2001 REU/PIA Projects

The Search for the Orbital Period of LMXB 4U1556-605

Gabriel Brammer - Williams College, Massachusetts

Abstract

We present photometry results of the search for the orbital period of the low mass X-Ray binary (LMXB) system 4U 1556-60. The orbital period is the first parameter needed before the other geometric parameters and physical properties of a binary system can be determined. The optical counterpart for the X-ray source of 1556-60 was identified by Charles et al. (1979), and subsequent observations failed to detect significant periodicity in optical variations. Smale (1991) proposed a possible period of 9.1hrs (0.3807 days) but added that a further independent study would be needed for confirmation.

The data obtained for this project were obtained on three separate runs, the first with the CTIO 1.5 meter telescope from April 29 to May 5, 1997, and the second and third with the CTIO 0.9 meter telescope from May 22 to 29, 2000 and from June 17 to 23, 2000. We have 170 400s V filter exposures from the 1997 run, and we have 174 450-900s V and 122 600-900s I exposures from the combined runs from 2000. We observed the source to vary between 0.2 and 0.6 magnitudes nightly as well as a larger time scale brightening visible in the 2000 light curve (Figure 2). The time coverage and size of our data sets are both improvements over previous published observations, and should be capable of revealing the orbital period if photometrically possible. Another goal of this project was to correlate simultaneous X-ray observations taken with the RXTE satellite over two days of the 1997 run, but there was not sufficient temporal overlap in the optical and X-ray data sets to perform the correlation.

We used the CLEAN program to search for periods in the data separated into two sets from the two epochs. We normalized the data from 2000 to the mean value to remove contributions from the overall brightening observed which we believe to be unrelated to the orbital period. The CLEAN program performs a fourier analysis of the data and iteratively subtracts spectral contributions due to sampling, such as gaps in the data and the finite width of the data set itself, and unfortunately, our light curve hints that the orbital period could be close to 1 day, which is confounded by the earth's rotation.

Analysis of the two data sets with CLEAN did not provide a definitive result for the orbital period of 1556-60. We did find candidate periods of 0.46229 days and 0.43993 days for the 1997 and 2000 data, respectively, and the data folded on those periods are presented in Figures 3 and 4. Neither data set provided any evidence for the 0.3807 day period proposed by Smale (1991). There are many factors that could complicate the search for the orbital period of this system. First, the inclination angle of the system could be low, greatly decreasing the amplitude of optical variations. Second, there are is a large nightly scatter in the light curve that could be caused by X-ray flickering of the accretion disk and nonuniform heating of the normal star due to the disk. We conclude that further photometry will not be able to detect the orbital period of 4U1556-60, and that spectroscopic radial velocity measurements are needed for further investigation of the system.

Figures:

Figure 1: Light Curve of LMXB 4U1556-60 from the 1997 observations.

Figure 2: Light Curve of LMXB 4U1556-60 from the 2000 observations.

Figure 3: Data from 1997 folded on the period candidate of 0.46229 days.

Figure 4: Data from 2000 folded on the period candidate of 0.43993 days.
 

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Star Formation in the Isolated Molecular Cloud NGC 1788
Kathy Cooksey - Valparaiso University, Indiana

Abstract

In the standard model of star formation, UV radiation from OB stars or supernovae shockwaves compress the cold material in giant molecular clouds (GMCs). The denser areas or cores collapse due to self-gravity and accrete material until there is enough pressure to ignite nucleosynthesis, marking the beginning of the proto-star phase. This model successfully describes the formation of large conglomerations of stars. However, the spatial distribution of stars in the universe cannot be fully explained if stars only form in large clusters. Thus, effective star formation in isolated molecular clouds, further from the massive complexes but most likely still induced by them, offers an explanation to the distribution of stars in our universe. The region around NGC1788 is such an isolated cloud. We are performing a multi-wavelength survey of the area around NGC1788 in order to characterize how star formation proceeds in isolated molecular clouds.

Background

NGC1788 is a reflection nebula about fifty parsecs to the west of the Orion (see Figure 1).1 LDN1616 is the dusty nebula encompassing NGC1788 and the surrounding area. Actually, LDN1616 merges with LDN1615 in a cometary shape (see Figure 2a & b). The exact distance to the nebula has many literature values ranging from 360 to 460 parsecs.2,3 The majority of the distance estimates are calculated by using the brightest star illuminating the cloud as a standard candle. In the case of NGC1788, that star is HD293815, a B9V star. The vast variation in distance estimates may be due to different calculations of the extinction, which stems from LDN1616/15. It is suspected that the star formation in NGC1788 has been induced most likely [by] the UV radiation from the past and/or present O stars in the Ori OB1 Association.3 This pressure may also be tearing the cloud apart (see Figure 3).2

Progress

We are cataloging what objects around NGC1788 appear in which wavelengths. We are nominally working with optical, infrared, and x-ray data but also with some radio data. We observe the region in optical [CTIO 0.9-m] and infrared [CTIO 1.5-m] ourselves and utilize collaboratorsA’ data [ROSAT X-ray sources and ESO 2.2-m optical] and web-based database [2-micron All-Sky Survey] for all wavelengths. We have taken optical spectra {CTIO 1.5-m] and have a collaborator's radio data. By comparing the objects in NGC1788 to regions of known star formation, we can see how effective and what stage the star formation is in NGC1788. For example, we can compare color-color and color-magnitude diagrams (CCD and CMD, respectively) for our region (see Figures 4 and Figures 5 a & b) to those of known star formation. The majority of our objects fall above the main sequence on the CMD, implying that they are young.

In general, we are examining NGC1788 at multiple wavelengths so that we can assess the state of star formation in the region. The standard model of star formation in GMCs does not sufficiently explain the spatial distribution of stars in our universe. We endeavor to characterize effective star formation in an isolated cloud so that we might find a process to compliment the standard model.

References

1 Ramesh, B. MNRAS 276:923-932. 1995 Apr. 27. p293
2 Racine, Rene. AJ 71:233-45. 1968 May. p233
3 Ogura, Katsuo and Sugitani, Koji. Publ. Astron. Soc. Japan 15:91-98. 1998 Aug. p91

Figures and Tables

NGC1788: UBVRI false color image (13.5'x13.5') ~ I made this from data my advisor Stefanie and I took at CTIOA’s 0.9-m telescope. We imaged NGC1788 in U, B, V, R, and I filters. Then I used LinuxA’s Gimp to color U, B, V, R and I images as purple, blue, green, red, and orange-red. I combined the short images with the long ones by having Gimp "multiply" the imagesA’ pixel values; this enabled the point source details of the short exposures to combine somewhat with the nebulosity of the long ones. I changed the contrast of each filter image in order to bring out the most detail of the center region and the nebula. I then overlaid V, R, B, U, and I (in that order) on each other and combined them with the "screen" function, which created a nice blend. JPG (59 kb)

Figure 1: Molecular clouds of Orion complex (30° x 40°) ~ NGC1788/L1616 is cloud number 13 at about (5h 6m, -3d 30m). "Schematic diagram of the molecular clouds: the lowest contour from Fig. 2. Dots with numbers corresponding to those in Table 1, indicate locations of CO emission peaks. Some NGC numbers indicate the optical prominent objects coincident with CO peaks. The extent of UV emission from BanardA’s loop is indicated by the shaded arc (from OA’Dell, York, and Henize 1967; Isobe 1973). The dashed line roughly indicates the extent of the lamda Ori ring of clouds." (Maddalena, Ronald J., Morris, Mark, Moscowitz, J., and Thaddeus, P. The Large System of Molecular Clouds in Orion and Monoceros. ApJ 303:375-391. 1996 Apr. 1. p379) JPG (77 kb)

Figure 2a: Cometary clouds around Orion complex (15° x 25°) ~ NGC1788/L1616 is cloud number 3 at about (5h 5m, -3d 30m). The image shows how most nebulae seem to be being blown away from Sigma Orionis. Ramesh believes Epsilon Orionis is the star inducing the star formation. "Surface distribution of objects in Table 1. Ticks indicate the directions of their tails." (Ogura, Katsuo and Sugitani, Koji. "Remnant Molecular Clouds in the Ori OB 1 Association." Publ. Astron. Soc. 15:91-98. 1998 Aug. p97) JPG (49 kb)
Figure 2b: Cometary shape (DSS 1st Gen. 30'x30') ~ HD293815 is marked, as well as L1616 (the head) and L1615 (the tail). The image is a thirty by thirty arcminute Digital Sky Survey 1st generation image. JPG (114 kb)

Figure 3: Proper Motion (DSS 2nd Gen 15'x15') ~ Tentative proper motion vector image of objects around NGC1788 region. A Digital Sky Survey (DSS) image (North up and East left) of 15'x15' around NGC1788 tentatively marked with the proper motion vectors of the Hipparcos and Tycho Catalogues objects (HIC and TYC, respectively). The HIC objects have five digit number corresponding to the catalogue number while the TYC objects are arbitrarily numbered. The TYC has larger errors than the HIC, and as of now, this is just a temporary, rough visual of the motions of the objects. HIP23837 is a ROSAT X-ray source but from the distance eximate (658pc), probably is not part of the cluster. (see Table 2) It is not known which are foreground, background and NGC1788 region objects. It could be that the cloud is disintegrating. JPG (75 kb)

Figure 4a: Near-Infrared Color-Color Diagrams ~ This for objects from the 2-micron All-Sky Survey (2MASS) Point Source Catalogue (PSC) in the central region (see Figure 5c). The x-ray sources (pink points) come from one of our collaborators, and we matched them visually on a 2MASS image, as well as coordinate wise to the PSC. The green and blue lines are the main sequence and giants III lines, respectively. Reddening vectors to account for dust obscuration have not been calculated. 2MASS has a faintness limit of 14.3 for K, which was not taken into account in these diagrams. JPG (29 kb)
Figure 4b: Color-Magnitude Diagram ~ This has the same properties as above (K is apparent magnitude). For the most part, the objects fall above the main sequence, as objects in a star-forming region should. The main sequence lines in this diagram were corrected for distance, which still needs to be verified. JPG (26 kb)
Figure 4c: CMD of IC348 ~ This is another star forming region, with main sequence line and reddening vectors (solid and dashed lines, respectively). (Lada, Elizabeth A. and Lada, Charles J. Near-Infrared Images of IC 348 and the Luminosity Functions of Young Embedded Star Clusters. AJ 109:1682-1996. 1995 Apr. p1687) JPG (58 kb)

Figure 5a: Optical Color-Color Diagrams ~ Color-color magnitude diagram. The optical data comes from one of our ESO collaborators. We calibrated their B, V, R, and I instrumental magnitudes with the transformation equations given and matched the objects in all filters. Once again, we took what we believe are the closest objects in ir.fits (see Figure 5.c). There are nine x-ray sources that have matches in the optical. The majority lie above the main sequence, as proto-stars or young stellar objects should. JPG (26 kb)
Figure 5b: Color-Magnitude Diagram ~ The tail end of the main sequence ends about M6. The X-ray sources are once again entering the main sequence. However, since this is optical, extinction is much worse than in the near-infrared. We have not taken that into account for this diagram. JPG (25 kb)
Figure 5c: Near-Infrared Image of Center ~ The objects in our CMD and CCD are taken from matching objects within the infrared image. The image is about 13A’ long and 8A’ wide. Only a few x-ray sources fall within this range, but they are all included in the diagrams. JPG (10 kb)
Figure 5d: Optical Image ~ This optical image is the same size, scale and location as the previous image. Comparing the two shows the extinction of the region. JPG (38 kb)

Figure 6a: Optical Spectra Part 1 ~ This is low-resolution optical spectra from CTIO's 1.5-m taken by Stefanie. They are of the ROSAT x-ray sources in our region of interest (see Table 1). We are looking for Lithium absorption at 6708A, which is a characteristic of T-Tauri stars (young, low-mass stars). JPG (38 kb)
Figure 6b: Optical Spectra Part 2 ~ More x-ray optical spectra... JPG (38 kb)
Figure 6c: Optical Spectra Lithium ~ This is an enlargment of the 6700A area to better see the Lithium absorption. JPG (38 kb)

Table 1: Multi-Wavelength Matches ~ View and/or download the table of correlated x-ray, near-infrared and optical objects. PDF

Table 2: Proper Motions ~ View and/or download the table of Hipparcos and Tycho Catalogue sources in the central region of NGC1788 (such data as distance and proper motion vectors). The objects are also cross-listed with the X-Ray sources. PDF

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HII Region Analysis in Dwarf Irregular Galaxies: The Slog and HIIphot
Shadrian Holmes - College of Charleston, South Carolina

Abstract

We have CTIO 4-m telescope + Mosaic wide-field CCD camera images taken in UBVRI and Halpha, [SII], and [OIII] of three Local Group dwarf Irregular galaxies: NGC 6822, WLM, and Phoenix, taken as part of the Survey project. As a step towards understanding the mode of star formation in these galaxies, we are employing an automated method to study the HII regions. A multi-step project, we are currently investigating the differences between HII region analysis done in a 1988 study of NGC6822 and the automated analysis, and developing a reference guide for Mosaic reductions. A complete sample of HII regions would be used to compare the properties of the HII regions (through luminosity functions and distribution of morphologies) with those in other Local Group galaxies.

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Kinematics of the Bipolar Planetary Nebula Sa 2-237
Rodolfo Montez - University of Texas, Austin

Abstract

We present H-alpha+[NII] images and long-slit, high resolution echelle spectra in the H-alpha and [OIII]500.7nm regions of Sa2-237, a possible planetary nebula. The image shows a bipolar nebula of about 34'' extent, with a narrow waist. The long slit spectra were taken over the long axis of the nebula, and show a distinct "eight'' shaped pattern and a maximum projected outflow velocity of 211 km/s, both typical of expanding bipolar planetary nebulae. By model fitting the shape and spectrum of the nebula simultaneously, we derive the inclination of the long axis to be 70° and the maximum space velocity of expansion to be 308 km/s. We use the IRAS fluxes, the radio flux, the energy distribution, and the projected size of Sa2-237 to estimate it's distance to be 2.1kpc. At this distance Sa2-237 has a luminosity of 340 Lsun, a size of 0.37pc, and assuming constant expansion velocity a nebular age of 624 years.

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Optical Light Curve of SN2000cx
Jorge Cuadra - Pontificia Universidad Católica de Chile, Santiago

Abstract:

We have measured the light curve of SN2000cx (Fig.1), a Type Ia supernova (SN) located in the outer part of NGC 524 (an S0 galaxy), using observations made at CTIO telescopes (10 nights UBVRI at 36inch telescope and 20 nights BVRI at the YALO telescope). Using the standard analysis, we derive a substantial negative reddening in the host galaxy. Implication of this result is that relations between colors and decline rate for type Ia SNe are not universal.

Observations and reductions

From the 36 inch telescope observations we measured aperture photometry of the SN and Landolt standard fields. Seven of these nights were photometric and we used them to determine the transformation equations between instrumental and library magnitudes. With these equations we determined the magnitude of the SN every night and the brightness of others stars in the field, "local standards".
With the YALO observations we measured PSF photometry of the SN field and used the local standards magnitudes to find the brightness of the SN each night (Fig. 2). For a more detailed description, see the procedure described by Suntzeff et al.(1999, AJ 117:1175)in sections 2.2 and 2.3.
Then we added Kevin Krisciunas's data, observed and reduced independetly of ours. We see that the two data sets aproximately agree, but in some phases there are big differences (worst case ~0.2mag). However, in the bands and times that we used in the following analysis they are very close (Fig. 3).

Analysis

We used Mark Phillips's programs to compute the reddening in the host galaxy (Phillips et al. 1999, AJ 118:1766), obtaining the surprising results:
The SN's (B-V) color at tail is bluer than we expected even for an unreddened SN. We are computing a physically impossible negative reddening: E(B-V)=-0.235+-0.051 (Fig. 4).
Using the "colors" defined by the differences between the maximum magnitudes of each curve we find with Bmax-Vmax a big reddening, E(B-V)=0.176+-0.104, but with  Vmax-Imax we again find a negative one, E(B-V)=-0.128+-0.88 (Fig. 5). All results are corrected by Galactic reddening.
Also we used the Tonry et al. distance to the galaxy (2001, ApJ 546:681), scaled to H0=65 km s-1 Mpc-1, and the Galactic absorption to compute the absolutes magnitudes at maximum light of this SN. The results are B=-19.093, V=-19.201, I=-18.709 (uncertaints ~0.2mag). This SN is also fainter than we expected (Fig. 6).

Conclusions

This SN is in the outer part of an early type galaxy. We expected it to have little reddening, but Bmax-Vmax implies a big one.
Others methods show a physically impossible negative reddening.
Finally, the light curve of this SN looks normal but we found this object having intrinsic colors different that we expected. This tell us that the relations between colors and decline rate are not as universal as we thought.

Figures

Figure 1: Color image of the SN field, the SN is below and right of the brightest star.
Figure 2: CTIO light curve.
Figure 3: CTIO + Kevin's light curve.
Figure 4: (B-V) color at tail, the blue line is what we expect for an unreddened galaxy.
Figure 5: Bmax-Vmax and Vmax-Imax "colors" of SNe vs. dm15 (taken from Phillips et al. 1999), the blue circle is SN2000cx.
Figure 6: Maximum magnitudes vs. dm15 (taken from Phillips et al. 1999), the red circle is SN2000cx.
 

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Type Ia SN 1999ac: The Most Complete Optical and IR Light Curves Ever Obtained for Such an Object
Erika Labbe - Pontificia Universidad Católica de Chile, Santiago

No abstract available. Sorry for the inconvenient

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