the average X-ray magnitude increases.

We used the CLEAN algorithm to remove the sampling window and search for periodicities. No convincing periodicities were found in the complete optical lightcurve. Due to the differences in optical variability during the high and low X-ray variability states the data was split into two sets for further analysis. For the low X-ray variability state a peak in the power spectrum occured at 1.06 days. For the high X-ray variability state the highest peak in the power spectrum occurs at 1.11 days. Figure 4 shows a strong periodicity at this peak. No evidence of a 5.2 day periodicity was found. While the complete data set does not show a 9.8 day periodicity, the optical lightcurve of just the high X-ray variability state folded on 9.8 days reveals a possible periodicity. No comparable (~10 day) periodicity was detected in the low X-ray variability state.


Figure 1: Optical (red circles) and X-ray (blue circles) lightcurve of X0614+091 showing anticorrelation of optical and X-ray brightness. A comparison star (black circles) is also plotted.

Figure 2: Optical (open circles) and X-ray (solid circles) lightcurve of X0614+091 showing rare state of decreased X-ray variability

Figure 3: Optical light curve of X0614+091 during decreased X-ray variability state, folded on a period of 1.06 days

Figure 4: Optical light curve of X0614+091 during increased X-ray variability state, folded on a period of 1.11 days.

Figure 5: Optical light curve of X0614+091 during increased X-ray variability state, folded on a period of 9.8 days.

RXTE ASM data provided by the ASM/RXTE teams at MIT and at the RXTE SOF and GOF at NASA's GSFC

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Optical Counterparts of X-ray Sources in the Small Magellanic Cloud
Tanya Tavenner - University of Washington


We searched for optical counterparts to X-ray sources in the SMC. We looked at thirteen fields observed with the CTIO 1.5-m telescope for potential canidates. These fields were selected from a ROSAT survey of the SMC that found about 250 X-ray sources. These thirteen fields were chosen because of their weak, hard X-ray emissions, which might correspond to cataclysmic variables. These fields also had the smallest pointing errors from the X-ray satellite. Each field was observed in the optical UBVRI passbands.

A region approximately twice as large as the pointing error given by the satellite was studied in each field. Standard magnitudes were obtained for every star in this region, from the standard stars which were viewed each night of the run. These standard magnitudes were then used in a ``poor man's spectrum'' by graphing them against their central wavelength. Cataclysmic variables should appear brightest in the blue, but also have substantial red flux. All of the stars that are strongest in the blue are considered to be candidates for the X-ray source. In the future, these candidate stars will be re-observed in order to determine which one is producing the X-rays in each field.


Figure 1: A representative field we observed. The green circle represents the pointing error of the ROSAT satellite. (The linear feature is an unfortunately-located satellite trail.)

Figure 2: X-ray sources are well-known for being located just outside the pointing error box, so this image shows the stars we actually analyzed.

Figure 3: In order to form a ``poor man's spectrum,'' the wavelengths of the filters are graphed against the calibrated magnitudes for each star. This is the poor man's spectrum of four example stars. The orange star stays fairly constant over the spectrum. The red star is brightest in the red wavelengths. The purple star is high in the blue, however it is even higher in the red. The blue star is fairly constant across the spectrum, then goes up sharply in the blue wavelengths. This is the best candidate in this example. JPG

Figure 4: The locations of the four stars in the field.

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Peculiar Motions with a Set of SN Ia distances
Axel Bonacic - Pontificia Universidad Católica de Chile, Santiago


We have studied the cosmological motion of a sample of 40 supernovae (SNe) type Ia, whose distances have been carefully determined, and corrected for reddening. The redshifts of the sample range from 3000-30,000 km/sec. Our best estimates show that the bulk of the SN Ia are at rest in the frame defined by the cosmic background radiation; i.e., the reflex motion of the Milky Way in the SN frame reproduces the CMB dipole with significant accuracy. The bulk flows in this frame are typically 200+/-200 km/sec. Most of the signal comes from the sample between 3000 and 15,000 km/sec. At higher redshifts the precision of the SN Ia distances (typically 10%) are insufficient to detect typical peculiar motions (our signal becomes dominated by noise). The "convergence depth" and scale of these motions appears to be about 10,000 km/sec. These results are at variance with the large bulk motions measured by Lauer and Postman (1994) and Hudson et al (1999). Our motions and scale appear to be consistent with cosmological simulations of CDM, which predict the gravitational accelerations of typical observers given the expected distribution of matter in the universe, when scaled by the abundance of rich clusters.


Figure 1: Redhisft bin vs.\ mean velocity in the bin for the data set.

Figure 2: Direction of the dipole determined from the SNe velocities. Asterisk = 0.001 < z < 0.09 shell; square = 0.01 < z < 0.05 shell; triangle = 0.05 < z < 0.09 shell; diamond = 0.035 < z < 0.09 shell. The crosses are, from ``north'' to ``south,'' the direction of dipoles of: (1) Riess result, (2) Schommer result, (3) CMB result, (4) Lauer and Postman result.

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Testing completeness of and refining the NGSS residual search methods
Juan Seguel - Universidad de Concepción


While observations of distant supernovae (SNe) are providing intriguing new information about cosmological parameters, there is still much work to be done on understanding the details of the various types of SNe and the possible systematic uncertainties in their use as precise distance indicators. Such work is best done with studies of relatively nearby SNe (z<0.1) which are bright enough to obtain detailed photometric and spectroscopic follow-up data for.

NGSS (Nearby Galaxies Supernovae Search) is a search for moderate to low redshift SNe using the mosaic camera at the Kitt Peak 0.9-m telescope, in Tucson, AZ. This 8K x 8K mosaic CCD array provides an areal coverage of ~ 1ºx1º and allows us to search for SNe along the celestial equator to a limiting magnitude of R ~ 21.

Due to the limitations in the search methods the NGSS team uses, it is important to investigate how effective we are in finding SNe candidates in residual images. In doing so, we will be able to determine accurate SNe rates from our search sample, and incompleteness in our searches due to our methods.

We added false stars into the images, used Daophot routines to obtain photometry for false stars, and ran NGSS software to combine and substract images. Finally, we used IRAF scripts to search for "false supernovae" to measure detection efficiency and improve detection algorithms. The preliminary results indicate that generating a good PSF is very important. Factors such as stars near the chip edges, bad pixels, or bad seeing must be carefully considered.


Figure 1: Example of false SNe added into an image field.
Figure 2: Sample PSF of field stars.


I would like to thank Don Hoard, Lou Strolger and Chris Smith for giving me the opportunity to work in the PIA program and NGSS project. This research was carried out as part of the 2000 Research Experiences for Undergraduates Program at CTIO, funded by the National Science Foundation (NSF).

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Updated on April 18, 2024, 6:45 am