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DV Ursae Majoris: An Eclipsing Dwarf Nova
Abstract
DV Ursae Majoris is a cataclysmic variable star system that is located in the Ursa Major constellation. DV Ursae Majoris has an inclination of ~ 90 ° in relation to earth, which means we are viewing it edge-on. This inclination makes the star system unique because light curve observations can be used to identify the patterns that are present in the binary system. DV Ursae Majoris has one white dwarf as its primary star and an M dwarf as its companion star. Each of these stars has a different brightness, and as they revolve around one another, clear maxima, minima, and eclipses can be seen during quiescence. This system experiences a change in brightness as the two stars in the system revolve around one another, but also experiences variations in brightness due to outbursts. An outburst is a change in brightness due to the collapse of the accretion disk onto the surface of the primary star. This paper presents an analysis of two sets of data, one taken in April 1997 during outburst and the second during quiescence in February 1999.Using this data, DV Ursae Majoris’ orbital period and a superhump period were determined.
Introduction
Stars that have masses of ~ 1.4 solar masses spend most of their lives on the main sequence. When all of the star’s initial hydrogen has been converted into helium by fusion reactions, the star grows in size and its outer envelope expands. The star’s core then collapses due to the great amount of gravity that is present within the star. The core will become very small, but fusion continues as the helium changes into carbon and then carbon into heavier elements. As the star continues to fuse into these heavier
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elements, it gives off less and less light and will eventually burn out- becoming first a white dwarf, and then eventually a black dwarf. (Berman) If the star is in a binary system and is the more massive star in the system, it will start to pull matter off the outer envelope of its companion star. Due to the angular momentum associated with the stars’ orbital motions, the matter pulled off cannot fall directly onto the surface of the massive star. The matter is transferred due to the gravitational differences of the stars, and ends up spinning around the primary star, forming a flat elliptical structure called an accretion disk. Gradually the transferred material making up the accretion disk spirals its way inward toward the primary star (Seeds). When the matter pulled off the secondary star comes in contact with the outer edge of the accretion disk, energy is released and a luminous “hot spot” is present (Figure 1). When the disk fills with material, the star system will experience an outburst. The timescale for these outbursts is one of the periods observed in this study. An accretion disk can be compared to a rubber band: when the disk can no longer hold any more matter it will break and all of the matter will fall onto the surface of the star (Armitage). This new matter that has been obtained gives the dying star a short rebirth. But it will soon burn all of this new matter and go back to pulling matter off its companion star. A new accretion disk will then form. (Snow)
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Figure 1: A picture of a binary star system during an eclipse, similar to DV Ursae Majoris, with the M dwarf on the left and the white dwarf on the right (Patterson).
The two stars in a binary star system revolve around a common center. In systems that have inclination ~ 90°, the differences in the light patterns are very vivid. When both stars are in view, and the hot spot is in the foreground as well, you will observe the greatest amount of light from the system, which corresponds to the lowest absolute magnitude value. When the hot spot is not in the foreground and not visible, you have a local minimum, which is brighter than the lowest minimum but not nearly as bright as the maximum. When the M dwarf is in front of the white dwarf you get the least amount of light. When the white dwarf is in front you have a secondary maximum, where the light is brighter then the minimum but dimmer then when both stars are in view. The orbital period, the second period that was observed in this study, was derived from the differences in the timings of the minima. (Seeds)
In DV Ursae Majoris, the primary, more massive star is a white dwarf and the secondary star is an M dwarf. White dwarfs have already left the main sequence and have fused all of their hydrogen into helium. An accretion disk has formed that is fed by its secondary star, the M dwarf. M dwarfs are still on the main sequences with temperatures
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below 4000° K and about 1/3 the mass of our sun. This binary system has an inclination ~90°, so maxima, minima and eclipses can easily be studied using light curves (Zeilik). Therefore, using the timing of these events, an orbital period and a superhump period can be determined. A superhump occurs during outburst, when a variable star system has one extremely bright maximum instead of two fainter, local maxima. (Warner)
Methods
Data was obtained by the CBA (Center for Backyard Astrophysics), an organization headed by Dr. Joseph Patterson. These amateur astronomers from around the world use telescopes that range from 0.25m to 1.3m in size with CCD imagers to obtain brightness (measured in magnitude) readings at exact time intervals (recorded in Julian date). The data was sent via email to Dr. Joseph Patterson in two column files, magnitude and Julian date. Using Gwbasic programs (plotxy), each file was graphed separately, showing only one-night’s data. The timing of each event, including maximum, minimum and eclipse, was measured and an O-C (observed value minus calculated value) analysis was done to refine the orbital period. A log of observations for all of these graphs was created and is shown in Table 1 and Table 2. Then the separate data from each night was zeroed and graphed in one single file. The zeroed files were then run in dft6000, where power spectra were created and then graphed in plotpowa. Finally, the graphs were analyzed to find the superhump period.
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Table 1: Log of observations for DV Ursae Majoris during outburst
UT Date JD
Start (2,450,000+) Duration Points
(V) (Mag)
April 15 548.3791 12.85 748 14.20
April 16 549.3785 4.96 150 14.25
April 18 551.6925 4.79 250 14.65
April 19 552.5375 2.59 128 14.57
April 20 553.3104 0.06 434 14.70
April 21 554.3034 14.38 809 14.93
April 22 555.3180 4.73 135 14.80
April 23 556.3332 4.68 141 14.96
April 24 557.6346 5.75 299 15.09
April 25 558.3263 3.78 112 15.09
Table 2: Log of observations for DV Ursae Majoris during quiescence
UT Date JD
Start (2,451,000+) Duration Points
(V) (Mag)
February 20 211.7061 6.54 192 18.77
February 21 212.6906 8.99 483 18.75
February 22 213.7672 2.34 127 18.78
February 24 215.8497 2.45 134 18.75
February 25 216.9011 0.79 221 18.74
Results
Graph 1 is a light curve of DV Ursae Majoris during quiescence. In the graph the maxima, minima and eclipses can be seen. Using the timing of the eclipses, an orbital period was estimated to be 0.08585 days. Graph 2 shows a light curve of DV Ursae Majoris during outburst. There is a change in the shape of the events that are present. The high maximum that appears is the superhump, and the local maxima and minimum in between these are no longer visible. In DV Ursae Majoris, the superhump period has been approximated to be 0.08865 days. Graph 3 is a power spectrum for DV Ursae Majoris during outburst; the highest peak is the superhump. Because of the change in the light
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curves between outburst and quiescence, when power spectra during quiescence are graphed, the high peak seen during outburst disappears. This difference verifies that the high peak in the power spectrum during outburst is the timing of the superhump, not the orbital period.
The secondary peaks that are present in the power spectrum are harmonic to the fundamental period, but their frequencies do not fall at exact integer multiples, as would be expected. The frequencies of the harmonics are occurring sooner than the multiple integers, which means that the frequencies are slightly too low. This result is different from that found in the research by Harvey and Patterson (1995) in their study of another variable system, CY Ursae Majoris, where the harmonic frequencies are larger than multiple integers of the fundamental.
Graph 1: Quiescence Graph 2: Outburst Graph 3: Power Spectra
Conclusion
DV Ursae Majoris is one of the few known binary stars with an inclination ~90°.
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Data from the Center for Backyard Astrophysics was used to determine the orbital period from the time interval between the eclipses, and the superhump period from the time between the brightness maxima at outburst. This is the first determination of the superhump period for DV Ursae Majoris. Based on the analysis of the harmonics present in the power spectrum, the harmonics were found to occur at lower frequencies than expected. This differs from the frequency shifts found in other systems and could eventually aid in the understanding of the origin of the harmonics.
References
Armitage, P.J., Livio, M., Pringle, J.E. Astrophysical Journal. 1996. 457: 332-
339.
Berman, Louis., Evans, JC. Exploring the Cosmos. 1980. Pg 319-326.
Harvey, D., Patterson, J. PASP. 1995, 107:1055-1058.
Patterson, J. Sky & Telescope. October 1998.
Seeds, Michael A. Foundations of Astronomy. 1990. Pg 202-219.
Snow, Theodore P. The Dynamic Universe. 1985. Pg 389-393.
Warner, B. 1985, in Interacting Binaries, NATO ASI, ed P. Eggleton and JE
Pringle, pg367.
Zeilik, Michael. Astronomy: The Evolving Universe. 1991. Pg 299- 306.