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GD 552: the Oldest Cataclysmic Variable

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GD 552: the Oldest Cataclysmic Variable Hubble Space Telescope Cycle 10 General Observer Proposal GD 552: The Oldest Cataclysmic Variable Principal Investigator: Dr. Joe Patterson Institution: Department of Astronomy, Columbia University 550 W. 120th St. New York NY 10027, USA Telephone: 212 854 3276 Electronic mail: [email protected] Scientic category: HOT STARS Scientic keywords: ERUPTIVE BINARY STARS AND CATACLYSMIC VARIABLES, PECULIAR BINARY STARS, VERY LOW MASS STARS AND BROWN DWARFS, WHITE DWARFS, ASTROMETRY Instruments: FGS, STIS Proprietary period: 12 Cycle 10 primary orbits: 12 Cycle 10 parallel orbits: 0 Abstract A terrible puzzle has long aicted our understanding of the evolution of cataclysmic variables (CVs). Angular momentum loss should grind the binaries down to orbital periods near 1.3 hr in2–4Gyr, and then slowly drive them apart again. Most CVs should therefore have undergone “period bounce” long ago, and be evolving towards longer period, with sec- ondaries 0.1M. However, not a single post-bounce CV has been conclusively identied. Where are the old CVs hiding? They should be hard to nd since they’re probably faint intrinsically, and because their accretion rates may be too low to trigger dwarf-nova erup- tions. One, and only one, good candidate appears in the Lowell proper-motion lists. This is GD 552: noneruptive, possessing a light secondary, and probably the least luminous CV yet found (MV > +12.5). An accurate FGS parallax will establish whether this object (clearly very nearby) signies a large population of very old CVs. A 1200 – 10000 A spectrum would likely represent a pure steady-state low-M˙ disk (the only one known), and the FUV region would provide a measurement of Te in a white dwarf long after eruptive heating episodes have stopped. The FUV observation obviously requires HST, and eorts to measure the parallax and visual/near IR spectrum from the ground are confounded by a background star 000.7 distant. Dr. Joe Patterson GD 552: The Oldest Cataclysmic Variable Investigator Institution Country PI: Dr. Joe Patterson Department of Astronomy, Columbia USA/NY University CoI: Dr. John Thorstensen Dartmouth College USA/NH CoI: Mr Jonathan Kemp Joint Astronomy Centre UK (& CA, NL) CoI: Dr. Edward Sion Villanova University USA/PA Total number of investigators: 4 Number of ESA investigators: 1 (indicated by after name) Observing Summary: Conguration,mode,aperture Total Target RA DEC V spectral elements orbits Flags GD552 22 50 +63 28 16.5 STIS/CCD SPECTRA 3 CVZ G230LB,G430L,G750L GD552 22 50 +63 28 16.5 STIS/FUV SPECTRA 3 CVZ G140L GD552 22 50 +63 28 16.5 FGS POS 6 CVZ Grand total orbit request 12 2 Dr. Joe Patterson GD 552: The Oldest Cataclysmic Variable Scientic Justication Close binary stars are the most accessible settings for the study of accretion, and provide unique opportunities for testing our physical understanding of stars. The cataclysmic vari- ables (CVs), in which matter is transferred from a small late-type secondary onto a white dwarf, have been especially fruitful. They provide a ready-made laboratory for the study of accretion disks and their instabilities, and the physics of binary star evolution. About 1-10% of all stars will eventually go down that highway of evolution, and at least 20% of stars > 2 M (Patterson 1984). Today’s CVs will tell us, once we learn to read the clues, what lies ahead for all these stars. An important insight of the last two decades has been the central role of angular mo- mentum losses (“J˙”) in driving the evolution of CVs. Over most of the life of a CV, J˙ grinds the binary down to shorter and shorter orbital periods as the secondary star shrinks. But eventually, near Porb = 1.3 hr, the timescale for mass transfer becomes shorter than the secondary’s thermal relaxation time, and the (somewhat counter-intuitive) result is that the secondary’s radius grows with time, causing the orbital separation and period to increase. This is known as “period bounce” in theories of binary evolution; it has been predicted for 20 years (Paczynski 1981; Rappaport, Joss, & Webbink 1982; Patterson 1984), but not yet rmly proved. Details depend on the exact prescription for J˙; gravitational radiation (GR) is the most predictable channel, and this is often assumed to dominate. At period bounce, the secondary mass M2 should be near 0.1 M, so the secondary soon loses the ability to burn H, and becomes an object sharing the properties of white dwarfs and brown dwarfs – H-rich, partly degenerate, radiating feebly with an energy left over from an earlier era (thermonuclear). Angular momentum loss continues to push the secondary farther out and make it into essentially a “warm planet”. This is what the oldest CVs should look like. Kolb (1993) calculated the timescales, and predicted that 70 % of CVs should already have passed period bounce. The actual fraction is likely to be even higher, since other J˙ losses will add to GR and only hasten evolution (Patterson 1998). But where are all these predicted stars? The vast majority of studied CVs have secondaries which are relatively “normal”, with masses and radii in rough agreement with the main sequence. This is illustrated in Fig. 1, which shows the masses and mass ratios deduced from the study of accretion-disk precession rates and calibrated by eclipsing systems (see Mineshige et al. 1992, Patterson 1998, Murray 2000, and Figure 2 for details). Theory predicts that CVs should evolve along a curve like the ones shown, and the distribution of points does suggest such a track (with a substantial preference for the “GR+” theory). But the rate of evolution, indicated by the numbers on the curves, shows that most CVs should have already bounced and be evolving on the lower branch, whereas the data clearly show that only a few can be on the lower branch – and even they can have bounced only recently. So this comparison of theory with observation suggests a severe undercount of very old CVs. In part this must arise from the intrinsic faintness of old CVs. Very low mass ratios render the accretion light feeble, with predicted mass transfer rates M˙ in the range 12 11 1 10 -10 M yr . And there is an additional, more serious barrier to discovery. Nearly 3 Dr. Joe Patterson GD 552: The Oldest Cataclysmic Variable Figure 1: The correlation of (the fractional period excess of Psuperhump over Porb) with Porb for superhumping CVs (Patterson 1998). The deduced mass ratio and secondary masses are shown at the right. Superposed are predicted evolutionary paths based on alternate assumptions. Solid curve, J˙ from GR alone. Dashed curve, J˙ = J˙GR + constant. The stars appear to evolve along a curve something like this, or possibly a family of related curves. The numbers accompanying the curves indicate how many Gyr are required to reach that point. Note that unless CV formation is relatively new in the Galaxy, we’d expect most CVs to be on the lower branch of the curve, past period bounce. A few might be (two?), but they are so close to period minimum that it is hard to be sure. GD 552 is at much longer period, a much better candidate. Since it does not erupt, it has no superhumps and hence no , so we depict it with the q upper limit from spectroscopy. 4 Dr. Joe Patterson GD 552: The Oldest Cataclysmic Variable Figure 2: The empirical correlation of with q for the 7 CVs (boxes) with a good-quality q independently available from eclipses, plus 3 X-ray binaries with q from other means (essentially spectroscopy). The good quality of the linear t, and the fact that it goes plausibly through the origin (precession should be innitely slow in the limit of no secondary), testify to the value of using to learn q. all the short-period systems in Fig. 1 were found by their dwarf nova outbursts, which are thought to occur when the disk accumulates enough material to undergo an instability. If the timescale for loading up the disk exceeds the disk’s diusion timescale (i.e., the timescale for steady accretion because of residual viscosity in the low state), the instability never develops, and the star never outbursts. We don’t know what the diusion rate is, but observations to 13 12 1 date (from X-ray emission in quiescent dwarf novae) suggest 10 to 3 10 M yr ; below this, the star hides from variability searches. It also hides from UV-excess searches, since the disk is too cool, and the white dwarf, largely unheated by accretion and quite old, is also too cool. But very nearby examples should turn up in proper-motion surveys. Only two CVs have been discovered this way; the better-studied case is GP Com, a helium double-degenerate. Unfortunately helium stars represent only 1% of all CVs, and so do not help us (much) with understanding the main line of CV evolution. The other star is the subject of this proposal, GD 552. Greenstein & Giclas (1978, hereafter GG) identied this emission-line star from the Lowell proper-motion lists, and suggested that the ux distribution very roughly ts a 9000 K white dwarf, with disagreement at the shortest and longest wavelengths. Stover (1985) later reported a spectroscopic s-wave in the H-beta emission, moving with a frequency of 15 1 cycle d1 and thereby verifying that the star is a CV. GG’s ux distribution suggests a combination of cool ( 5000 K) continuum from an optically thin disk, plus a smaller, hotter source in the UV which is presumably the white dwarf. All other H-rich stars of this type are dwarf novae, so GD 552 is distinctive in having no eruptions – none in the star’s history, and none in detailed coverage by the Center for Backyard Astrophysics over the last 5 Dr.
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