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Department of Astronomy, University of Minnesota, 116 Church Street, S.E., ... Planetary Science Institute, Astrophysics Group, 620 North Sixth Avenue, Tucson, ...
The Astronomical Journal, 124:2888–2898, 2002 November # 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A.

HUBBLE SPACE TELESCOPE NICMOS OBSERVATIONS OF CLASSICAL NOVA SHELLS1 Joachim Krautter Landessternwarte, Ko¨nigstuhl, Universita¨t Heidelberg, D-69117 Heidelberg, Germany

Charles E. Woodward, Michael T. Schuster, Robert D. Gehrz, and Terry J. Jones Department of Astronomy, University of Minnesota, 116 Church Street, S.E., Minneapolis, MN 55455

Kunegunda Belle Planetary Science Institute, Astrophysics Group, 620 North Sixth Avenue, Tucson, AZ 85705

A. Evans and S. P. S. Leyers Department of Physics, Keele University, Keele, Staffordshire ST5 5BG, UK

Sumner Starrfield Department of Physics and Astronomy, Arizona State University, P.O. Box 871504, Tempe, AZ 85287

James Truran Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637

and Matthew A. Greenhouse NASA Goddard Space Flight Center, Code 685, Greenbelt, MD 20771 Received 2002 May 6; accepted 2002 July 29

ABSTRACT We present results of a campaign to obtain near-infrared images of evolved classical nova shells by using the Near-Infrared Camera and Multi-Image Spectrometer (NICMOS) on the Hubble Space Telescope. Images were obtained in six filters from 1.87 to 2.37 lm. Three of our four targets, QV Vulpeculae (Nova Vul 1987), QU Vulpeculae (Nova Vul 1984 No. 2), and V1974 Cygni (Nova Cyg 1992) exhibit pronounced shell emission in the narrowband Pa (F187N) filter. In the other five filters, the shells are only marginally detected with the exception of QU Vul, which exhibits spatially extended low surface brightness emission in the medium-band filter (F222M) centered near 2.22 lm. Spatially extended emission from V723 Cassiopeiae (Nova Cas 1995) was not detected above the image noise in any filter. The observed ejecta geometries lend further support to the suggestion that the speed class of a nova may correlate with the shape of the shells. The NICMOS images, in combination with ground-based spectroscopic determination of the ejecta expansion velocities, enable us to estimate the expansion parallaxes for V1974 Cyg, QU Vul, and QV Vul. Of note, the expansion velocity (in milliarcseconds per day) determined for V1974 Cyg has remained constant over the 4 year period (1993–1998) after onset of the optical thin phase. Lastly, more structures in the shell of V1974 Cyg have appeared since 1993. Key words: novae, cataclysmic variables — stars: individual (V723 Cassiopeiae=Nova Cassiopeia 1995, V1974 Cygni=Nova Cygnus 1992, QU Vulpeculae=Nova Vulpecula 1984 No. 2, QV Vulpeculae=Nova Vulpecula 1987) M of material into space at velocities that can exceed 5000 km s1, with strongly nonsolar compositions (for a review, see Gehrz 1999; Gehrz et al. 1998). About 5 to 10 Galactic novae are discovered per year in the solar neighborhood, while the total Galactic novae rate is 35 per year (Shafter 1997). Spatially resolved images of nova shells enable determination of fundamental physical parameters (e.g., luminosity and degree of clumpiness) of the thermonuclear runaway (TNR) event that are indispensable for realistic modeling of the nova outburst. It is now well established that nova shells are not spherically symmetric. Often, nova shells exhibit complicated structure consisting of many individual emission knots and streamers that evolve temporally (e.g., Paresce et al. 1995). The actual geometry of the ejected shells provides information about the inclination of the underlying binary system and the physics of the common envelope phase when the binary is engulfed by the ejected material. There are also indications of possible abundance gradients in nova ejecta (Gehrz et al. 1992; Evans et al. 1992). In

1. INTRODUCTION

A nova event occurs on the white dwarf (WD) component of a cataclysmic variable binary star system in which a Roche lobe–filling secondary is losing hydrogen-rich material through the inner Lagrangian point onto a WD primary. Theoretical studies show that the accreted layer grows until it reaches a temperature exceeding 107 K at its base, and this layer is sufficiently dense to initiate a thermonuclear runaway (e.g., Starrfield 2001). While classical novae are the thirdmost violent of the stellar explosions that can occur within a galaxy, after -ray bursts and supernovae, they are far more numerous and typically closer. At maximum light, their bolometric luminosities can exceed 105 L (super-Eddington), and they eject more than 104 1 Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

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CLASSICAL NOVA SHELLS principle, such delineation of abundance gradients can be identified from spatially resolved imagery. However, one of the most fundamental parameters that spatially resolved images can provide is a measure of the distance to the novae via expansion parallax measurements (e.g., Martin 1989; Baade 1940). A variety of radio, optical, and near-infrared (IR) observational techniques can produce very high spatial resolution images of nova shells and resolve details as small as 0>05 to 0>2 in extent. In particular, the expansion parallax, thus nova distance, derived from spatially resolved IR images can be directly compared with an independent distance estimate derived from measurement of the blackbody radius determined from measurements of the IR spectral energy distribution during the early optically thick ‘‘ fireball ’’ stage of the post-TNR evolution (e.g., Gehrz 2002). In addition, (1) molecules can be seen in the IR but not in the optical and (2) extinction—which is often not well known for novae—is less important in the IR. Here we present deep Near-Infrared Camera and MultiImage Spectrometer (NICMOS) near-infrared (IR) observations of the remnants of four classical novae, QV Vul, QU Vul, V1974 Cyg, and V723 Cas. These four novae exhibit a wide variety of individual properties (including both oxygen-neon-magnesium [ONeMg] and carbon-oxygen [CO] WD progenitors, variation in t3 speed class [defined as the time required for the nova to decline 3 mag from maximum at visible wavelengths], and dust formation efficiencies) that are useful for interpreting the TNR event and the subsequent temporal evolution of the ejecta material. 2. OBSERVATIONS

Observations of the four novae were obtained with the NICMOS 2 on the Hubble Space Telescope (HST) between 1998 February 10 UT and 1998 August 13 UT. The NICMOS camera has a plate scale of ’0>0757 per pixel and a total field of view of 19>4  19>4. Our observations were conducted in three near-IR emission line filters, as well as three corresponding continuum filters (for a complete description of the NICMOS filters see MacKenty et al. 1997). Total on-source integration times were between 160 and 224 s. Table 1 summarizes the NICMOS 2 IR imagery of the three classical novae detected in our study. Standard STScI pipeline processing packages, CalnicA and CalnicB (Voit 1997), were used to reduce the raw images. Any residual bias, the ‘‘ pedestal effect,’’ which is a quadrant-dependent, time-varying DC offset, was removed with the Pedestal Estimation and Quadrant Equalization software package provided by the STScI (e.g., Bushouse, Dickinson, & van der Marel 2000). Since our HST images are essentially devoid of any stellar point sources from which point-spread functions (PSFs) can be derived, a model PSF was constructed using the Tiny Tim software package (Krist & Hook 1997).2 The advantages of the Tiny Tim PSFs, when compared with PSFs derived from stellar objects in the image fields, are that the Tiny Tim PSFs are noiseless and they can be created at any (x, y) location on the detector with precise subpixel recentering. Of paramount importance in the process of the PSF construction were the pupil alignment mechanism position, 2

See http://www.scivax.stsci.edu/~krist/tinytim.html.

2889 TABLE 1 Observation Log Target Nova

Parameter

V1974 Cyg

QU Vul

QV Vul

Observation date (UT) ........ Days since outburst............. Narrowband filters:a Total integration (s)......... Dither steps ..................... Individual frame exposure (s) ................. Samples per exposure ...... Medium-band filters:b Total integration (s)......... Dither steps ..................... Individual frame exposure (s) ................. Samples per exposure ......

1998 Feb 10 2184

1998 May 7 4881

1998 Aug 13 3925

1152 6

1120 5

1120 5

192 12

224 15

224 15

1152 6

1152 6

960–1120c 6–7d

192 12

192 12

160 13

a

Filters F187N, F190N, F216N, and F215N. Filters F237M and F222M. c Integration time 960 s for F222M and 1120 s for F237M. d Six dither steps for F222M and seven for F237M. b

use of a polychromatic PSF, and the PSF size. Focus can also be manipulated in the Tiny Tim algorithm. However, our analysis showed that sufficient high-quality PSFs could be generated solely by the use of the mean of the associated dither pattern foci, with the PSF centered on the optical axis of the detector (Schuster 2001). The PSF obtained from the above procedure was then used to subtract the central stellar source (WD remnant) in each nova image. For the fit to the stellar source, the PSF had to be aligned and normalized. This procedure was iterated until the residuals were minimized. In the best case, the residuals were comparable to the background image noise; however, optimal minimization was not achieved in all cases. Figure 1 demonstrates the effect of appropriate PSF subtraction in the Pa- continuum images of V1974 Cyg and QU Vul. In the latter case, the residuals are within the background noise, whereas in the case of V1974 Cyg the residuals are somewhat larger than the background noise. However, the residuals are still small compared with the intensity before subtraction. A complication arose when the nebular emission, overlapping the central source, was not negligible, as seen in the cases shown in Figure 1. Two such cases are presented in Figure 2, which shows center cuts across the Pa- emissionline images of QU Vul (Fig. 2a) and QV Vul (Fig. 2b). In these cases the model PSF was given an offset to fit the shape of the shell. 3. RESULTS

3.1. Morphology A shell is detected around three of our four targets, QU Vul, QV Vul, and V1974 Cyg. No shell is detected around V723 Cas. The negative result in the latter case can be ascribed to the fact that the shell of this very slow nova (vexp  300 km s1; Evans et al. 2002) is only slightly larger than the diffraction resolution limit of the HST at IR wavelengths. In addition, because of the brightness of the central object, any faint shell of low surface brightness that extends

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Fig. 1.—Flux density intensity profiles as a function of position (x-axis) through Pa- continuum images (F190N) before (solid line) and after (dotted line) PSF subtraction. (a) QU Vul. The fit residuals are of order the background noise in the image. (b) V1974 Cyg. In this case, the fit residuals are large compared with the background noise in the image and are larger than those in (a).

only marginally beyond the PSF would not be detected because of contrast effects. Thus, V723 Cas will be excluded from further discussion in this text. To determine the extension of the shell, isophotes were fitted to the intensity distributions of the shell. These measurements were performed with the IRAF/STSDAS task ELLIPSE, which fits a family of elliptical isophotes for a specified range of semimajor axes (Jedrzejewsky 1987). The ellipticity and position angle (P.A.) of an isophote are determined by minimizing the error in the mean intensity (Busko 1996). Because of the pixel structure of the detector, the accuracy of the isophote fits is ’0.5 pixels, which corresponds to 37.9 mas. To determine the extensions of the nova shells, we fit the Pa- images since those data have the best signal-to-noise ratio. The isophote used to determine the dimension of the semimajor and semiminor axes of the shells had a mean intensity equal to the mean background intensity plus 5 bkg. By using this rather conservative limit, the size of the shells may be slightly underestimated. Shell

dimensions, as well as ellipticities [e  1  (b/a)] and position angles, are summarized in Table 2. 3.2. V1974 Cygni V1974 Cyg, a moderately fast (t3 = 37 days) ONeMg nova, reached a visual magnitude at maximum brightness of 4.4 mag and was the brightest nova within the last 20 yr. V1974 Cyg was the first nova to be observed in all wavelength ranges from the -ray to the radio regime. The structure and evolution of the shell during the first two years after outburst were studied with the HST in the optical and UV spectral ranges by Paresce (1994) and Paresce et al. (1995). Infrared Satellite Observatory observations conducted by Salama et al. (1996) found evidence that Ne and O emission arose in physically distinct regions in the ejecta. Figure 3 shows the reduced images of V1974 Cyg. The Pa- image of V1974 Cyg exhibits the same shell structure morphology in the IR as observed in the optical by Paresce (1994) and Paresce et al. (1995). There are two components,

Fig. 2.—Flux density intensity profiles as a function of position (x-axis) through Pa- line emission images (F187N) before (solid line) and after (dotted line) PSF subtraction. (a) QU Vul. (b) QV Vul. Note the presence of emission over the central source.

TABLE 2 Measured Shell Dimensions, Geometries, and Distances

Nova

Semimajor Axis (arcsec)

Semiminor Axis (arcsec)

Position Anglea (deg)

Ellipticity

Derived Distanceb (kpc)

V1974 Cyg ...... QU Vul ........... QV Vul............

0.983 1.069 0.542

0.983 1.069 0.476

12.0  5.0 0.0 355.0  5.4

0.00 0.00 0.121

3.72 3.14 3.35

a b

The P.A. of the semimajor axis, counterclockwise from north. Expansion parallax–derived distance (see x 4 of text).

Fig. 3.—V1974 Cyg: PSF-subtracted line and continuum gray-scale images. The field of view is 3>56  3>56 with a ’0>0757 per pixel spatial resolution. North and west are indicated by the arrow. (a) Pa- 1.87 lm. (b) Pa- continuum 1.90 lm. (c) Br- 2.16 lm. (d ) Br- continuum 2.15 lm. (e) CO band 2.37 lm. ( f ) CO continuum 2.22 lm. Note that the PSF-subtraction residuals increase with increasing filter width.

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Fig. 4.—Elliptical isophote fit to the Pa- (F187N) image of V1974 Cyg (Fig. 3a). North and west are indicated by the arrow. The isophote fit has a P.A. = 101=61 measured counterclockwise from north and an ellipticity of 0.255.

a bright ring and an ellipsoidal shell whose largest principal axis is perpendicular to the plane of the ring. The projection of this axis (shell) is 12 counter-clockwise from north. The Pa- isophote fit to the maximum of the ring emission is shown in Figure 4. The isophote has a P.A. = 101=6  5=0, in good agreement with the 97 found by Paresce et al. (1995) at optical wavelengths or the 115 derived from Very Large Array (VLA) 2 cm radio observations of Hjellming (1995). The semimajor axis a of the Pa- isophote of maximum ring emission is 0>638, while the semiminor axis b is 0>475, resulting in an axial ratio of 1.34. Utilizing the measured axial extent from the Pa- images, we derive expansion rates of 0.292  0.011 and 0.218  0.020 mas day1, respectively, for the major and minor axis of the shell, assuming uniform expansion. These expansion rates agree well, within our measurement uncertainty, with those found by Paresce et al. (1995) of 0.297 and 0.218 mas day1, or Hjellming (1995) of 0.296 and 0.219 mas day1. Comparison of the expansion rates derived from the IR and optical observations strongly indicates that the expansion velocity of the shell remained constant (between the epochs of the VLA radio observations [1994], the HST Faint Object Camera [FOC] images [from 1993 through 1994], and the NICMOS images [1998 February]) for nearly 4 years after the onset of the optically thin phase. The length of the shell’s semimajor axis is 0>983. We note that these observations place limits on the amount of circumstellar material that the ejecta are plowing into. Kinematically, one would expect that the ejecta would decelerate once it sweeps up its own mass of surrounding material. We observe no deceleration, implying that the local circumstellar density  is 5.8  1022 g cm3, assuming an ejected mass of 2  104 M and an expansion velocity of 2900 km s1 (Woodward et al. 1995).

Fig. 5.—Flux density isophotes of V1974 Cyg derived from the PSFsubtracted Pa- line (F187N) image (Fig. 3a). The outermost contour line ˚ 1 (sky + 5 ), with has a value of 2.121  1019 ergs cm2 s1 A ˚ 1. contour intervals (4 ) separated by 1.619  1019 ergs cm2 s1 A North and west in the image are indicated by the arrow. The knot and the limb-brightened arcs are clearly distinguished from the inner ring structure in the isodensity contours.

In principle, the observed shell geometry can be used to infer the possible inclination of the system. Under the assumption that the bright ring of emission observed in V1974 Cyg is nearly circular, the inclination of the system on the plane of the sky can be estimated from the ellipticity of the ring. With i = arccosðb=aÞ, we derive i  42 . The ring exhibits pronounced density inhomogeneities. Several knots, visible in the earlier epoch HST optical observations of V1974 Cyg (Paresce et al. 1995), can also be identified in the NICMOS Pa- images as shown in Figure 5. Because of the continued expansion of the shell, over a baseline of nearly four years, more details of the density knots can be spatially resolved. For instance, on the FOC images only one northern knot can be seen, whereas on the Pa- image this knot is split into two separate components. Most pronounced is the southern knot, which shows an excess emission compared with the mean ring emission, ’22%. The two northern knots have an excess emission of 11% and 15%, respectively. 3.3. QU Vulpeculae QU Vul, the archetypal ‘‘ neon nova,’’ provides the paradigm for IR studies of the physical characteristics of a TNR on ONeMg WDs. Within one year after outburst, QU Vul entered into a persistent IR ‘‘ coronal line ’’ phase characterized by a thermal bremsstrahlung continuum with strong emission from IR forbidden fine structure lines (Gehrz et al.

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1995; Saizar et al. 1992; Greenhouse et al. 1990; Gehrz, Grasdalen, & Hackwell 1985). QU Vul also condensed a small amount of dust (107 M) composed of oxygen-rich silicate grains (Gehrz et al. 1995). The expansion velocities inferred for the coronal line and the free-free emitting regions by radio (Taylor et al. 1988) and IR techniques (Greenhouse et al. 1990, 1988) vary by a factor of 8. Recent ground-based IR observations of QU Vul in the K band (broadband 2.2 lm) suggest a shell extension of ’1>3 (Shin et al. 1998), smaller than that predicted from an extrapolation of the ejecta expansion velocities. QU Vul had a t3  40 days (Gehrz et al. 1995). Figure 6 shows the PSF-subtracted line and continuum images of QU Vul. In all three emission-line filters the shell is clearly visible. In the continuum filters, shell emission could be detected in the CO continuum at 2.2 lm. While this emission is even stronger than the corresponding line emission (see discussion below), the Br- continuum at 2.15 lm is barely visible and no continuum at all could be detected at the Pa- continuum filter at 1.90 lm. Semicircular arcs of emission are evident (radius of 0>6) because of limb brightening (Fig. 7). As the Pa- image shows (Fig. 7), the intensity distribution of both the ‘‘ ring ’’ and the inner shell structure are rather inhomogeneous. The most pronounced knot of the east-southeastern quadrant exhibits excess emission, 39% higher compared with the mean flux of the ring. From our Pa- images, we derive an angular expansion rate of 0.219  0.015 mas day1. 3.4. QV Vulpeculae QV Vul is a CO nova that reached a maximum visual magnitude of 7.0 within a few hours of the moment of mass ejection. Subsequent observation and the analysis of the spectral energy distribution of QV Vul conducted over an 700 day period suggested that an optically thick carbon dust shell formed within 83 days after discovery of the outburst. The observed t3 interval for QV Vul was 60 days (Gehrz et al. 1992). QV Vul was an unusual nova event. The measured Doppler expansion velocities (inferred from the FWHM of emission-line profiles) of the gas pseudophotosphere obtained from optical observations and the expansion velocity of the optically thick carbon grain shell inferred from blackbody angular diameter measurements (Gehrz et al. 1992) were distinctly different. The carbon dust shell was moving at nearly 3 times the velocity of the principal ejecta (vprc  460 km s1; Wagner 1987). These observations strongly suggest that large velocity gradients were present in the ejecta.

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As can be seen from Figure 8, a clearly detectable shell around QV Vul is visible only in the Pa- image. In all other line and continuum filters, the shell is not evident ( 3 ). On all images, a stellar-like knot of emission is visible on the western edge of the shell. We attribute this emission to a serendipitous (stellar) source along the line of sight to the nova. Figure 9 shows the isodensity contour plot of the Pa- shell emission in QU Vul. An ellipsoidal structure is clearly present, with a semimajor axis a = 0>542 and a semiminor axis b = 0>476, resulting in an axial ratio of 1.14. Because of the compact structure of the ejecta, no ringlike feature has thus far evolved, in contrast to the structures observed in V1974 Cyg and QU Vul (both fast ONeMg novae). The lack of a ringlike feature in QV Vul may be indicative of the lower ejection velocities associated with CO novae. Using the Pa- image, we find that the semimajor axis is expanding at 0.138  0.001 mas day1, while the semiminor axis is expanding at 0.121  0.003 mas day1. These values are a factor of 2 larger than the 0.056 mas day1 derived by Gehrz et al. (1992) using measurements of the blackbody angular radius of the dust shell. However, this discrepancy is not unanticipated, given that the ionized gas and the dust envelope were moving at different velocities. 3.5. Fluxes Integrated fluxes for the shell emission are listed in Table 3. The fluxes were obtained from the PSF-subtracted images. A mean background was subtracted from the remaining shell emission. In those cases for which no shell flux was present, 3  upper limits are given in the table. The upper limit of the Pa- continuum filter of QU Vul enables an estimate of whether or not the flux in the CO line filter F237M can be due to continuum emission. If we assume as the steepest possible continuum distribution an exponential decay, one obtains a flux ratio F190N = 0.73 F237M. Hence, if the flux of 4.42  1015 ergs cm2 s1 of the QU Vul shell at 2.37 lm was due to continuum emission, the flux at 1.90 lm should be at least 3.24  1015 ergs cm2 s1. Since the 3  limit of 1.29  1015 ergs cm2 s1 at 1.90 lm is considerably lower, one can safely assume that, if any at all, only a small fraction of the flux in the CO filter at 2.37 lm is due to continuum emission. Unexpectedly, the CO continuum filter F222M at 2.22 lm shows a rather strong shell flux of 18.8  1015 ergs cm2 s1. The transmission profile of this filter overlaps the narrow F216N Br- filter, and Br- line emission may contribute up to 40% of the observed F222M emission. Unfortunately, there is also modest wavelength overlap

TABLE 3 Measured Nova Shell Fluxes Measured Fluxa Filter

Centralb  (lm)

FWHMb (lm)

Name

V1974 Cyg

QU Vul

QV Vul

F187N ....... F190N ....... F215N ....... F216N ....... F222M....... F237M.......

1.8740 1.9005 2.1488 2.1642 2.2160 2.3677

0.0192 0.0174 0.0200 0.0208 0.1432 0.1546

Pa- Pa- cont. Br- cont. Br- CO cont. CO

60.3  0.693 1.31 ’2.27 5.16  0.342 ... ...

83.0  0.444 1.17 ’1.77 7.72  0.307 18.8  0.528 4.42  0.647

14.1  0.271 0.600 0.505 0.473 0.813 0.997

a b

Flux units are times 1015 ergs cm2 s1. MacKenty et al. 1997.

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Fig. 6.—QU Vul: PSF-subtracted line and continuum gray-scale images. The field of view is 3>56  3>56 with ’0>0757 per pixel spatial resolution. North and west are indicated by the arrow. (a) Pa- 1.87 lm. (b) Pa- continuum 1.90 lm. (c) Br- 2.16 lm. (d ) Br- continuum 2.15 lm. (e) CO band 2.37 lm. ( f ) CO continuum 2.22 lm. Note the strong shell emission in the CO continuum image.

between the F222M filter and the F237M CO filter. As the flux observed in the F222M continuum filter is a factor of 4.25 greater than that observed in the F237M CO line filter, a significant fraction of the flux observed in the F237M filter could be due to continuum contamination. Greenhouse et al. (1988) detected [Ca viii] coronal line emission at 2.32 lm on days 581 and 828 in QU Vul. However, from day 581 to day 828 the line flux had decreased by 75%, and it seems

unlikely that any [Ca viii] line emission would be present on day 4881, the epoch of the HST NICMOS observations. Alternatively, the excess emission could arise from the He i 2.229 lm (1Po–1S), a strong emission line evident in the spectra of many novae. However, in the absence of additional spectroscopic observations (at resolution / D ’ 2000), the origin of this excess emission observed in the F222M and F237M filters remains not well understood.

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are obtained from expansions in the plane of the sky. Two other sources of uncertainty are the determination of the expansion velocity and the radius of the shell. Often it is difficult to unambiguously assign specific expansion velocities (measured using different emission lines) to a given shell radius. Hence, any distance estimate derived by the expansion parallax method could be affected by errors of order a few tens of percent (e.g., Wade, Harlow, & Ciardullo 2000). Nevertheless, expansion parallaxes are of crucial importance for distance estimates since the errors of other methods, such as MMRD, are much larger in magnitude (within a few kiloparsecs) than parallax techniques, which are real measurements as opposed to derived regression lines through scatter plots. 4.1. V1974 Cygni

Fig. 7.—Flux density isophotes of QU Vul derived from the PSFsubtracted Pa- line (F187N) image (Fig. 6a). The outermost contour line ˚ 1 (sky + 5 ), with contour has a value of 1.638  1019 ergs cm2 s1 A ˚ 1. North and intervals (5 ) separated by 1.598  1019 ergs cm2 s1 A west in the image are indicated by the arrow. The dense equatorial knots are clearly distinguished from the limb-brightened arcs that form nearly a complete ring. The outer shell, with radius 1>07, shows a nearly spherical morphology.

For this nova, a variety of expansion velocities (primarily derived from observations of hydrogen recombination lines) have been published. Shore et al. (1993) determined a value of 1500 km s1; Chochol et al. (1993) found 830– 895 km s1; Chochol et al. (1997) inferred a rate of 1100 km s1; and Rosino et al. (1996) report velocities of 700–950 km s1, while Woodward et al. (1995) derive expansion velocities of 2400–3400 km s1 from the near-IR hydrogen, helium, nitrogen, and oxygen recombination lines. Since we use for our distance determination the maximal extension of the shell, which represents the material with the highest velocities, we chose a value of 2900 km s1 as determined by Woodward et al. (1995) from H recombination lines. With this expansion velocity, we derive a distance of D = 3.72 kpc. This value is greater than the 2.6 kpc determined by Hjellming (1994) using radio expansion parallax observations or that derived by Chochol et al. (1993) of 1.77 kpc. However, our value is comparable to the distance of 3.2 kpc derived by Paresce (1994) and Paresce et al. (1995) using expansion rates from UV and optical HST images. We note that adopting different expansion velocities yields differing distance estimates, while not affecting the expansion rates.

4. EXPANSION PARALLAXES

4.2. QU Vulpeculae

The sizes of the shells (Table 2) can be used to estimate distances to the nova via the expansion parallax, which combines the angular expansion rate with observed expansion velocities. This method provides an alternative independent means of estimating distance that can be compared with distances derived from maximum magnitude versus rate of decline relationships (MMRDs) using optical light curves (e.g., della Valle & Livio 1995). Using spatially resolved images, the distance D in kiloparsecs can be derived from

A wide range of expansion velocities are also reported in the literature for QU Vul. Rosino & Iijima (1987) reported 790 km s1, della Valle et al. (1997) obtained 1190 km s1, and Taylor et al. (1987), 1200 km s1, while Andrea¨ (1991) measured 1500 km s1. Since Andrea¨’s value of vexp = 1500 ˚ ), km s1 is based on only one emission line ([O iii] 5007 A 1 we adopt the value of 1190 km s (FWHM) determined by della Valle et al. (1997) from the average measurement of numerous H emission lines. Using this value and equation (1) we derive a distance of 3.14 kpc. This value is close to the distance of 3.0 kpc determined by Gehrz et al. (1985) using the decline rate of the visual light curve. Previous expansion parallax methods give both higher and lower distance estimates, 3.6 kpc (Taylor et al. 1987), 2.6 kpc (della Valle et al. 1997), and 1.75 kpc (Downes & Duerbeck 2000).

D ¼ 5:77  107 Dtvexp 1 ;

ð1Þ

where Dt is the time since outburst (in days), vexp is the expansion velocity (in kilometers per second), and  is the angular radius of the shell (in arcseconds). Adopting the measured semimajor axis given in Table 2 as the shell radius , we can derive distances to the novae. However, there are several uncertainties that affect the accuracy of the expansion parallax method. As Gill & O’Brien (1998) remark, distance estimates based on the expansion parallax generally assume spherically symmetric ejection; i.e., the expansion velocity is obtained from Doppler shifts along the line of sight, while shell diameters

4.3. QV Vulpeculae From the full-width zero-intensity (FWZI) of the [O iii] ˚ emission line, Andrea¨ (1991) finds an expansion 5007 A velocity of 800 km s1. With this velocity, we obtain the distance D = 3.35 kpc. However, there were indications of components in the ejecta with higher velocities, 900 to 1240 km s1 (Andrillat 1987; Wagner 1987), in spectra obtained

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Fig. 8.—QV Vul: PSF-subtracted line and continuum gray-scale images. The field of view is 3>56  3>56 with ’0>0757 per pixel spatial resolution. North and west are indicated by the arrow. (a) Pa- 1.87 lm. (b) Pa- continuum 1.90 lm. (c) Br- 2.16 lm. (d ) Br- continuum 2.15 lm. (e) CO band 2.37 lm. ( f ) CO continuum 2.22 lm. Note the stellar-like emission slightly above the center of the field in images (b)–( f ).

soon after outburst. Thus, the distance we derive using Andrea¨’s (1991) result represents a lower limit. If vexp = 1200 km s1, we would derive a distance of 5 kpc. Downes & Duerbeck (2000) derive a considerably lower distance, 2.7 kpc, to QU Vul by using similar methods and data. The difference between our results arises from the fact that Downes & Duerbeck (2000) adopt the FWHM value for the expansion velocity and not the FWZI value. From several photometric techniques, Gehrz et al. (1992) derive a distance of 4.6 kpc.

5. DISCUSSION

The physical parameters derived from the HST images of QU Vul disagree with those found by Shin et al. (1998) in several key areas. First, we find a much larger shell size. Shin et al. estimated the shell size by deconvolving azimuthally averaged ground-based images. The NICMOS results extend to fainter brightness levels than could be obtained from ground-based observations and show a shell about 1.5 times the diameter (taking into account shell expansion

No. 5, 2002

CLASSICAL NOVA SHELLS

Fig. 9.—Flux density isophotes of QV Vul derived from the PSFsubtracted Pa- line (F187N) image (Fig. 8a). The outermost contour line ˚ 1 (sky + 5 ), with has a value of 2.034  1019 ergs cm2 s1 A ˚ 1. contour intervals (3 ) separated by 1.197  1019 ergs cm2 s1 A North and west in the image are indicated by the arrow. The knotlike feature on the western edge of the shell has exactly the same position as the background star in Fig. 8.

since the epoch of the Shin et al. observations). The much smaller shell size determined by Shin et al. is due to the fact that our NICMOS observations are much more sensitive to very faint surface brightness emission at larger shell radii. Second, Shin et al. measured a K-band shell flux, ˚ 1, which is only about F = 7.4  1018 ergs cm2 s1 A ˚ 1 half of the shell flux F = 13.1  1018 ergs cm2 s1 A found from the NICMOS images. Shin et al. (1998) assumed that the emission was completely free-free. This model produced a reasonable fit to the sparse visual and K-band measurements but predicted greater 3.6 cm radio emission than the observed upper limit. Our NICMOS observations strongly suggest that the K band contains significant line emission. Moreover, the missing continuum emission within the Pa- continuum filter (Table 3) strongly suggests that only very little of the emission at 2.2 lm is due to free-free continuum emission. This, of course, implies that the free-free component is much weaker than assumed by Shin et al. and would explain the lack of 3.6 cm flux compared with their model. However, this explanation also alters several of their conclusions, including the mass determination. These discrepancies clearly show that the high-resolution NICMOS images provide a more accurate picture of the physical characteristics of the QU Vul shell. Our NICMOS images also support the opinion of Wade et al. (2000) that the method used by Shin et al. is adequate to demonstrate angular expansion but does

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not give correct numerical results because the methodology incorrectly assumes that the profile of a seeing-convolved image of a shell is Gaussian. The shell sizes determined here differ considerably in all three cases from those derived by Downes & Duerbeck (2000) from the same NICMOS data that they obtained from the HST archive. For QU Vul, we found a nearly spherical shell with a maximum radius of 1>07, whereas Downes & Duerbeck (2000) report a smaller ellipsoidal shell with a (total) size of 1>6  1>3. For QV Vul, Downes & Duerbeck (2000) found a spherical shell with a diameter of 1>6, as compared with an ellipsoidal shell with a semimajor and semiminor axis of 0>542 and 0>476, respectively, based on our analysis of the images. For V1974 Cyg, they found again a nearly spherical shell (total size of 1>4  1>3), whereas our radius for the outer shell is 0>983, exterior to an elliptical (’0>292) ring of material. The differences are probably due to the fact that the NICMOS images Downes & Duerbeck (2000) utilized were reduced using only standard procedures. Apparently (based on their published description), Downes & Duerbeck (2000) neither corrected for the pedestal effect nor subtracted the central object (WD remnant) with the proper PSF as we described in x 2. This clearly demonstrates that for many objects the standard pipeline reduction process is insufficient, and that more sophisticated procedures (as demonstrated herein) are required. Slavin, O’Brien, & Dunlop (1995) found a correlation between the speed class of a nova (established by t3-MMRD relations) and the shape of the shells. They asserted that the faster novae produce nearly spherical remnants, whereas slower novae exhibit more pronounced ellipsoidal remnants. The nearly spherical shell of QU Vul (t3  40 days) and the slightly ellipsoidal shell of QV Vul (axial ratio 1.14, t3  60 days) fit well into the model of Slavin et al. (1995; see Fig. 8 of their paper). 6. CONCLUSIONS

NICMOS images of four classical novae were obtained in six different filters between 1.87 and 2.37 lm. No shell could be detected around V723 Cas. The other three novae, QU Vul, QV Vul, and V1974 Cyg, show pronounced shell emission in the narrowband Pa- filter. In the other filters, the shells are only marginally visible, with the exception of QU Vul, which shows somewhat stronger emission in the medium-band filter centered around 2.22 lm, suggesting the possibility that there is significant line emission contributing to the observed flux. All three shells exhibit more or less pronounced density inhomogeneities. QU Vul and V1974 Cyg have twocomponent structure with brighter ringlike emission superposed on a more extended low surface brightness shell emission. Two of the nova shells (QV Vul and V1974 Cyg) exhibit an ellipsoidal morphology, whereas the shell of QU Vul has a spherical structure. The shell of QU Vul is larger than expected compared with previous ground-based observations (Shin et al. 1998). QU Vul and QV Vul fit well into a proposed relation equating the degree of ellipticity with nova speed class (Slavin et al. 1995). The high-resolution, spatially resolved HST IR images enable a more precise determination of the shell radii than is possible from current ground-based observations. Hence, the NICMOS images are well suited to derive, in combina-

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tion with expansion velocities from ground-based spectroscopic observations, expansion parallaxes. Our NICMOS results demonstrate that highly resolved spatial imagery in the IR is a powerful tool to study the shells of novae. J. K. and this work were supported by DLR under grant Verbundsforschung 50OR9910/9. C. E. W., R. D. G., T. J. J., M. T. S., and K. B. were supported by grant GO07831.04-96A from the Space Telescope Science Institute,

which is operated by the Association of Universities for Research in Astronomy, Inc., under contract NAS 5-26555. Additional support for IR astronomy at Minnesota is provided by NSF, the Department of Defense, and the Graduate School of the University of Minnesota. S. Starrfield acknowledges support to Arizona State University from NASA and NSF grants. The authors also thank an anonymous referee for helpful suggestions, which improved the manuscript.

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