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The Recurrent Nova T Pyx: Distance and Remnant Geometry from Light Echoes

Columbia University

Stephen Lawrence (Hofstra University), Jennifer Sokoloski, Arlin Crotts and Helena Uthas (Columbia University)

Figure 1: Series of HST WFC3/F547M images, from 2011 Sep 19, Sep 26, November 16, December 10, and 2012 January 22, showing the visual-light echo. In the left two panels, the top and bottom arrows point to echos that appeared in regions N1 and S1 in Figure 2, respectively. The arrows in the center panel indicate echos that appeared in regions N2, NW, W, and S2 (clockwise from top).


WFC3 F656N 2011-11-15

The recurrent nova T Pyxidis (T Pyx) is well known for its small binary separation, its unusually high luminosity in quiescence, and the spectacular Hubble Space Telescope (HST ) images of its surrounding remnant. In 2011 April, T Pyx erupted for the first time since 1966. Here we describe HST observations in late 2011 of a transient reflection nebula around the erupting white dwarf (WD). Our observations of this light echo in the pre-existing remnant show that it is dominated by a clumpy disk with a radius of about 5′′ and an inclination of 30◦ – 40◦, with the eastern edge tilted toward the observer. The delay times between the direct optical light from the central source, and the scattering of this light from dust in several clumps with the same foreground distance as the central source, give a distance to T Pyx of 4.8±0.5 kpc. The large distance of 4.8 kpc supports the contention that T Pyx has an extraordinarily high rate of mass transfer in quiescence, and thus that nova explosions themselves can enhance mass loss from a donor star, and reduce the time between eruptions in a close binary. BACKGROUND AND HST OBSERVATIONS T Pyx is an unusual cataclysmic variable (CV) that has suffered recurrent nova eruptions roughly every 20 years from 1890 to 1966. Compared to other CVs with similar orbital periods, T Pyx has a much higher accretion rate, and hence luminosity (e.g., Patterson et al. 1998). It can also have a high ejecta mass for a recurrent nova (Nelson et al. 2013; Patterson et al. 2013). Both the high luminosity and frequent nova eruptions may be part of a self-perpetuating cycle in which nova eruptions trigger enhanced mass-loss from the donor star, as first proposed by Knigge et al. (2000). Previous images of the remnant around T Pyx in Hα and [NII] line emission revealed thousands of knots within about 6′′ of the central binary and a halo possibly extending out to 10′′ — resulting from multiple recent nova eruptions (Shara et al. 1989, 1997; Schaefer et al. 2010). The most recent eruption was discovered on 2011 April 14.29 (Waagen 2011). Because T Pyx is the only nova confirmed to have erupted within the remnant of its own pre-existing ejecta, it provides a rare opportunity to examine the recent ejecta from a nova using a light echo. From 2011 May through 2012 May we collected 12 epochs of HST images and spectra in an effort to detect and track scattered light in the pre-existing nova ejecta. We were successful in four imaging epochs in late 2011, using the WFC3 F547M (Str¨omgren-y) filter. During our 2011 December STIS visit we also successfully captured an actively echoing region within the 52x2 slit with the G430L and G750L low resolution gratings. Our imaging and spectral reductions followed the standard STSDAS pipeline tasks, with additional manual screening for remaining cosmic rays, chip defects and charge-transfer artifacts. We subtracted the bright central source in each WFC3 image using the 2011 July image as a PSF template.




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Figure 2: Left panel: V -band light curves of the erupting WD (solid lines), with WFC3/F547M light curves (symbols) from the six strongest echo regions overplotted, after shifting by the delay times found in our analysis. Center panel: Extraction regions for the echo light curves, superimposed on the PSF-subtracted WFC3/F547M image from 2011 July. Right panel: a) The STIS slit position on 2011-12-05 superimposed on a PSF-subtracted WFC3/F656N image from 2011-11-15. The blue rectangle shows the region of the spectral extraction. b) A summed, one-dimensional extraction of the combined G430L and G750L spectra. Colored rectangles show shows the expected spectral range of diffuse emission uniformly filling the slit for [O III] 5007 (green), Hα (red) and the WFC3/F547M filter (yellow) used in our light-echo analysis; there is no strong line emission in this passband.

ANALYIS AND RESULTS For a dust echo seen at time t, the foreground distance z is given by: ct ρ2 − , z= 2ct 2


where ρ = θD is the distance projected onto the plane of the sky, θ is the angular separation from the nova, D the distance to T Pyx and c is the speed of light. With measurements of θ and t, if z can be determined, then D is calculable. The morphology and progression of the light echo reveals that the remnant around T Pyx is a clumpy disk inclined 30◦–40◦ on a N-S axis. Assuming the CV is coplanar with this disk, the delay times for the four echo regions to the north and south of the central binary give the distance to T Pyx. Assuming z = 0, we obtain four distinct distance estimates that cluster tightly around D = 4.8 ± 0.5 kpc (with an uncertainty increased by our geometrical assumptions and the complexity of the nova light curve). Finding a disk-shaped remnant around T Pyx, with an inclination comparable to that of the orbital plane of the binary, strongly suggests that the donor star helped shape the ejecta. Hydrodynamical simulations demonstrate that a companion can produce rings such as those seen in images of old novae. Alternatively, we cannot rule out the possibility that some of the disk material emanated directly from the heated donor.

In T Pyx, the increase in orbital period during quiescence indicates than even between outbursts, the binary blows a strong wind. A distance to T Pyx of 4.8 ± 0.5 kpc means that the quiescent-state accretion rate in this system is inescapably high, approximately three orders of magnitude higher than expected for mass transfer driven by gravitational radiation (e.g., Knigge et al. 2000). If such high accretion rates can be stimulated by nova eruptions in close binaries, and if they can be maintained for even a few decades following an explosion, then novae must strongly impact the evolution of a close binary. Our confirmation of the large distance and high quiescent-state rate of accretion for T Pyx could have bearing on the problem of how the WDs in CVs can grow to surprisingly large masses and how they might possibly evolve to become type Ia supernovae. Knigge, Ch., King, A. R., & Patterson, J. 2000, A&A, 364, L75 Nelson, T. 2013, arXiv:1211.3112 Patterson J. et al. 1998, PASP, 110, 380 Patterson, J., et al. 2013, arXiv:1303.0736 Schaefer, B. E., Pagnotta, A., & Shara, M. M. 2010, ApJ, 708, 381 Shara, M. M., Moffat, A. F. J., Williams, R. E., & Cohen, J. G. 1989, ApJ, 337, 720 Shara, M. M., Zurek, D. R., Williams, R. E., Prialnik, D., Gilmozzi, Moffat, A. F. J. 1997, AJ, Waagen, E. 2011, CBET 2700

Accepted in Astrophysical Journal Letters, 2013:

Stephen Lawrence